Near‐Field Thermal Profiling and 3D Anisotropic Thermal Analysis of Quantum Cascade Lasers

Abstract

Quantum cascade lasers (QCLs) are unipolar light sources wherein intersubband transitions of electrons occur in repeated layers of superlattices composed of semiconductor gain media. [1] The emission wavelength of QCLs is determined by the superlattice layer thickness and the material composition. Therefore, it can be tuned over a wide range in a given material system. [2] The typical operation wavelength of QCLs is in the mid-infrared (3.5-24 μm) and terahertz (1.2-4.9 THz). [3,4] Because the vibrational modes of many chemical compounds lie in the wavelength range of 3 to 15 μm, mid-infrared QCLs have found various applications in biological and chemical warfare agent detection [5,6] and pollutant detection, [7] and noninvasive medical diagnostics. [5] They can also be used in optical wireless communication: [8] the "terahertz gap" appears due to the absence of radiation sources in this frequency range, and THz QCLs fill the terahertz gap and have been used in imaging and spectroscopy. [9][10][11] However, the low thermal conductivity of QCL's multilayer superlattice gain medium and the ample operational electrical power cause significant self-heating in QCLs, [12,13] which, over time, results in a rise in lasing threshold current, [6] a decrease in output power, [14] and even device failure. [15] Therefore, to better understand QCL's failure mechanism, thermal analysis and monitoring are necessary for designing QCLs with optimal performance.Techniques for temperature monitoring include microphotoluminescence, [16] Raman spectroscopy, [17] thermoreflectance spectroscopy, [18] lock-in IR Thermography, [19] and infrared scanning near field optical microscopy (IR-SNOM). [20] For example, Raman spectroscopy as a non-contact thermometer was used to study output facet heating in an uncoated high-power continuous-wave QCL emitting at 8.5 μm. [17] A comparison of the spatial and temperature resolutions of these techniques can be found in Ref. [21] In this work, we report on developing an IR-SNOM technique with high temperature and spatial resolutions, both achieved by opening a subwavelength aperture at the tip of an IR fiber optic probe. We apply this technique to study the time constants in InP/InAlAs/InGaAs buried heterostructure (BH) mounted epi-layer side down QCLs--a geometry reported to have the best thermal performance by Pieŕscínska et al. [22] To verify the experimental findings, we develop an anisotropic 3D model to study steady-state and