NASA

Technical

Memorandum

NASA TM - 108437

/ .

(NASA-TM-108431) DEVELOPMENT OF

INFRARED SENSORS USING ENERGY

TRANSFER/ENFRGY UPCONVERSION

PROCESSES: STUOY OF LASER EXCITED

FLU@RESCENCE IN RARE EARTH ION

DOPED CRYSTALS Fin_] RepoFt (NASA)

18 p

f-

/i t /

N94-24073

Unc|as

G3/25 0204779

DEVELOPMENT OF INFRARED SENSORS USING ENERGYTRANSFER/ENERGY UPCONVERSION PROCESSES:

STUDY OF LASER EXCITED FLUORESCENCE IN RARE

EARTH ION DOPED CRYSTALS

(MSFC Center Director's Discretionary Fund Final Report,Project No. 91-10)

By S.K. Nash-Stevenson, B.R. Reddy, and P. Venkateswarlu

Astrionics Laboratory

Science and Engineering Directorate

February 1994

Nf ANatMonal Aeronautics andSDace Administration

George C. Marshall Space Flight Center

MSFC- Form 3190 (Rev. May 1983)

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Public reoo_incl burden for this collection of information is estimated to av.e_age ,, _o I_. r^aes_..._r=.,_ n _ comments re(larding this burden estimate or any other a sO_, of th_

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE

February 1994

4. TITLEANDSUBTITLEDevelopment of Infrared Sensors Using Energy Transfer/Energy I

Upconversion Processes: Study of Laser Excited Fluorescence in Rare Earth IonDoped Crystals--Final Report Marshall Space Flight Center Center Director's

6. AUTHOR(S)

S.K. Nash-Stevenson, B.R. Reddy,* and P. Venkateswarlu*

7. PERFORMINGORGANIZATIONNAME(S)ANDADDRESS(ES)

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama 35812

9. SPONSORING/ MONITORINGAGENCYNAME(S)ANDADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546

3. REPORTTYPEAND DATESCOVEREDTechnical Memorandum

,5. FUNDINGNUMBERS

8. PERFORMING ORGANIZATIONREPORT NUMBER

10. SPONSORING / MONITORINGAGENCY REPORT NUMBER

NASA TM - 108437

11. SUPPLEMENTARY NOTES

Prepared by Astrionics Laboratory, Science and Engineering Directorate.

*Alabama A&M University, Normal, Alabama 35762

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified--Unlimited

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

This report presents a summary of the spectroscopic study of three systems: LaF3:Ho 3+,

LaF3:Er3* and CaF2:Nd 3÷. When the D levels of Ho 3+in LaF 3 were resonantly excited with a laser beam

of 640 nm, upconverted emissions were detected from J (416 nm), F (485 nm), and E (546 nm) levels.

Energy upconverted emissions were also observed from F and E levels of Ho 3+when the material was

excited with an 800 nm near infrared laser. When the D levels of Er 3÷in LaF 3were resonantly excited

with a laser beam of 637 nm, upconverted emissions were detected from the E (540 nm) and P (320,

400, and 468 nm) levels. Energy upconverted emissions were also observed from F, E, and D levels of

Er 3÷ when the material was resonantly excited with an 804 nm near infrared laser. When the D levels o

Nd 3+in CaF 2 were resonantly excited with a laser beam of 577 nm, upconverted emissions were de-

tected from the L (360 and 382 nm), K (418 nm), and I (432 nm) levels. Very weak upconverted

emissions were detected when this system was irradiated with a near infrared laser. The numbers in

parentheses are the wavelengths of the emissions.

14. SUBJECT TERMS

LaF3:Ho 3÷, LaF3:Er3* and CaFz:Nd 3÷, upconverted emissions

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATIONOF REPORT OF THIS PAGE OF ABSTRACT

.,T , _ __" c." _ J T 1_ _1_, _ c,;._"; _.A 1 lr,,q .ce;¢;,_cl

NSN 7540-01-280-5500

" 15. NUMBER OF PAGES

1916. PRICE CODE

NTIS

20. LIMITATION OF ABSTRAC

I T,.-, 1 ; _:'-_

Standard .,_--,-_,r_v'_ . 2-89)

TABLE OF CONTENTS

INTRODUCTION .........................................................................................................................

