Characteristics of a laserpumped 1.5μm infrared quantum counterS. E. Stokowski Citation: Journal of Applied Physics 45, 2957 (1974); doi: 10.1063/1.1663708 View online: http://dx.doi.org/10.1063/1.1663708 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/45/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Large and smallsignal gain characteristics of 1.5 μm multiple quantum well optical amplifiers Appl. Phys. Lett. 56, 1201 (1990); 10.1063/1.102559 Gain characteristics of a 1.5 μm nonlinear split contact laser amplifier Appl. Phys. Lett. 53, 1577 (1988); 10.1063/1.99954 Cancellation of fiber loss by semiconductor laser pumped Brillouin amplification at 1.5 μm Appl. Phys. Lett. 48, 1329 (1986); 10.1063/1.96950 Detection of 1.5μm wavelength laser light emission by infraredexcitable phosphors Appl. Phys. Lett. 39, 587 (1981); 10.1063/1.92833 Sensitivity of cw and pulsed laserpumped Pr infrared quantum counters J. Appl. Phys. 50, 4509 (1979); 10.1063/1.326557

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Characteristics of a laser-pumped 1.5-}-I-m infrared quantum counter

S. E. Stokowski

Martin Marietta Laboratories. Baltimore. Maryland 21227 (Received 7 March 1973)

The characteristics of room-temperature laser-pumped Er3+ infrared quantum counters (IRQC's) have been investigated. One such detector, employing BaY 2 Fs : Er J+ and a GaAs laser pump, exhibited an NEP of 3 X 10-9 W/Hz 112 at 1.493 /-lm, the smallest NEP reported thus far for an IRQC. The limiting source of noise was found to be a' generation of visible emission by the laser light alone, which provides an output equivalent to the dark current of a conventional detector. It is proposed that this output, which has a quadratic dependence on the laser power, arises largely from absorption of the . 8S4-nm pump light by the anti-Stokes sideband of the 4/9/2 state of Er 3+. The absorption coefficient for this process was determined to be approximately 3 X 10-6 cm -I. Minor modifications of the present. system appear capable of improving the NEP to 2 X 10-10 W/HZI/2, and further improvements may be possIble by selecting an IRQC host crystal that has a smaller absorption of the pump radiation than does BaY2Fg: ErH.

I. INTRODUCTION

Incoherent infrared-to-visible upconversion employing rare -earth ions has been considered for use in detectors based on Bloembergen's quantum counter concept1 and also in infrared-pumped visible phosphors. 2 Although much work has been done on infrared quantum counter (IRQC) schemes in rare-earth-doped materials, the results have been disappointing insofar as detector applications have been concerned. Noise equivalent powers (NEP's) for various IRQC detectors measured in previous work3

-s are listed in Table I. The primary

limiting factor in the previous work has been the use of low-power pump sources; the low oscillator strengths of the rare -earth transitions require high pump powers (-100 W / cm2) in order to achieve useful IRQC quantum efficiencies. Practically, this requirement means the use of lasers as pump sources. In the past, little work has been done on laser-pumped IRQC detectors, the only experiments reported thus far being that of Wright et al. 6 on Pr3

+ in LaCl3 and LaF3 and Bakumenko et al. 7

on E r 3+ in CaW04 • In these works, however, noise pro­cesses in the IRQC detector were not investigated and, thus, no NEP's were obtained ..

We report here on the characteristics of a laser­pumped Er 3+ IRQC for detecting 1. 5-}..Lm radiation, in­cluding an investigation of the noise processes. An NEP of 3 x10-9 W /HZ

lI2 at 1. 493 }..Lm and at room tempera­

ture, the lowest reported so far, was obtained in a BaY2FS: Er 3+ detector. The detection scheme used is shown in Fig. 1. The Er 3+ ions raised to the first ex­cited state (I 2 ), 4113 / 2) by the absorption of infrared radiation near 1.5 }..Lm were pumped up to the 14) state (4S3/2) by radiation, near 850 nm, from a GaAlAs laser or from a GaAs laser array operating near 77 K. Emission from the 14) level occurred in several lines between 540 and 555 nm and was detected by a photo­multiplier tube.

