NAME OF AUTHOR/NOI# DE L'AUTEUR, Mr. 'Ian S. GORDON h

T tE OF THESIS/TfTRE DE LA TH& % A Nitrogen-Laser-Pumped Dye . -

P h i s s i o n is haQy (panted to ths NATIONAL LlEWW OF L'autwisetiim rst, par !a pr~*(nte: K C O I ~ @ 4 ,le ~ U O T H ~ - - . 2 - , )-- > -

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The euB## reserves other publication r a w , and neithgr the L'sutew se @serve fes eutrtw &&Is do puliHc&tion; ni la -

thesis M* extbisiw extpzts from it m y be printed or olheii thdsenl de longs eqreits aGP c s l t ~ i ne doivent &re impritnds

A NITROGEN- LASER-PUMPED

- -

OF T~ REQUIREMENTS FOR THE DE6mE OF *

MASTER OF SCIENCE*

i n the Dapartnent

A > - A

i s m y not be ..

reproduced in whole or in part, by photocopy or other means, without @rmission of the author.

%

APPROVAL . . \

. . I '\

Name : Ian Sydney 'Gordon

Degree : ast tar of Science t

-~itl$ o f Themis : A Nf trogen Laser Pumped Dye Lmer

- Examining C o w ttee :

Chairman: Be ?. Clayman

J. C. Irwin . , -

Senior Supervisor 0

I hereby g r a n t t o Uproa F -ty t h ~ r- 8

,

' my t h e s i s o r d ' i e e e r t a t i o n (the t i of "whgch =--below) t o u s e n

of t h e Simon F r e s e r u n i v e r s i t y Libr,ary, and t o make p a r t i a l o r e i n g l c

@ c o p i e s on ly f o r such u s e r s o r i n response t o a r e q u e s t from t h e l i b r a r y :

of any o t h e r u n i v e r s i t y , or o t h e r e d u c a t i o n a l i i t s t i t u t i m , on i'te 'own s

behalf or f o r one of fts u s e r e . I f u r t h e r agree t h a t permisatcan f o r A

- m7riprripre-cFpTing of this t h e s i s f o r s c h o l a r l y purposes &y be g r a n t e d

by me o r t h e Dean of Graduate S tud i e s . It is understood t h a t copying

wi thout my written permiss ion.

T i t l e of i h e d i s / ~ i s s e r t s t i o n :

* A Nitrogen-Laser-Pumped Dye Laser system ? ' a

( s i g n a t u r e )

I +. > 1 Pfr. Ian S. Gordon

( d a t e )

I

, , . .

P . . . . .

. , .

STRACT . ~ . .

1 1 .

. . . .

%

, A fiitrogen-'l~ser~pu~~ped dye l a s e r system has been ,.- .\

designed. constructed, and evaluated. me ni t rogen laser #

?

-ou'tput was i n the form of p u b e s whose width between the

half-pawer poin ts wds about. 9 nsec. The nitrogen laser Y gave a peak power of 500 kW i n each pulse and a t the

average pa re r of 38 xWwa.8 obtained, The n i t r o g e n - l a s e r , ,

was i n tu rn used t o pump a dye laser; The dye laaer, -- -- -- ----

with Rhodamine 60 a s the ac t ive medium, gave a maximum " 4 i

. v

average pqJer output of approximately .064 mW. The d=

laser system h a s been used t'og perform preliminary Rman

s c a t t e r i n g experiments'on ZnSe and ZnTe. The fizst-order , I

. Raqan spectra of both ccrmpbunds have been obtained and I

\ ' i n addi t ion a very brief inves t iga t ion of the Resonant I

- . Raman effect i n ZnTe has been car r ied out. The fe t t s ib i t i t y

-

of us ingsoch &laser aystera for Raman sca t te r ingexpez iments ' -

is discussed and p o e s h l e improvements suggested.

e pleasure working with , him. A Thankrr are due to Frank Wick . e '

and-others in the machina shop twit weill aa Wally Hall kn the P

/ ' r o n i c shop. USO, thank you' T.B. B for stixnulaeing

4% -Y/ c.

many usef discussions. I also thank my colleagues,

----

many valuable

Acknowl dgment is made to'the National Research Council '?

Assistantships, aild to J . Q . lmii who provided f inanciai P""

assistance through his .grant. i d

Table of Contents I

Abstract

Acknowledgements 1'-

- * a - i v

~ i s t of Tables vii k , ,

\

. L i s t of Figures + v i f i d

Chapter 1 Introduction 1 . \

Chapter 2 The.Pulsed Molecular Nitrogen -Laser 6 0

-

e r Tr*si#$ons i n "the Nitrogen ' 6

. 2 The Rate Equations for the 8 U

i

- - -

'Inversion Criterion 11

1 4 ' 2.4 Power Output

2.5 The Nitrogen-Helium, ~ a s e r

2.6 Descript~onZof the Nitrogen Laser q. I I _

2,6a The Laser Box , 20 a . .. -

2.6b The ~ i e c t r o d e a 22

2 . 6 ~ ' The Gas Supplr system A t

24

I I 2.6d The Electrical Circuit L

0 . - 2 -?+ De.tmztora 27

* ~ 0 7 I

+ 2; 8 ~ i s c h a r ~ e Characteristics of the 27

Nitrogen Lasee

2 .9 Output Energy of the'gitrogen Yes 29

$

b.

r Chapter 3 T u n e a b l e p u l s e d Dye L a s e r s 37 - - .

-". <

3.1 t he Dye ~olecule 37 - P r=

3 . 2 L i g h t Ab$Mpti% by Dyes ' 38 i

, 3 . 3 Dye Laser. Stirnulation W f t h a - 4 3 Pulsed Molecular N i t r o g e n L a s e r

i

.3.4 The Description of the Dye. ~ s a e r , 44 '

3 . 4 a The Dye C e l l F 4 6

3 . 5 The Output of the Dyepaset 51 * r

3.6 Dye8 52

- I

Etfrapted---Sc-- -- - --- 4 . 1 ~ntxoduction a 5 6

56 '

4.2 The Apparatp ,

4

4 . 3 Results 60 ?\

h f

e 4 . 4 R e s o n a n t Rman E f f e c t 63 4 1

.Chapter 5 Cone lusion's - 69

L i s t of References - 72

9

' . 4"

\

7 4 i - d

/

- - - - --- ---A- - -- --

A- - L

%

v - - - - - - - - - - --- - - - -- - - - - - - - - - - -

w

Table * ,.

I The Lasing L i m i t s and Laser output for Dyes Used

Page

LIST; OF FIGURES - - d

Figure Page

An Energy Level Diagram of t h e Nitrogen 7 Molecule P S P '

The Pump cycle of Nitrogen Molecules 9

Calcula ted Laser Power Density Using t h e 12 S a t u r a t i o n ~ ~ p p r o x i m a t i o n ~ - . The,. Laser Box Cross-Section, 23 '

-- - -- --7 T--- u------L-~ - - A- -ALL

w-

The Electrical C i r c u i t - f

?he Bias C i r c u i t f o r t h e Phototube 28 -- - - -- - -

T& Dependence of Pulse-Energy P-~=sNEF- - -31-

The Dependence o f Pu l se Energy on Repe t i t i on R a t e wi th a ~ c t r o g e n Flow 0 of 3 L i t e t s pet Minut

* *.

The Dependence of Pu l se Energy on Repe t i t ion ' 33 t

R a t e wi th a ~ i t r o g e n Flow of 5 L i t e r s per Minute

34' The Dependence o f Average Power Output on ~ e p e t i t i o n Rate

Pulse Shape Measurement f i s 36

The Molecule Butadiene 37 7

,. Eigens t a t e s of a Typica l Dye Molecule wdth 42 Radia t ive and Non-Radiative T r a n s i t i o n s

. . A Simple Tuneable Dye Laser. 44

The Pump Cycle of Dye Molecules ' 4 5

The Layout of the Dye -Laser 47 - - - - - - -

t ,-46 - -- The Frame of t* Dye C e l & Cr

--

Relative Laaer I n t e n s i t y a s a L'unction o r 53 *

dave l eng th for Rhodamine 66 and 7-Diethylamino-4-Methylcoumkfin

Page

Sek-up for Scattering Experiments '. 57

A Ranfan Spectr of ZnSe at R o q n Temperatare ,, ' 7 '61

d k Spectra o ZnTe at Roam Temperature + - , 62

Scattering Amplitude as a Function of the Rdduced Resonance Enesg)', Ai

The Absorption y g e in undbped\ ZnTe at. 300 K -

? .

'he i d e a l l i g h t ' s o u r c e f o r luminescence and l i g h t

,scatterlYng s tud ies would provide an a r b i t r a r i l y i s t e n s e ,

m&ochromatic beam whose frequency could be varied

continuously throughout the electro-magnetic spectruxu.'

d s s c ~ v e r y i n 1966 (Schaf , 1973, p. 1) of t h e dye l a s e r , I - i

provides an approximate equivalence i n the v i s i b l e - - - - - - - -- --- - - - - - - - - - ----- - --- 2

region of thqfspectrum. W i t h a s u i t a b l e s e l e c t i o n of A *i =

\

dyes and exc i t a t ion source, t h e l a s e r can be tuned e

throughout the v i s i b l e spectrum. A t present , t he re a r e ,

e s e e n t i a l l y t h r e e types of dye lasers. c l a s s i j i e d .acc&ding * &

t o t h e type of exc i t a t ion , These three sources of *

exc i t a t ion a r e t h e flashlamp. cw argon ion l a s e r s , and %

t h e ni trogen l a s e r .

