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Systematic alterations of excitonic spontaneous emission through continuous uning of emission wavelength in AlGaAs quantum microcavities
Surface Science 267 (1992) 612-615 North-Holland
surface science
Systematic alterations of excitonic spontaneous emission through continuous tuning of emission wavelength in A1GaAs quantum microcavities Y. Honda 1, y . Lee, M. Yamanishi, N. Ochi
2 T.
Shiotani and I. S u e m u n e
Department of Physical Electronics, Faculty of Engineering, Hiroshima Unicersity, Kagamiyama 1-chome, Higashihiroshima 724, Japan Received 7 June 1991; accepted for publication 22 July 1991
In our experiments, we performed a systematic investigation on the controllable enhancement of spontaneous emission through the continuous tuning of the emission wavelength by quantum confined Stark effect in AIGaAs quantum microcavities. It is found that intensities and radiation patterns of the spontaneous err,ission can be continuously controlled by electric fields applied to the microcavities.
I. Introduction in the 1940's, spontaneous emission from an atom was predicted to be controllable by means of the modification of the vacuum field fluctuations surrounding the atom [1]. A confirmation of this prediction was recently made by extensive studies in a variety of systems, such as atoms [2-5] and semiconductors [6-8]. There are basically two ways of controlling the spontaneous emission. One is the modification of the structure of microcavities. Another is the modification of the emission wavelength from the microcavities. However, all the work reported so far, except ref. [4], have been performed only with the former This manner obviously raises difficulties Jn a systematic and quantitative measurement ot the alterations of the spontaneous emission since the fabrication of a number of samples is required. On the other hand, the latter not only enables us to perform such systematic study in a simpler way but also is attractive and of great importance Semiconductor Research Laboratory, Matsushita Electric Works Ltd., Kadoma, Osaka, 571, Japan. 2 Material Laboratory, Komatsa Ltd., Manda, Hiratsuka-shi, Kanagawa, 254, Japan.
from device-application point of view. In this paper, we present a demonstration of the systematic alterations of the excitonic spontaneous emission through the continuous tuning of the emission wavelength by the quantum-confined Stark effect (QCSE) in a GaAs single quantum well (QW) sandwiched between pairs of A l A s / AIGaAs distributed Bragg reflectors (DBR's) [9].
2. Experimental results and discussion We fabricated two kinds of devices with and without well-designed quantum microcavities, named quantum microcavity (QMC) and weak microcavity (WMC), respectively, The detailed structures are depicted in fig. la and b. The QMC-device was designed so that the GaAs QW is located at an antinode position of the standing w a r p ~ f tho
V~O1,nm
fiolrt fillc, hl~t;r~nc ;n tla,~
I 1
cavity. Consequently, the spontaneou~ emission into n normal direction to the layers is expected to be enhanced in the ½A-cavity when the excitonic emission wavelength in the QW is tuned to the resonant wavelength of the QMC, Ar. The WMC-device with a single DBR layer, providing free space emission, was fabricated as a reference
0039-6028/92/$05.00 ~C~ 1992 - Elsevier Science Publishers E.V. and Yamada Science Foundation. All rights reserved
Y. Honda et al. / Systematic alterations of excitonic spontaneous emission
to the QMC-device in order to see the microcavity effect clearly. Fig. 2 shows the spontaneous emission (PL) spectra, detected along the axis normal to the device surface, under different applied voltages in the QMC- and WMC-devices at low temperature, ~ 50 K. The GaAs QW's in both devices were pumped selectively by a dye laser (or T i : A I 2 0 3 laser) beam with a photon energy of 1.72 eV, smaller than the band gap of Al0.2Ga0.sAs ( ~ 1.84 eV at T ~ 50 K) so that the generation rates of photoexcited carriers are the same. Also, the photon energy of the pump beam
~
l
613
was chosen to be sufficiently higher than the band gap of the GaAs QW's so that the generation rates of photoexcited carriers for both devices are expected tc be independent of electric fields in the QW's. In both devices, the peak emission wavelength of l e - l h h exeitonic transitions in the QW's varies due to the well-known QCSE. However, there is a remarkable difference between the PL data in these two devices. In the QMC-device, a dramatic change in the emission intensity can be seen with variation in the applied field. When the peak emission wavelength is tuned to the resonant wavelength of the cavity by the
Au/Cr
I DBR ] P p - AI As 67. I nm "~ p - Alo.2Gao.8As 5 8 . 0 n m . . / x l 0 i - AlAs 67.1nm (Barrier Layer) i - GaAs 10A)nm ( O W ) ' ~ i - AlAs 67.1nm (Barrier Layer)
~,
DBR
.~
n - AI As 67.iom'~ n - AI0.2Gao.sAs 58.0nra/
xlO
AuoSn
(a) Q M C
Au thin film p-Al0.2Ga0.8As 567nm
" ~ - , ; ,., - AlAs 134.2nm (Barrier Layer) i - GaAs !0.0nm (QW) ::-:,:-~...............~........,................,................:.............................:,:: ~ _ - - , d A s 134.2nm (Barrier Layer)
~_.:::÷:~.::,..:
[
,~:~:...~
Sub,rate
~..:.1~
n - AI .2G . . . .
