REVIEWS

OSCILLATION BY COMPLEX ORGANIC COMPOUNDS*

B. I. Stepanov UDC 535.35

There are now several hundred papers on oscillation by dye solutions, which was first reported in 1966; research on this topic has become a major trend in quantum electronics, and this review deals with the general state of the problem together with a brief exposition of some studies made at the Institute of Physics, Academy of Sciences of the Belorussian SSR [1-5].

It is no accident that oscillation by solutions of dyes, scintillators, and other compounds has attracted particular attention. Lasers based on dyes have unique properties, which are lacking from lasers of other types; they allow one to produce oscillation at any wavelength in the range 280-1170 nm and to adjust smooth- ly the frequency over a wavelength range of the order of 10 n m (up to i00 nm for eertainphthalimides). Some- times it is desirable to work with a broad oscillation band. The dye solution can be pumped through the cell, so that it is easy to realize systems operating at high repetition frequencies (up to 50-100 Hz). It is also possible to have steady-state oscillation. In the free-running state, there are no pulsations in the emission, which is also of practical value. A wide-band amplifier is readily based on solutions of'organic compounds.

These properties of dye solutions provide the basis for novel lasers, but even recently doubts have been expressed as to their practical significance, since it was uncertain whether one could obtain sufficiently intense emission. It has now been shown that these doubts were without basis. Theoretical calculations and experimental studies have shown that the power and energy of a laser based on a good dye can be close

to that of a ruby or neodymium laser.

Two types of pumping are used in producing oscillation by dyes: laser excitation and excitiation by flash lamps. In the first case, the conversion factor in certain instances is as high as 75%. Table I com- pares the oscillation parameters for ruby and rhodamine 6G when excited by flash lamps; the values in the last column are of a preliminary nature and optimization of the conditions could result in much better values. In many technological applications, dye lasers can compete successfully with ruby and neodymium ones;

recently, pulse energies of over I00 J have been obtained.

The principal distinctive feature of the practical applications of dye lasers is their use as a source of strong coherent monochromatic light of variable frequency; such sources are of value especially in laser spectroscopy and in research on the spectra of strongly absorbing objects, as well as in measuring excita- tion spectra for luminescence and photochemical reactions, spectra of multiphoton absorption and excitation, nonlinear optical effects, saturation, oscillation in semiconductors with optical pumping, frequency depen- dence of spontaneous and induced scattering of various types, phototropy in colored materials, etc. Dye lasers are best used as light sources also in plasma diagnosis, atmospheric research, and similar appli-

cations.

Dye lase r tuning can be used in many branches of chemist ry , color holography, and medicine; in chemis t ry one expects par t icular ly great advances, because s trong monochromat ic radiation at resonance frequencies can cause controlled t ransformat ion of molecules .

* Read at the Plenary Session of the Fifth Conference on Nonlinear Optics (Kishinev, November 10, 1970) and at the Ukrainian Seminar on Quantum Electronics (Khar'kov, November 17, 1970).

Translated f rom Zhurnal Prikladnoi Spektroskopii, Vol. 15, No. 2, pp. 359-370, August, 1971.

�9 }974 Consultants Bureau, a division o f Plenum Publishing Corporation, 227 ~'est i7th Street, New York, N. Y. 10011. No part of this publication may be reproduced, storecl in a retrieval system, or transmitted, in any form or b) ~ any means,. electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. /l copy of this article is available from the publisher for $15.00.

