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Efficient doubling and CW difference frequency mixing in the infrared using the chalcopyrite CdGeAs2
Volume t0, number 4
OPTICS COMMUNICATIONS
April 1974
EFFICIENT DOUBLING AND CW DIFFERENCE FREQUENCY MIXING IN THE INFRARED USING THE CHALCOPYRITE CdGeAs 2 H. KILDAL and J.C. MIKKELSEN Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 021 73, USA Received 14 January 1974
We have obtained fifteen percent average internal power conversion efficiency for second harmonic generation in CdGeAs2 of radiation from a TEA CO2 laser operating at 10.6 #m. This is the highest reported doubling efficiency for a CO2 laser. We have also observed cw difference frequency mixing in CdGeAs2 at wavelengths between 11.4 and 16.8 ,,m using grating-tuned CO and CO2 lasers. The CdGeAs2 crystals are significantly improved in size and optical quality over those previously available.
We report efficient doubling of 10.6 #m radiation and cw difference frequency mixing out to 16.8/am in CdGeAs 2. This chalcopyrite is a useful nonlinear optical material between 3 and 18/am. Its linear and second order nonlinear optical properties have been measured by Byer et al. [1] and Boyd et al. [2]. Recently, phasematched third harmonic generation has also been observed [3,4]. The nonlinear efficiency in these earlier experiments was limited because the phasematching lengths were less than 2 ram. For the second harmonic generation experiment we used a 10.6/am TEA CO 2 laser operating in the TEM00 mode with a pulse length of 160 nsec. The laser was focused by a 21-inch focal length mirror on the CdGeAs 2 crystal, and polyethylene attenuators were used to control the incident CO 2 power. The second harmonic output was measured with a calibrated thermopile; a sapphire filter was used to block the CO 2 radiation. The highest conversion efficiency was observed for a 9 mm long uncoated p-type crystal cut for type 1I phasematching. With an incident CO 2 power density of 20 MW/cm 2 the observed average external power conversion efficiency was found to be 7% with a maximum 5.3 tam peak power of 2 kW. This corresponds to internal conversion efficiency of 15 %, the highest observed for doubling CO 2 laser radiation. The best previous result was 5 % internal conversion in tellurium [5]. The measured optical damage threshold in CdGeAs 2 306
Fig. 1. Second harmonic pulse shape for increasing incident power densities (all horizontal time scales are in 200 nsec/div): a) 7 MW/cm2 (vertical scale: 10 mV/div); b) 21 MW/cm2 (vert. sc.: 20 mV/div); c) 34 MW/cm2 (vert. sc.: 20 mV/div); d) 34 MW/cm2 (vert. sc. : 4 mV/div) with the crystal tuned slightly off phasematching position. is as high as 38 MW/cm 2, and a further enhancement in conversion efficiency had been expected for input power densities higher than 20 MW/cm 2. Instead a
Volume 10, number 4
OPTICS COMMUNICATIONS
(a)
2.46*
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o
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/~
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0
(b)
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Fig. 2. Second harmonic output power from a 9 mm CdGeAs2 crystal, cut for type II phasematching, when the crystal is scanned through its phasematching position at an incident power density of: a) 0.4 MW/cm 2, and b) 31 MW/cm 2.
