| Author: | Wang, Yifan |
| Title: | Suspended gallium phosphide thin film platform for nonlinear photonics and optomechanics |
| Advisors: | Chai, Yang (AP) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Subject: | Thin film devices Gallium compounds Optomechanics Nonlinear optics Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xxv, 185 pages : color illustrations |
| Language: | English |
| Abstract: | This thesis focuses on the design, fabrication, characterization, and application of high-performance suspended gallium phosphide (GaP) thin-film photonic and acoustic devices, with emphasis on Bound States in the Continuum (BIC) waveguides, nonlinear optical properties, ultrafast all-optical modulation, and GHz photoacoustic vibration modulation. First, in terms of waveguide design, a hybrid waveguide structure based on a low-refractive index polymer/high-refractive index GaP single-crystal substrate is proposed and realized, which utilizes the BIC principle to realize the complete decoupling of the TM modes from the TE continuum spectrum, thus obtaining zero radiation loss in the continuum spectrum band. Theoretical and finite element simulation analyses show that by precisely controlling the waveguide geometrical parameters (especially the width), phase-canceling interference can be achieved in both straight and curved waveguides with significant suppression of propagation loss and high fabrication tolerance. The method avoids direct etching of brittle or expensive single-crystal materials, expanding the freedom of material and structure design. In terms of materials and processes, GaP/AlGaInP/GaAs trilayer structures were epitaxially grown on GaAs substrates by metal organic chemical vapor deposition (MOCVD), and suspended GaP films were prepared by removing the sacrificial layer using wet etching. The thickness uniformity and low surface roughness of the films were confirmed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Combining photolithography and inductively coupled plasma (ICP) etching, suspended waveguide arrays were successfully constructed. Process optimization shows that precise control of etching rate and morphology can be achieved while maintaining sidewall perpendicularity and low roughness. In terms of waveguide propagation performance, the propagation losses of BIC waveguides with different widths were measured at 1550 nm and 775 nm wavelengths, respectively. The results show that the experiments and simulations are highly consistent with each other in terms of the width-dependent trend, but the experimental values are overall high, which is mainly attributed to the non-ideal factors such as material absorption and sidewall scattering. At 1550 nm, the TM fundamental mode loss is about 24.35 dB/cm; at 775 nm, the loss is reduced to 11.83 dB/cm, reflecting the smaller area of the short-wavelength modes and their lower sensitivity to defects. In the nonlinear optical research, the third-order nonlinear optical response of GaP in the range of 650-850 nm, including two-photon absorption (TPA) and nonlinear refraction, was systematically measured using the Z-scan technique. The results show that GaP exhibits anti-saturation absorption characteristics and self-focusing effect at 800 nm, and TPA coefficients and nonlinear refractive indices similar to those in the literature are obtained. The theoretical model combines the measured spot size, pulse width and linear absorption coefficient to realize the quantitative fitting of the nonlinear coefficients. For all-optical modulation, the transient reflectance changes of different suspended GaP device structures (grating, waveguide, homogeneous film, bare film, and glass substrate film) are compared using femtosecond pump-probe technique. The results show that the grating and waveguide structures achieve up to 2.3% transient reflectance change under 775 nm pumping/600 nm probing, much higher than the bare and glass base films due to field localization and mode modulation effects. All structures exhibit sub-picosecond fast rise and picosecond decay processes, where the slow decay component of the localized structures is significantly attenuated, favoring high-speed modulation applications. Comparative performance analysis shows that the suspended GaP films can achieve a 387 fs response with a 16.4 dB switching ratio without the introduction of a sub-wavelength resonant structure, but there is room for further reduction of the switching energy (509.7 J/m²), which can be achieved by the introduction of a high-Q resonant cavity or a photonic crystal structure. In the GHz photoacoustic research, a suspended GaP fishnet membrane structure is proposed and compared with the conventional nanodisk structure in finite element simulations and ultrafast pump-probe experiments. The fishnet membrane consists of a periodic array of circular holes with a thickness of 550 nm and a period of 4 μm. It is found that the fishnet membrane exhibits significantly different vibrational mode distributions and boundary-condition effects in photoacoustic interactions compared to the disc structure, and can support long-lived higher-order singular modes with an order-of-magnitude enhancement of the quality factor (Q) of up to more than 90. This reveals the difference between the behavior of geometrically complementary structures in the photoacoustic and electromagnetic domains, and provides new ideas for the design of high-Q optomechanical devices. In summary, this thesis systematically investigates suspended GaP thin-film devices in terms of device design principle, fabrication process, linear and nonlinear optical properties, all-optical modulation performance, and photoacoustic vibration modulation. The results not only verify the feasibility of BIC waveguides for low-loss propagation, but also demonstrate the potential of GaP materials in the field of high-speed, low-power all-optical switching and high-Q optomechanical resonators. In the future, the combination of sub-wavelength resonant structure and local field enhancement is expected to further reduce the energy consumption, enhance the modulation depth and device integration, and promote its application in the direction of on-chip optical computing, quantum optics and high-speed optical communication. |
| Rights: | All rights reserved |
| Access: | open access |
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