1112

TABLE 1. Excitation

Ruby

single pulse

0,1--0,5 J

100 MW 10-Ssec.

N0,2%

Comparison of Ruby and Rhodamine 6G Lasers with Lamp

free running

2--4 J

50 kW I0 -s see

1--4%

Energy from t c m a Power PuRe length

Efficiency

Rhodamine 6g;

ordinary lamps

0,6J

50 kW 10 -5 sec

0,75%

pinch lamps

2,5J

10 MW 10 -~ sec

0,2--0,4%

F u r t h e r , r e s e a r c h on o s c i l l a t i o n by c o m p l e x m o l e c u l e s has a l r e a d y p r o v i d e d c o n s i d e r a b l e i n f o r m a - t i on on s p e c t r a and l u m i r m s c e n c e ; in the fu tu re , such r e s e a r c h e s wi l l be one of the m o s t e f f ec t i ve ways of s t u d y i n g p r o c e s s e s in e x c i t e d s t a t e s of m o l e c u l e s , i nc lud ing p h o t o c h e m i c a l r e a c t i o n s .

C o m p l e x M o l e c u l e s as S o u r c e s of L a s e r E m i s s i o n . It was shown in 1960 tha t c o m p l e x m o l e c u l e s tha t had been e x p o s e d to l i gh t cou ld a m p l i f y r a d i a t i o n of l o w e r f r e q u e n c i e s ; n e v e r t h e l e s s , m o s t s p e c i a l i s t s a s s u m e d tha t o s c i l l a t i o n by o r g a n i c compounds was v i r t u a l l y i m p r a c t i c a b l e , t h i s poin t of v iew b e i n g b a s e d

on the o r d i n a r y f o r m u l a

kga(V) = hv B21(v) (n 2 _ n l ) (1) V

fo r the ga in c o e f f i c i e n t ; f o r n 2 to be l a r g e , one needs h igh m e t a s t a b i l i t y in l e v e l 2, i . e . , a s m a l l va lue of t he E i n s t e i n c o e f f i c i e n t A2i. H o w e v e r , s m a l l A2i m e a n s tha t the i n t e g r a l B21(Y)dg is s m a l l , and h igh va lue s of B2i(/;) c an be a t t a ined at the peak in t he a b s o r p t i o n bands only f o r s u b s t a n c e s hav ing v e r y n a r r o w f luo ~- r e s c e n c e b a n d s . Th is would m a k e i t s e e m tha t a m a t e r i a l wi th a b r o a d e m i s s i o n band would be u n s u i t a b l e .

H o w e v e r , (1) is i n c o r r e c t f o r s y s t e m s with b r o a d f l u o r e s c e n c e s p e c t r a ; the ga in f a c t o r fo r a s y s t e m of p a r t i c l e s wi th two e l e c t r o n - v i b r a t i o n a l l e v e l s is d e t e r m i n e d by a d i f f e r e n t f o r m u l a , which t a k e s into account the e q u i l i b r i u m d i s t r i b u t i o n of the p a r t i c l e s o v e r the v i b r a t i o n a l s u b l e v e l s :

kga(V) = h_~_~ B~ 1 (v) [n~ - - nle-h(Vel -v)/kr ]. (2) U

A p o s i t i v e va lue of k g a ( V ) d o e s not r e q u i r e a popu la t ion i n v e r s i o n in the e l e c t r o n i c l e v e l s ; if the f l u o r e s c e n c e band is v e r y b r o a d , and V e l - V i s , f o r i n s t ance , 800 c m -1, t hen ga in a r i s e s at r o o m t e m p e r a t u r e s when the u p p e r l a s e r l e v e l c o n t a i n s about 2% of the p a r t i c l e s . One needs l a r g e va lue s of B21(v) in o r d e r to p r o v i d e h igh gain, and th i s i s ob t a ined with b r o a d s p e c t r a only ff the u p p e r l a s e r l e v e l is l a b i l e ; t h e r e f o r e , we at once l e a v e behind the two r e q u i r e m e n t s c h a r a c t e r i s t i c of s o l i d - s t a t e l a s e r s and do not r e q u i r e m e t a s t a b i l i t y in t he l a s e r l e v e l o r n a r r o w n e s s in the f l u o r e s c e n c e s p e c t r u m .

