Although Quantum cascade lasers (QCLs) are frequently used in sensing, spectroscopy, and free space communication applications, their poor thermal properties lead to high temperature gradients in the devices. To diagnose failure mechanisms of mid-wave infrared (MWIR) QCLs, it is critical to understand their thermal generation and transport characteristics. In this work, we use 3D anisotropic steady state heat transfer analysis to investigate the thermal behavior in lattice matched InP/InAlAs/InGaAs buried heterostructure (Bh) mounted epi-layer side down QCLs. We introduce anisotropic thermal conductivities in the in-plane and cross-plane directions in QCL’s superlattice active region, and study the temperature distribution inside the laser. We consider several configurations, including the overhanging of the laser chip on the submount by different amounts, the choice of front facet dielectric coating materials and their thicknesses, and the width of the active region. Combining these effects, we optimize QCL’s thermal performance. This work aims to provide guidelines for the design of durable QCLs as well as to help diagnose QCL failure mechanisms.
The fundamental optical diffraction in infrared microscopes limits their spatial resolution to about ~5μm and hinders the detailed observation of heat generation and dissipation behaviors in micrometer-sized optoelectronic and semiconductor devices, thus impeding the understanding of basic material properties, electrical shorts and structural defects at a micron and sub-micron scale. We report the recent development of a scanning near-field optical microscopy (SNOM) method for thermal imaging with subwavelength spatial resolution. The system implements infrared fiber-optic probes with subwavelength apertures at the apex of a tip for coupling to thermal radiation. Topographic imaging and tip-to-sample distance control are enabled by the implementation of a macroscopic aluminum tuning fork of centimeter size to support IR thermal macro-probes. The SNOM-on-a-fork system is developed as a capability primarily for the thermal profiling of MWIR quantum cascade lasers (QCLs) during pulsed and continuous wave (CW) operation, targeting QCL design optimization. Time-resolved thermal measurements with high spatial resolution will enable better understanding of thermal effects that can have a significant impact on a laser's optical performance and reliability, and furthermore, will serve as a tool to diagnose failure mechanisms.
Dyakonov–Tamm (DT) waves are highly sensitive to the constitutive properties of the partnering materials near the interface. DT waves are excited at the interface of two dielectric materials of which at least one is anisotropic and periodically nonhomogeneous normal to their interface. Sculptured nematic thin film (SNTF) is a good candidate for the periodically nonhomogeneous dielectric partner for optical sensing of a fluid due to its porosity. The nanoscale parameters of an uninfiltrated SNTF obtained from the inverse Bruggeman homogenization formalism were used in the forward Bruggeman homogenization formalism to determine the constitutive parameters for the infiltrated SNTF. The sensitivity of DT waves to the refractive index was analyzed for two possible sensing modalities and it was found that the sensitivity was comparable to that of the chiral sculptured thin films (STFs) made of the same material as of the SNTF. This implies that the sensing with DT waves is robust, is independent of the morphology of the partnering nonhomogeneous dielectric material and could make the sensing easier since SNTFs are easier to fabricate than the chiral STFs.