One pods ib i l i ty of pumping a dye l a s e r i s t o use

a f lashlamp. T h b e , however, a r e not very e f f i c i e n t

f o r severa l reasons. F i r s t oT a l l , they pr0duce.a l o t

of l i g h t which 4s not of a s u i t a b l e wavelength t o pump

the dye; only a f r a c t i o n of t h e l i g h t produced can be - - - - -

- - P--- - --- - -- --

A~

used for t h i s puzpose, I t is also-necessary t o f i l t e r ---*.--- out phot&herai~ally ac t ive wavelength; which w i l l decom-

'pose the - me, Secondly, itcis an extended, d i f f u s e -

* .

g 5' A

7 1 1 r i se t ime of t h e pulse i s slow, In the dye rnolecules,~ ' i I a t h e maximum population of t h e exci ted s t a t e responsible 2 ii i

f + f o r t h e l a s e r t ranj i i t ion can be reachid b e f o r e the & I . 5 ,

i n t e n s i t y of t h e f lashlamp is a mqximum: Furthermore, f - 9

[ t he re are many problems associa ted with flashlamps i n

- general , These include a lack of pulse t o pulse repro-

>" - - - - -- -- -- ->

cw l a s e r s , Un t i l very recent ly , 'cw l a s e r s operated only a.,

down t o t h e green, s o t h a t they were unable to pump t

; the dyes t o lower wavelepgths. Recently, however, an

.argon laser operat ing i n the u l t r a - v i o l e t has became a

comerckal ly avai lable . A t present this is t h e most

I ,

h e f f i c i e n t and noise f re&i ta t ion source f o r use with

I dye l a s e r s bu t it is- very expensive and has a r a t h e r

s h o r t lifetime. I

\

F t The t h i r d exc i t a t iog aource for dyes is the ni t rogen L

1 e

1 b

laser. U n t i l r ecen t ly r t h i s was t h e only laser which

I operated i n t h e near ul t ra -v io le t with a reasonqble - -- ----

power output; ~ h e strong l irX6f t h i C l i i s F i* a t - % ~ 2 - .

p-pppp

. ma iind is thus well s u i t e d for pumping dyes. ~ T ~ t r o g e n .. d

T- laser output is i n t h e form o f zt pulse l a s t h g f o r about c' - 8 I

- d" - 10 . se&nda, so t h a t with a t y p i c a l peak Gwer of 100

kilowatts and pulse repetition rate of 100 pulses per ~

second, a cw output of .lwatts is obtained. This laaer

can be built of readily available materials, and the

gases required, .nitrogen and helium, are readily available.

Furthemre, the durability of the laser. is very good. L .

There are two main disadvantages of this particular '1

- - excitation - - - -- source. - - A - - Firstly, A- the -- laser -- -- -- produces - -- a ---A large - - P

amount of electrical noise interferes with any

- 'sursounding equipment, difficult to separate

low. ?.An output of .1 watts, wheh combi&ed with a dye

laser efficiency of somewhat less than 101, yields an

output of less than 10 milliwatts. Thus, to obtain

results from a scattering experiment, qui- a sophisticated

detection procedure is required. These two main dis-

-advantages, however, are.also shared with the flashlamp r* -4

excitation source.

The nitrogen laser has many advantages over the

flashlamp; its output beam is monochromatic and collimated 9

to allow easy focussing, its risetime is short, and its ... - * .

reproducibiXity is good. It was therefore decided to 7 -

- - - - -- - -- - -- - - - -

build a n~t~gen-fas&-Pumpesd dye laser ~~st&. This @

laser syst- has been designed and constructed, and then

used to prform preliminary scattering experintents on - -

single crystals of ZnSe and ZnTe. The first order spectra

of both these crystals have been observed and the spectrum

of ZnTe has been investigated as a function of the excita-

tion energy. Although both compounds are familiar and

have been extensively studied, the resonant Raman effect

in these compounds has not been investigated for a lack

of a suitable source.

The organization of the thesis is as follows.

Chapter 2 contains the theory of the nitrogen laser and

explains why this laser when operated in the ultra-violet

cannot be made to work continuously. It describes the

nitrogen laser, giving details on design considerations

and construction of the laser cavity, the electrodes,

the gas supply system, and the electrical circuitry.

The discharge characteristics, pulse shape, and power

output as a function of various parameters such as gas

mixture and pulse repetition rate are also given.

Chapter 3 contains a brief review of the structure of

dye molecules, and the important transitions concerned

in laser operation. The dye laser is then described;

its various components are described and the output

characteristics for 4 different dyes are given. The

next chapter describes Raman scattering experiments

performed on ZnSe and ZnTe using the nitrogen-laser-

pumped dye laser system. These were difficult because

electrical noise from the nitrogen laser interfered with

p. 4

the s igna l s , and the s ignal was weak. To help r e c t i f y

the s i tuat ion , the detection system was physically removed

from the room of the nitrogen laser , and the experiments

carried out i n t h i s manner. The final chapter suggests 8

possible improvements t o the system that would further

s i m p l i f y the of such experiments and discusses

CHAPTER

THE PULSED MOLECULAR NITROGEN LAS

2 . 1 Laser Transi t ions i h the Nitrogen Molechle

T h e n i t rogen molecule .is diatomic and has many , s

bands of e l e c t r o n i c t r a n s i t i o n s . An energy l e v e l diagram

for the ni t rcqeh molecule is shawn i n f igu re 1, and the

f irsz a n d secon-dt -posZC1ve bbahldZlllare fa?. ed . ~ ~ e x - X t % ' & % ~ ~ ~ ~ -

i n both these first and second pos i t ive bands has been

observed. - -- - -- - The, l a s e r l i n e s corresponding -- - - -- t o - - t he - - - - e l e c t r o n i c - -- -- --

- G - - -- -

t r a n s i t i o n B'E +. A" or, t h e f i r s t ' p o s i t i v e group; was g &' F

reported by Mathias

Associated w i t h the

and Parker (1963) i n March 1963.

e l e c t r o n i c t r a n s i t i o n a r e maw

v ib ra t iona l l i n e s and t h e r e s u l t i n g bands f a l l i n the

near infra-red. The l a s e r l i n e s corresponding t o the

e l e c t r o n i c t r a n s i t i o n c3nU -+ B ' I ~ ~ , o r the second pos i t ive

group,, occur i n t h e near u l t ra -v io le t . Lasing ac t ion i n .

t h i q band w a s reported by Heard (1963) i n November 1963. I

There a r e a t l e a s t seventeen l i n e s corresponding t o the

0-0 r o t a t i o n a l band of this e l e c t r o n i c t r a n s i t i o n (Kasuya #

and L i d e , 1967). These l i e bhtween 337.139 nm and' f

337.24 nm i n vacuum, and range i n - -- i n t e n s i t y -- - from -- very - - --- - -

s t rong (337.18 nm) t o weak (7 d i f f e r e n t -- l i n e s ) . T h i s

discovery caused a g r e a t dea l of -excitement s ince it was

t h e highest frequency las-er a v a i l a b k _at that ti=. - -

F'ig. 1. An Energy Level Diagram @f the ~itrogen Molecule (Herzberg, 1967. p .449) . (

a

c o l l i s i o n l a s e r , Electrons of s u f f i c i e n t l y m g h energy &

(produced by a f i e l d of geveral thousand v o l t s per inch)

c o l l i d e with ni t rogen molecules which a r e exc i ted i n t o

higher energy l eve l s , The populat ' on densi ty of each d- l B

l e v e l can be determined from a s u i t a b l e set of rate equa- - t i o n s and these equations can i n turn be used t o provide a

t

guide f o r t h e d e s i Q a n d constrkction of t h e ni trogen Laser.

" t he re fo re be described.

* . 2 .2 The ate bquat ions f o r the Nitroqen Laser

7 d The u l t r a - v i o l e t n i t roge laser is bas ica l ly a a /r'

three- level laser, s ince .the n#trogen molecule is first C

r a i s e d from its ground s t a t e t o the C e l e c t r o n i c s t a t e ,

ahd then l a s ing ac t ion between the C and B e l e c t r o n i c .

s t a t e s * occurs. These t h r e e s t a t e s a r e shown on the

energy le 2 e l diagram of the nitrogen molecule ( f igure 1)

and i n f igu re 2 with abbreviated notat ion. Udng t h i s

no ta t ion , t h e equations governing a three-level l a s e r i n

general , and t h e ni trogen l a s e r i n p a r t i c u l a r , can be - - - - - -- - - - - - - - - -

w r i t t e n as ( A l i , KoLb , 46% Anderson, 1967) : -- ---- -- - -- -- - --

337.1 nnt

F i g . 2. The Pump Cycle of N i t r o u e n Molecules. - - - -- - - - -

dN1 and - = -(X12+Xtt)N~+(r;:+Y~I)N~+(r;:+Y,1)~~e . (3)

d t

, . c*

Here N l rN2ir and N are the population d e n s i t i e s ~f the

o the r tentis i n quest ion; and gz=gs ( the B and C s t a t e s

a r e both triplet states and have t h e same' i s t a t i s t i c a l I - -

and ( 2 ) may now be wr i t t en as

is the r a t e of c o l l i s i o n a l ekc i t a t ion from l e v e l i 'ij .

t o l e v e l j where i < j ; Y " is the- . ra te of c o l l i s i o n a l ji

- 1

deexci ta t ion fro? j t o i ; 3 i is the rate of r ad ia t ive

decay from j t o i; Ri is the r a t e of induced emission;

and 9 3 and g2 a r e t h e s t a t i s t i c a l weights of t h e upper

and lower l e v e l s respectively. .

In the case of the ni t rogen molecular lmer , N l r .

