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Stimulated spin-flip Raman scattering pumped by stimulated recombination radiation in InSb
Volume 17, number 1
OPTICS COMMUNICATIONS
April 1976
STIMULATED SPIN-FLIP RAMAN SCATTERING PUMPED BY STIMULATED RECOMBINATION RADIATION IN InSb W. RICHTER, G. APPOLD, H. PASCHER and H.G. HAFELE Physikalisches Institut der Universit#t Wfirzburg, 8700 Wfirzburg, Fed. Rep. Germany Received 19 February 1976
The generation of stimulated spin-flip Raman scattering (SFR) by stimulated recombination radiation (SRR) has been observed for the first time in InSb. The SRR was pumped by a Q-switched carbon monoxide laser. By magnetic tuning of the SRR, we found a new low intensity stimulated radiation, whose frequency was shifted away from the SRR by about g*uBB/h. The tuning range of this radiation lies between 1879 cm -1 and 1868 em-1 for magnetic fields between 0.4 T and 1.3 T.
The generation of stimulated spin-flip Raman scattering (SFR) in InSb, which results in a magnetically tunable coherent radiation with a halfwidth of for example 200 MHz, has been thoroughly examined, both experimentally and theoretically [ 1 - 3 ] (only some of the numerous references are listed). Also the generation of stimulated recombination radiation (SRR) in InSb, originally by electric excitation [4], and later by optical pumping with lasers [5], is well understood as far as the energy levels [6], resonant pumping phenomena [7], and the tunable lasing action of the SRR is concerned [8]. Recently, an interaction between SFR and SRR, both pumped by a Q-switched carbon monoxide laser, has been found [9]. We report here the first observation of spin-flip Raman scattering in n-InSb with the stimulated recombination radiation operating as the pumping source. The halfwidth of the SRR in our InSb-samples was about 2 cm -1 (= 60 GHz) and the ensuing SFR had nearly the same halfwidth. The spectral resolution in our experimental arrangement does not allow one to separate the longitudinal modes of the SRR (about 6 modes in the spectral halfwidth). We assume, tfiat all of the longitudinal modes of the SRR independently pump the SFR. The total power of the SRR inside the sample was estimated to be more than 100 mW. This indicates, that the power for
several of the longitudinal modes of the SRR lies above the threshold power for SFR. However, the poor resolution and the asymetric line shape resulting from atmospheric water vapour absorption made it difficult to measure the linewidth of the SRR. It appears to be nearly as large as the linewidth of the SRR. We investigated the intensity, polarization and frequency of the new SFR in magnetic fields up to 1.3 T. A Q-switched liquid-nitrogen cooled carbon monoxide laser with a maximum output of 100 W was used as a pump source for the SRR. The selection of the different vibrational-rotational laser frequencies was carried out with an echelette grating in the resonator. Continuous attenuation of the pump power was achieved by varying the discharge current of the laser. The laser beam was focused on the InSb-sample. Its dimensions were 3 × 3 × 4 mm 3, the carrier concentration was 1.35 X 1015 cm -3 and the mobility was 3.2 × 105 cm2/Vs at 77 K. Special care was taken to get plan-parallel surfaces which were flat to within ~/5. The sample was kept at 6 K between the pole faces of a 9-inch Bruker magnet with k 1 B and Ep .I.B - the subscript p refers to the pump radiation (see fig. 1 inset). The collinear output of the SRR was analyzed by a/'/6 Jobin-Yvon grating monochromator and monitored with a photoconductive InSb detector. The 100-nsec signal was fed into a conventional boxcar integrator and recorded. 21
Volume 17, n u m b e r 1
OPTICS COMMUNICATIONS
April 1976
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By exam•nat•rig the SFR power as a function of the pump power, we were able to establish that we are dealing with a stimulated process (fig. 1). The steep rise of over two orders of magnitude in the SFR power upon enlarging the CO laser pump power for the SRR by a factor 2 (this leads to an increase in SRR power of only one order of magnitude) is characteristic for a stimulated process. The maximum output power of the SFR was about 20 mW. The magnetic field dependence of the SFR together with that of the pump source, i.e. the stimulated recombination radiation, is shown in fig. 2. The frequency interval between the measured SRR and SFR radiation is equal to the Stokes shift connected with a spin-flip transition. This interval is represented in fig. 3 by dots. The solid line represents the known Stokes shift A ~ = g*~BB/hof a normal spin-flip Raman laser. Also other measurements of the new SFR obtained by pumping with different CO laser lines 1905 cm -1 and 1948 cm -1 show the same
frequency shift behaviour. The structure in the tuning curve of the SRR at about 0.6 T (fig. 2), which is a result of the overlap of the SRR from two different initial levels [6], also exists in the tuning curve of the SFR. This corroborates very well the observed connection between these two stimulated radiations. Furthermore the polarization of the SFR was found to be
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I 1.0 FIELD (Tesla)
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Fig. 1. Output power of the stimulated spin-flip Raman scattering at 0.9 T versus CO laser pump power. The inset shows the experimental configuration.