Although much of the initial materials researchs was done on oxygen-fired CdF2 : Er3+, BaY2Fa doped with 0.5% Er 3+ was chosen for detailed investigation, be­cause it can be grown as large clear specimens, where­as oxygen-fired CdF2 : Er3+ turns a milky-white color when heat treated in an oxygen atmosphere. The IRQC parameters for BaY2Fs : Er 3+ are all within a factor of

2957 Journal of Applied PhYSics, Vol. 45, No.7, July 1974

2 of those reported previouslys for oxygen-fired CdF2 : Er 3+. Some of the experiments on BaY2 Fs : Er 3+

described here were repeated on oxygen-fired CdF2 : Er3+, with similar results.

II. EXPERIMENTAL

The BaY2 Fs crystal used in the IRQC detector had dimensions of 1 X3 x10 mm3 and was oriented such that the ir light was inCident on one of the 1 X3-mm2 faces, with the laser light illuminating either the opposite 1 x 3-mm2 or the 1 x10-mm2 face. The viSible output was observed from the 3 x10-mm2 face. The CdF2 : Er 3+

samples were slightly smaller (-lx3x7 mm3). The ir

source was a quartz iodine lamp passed through a Spex Minimate, and the laser source was either a GaAlAs diode operated near room temperature or a 1-W GaAs array operated near 77 K. Both laser pumps were placed on heat sinks, the temperatures of which could be varied to provide wavelength tuning of the laser out­put. The visible output in the 540-nm band was detected by an EMI 9635B photomultiplier tube. Power levels were calibrated USing an Eppley thermopile, and the laser power was monitored by a Si photodiode.

III. RESULTS AND DISCUSSION

Employing the detector configuration described in Sec. II, the IRQC output at 300 K was measured as a function of both the infrared Signal wavelength As and the pump wavelength Ap• It was found that the IRQC output as a function of As followed closely the absorption spec­trum of the 1. 5-}..Lm band, as shown in Fig. 2. Further, the output varied linearly with the pump power up to 5 W/cm2, as expected from the theory9; saturation of the pump transition is expected to occur only when the

TABLE 1. NEP's of ffiQC detectors measured previously.

Material NEP Temp. A Reference <W/Hz1l2) (K) iJLm)

Pr3+: LaCl3 4 xlO-4 300 1.48 3

Pr3+ : LaCl3 3 X10-9 4.2 2.03 4

Tm3+: LaCl3 3 xlO-7 300 1. 76

H 03+ :CaW04 2.5 xlO-6 300 1. 95 5

Copyright © 1974 American Institute of Physics 2957

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2958 S.E. Stokowski: Infrared quantum counter

20 15>----14>

\ \

\ \

15 13>

-, It> E u

""0 -~

10 Ir>

<.:> a:: LW ;z LW

12>

5

o 11>

FIG. 1. Er 3+ energy levels. showing the IRQC scheme with green emission from the I 4) level and red emission from the r 3) level.

pump power exceeds"" 104 W /cm2• The IRQC output as

a function of Ap, obtained by temperature tuning the lasers, is shown in Fig. 3. The NEP measurements were made with Ap = 854 nm, at one of the peaks in this spectrum.

The NEP of the IRQC detector at room temperature was determined to be 3 x 10-9 W /HZl/2 at 15 Hz and a pump power of 2 W / cm 2. For reasons not yet known, the PMT used here was discovered to have a rms noise current at 15 Hz four to five times higher than the shot­noise limit. Thus, an NEP of 8x10-10 W/Hzl / 2 would have been obtained uSing a shot-noise-limited PMT.

