Ultrafast all-optical modulation using nearinfrared intersubband transition

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Intersubband transition (ISB-T) in quantum wells (QWs) has been drawing much attention in terms of novel device applications due to their unique physical properties, such as an ultrafast relaxation time. We have proposed an ultrafast all-optical modulation scheme by using n-doped QWs, where an interband transition (IB-T) is modulated by using the ISB-T. In this scheme, the modulation speed is considered to be limited by the relaxation time of the electrons excited by ISB-T, which is expected to be as fast as 〜 ps. However, the available transition wavelength of the ISB-T was limited to the midfar-infrared region (4 〜 20 μm) until recently, and there has been a strong need to shorten the transition wavelength for some applications such as all-optical modulators for optical fiber communications. Recently, we have achieved an ISB-T wavelength of 1.90 μm in 7-ML-wide InGaAs/AlAs multiple QWs grown coherently on a GaAs substrate. However, in our previous studies, it was observed that the ab … More sorption magnitude decreases as the transition wavelength is shortened by narrowing the well width. It is required to enhance the absorption magnitude of the short-wavelength ISB-Ts to as much as that of the conventional wavelength ISB-Ts in order to enable application for our ultrafast all-optical modulators.In this study, we tried to optimize the QW structure to obtain large absorption while conserving the short transition wavelength. First, we reviewed the results presented in the previous reports and discussed the origins of the decrease of the absorption. Theoretical calculation of the carrier distribution shows that the reduction of the absorption is caused by carrier leakage to the X minimum of the AlAs barrier from the Γ minimum of the InGaAs well. In order to regain the absorption magnitude, we undertake to suppress the carrier leakage by reducing the density of states in the AlAs X minimum and by increasing the energy spacing between Γ and X minima by (a) increasing the In composition of the well from 0.2 to 0.4, (b) suppressing the In segregation during the growth by decreasing the substrate temperature from 480 〜 520 ℃ to 400 ℃ and c decreasing the barrier width form 36ML to 8ML. The optimizations lead to the enhancement of the absorption magnitude of more than twelve-fold, and a peak absorption coefficient of 14000cmィイD1-1ィエD1 is obtained with an optimized structure. The results obtained here are considered very useful for device applications of the short-wavelength intersubband transitions.Then, we have carried out a femtosecond time-resolved measurement on InGaAs/AlAs and AlGaAs/GaAs quantum wells, of the proposed modulation scheme. At first, electron relaxation dynamics in short wavelength (〜 2.5 μm) intersubband transition in InGaAs/AlAs quantum wells was investigated. Femtosecond pump and probe measurement yields a relaxation time of 〜 2.7 ps, which is as fast as that observed for 〜 7.2 μm ISB-T in GaAs/AlGaAs QWs (〜 0.6 ps). The ISB relaxation time increases more rapidly than that predicted from the usual intersubband optical-phonon scattering model as the ISB-T energy increases. Our theoretical calculation, which takes into account not only intersubband scattering but also intrasubband energy relaxation process, agrees well with the experimental results. The intrasubband energy relaxation process is found to be important in the short wavelength region. To the next, we carried out the all-optical modulation experiment with sub-picosecond time resolution. Ultrafast modulation of the signal light has been clearly observed with relatively low control light pulse energy of 〜 5 fJ/μmィイD12ィエD1 the control light wavelength is tuned to the ISB absorption peak (7.2 μm). The modulation depth was 〜 8.5% (which corresponds to an increase of 1000cmィイD1-1ィエD1 in the IB absorption coefficient) and the modulation speed (measured from FWHM of the modulation curve) was as fast as 〜 1.3 ps. The modulation depth was observed to decrease when the control light wavelength was detuned from the ISB absorption peak, which clearly indicates that the modulation is induced by the ISB-T. The results show that the proposed modulation scheme is promising for ultrafast all-optical modulation or switching devices. Less