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F3 High power far infrared generation in GaAs
Volume 18, n u m b e r l
OPTICS COMM UN1CATIONS
The focal spot size is chosen to give a combined incident power density o f approximately 5 MW cm - 2 , which was f o u n d to be below the damage threshold of the crystal for these short pulses. The infrared difference signal is detected with a HgCdTe detector. To satisfy non-critical (90 °) phasematching, b o t h dye lasers m u s t be t u n e d simultaneously. The input wavelengths necessary for each infrared difference frequency were calculated using the Sellmeier equations given by Bahr and Smith [ 1 ]. To cover the entire range, the e-ray was tuned from 5385 A to 5800 A and the o-ray from 5550 A to 6200 A. The experimentally measured wavelengths were within 10 A of the calculated ones. This d e m o n s t r a t e s that the Sellmeier equation for the infrared, which was derived from index data to 13 microns, is useful to at least 18 microns. The conversion efficiency was f o u n d to decrease at about 13 microns, but to rise again at 15 microns. This decrease in efficiency can be attributed to a two p h o n o n absorption [4]. However, this absorption does n o t limit the usefulness of the crystal in this region, and, in addition, it was f o u n d to have only a minimal effect on the indices. The cut-off at 18 microns is due to the detector response. With one of the dye laser frequencies fixed, the other dye laser was scanned through the corresponding phasematching frequency. The expected (sin x/x) 2 signature was observed and was measured to have a width (fwhm) o f 5 cm - I , which agrees with the calculated value to within the b a n d w i d t h s of the dye lasers. This experiment d e m o n s t r a t e s the feasibility of using a nitrogen laser-dual dye laser c o m b i n a t i o n for tunable infrared generation in the fingerprint region of the infrared. Its relative simplicity and low cost m a k e it an attractive alternative to parametric oscillators and spin-flip R a m a n lasers. We wish to thank Prof. R.L. Byer of Stanford Univ. lor providing the crystal of AgGaS 2. References [ 1 ] G.C. Bahr and R.C. Smith, IEEE J. Quantum Elektron. QE-10 (1974) 546. 12] S.A. Myers, Opt. C o m m u n . 4 (1971) 187. [3 ] D.C. Hanna, P.A. Karkkainen, and R. Wyatt, Optics and Quant. Elect. 7 (1975) 115. [4] J. Jerphagnon, private c o m m u n i c a t i o n .
F3
HIGH POWER F A R I N F R A R E D G E N E R A T I O N IN GaAs* N. LEE, B. L A X * * and R.L. A G G A R W A L * *
Francis Bitter National Magnet Laboratoryt, Massachusetts Institute o f Technology, Cambridge, Massachusetts 02139, USA Previously [ 1,2] reported generation o f far infrared (FIR) radiation by noncollinear mixing o f two pulsed T E A CO 2 lasers in a 1 cm crystal o f GaAs at liquid helium temperatures produced step-tunable radiation from ~ 70 # m to 2 m m . With
50
July 1976
200 kW input peak power from each ('O 2 laser ~ 20 mW peak o u t p u t power at a wavelength of ~ 100 u m was obtained. In this paper we report the generation of FIR o u t p u t ~ 4 kW at ~ 100/am, which is orders of magnitude higher than those obtained previously. These new experimental results were achieved by using noncollinear folded mixing geometries [ 3 ]. With a 10 cm long crystal of a simple folded geometry [4] in liquid helium temperatures and 130 kW peak input power incident on the crystal from each CO 2 laser, the H R o u t p u t was measured to be ~ 50 W at 100 # m at the o u t p u t face of the crystal [5 ]. This result is within an order of magnitude of the theoretical estimate neglecting the absorption losses. The orders-of-magnitude i m p r o v e m e n t o f present results is largely due to a better GaAs crystal and more reliable calibration of the FIR detection system. Much higher input powers could not be used in this crystal since laser damage was observed at an energy density as low as 0.3 J / c m 2 which is more than an order o f magnitude less than the generally quoted value of 10 J/cm 2. The cause for this lowering of the damage threshold was f o u n d to be due to the trapping o f the CO 2 laser b e a m s inside this crystal geometry. The trapped multiple passed CO 2 laser b e a m s interfere with each other constructively at some points in the crystal creating high peak intensity