22
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Fig. 2. Magnetic field dependence of the emission wavenumber of the stimulated recombination radiation and the stimulated spin-flip Raman scattering. Co laser pump frequency ~'p = 1947.5 cm -1 .
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Fig. 3. Frequency shift of the spin-flip Stokes output with respect to the stimulated recombination radiation versus magnetic field. The solid line shows the frequency shift , x ~ = g*~BB/~of a spin-flip Raman laser.
Volume 17, number 1
OPTICS COMMUNICATIONS
rotated 90 ° relative to the SRR, which is consistent with the single-particle spin-flip selection rules if the SRR is the pump source. In summary, we report the observation of a two-step pumping process for stimulated spin-flip Raman scattering in InSb working via the CO-laser pumped SRR. The identification of this radiation as spin-flip Stokes radiation is made from its polarization, tuning curve and frequency shift away from the SRR. The nonlinear behaviour of the Stokes output power to pump power indicates that the radiation was stimulated. The authors wish to express thanks to the Deutsche Forschungsgemeinschaft for financial support.
April 1976
References [1] P.A. Wolff, Phys. Rev. Lett. 16 (1966) 225. [2] C.K.N. Patel and E.D. Shaw, Phys. Rev. B3 (1971) 1279. [3] S.R.I. Brueck and A. Mooradian, Appl. Phys. Lett. 18 (1971) 229. [4] R.J. Phelan, A.R. Calawa, R.H. Rediker, R.J. Keyes and B. Lax, Appl. Phys. Lett. 3 (1963) 143. [5] R.J. Phelan and R.H. Rediker, Appl. Phys. Lett. 6 (1965) 70. [6] W. Richter, G. Appold, H. Pascher and H.G. H~/fele,Phys. Star. Sol. B (to be published). [7] R. Grisar, C. Irslinger, W. Waehernig and H.G. H~fele, Optics Commun. 3 (1971) 415. [8] A.S. Pine and N. Menyuk, Appl. Phys. Lett. 26 (1975) 231. [9] G. Appold, W. Richter, H. Pascher and H.G. H~ffele,Opt. Commun. 15 (1975) 147.
23
OPTICS COMMUNICATIONS
April 1976
STIMULATED SPIN-FLIP RAMAN SCATTERING PUMPED BY STIMULATED RECOMBINATION RADIATION IN InSb W. RICHTER, G. APPOLD, H. PASCHER and H.G. HAFELE Physikalisches Institut der Universit#t Wfirzburg, 8700 Wfirzburg, Fed. Rep. Germany Received 19 February 1976
The generation of stimulated spin-flip Raman scattering (SFR) by stimulated recombination radiation (SRR) has been observed for the first time in InSb. The SRR was pumped by a Q-switched carbon monoxide laser. By magnetic tuning of the SRR, we found a new low intensity stimulated radiation, whose frequency was shifted away from the SRR by about g*uBB/h. The tuning range of this radiation lies between 1879 cm -1 and 1868 em-1 for magnetic fields between 0.4 T and 1.3 T.
The generation of stimulated spin-flip Raman scattering (SFR) in InSb, which results in a magnetically tunable coherent radiation with a halfwidth of for example 200 MHz, has been thoroughly examined, both experimentally and theoretically [ 1 - 3 ] (only some of the numerous references are listed). Also the generation of stimulated recombination radiation (SRR) in InSb, originally by electric excitation [4], and later by optical pumping with lasers [5], is well understood as far as the energy levels [6], resonant pumping phenomena [7], and the tunable lasing action of the SRR is concerned [8]. Recently, an interaction between SFR and SRR, both pumped by a Q-switched carbon monoxide laser, has been found [9]. We report here the first observation of spin-flip Raman scattering in n-InSb with the stimulated recombination radiation operating as the pumping source. The halfwidth of the SRR in our InSb-samples was about 2 cm -1 (= 60 GHz) and the ensuing SFR had nearly the same halfwidth. The spectral resolution in our experimental arrangement does not allow one to separate the longitudinal modes of the SRR (about 6 modes in the spectral halfwidth). We assume, tfiat all of the longitudinal modes of the SRR independently pump the SFR. The total power of the SRR inside the sample was estimated to be more than 100 mW. This indicates, that the power for
