Terahertz interlevel spectroscopy of quantum well excitons


Terahertz interlevel spectroscopy of quantum well excitons

Schneider, H.; Bhattacharyya, J.; Zybell, S.; Winnerl, S.; Helm, M.

According to the proceedings of the first conference on intersubband transitions in quantum wells (ITQW), which took place in 1991 in Cargèse [1], research on this topic in the early 90s concentrated mostly on mid-infrared detectors (quantum well infrared photodetectors, QWIPs) for applications in thermal imaging. The following reasons make ITQW quite interesting for infrared optoelectronics: (a) III-V semiconductor technology supply a mature technological basis, (b) spectral properties of ITQW are determined basically by layer thicknesses and not by material bandgaps, and (c) theoretically engineered layer structures enable novel device concepts and huge resonant optical nonlinearities [2]. In 1994, the invention of the quantum cascade laser (QCL) [3] has marked the birth of another exciting research field and paved the way for a wide range of new applications for ITQW. Later on, both the QWIP and QCL concepts have been applied to longer wavelengths beyond the Reststrahlen band [4,5], thus giving rise to important devices in the emerging field of terahertz (THz) technology.
Going from the 2D quantum well (QW) system to 1D quantum wires, and even 0D quantum dots (QD), there has been a long debate whether device performance, in particular at high operation temperatures, will become superior due to the restricted phase space for electron scattering. Most importantly, a phonon bottleneck has been predicted [6], which gave rise to the expectation that quantum dot infrared photodetectors (QDIP) [7] and QD-based QCLs could exhibit improved device properties. Even though the formation of a "true" phonon bottleneck is prevented by polaron effects, very long relaxation times up to ~ 1 ns have in fact been observed for intersublevel transitions in n-type QD systems at appropriate transition energies [8]. Nevertheless, the performance of QD-based infrared devices is limited as yet mainly by technological difficulties to realize QD ensembles with sufficient homogeneity, spatial density, and, in particular, optimized QD shape.
Besides QD and QW, yet another interesting system is the quantum well exciton (QWE), which is caused by the Coulomb attraction between a – usually photogenerated – electron and hole inside a QW. This results in a hydrogen-like quasi atom with discrete bound states. In contrast to ITQW, interlevel transitions in QWEs have their dipole matrix elements in the QW plane, such that optical transitions between QWE levels occur under normal incidence. Since QWE can move freely in the QW, the center-of-mass momentum gives rise to a 2D continuum, which is in close analogy to the in-plane dispersion of QW subbands. Being a two-particle molecule, the QWE exhibits more complex physics than an electron in a QW or QD, including angular momentum as a new degree of freedom, finite recombination lifetime, and various optically active (bright) or inactive (dark) exciton states. For possible applications, an intriguing property of QWE is the opportunity of controlling near-infrared (NIR) interband processes (eV range) via intra-exciton transitions (few meV range) between bright and dark exciton states [9-11].
This talk will focus on the internal dynamics of quantum well excitons, which are excited into higher levels using a THz free-electron laser, by monitoring intra-exciton relaxation via time-resolved interband photoluminescence.
Acknowledgements – The authors thank E. Rosencher for initiating the ITQW conference series. We are also indebted to L. Schneebeli, C. N. Böttge, B. Breddermann, S. Chatterjee, M. Kira, and S. W. Koch (U. Marburg, Germany) for fruitful discussions and collaboration, to A.M. Andrews and G. Strasser (TU Vienna) for sample growth, and to P. Michel, W. Seidel (HZDR Dresden) and the ELBE team for their dedicated support.
References
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[10] W. D. Rice, J. Kono, S. Zybell, S. Winnerl, J. Bhattacharyya, H. Schneider, M. Helm, B. Ewers, A. Chernikov, M. Koch, S. Chatterjee, G. Khitrova, H. M. Gibbs, L. Schneebeli, B. Breddermann, M. Kira, S. W. Koch, "Observation of Forbidden Exciton Transitions Mediated by Coulomb Interactions in Photoexcited Semiconductor Quantum Wells", Phys. Rev. Lett. 110, 137404 (2013).
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Keywords: Terahertz free-electron laser; semiconductor quantum wells; excitons; Coulomb scattering

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