Nz, and Ng denote the 'popula t ion of the ground s t a t e ,

, and the e l e c t k n i c l eve l s B" and C 'll respect ively . ' ' Q U

Also r;; << T;: : Y 1 and Y2 1 a& very small compared t o

- 11 - ,

dNp b a z

and - .= XI~NI+~~;;+Y~~)N)-(T;;+X~ 3 ) ~ 2 + ~ 3 2 (NI-NI) (5 )

* J I n t h e case of the ni t rogen laser, equat ions ( 4 )

and (5) above are n o t exac t , s i n c e t h e r e are a d d i t i o n a l

l e v e l s above the C l e v e l . Furthermore, there i s the 1

effect of e l e c t r o n s c o l l i d i n g w i t h the exc i t ed moladules

i

t o t h e s e equat ions are those which inc'lude t h e i o n i z a t i o n

s ign3 f i can t ly altered; peak power is reduced by 3% and t

t h e half-width is reduced by 10%. These equation- 8- -+

thus ' i n a s u i t a b l e form f o r obtai 'ning estimates of how

long it is p o s s i b l e f o r the invers ion c r i t e r i o n td hold.

2 .3 h e Invers ion ' c r i t e r i o n

I n this s e c t i d n it is shown that the condi t ion

N+N* holds on ly f o r t imes smaller. , . than ~ 3 2 (40 nsec,.

A l i , Kolb, and Anderson, i

n i t rogen laser can never 6

..pulsed laser.

2

1967), s o t h a t the u&t ra -v i a l e t

be comtinuous~ it must be a -

F i g . j. Calculated Laser Power Density Using the Saturation Approximation (Ali, Anderson, .and Kolb, 1967).

which can be integrated, assuming N l and the pumping .

ra te constant, to y e i l d - - -A - - A -- - - --- -- - - - 7 " - - - I - --

&

N2+N3 X 1 3 N l t 0 (7 ) .

3 ,

t h j s term the inversion cr i t er ion w i l l only hold for "

Y even shorter times), and subst i tut ing the &-ove equation , -

i n t o ( 4 ) , it is found that

Solving for N /

4" C

Thus, f o r N s>N2 t o hold ,

This means invers ion can pnly t ake p lace i n times smal l

t h i s .time. Y 3 2 , "the c o l l i s i o n a l deexci ta t ioh. term. is 1

not always small canpared t o r Y ' 2 , tm r a d i a t i v %r decay term. 'In f a c t , with a high densi ty of l o w energy e l ec t rons

p re sen t , e l e c t r o n - excitedan~f trogen morecuke col l i s . ions

may drop more n i t rogen molecules from t h e e l e c t r o n i c C - s t a t e ( A l i , Kolb, and Anderson, 1967) than spontaneous .

'a

r a d i a t i v e decay. W e w i l l , how&&, alway* have the

-maximum condi t ion t < ~ ~ ~ ; that $s, invers ion can only

l as t for times less than. T 3 1 (40 nsec) . K It & thus seen +at the output of $he molecular

n i t rogen laser w i l l be i n t h e form of pulses of durat ion

I n order t o p r e d i c t the power output of the ni t rogen

laser, the energy dens i ty wi-thiri the laser is required.

This can be determined from ' the rate eqiiatio s, s inde I

4 "s,

the change i n population densi ty bf the l a s i n g s t a t e s

w i l l give the photon dens.ity , and E ~ T , t h e energy of the

l a s e r p8oton is knwn, The r a t e equations, however, * -

are c lose ly coupled t o t h e - ion iza t ion r a t e .equation and. j

- d e e l e c t r o n energy equation ,* s ince the energy of the

e lec t rons are responsible f o r exc i t ing t h e ni t rogen d

The change i n energy densi ty p i n the laser a t ' t h e

laser frequency can be obtained 'from

El d t 9 ' i C

--a

. The rate 'of ion iza t ion , assuming binary coklisions n

t where Ne is. t h e e l ec t ron dens i ty , ai is t h e i i a t i o n t

7

0 ,

cross-section, and v is t h e ele rori welocity. 9 d- B

n a l l y , t h e r a t e of c h n g e of e lec t ron energy is - - - - - - - - -- -- - - - - - - - - - - - - - \ - - -

" given by

. . P

- 16 - , C

- where k is Boltztaann's constant , Te is t h e e l e c t m n .. -

temperature, 3 is'the cur ren t densi ty , E are the i ,

j i energies of the t r a n s i t i o n j t o i, E, is t h e ion iza t ion

- energy of the ni trogen molecule, and XVE, i s t h e r~>

of energy l o s s by, t h e ,electrons t o t h e ground s t a t e -L-j

- 3

1

' v ib ra t iona l leve ls . * .- i

To obta in t h e cu r ren t densi ty from t h e above equation,

t h e remainder of t h e e l e c t r i c a l c i r c u i t must be considered. .- The c i r c u i t , shown i n d e t a i l i n f igu re 5, e s s e n t i a l l y ,

, r

vol tage Vo being discharged through t h e l a s e r oavity . %

This c i p u i t i nev i t ab ly has a small but f i n i t e inductance 2-

where 1 is t h e discharge length, -wd A i s t h e &ischarge

area. - 1

The ekc i t a t ion r a t e coe'rf i c i e n t (<a 1 ~ v > = X 1 rNe f , " - t h e ioniaaltion rate c o e f f i c i e n t (,<o )- , t h e 'rate o'f * - energy 106s t o t h e ground s t a t e v ib ra t iona l l e v e l s (XVE; -

-- ' _ A , A (involving terms such as , - i j ijNe . yd j i) o r t h e

ni t rogen &aser have been measured by various authors 9 .

3

-

m ' C

( ~ e r j , 1965; A l i , Kolb, and Addereon, 1967);' Once t h e s e L

*

J t

es a r e known, it i s poss ib le t o so lve t h e equations .

previously developed numerically (equations (4) , (5) I' and

(13) t o (16) ) . The l a s e r power densi ty n& then be p lo t t ed

as a fqnctfon of t i m e f o r d i f f &! e n t c i r c u i t parameters

( including d i f f e r e n t inductances, i n i t i a l capaci tor vo l t -

press,ures&of n i t r o m n . It is thus poss ib le t o p red ic t

condit ions f o r maximizing l a s e r output as a function of

equations f u r t h e r r a t h e r than obtaining the exact' solut ion.

If t h e laser t r a n s i t i o n is sqturated ( sa tu ra t ion has

been observed, Gerry, 1965) , then NI-N2 <<N and m P r N 2 r N . 8

~lso, RS1 ( N 3 - N r ) =P wh&e PE1 is8 t h e sa tu ra ted pa re r output

per u n i t volume. with t h e s e . aieumptions , t h e sum and

h" d i f fe rence of ' equations ( 4 ) and ' ( 5 ) are

'---. ,

and . .

- - - a - - - - -- - - -- -- - - - -- - - - - - -- --

W i -- --- P = -(X13-X12) - -- + (-1 - N ( Y 3 2 - X 2 3 ) . (18) A-

2- 7 ' - T 3 2 2 T 2 I - 3 '. 4

This s i m p l i f i e s equations ( 4 ) and (5) so t h a t one may - - -

obtain' numerical so lu t ions of power,putput vs. @ b e 9 L

as a function of t i m e is shown i n f i g u r e 3.

The maximum gain of the l a s e r may be ca l cu la t ed -t

from the equation (Fowles, 1968, p.2671,

H e r e , m is t h e mass of t h e ni t rogen molecule, T is t h e

abso lu te ~ temperature of t h e ni trogen gas, , c , is t h e speed r;*--~ -- --- ~ - -- -~ ~ -

of l i g h t , f is t h e frequency of' t h e laser l i g h t , and A 2 I

i s E ins t e in ' s c o e f f i c i e n t (l/lOnsec; A l i , Kolb, andp-- --

Anderson, 1967) . A l l k i t s are MKS. Typical values of -- ---

t h e invers ion dens i ty , N3-N2, are of t h e order of 10" . +

( A l i , Kolb, and Anderson, 1967). This w i l l y i e l d a gain

of about 20dB/m i n the ni t rogen laser. =e

2.5 The Nitrogen-Helium

When t h e n i t rogen l a s e r h pulsed a t more th& twb 4

or t h r e e pu lses per second, t h e power output per pu lse

drops noticeably. This is most l i k e l y due t o a decrease

i n tde population dens i ty of t h e ground state ni trogen. r- - --

Thus, i n o rder t o puIse- t h e l aser %any tmes per s e c o n d , ~ - -

1

llr is n e c e s s r q t o reduce t h e relaxationtixneof t h e * .

molecules t o t h e i r ground s t a t e . I n n i t rogen lasers -

opera t ing i n t h e i n f r a - r e d , it has been observed t h a t

t h e recovery t i m e . of t h e laser ( the &nimumrequired

waiting t i m e between pulses) has a l i n e a r dependence

upon gas pressure i n t h e range of 0.5 t o 5 t o r r (Kasuya 9 7 .

and Lide, 1957). T h i s suggests t h a t t h e process of

molecyles c o u i d i n g with each o t h e r - i s t h e dominant *

f a c t o r i n t h e re laxa t ion t i m e , which is t h e c r i t i c a l

f a c t o r i n the recovery time. For ni trogen l a s e r s Qpr-

pqocess is a l s o l i k e l y very important s ince i n both cases

s t a t e A t o the ground s t a t e . I f helium is ~added t o t h e --

system, t h i s process is hastened, and the re laxa t ion

t i m e t o t h e ground state is reduced. -n

Another way t o increase t h e pulse r e p e t i t i p a r a t e

is t o rap id ly flow t h e ni trogen gas through t h e l a s e r .

This process removes long l ived ionized p a r t i c l e s and . r

n e u t r a l m l e c u l e s not i n t h e ground s t a t e and renews

t h e supply of ground s t a t e , ni trogen.

2.6 Descript ion of the Nitrogen Laser.

I n order t o produce e l ec t rons of s u f f i c i e n t l y high *

energy to c o l l i d e with and e x c i t e . t h e ni t rogen molecules ,

t o t h e e&ctronic C state, it -is r r e c e s s a r y - t w p m e - - - - -

- - --- -

a ~ r e i e t - . . a n w

k t h i n t h e l a s e r cavi ty . Many problems w e r e encountered ,

i n t r y i n g t o g e t a good d e s i g n s o t h a t t h e discharge

would indeed be f a i r l y uniform along the length of the

laser. These problems w i l l be d e a l t with

p r i a t e s e c t i o n s descr ib ing t h e laser box, i

l y system, and t h e e l e c t r i c a l

2.6a The Laser Box a'

under, t h e appro-

t h e electrodes,

c i r c u i t r y .