several of the longitudinal modes of the SRR lies above the threshold power for SFR. However, the poor resolution and the asymetric line shape resulting from atmospheric water vapour absorption made it difficult to measure the linewidth of the SRR. It appears to be nearly as large as the linewidth of the SRR. We investigated the intensity, polarization and frequency of the new SFR in magnetic fields up to 1.3 T. A Q-switched liquid-nitrogen cooled carbon monoxide laser with a maximum output of 100 W was used as a pump source for the SRR. The selection of the different vibrational-rotational laser frequencies was carried out with an echelette grating in the resonator. Continuous attenuation of the pump power was achieved by varying the discharge current of the laser. The laser beam was focused on the InSb-sample. Its dimensions were 3 × 3 × 4 mm 3, the carrier concentration was 1.35 X 1015 cm -3 and the mobility was 3.2 × 105 cm2/Vs at 77 K. Special care was taken to get plan-parallel surfaces which were flat to within ~/5. The sample was kept at 6 K between the pole faces of a 9-inch Bruker magnet with k 1 B and Ep .I.B - the subscript p refers to the pump radiation (see fig. 1 inset). The collinear output of the SRR was analyzed by a/'/6 Jobin-Yvon grating monochromator and monitored with a photoconductive InSb detector. The 100-nsec signal was fed into a conventional boxcar integrator and recorded. 21
Volume 17, n u m b e r 1
OPTICS COMMUNICATIONS
April 1976
1900
''"1
./
101
eo oe °
../
z LU > <
/
..Q
t
100 UJ
r
~
0 O-
e•
'[
Qooeo °e
oooo oQeo~e
Z 1880 O
l • •toe
03
(,O ~E UJ ee*eeeQ%~e
1870 t
10_1 I III[
10 0
I
l
l l i LLI]
'
L
By exam•nat•rig the SFR power as a function of the pump power, we were able to establish that we are dealing with a stimulated process (fig. 1). The steep rise of over two orders of magnitude in the SFR power upon enlarging the CO laser pump power for the SRR by a factor 2 (this leads to an increase in SRR power of only one order of magnitude) is characteristic for a stimulated process. The maximum output power of the SFR was about 20 mW. The magnetic field dependence of the SFR together with that of the pump source, i.e. the stimulated recombination radiation, is shown in fig. 2. The frequency interval between the measured SRR and SFR radiation is equal to the Stokes shift connected with a spin-flip transition. This interval is represented in fig. 3 by dots. The solid line represents the known Stokes shift A ~ = g*~BB/hof a normal spin-flip Raman laser. Also other measurements of the new SFR obtained by pumping with different CO laser lines 1905 cm -1 and 1948 cm -1 show the same
frequency shift behaviour. The structure in the tuning curve of the SRR at about 0.6 T (fig. 2), which is a result of the overlap of the SRR from two different initial levels [6], also exists in the tuning curve of the SFR. This corroborates very well the observed connection between these two stimulated radiations. Furthermore the polarization of the SFR was found to be
5FR °e o
I 1.0 FIELD (Tesla)
I 0.5 MAGNETIC
I(]I
Fig. 1. Output power of the stimulated spin-flip Raman scattering at 0.9 T versus CO laser pump power. The inset shows the experimental configuration.
22
"'"
I
PUMP P O W E R (orbitrory units)
between
mo °
g" duB B/'h
o_
rY U, 03
SRR
l•
n,LU 1890 133
/ o
o~ Q
'E u
/ ~n
I
o,,¢
I 1.5
Fig. 2. Magnetic field dependence of the emission wavenumber of the stimulated recombination radiation and the stimulated spin-flip Raman scattering. Co laser pump frequency ~'p = 1947.5 cm -1 .
/
30
/
?
2o
7_ -r u'l
/
/
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Fig. 3. Frequency shift of the spin-flip Stokes output with respect to the stimulated recombination radiation versus magnetic field. The solid line shows the frequency shift , x ~ = g*~BB/~of a spin-flip Raman laser.
Volume 17, number 1
OPTICS COMMUNICATIONS
rotated 90 ° relative to the SRR, which is consistent with the single-particle spin-flip selection rules if the SRR is the pump source. In summary, we report the observation of a two-step pumping process for stimulated spin-flip Raman scattering in InSb working via the CO-laser pumped SRR. The identification of this radiation as spin-flip Stokes radiation is made from its polarization, tuning curve and frequency shift away from the SRR. The nonlinear behaviour of the Stokes output power to pump power indicates that the radiation was stimulated. The authors wish to express thanks to the Deutsche Forschungsgemeinschaft for financial support.
April 1976
References [1] P.A. Wolff, Phys. Rev. Lett. 16 (1966) 225. [2] C.K.N. Patel and E.D. Shaw, Phys. Rev. B3 (1971) 1279. [3] S.R.I. Brueck and A. Mooradian, Appl. Phys. Lett. 18 (1971) 229. [4] R.J. Phelan, A.R. Calawa, R.H. Rediker, R.J. Keyes and B. Lax, Appl. Phys. Lett. 3 (1963) 143. [5] R.J. Phelan and R.H. Rediker, Appl. Phys. Lett. 6 (1965) 70. [6] W. Richter, G. Appold, H. Pascher and H.G. H~/fele,Phys. Star. Sol. B (to be published). [7] R. Grisar, C. Irslinger, W. Waehernig and H.G. H~fele, Optics Commun. 3 (1971) 415. [8] A.S. Pine and N. Menyuk, Appl. Phys. Lett. 26 (1975) 231. [9] G. Appold, W. Richter, H. Pascher and H.G. H~ffele,Opt. Commun. 15 (1975) 147.
23