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Open Access Publications from the University of California

Scholarly Works (6 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2021)

Mid-infrared Quantum Cascade Lasers (mid-IR QCLs) have been rapidly developing andbecoming the practical mid-IR source for a multitude of applications. They are of particular technological interest as high efficiency lasers designed for the mid-wavelength infrared (MWIR) (3-5 μm), long-wavelength infrared (LWIR) (8-13 μm), and very-long-wavelength (VLWIR) (16-25 μm) atmospheric windows. Even though reasonably high efficiency has been achieved in mid-IR QCLs, a study of their efficiency based on an accurate transport modelling such as Non-equilibrium Green's Function (NEGF) is missing, probably due to its complicated and computationally intensive nature. In this work I focused on two subprojects; first is the development of high efficiency QCLs at 5 μm, and second is the design of a QCL that operates at 20 μm. For both projects, I present the design, bandstructure modeling and simulation, detailed micro-fabrication process, and characterization results and methods.

    Cover page: Development of Mid-infrared and Far-infrared QCLs
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2023)
    Radiofrequency (RF) injection locking and spectral broadening of a terahertz (THz) quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) is demonstrated. An intracryostat VECSEL focusing cavity design is used to enable continuous-wave lasing with a cavity length over 30 mm, which corresponds to a round-trip frequency near 5 GHz. Strong RF current modulation is injected to the QC-metasurface electrical bias to pull and lock the round-trip frequency. The injection locking range at various RF injection powers is recorded and compared with the injection locking theory. Moreover, the lasing spectrum broadens from 14 GHz in free-running mode to a maximum spectral width around 110 GHz with 20 dBm of injected RF power. This experimental setup is suitable for further exploration of active mode-locking and picosecond pulse generation in THz QC-VECSELs.
      Cover page: RF Injection Locking of THz Metasurface Quantum‐Cascade VECSEL
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2024)
      Design strategies for improving terahertz (THz) quantum cascade lasers (QCLs) in the 5-6 THz range are investigated numerically and experimentally, with the goal of overcoming the degradation in performance that occurs as the laser frequency approaches the Reststrahlen band. Two designs aimed at 5.4 THz were selected: one optimized for lower power dissipation and one optimized for better temperature performance. The active regions exhibited broadband gain, with the strongest modes lasing in the 5.3-5.6 THz range, but with other various modes observed ranging from 4.76 to 6.03 THz. Pulsed and continuous-wave (cw) operation is observed up to temperatures of 117 K and 68 K, respectively. In cw mode, the ridge laser has modes up to 5.71 THz - the highest reported frequency for a THz QCL in cw mode. The waveguide loss associated with the doped contact layers and metallization is identified as a critical limitation to performance above 5 THz.
        Cover page: Continuous-wave GaAs/AlGaAs quantum cascade laser at 5.7 THzCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2021)
        An amplifying quantum-cascade (QC) metasurface, the key component of the QC vertical-external-cavity surface-emitting-laser (VECSEL), is studied as a function of injected current density using reflection-mode terahertz time domain spectroscopy. Nearly perfect absorption is measured at zero bias, which is associated with the transition from the weak to strong coupling condition between the metasurface resonance and an intersubband transition within the QC material. An increase in reflectance is observed as the device is biased, both due to reduction in intersubband loss and the presence of intersubband gain. Significant phase modulation associated with the metasurface resonance is observed via electrical control, which may be useful for electrical tuning of QC-VECSEL. These results provide insight into the interaction between the intersubband QC-gain material and the metasurface and modify the design rules for QC-VECSELs for both biased and unbiased regions.
          Cover page: THz time-domain characterization of amplifying quantum-cascade metasurface
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2022)
          An active amplifying metasurface based on a quantum-cascade gain material at 2.7 THz is studied. The metasurface is first evaluated as the active component of an external cavity laser with excellent beam quality and frequency tunability from 2.55–2.8 THz. Amplification and absorption of the metasurface alone are then separately measured at a single frequency using a probe signal from a CO2-pumped gas laser operating at 2.743 THz. The metasurface reflectance vs bias is measured and compared with expectations from non-equilibrium Green’s function simulations of the quantum-cascade gain material and FEM simulations of the metasurface reflectance. A peak amplification on the order of 0.3 dB is measured. Design strategies are discussed for increasing single-reflection amplification (upward of 10 dB) and reducing power dissipation. Further increased amplification could be achieved by cascading multiple metasurfaces.
            Cover page: Measurement of amplification and absorption of a THz quantum-cascade metasurface free-space amplifierCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2021)
            We compare the performance of 10 and 5 μm thick metal-metal waveguide terahertz quantum-cascade laser ridges operating around 2.7 THz and based on a 4-well phonon depopulation active region design. Thanks to reduced heat dissipation and lower thermal resistance, the 5 μm thick material shows an 18 K increase in continuous wave operating temperature compared to the 10 μm material, despite a lower maximum pulsed-mode operating temperature and a larger input power density. A maximum continuous wave operating temperature of 129 K is achieved using the 5 μm thick material and a 15 μm wide ridge waveguide, which lased up to 155 K in the pulsed mode. The use of thin active regions is likely to become increasingly important to address the increasing input power density of emerging 2- and 3-well active region designs that show the highest pulsed operating temperatures.
              Cover page: Thin THz QCL active regions for improved continuous-wave operating temperatureCreative Commons 'BY' version 4.0 license
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