regions at the surface. By going to a folded parametric geometry [6] with input and o u t p u t couplers for the CO 2 laser beams, we have eliminated the internal trapping o f CO 2 laser radiation responsible for lower damage threshold. A 10 cm long crystal o f folded parametric geometry was prepared by optically contacting input and o u t p u t couplers for the CO 2 laser beams to the main crystal. The transmission loss of CO 2 laser beams across such an optically contacted interface was found to be about one percent. With about 1.7 MW and 3 MW peak power from the two CO 2 laser b e a m s incident on the sample, we obtained ~ 4 kW FIR power at ~ 100 tam at the o u t p u t face of the crystal. Presently we are developing the fine tuning capability of our TEA lasers so as to take advantage o f resonance cavity for FIR radiation. We h o p e this will improve the mixing efficiency for the FIR generation. References [ 1 ] R.L. Aggarwal, B. Lax and G. Favrot, Appl. Phys. Lett. 22 (1973) 329. [2] B. Lax, R.L. Aggarwal and G. Favrot, Appl. Phys. Lett. 23 (1973) 679. [3] N. Lee, R.L. Aggarwal and B. Lax, Opt. C o m m u n . 11 (1974) 339. [4] See fig. 2 o f r e f . [3].
* Supported in part by the Advanced Research Projects Agency through the Office of Naval Research, and in part by the National Science Foundation. ** Also Physics Department, M.I.T. "~ Supported by the National Science Foundation.
N O N L I N E A R OPTICS I (solids and liquids)
[ 5 ] The input peak powers given here have been deduced on the a s u m p t i o n that half of the CO2 pulse energy is contained in an initial spike of 100 nsec (fwhm) with the remaining half of the energy in a relatively long tail. The FIR peak powers are obtained by assuming that the output energy is contained in a 100 nsec (fwhm) pulse. I6] See figs. 3 of ref. [31.
F4
SECOND HARMONIC G E N E R A T I O N IN R A R E E A R T H 1ON DOPED N O N L I N E A R C R Y S T A L R. BONNEVILLE and F. A U Z E L Centre National d'Etudes des T~l~communications, 196 rue de Paris, 92220 Bagneux, France
Interactions between rare earth ions in solids are k n o w n to induce, besides energy transfers, cooperative effects [ 1 ]. A m o n g t h e m , cooperative luminescence has been reported by Nakasawa and Shionoya [2]: when exciting YbPO 4 by incoherent, near infrared (Yb 3+ absorption) light, they observed an anti-stokes luminescence at twice the excitation energy, the intensity of which exhibits a quadratic dependence on the incident flux. The recorded spectra can be understood by considering the coalescence of two excited Yb 3+ ions into one p h o t o n o f twice their energy. If this effect could be produced coherently it will be some sort of second h a r m o n i c generation and could therefore possibly e n h a n c e the nonlinear optical properties of a crystal. Since Yb 3+ absorption is n o t too far from n e o d y m i u m - Y A G laser at 1.06 t~ [ 1], our choice as nonlinear h o s t has been gaddinium m o l y b d a t e (GMO) which already contains rareearth ions, so that partial substitution of Gd 3+ by Yb 3+ ions is rather easy [3] ; good optical quality single crystals can be obtained, t h o u g h care m u s t be taken to eliminate ferroelastic d o m a i n s [4]. Nonlinear optical properties of pure GMO have already been investigated [7]. X-ray studies show that GMO in the paraelastic phase has point-group s y m m e t r y mm2(C2v) and that Gd 3+ site has no s y m m e t r y elements (and particuliarly no inversion center [8]. The five nonlinear optical coefficients of Gd 2 _ x Y b x (MOO4) 3 were measured by a standard Maker's fringes method. A significative e n h a n c e m e n t o f SHG was observed, variable with the investigated coefficient, from about 30% for d31 and d32 , 60% for d 15 and d 2 4 , to more than 100% for d33, with a 10% Yb 3+ concentration. In order to evaluate theoretically the contribution of Yb 3+ ions to nonlinear susceptibility, and to determine what kind of resonant effect, if any, is involved, several steps in our calculations have been considered: - Contribution of a one ion resonance in the vicinity o f 1.06 alone, Contribution o f a resonant coherent cooperative emission, - Contribution due to second h a r m o n i c fluorescence from the equivalent rare-earth ions at sites lacking inversion center, enhanced by one ion resonance.