I
(b) W M C Fig. 1. S c h e m a t i c s t r u c t u r e s o f (a) t h e q u a n t u m microcm, ity ( Q M C ) device a n d (b) t h e w e a k m i c r o c a v i t y ( W M C ) d e v i c e .
614
Y lfonda et aL / Systematic alterations of excitonic spontaneous emission | V:0V(I'- 104kV/cm) I O'SV(TlkV/¢m) ~
I ~0v(36~v,cm)\\ ~,r 10 4 I ' t5V(OkV/cm~.~ ..~.._ " _~. ..
_-0.SV(137kV/cm) / - I 0V(169kV,'cm)
>
/l /
/ I ".Sv(2OlkV'cm) -2.1IV( 232kV.crn
o
E
.... -20
I
,o,I
• jo
///
,',I
l u i/ - ,%".'X" ,'Z/
"~I~. i ,;,.\/lOVC54kVlcm)il / 10°[ "\v="°lv(~" m'V/cm) I
790
%
',~ ~",/ ' t
-40
o
n
~il 200
I 100
0
P - waves
-30 ..........~ ~
~
polalized
o
•
On-resonance
•
0ex~_15°
0o _
~ ......... J "
"~0
o
p- pola_~rizcd --
• " \' t~¢/] / ~7 ~'",~,.7~r"' . ~_ ,'~ ~.-,,..~ I" "." " '/* *~. Y~I.¢~I'[ V "/~, /~----/~..,)11111 •" ~
/v,::/ 1
* 300
more, the estimated external quantum efficiency, defined as the ratio of the emission rate of detected photons to the generation rate of elect r o n - h o l e pairs in a Q W is as high as ~ 3.0% in the maximized condition of the QMC-devices. In fig. 3, peak energy shifts in the Q M C - and WMC-devices are shown as a function of the applied field. The peak shifts in the WMC-device almost keep track of the theoretical curve obtained by the Q C S E theory including a correction due to the exciton effect, while in the Q M C d : -
0ex~-15 °
Scan 4- -
_ %. _ _ \
Fig. 3. Shifts of peak photon energies of the PL as functions of the applied field. Triangles show the shifts of shoulders in PL, corresponding to le-lhh optical transition lines, indicated by down arrows in fig. 2.
,I
A~.
WMC "1 slaouldcr j~ QMC
Internal Electric Field (kV/cm)
applied bias voltages, the spontaneous emission intensity is maximized, indicating the enhancement of the spontaneous emission, in other words, the enhancement of the coupling efficiency (fl) of the spontaneous emission into the resonant mode in the cavity. By a direct comparison between the PL data in two devices, we found that the on-resonance PL intensity is enhanced up to about 40 times compared to the PL, involving only free space emission, from the WMC-device. Further-
', p -
a 0
WMC
810 820 830 Wavelength (nm) Fig. 2. Spontaneous emission (PL) spectra for different bias voltages (different electric fields) in QMC- and WMC-devices at ~ 50 K.
Scanf~~"
le-lhh
QCSEThe~ory
g.
800
A~>O
A
Peak
N°
-30
d¢
i
Peak
I,,I
MC •
le-lhh
o
-10 r/3
"~
•
0.5
0
V=0.7V
A )~=0.0nm
V=0.4V
A ~.=l.3nm
V=0.0V
A L=4.2nm
0.5
1
Radiation Intensity (a.u.) Fig. 4. Measured radiation patterns for different bias voltages in another OMC-device with )t~ ~ 803 nm.