]( abs

!

kga

Fig. 1 Fig. 2

Fig. I. Shape of absorption (gain) band as a function of pump- ing intensity: I) no pumping; 2-4) pumping increasing. The horizontal line is the loss factor.

Fig. 2. Electronic-vibrational levels and main transitions.

1113

TABLE 2. Comparison of Ruby and Dye Characteristics

CharacterRtics Ruby Dye

n, c m -3

B21 ( Y ), C G S

k ~im (v), cm -I

thr erg.cm-3 U purr) ,

1.6.1019 I00

0,3

1 . 5 . 1 0 V

5 . 1 0 6

20

6/.

Table 2 compares the characteristics of ruby with good dyes ; the limiting gain, i.e., the value of kga(V) for n 2 = n, is substantially higher for the dye than for the ruby. The threshold density for the exci- ting radiation Upu m is calculated in the usual way from the condition kga(V) = klos; the thresholds for the best dyes are only 3-5 tittles those for ruby (~ is the quantum yield in fluorescence).

These arguments go back to 1964 and led to the prediction that oscillation by organic compounds is not only possible but quite realizable, In the subsequent two years, the principles were developed for a quantitative theory of the phenomenon, which provided prediction of the optimal experimental conditions and the basic features of the possible oscillation, especially the scope for tunable lasers, i.e., that feature of dye lasers that has attracted the most attention,

The essence of the arguments is simple. Curve 1 in Fig. 1 represents the absorption band of the dye in the absence of pumping, while curves 2-4 represent the effects of successively increasing pumping levels ; as Upu m increases, so does oscillation of the upper laser level, and this increases the gain, with the peak in the gain band shifting towards larger frequencies. Oscillation arises when the peak kga(V ) becomes com- parable with the loss factor (horizontal curve). The point of contact of the kKa(V ) and k[o s curves deter- mines the oscillation frequency v0; the higher klos, the higher u 0. The dependence of v 0 on concentration n is the opposite to this ; ff n decreases , the same gain equal to klo s will be attained at a higher pumping level and consequently at higher v 0. It follows f rom (2) that the gain is dependent on T, which is one reason for the tempera ture coefficient of the oscillation frequency. In some cases (for instance, phthalimides), the strong dependence of v 0 on T is due to tempera ture shift in the f luorescence band. In 1966, dye oscillation was recorded with excitation by a ruby single pulse , and in 1967 with excitation by lamps, which were o r - dinary tubular xenon tamps.

Specifications for Promis ing Organic Compounds. The threshold and oscilkation power are dependent on the f luorescence spectrum, the propert ies of the cavity, and the pumping conditions; the details of these relationships are largely now known, and the resul ts of theory and experiment, where they are comparable, agree closely. To calculate the threshold, power, and oscillation frequency requires a sys tem with five e lec t ron ic -v ib ra t iona l levels as shown in Fig. 2. It is neces sa ry to know the absorption coefficient in the absence of pumping kabs(V), the f luorescence power level Wfiux(V), the transi t ion probabilit ies A31, P32, P21, the quantum yield in f luorescence 7, the absorption c ross sections in the channels 3 -~ 5 and 2 --~ 4, the part icle concentrat ion n, the cavity loss coefficient, the mode of disposition of the m i r r o r s , and the density of the exciting radiation.

t h r thr u u

pure ~ uPUm i u ~ [ pum !

~te ~ ~ te Fig. 3. 1) Threshold as a function of t ime; 2) shape of

primping pulse.

1114

TABLE 3. Classes of Laser Compounds

Red to IR

Polymethynes

Phthalocyanins Thiazines Tr iphenylmethanes

Green - yellow - red

Rhodamines

Acridines Phthalimides Fluoresceins Pyronins

Green - yellow

Phthalimides Pyrilenes Benzotrenes

Blue - green

Oxazoles Oxadiazoles Coumarins

If one knows how utnh, rm and Wos c vary with all these parameters, one can formulate the requirements

for promising molecules; in all cases, a molecule is suitable if its absorption in the 3 -~ 5 channel is small

and it has high values of A31 and ~?, together with a broad fluorescence band, and also a large distance be-

tween the absorption and fluorescence band. Unsuitable compounds are those with high P32 and small P21. It is disadvantageous for particles to accuraulate in the me[as[able level, especially when there is strong

absorption in the 2 --~ 4 channel.

