Candidate: Siyi Wang
Title: Thermal Dynamic Imaging and Thermal Management for Quantum Cascade Lasers
Date: September 2, 2022
Time: 9:00 am
Place: EIT 3142
Supervisor(s): Ban, Dayan
Abstract:
Quantum cascade lasers (QCLs) are semiconductor-based electrically pumped laser sources that are based on electron transitions between the subbands in the conduction band of multiple-quantum-wells (MQWs) heterostructure. The light emission wavelength range of QCLs can be tuned from mid-infrared (3-30 m) to far-infrared or so-called terahertz range (1-10 THz, 30-300 m, ~4-40 meV), respectively, which refers to MIR QCLs and THz QCLs. So far, the maximum operation temperature (Tmax) for MIR QCLs is far beyond room temperature (RT); the Tmax for THz QCLs is up to 250 K via thermal electrical cooler (TEC) system. However, the electrical-to-optical power conversion efficiency, the so-called wall-plug efficiency (w) for such devices are still limited. For MIR QCLs, w is up to ~31% on pulsed mode and ~22% on continuous-wave (cw) mode at room temperature, respectively. For THz QCLs, w is even lower, which is less than 2% at 10 K even for the record high peak output power (~1.01 W) of THz QCL, which means that most of the injection power generates heat rather than light. Therefore, as such lower w, to minimize Joule heating and to improve thermal management are still the critical issues for such QCL devices towards higher device performance.
In order to better understand the internal heat dynamics of such laser devices, it is requisite to directly observe the transient temperature profile on device’s active region (MQWs heterostructure), where heat is generated and how heat is dissipated, providing insights based on where the best strategy on device thermal management can be found. However, early works on direct observations of temperature imaging profiles on QCLs are still insufficient and limited by accurately monitoring the thermal dynamics in high spatial and temporal resolution simultaneously. Recently, CCD-based time-domain thermoreflectance (TDTR) microscopy have been employed to monitor pulse injected MIR QCLs, providing a non-intrusive methodology capable of measuring real-time two-dimensional (2D) temperature profiles measurements with high temporal and spatial resolution simultaneously.
In this study, a series of transient temperature imaging profiles of an actively biased ridge-waveguide MIR QCL were obtained with ultrafast temporal resolution down to 50 ns and sub-micrometer spatial resolution down to 390 nm simultaneously at RT. This study reveals that heat accumulates and temperature starts to rise in device’s active region up to ~100 degree above RT within 500 ns under a short pulsed high current injection (~112W of peak power injection, I=6A, V=18.7V. Further study reveals that, with epi-side up mounting strategy, within 1-2 µs, the heat dissipation to the top cladding layer is substantially suppressed, and most of the heat is drained to the substrate through the bottom cladding layer. This insufficient heat dissipation eventually leads to thermal-induced lasing quenching after 2 µs, which is also confirmed by combining light-current-voltage (L-I-V) measurements and theoretical thermal modeling.
In the following study, a novel thermal management is under development for THz QCLs. Instead of metal-metal (MM) bonding on gallium arsenide (GaAs) receptor for THz QCL MM ridge waveguide, the THz QCLs are with hybrid integration on hetero substrate (e.g., silicon (Si) or aluminum nitride (AlN)) as heat sink submount. The selected hetero substrates are with higher thermal conductivity and lower coefficient of thermal expansion (CTE) mismatch compared with GaAs based THz QCLs, providing superior heat dissipation properties and facilitates photonic integration. The results show that the maximum operating temperature of transfer bonded THz QCLs on AlN is 97 K which is comparable with 100 K by conventional metal-metal waveguide on GaAs receptor carrier on pulsed mode. Further investigations are carried out to study the transient thermal behaviours towards long pulse injection or high duty cycle towards cw operation. The novel thermal management structure by hetero substrate integration demonstrates improved heat extraction and facilitate heat dissipation.