saturation o f the second harmonic peak power was observed. This is illustrated by figs. la, b, and c, which show the second harmonic pulse shape for increasing incident CO 2 power densities. The saturation is not due to normal pump depletion since the observed maximum internal peak power conversion was only 17%. For a constant incident power density the saturation disappears when the crystal is tuned slightly off its phasematching position, as illustrated by figs. lc and d. This demonstrates that the magnitude of saturation depends on the power at the second harmonic wave-
April 1974
length. Figs. 2a and b show the average second harmonic output power as the crystal is scanned through the phasematching angle at small and large incident power densities. Again saturation is apparent for large power densities. The CO 2 pump depletion shows no similar saturation. It continued to increase, reaching almost 60% at an incident power density of 32 MW/cm 2. We have used crystals cut from different boules and all show saturation. The maximum conversion efficiency is approximately proportional to the crystal length. We believe saturation is due to a nonlinear absorption mechanism involving the 5.3/am radiation. There is no evidence for optically generated free carriers, which are responsible for limiting the conversion efficiency of tellurium [5]. The best crystal of CdGeAs 2 used for second harmonic generation was also used to perform difference frequency mixing with grating-tuned cw CO and CO 2 lasers. A 12.7 cm focal length Ge lens, AR-coated at 10.6/am, focused the two laser beams into the crystal. The output was focused by a 5 cm uncoated Ge lens into a 1 m grating spectrometer and monitored by a Ge:Cu detector. The output wavelength ranged between 11.4 and 16.8/am. The effective linewidth determined by the frequency jitter was approximately 0.5 MHz. As an example of conversion efficiency, the generated power at 12.87/am was 4/aW with incident powers of 1.25 W at 10.22/am and 0.097 W at 5.70/am. Attenuation by the uncoated Ge lens and the spectrometer reduced the power to 0.7/aW at the detector. The resulting signal-to-noise ratio observed on the oscilloscope display was 50 at a 3 kHz bandwidth. We estimate that absorption by the crystal reduced the conversion efficiency by a factor of two. In addition crystal reflection losses reduced the efficiency by a factor of three. Fig. 3 shows the measured mixing wavelengths as a function of phasematching angle. The CO 2 laser wavelength was fixed at 9.59 or 9.54/am while the CO laser was tuned from 5.21 to 5.92/am or from 5.80 to 6.09 /am, respectively. The figure also shows the phasematching angles calculated with the Sellmeier equations 6.5336 1.1204 + n~ = 6.4303 + 1 (0.843 68/~.)2 1 - ( 3 6 / ) t ) 2 and 8.9801 1.4378 g/o2 = 6.4141 + - 1 - ( 0 . 5 6 0 36/X) 2 1 (36/X) 2 ' 307
Volume 10, number 4
OPTICS COMMUNICATIONS
April 1974
1Or 17 t
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Pig. 3. Difference mixing wavelength versus measured and calculated phasematching angles for type II phasematching in CdGeAs2. The CO2 laser is fixed at 9.59 or 9.54 um while the CO laser is tuned from 5.21 to 5.92/am or from 5.80 to 6.09 urn, respectively.
CdGeAs2.
for the extraordinary and ordinary indices of refraction as determined from the data in ref. [2] for wavelengths out to 12.5 tam. There is a systematic difference of 2 - 3 ° between the measured and calculated phasematching angles. We see that tuning between 11 and 17 tam is accompanied by a change in the phasematching angle of only one degree, which corresponds to a crystal rotation of less than four degrees. One crystal will therefore cover the entire tuning range. We have observed similar disagreement between measured and calculated phasematching angles for doubling of 10.6 tam radiation. For the best crystal the measured angle of 48.4 ° -+0.3 ° is 2.4 ° smaller than the calculated angle of 50.8 °. For a crystal cut from a boule with larger optical absorption, the measured phasematching angle was 50.7 ° -+0.5 °. The difference in the measured angles for the two boules may be due to a slight change in stoichiometry or carrier concentration. Boyed et al. [6] report a change in the birefringence of 0.001 between different boules of ZnGeP2, which they attribute to stoichiometric variations. It has been estimated that the type II phasematching angle is reduced by one degree for a decrease in hole concentration o f 5 × 1015 cm -3 in p-type CdGeAs 2 [7]. The crystals used in this work were grown by the vertical Bridgman Stockbarger technique with fur-
nace temperature gradients between 50 and 100 ° C/cm. We have grown crack-free single crystals up to 3 cm long and 11 mm in diameter, whereas the largest such crystals previously reported [1] have been only a few mm 3 in volume. Our crystals show optical nonuniformities, which are probably due to inclusions and also variations in carrier concentration. Fig. 4 shows the measured optical absorption as a function o f wavelength for a sample cut from the best crystal. The minimum absorption is 0.23 cm -1 at 9 to 11/am, the smallest measured absorption for this material. (Byer et al. [ 1 ] report 0.4 cm-1.) The absorption maximum at 5.3 tam, which is seen only in p-type material, is due to hole transitions between valence bands. By fitting a theoretical curve to the measured absorption data we obtain a valence band splitting of 0.15 eV, in good agreement with the value of 0.16 eV obtained from electroreflectance measurements [8]. We expect further improvements in the optical quality by controlling the carrier concentration through annealing and compensation techniques. In conclusion we have observed 15 % average internal power conversion efficiency for second harmonic generation in CdGeAs 2. The detailed nature of the mechanism giving rise to the observed saturation in efficiency is not known. CW difference frequency mixing has been obtained between 11.4 and 16.8 tam.