F 5
References I1] F. Auzei, Proc. IEEE 61 (1973) 758. 12] E. Nakasawa, S. Shionoya, Phys. Rev. Lett. 25 (1970) 1710. 131 H.J. Borchardt, P.E. Bierstedt, Appl. Phys. Letter 8 (1966) 50. [4] A. Kumada, Ferroelectrics 3 (1972) 115. R.C. Miller, W.A. Nordland, K. Nassau, Ferroelectrics 2 (1971)97. [6] W. Jeitschko, Acta Cryst. B 28 (1972) 60.
F5
M O L E C U L A R MECHANICS OF THE F E R R O ELECTRIC TO P A R A E L E C T R I C PHASE TRANSITION IN B a 6 T i 2 N b s O 3 0 VIA OPTICAL SECOND HARMONIC G E N E R A T I O N J.G. BERGMAN Bell Laboratories, Holmdel, New Jersey 0 7733, USA
Second h a r m o n i c generation (SHG) can be used to determine the positions of a t o m s in crystals [ 1 ]. The essence of the method relies on the simple relation 1 ~ f(O), where I represents the intensity of the 2nd h a r m o n i c (the square root o f / i s proportional to the bulk nonlinear polarizability d) and 0 represents the relative position of some bond in the unit cell. The aforem e n t i o n e d technique has now been successfully applied to structural problems involving rigid body rotations and deformations o f tetrahedra as well as trigonal and tetragonal deformations of octahedra. In the case of the ferroelectric [2] tungsten bronze-type crystal, B a 6 T i 2 N b 8 0 3 0 we find that the
®=~-90
o
200
240
I 6 2
J
0
40
80
420
460
I
280
~°C P Fig. 1. Temperature dependence o f the M O 6 d i s t o r t i o n angle 0 (0 = ~ - 90 °) as determined f r o m the SHG coefficient d333 (O's) and the spontaneous polarization Ps (A's) •
51
OPTICS COMM UN1CATIONS
The focal spot size is chosen to give a combined incident power density o f approximately 5 MW cm - 2 , which was f o u n d to be below the damage threshold of the crystal for these short pulses. The infrared difference signal is detected with a HgCdTe detector. To satisfy non-critical (90 °) phasematching, b o t h dye lasers m u s t be t u n e d simultaneously. The input wavelengths necessary for each infrared difference frequency were calculated using the Sellmeier equations given by Bahr and Smith [ 1 ]. To cover the entire range, the e-ray was tuned from 5385 A to 5800 A and the o-ray from 5550 A to 6200 A. The experimentally measured wavelengths were within 10 A of the calculated ones. This d e m o n s t r a t e s that the Sellmeier equation for the infrared, which was derived from index data to 13 microns, is useful to at least 18 microns. The conversion efficiency was f o u n d to decrease at about 13 microns, but to rise again at 15 microns. This decrease in efficiency can be attributed to a two p h o n o n absorption [4]. However, this absorption does n o t limit the usefulness of the crystal in this region, and, in addition, it was f o u n d to have only a minimal effect on the indices. The cut-off at 18 microns is due to the detector response. With one of the dye laser frequencies fixed, the other dye laser was scanned through the corresponding phasematching frequency. The expected (sin x/x) 2 signature was observed and was measured to have a width (fwhm) o f 5 cm - I , which agrees with the calculated value to within the b a n d w i d t h s of the dye lasers. This experiment d e m o n s t r a t e s the feasibility of using a nitrogen laser-dual dye laser c o m b i n a t i o n for tunable infrared generation in the fingerprint region of the infrared. Its relative simplicity and low cost m a k e it an attractive alternative to parametric oscillators and spin-flip R a m a n lasers. We wish to thank Prof. R.L. Byer of Stanford Univ. lor providing the crystal of AgGaS 2. References [ 1 ] G.C. Bahr and R.C. Smith, IEEE J. Quantum Elektron. QE-10 (1974) 546. 12] S.A. Myers, Opt. C o m m u n . 4 (1971) 187. [3 ] D.C. Hanna, P.A. Karkkainen, and R. Wyatt, Optics and Quant. Elect. 7 (1975) 115. [4] J. Jerphagnon, private c o m m u n i c a t i o n .