LaF3:Ho 3÷ .......................................................................................................................................

LaF3:Er 3÷ ........................................................................................................................................

CaF2:Nd 3+ .......................................................................................................................................

CONCLUSIONS ............................................................................................................................

Page

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Figure

1.

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10.

11.

12.

13.

LIST OF ILLUSTRATIONS

Title

Partial energy level diagram of LaF3:Ho 3÷ showing observed fluorescence

transitions .....................................................................................................................

Upconverted fluorescence spectrum of LaF3 :Ho 3÷ obtained by pumping thematerial with a near infrared laser (800 nm) ...............................................................

Power versus intensity plot of the F---)Z band of LaF3 :Ho 3+ using 800-nmlaser excitation .............................................................................................................

Fluorescence spectrum of LaF 3 :Ho 3÷ using 640-nm laser excitation ...........................

Power versus intensity plot of the F---)Z band of LaF3 :Ho 3÷ using 640-nmlaser excitation .............................................................................................................

Partial energy level diagram of LaF3:Er 3÷ showing observed fluorescence

transitions .....................................................................................................................

Fluorescence spectrum of LaF 3 :Er 3÷ obtained with 637-nm laser pumping ................

Fluorescence spectrum of LaF 3 :Er 3÷ obtained with near infrared (804-nm)

laser pumping ...............................................................................................................

Absorption spectrum of CaF2:Nd 3÷ .............................................................................

Partial energy level diagram of CaF2:Nd 3÷ .................................................................

Fluorescence spectrum of CaF2:Nd 3+ obtained on excitation with an Ar ÷

laser (514.5 nm) ...........................................................................................................

Upconverted fluorescence spectrum of CaF2:Nd 3+ obtained with dye

laser (577-nm) excitation .............................................................................................

Partial energy level diagram of CaF2 :Nd 3÷ showing observed fluorescencetransitions .....................................................................................................................

Page

2

6

7

8

9

10

11

11

12

iv

Symbol

LaF3:Ho 3+

LaF3:Er 3+

CaF2:Nd 3+

DEFINITION OF SYMBOLS

Lanthanum fluoride with triply ionized holmium

Lanthanum fluoride with triply ionized erbium

Calcium fluoride with triply ionized neodymium

TECHNICAL MEMORANDUM

DEVELOPMENT OF INFRARED SENSORS USING ENERGY TRANSFER/ENERGY

UPCONVERSION PROCESSES: STUDY OF LASER EXCITED FLUORESCENCE

IN RARE EARTH ION DOPED CRYSTALS

(MSFC Center Director's Discretionary Fund Final Report, Project No. 91-10)

INTRODUCTION

Rare Earth ion doped materials have been investigated extensively and have proven to be veryuseful in the generation of laser sources, luminescent materials, detection of infrared radiation, and in the

production of upconversion lasers. Specifically, these materials can convert infrared energy into visible

energy or long wavelengths into short wavelengths. Sequential two-photort/multiphoton excitation,

energy transfer interaction, and excited state absorption were found to be the mechanisms responsible

for the production of the energy upconverted emissions in rare earth ion doped crystals. Utilizing one or

more of these mechanisms, ions in a lower energy state get upconverted to a higher energy state whose

energy is greater than that of the incoming photon, and subsequently emit high energy photons.

Development of infrared sensors using energy upconversion processescould eliminate the cool-

ing required for some infrared sensors. However, the process and specific material would have to beidentified. Under CDDF Project No. 91-10, laser spectroscopic studies of different rare earth ion doped

CaF2 and LaF3 crystals were carried out at room temperature for this purpose. Three systems were iden-

tified which generated upconversion radiation and a discussion of the results is presented.