This NEP is, nevertheless, considerably larger than the theoretical value, 1. 5 x 10-12 W /Hzl / 2

, estimated by Byer10 for CdF2 :Er 3+ at a pump power of 2 W/cm2

• The

WAVELENGTH ~,LLm)

FIG. 2. IRQC output and absorption coefficient at 300 K vs infrared wavelength of a BaY2Fs crystal doped with 0.5% of E r 3+.

J. Appl. Phys., Vol. 45, No.7, July 1974

~ l-

V'> Z ~ Z

CI z ...: cc E c

~

840 PUMP WAVELENGTH Inm)

FIG. 3. IRQC output at 300 K vs laser-pump wavelength.

2958

discrepancy was determined to arise from noise asso­ciated with a signal-independent visible emission gener­ated by the laser-pump radiation. That is, the laser illumination by itself was found to produce emission in the 540 and 560 nm bands, with a lesser intensity in the 525-nm band from the 2Hll/2 state of Er3

•• Fluctuations of this emission, which overlapped the 540-nm emission produced by the infrared double excitation, proved to be the limiting source of noise in the IRQC. The intensity of this emission varied as the square of the pump power

105

)04

~ c:: ::>

'"' e :e ~ >-~

'" i5 >-~

102

10 / 103

REPETITION RATE (sec-I,

BaY2 F8 . E,3+

300K

/ . I nte ns ity C( (Pow.,,2

FIG. 4. Intensity of green emission of BaY2Fs at 300 K with the laser alone incident on the crystal vs repetition rate of the pulsed laser. The energy density of each pulse was approxi­mately 0.2 mW/cm2•

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2959 S.E. Stokowski: Infrared quantum counter

14> ----.--

13> ---4--

12) -r-'--

11> ...1... __

(a) (b)

4 --1- F9/2

4 ---1- 19/2

4 +t-+- 11lI2

I RQC Absorption of Raman excitation pump light

FIG. 5. Processes by which the green fluorescent emission (483 / 2 to 4115 / 2) of Er3+ is excited. The laser-induced transi­tions are indicated by the heavier lines. Not shown are cooper­ative processess that involve a pair of Er 3+ ions.

over two orders of magnitude of laser power, as shown in Fig. 4. The presence of a visible output from the material with only the laser light incident is equivalent to a dark current from the IRQC detector. Because this dark current varies as the square of the power and the signal efficiency increases only linearly, the NEP of the detector will be independent of pump power, once the power is sufficiently high that the photomultiplier dark­current noise is not the limiting noise source.

We have considered various processes that may give rise to this nonlinear visible emission: (i) cooperative processes involving pairs of Er3

< ions, (ii) two-photon absorption, (iii) phonon-assisted absorption from the ground state, and (iv) Raman scattering. The latter two mechanisms are illustrated schematically in Figs. 5(b) and 5(c), respectively.

Cooperative processes will not be discussed further, because the experiments performed here cannot dis­tinguish them from processes involving a single ion only.

The two-photon absorption mechanism does not appear to be a significant contributor to the output. In the first place, the spectral dependence of the dark current, ID vs Ap , of the IRQC detector is closely similar to that of the signal current, Is vs Ap, indicating that the emission is produced by laser pumping from the 4113 / 2 state. Second, the visible emission arising from the pump radiation alone has a growth time constant of approxi­mately 14 ms and is not strictly exponential; whereas, if the emission were due to two-photon absorption the time constant would be expected to have a value close to the lifetime of the 4S3 / 2 state, T = 0.7 ms.

Experimentally, it is diffic~lt to distinguish between the Raman effect and the ground-state absorption shown

J. Appl. Phys., Vol. 45, No.7, July 1974

2959

in Fig. 5. It is estimated, however, that the Raman effect has an equivalent absorption strength of approxi­mately 10-11 cm -1, much smaller than the strength found here. Further, upon COOling to 90 K, the green emission decreased by a factor of 50, indicating the participation of a phonon, which is not required by the Raman process.

