Abstract
In this work, AlGaAs/GaAs superlattice, with layers’ sequence and compositions imitating the active and injector regions of a quantum cascade laser designed for emission in the terahertz spectral range, was investigated. Three independent absorption-like optical spectroscopy techniques were employed in order to study the band structure of the minibands formed within the conduction band. Photoreflectance measurements provided information about interband transitions in the investigated system. Common transmission spectra revealed, in the target range of intraband transitions, mainly a number of lines associated with the phonon-related processes, including two-phonon absorption. In contrast, differential transmittance realized by means of Fourier-transform spectroscopy was utilized to probe the confined states of the conduction band. The obtained energy separation between the second and third confined electron levels, expected to be predominantly contributing to the lasing, was found to be ~9 meV. The optical spectroscopy measurements were supported by numerical calculations performed in the effective mass approximation and XRD measurements for layers’ width verification. The calculated energy spacings are in a good agreement with the experimental values.
Highlights
The development of very precise and controllable techniques of material deposition, such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD), allowed the realization of a concept of a quantum cascade laser (QCL) operating in the range of terahertz (THz)
We present the results of experimental and theoretical studies of GaAs/AlGaAs superlattice designed to imitate the active and injector regions of a QCL aimed at much longer wavelengths, corresponding to the terahertz range of radiation
The AlAs-related Reststrahlen region includes the transverse optical modes (TO) and longitudinal optical modes (LO) modes marked by blue arrows in Fig. 2 [23]
Summary
The development of very precise and controllable techniques of material deposition, such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD), allowed the realization of a concept of a quantum cascade laser (QCL) operating in the range of terahertz (THz). Many bio-chemical molecules have strong spectral fingerprints at terahertz frequencies that enables a variety of applications in the field of spectroscopy and sensing [1]. More than a decade after the first demonstration of a working THz-QCL device [3], the room temperature operation has still not been reached. The highest reported operating temperature of 199.5 K was realized by the well-optimized Al0.15Ga0.85As/GaAs structure within the longitudinal optical phonon depopulation scheme by Fathololoumi et al in 2012 [4]
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