As e a r l i e r mentioned, t h e gain i ni t rogen - * .- - A -2L-- -- "- - -- ----

laser could be 20 dB/m. In order to n as much

power output as poss ib l e , it is d e s i r a b l e t o make t h e

- - - - - f aser - long tcrtakeadtvarrtage--of -this gain; The -ferigth - --- - - - -

3 od the l a s e r , however, i s l i m i t e d by t h e s h o r t du a t ion

of\the gain. A f t e r about 15 nsec, t h e pulse is absorbed

and extinguished because the lower l a q e r l e v e l is meta-

s t a b l e , and t h e per laser s t a t d is no longer populated. 7' 12 15 nsec, l i s h q t r a v e l 3 a d i s t ance (d=ct) of h u t 4.5

metres. I f a s i n g l e pass laser is b u i l t wi th a r e f l e c t i n g

mirror a t one end and ,an output window at t h e other,

som2 of t h e l i g h t w i l l t r a v e l t w i c e t h e length of M e

laser. Since this l i g h t aannot t r a v e l g r e a t e r t h h , 1

about 4.5 metres before being absorbed, it is sense less

t o_bu i ld a laser longer than roughly 2 metres.

electric f i e l d wi thin t h e cav i ty , it*was necessary t o '.%

a - use a trar lsverse discharge geometry. S i n c e It2 is -d i f f i -

c u l t t o produce an even d i scha rge i n a l a r g e volume, t h e

wid th of t h e l a s e r c a v i t y i s l i m i t e d t o about 1 inch.

To m e e t t h e l e n g t h and width requi rements , t h e l a s e r

c a v i t y b u i l t w a s 1 inch wide and 6 f e e t long.

C a r e must be taken when c o n s t r u c t i n g t h e laser box

t h a t a r c i n g cannot t a k e p l a c e a long o r n e a r a c a v i t y

w a l l . This i s complicated by t h e f a c t t h a t a t t h e h igh

v o l t a g e s be ing used, t h e e l e c t r i c a l pa th a long a non-

conduct ing s u r f a c e o f f e r s a lower r e s i s t a n c e t o a d i s c h a r g e

than i n a g a s n o t c l o s e t o any s u r f a c e . Therefore , i n

o r d e r tp o b t a i n a d i s c h a r g e w i t h i n t h e gas and n o t near

a c a v i t y w a l l , it i s necessary t o make t h e d i s t a n c e between

t h e e l e c t r o d e s which a r e conta ined i n t h e c a v i t y less

than one-half t h e d i s t a n c e a long any o t h e r p o s s i b l e pa th

t h e d i s c h a r g e can t a k e when it t r a v e l s nea r a c a v i t y

w a l l . I n t h i s way, t h e d i s c h a r g e w i l l n o t t r a v e l from

t h e ca thode , t o a c a v i t y w a l l , a long t h e c a v i t y w a l l ,

and t o t h e anode.

Also, it may be noted t h a t t h e l a s e r c a v i t y does

n o t have t o main ta in a h igh vacuum. This i s due t o t h e

f a c t t h a t a i r i t s e l f i s 80% n i t r o g e n ( t h e a c t i v e medium

o f t h e l a s e r ) and t h a t t h e l a s e r o p e r a t e s b e s t a t about

100 t o r r o r h ighe r .

The laser box i t s e l f was c o n s t r u c t e d of o r d i n a r y

materials, namely, p l e x i g l a s s ( l u c i t e ) , aluminum, and

brass . The l a s e r box had ins ide dimensions of 1-1/Zmx.

2-3/4"x6'-On, The individual pieces f o r t h e laser were.

- screwed tdgether. O-rings w e r e compressed between t h e 'B

pieces of material sv a s t o make a seal. A cross-section 9

LJI of t h e laser cqvi ty is shown i n f igu re 4 . The end

pieces were of l u c i t e and w e r e bol ted t o t h e ends of t h e

l a s e r box. A gasket between t h e end pieces qdd t h e l a s e r -- - --A - - ---Lu>L ---- -- - D

box was used t o make a s e a l , A length of 2-112" diameter

d , l u c i t e tubing protruded from each end piece, upon which - - - - -- - - - - - - - ---- - - - - -- -- - 7 - - - - - - - - -- - - -- - - - - - - - - - --

was attached a 4" . diameter window (obtained from ESCO

Products and was o p t i c a l grade) a t brewster's angle.

The l a s e r box was bolted onto an inverted U-beam, which

was a t tached t o a heavy t ab le .

As mentioned e a r l i e r , only one mirror was used i n .,

t h i g l a s e r . A f r o n t s i lve red mirror was mounted i n a

.holder which was i n tu rn bolted securely onto t h e inver ted

U-beam t h a t t h e l a s e r r e s t e d on. i

2.6b The Electrodes @

The cathode cons is ted of a bandsaw blade 1 4

t e e t h per inch, and w a s 5'10," long so as t o f a l l 1" 9

- - s h o r t of t h e e n d s o f the caviky. Itprotruded-approx0 -- - - -

, imately 1f2" i n t o t h e i n s i d e of t he l age r box.

The purpose of t h e bandBaw blade was t o produce an

- even discharge -originating-from -the t e e t h o f the hl'ade;

- -

Aluminum

1-1 Plexigloas

Fig. 4. The Laser Box Cross-Section.

t would be easy for t h e discharge t o s tart -A

pointed too th of t h e bandsaw blade.' I f t h e a

simply a @inted edge, alignment between t h e

would be much more c r i t i c a l i n o rder t o ob ta in an even I

discharge along t h e s i x foo t length of t h e cav i ty . -

The anode consis ted of a 6'-On long p iece of aluminum,

which protruded 1" i n t o t h e laser box: I t ~ ~ s i d e s were

tapered down from 1/4" t o a narrow rounded edge. Its '

e

ends were a l s o rounded.

2 . 6 ~ The Gas Supply System ,-

The ni t rogen gas was obtained from C

pressurized, a t between 15 and 20 -p .s . i .

a w a l l supply

The helium f

source was a pressur ized tank with a' s tandard regula tor

t o reduce pressur! t o 20 p. , s . i . Prec is ion nebdle valves

cont ro l led t h e f l o w of t h e gases, The gases w e r e mixed'

in' a *ing chamber and passed from there i n t o t h e laser i

chamber. The Gases w e r pumped t h r o z h t h e laser chamber 'a and o u t t h e opposite end. A manometer a t t h e pump end -

was used t o measure t h e pressure of t h e gases i n toxr .

Using t h i s gas supply system, it w a s poss ib le t o ~ o m p ~ l e t e l y 4

accomplished two thing;. F i r s t l y , . a f r e sh --

supplk of n i t rogen i n i t s ground s tate would replace

ni t rogen molecules t h a t would no longer be i n the ir ground -

s t a t e . Thus a better power output w a s achieved. Secondly, .

2.6d The E l e c t r i c a l C i r c u i t 9

The e l e c t r i c a l c i r c u i t used i e sh i n f i g u r e 5.

A high voltage supply charges up the .(e5 microfarad

capaci tor . When t h e thyrat ron is t r i g h r e d , t h e capaci tor

l a s e r a r e twenty 500 picofarad ceramic ca a c i t o r s . The' 4 l eads from t h e capac i tor t o the l a s e r

--- --,- ---L--- - - - - -- - -- -- - - - -- -

cables , . each about 4 ' ' long, The small

and t h e cable& a r e evenly spaced along t h e 6 foo t length

of t h e l a s e r . The purpose of t h i s arrangement of t h e

'cables and capac i tors i s t o d i s t r i b u t e the charge evenly r9

along the length of t h e laser, s o t h a t tffe discharge

within t h e cavi ty w i l l be f a i r l y uniform along i ts lgngth,

and- the discharge everywhere within .khe cavi ty w i l l occur

simultaneously.

The t r i g g e r u n i t consis ted of a timex and pulse

generator , which t r igg d an SCR> The SCR was r e q d r e d F - t o produce a pulse whohi r a t e of rise of $ltage was a t

l e a s t 1800 v o l t s per microsecond and whose peak, voltage , - - - - -- -- - - - - - - - - - - - -- - - --- - - - - - - - - --

w a s a minumurtl of 550 v o l t s . This pulse was fed i n t o a

1:l pulse transformer which w a s required t o d r i v e t h e

thryrat ron. The thyrat ron would thqn be i n a conducting - - - - - - -

c s

Volts v PC

2d-500pf ' ceramic capacitors

Fig. 5 . The Electrical Circuit

state fo r ap~roximate ly 2 microseconds during which time r

-

the capaci tor would dump its charge across t h e laser .

2.7 Detectors

Two d i f f e r e n t types of detectors were used i n t h i s

experiment? one was a phototube, and the other was .a

'" "py roe l ec t r i c detector ;

and was used t o measure'the pulse shape of t he nitrogen b

l a se r . The b i a s c i r c u i i f o r the'phototuba (f igure 9 reduced

p.160). I t was thus ,poss ib le t o observe pulses $hose

width was t he -o rde r of 1 0 ngec,

A pyroeledtr ic de tec tor , Molectron model PI-71, was i

1

used t o measure t he energy output of t he nitrogen laser.