Y. Honda et al. / Systematic alterations of excitonic spontaneous emission
vice the peak shifts are deviated from the curve so as to be closer to the resonance wavelength, particularly ,,,,hen the peak wavelengths are longer than the resonant wavelength. This may result from the fact that the inhomogeneously broadened emissions with wavelengths closer to the resonant wavelength of the cavity are preferentially enhanced. However, shifts of PL-shoulders seen in the high-field regime in fig. 2, corresponding to l e - l h h optical transition lines, fit well with the theoretical curve. Fig. 4 shows the result of the radiation patterns for different bias voltages at ~ 50 K. In this measurement, we chose another QMC-device with slightly thinner DBR layers; resulting in a slightly shorter resonant wavelength, Ar = 803 nm, because the photocurrents in the low-field regime are negligibly small so that a pure cavity effect on the radiation pattern can be observed. The data were obtained by scanning along the center lines of the two-dimensional symmetric patterns as shown in the insertion of fig. 4. Whep the peak wavelength of the spontaneous emission, Ap, is shorter than the cavity resonant wavelength, Ar, i.e., AA = Ap - Ar < 0, the conical beam pattern is observed, peaked at the emission angle 0ex defined as 0ex=sin-~{neff sin 0i,} where 0in = cos-~(Ap/Ar) and r/eft is the effective index of the refraction of a microcavity [6]. When Ap > At, i.e., AA > 0 , the radiations are concentrated around 0ex = 0 [6], and a monotonous reduction in the intensity of the radiations is observed.
3. Conclusion We have demonstrated the controllable excitonic spontaneous emission through the continuous tuning of the emission wavelength by DC electric fields applied to GaAs single QW's located at the center of half-wavelength microcavities. The control of the spontaneous emission by this means sheds light on a great improvement in the conventional optical device performance. For instance, incorporation of the quantum microcavity into recently developed field effect light emitters [10] may promise a remarkable improvement in the external efficiency for light output of the
615
devices, keeping the high speed capability inherent in the field-effect light emitters. Also, the possibility of an ultrawide band modulation of light output intensity in light-emitting devices with quantum microcavities through the field-induced /3-switching was theoretically predicted [11] very recently.
Acknowledgements This work was partly supported by a Grant-lnAid for Scientific Research on Priority Areas, "Electron Wave Interference Effect in Mesoscopic Structures" from the Ministry of Education, Science and Culture of Japan. The authors express their thanks to Dr. Y. Yamamoto, NTT Basic Research La0s., and Dr. A. Shimizu, Sakaki Quantum Wave Project, for fruitful discussions. The radiation patterns were measured with an experimental set of NTT Basic Research Labs. 'Fhe authors thank Dr. S. Machida, NTT Basic Research Labs. for his kind arrangements in the measurement.
References [1] E.M. Purcell, Phys. Rev. 69 (1946) 681. [2] P. Goy, J.M. Raimond, M. Gross and S. Haroche, Phys. Rev. Lett. 50 (1983) 1903. [3] G. Gabrielse and tl. Dchmclt, Phys. Rev. Lett. 55 (1985) 67. [4] R.G. Hulet, E.S. Hiller and D. Kleppner, Phys. Rev. Lett. 55 (1985) 2137. [5] D.J. Heinzen, J.J. Childs, J.E. Thomas and M.S. Feld, Phys. Rev. Lett. 58 (1987) 1320. [6] Y. Yamamoto, S. Machida, K. Igeta and G. Bj6rk, in: Coherence, Amplification, and Quantum Effects in Semiconductor Lasers, ed. Y. Yamamoto (Wiley, New York, 1991) pp. 561-615. [7] H. Yokoyama, K. Nishi, T. Anan, H. Yamada, S.D. Bronson and E.P. Ippen, Appl. Phys. Lett. 57 (1990) 2814. to] l.J. ~ogers, D.G. Dcppc and B.G. Siieetf, a i i , ",~ppi. .... Phys. Lett. 57 (1990) 1858. [9] N. Ochi, T. Shiotani, M. Yamanishi, Y. Honda and [. Suemune, Appi. Phys. Lett. 58 (1991) 2735. [10] Y. Kan, M. Okuda, IVl. Yamanishi, T. Ohnishi, K. Mukaiyama and I. Suemune, Appl. Phys. Lett. 56 (1990) 2059. [11] M. Yamanishi, Y. Yamamoto and T. Shiotani, IEEE. Photonics Tech. Lett. 3 (1,991) 888. r,al
~
•
n
surface science
Systematic alterations of excitonic spontaneous emission through continuous tuning of emission wavelength in A1GaAs quantum microcavities Y. Honda 1, y . Lee, M. Yamanishi, N. Ochi