F3
HIGH POWER F A R I N F R A R E D G E N E R A T I O N IN GaAs* N. LEE, B. L A X * * and R.L. A G G A R W A L * *
Francis Bitter National Magnet Laboratoryt, Massachusetts Institute o f Technology, Cambridge, Massachusetts 02139, USA Previously [ 1,2] reported generation o f far infrared (FIR) radiation by noncollinear mixing o f two pulsed T E A CO 2 lasers in a 1 cm crystal o f GaAs at liquid helium temperatures produced step-tunable radiation from ~ 70 # m to 2 m m . With
50
July 1976
200 kW input peak power from each ('O 2 laser ~ 20 mW peak o u t p u t power at a wavelength of ~ 100 u m was obtained. In this paper we report the generation of FIR o u t p u t ~ 4 kW at ~ 100/am, which is orders of magnitude higher than those obtained previously. These new experimental results were achieved by using noncollinear folded mixing geometries [ 3 ]. With a 10 cm long crystal of a simple folded geometry [4] in liquid helium temperatures and 130 kW peak input power incident on the crystal from each CO 2 laser, the H R o u t p u t was measured to be ~ 50 W at 100 # m at the o u t p u t face of the crystal [5 ]. This result is within an order of magnitude of the theoretical estimate neglecting the absorption losses. The orders-of-magnitude i m p r o v e m e n t o f present results is largely due to a better GaAs crystal and more reliable calibration of the FIR detection system. Much higher input powers could not be used in this crystal since laser damage was observed at an energy density as low as 0.3 J / c m 2 which is more than an order o f magnitude less than the generally quoted value of 10 J/cm 2. The cause for this lowering of the damage threshold was f o u n d to be due to the trapping o f the CO 2 laser b e a m s inside this crystal geometry. The trapped multiple passed CO 2 laser b e a m s interfere with each other constructively at some points in the crystal creating high peak intensity regions at the surface. By going to a folded parametric geometry [6] with input and o u t p u t couplers for the CO 2 laser beams, we have eliminated the internal trapping o f CO 2 laser radiation responsible for lower damage threshold. A 10 cm long crystal o f folded parametric geometry was prepared by optically contacting input and o u t p u t couplers for the CO 2 laser beams to the main crystal. The transmission loss of CO 2 laser beams across such an optically contacted interface was found to be about one percent. With about 1.7 MW and 3 MW peak power from the two CO 2 laser b e a m s incident on the sample, we obtained ~ 4 kW FIR power at ~ 100 tam at the o u t p u t face of the crystal. Presently we are developing the fine tuning capability of our TEA lasers so as to take advantage o f resonance cavity for FIR radiation. We h o p e this will improve the mixing efficiency for the FIR generation. References [ 1 ] R.L. Aggarwal, B. Lax and G. Favrot, Appl. Phys. Lett. 22 (1973) 329. [2] B. Lax, R.L. Aggarwal and G. Favrot, Appl. Phys. Lett. 23 (1973) 679. [3] N. Lee, R.L. Aggarwal and B. Lax, Opt. C o m m u n . 11 (1974) 339. [4] See fig. 2 o f r e f . [3].
* Supported in part by the Advanced Research Projects Agency through the Office of Naval Research, and in part by the National Science Foundation. ** Also Physics Department, M.I.T. "~ Supported by the National Science Foundation.
N O N L I N E A R OPTICS I (solids and liquids)
[ 5 ] The input peak powers given here have been deduced on the a s u m p t i o n that half of the CO2 pulse energy is contained in an initial spike of 100 nsec (fwhm) with the remaining half of the energy in a relatively long tail. The FIR peak powers are obtained by assuming that the output energy is contained in a 100 nsec (fwhm) pulse. I6] See figs. 3 of ref. [31.
