The quantum cascade lasers (QCLs) are the most advanced class of semiconductor sources operating in the midinfrared wavelengths (3.5 – 24 µm) and also in the terahertz range (1.2– 4.9 Thz). QCLs are unipolar devices based on transitions between intersubband states created by quantum confinement in ultrathin alternating layers of semiconductors.
The first demonstration of emissions in GaAs/ AlGaAs QCL was reported in 1998 by C. Sirtori group. From this time QCL based on this materials have undergone a rapid development over the last few years. However still, the highest operating temperature in continuous wave are 150 K. The main limiting factors are the large electrical power density required for operation, and the low thermal conductivity characteristic of ternary alloys and complex multilayer heterostructure. Those factors contribute to high temperature gradients in the device. Additionally, temperature causes the leakage of the electrons into delocalized continuum states, lowering the population inversion. These effects are the main limiting factors of the high temperature operation of the devices.
In this paper we use spatially resolved thermoreflectance (SRTR) to measure temperature distribution over the facet of pulsed operated quantum cascade lasers . The method is based on the measurement of the change in the refractive index caused by current-induced heating of working device. The technique has a spatial resolution of about ~1 µm and temperature resolution better than 1 K. It has been previously applied to study facet heating in edge emitting lasers,. In conventional junction lasers facet temperature can be significantly higher than the temperature inside device because of nonradiative surface recombination. This process is eliminated in unipolar lasers, which allows one to use facet temperature as an estimate of true temperature inside device. The presented method gives an insight into distribution and relative importance of heat sources within the laser. It also allows for determination of thermal resistance of the laser and to evaluate the in-plane kII and the cross-plane k⊥ thermal conductivities of the active region which enables validation of a two-dimensional model for the anisotropic heat diffusion in QCLs.
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