2 T.
Shiotani and I. S u e m u n e
Department of Physical Electronics, Faculty of Engineering, Hiroshima Unicersity, Kagamiyama 1-chome, Higashihiroshima 724, Japan Received 7 June 1991; accepted for publication 22 July 1991
In our experiments, we performed a systematic investigation on the controllable enhancement of spontaneous emission through the continuous tuning of the emission wavelength by quantum confined Stark effect in AIGaAs quantum microcavities. It is found that intensities and radiation patterns of the spontaneous err,ission can be continuously controlled by electric fields applied to the microcavities.
I. Introduction in the 1940's, spontaneous emission from an atom was predicted to be controllable by means of the modification of the vacuum field fluctuations surrounding the atom [1]. A confirmation of this prediction was recently made by extensive studies in a variety of systems, such as atoms [2-5] and semiconductors [6-8]. There are basically two ways of controlling the spontaneous emission. One is the modification of the structure of microcavities. Another is the modification of the emission wavelength from the microcavities. However, all the work reported so far, except ref. [4], have been performed only with the former This manner obviously raises difficulties Jn a systematic and quantitative measurement ot the alterations of the spontaneous emission since the fabrication of a number of samples is required. On the other hand, the latter not only enables us to perform such systematic study in a simpler way but also is attractive and of great importance Semiconductor Research Laboratory, Matsushita Electric Works Ltd., Kadoma, Osaka, 571, Japan. 2 Material Laboratory, Komatsa Ltd., Manda, Hiratsuka-shi, Kanagawa, 254, Japan.
from device-application point of view. In this paper, we present a demonstration of the systematic alterations of the excitonic spontaneous emission through the continuous tuning of the emission wavelength by the quantum-confined Stark effect (QCSE) in a GaAs single quantum well (QW) sandwiched between pairs of A l A s / AIGaAs distributed Bragg reflectors (DBR's) [9].
2. Experimental results and discussion We fabricated two kinds of devices with and without well-designed quantum microcavities, named quantum microcavity (QMC) and weak microcavity (WMC), respectively, The detailed structures are depicted in fig. la and b. The QMC-device was designed so that the GaAs QW is located at an antinode position of the standing w a r p ~ f tho
V~O1,nm
fiolrt fillc, hl~t;r~nc ;n tla,~
I 1
cavity. Consequently, the spontaneou~ emission into n normal direction to the layers is expected to be enhanced in the ½A-cavity when the excitonic emission wavelength in the QW is tuned to the resonant wavelength of the QMC, Ar. The WMC-device with a single DBR layer, providing free space emission, was fabricated as a reference
0039-6028/92/$05.00 ~C~ 1992 - Elsevier Science Publishers E.V. and Yamada Science Foundation. All rights reserved
Y. Honda et al. / Systematic alterations of excitonic spontaneous emission
to the QMC-device in order to see the microcavity effect clearly. Fig. 2 shows the spontaneous emission (PL) spectra, detected along the axis normal to the device surface, under different applied voltages in the QMC- and WMC-devices at low temperature, ~ 50 K. The GaAs QW's in both devices were pumped selectively by a dye laser (or T i : A I 2 0 3 laser) beam with a photon energy of 1.72 eV, smaller than the band gap of Al0.2Ga0.sAs ( ~ 1.84 eV at T ~ 50 K) so that the generation rates of photoexcited carriers are the same. Also, the photon energy of the pump beam
~
l
613
was chosen to be sufficiently higher than the band gap of the GaAs QW's so that the generation rates of photoexcited carriers for both devices are expected tc be independent of electric fields in the QW's. In both devices, the peak emission wavelength of l e - l h h exeitonic transitions in the QW's varies due to the well-known QCSE. However, there is a remarkable difference between the PL data in these two devices. In the QMC-device, a dramatic change in the emission intensity can be seen with variation in the applied field. When the peak emission wavelength is tuned to the resonant wavelength of the cavity by the
Au/Cr
I DBR ] P p - AI As 67. I nm "~ p - Alo.2Gao.8As 5 8 . 0 n m . . / x l 0 i - AlAs 67.1nm (Barrier Layer) i - GaAs 10A)nm ( O W ) ' ~ i - AlAs 67.1nm (Barrier Layer)
~,
DBR
.~
n - AI As 67.iom'~ n - AI0.2Gao.sAs 58.0nra/
xlO
AuoSn
(a) Q M C
Au thin film p-Al0.2Ga0.8As 567nm
" ~ - , ; ,., - AlAs 134.2nm (Barrier Layer) i - GaAs !0.0nm (QW) ::-:,:-~...............~........,................,................:.............................:,:: ~ _ - - , d A s 134.2nm (Barrier Layer)
~_.:::÷:~.::,..:
[
,~:~:...~
Sub,rate
~..:.1~
n - AI .2G . . . .