The peak voltage output of the pyroelec t r ic de tec tor i s

di ' rectly proportional t o t h e energy of t he l a s e r pulse

2.8 Discharge Charac ter i s t ics of the Nitrogen Laser

mat is uniform along the length of the electrodes i s . 0

obtained here was not hrfectly uniform but cons is tedL&

a ae r i e s of narrow channels or ig ina t ing at each tooth

of t h e bhndsaw blade cathode and spreading out towards - - - - - -

GR short circuit - - - -

Tektronix GR coaxial type 847-WN cgpacieor typq 847-K

Laser light pulse

4

50Q coaxial /

~i-a1 BNC

Battery BNC .

L - - - - -- - - - - - - -- -

Fig. 6. The ,Bias Circuit for the Phototube. I

- - - -

the anode. The discharge characteristics depended upon

the voltage, gas composition, and pulse repetition rate.

There was a distinct range of values of these parameters

beyond which the discharge became irregular, with most

of the discharge passing through one or several points.

In this case, most of the energy would be in one or two

bright arc discharges. Most of the gas would remain

unexcited, and c~ns&~uentl~ the laser output would be

reduced and become highly irregular.

With an optimum choice of the parameters mentioned

above a fairly uniform discharge was'obtained, although

some discharge channels always appeared somewhat brighter

than others.

2.9 Output Energy of the Nitrogen Laser

An attempt was made to find the best operating condi-

tions for the laser. The power output of the laser in-

creased with increasing voltage. Therefore, in all measure-

ments, the voltage used was 26 kilovolts, the highest volt-

aqe available from the power supply. It was found that a

mixture of nitrogen and helium was best, with helium

being 2.7 times more concentrated than nitrogen. A

variation of the relative concentrations of these gases

from this ratio would result in a drop in the output.

When 3 liters per minute of nitrogen, and 8 liters

per minute of helium were flowed through the cavity, the

h - optimum pressure was about 120 torr and r e s u l t e h i n M

v

output of 3.8 m i l l i j o u l e e per pulse a t a r e p e t i t i o n rate' , 1

\of 6 ~ u l s a s per second; while when 5 liters per &.nuts 'b I

of nitroq*n, and '13.5 liters of helium were flawed through I

t h e cavi ty , the optimum pressure was about l40 tom.. '

The output pex pulse here increaged t o 5 m i l l i j o u l e s pe r 1

- I

pulse at 6 pulses .per second. Also, it was noted t h a t -- - -- -L---L--L A --A- : &

ps.

t h e optimm pressure seems to f a l l s l i g h t l y as the r e p e t i t i o n cC

rate is ihcreased. These above results a r e shown i n

above, t h e p a r t i a l p r e s s u b of k t r o g e n is 32 torr and .

38 torr respectively. These p ~ s s u r - e s 'ckpare w i t h a

t h e o r e t i c a l opt%mum of about 28 t o r r (~ l i , 1969). .

As the pulse r a t e is increased, the energy output / . -

, per pulse i a reduced, as shown i n 'figures 8 9.

, However, &n though the energy output fa l l s C,

as the r e g e t i t i o r r r a t e ,is increased, t h e total average -b

+

power output ( the energy per pulse times t h e puloe

r e p e t i t i o n rhte) is increased. Thus, the a*erage power

+output inqxeases from about 19 mi l l%~at t s a t 2 pulses

n for a fill pres~ure of 100 ton, and a flow ra te rod

%

5 liters per ainute 6f nitrogen and 13.5 liters per a

- A - - --- - - -

minute of helium, as shown i n f igu re 10. A t r e p e t i t i o n -

- J *

Fig. 7- .The Dependence of PuSsa Energy on Pressut&. ..&

- - - &,6 pulses per second, 5 liters per minute of N A 0 purmr -parse-; 5- literrpar minute-of N$ - --

+ 6 pulses per 'second, 3 liters per 'dnute of N2 010 pulses per second, 3 liters per minute of NZ.

2 * . 4 6 8 10 % Putse Repetition Rate (secaV

t -- Fig. 8. The Dependence of Pulse Energy on Repe'tition

WZth a Nitrogen -3' biters per*-. A 80 torr pressure X 100 torr pressure 0'120 torr pressure - . L - -

fJ 140 tori pressure + 160 torr pressure.

Pulse Repetition Rate kec-'1 '

- - - - P - - - - -- - - - - - - - - -

Pig. 9 - ~ h a ~ e p e n d m c e of 6ul.e Energy on Repetition RateikmgenFZmudNitrocranPlowofrs -@r Min uke~. + 100 torr pressure

X 140 torr preeske 0 160 torr pressure.

F i g . 10. The Depehdence of Average Power Output,on Repetition Rate. + 80 torr pressure, 3 liters per minute of nitrogen f low-

X 120-torr pressure, 3 liters per minute of nitrogen flow 140 torr greesure, 5 liters per minute of nitrogen flow.

rates greater than 10 pu lses per second, however, the

discharge becomes highxy i r r e g u l a r and a rc ing takes place.

Pulse to p u h e consistency is very poor, and power out- ,

pu t fal ls . * .

The shape of t h e output pulse of the ni t rogen laser "

w a s measured as a function of t i m e . It is seen from

f i g u r e 11 tha t the pulse width a t haXf maximum is 9 nsec ---L--u-L -- -" a2u--------u-----~---- -

and the t o t a l dura t ion is 15 nssc. A 5 m i l l i j o u l e pulse,

pu l se as a func t ion of pressure , while f i g u r e s 8 and 9

give the energy output p e r p u l s e as a* funct ion of pulse

r e p e t i t i o n rate. From these f igu res , it is seen t h a t t he

maximum output energy per pu lse at 6 pu l ses per second is

5 m i l l i j o u l e s , wi th a pulsewidth of 9 naec at ha l f

maximum ( f igu re 1311, corresponding t o a peak power of =SO0

k i lowat t s , and an average power of 30 nriLliwatts ( f igure

P gure 10 also shows t h a t a power output of 38 7. e h l l i w a t t s is obta ined a t the maximum obtainable repetition

a

rate o k l 0 pu l ses per sepnd. .srr

3

TUNEABLE PULSED DYE LASERS

/ '

3.1 The Dye Molecule

The chemical d e f i n i t i o n of a bye (Schaf fer , p.6) is any substance which contains conjugated double

bonds are separated by a s i n g l e bond, a s i n t h e molecule L .

Fig. 1 2 . The Molecule Butadiene.

A l l dyes contain a t least one conjugated s e t of double

. bonds.

To understand how these conjugated double bonds

give rise t o t h e nature of dye@,

which is all as & a c t i v e

medium of the dye l a s e r , i t ' i s secesaary t o review the

two - - types - of -- bonding - - - e lec t rons , u - - e lec t rons - -- and --- a e lec t rons - - -

I - -

found i n the conjugated set of double bonda formed by t h e I

A-

\ carbon or lieteroatom i n t h e conjugated chain. There are

three-q e lec t rons , which a r e character ized by rota-t ional

symmetry of t h e i r wave functions with respect ti t h e I

bond diregt ion. These u e l e c t b n s form t h r e e 6 bonds.

Using the example of butadiene above, t h e two end carbon

atoms form two cf bonds with t h e two adjoining hydrogen

L atoms and one a bond with- a second carbon atom, and t h e i

carbon atoms i n t h e middle of t h e chain f o p one a bond a

- with a hydrogen atom and two a bonds wikh t h e tm o the r

, \ l e f t over, which is a , n e lect ron. This R e lec t ron is

I nucleus and has r o t a t i o n a l symmetry about an axis normal 1

- t o t h e $lane subtended by t h e o r b i t a l s of t h e t h r e e

e lect rons . A T bond is formed by t h e l a t e r g l overlap

of these n e lec t rons and is a maximum when t h e &nunetry -

axes of the o rb i t ads are p a r a l l e l . In t h i s pos i t ion ,

then, the bond energy i s minimized. As a r e s u l t , a

conjugated .chain, which cons i s t s of a t o m l inked byp both

. - o bonds and these n bonds is plans; and of high r i g i d i t y .

3.2 Light Absorption by Dyes

The l i g h t absorption by a e s can be ,understood

q u a l i t a t i v e l y (Schaffer , 1973, p.9) using a highly sim- /

of t h e conjugated chain ly ing i n a common plane and l inked - -

by o bonds. he n e lec t rons have a node i n t h e plane of P

' f

1 the molecule and form a charge clovd above dnd below this '

* i I

/ J

plane. The cent res of t h i s cloud are about one-half a

bond length d i s t a n t (above and below) from t h e nodal

plane. The electrostatic p o t e n t i a l , t o a first approx- >

imation, is constant along t h e length of t h e conjugated

'and l a r g e outs ide t h e lengthy of t h e chain. I n a

othe cha$ words, . . the T e lec t ron appears a s a par t i -c le i n a

-- - --- - - - - - me dimensional 4ox. -c& hngkh-L ,- Inthiscase tha._Len~_th _,

eL i s t h e d is tance between t h e end atoms of t h e conjugated

chain, plus two bond lengths (one bond lengeh on each

end of t h e cha in) . The quantum mechanical r e s u l t of a . I .---

p a r t i c l e i n a box i s well known (Schiff , 1968, p.37) .

The energy En of t h e nth e igans ta t e of t h e e l ec t ron is

'where h i s Planckls constant , and m is the mass of khe 9

electron.

Each state can be f i l l e d by exactly two e lec t rons - 7

of opposite spin. Hence, f d r N e lec t rons , we have N/2 ,

s t a t e s , for i n dyes hl is always even. The t r a n s i t i o n ,

then, of the highest oecrtpied%e the-lowerst-uj'toc-bd- -- - - -

1 .

The corresponwng wavelength of a photon abeorbed

i n such a t r a n s i t i o n is given by

where c is t h e speed of l i g h t .