The nonlinear viSible emission can be accounted for satisfactorily, however, on the basis of phonon-assisted absorption from the ground state. In particular, absorp­tion of laser light by Er3., with the absorption or emis­sion of a phonon, places the Er3

< in the 419/2 or 4111/2

state, from which decay to the 4113 / 2 state may occur. The output signal or "dark current", 1m from the photo­multiplier due to this process is

(1)

where G is the gain of the PMT; 7)c' the photocathode efficiency; Q, the total IRQC efficiency; FpA=PL , the pump power; a(Ap), the spectral absorption coeffiCient; l, the path length for the pump light; and (3, the fraction of Er3

< ions_ excited by the laser light that decay to the 12) level (Fig. 5).

The IRQC output for a fixed ir signal can be written

Is(Ap)~ Ge(At/hc) 7)cQPi rY, (2)

where P ir is the incident infrared power; and "I, the fraction of P ir absorbed by the IRQC crystal.

Dividing I D by Is results in

ID _ P LAp[a(Ap)l{3] • Is - PirAtrY

(3)

This relation may be used to obtain two independent estimates of the ground-state absorption coefficient at 854 nm, a(854):

(i) If the ratio I nAl.! Is Ap P L is plotted as a function of Ap , the relative absorption coefficient a(Ap) shown in Fig. 6 is obtained. By comparing the relative magni­tudes of ID for Ap=790 nm and Ap =833 nm, it has been determined that a(833)/a(790)~ 0.1. Direct measure­ments of the ground-state absorption at 790 nm yielded a(790)=3.3x10-2 cm-1

; thus, a(833)~:L3xlO-3 cm-I, and from Fig. 6, therefore, we estimate a(854) "" 3 x 10-6 cm -1 •

(ii) The ground-state absorption coefficient at 854 nm can be estimated also from the IRQC parameters. Using the values T2=11.0 ms, 0"23=4x10-21 cm2

, and 7)41",,0.5, obtained from separate experiments in the manner ex­plained in Ref. 7, the internal infrared-to-visible con­version efficiency is calculated to be 2 x 10-4 for A. = 854 nm and 2 W Icm2 of pump flux. For a photocathode effi­ciency 7)c = 6 X 10-3, the total quantum efficiency (incident infrared photon to photoelectron conversion) is calcu­lated to be 1. 2 x 10-6 Y. Comparing this value with that determined experimentally, 1.2 X 10-7

, we conclude that Y~ 0.1, which is consistent with the experimental ob­servation that only 10-20% of the incident infrared light was absorbed in the IRQC crystal. Using this value of "I in Eq. (3), with measured values of IDlIs at Ap =854 nm, the product a(854){3 is found to have the value 1. 3 x 10-6 cm -1. From measurements of the efficiency with which the IRQC crystal converts 790-nm pump light

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2960 S.E. Stokowski: Infrared quantum counter

I~'-------------------------------~

§ HI u i:::: ... 0_ u ~

~§ ~ ~ ~~ "'.0 '" ~ «.!l! ... >

S ... '" 10

1 834 842

BaYz F8 : Er3+

300K

Relative absorption coefficient

TO a • constant x -

ISAPPL

PL' laser power Io' PMT output with laser al!H1e

IS • PMT output with l.5fJ-m radiation + laser

PUMP WAVELENGTH, Ap (nm)

FIG. 6. A plot of the relative absorption coefficient of BaY2F8 : Er3+ at 300 K vs the wavelength of the laser pump.

to emission in the 540-nm band, the fraction,8 has been determined to have a value near 0.16. Thus, by this method of calculation, a(854) is found to have the value 8 X 10-6 cm-1, in reasonable agreement with the value found from the spectral variation of the, absorption in­ferred from the ratio IDAt.//sApPL' This agreement provides support for the interpretation that the nonlinear visible emission, thus 1m arises from ground-state absorption of the pump radiation.