I
(b) W M C Fig. 1. S c h e m a t i c s t r u c t u r e s o f (a) t h e q u a n t u m microcm, ity ( Q M C ) device a n d (b) t h e w e a k m i c r o c a v i t y ( W M C ) d e v i c e .
614
Y lfonda et aL / Systematic alterations of excitonic spontaneous emission | V:0V(I'- 104kV/cm) I O'SV(TlkV/¢m) ~
I ~0v(36~v,cm)\\ ~,r 10 4 I ' t5V(OkV/cm~.~ ..~.._ " _~. ..
_-0.SV(137kV/cm) / - I 0V(169kV,'cm)
>
/l /
/ I ".Sv(2OlkV'cm) -2.1IV( 232kV.crn
o
E
.... -20
I
,o,I
• jo
///
,',I
l u i/ - ,%".'X" ,'Z/
"~I~. i ,;,.\/lOVC54kVlcm)il / 10°[ "\v="°lv(~" m'V/cm) I
790
%
',~ ~",/ ' t
-40
o
n
~il 200
I 100
0
P - waves
-30 ..........~ ~
~
polalized
o
•
On-resonance
•
0ex~_15°
0o _
~ ......... J "
"~0
o
p- pola_~rizcd --
• " \' t~¢/] / ~7 ~'",~,.7~r"' . ~_ ,'~ ~.-,,..~ I" "." " '/* *~. Y~I.¢~I'[ V "/~, /~----/~..,)11111 •" ~
/v,::/ 1
* 300
more, the estimated external quantum efficiency, defined as the ratio of the emission rate of detected photons to the generation rate of elect r o n - h o l e pairs in a Q W is as high as ~ 3.0% in the maximized condition of the QMC-devices. In fig. 3, peak energy shifts in the Q M C - and WMC-devices are shown as a function of the applied field. The peak shifts in the WMC-device almost keep track of the theoretical curve obtained by the Q C S E theory including a correction due to the exciton effect, while in the Q M C d : -
0ex~-15 °
Scan 4- -
_ %. _ _ \
Fig. 3. Shifts of peak photon energies of the PL as functions of the applied field. Triangles show the shifts of shoulders in PL, corresponding to le-lhh optical transition lines, indicated by down arrows in fig. 2.
,I
A~.
WMC "1 slaouldcr j~ QMC
Internal Electric Field (kV/cm)
applied bias voltages, the spontaneous emission intensity is maximized, indicating the enhancement of the spontaneous emission, in other words, the enhancement of the coupling efficiency (fl) of the spontaneous emission into the resonant mode in the cavity. By a direct comparison between the PL data in two devices, we found that the on-resonance PL intensity is enhanced up to about 40 times compared to the PL, involving only free space emission, from the WMC-device. Further-
', p -
a 0
WMC
810 820 830 Wavelength (nm) Fig. 2. Spontaneous emission (PL) spectra for different bias voltages (different electric fields) in QMC- and WMC-devices at ~ 50 K.
Scanf~~"
le-lhh
QCSEThe~ory
g.
800
A~>O
A
Peak
N°
-30
d¢
i
Peak
I,,I
MC •
le-lhh
o
-10 r/3
"~
•
0.5
0
V=0.7V
A )~=0.0nm
V=0.4V
A ~.=l.3nm
V=0.0V
A L=4.2nm
0.5
1
Radiation Intensity (a.u.) Fig. 4. Measured radiation patterns for different bias voltages in another OMC-device with )t~ ~ 803 nm.