308
2
5
5
WAVELENGTH (/xm) Fig. 4. Optical absorption in the [110] direction for p-type
Volume 10, number 4
OPTICS COMMUNICATIONS
To obtain continuous tuning o f the difference frequency it should be possible to replace the CO laser by a cw InSb spin-flip laser of comparable output power. This would give a 10 MHz linewidth source [9] with adequate power for spectroscopy in the spectral region from 11 out to 18 t~m (where the infrared cut-off occurs). We are indebted to P.L. Kelley and A.J. Strauss for many helpful suggestions and their critical reading o f this manuscript. We also acknowledge helpful discussions with A. Mooradian and S.R.J. Brueck and thank S.N. Landon for aid in the computer calculations and F. Leonberger for ti:ansmission measurements. The technical assistance of E. Mastromattei, S. Duda, and R.M. Morandi in X-ray orienting, cutting, and polishing the crystals used in this work is greatly appreciated. This work was sponsored by the Department o f the Air Force.
April 1974
References [ 1 ] R.L. Byer, H. Kildal and R.S. Feigelson, Appl. Phys. Lett. 19 (1971) 237. [2] G.D. Boyd, E. Buehler, F.G. Storz and J.H. Wernick, 1EEE J. Quantum Electron. QE-8 (1972) 419. [3] H. Kildal, R.F. Begley, M.M. Choy and R.L. Byer, J. Opt. Soc. Am. 62 (1972) 1398. [4] D.S. Chemla, R.F. Begley and R.L. Byer, to be published in IEEE J. Quantum Electron. [5] W.B. Gandrud and R.L. Abrams, Appl. Phys. Lett. 17 (1970) 302. [6] G.D. Boyd, W.B. Gandrud and E. Buehler, Appl. Phys. Lett. 18 (1971) 446. [7] H. Kildal, Air Force Materials Laboratory Technical Report AFML-TR-72-277, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio, (1972). [8] A. Shileika, Surface Science 37 (1973) 730. [9] S.R.J. Brueck, personal communication.
309
OPTICS COMMUNICATIONS
April 1974
EFFICIENT DOUBLING AND CW DIFFERENCE FREQUENCY MIXING IN THE INFRARED USING THE CHALCOPYRITE CdGeAs 2 H. KILDAL and J.C. MIKKELSEN Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 021 73, USA Received 14 January 1974
We have obtained fifteen percent average internal power conversion efficiency for second harmonic generation in CdGeAs2 of radiation from a TEA CO2 laser operating at 10.6 #m. This is the highest reported doubling efficiency for a CO2 laser. We have also observed cw difference frequency mixing in CdGeAs2 at wavelengths between 11.4 and 16.8 ,,m using grating-tuned CO and CO2 lasers. The CdGeAs2 crystals are significantly improved in size and optical quality over those previously available.
We report efficient doubling of 10.6 #m radiation and cw difference frequency mixing out to 16.8/am in CdGeAs 2. This chalcopyrite is a useful nonlinear optical material between 3 and 18/am. Its linear and second order nonlinear optical properties have been measured by Byer et al. [1] and Boyd et al. [2]. Recently, phasematched third harmonic generation has also been observed [3,4]. The nonlinear efficiency in these earlier experiments was limited because the phasematching lengths were less than 2 ram. For the second harmonic generation experiment we used a 10.6/am TEA CO 2 laser operating in the TEM00 mode with a pulse length of 160 nsec. The laser was focused by a 21-inch focal length mirror on the CdGeAs 2 crystal, and polyethylene attenuators were used to control the incident CO 2 power. The second harmonic output was measured with a calibrated thermopile; a sapphire filter was used to block the CO 2 radiation. The highest conversion efficiency was observed for a 9 mm long uncoated p-type crystal cut for type 1I phasematching. With an incident CO 2 power density of 20 MW/cm 2 the observed average external power conversion efficiency was found to be 7% with a maximum 5.3 tam peak power of 2 kW. This corresponds to internal conversion efficiency of 15 %, the highest observed for doubling CO 2 laser radiation. The best previous result was 5 % internal conversion in tellurium [5]. The measured optical damage threshold in CdGeAs 2 306
Fig. 1. Second harmonic pulse shape for increasing incident power densities (all horizontal time scales are in 200 nsec/div): a) 7 MW/cm2 (vertical scale: 10 mV/div); b) 21 MW/cm2 (vert. sc.: 20 mV/div); c) 34 MW/cm2 (vert. sc.: 20 mV/div); d) 34 MW/cm2 (vert. sc. : 4 mV/div) with the crystal tuned slightly off phasematching position. is as high as 38 MW/cm 2, and a further enhancement in conversion efficiency had been expected for input power densities higher than 20 MW/cm 2. Instead a
Volume 10, number 4
OPTICS COMMUNICATIONS
(a)
2.46*
g o
o
E
~U r~
/~
n,* U.J
0
(b)
Q_
(,3 Z 0
a Z
8lad
-9
-6
-3
0
3
6
9
CRYSTAL ROTATION ANGLE (deq)
Fig. 2. Second harmonic output power from a 9 mm CdGeAs2 crystal, cut for type II phasematching, when the crystal is scanned through its phasematching position at an incident power density of: a) 0.4 MW/cm 2, and b) 31 MW/cm 2.