LaF3:Ho 3+

A partial energy level diagram of Ho 3÷ is shown in figure 1. The dye laser excited fluorescence isidentified with solid lines, and the infrared laser excited fluorescence is identified with dashed lines.

When the crystal was exposed to 800-nm laser radiation, a bright green emission from the material was

detected and visible to the naked eye. The resulting fluorescence was analyzed and found to arise from

the E, F, and D levels as identified in figure 2. The 800-nm laser excites only the phonon coupled levelsof the B state. However, the laser is in exact resonance from the excited state A to the higher G state. A

plot of F--->Z fluorescence versus laser power exhibited a gradient of 1.5 (fig. 3). This indicates that more

than one photon is responsible for the production of green emission. It was determined that the excited

ion relaxes to the A state and is then excited to the G state by another laser photon. In this case, the

800-nm photon is resonant with the A--->G transition. The G state relaxes to the F and E states in cascade

which emit the blue, 485-nm (F--->Z), and green, 546-nm (E--->Z), radiation.

When the D levels of Ho 3+ were resonantly excited with a 640-nm dye laser beam, upconvertedemissions were detected from the E, F, and J levels at 546 nm (E--->Z), 485 nm (F--->Z), and 416 nm

(J--->Z), respectively (fig. 4). The sample exhibited bright green fluorescence visible to the naked eye

even for 30 mW of input laser power. A plot of upconverted blue signal intensity (F-->Z) versus laser

power exhibited a gradient of 1.75 when the laser beam was focused in the sample and a value of 2.0

whenit wasdefocused(fig. 5).This is aclearindication that excited state population is getting saturated.

The 640-nm laser photon is resonant with Z--->D and A--->J transitions, and such a sequential two-photon

excitation process is responsible for the production of the upconverted emission. The lower F and E

levels were populated in cascade by multiphonon emission from the J state and emit blue and greenlight. The estimated energy upconversion efficiency was found to be ~ 1.1 x 10 --4.

32

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Figure 1. Partial energy level diagram of LaF3:Ho 3÷ showing observed fluorescence transitions.

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Wavelength (nm)

Upconverted Fluorescence

Figure 2. Upconverted fluorescence spectrum of LaF3:Ho 3÷ obtained by pumping thematerial with a near infrared laser (800 nm).

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LaF3: 14o3. (1%)Laser: 800 nmFluorescence: F -_ Z

SLOPE = 1.5

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Figure 3. Power versus intensity plot of the F---)Z band of LaF3 :Ho 3÷ using 800-nm laser excitation.

I

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Wavelength (nm)Fluorescence

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Wavelength (nm)Upconverted Fluorescence

Figure 4. Fluorescence spectrum of LaF3:Ho 3÷ using 640-nm laser excitation.

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Figure 5. Power versus intensity plot of the F---)Z band of LaF3 :Ho 3+ using 640-nm laser excitation.

LaF3:Er 3+

The next system discussed is LaF3:Er 3+. A partial energy level diagram of LaF3:Er 3+ is shown in

figure 6. Using a 637-nm dye laser beam, the D level of Er 3÷ was resonantly excited. Upconvertedemissions were observed at 317-nm (P---)Z), 398-nm (P---)Y), 468-nm (P_A), and 540-nm (E---)Z) as

shown by the fluorescence spectrum (fig. 7). These transitions are shown on the energy level diagram

(fig. 6) and are identified with the solid lines. Plotting P---)Y fluorescence versus laser intensity yielded a

gradient of 2.3, indicating that more than two photons were responsible for this emission. This was alsotrue for P--)A and P--)Z fluorescence. A plot of the E---)Z fluorescence versus laser intensity yielded a

gradient of 1.6. This indicates that two photons were responsible for this emission.

The fluorescence spectrum of LaF3 :Er 3+ obtained when the sample was excited with an 804-nm

laser beam is shown in figure 8. Using this laser, the B level of Er 3+ was resonantly excited and upcon-

verted emissions were observed at 524 nm (F_Z), 540 nm (E_Z), and 657 nm (D---)Z). Since the

804-nm laser beam is resonant with the Z---_B transition as well as the B--_K transition, it appears that a

direct stepwise two-photon excitation is responsible for the emissions.