- Thus,-tMe-atode&-indieate~lthat toa-first approx- - - --- -

-- -

intation, t h e pos i t ion of the absorption band is determined

only by L (the length of t h e conjugated chain) and N

( t h e number of n e leg t rons ) . The minimum value thatgmax

can have is about 200 nm. Different dyes have d i f f e r e n t

values of Amax; these vary from t h e near u l t ra -v io le t t o +% 8

t he near i n f ra-red, i'

An absorption hand of. a we usual ly cowre t e n s of

nanometers {Schaffeq, 1973, p.18). This l a r g e width is I . " I

due t o $ f a c t , t h a t s ince t h e dye molecules contain a . -

large number qf at- ( typ ica l ly 50 o r more), it has a '

l a r g e number of normal vibra t ions ( typ ica l ly 150 o r

cm-', and t h i s energy i n t e r v a l is superimposed on the

- electrorric t r a n s i t i o n s . Ul t ra-viole t absorpti6nt w I l l

general ly cover e i t h e r thb first o r t h e second absorption qa

i %

e l e c t r o s t a t f c . perturbations. caused by t h e surrounding

solvent molecuSes. These per turbat ions f u r t h e r broaden I

the individual l i n e s of a v ib ra t iona l series. Further- \

more, we hhve superimposed on these l i n e s a ladder of

r o t a t i o n a i l y excited l eve ls . These a r e very broa6, s ince - - - - - - -- ----

the c o l l ~ f t % s - ~ x ~ h t%e s o k n t molecules are pve'lp"fre- n,-r

quent, reaching t h e order of l o L 2 c o l l i s i o n s per second. I +

These_-lines -are b r o a d e n ~ d ~ s ~ much_ that-we hue -aaa?ntially_ _- -

a continuum i n . t h e absorption band. Consequently, 4

absorption i s near ly continuous over t h i s sand. The

same i s t r u e , of course, f o r t h e f lorescent emission .

which corresponds t o a reverse t r a n s i t i o n from an elec- i -

t r o n i c a l l y exc i ted s t a t e . to t h e ground e l ec t ron ic , s t a t e .

The dye molecule has an even number of n electrons .

This means t h a t t h e exc i ted states of the dye molecule

ean be s i n g l e t states o r triplet s t a t e s , and t h a t f o r . each s i n g l e t state t he re i s a t r i p l e t s t a t e af lower

/ P

* -BT energy ( S ~ h a f f e r , 1973, p. 26) .

Fina l ly w e have a p ic tu re of the e igens ta t e s of the I

i A dye- (see figure 13 1 , The -transi$ian, Sp ~ o - SJ IS- radia t ion- --- -

T h e t r a n s i t i o n s from a s i n g l e t state t o a t r i p l e t state t

4

are' sp in forbidden and are induced through p e r t u ~ b a t i o n s .

Florescence / I

~zlnsec / 1 .

The t r a n s i t t o n TI t o G is a l s o sp in forbidden. Usually Tt

non-gadiative processes whose l i f e t i m e i s general ly of B

4

the order of % microsecond dominate t h i s t r a n s i t i o n .

However, i f t h i s t r a n s i t i o n is r a d i a t i v e , it is termed % -

phosphorescent. This fphosphorescent li f e t i n e usual ly

ranges between mil l iseconds and seconds. . * * . .

*

1:

Nitrogen 'Laoer * -

C

pumps~urcefor - - -- h B

nanoseconds. 't

I n t h i s t i m e , it &s imposs ibh t o t r a n s f e r many of t h e J .

dye m a l e c u l e s ~ r o m an exci ted s i n g l e t s t a t e t o a t r iplet

state, s ince t h i s t r a n s f e r time is of % he order of 100

nqec. Hence, t o a first appxoxihration w e may neglect

a l l tr iplet ' s t a t e s - s i n c e they a r e not populateds

The d j e nblecules absorb pump r a d i a t i o n and a r e . "

l i f t ed from t h e around s t a t e i n t o a higher v f i o n i c I

oi the first or second exc i ted singl=t state. Radiation- A

less deac t iva t ion of t h e dye molecules t o t h e larest r-

v ib ra t iona l l e v e l the first e lec t ron ic s i n g l e t state i

.owurs_ i n picoaeeonds , and- sti$rmlate&emissfc)n- t h e n ~ ~ ~ ~ r s -- - - -

# - - - 4

from the first excited s i n g l e t ska te t o highervibronic-

l e v e l s of t h e ground s t a t e . A f u r t h e r r a d i a t i o n l e a s ' ,

*

- 7 w .-

- 44 - ;I 9

s t a t e within picoseconds. This p h p cycle isshown inr ?' .?

f igure 15,

A simple tuneable dye laser - consis ts of a dye

solution' contained within a c e l l , a d i f f rac t ion grating, e

4 m d an output window as shown i n . t h e diagr 3 below, 2

i

e

t d i f f ract idn

dye A-

>-- - - - - - - - . - - - -- - -- =-- - l a se r axis c e r .

3 - L - - -

a F i g . 14 . A simple Tpneable Dye Laser , s

When & photon of the l a s e r frequency is emitted

along the axis of the l a s e r and s t r i k e s the grat ing, it II

i b r e t u f n e d back along the laser axis , but a p h o t ~ of

a d i f f e r en t ' frequency is returned along a d i f f e r en t axis. s'

When the grating is rota tedr the l a s e r frequency

a&cordingly so t h a t t he photon of t he new laset

is returned'back along the laser axis. It is t h i s sblectn I I

L ive feedback process provided by the grat ing t h a t make

the dye laser tuneabls.

---

The essen t ia l components of tIiiZ@i l a s e r werb a I c

'8 dye cell t o contain the ac t ive medium, a d i f f r a c t f ~ n

. . c - _ - - - -

grat ing t o t he l a s e r cavity t o the desired wavelength, 9

/ Stimulated Emission

e- Fig- 15 -me Pulrlp Cycle of Dye MQle&ules_._ - - ---- - - - - -

- -

bF

7 in,, +& i . scope expand* t h e beam s o as t o cover more l i n e s of t h e

$lidfraction gra t ing , thereby increasing A * t h e reso lu t ion , ,

' ' of t h e g ra t ing and reducing the- linewidkh of - the- laser \

beam. A schematic o u t l i n e of t h e component ]configurat&n

is ehown in f i g u r e 16. Note t h a t t h e dye l a s e r u axis i > - lliilL1---lll - UI-Li--i--̂ ----- fC

p = + i ? n d i c u l e r t o t h e ~ i s of t h e ni trogen l a s e r . A brief

descr ip t ion of t h e components of t h e dye laser is given )

3.4a The a3ye.eell

The dye cel l consisted of a s t a i n l e s s steel frame

(shown i n f igu re 17) , a quartz f r o n t wiodw , and two

&ti-reflective coated side window. The f r o n t p a r t of

t h e frame had.a sec t ion 2.6 cm x 0.1 9 t h e width if I

t h e frame (1.0 cm) milxed out. The quar tz window, through

which t h e ni trogen l a s e r beam was passed, was glued onto

t h e f r o n t of the frame, and t h e an t i - r e f l ec t ive coated

g l a s s was glued onto t h e s i d e s of t h e frame. Thus a

cavity 1.0 dm long and 0.1 cm deep w a s f amed t o contain

t h e dye. The nonnal of the an t i - r e f l ec t ive coated g la s s - - - - - - - - - - - - - - - - - - - - - - - - - - -- - -- --

~ 4 s a t 5'. & the axis of the laser .so that ,any r e f l e c t i o n s

from t h i s -+lass would. no t be amplf i ed when returned back 5

t o w k d s t h e aye cell.

The dye had to be circulated within t he dye cell and

---

Fig.17. The E'rP&te of the Dye Cell.

dye was pumpcd through the cell at a speed of &out a '

metre par second. ~oles were drilled through the hack i

+of the cell cavity, and appropriate attachments made ta !

a dye pump and reservoir- me dye was pumped fro? the 1 ,

reservoir, through the pump to the dye cell. and through

dye was regulated by keeping the pump in a water bath.

For _the dye ? h d a m i n e b G ,-_the temperatzurk of_-the-_b&h- - _- - -- -

was room temperature or slightly higher, whereas q r other dyes used (7-Diethylamino-4-Methylcoumarin, POPOP,

and Dimethyl POPOP), the bath was ice -water. 1

Using the above- arrangement of .the dye pump .and .

reservoir, it was also a simple mattex to changb from

one dye to another. - i

3.4b & t i d e 1

The diffraction grating wad b l y e d at 1.0 microns l

and had 1200 grooves per ailiimatra. The grating was used

in second ordek for all &yes used in this experiml?nt.

The holder forpthe grating had fine adjustments so that I

_ _- _ - _ -- - - _ _ - - - - - - - - - - - - - -

the gaat&'eoufd be accurately positioned an& the laser a -

--

easily tuned to a particular wavelength. e

The beam expanding telescope consisted of two - > _ _ - - _ - a _ - - - - - "

coated &hr&tic &ensas with foca l hpq@s 122 naa and, *

. ,

20.6 mm,,to g i v e a magnification of about 6. The dhtance -I

b between t h e lenses could be accurate ly set through the

use of a f i n e adjustment. The axes of t h e lenses did qot

exac t ly cokncide w i t h t b % a x i s of the l a s e r itself, so . \

-that- any- ~ E t e e t i o m r ~ - t h ~ ~ ; e r r s e s - ' o f - t ; h . ~ - - - - - - -

would not be amplified. i-'

1 mi

with

were

t he te lescope i n s t a l l e d .

