LU2010/91 Acoustic Vibrational Modes in THz Quantum Cascade Laser Structures: a Picosecond Ultrasonic Investigation

AXEL BRUCHHAUSEN; MIKE HETTICH; JAMES LLOYD-HUGHES; JÉRÔME FAIST; THOMAS DEKORSY

Bordeau

Simposio; Second International Symposium on Laser-Ultrasonics (LU 2010); 2010

Universidad de Bordeau

Quantum cascade lasers (QCLs) have attracted much interest during the last decade, since they emerged as a promisingand interesting possibility for providing ecient coherent sources for terahertz electromagnetic radiation. The realizationof such ecient semiconductor lasers operating in the THz spectral range is highly desirable, and could openthe door to a countless number of applications. However, dierent aspects regarding the QCL technology still remainchallenging, and serve as motivation of ongoing investigations. These semiconductor based devices, resulting fromthe convenient band structure engineering, essentially rely on intersubband electronic transitions arising from the sizequantization and connement in heterostructures. The energy levels corresponding to THz frequencies are quite narrow,and therefore the requirements for nanofabrication of such quantum-well structures with adequate accuracies arevery demanding. Due to the narrow separation between the subband levels, heating, electron-electron scattering, andspecially the scattering of the carriers with phonons play a crucial role, limiting the operation of such devices to lowtemperatures. In this work we present an investigation of the vibrational dynamics in AlGaAs QCLs based on highspeed picosecond ultrasonics. The active part of the QCL is basically a superlattice (SL), formed by dierent dopedlayered materials. Due to the new periodicity of the SL, the acoustic branches are modied and folded into a new reducedBrillouin zone, giving rise to very distinctive vibrational modes of the structure. We performed a characterizationof the phonon acoustic states associated to the complex QCL structure, paying special attention to the inuence onthese phonon states with varying interface roughness between layers, establishing accurate structural parameters, andanalyzing the lifetimes of the involved phonon states which may limit the high-temperature performance of quantumcascade lasers.Number