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CaF2 :Nd 3+

The last system discussed is CaF2:Nd3+. The absorption spectrum (fig. 9) of this crystal was

recorded and revealed strong peaks at 349 nm (Z_L), 521 nm (Z_E), 579 nm (Z---_D), 539 nm (Z_A),and 792 nm (Z_S). From the absorption spectrum, an energy level diagram, shown in figure 10, was

drawn. When the F levels of CaF2:Nd 3÷ were resonantly excited with the 514.5-nm line of an Ar ÷ laser,

fluorescence was observed (fig. 11) at 586 nm (D_Z), 665 nm (B_Z), 788 nm (S_Z), and 861 nm

(R---_Z). However, when the D levels of CaF2:Nd 3÷ were resonantly excited using the 577-nm line of a

dye laser, regular fluorescence was detected as well as upconverted emissions at 360, 382, and 418 nm.

The upconverted fluorescence spectrum is shown in figure 12. These transitions are identified as L---_Z,L--_Y, and K---_Z and are shown on the energy level diagram in figure 13.

Attempts were also made to record the upconverted fluorescence of CaF2:Nd 3÷ by pumping the

sample with a near infrared laser. Unfortunately, the upconverted signals were too weak to get a goodspectrum. Therefore, no further studies were done on this particular system.

N

LU

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Figure 9.

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Wavelength (nm)

Absorption spectrum of CaF2:Nd 3+.

l

915

CONCLUSIONS

Of the three materials studied, LaF3:Ho 3+ and LaF3:Er 3+ would be very beneficial to the devel-

opment of uncooled infrared sensors and upconversion lasers operating at room temperature. InLaF3:Ho 3+, when a red laser beam and an infrared laser beam were used for excitation, green and blue

emissions were detected, with bright green being visible to the naked eye in both cases. Also, when

LaF3:Er 3+ was pumped with a red laser beam, violet, blue, and green emissions were detected. Green

and red emissions were detected when the material was pumped with an infrared beam. These detectedemissions were all results of energy upconversion. It was determined that sequential two-photon excita-

tion is the dominant mechanism in upconverting energy in these systems. The upconversion efficiency

of LaF3:Ho 3+ was found to be ~0.01 percent. The efficiency of LaF3:Er 3+ was not determined.

As a result of this research, a paper entitled "Energy Upconversion in LaF3 :Ho 3+'' was presented

at the Optical Society of America Annual Meeting/Ninth Interdisciplinary Laser Science Conferenceheld in Toronto, Canada, on October 3-8, 1993. Also, a paper entitled "Near Infrared to Blue Energy

Upconversion in LaF3:Ho 3+'' was submitted to the Journal of Optical Society of America B.

9

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Wavelength (nm)

Fluorescence spectrum of CaF2:Nd 3+ obtained on excitation with an Ar + laser (514.5 nm).

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Upconverted fluorescence spectrum of CaF2:Nd 3÷ obtained with dye laser (577-nm)excitation.

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Figure 13. Partial energy level diagram of CaF2:Nd3+ showing observed fluorescence transitions.

12

APPROVAL

DEVELOPMENT OF INFRARED SENSORS USING ENERGY TRANSFER/ENERGY

UPCONVERSION PROCESSES: STUDY OF LASER EXCITED FLUORESCENCE

IN RARE EARTH ION DOPED CRYSTALS

(MSFC Center Director's Discretionary Fund Final Report, Project No. 91-10)

By S.K. Nash-Stevenson, B.R. Reddy, and P. Venkateswarlu

The information in this report has been reviewed for technical content. Review of any informa-

tion concerning Department of Defense or nuclear energy activities or programs has been made by theMSFC Security Classification Officer. This report, in its entirety, has been determined to be unclassified.

H2Lr,ector, Astrionics Laboratory

"_" LIS. GOVERNMENT PRINTING OFICE 1994 533--108 /00013

13