The specifications for the molecules are dependent on the excitation conditions; the position of the

absorption band should correspond to the pumping source spectrum.

If one uses excitation by short high-intensitylaser pulses, most of the above requirements are of no

great importance; in fact, the density of the exciting" radiation from a ruby single-pulse laser of output

energy about 1 J is roughly 104 erg/cm -3. The threshold is about 6 erg. cm-3/~ and so it can be exceeded by

a ['actor of a thousand or more if V is of the order of 1. This is why laser excitation with ~ ~ 1 will trans-

fer almost all the particles to the upper level (n 3 >> nl), kga(V) ~ 10 cm -i. As a result, one sometimes

finds lasing without mirrors, and superfluorescence, If the excitation is strong, it is possible to obtain

oscillation by dyes of methylene blue type, the quantum yield of which in fluorescence is less than 0.001%.

It has been found that nearly all compounds that have been examined (several hundred) will show oscil-

lation; we can say that lasing is one of the most general optical properties of complex molecules, although

the threshold in certain cases is very high. Absence of oscillation is to be taken as an exception requiring

a special explanation.

Oscillation is k~herently impossible only for compounds for which B35(v ) > B~I(~), i.e., those for

which the absorption in the 3 -~ 5 channel exceeds the gain in the 3 -+ 1 channel. For this reason it has

proved impossible to obtain lasing with some phthalocyanins.

Accumulation of particles in the metastable state is not important with laser excitation provided that

P32 is less than 108 see-i; in that case the oscillation is completed before the particles have passed to level

2. The shape of the oscillation pulse reproduces that of the pumping one.

Lasers employing organic compounds now cover the entire range from 280-1170 n m; Table 3 lists

some classes of compounds that oscillate in various spectral regions, with excitation by single pulses from

ruby or niodymium lasers operating at their fundamental frequencies or with their harmonics. There is

now no point in examining lasing by every member of a series of types of complex molecule; instead, the

problem is to locate these compounds most promising for use in particular practical applications.

F i g . 4 , O s c i l l o g r a m o f p u m p i n g p u l s e ( u p p e r

c u r v e ) a n d l a s i n g .

Fig. 5. a) Oscillation spectrum as a function

of time; b) divergence.

1115

TABLE 4. Basic Characteristics of Laser

Power in single-pulse ranges

Pulse length Spectrum width Divergence Interval between pulse

700-1080nm 5-20 MW 347-650 nm 0.1-0, 5 MW 20 nsec 0.3-1 nm 10' - 20' 15 sec

The requirements as to compound performance are rather more stringent with lamp excitation, on account of the small density of the exciting radiation and the longer irradiation time. With ordinary lamps, even the best dye (rhodamine 6G) has a threshold less than the pumping level by only a factor 3-5. Com- pounds with low ~7 do not lase.

Accumulation in the metastable level 2 is very much disadvantageous, for it raises the threshold and often causes lasing to stop long before the lamp emission has ceased.

In Fig. 3 two typical relationships between the lasing threshold and time are shown (curve I). In the first case, increase in n 2 is accompanied by rise in the threshold towards infinity, while in the second the threshold tends to a limiting value. The second ease can be realized only for systems for which one has

k lim B31 (v o) = ga ( v& P~I/

(3)

Condi t ion (3) is r a r e l y m e t .

