Figure 1: Left: Schematic diagram of conduction subbands in an InAs/AlSb quantum well, showing instersubband absorption (E1 –> E2) and photoluminescence.  Right: Measured intersubband absorption and photoluminescence energies in InAs/AlSb quantum wells at 77 K as a function of well width.

Figure 2: Left: Schematic diagram of asymmetric double quantum well structure designed for resonant difference frequency generation with mid-infrared pumps.  Right: Measured intersubband absorption coefficient of the sturcture at left at 77 K and room temperature.

Antimonide-based compound semiconductors (InAs/GaSb/AlSb) are a promising system for intersubband wavefunction engineering.  Their large conduction band offsets–2.1 eV between InAs and AlSb, compared to 0.5 eV between GaAs and AlGaAs–permit short wavelength intersubband interactions.  We have measured intersubband absorption in a variety of InAs/AlSb quantum wells and observed E1-E2 absorption at energies as high as 0.67 eV (1.85 um) (see Fig. 1).

We are attempting to generate THz radiation via difference frequency mixing of mid-IR lasers in resonant InAs/AlSb double quantum wells. Optical pumping has a significant advantage over electrical pumping for THz generation: it does not require heavily-doped contact layers, which cause strong FIR absorption.  Using a double quantum well structure, as shown in Fig. 2, we can make the intersubband transitions resonant with the difference frequency transitions.  This strongly increases the second order nonlinear susceptibility, up to ~ 4E-6 m/V in our best sample, without causing significant pump absorption.  The interaction is not phase matched.

References:

S. Sasa, Y. Nakajima, M. Nakai, M. Inoue, D. C. Larrabee, and J. Kono, “Near-Infrared Intersubband Transitions in Delta-Doped InAs/AlSb Multi-Quantum Wells,” Applied Physics Letters 85, 5553 (2004). (abstract, full text)

J. Li, K. I. Kolokolov, C. Z. Ning, D. C. Larrabee, G. A. Khodaparast, J. Kono, K. Ueda, Y. Nakajima, S. Sasa, and M. Inoue, “Microscopic Modeling of Intersubband Resonances in InAs/AlSb Quantum Wells,” Physica E 20, 268 (2004). (full text)

D. C. Larrabee, G. A. Khodaparast, J. Kono, K. Ueda, Y. Nakajima, M. Nakai, S. Sasa, M. Inoue, K. I. Kolokolov, J. Li, and C. Z. Ning, “Temperature Dependence of Intersubband Transitions in InAs/AlSb Quantum Wells,”Applied Physics Letters 83, 3936 (2003). (abstract, full text)

D. C. Larrabee, J. Tang, M. Liang, G. A. Khodaparast, J. Kono, K. Ueda, Y. Nakajima, O. Suekane, S. Sasa, M. Inoue, K. I. Kolokolov, J. Li, and C. Z. Ning, “Intersubband transitions in narrow InAs/AlSb quantum wells,” in:Proceedings of the 26 th International Conference on the Physics of Semiconductors, edited by A. R. Long and J. H. Davies (Institute of Physics Publishing, Bristol, 2003), P129. (full text)

J. Li, K. I. Kolokolov, C. Z. Ning, D. C. Larrabee, G. A. Khodaparast, J. Kono, K. Ueda, Y. Nakajima, S. Sasa, and M. Inoue, “Intersubband Transitions in InAs/AlSb Quantum Wells” (invited paper), in: MRS Proceedings Volume 744, Progress in Semiconductors II – Electronics and Optoelectronic Applications, edited by B. D. Weaver, M. O. Manasreh, C. C. Jagadish, and S. Zollner (Materials Research Society, 2003), pp. M9.2.1-M9.2.12. (full text)

Collaborators:

Sample Growth: Masataka Inoue and Shigehiko Sasa, Osaka Institute of Technology

Theory: Alexey Belyanin, Texas A&M University

Theory: Cun-Zheng Ning, Nasa Ames Research Center

Quantum Cascade Lasers: Claire Gmachl, Princeton University