Candidate: Boyu Wen
Title: Development of Terahertz Quantum Cascade Lasers with Novel Quantum Designs
Date: May 5, 2021
Time: 2:00 PM
Place: REMOTE ATTENDANCE
Supervisor(s): Ban, Dayan
Terahertz (THz) quantum cascade lasers (QCLs) are arguably the most promising THz radiation source, as they have high output power and efficiency. The main limitations of THz QCLs are the need for a cooling system due to the below-room-temperature operation and their relatively low frequency-tuning ability compared with gas lasers. Therefore, achieving room temperature operation and exploring effective frequency-tuning technology are essential for many potential applications of THz QCLs. This thesis simulates THz QCLs’ operation, designs and demonstrates the possible THz QCLs with novel quantum designs that have potential to improve the maximum lasing temperature (Tmax) and frequency-tuning ability of THz QCLs.
Resonant-phonon (RP) and scattering-assisted (SA) schemes are two widely used THz QCLs quantum schemes that show good temperature performance at different frequency ranges. However, both schemes have limitations, such as the pre-threshold electrical instability in RP designs and thermally activated leakage to continuum in SA designs, which have prevented significant temperature improvements in the last eight years. To overcome those limitations, this thesis develops a six-level hybrid extraction/injection design (HEID) scheme in which the RP and the SA-based injection/extraction are combined within a single Al0.15Ga0.85As/GaAs based structure. By utilizing extra excited states for hybrid extraction/injection channels, this design minimizes the appearance of an intermediate negative differential resistance (NDR) before the lasing threshold. The final negative differential resistance is observed up to 260 K, and a high characteristic temperature of 259 K is measured. These observations imply very effective suppression of pre-threshold electrical instability and thermally activated leakage current.
Broadband emission is another challenge that is yet to be addressed. One possible way of extending THz QCLs’ frequency coverage involves activating multiple-lasing transit channels in the device active region (AR). This thesis discusses a dual-lasing channel THz QCL both theoretically and experimentally. The dual-lasing channel device combines two optical transitions at different frequencies under different device biases. The device exhibits a low threshold current density of 550 A/cm2 at 50 K and a maximum operating temperature of 144 K. It provides 0.3 THz emission frequency coverage with the lowest reported threshold current density among SA THz QCLs. The combination of a dual-lasing channel operation, low lasing threshold current density, and high-temperature performance makes such devices ideal candidates for broadband emission applications and paves the way for achieving high-temperature-performance THz QCLs with a greater frequency-tuning ability.
The thesis also theoretically investigates two further novel designs. One design addresses an issue observed in the first reported HEID structure for Tmax improvement. The second design is a quasi one-well (Q1W) design consisting of the fewest number of layers (three) and lowest thickness per period (~20 nm) of all the THz QCL quantum structures. The quasi one-well design exhibits sufficient high optical gain in the positive differential resistance (PDR) region up to a lattice temperature above 250 K.