On the basis of this interpretation, the NEP asso­ciated with I D• in the shot-noise limit, can be deter­mined from the relation10 :

NEP= hV12 (2..!lL)1/2. Q17c Ge

(4)

Using the values of the parameters estimated above, we calculate NEP~ 10-9 W/Hz1/2, in good agreement with the experimental data as extrapolated to measurements made with a shot-noise -limited PMT (8 x 10-10 W IHz1 /2).

It is suggested that this absorption at the laser-pump wavelengths arises from the anti-Stokes sidebands of transitions from the ground state to 4[9/2' The fact that

J. Appl. Phys., Vol. 45, No.7, July 1974

2960

this absorption decreases upon cooling the crystal to 90 K is qualitatively consistent with this proposal, but the magnitude of the change poses a problem: At 90 K, the contribution due to this anti -Stokes sideband would be expected to decrease to 3.7 xl0-5 of the room-tem­perature value, whereas the observed decrease is only to 0.02. It appears necessary, therefore, to assume that an additional small absorption is present that is in­fluenced by temperature less strongly than is the anti­Stokes absorption. It is reasonable and consistent to attribute this additional absorption to the Stokes phonon Sideband of the 4/11/2 state, although impurity absorption also may contribute.

In summary, this investigation has shown that high quantum efficiencies are attainable in a laser-pumped BaY2FS :Er 3+ IRQC. The resulting NEP of 3 X 10-9 wi Hz1 / 2 measured for this IRQC at 1. 493 /-Lm and room temperature appears to be the smallest reported thus far for an IRQC. Moreover, substantial improvements, possibly to 2 X 10-10 W IHzl /2, seem attainable by using a better PMT and a higher concentration of Er 3+ ions, and by capturing the incident radiation more effectively through the use of reflective coatings on the passive surfaces of the IRQC crystal. Improvements in the NEP beyond this value would require an IRQC material with an absorption coefficient at the pump wavelength signi­ficantly smaller than that observed in this study of BaY2 Fs :Er 3+. Perhaps the fact that the Er 3+ ion usually haS a weaker coupling to the phonons in chlorides than in fluorides can be used to advantage by making one of the rare-earth trichlorides, or a related compound, the host crystal for an Er 3+ IRQC.

ACKNOWLEDGM ENTS

The author wishes to acknowledge helpful discussions with N.E. Byer, W.M. Mularie, and R.G. Lye, and to thank L. F. Johnson for the loan of the BaY2 Fs material, which was grown by H. J. Guggenheim.

IN. Bloembergen, Phys. Rev. Lett. 2, 84 (1959). 2L.F. Johnston, J.E. Geusic, H.J. Guggenheim, T. Kushida, S. Singh, and L. G. VanUitert, Appl. Phys. Lett. 15, 48 (1969).

3J.F. Porter, 'Jr., IEEE J. Quantum Electron. QE-l, 113 (1965). '

4W.B. Gandrud and H. W. Moss, IEEE J. Quantum Electron. QE-4, 249 (1968).

·J.G. Gualtieri, G. P. deThery, T.R. AuCOin, and J.R. Pastore, Appl. Phys. Lett. 11, 389 (1967).

6J.C. Wright, D.J. Zalucha, H. V. Lauer, D. E. Cox, and F.K. "Fong, J. Appl. Phys. 44, 781 (1973).

7V.L. Bakumenko, A.N. Vlasov, E.S. Kovarskaya, G.S. Koz ina , and V. N. Favorin, JETP Lett. 2, 16 (1965).

8N.E. Byer, T.C. Ensign, W.M. Mularie, andS.E. Stokowski, J. Appl. Phys. 44, 1733 (1973).

9W.F. Krupke, IEEEJ. Quantum Electron. QE-l, 20 (1965). ION. E. Byer, J. Appl. Phys. 43, 3567 (1972).

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