The outputbmirror was interchangeable and two types 4 .

used. One was a quar tz f l a t with a 4% r e f l e c t i v & , and t h e o the r A s a coat& mirror whose r e f x e c t i v i t y

var ied f r ~ m 20% a t 385 nm t o 57% a t 475 nm, and 35% at'

The beam from t h e n i t rogen laser was focuesed t o a

t h i n l i n e on. the i n s i d e of t h e dye cell ca i ty , . by the use Y of two fucred s i l i c a lenses; one plano-convdt wd the o the r

c y l i n d r i c a l . Both these lenses w e r e required to reduce n

ni t rogen beam t o 6 l i n e of suitable - ---- - - - - - - --- - - - - - - - - - - - - r- -

length t o match the dye cell; the plano-convex lens to

match t h e length of the 'be& with. the dye cell, and the

c y l i n d r i c a l lentil t o f u r t h e r focu8 - t h e bean- to a l ine.- -

This t h i n l i n e excited t h e dye molecules and' defihad the \

a x i s of t h e dye laser.

i

3.5 The 0 of the Dye Laser

he output of the dye l a s i r con&ted a f the laser

line, and also of a weaker broadband output due to t h e

spontaneous f lorescence of the dye. SSnce *is-broad f

background' f lorescence tends t o obscure s i g n a l s t o be i

observed, it w a s necessary t o dass the laser output beam

through- a ~ h ; r = o ~ \ & ~ ~ - I t e & t ~ - o A L ~ e - ~ ~

A f t e r e l imina t ing t h i s baqkground, t h e measured beam

POPOP, and a f a c t o r of twenty using t h e dye Rhodamfne 66.

The ~~anochromator ,used w a s a 0.25 m g r a t i n g spectro- - meter (Spex minimate). 0.25 h slits were %;.sad to ob ta in

a bandpass of approximate$ 1 nm and a r e j e c t i o n bf about

10' a t 3 nm from the c e n t r e of t h e bandpass. A f t e r t h e

beam had passed through t h e monochromator, measuremnts~

w e r e .mde to determine t h e t u n a a b i l i t y of t h e b e -laser.

The power output of the dye laser as a func t ion b

of wavelength w a s determined. .To determine the wavelength

of t h e , ' l a s e r , t h e beam' was at tenuated and then passed

through a Spex model 1460 nbnochramtor, and de tec ted

with an ITT FW 130 photomult ip l ier tube . The autput of

t h e + p h o t o m u l t i p l i k was pasaed through a preampl i f ie r

( O r t e c modal<l3) and amplifier ( O r t e c model 454) t o a ,

boxcar i n t e g r a t o r (PAR model CW-1) which w a s t r i gge red - - - - - - A -- - - - - - - -

by thb elqctrical noise from t h e ni t rogen laser. The

monochromater was then tuned u n t i l t h e maximum output

from the boxcar was achieved. T o - d e t e d n e t h e power b

output of t h e dye laser, t h e output (at tenuated if

necessary) was focussed dnto the pyroe lec t r ic detector .

The detectox t h e n ' m e a s ' e d t h e energy-output of the d y i F'-

r e f l e c t i v e mirror. The i n t e n s i t y of W e laser l i n e as.

a function of wavelength was p lo t t ed as shown i n figure - ..

18. The m a x i m u m obtainable cw power w a s ,022 mil l iwat t s

a t 455 nm. *

With a 5 x 1 0 ~ ~ molar concentrat ion of R h o d k n e 66

as t h e dye, t h e l a s e r worked between t h e l i m i t s of 572 nm

arid 606 nm, with a 38% r e f l e c t i v e mirror. The maximum C

average power output w a s ,964 mi l l iwat t s a t 590 nm, as 2

shown i h f i g u r e 18.

.Replacing the r e f l e c t i v e mirror w i t h a quar tz p l a t e ,

of 4 o r 5 i n each of the above C-L -- -

Two oeher dyes w e r e 180 testad f q r t h e i r upper and r

lower laser l imi , ts , as- w e l l 48 their ma- cw- power

WAVELENGTH lnml - - - - -

F i g . 18. Relative L a s e r In s i t y 9s a Function of Wavelength for Rhodamine 6 G d ' 7 - ~ i e t h y l d n o - 4 - +

Methylcoumsrin.

output . A 7 . 5 x 1 f 4 molar so lu t ion of POPOP i n t o l u p e 4 lased between t h e l i m i t s of 4 1 3 nm and 427. nm with a

44% r e f l e c t i v e mirror. Its'rnaximum cw power output I

w a s .011 m i l l i w a t t s . AXSO, a 7 . 5 ~ 1 0 ~ molar so lu t ion -

of Dimethyl POPOP i n to luene l a sed between t h e limits

- - - A - A L - -- of - 4-zE-x~~ -&- 44-1--m. - - E e - - e ~ ~ h : m r - w ~ - 4 - 9 % ~ - - - - - - - *

r e f l e c t i v e , and t h e m a x i m u m cw power w a s .050 mil l iwat t s .

M of t h e mirror f o r each of t h e four 'dyes used is stqmarizea

i n Table -I. It is noted t h a t using t h e combination of k

t h r e e dyes, -namely, POPOP, Dimethyl POPOP, and 7-Diethyl*

amino-4Methlycoumarin, it w a s poss ib le t o continuously I

tune t h e dye l a s e r between t h e L i m i t s of 413 nm'and 477

nm, a range a 64 nm. Additional dyes ( B e l l and Tyte, -

1974) can be obtained t o cover t h e remaindeq of t h e v i s i b l e '4

s p e c t r a l kegion.

$ With a l l dyes, it w a s observed t h a t the tuning range

dropped considerably i f t h e quar tz f l a t replaced t h e .

r e f l e c t i v e mirror. I n Dimethyl POFOP, f o r example, the

r-

POPOP

I I k*

Dimet

The Lasing L i m i t s and Laser Output for

I

L ?a&$, L i m i t s (m) Maximum E ~ - I

,Power Ou

1 OPOP 422 - 441 ' .OSO

Dyes Us . .

w , :put (mw

Reflectivity, of Mirror

C

14 4 + - 56 - 8

b' I

cy- a, . - t

SCATTERING EXPERIKENTS

-- 4.1 Introduction 4

- 1 \ To test the a p p l i c a b i l i t y and perfomancs of t h e

b I+

nitrogen-laser-pynped dye l a s e r sys tep preliminary 'Rmh LL- -- -- A----------.--A- -Ap--

s c a t t e r i n g experiments were ca r r i ed out on two wide band 5

semiconductors, ZnSe and ZnTe. The f i r s t order Raman

l y (Irwin and e, 1972) an'd t h e spectra are we11 3

knbwn. The resgant Raman e f f e c t (RRE) has no t been

s tudied i n d e t a i l i n e i t h e r ZnTe of XnSe, however,

because of the l ack of a s u i t a b l e source. .BeOause of ,

t h e i r r e l a t i v e l y simple c r y s t a l s t r u c t u r e (zincblende) - they are i d e a l c r y s t a l s i n which t o study t h e RRE and ir

the-prel iminary r e s u l t s of -such an inves t iga t ion are

, . . presented i n t h i s chapter. P'

4.2 The ~ ~ p ~ r a & s

m e experimental apparetus i s as shown i n figuke T

. monochrontat@, is f ocuss$ onto a cyys ta l , employing-a - -

90 degree seateerzng geometry: The s c a t t e r e d l i g h t from

double monochromator (Spex model 1400) . The output of

t- J

the monochromator is detected by an ITT E'W 130 photo-

multiplier contained in a thermoelectrically cooled housing.

Pulses from the photomultiplier are amplified by an Ortec

model 113 preamplifier and model 454 amplifier and are

then passed into a boxcar detector (PAR model 160). The

boxcar is operated as a linear gate and is triggered by

the electrical noise,from the nitrogen laser which is

picked up by an antenna. The boxcar gate is open for

approximately 1 microsecond and thus dark pulses occuring

outside this time interval are prevented from reaching

the counting electronics. The output of the boxcar is

applied to a discriminator (Ortec model 486) whose threshold

is set to reject electrical nodse, but will accept

single photon pulses. The output pulses from the discrim-

inator are then counted on a Hewlett-Packard model 3734A

electronic counter.

Although the system will accept single photon pulses,

if there is more than one photon per pulse, the electronics

will not resolve the individual photons; the system cannot

resolve separate counts within one pulse that is only a

few nanoseconds long. This leads to the problem that it

is possible to have at most one count per pulse, and at

high counting rates, some counts represent two or more

photons reaching the detector. Let us therefore consider

closely what is being counted (Bell and Tyte, 1974).

probabi l i ty that a s i g n a l is produced.by the laser pulse,

t h e counting rate is ' . . I

- - -~ ~ ,- --

"

I n s c a t t e r i n g experiments, however, t h e value r is

very s m a l l (0; the order of 10-'), s o t h a t we may approx-

imate i n ( ~ - R / N ) ~ by -R: Thus we f ind t h e t e l a t ionsh ip , &

- Therefore,,yhen a counting rate i s ob.served, w e must e

use equation 425) i n order t o f i n d ou t the true counting \

r a t e : If the observed counting rate is small, then t h e B

t i +J

true counting r a t e is v i r t u a l l y the same, but f o r high i'

- cawrting r aZss , -the_ 03semd and - t i n e coumlng -rqtes a r e - - -

J

iie dliffekmt. ( --

While a spectrum is being taken, M a t is, while *

s c a t t e r e d p h o t o n s a r e being c o u n t d , it is des i rab le t o

monitor t h e i n t e n s i t y of t h e dye l a s e r beam. For t h i s Q

11 f r a a gf t& . I A r

diver tad , nuated i f n k e s s a r y , and focuss& w pyroelcrctrid detector . The output of t h e defec tor is,

V b

i

then passed i n t o a boxcar in t eg ra to r . ~ h u s the intensity of t h e dye l a s e r beam can be monitored while a spac t f" - is taken.

4 . 3 Results

Preliminary measurements indicated t h a t Raman spec t ra

can

The

and I

Per

be obtained with the present experimental set- we ' . ,

s l i t width of t h e

t h e maximum count

I. "

monoahromater w a s about 4 cnt , - rates obtained wede about 25 counts

minute.' Three spec t ra are shown i n f igu res 20 and 21.

I n figure 2 0 , t h e c r y s t a l i s polycrys ta l ine ZnSe.