saturation o f the second harmonic peak power was observed. This is illustrated by figs. la, b, and c, which show the second harmonic pulse shape for increasing incident CO 2 power densities. The saturation is not due to normal pump depletion since the observed maximum internal peak power conversion was only 17%. For a constant incident power density the saturation disappears when the crystal is tuned slightly off its phasematching position, as illustrated by figs. lc and d. This demonstrates that the magnitude of saturation depends on the power at the second harmonic wave-
April 1974
length. Figs. 2a and b show the average second harmonic output power as the crystal is scanned through the phasematching angle at small and large incident power densities. Again saturation is apparent for large power densities. The CO 2 pump depletion shows no similar saturation. It continued to increase, reaching almost 60% at an incident power density of 32 MW/cm 2. We have used crystals cut from different boules and all show saturation. The maximum conversion efficiency is approximately proportional to the crystal length. We believe saturation is due to a nonlinear absorption mechanism involving the 5.3/am radiation. There is no evidence for optically generated free carriers, which are responsible for limiting the conversion efficiency of tellurium [5]. The best crystal of CdGeAs 2 used for second harmonic generation was also used to perform difference frequency mixing with grating-tuned cw CO and CO 2 lasers. A 12.7 cm focal length Ge lens, AR-coated at 10.6/am, focused the two laser beams into the crystal. The output was focused by a 5 cm uncoated Ge lens into a 1 m grating spectrometer and monitored by a Ge:Cu detector. The output wavelength ranged between 11.4 and 16.8/am. The effective linewidth determined by the frequency jitter was approximately 0.5 MHz. As an example of conversion efficiency, the generated power at 12.87/am was 4/aW with incident powers of 1.25 W at 10.22/am and 0.097 W at 5.70/am. Attenuation by the uncoated Ge lens and the spectrometer reduced the power to 0.7/aW at the detector. The resulting signal-to-noise ratio observed on the oscilloscope display was 50 at a 3 kHz bandwidth. We estimate that absorption by the crystal reduced the conversion efficiency by a factor of two. In addition crystal reflection losses reduced the efficiency by a factor of three. Fig. 3 shows the measured mixing wavelengths as a function of phasematching angle. The CO 2 laser wavelength was fixed at 9.59 or 9.54/am while the CO laser was tuned from 5.21 to 5.92/am or from 5.80 to 6.09 /am, respectively. The figure also shows the phasematching angles calculated with the Sellmeier equations 6.5336 1.1204 + n~ = 6.4303 + 1 (0.843 68/~.)2 1 - ( 3 6 / ) t ) 2 and 8.9801 1.4378 g/o2 = 6.4141 + - 1 - ( 0 . 5 6 0 36/X) 2 1 (36/X) 2 ' 307
Volume 10, number 4
OPTICS COMMUNICATIONS
April 1974
1Or 17 t
[
"
[
!
[
I
I
T l T-1
r T TT
r
j
: 9.5 9/x m ..~
11/
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47 48 49 PHASEMATCHING ANGLE (deg)
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50
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l
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~'
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20
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Pig. 3. Difference mixing wavelength versus measured and calculated phasematching angles for type II phasematching in CdGeAs2. The CO2 laser is fixed at 9.59 or 9.54 um while the CO laser is tuned from 5.21 to 5.92/am or from 5.80 to 6.09 urn, respectively.