Y. Honda et al. / Systematic alterations of excitonic spontaneous emission
vice the peak shifts are deviated from the curve so as to be closer to the resonance wavelength, particularly ,,,,hen the peak wavelengths are longer than the resonant wavelength. This may result from the fact that the inhomogeneously broadened emissions with wavelengths closer to the resonant wavelength of the cavity are preferentially enhanced. However, shifts of PL-shoulders seen in the high-field regime in fig. 2, corresponding to l e - l h h optical transition lines, fit well with the theoretical curve. Fig. 4 shows the result of the radiation patterns for different bias voltages at ~ 50 K. In this measurement, we chose another QMC-device with slightly thinner DBR layers; resulting in a slightly shorter resonant wavelength, Ar = 803 nm, because the photocurrents in the low-field regime are negligibly small so that a pure cavity effect on the radiation pattern can be observed. The data were obtained by scanning along the center lines of the two-dimensional symmetric patterns as shown in the insertion of fig. 4. Whep the peak wavelength of the spontaneous emission, Ap, is shorter than the cavity resonant wavelength, Ar, i.e., AA = Ap - Ar < 0, the conical beam pattern is observed, peaked at the emission angle 0ex defined as 0ex=sin-~{neff sin 0i,} where 0in = cos-~(Ap/Ar) and r/eft is the effective index of the refraction of a microcavity [6]. When Ap > At, i.e., AA > 0 , the radiations are concentrated around 0ex = 0 [6], and a monotonous reduction in the intensity of the radiations is observed.
3. Conclusion We have demonstrated the controllable excitonic spontaneous emission through the continuous tuning of the emission wavelength by DC electric fields applied to GaAs single QW's located at the center of half-wavelength microcavities. The control of the spontaneous emission by this means sheds light on a great improvement in the conventional optical device performance. For instance, incorporation of the quantum microcavity into recently developed field effect light emitters [10] may promise a remarkable improvement in the external efficiency for light output of the
615
devices, keeping the high speed capability inherent in the field-effect light emitters. Also, the possibility of an ultrawide band modulation of light output intensity in light-emitting devices with quantum microcavities through the field-induced /3-switching was theoretically predicted [11] very recently.
Acknowledgements This work was partly supported by a Grant-lnAid for Scientific Research on Priority Areas, "Electron Wave Interference Effect in Mesoscopic Structures" from the Ministry of Education, Science and Culture of Japan. The authors express their thanks to Dr. Y. Yamamoto, NTT Basic Research La0s., and Dr. A. Shimizu, Sakaki Quantum Wave Project, for fruitful discussions. The radiation patterns were measured with an experimental set of NTT Basic Research Labs. 'Fhe authors thank Dr. S. Machida, NTT Basic Research Labs. for his kind arrangements in the measurement.
References [1] E.M. Purcell, Phys. Rev. 69 (1946) 681. [2] P. Goy, J.M. Raimond, M. Gross and S. Haroche, Phys. Rev. Lett. 50 (1983) 1903. [3] G. Gabrielse and tl. Dchmclt, Phys. Rev. Lett. 55 (1985) 67. [4] R.G. Hulet, E.S. Hiller and D. Kleppner, Phys. Rev. Lett. 55 (1985) 2137. [5] D.J. Heinzen, J.J. Childs, J.E. Thomas and M.S. Feld, Phys. Rev. Lett. 58 (1987) 1320. [6] Y. Yamamoto, S. Machida, K. Igeta and G. Bj6rk, in: Coherence, Amplification, and Quantum Effects in Semiconductor Lasers, ed. Y. Yamamoto (Wiley, New York, 1991) pp. 561-615. [7] H. Yokoyama, K. Nishi, T. Anan, H. Yamada, S.D. Bronson and E.P. Ippen, Appl. Phys. Lett. 57 (1990) 2814. to] l.J. ~ogers, D.G. Dcppc and B.G. Siieetf, a i i , ",~ppi. .... Phys. Lett. 57 (1990) 1858. [9] N. Ochi, T. Shiotani, M. Yamanishi, Y. Honda and [. Suemune, Appi. Phys. Lett. 58 (1991) 2735. [10] Y. Kan, M. Okuda, IVl. Yamanishi, T. Ohnishi, K. Mukaiyama and I. Suemune, Appl. Phys. Lett. 56 (1990) 2059. [11] M. Yamanishi, Y. Yamamoto and T. Shiotani, IEEE. Photonics Tech. Lett. 3 (1,991) 888. r,al
~
•
n