There 'are four l i n e s i n evidence. These a*' a t 1 4 3 clam',

204 cmv I , 251 cm-l, and 501 c m o l , and have an experimental - 1

error of about *6 cm . These values compare favourably

' with t h e previously observed values of 139 6 ' (2TA) ,

r and Lacombe, 1972). ? % '3 )?j In f igu re 21, t h e e t a 1 , i s a s i n g l e c r y s t a l of ZnTe,

-- - - - - n - - - - - - - - - - - - - - -- - - - -- -- - -- -- - - -- -

c u t w i t h one 100 face and two 110 faces. Incident focussed

, l i g h t s t ruck t h e 100- face and w a r ~ a t t e r e d out t h e 110

face. Two l i n e s corresponding t o the LO and M phonons - - - - - + - -

- 1- - - -

are i n evidence, These l i n e s occur a t 207 +-

a Po and 177 .

. To- A Ramun Spectrum of XnSc at' ?oom T e b r a t k I

P-"L? I

+4

. Fig. 21. Raman Spectra of ZnTe at Room Temperature. .P

C

d * 2

1 I I - 1 1

.,an and compare favourably t o the values of 208.3

and 1!7.5 cm-f (L~tCombe,~J971). ,

4: 4 Resonant Raman ~ f f e c t a , - *

e

The ratio of t e intensity of t h e m + l i n e t o t h e .

3, . <

. i n t e n s i t y of t h e sca t t e red lager l i n e was measured as

a function of wavelength i n ZnTe. '

According t o Martin (1974) , . it is predicted, t h a t

' the amplitude of the LO l i n e should increase relativet

of -photon i s increased towards the band gap energy. '

-

F i g p a 22 shows t h i s t h a o r e t i c a l curvef where A 0 - h~ p/E lg

- . which for ZnTe is 10-13 meV, hua is t h e energy of t h e

+ - - . - phonon which is 25.6 meV (207 CIC?), and hui is the bhergy

d i f fe rence between the l a s e r Zine and t h e band gap energy.

The results of t h e measured i n t e n e i t y of the-scattereiX

l a s e r line, and t h e ff jpeak. are shown i n T a b l e XI: The % . . . a

Value of" ths inten& of- t he LO l i n e divided by .the , I

i n t e n s i t y of the scattered laser lide js shown under t he * - r

column IdIli lser. These values w e r e then acorrected f o r

not well defined a t room temperature, and may bk reuf ted . 0

- - - - - - - - as well 'because -oT i&*i

8 T

3

a T h e - & s o r p r i o n c o e r r i c i e n t 'was takein rrm rigure 23 0

distance Qas taken t o be 1 millirmetre. The column *

under I~d' laser is corrected f o r absorption and is -\ *

r e l a t i v e t o t h e lowest value of ILdIlaser . he values 1 ' 1

predicted by Martin (1974) f o r t h e values given above

and an energy gap of 2.25 e V are, a l s o given i n able 11 a -L - -- - -- ---

under t h e ~ c o l u m t h e o r e t i c a l ILO/Ilaset. ' J ' + . There a r e many poss ib le sources of error i n *he*

band gap be s h i f t e d , bu t impuri t ies may d r a s t i c a l l y 4' change absorption.

4

. .

The laher l i n e i n t h i s geometry may s c a t t e r off t h e

face of the c rys ta l .w i thou t ac tua l ly penetra t ing t h e ,

crystal. Furthermore, t h i s s c a t t e r i n g may also be a , '

function of wavelength.

In t h i e experiment, because of many uncertaf n t i e s .c

the agreement between the experimental and t h e o r e t i c a l . values o f Im/Ilaser must be considered sat isfactory ' ,

#

The value is the' righfiorder of magnitude and fs a l s o

increasing a s the band edge is appraached, as expected.

- t has thus beerrdemonstrat& that-the-nktmgen--7 - L -

Raman scatteltfng experimen,ts and also study t h e resonant - Raman effect . Although tgia B y s t e m sky b = ~uccessfuXLy

Fig. 23. The Ab8orption Edge in Undopsd lnTe at 3 0 0 ~ ~ . A

- - - - , - -- '* -

- used, it has t h e disadvantage that the cw power output

is low, and it is therefore necessary t o count for long J

i periods of t i m e and to oqpate with very poor resolution. \

As a result the experiments are very tedious and lengthy

and the results lack precieion. . . d-

CHAPTER 5

. A n i t r ~ ~ e h - l a s e r - ~ ~ e d dye l a s e r system has bean

constructed ' a d used t o perform preliminary Raman scatter- , .R

ing and..resonant Raman experiments on ZnSe and ZnTe. 8

. ~h?e nitrogen laser output was i n t h e form of pulses with - .

a width at half maximum of 9 nsec, and a t o t a l durat ion

k i lowat t s peak power from t h e l a s e r operat ing a t 6 pu lses . I'

30 mi l l iwat t s . If it were pwsib le t o operate a t con-

s iderab ly higher pulse r e p e t i t i o n r a t e s , more cw power

, output could be obtained from the 1 yer . For r e p e t i t i o n

li r a t e s greater than 10 puJses per nd w i t h the pres n t + - s" '

became very along ' t h e

length of t h e l a s e r , t h e energy per pulse dropped con-

s iderab ly , a n d . t h e pvlse t o pu lse consi$tency b g c w

extremely wr. 1t would ~ S O be possule lo i n c r f ase

L , e - t h e power : t p b t $F the l a s e r by reducing the inductance

i n t h e e l q c t r i c a l c i r c u i t r y , since a ma~imum discharge , f , , " 6 *

within a nanosecLhds is desi rable . A meaningful ..

reducticm ,in. would_ he -ach;haued iCth_a ty - -- -- -- --

were shortened and capperbar instead of cab - les were

b8ed t o c a t r y t h e discharge current*. I n designing the . ni t rogen laser, a gas- f l o w &stem vas&viaad- to re&ve - -

4-

excited and ionized molecules a d .replace them with F' 1

molecules in their ground state. A more **id egubange c

of' gas, for example by means of a transverse gee-try, ?

would also enable higher repetition rates to be employed. L--

With-the dye laser constructeq and 4.differTt 5 1

dyes used, the laser output could be tuned from 413 + to

varied from .Oil milliwatts for POPOP to ,064 milliwat P for ~hbdamine 6 6 . Using other dyes (Bell and Tyte, 1970,

I

-

. .;between 437 and 602 nm, The linewidth of the dye laser

1, b

was about 0.3 nm. I~rovements in both power outpuf anb ,

a reduction of the linewidth may be possible by increasing ,

the power of the telescope 'and tnsertfng a Fabry-Pexot - . R' - .,

etalon betwe the telescope and diffraction grating. 7 I

The laser system was use4 to perfarm preliminary a ,,

scattering experiments. A first order Raman spectrum " * . ,

of both ZnSe and ZnTe was obtained, and.the results were ' I

in gqod agreement with previous work (Irwin d IaCombe, 4' d e

1972). Thei resonant Ranran effect wa$ also %

ZnTe, and satisfactory agre-nt with the theory of . . - - - - - - - - -- - - --- - - - - - - - -- - - - - - -- - - - - --

IWrtin (1974) was nated. In performing kheae ex@rhts,

the problem encountwed were the electrica1,noise

. . I .

t-n a a t OE .th& - ,

nitrogen laser. Further improvements may be made by

either shieldingL the &trogen lalrer, or shielding the \

detecting equipme&. To telieva the tedium caused by

long counting times due to the &ow power o the laser. 3 9

automatic data acquisition equipment lady be used. Also. - - - - - -- ---

t h e power output of the l a ~ e r my

high& repetition rates. .The incorporation of some or

- - - - - - a L I ef -these i~rovementa -to_ the@xe_ee&-lasgr~~ s temp-

would provide an extremely useful'excitation souroe for +

5

. .

luminescence and scattering studies.

*

- - . , - , t A

i6

r \ I

1 4

i B ,-

I

,J - - - - % - L- - - -- --

> - - - - - -- - - - --p - -- -- -- - -- -

f - - &

f k

Y B

b

-

- -+ - - - - - - - - - - - - " --- - A

? \

t 1 t

jL4

'd I-ri I

1 Ali , A.W., Kolb, A.C., and Anderaon, A.D. 1967.-

Appl. Opt.' 6, 2115. a- - A1iI A.W. 1969. Appl. Opt.' 81-993. . -

I Brtmea, PoAo 1970. wudies of Laser Lnduded BreakBawn Pheria~na in Liquid Water, Ph.D. Thesis, Sinon F r a a e ~ University, Burnaby, B.C.

Bell, M.I., and Tyte, R.N.' 19?4> ~ p ~ l . Opt.33, 1610.. - - - ---- -- --- -- --

Heard, HOG. 1963. ~atase,%N, 667. - Xeraberg, G. 1967. . HoLe'culeir' Spec'tta ah&8"~o:le'c~l'ar

S tmcture I. ' ' Spim'tYaa 'o'f DiatoWC I@'Xd'cp'Xea, van Nostrand-Co. , Inc. e J',

Irwin, Jd!., and I;aCombe,*Je 1972. Can. J. Phy&"fiO, 2596. - -

Laconbe, J? 1971. R& Studiea of ~honon Disper~ion ' in Zincblende Semiconductore, PFm,*. simn

e

Praser University, Burnahy, L C

1 k

tbrtin-, R.M. , 1974. Privpe .Conmunication. P .

Mathiae , 5. , and Parker, *l963. A&. p h p . Lett , 2, I

f . 16..

. L Schaf fez, F.P. 1973. Dye' L'aserr, Springer-Verlag, Betlin. ,' ,

5

Ahif f , L. I. 1968. " puah'tmi Tag'c'ffahics, ~~Graw- ill. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ?+-

Ad - - /"- b >

1-

- F v

2. . .: % - -

'. * - h

C

. t r' m .

r' > -