Curves 2 in F ig . 3 show the shape of the pumping pu l se ; at the point where c u r v e s 1 and 2 i n t e r s e c t ,

l a s i n g inev i t ab ly t e r m i n a t e s . The t i m e t te v a r i e s with the spec ies but is r e a d i l y ca l cu la t ed . If (3) is v io -

la ted, t e r m i n a t i o n is inev i t ab le even for ve ry high exc i t ing i n t e n s i t i e s . A typica l o sc i l l a t i on pu l se is shown in F ig . 4.

The n u m b e r of m o l e c u l e s tha t wil l o sc i l l a t e in l amp exc i ta t ion can be improved , and the e n e r g y c h a r - a c t e r i s t i c s can a l so be be t t e red , if one has ava i l ab le s t anda rd f lash tubes with outputs of dura t ion 10 -6 to 10 -5 sec.

If (3) is met, such a system can produce steady-state lasing; calculations show that the threshold for steady-state lasing by rhodamine 6G is only three times the initial threshold.

In lamp pumping, the rate of rise of the light intensity is as important as the intensity itself; if this rate is less than the rate passage of particles to the metastable state, many compounds are unable to lase even at very high pumping levels. This sets the specification for the steepness of the leading edge in the pumping pulse.

Lamp excitation does not produc e the peaky output so characteristic of solid-state lasers; small in- tensity pulsations can be observed only at the start and end of the process, or near the threshold. In the initial stage, when no particles have accumulated in the metastable state, the lasing pulse reproduces the

Eose, Upum ~ f

!

~oo iso 8'aa '~,nm

Fig . 6. T h r e s h o l d (1); and o s - c i l l a t ion power (2) as func t ions of f r e q u e n c y ; 3) f l u o r e s c e n c e band .

1116

t L!

!

P

~ . a a .

l l l l l l l l l l ~

p = pumping L = Lasing

I L

Fig. 7.

b

d

Pumping systems.

lamp pulse; the lifetime of the upper laser level is very small, so such a system is Mmost free from lag,

and for this reason complex molecules are not promising for producing single pulses in a switched-Q state,

but this is not a serious disadvantage.

Increase in lamp temperature is advantageous for dye rasing; not only does it increase the brightness

but also it shifts the spectrum towards shorter wavelengths. In the case of ruby, this leads only to a use-

less absorption of radiation by the lattice, while neodymium glass may be damaged. On the other hand, the absorption bands of a dye cover practically the entire spectrum, and therefore the tamp radiation is most

efficiently used. Good results are obtained with pinch lamps employing plasma compression (T about

30, 000~ Table I).

Tuning. The possible lasing band for an organic compound is usually I00 ~ or more wide and con-

stitutes about 0.I of the fluorescence band width with laser excitation; the banded structure seen in the spec-

trum arises from interference effects in the cavity and can be eliminated. With lamp excitation, there are

additional reasons for band broadening on account of accumulation in the metastable state and introduction

of additional time-dependent loss sources. Also, the band position is dependent on klo s. Figure 5 shows a

time scan of the lasing spectrum, the instantaneous width of which is about 60 ~. The oscillation shifts to

shorter wavelengths as time passes.

Tuning can be performed by adjusting the concentration, ceil length, cavity Q, and temperature; but

the most effective method of tuning is to introduce a selected element into the cavity (diffraction gratin G

prism, or interferometer), which also greatly narrows the lasing spectrum, to 2-5 X or even to a fraction

of a unit. The threshold and lasing power are dependent on frequency, and Fig. 6 shows the observed re-

lationship for rhodamine 6G. The calculations agree with experiments. The maximum lasing power occurs

at the long-wave edge of the fluorescence band. Tuning is often accompanied by a substantial reduction in

the power output. The higher the lamp intensity, the wider the tuning range.