F4
SECOND HARMONIC G E N E R A T I O N IN R A R E E A R T H 1ON DOPED N O N L I N E A R C R Y S T A L R. BONNEVILLE and F. A U Z E L Centre National d'Etudes des T~l~communications, 196 rue de Paris, 92220 Bagneux, France
Interactions between rare earth ions in solids are k n o w n to induce, besides energy transfers, cooperative effects [ 1 ]. A m o n g t h e m , cooperative luminescence has been reported by Nakasawa and Shionoya [2]: when exciting YbPO 4 by incoherent, near infrared (Yb 3+ absorption) light, they observed an anti-stokes luminescence at twice the excitation energy, the intensity of which exhibits a quadratic dependence on the incident flux. The recorded spectra can be understood by considering the coalescence of two excited Yb 3+ ions into one p h o t o n o f twice their energy. If this effect could be produced coherently it will be some sort of second h a r m o n i c generation and could therefore possibly e n h a n c e the nonlinear optical properties of a crystal. Since Yb 3+ absorption is n o t too far from n e o d y m i u m - Y A G laser at 1.06 t~ [ 1], our choice as nonlinear h o s t has been gaddinium m o l y b d a t e (GMO) which already contains rareearth ions, so that partial substitution of Gd 3+ by Yb 3+ ions is rather easy [3] ; good optical quality single crystals can be obtained, t h o u g h care m u s t be taken to eliminate ferroelastic d o m a i n s [4]. Nonlinear optical properties of pure GMO have already been investigated [7]. X-ray studies show that GMO in the paraelastic phase has point-group s y m m e t r y mm2(C2v) and that Gd 3+ site has no s y m m e t r y elements (and particuliarly no inversion center [8]. The five nonlinear optical coefficients of Gd 2 _ x Y b x (MOO4) 3 were measured by a standard Maker's fringes method. A significative e n h a n c e m e n t o f SHG was observed, variable with the investigated coefficient, from about 30% for d31 and d32 , 60% for d 15 and d 2 4 , to more than 100% for d33, with a 10% Yb 3+ concentration. In order to evaluate theoretically the contribution of Yb 3+ ions to nonlinear susceptibility, and to determine what kind of resonant effect, if any, is involved, several steps in our calculations have been considered: - Contribution of a one ion resonance in the vicinity o f 1.06 alone, Contribution o f a resonant coherent cooperative emission, - Contribution due to second h a r m o n i c fluorescence from the equivalent rare-earth ions at sites lacking inversion center, enhanced by one ion resonance.
F 5
References I1] F. Auzei, Proc. IEEE 61 (1973) 758. 12] E. Nakasawa, S. Shionoya, Phys. Rev. Lett. 25 (1970) 1710. 131 H.J. Borchardt, P.E. Bierstedt, Appl. Phys. Letter 8 (1966) 50. [4] A. Kumada, Ferroelectrics 3 (1972) 115. R.C. Miller, W.A. Nordland, K. Nassau, Ferroelectrics 2 (1971)97. [6] W. Jeitschko, Acta Cryst. B 28 (1972) 60.
F5
M O L E C U L A R MECHANICS OF THE F E R R O ELECTRIC TO P A R A E L E C T R I C PHASE TRANSITION IN B a 6 T i 2 N b s O 3 0 VIA OPTICAL SECOND HARMONIC G E N E R A T I O N J.G. BERGMAN Bell Laboratories, Holmdel, New Jersey 0 7733, USA
Second h a r m o n i c generation (SHG) can be used to determine the positions of a t o m s in crystals [ 1 ]. The essence of the method relies on the simple relation 1 ~ f(O), where I represents the intensity of the 2nd h a r m o n i c (the square root o f / i s proportional to the bulk nonlinear polarizability d) and 0 represents the relative position of some bond in the unit cell. The aforem e n t i o n e d technique has now been successfully applied to structural problems involving rigid body rotations and deformations o f tetrahedra as well as trigonal and tetragonal deformations of octahedra. In the case of the ferroelectric [2] tungsten bronze-type crystal, B a 6 T i 2 N b 8 0 3 0 we find that the
®=~-90
o
200
240
I 6 2
J
0
40
80
420
460
I
280
~°C P Fig. 1. Temperature dependence o f the M O 6 d i s t o r t i o n angle 0 (0 = ~ - 90 °) as determined f r o m the SHG coefficient d333 (O's) and the spontaneous polarization Ps (A's) •
51