CdGeAs2.
for the extraordinary and ordinary indices of refraction as determined from the data in ref. [2] for wavelengths out to 12.5 tam. There is a systematic difference of 2 - 3 ° between the measured and calculated phasematching angles. We see that tuning between 11 and 17 tam is accompanied by a change in the phasematching angle of only one degree, which corresponds to a crystal rotation of less than four degrees. One crystal will therefore cover the entire tuning range. We have observed similar disagreement between measured and calculated phasematching angles for doubling of 10.6 tam radiation. For the best crystal the measured angle of 48.4 ° -+0.3 ° is 2.4 ° smaller than the calculated angle of 50.8 °. For a crystal cut from a boule with larger optical absorption, the measured phasematching angle was 50.7 ° -+0.5 °. The difference in the measured angles for the two boules may be due to a slight change in stoichiometry or carrier concentration. Boyed et al. [6] report a change in the birefringence of 0.001 between different boules of ZnGeP2, which they attribute to stoichiometric variations. It has been estimated that the type II phasematching angle is reduced by one degree for a decrease in hole concentration o f 5 × 1015 cm -3 in p-type CdGeAs 2 [7]. The crystals used in this work were grown by the vertical Bridgman Stockbarger technique with fur-
nace temperature gradients between 50 and 100 ° C/cm. We have grown crack-free single crystals up to 3 cm long and 11 mm in diameter, whereas the largest such crystals previously reported [1] have been only a few mm 3 in volume. Our crystals show optical nonuniformities, which are probably due to inclusions and also variations in carrier concentration. Fig. 4 shows the measured optical absorption as a function o f wavelength for a sample cut from the best crystal. The minimum absorption is 0.23 cm -1 at 9 to 11/am, the smallest measured absorption for this material. (Byer et al. [ 1 ] report 0.4 cm-1.) The absorption maximum at 5.3 tam, which is seen only in p-type material, is due to hole transitions between valence bands. By fitting a theoretical curve to the measured absorption data we obtain a valence band splitting of 0.15 eV, in good agreement with the value of 0.16 eV obtained from electroreflectance measurements [8]. We expect further improvements in the optical quality by controlling the carrier concentration through annealing and compensation techniques. In conclusion we have observed 15 % average internal power conversion efficiency for second harmonic generation in CdGeAs 2. The detailed nature of the mechanism giving rise to the observed saturation in efficiency is not known. CW difference frequency mixing has been obtained between 11.4 and 16.8 tam.
308
2
5
5
WAVELENGTH (/xm) Fig. 4. Optical absorption in the [110] direction for p-type
Volume 10, number 4
OPTICS COMMUNICATIONS
To obtain continuous tuning o f the difference frequency it should be possible to replace the CO laser by a cw InSb spin-flip laser of comparable output power. This would give a 10 MHz linewidth source [9] with adequate power for spectroscopy in the spectral region from 11 out to 18 t~m (where the infrared cut-off occurs). We are indebted to P.L. Kelley and A.J. Strauss for many helpful suggestions and their critical reading o f this manuscript. We also acknowledge helpful discussions with A. Mooradian and S.R.J. Brueck and thank S.N. Landon for aid in the computer calculations and F. Leonberger for ti:ansmission measurements. The technical assistance of E. Mastromattei, S. Duda, and R.M. Morandi in X-ray orienting, cutting, and polishing the crystals used in this work is greatly appreciated. This work was sponsored by the Department o f the Air Force.
April 1974
References [ 1 ] R.L. Byer, H. Kildal and R.S. Feigelson, Appl. Phys. Lett. 19 (1971) 237. [2] G.D. Boyd, E. Buehler, F.G. Storz and J.H. Wernick, 1EEE J. Quantum Electron. QE-8 (1972) 419. [3] H. Kildal, R.F. Begley, M.M. Choy and R.L. Byer, J. Opt. Soc. Am. 62 (1972) 1398. [4] D.S. Chemla, R.F. Begley and R.L. Byer, to be published in IEEE J. Quantum Electron. [5] W.B. Gandrud and R.L. Abrams, Appl. Phys. Lett. 17 (1970) 302. [6] G.D. Boyd, W.B. Gandrud and E. Buehler, Appl. Phys. Lett. 18 (1971) 446. [7] H. Kildal, Air Force Materials Laboratory Technical Report AFML-TR-72-277, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio, (1972). [8] A. Shileika, Surface Science 37 (1973) 730. [9] S.R.J. Brueck, personal communication.
309