Loss, Energy, and Efficiency. A major research problem at the present time is concerned with the loss sources, which vary with the compound, and hence development of methods of increasing the efficiency

and output power. The following is the factor for conversion of the absorbed pumping energy into laser

emission in the transverse form of cavity

vo kr [ 1 l n X ] 7 = - - 1 (1 --~O)(1--~p), Vpu m kr q- p X X

where

(4)

k~= __1 In __1 (5) 2! q r 2

1117

Fig. 8. Tunable laser.

is the loss factor for the mirrors, I is the length of the active material, r I and r 2 are the effective reflec-

tion coefficients of the mirrors, p is the unwanted cavity loss factor, X is the number of thresholds, ~o is

the fraction of the oscillation power absorbed in channels 2 --~ 4 and 3 -~ 5, and ~p is the proportion of the

pumping power absorbed in these channels. The I/~X term takes account of incomplete absorption of the

pumping radiation in the active material, while the In X/~ term takes account of the loss to fluorescence

and heat production in the 3 --~ I and 3 ~ 2 -~ 1 channels.

The limiting 7 is determined by Vo/V , i e , by the Stokes losses, the usual value being 0.7-0.9, pum �9 " which is the value (70-75%) attained with strong laser excitation of rhodamine 6G. If X is very large, while

k r -> p, the main losses are associated with the factor (I-~r); for instance, cryptocyanin has a high P32 and up to 45% of the molecules accumulate in the metastable state even With laser excitation, with the re-

sult that the output radiation is absorbed in the 2 -* 4 channel and the efficiency is reduced to 0.3.

The theoretical analysis implies that the efficiency is very much dependent on the working conditions;

for instance, there is an optimal value for the loss coefficient of the mirrors. If this tends to zero (as is

often the case when attempts are made to reduce the threshold), there is a marked increase in the propor-

tion of loss in the 2 -~ 4 and 3 --~ 5 channels, and the efficiency tends to zero. There is a similar depen-

dence on the tuning frequency of the cavity.

All forms of loss are important in lamp pumping; since X is small, there is a marked increase in the

proportion of the loss governed by the terms in I/X and In X/X. For instance, if X = 2, the third factor in

(4) becomes 0.15; one always reduces k r to reduce the threshold, and this results in increase in the second, fourth, and fifth factors in (4). Also, allowance must be made for the long pumping pulse and broad exciting

spectrum in lamp excitation.

The oscillation is pro!onged, so almost always there are important losses in the 2 --~ 4 channel, which

arises from accumulation in the metastable state; these losses are particularly large if (3) is not met, and

unduly early termination of lasing may occur (Fig. 4).

The effects of triplet levels can be substantially suppressed by adding to the solution substances that

quench the metastable triplet state, i.e., that increase P21. Oxygen is the most effective quenching agent

for rhodamine 6G, and saturation with oxygen substantially reduces the threshold and increases the output

power. On the other hand, removal of oxygen, i.e., reduction in P21, leads to complete loss of lasing. One can use other quenching agents, but they quench the labile state as well as the metastable one.

Not only does triplet-triplet absorption have a substantial effect on the lasing with lamp pumping, but there is also the effect from irreversible photochemical decomposition of the dye in response to the UV pumping; the decomposition products absorb the output radiation and so increase p and the threshold.

One can use filters to cut off part of the ultraviolet to reduce the losses and thus increase the output

power; unfortunately, filters reduce the overall pumping power, and there is an optimal filter such that the pumping efficiency is not greatly affected but the loss to photolysis is substantially reduced. In the case of

rhodamine 6G, it is best to cut off pumping radiation below 230 rlm. The importance of photolysis is much greater at high pumping intensities; photolysis essentially restricts the fusing energy and pulse Length, and

in some cases for example, that of rhod~mine, it is the main factor preventing the achievement of steady-

state fusing by dyes.

1118

The basic means of dealing with photolysis is to induce the solution to flow through the cell; to keep the loss factor unchanged and obtain steady-state oscillation the solution should pass through at about 50

m / s e e .

There is also marked thermal distortion of the cavity in lamp pumping, which reduces the output pulse lengt h, increases the divergence angle (Fig. 5b), and broadens the band (Fig. 5a). Increase in the loss coefficient reduces the conversion factor y .

