Optical and quantum coherence study of nanolasers with III/V-QW and 2D gain materials

The pursuit of nanophotonic devices optimized for energy efficiency, minimal size, and cost-effectiveness continues to be a dynamic challenge in both research and industry. Among promising candidates, semiconductor lasers operating in the regime of cavity-enhanced quantum electrodynamics (cQED) stand out to open up new avenues in the field of nanophonics. Due to their low optical losses and ultralow mode volumes, these nanolasers can harness a large portion of spontaneous emission and achieve substantially lower lasing thresholds compared to traditional laser structures. Metal-cavity nanolasers, in particular, exhibit remarkable mode confinement, surpassing the fundamental half-wavelength limit by facilitating plasmonic mode lasing. However, intrinsic losses in metallic components, especially in the telecom emission band essential for commercial communications, present a key limitation. Accurate characterization and control of these losses are critical for further optimization. Simultaneously, exploring novel gain materials such as transition metal dichalcogenides (TMDCs) with high exciton binding energies holds the promise of achieving room-temperature lasing in single-layer nanolasers. Despite their potential, TMDC-based lasers remain relatively unexplored, particularly concerning their unique lasing mechanisms and coherence properties. This thesis aims to bridge this gap by conducting optical and quantum coherence studies of 2D-material-based, cavity-enhanced nanolasers. To establish a foundation, this work first investigates silver-coated nanolasers with InGaAsP multiple quantum wells (MQWs) as the active material. As a well-characterized system, this allows the consolidation of high-β-lasing fundamentals. Power-dependent optical studies conducted at 10 K confirm telecom emission at 1437 nm, while the observed nonlinearity and eightfold linewidth narrowing indicate high-β lasing behavior, with an estimated β-value of (0.088 ± 0.008) and threshold pump power Pth of (68.8 ± 6.2) kWcm−2. Quantum optical studies, including photon autocorrelation measurements, reveal a power-dependent decrease in the photon correlation function from g(2)(0) = (1.99 ± 0.23) below threshold to g(2)(0) = (1.10 ± 0.09) above threshold, confirming the transition from thermal spontaneous emission to coherent, stimulated emission. Despite the success of single-mode quantum optical models in capturing many observed behaviors, they fall short of explaining a compelling phenomenon: a lineshape transition from a Lorentzian profile (μ = 1) below threshold to a Gaussian profile (μ = 0.2) above threshold. While previously suggested extrinsic explanations, such as charge density fluctuations, have been ruled out, this Gaussian component is now linked to the onset of stimulated emission. A semiclassical multimode model, incorporating coupled free-space and cavity modes, attributes the lineshape transition to frequency pulling among modes above threshold, positioning this transition as a potentially universal lasing indicator for future study. Building on the 10 K lasing operation, this research further explores temperature-dependent performance limitations in silver-coated nanolasers. Absorption losses in silver, the primary factor reducing the quality factor (Q-factor) at elevated temperatures, underscore the need for accurate silver permittivity values in the telecom range, particularly at cryogenic temperatures. By combining numerical cavity simulations with experimental power-dependent data from both disk-shaped and coaxial silver-coated nanolasers, this study extracts the imaginary part of silver permittivity across a temperature range from 10 K to 180 K, demonstrating a six- to ten-fold reduction in losses as temperature decreases. This work affirms that precise knowledge of material optical constants enables targeted optimization for metallic cavity lasers and, more broadly, for nano- and microlasers. The groundwork laid by the metallic InGaAsP MQW-based nanolasers opens the door for investigating TMDC-based microlasers. Here, dielectric microcavities avoid the intrinsic absorption losses of metal-cavity designs. Embedded within a distributed Bragg reflector (DBR) is a monolayer (ML) of WSe2, encapsulated in hexagonal boron nitride (hBN). Fascinatingly, encapsulation introduces air bubbles between the monolayer and hBN, forming photonic defects in the DBR visible under microscopy. Atomic force microscopy reveals a range of defect morphologies, with base diameters and heights spanning 2.5 - 12.9 μm and 53 - 371 nm, respectively. These defects enable three-dimensional confinement, in contrast to the one-dimensional confinement in planar DBRs, significantly reducing mode volume. Angle-resolved measurements reveal that resonance wavelength tuning aligns with the trapped air layer thickness, resulting in Q-factors between 550 and 3250, which enhance light-matter interaction. Power-dependent optical measurements on two TMDC structures indicate high-β lasing through a soft S-shaped curve, slight coherence time increase, and lineshape narrowing with estimated β-factors of (0.079 ± 0.009) and (0.026 ± 0.003) and corresponding threshold pump power densities of (10.2 ± 3.2) kWcm−2 and (17.2 ± 3.5) kWcm−2, respectively. This leads up to the pivotal measurement of the work: quantum-optical verification of lasing in one structure, with g(2)(0) decreasing from (1.70 ± 0.28) near threshold to (1.11 ± 0.10) above threshold. Furthermore, based on the TMDC resonator concept, this thesis explores a TMDC-based nanobeam laser in numerical simulations, yielding a high nominal Purcell factor value FP of 1000, which could be further optimized to 6100 with algorithmic adjustments. The findings of this thesis underscore the importance of quantum-optical studies in understanding lasing transitions and highlight the need for continued exploration of high-β lasing mechanisms, such as the lineshape transition. The promising outcomes of TMDC-based lasing present a strong foundation for future research into next-generation nanophotonic devices.