Theoret ical analysis shows that it is often important to consider the losses related to the large spec- tra[ width of the exciting lamps; the amplification band is also very broad, so the low-frequency part of the pumping radiation is not absorbed but is amplified, and these losses are the basic cause of lasing t e rmin- ation as ~o is reduced. So far, this aspect has not been examined by exper iment : It is best to f i l ter off the low-frequency part of the pumping radiation in order to increase the efficiency.

Figure 7 shows methods using laser" excitation; the t r ansve r se pumping form (a) is usually bet ter than the longitudinal variant (b). The t r ansve r se variant makes it substantiMly eas ier to optimize the cavity and length of the dye layer . The longitudinal sys tem can give comparable per formance only when using a se lec - t ive ref lector , i . e . , a ref lec tor that t ransmi ts the pumping radiation well and ref lects the output radiation (Fig. 7c). The scheme of Fig. 7d is also possible.

Polar izat ion. The degree of polarization in the output is dependent on the type of dye, the pumping intensity, the solvent viscosity, and the orientation of the electr ic vector in the pumping radiation relat ive to the cavi ty axis. Competition between individual modes is found here . The polarization features of the output give very valuable information on the propert ies of molecules and p rocesses in oscil lat ion.

Ins t ruments . We now have sufficiently complete practical and theoret ical information on dye l a se r s to be able to produce designs, organize routine production, and undertake practical applications in science and technology.

One of these instruments has been developed at the Institute of Physies, Academy of Sciences of the Beloruss ian SSR (Fig. 8).* The sys tem is excited by a single ruby pulse at the fundamental or the second harmonic ; a revolving drum has 8 places for cells with different dyes. Table 4 gives the basic c h a r a c t e r - is t ics of the device, which is meant for scientific research. The output characteristics of the apparatus

could be improved by further dye selection.

Lasers with lamp excitation are very convenient and cheap; the solution can be made to flow through �9 the cell to produce high pulse repetition rates, and such lasers are needed in many practical instances. On the other hand, heating of a solid-state laser substantially interferes with increasing the pulse repetition rate or output power. This makes clear the advantages of dye lasers, which also provide for tuning.

The problems for research at the present time are as follows:

!. A search for organic compounds similar in energy characteristics to rhodamine 6G but lasing in other spectral regions,

2. Extension of the oscillation region to the UV and (especially) the IR.

3. Search for new pumping methods; nitrogen lasers have already been used in certain studies for example, and argon lasers are also promising.

4. Design of new lamps of high output power and flash duration I0-~-I0 -5 sec, with the UV filtered off or (better) the UV transformed to visible radiation.

5. Means of diminishing sources ofqoss, especially effective quenching agents for the metastable

state.

6. Design of dye lasers for particular applications; in particular, the design of spectrophotometers

with automatic recording of absorption spectra.

LITERATURE CITED

i. B.I. Stepanov and A. N. Rubinov, Dye Lasers [in Russian], Preprint, Physics Institute, Academy of Sciences of the BSSR, Minsk (1970); Zh. PriM. Spektr., 7, 505 (1967); Usp. Fiz. Nauk, 95, 45

(1968).

* The designers of the instrument have been awarded National Economy Medals of the USSR.

1119

2. B.I. Stepanov, Calculation Methods for Dye Lasers with Monochromatic Excitation, Part I [in Russian], Preprint, Physics Institute, Academy of Sciences of the BSSR, Minsk (1969).

3. A.M. Sgmson, Calculation Methods for Dye Lasers with Monochromatic Excitation, Part 2, Tran- sient State [in Russian], Preprint, Physics Institute, Academy of Sciences of the BSSR, Minsk (1970).

4. B.B. Snavely, Proc. IEEE, 57, No.8 (1969). 5. Quantum Electronics and Laser Spectroscopy [in Russian], Institut Fiziki AN BSSR, Minsk (1971).

1120