Integrated Polymer Photonics: Thermo-Optic Properties and Low-Loss Fiber-to-Chip Couplers for Cryogenic and Broadband Applications

Abstract

Integrated photonics consolidates multiple photonic functions onto a compact platform. It enables high-speed data transmission, advanced sensing technologies, and energy-efficient optical computing. While conventional photonic integrated circuits (PICs) rely on established semiconductor platforms, polymer-based photonics offer a low-cost, flexible alternative with tunable optical properties. This dissertation explores the role of polymer materials in integrated photonics, focusing on two key areas. The first involves the development of low-loss fiber-to-chip couplers for polymer-based photonic platforms, specifically SU-8 in the C and L bands, as well as III-V (AlGaAs) photonic devices in the visible range. The second focuses on the thermo-optic characterization of SU-8 at cryogenic temperatures, utilizing these fiber-to-chip couplers for efficient device integration and packaging, enabling precise optical measurements in cryogenic environments. In the first part of this dissertation, a 3D low-loss, broadband fiber-to-chip coupler is developed for polymer-integrated photonics, incorporating custom-designed fiber receptacles that enable a self-aligning structure. The polymer coupler, fabricated via two-photon polymerization (TPP), facilitates seamless light transition between standard optical fibers and on-chip waveguides, significantly reducing coupling losses by integrating mode field adapters and a hybrid coupler-waveguide tapered structure. Custom-designed fiber receptacles ensure stable and consistent fiber positioning, eliminating the need for high-precision alignment and facilitating robust fiber-to-chip packaging. In the second part of this dissertation, the thermo-optic coefficient (TOC) of SU-8 is characterized at cryogenic temperatures to better understand its behavior in superconducting and quantum photonic applications. SU-8 is widely used in photonic devices due to its excellent optical properties, low-loss characteristics, and ease of fabrication. However, its TOC at ultralow temperatures remains largely unexplored despite its critical importance in designing stable and efficient photonic circuits for cryogenic environments. To address this gap, the TOC of SU-8 is systematically measured down to 3 K using an integrated microring resonator approach. The results reveal a significant reduction in TOC, decreasing by nearly two orders of magnitude as the temperature drops from room temperature to cryogenic levels, providing key insights for future low-temperature photonic designs. A critical connection between the two parts of this dissertation is established through the integration of the same 3D fiber-to-chip couplers developed in Part 1. These couplers enable efficient and stable fiber-to-SU-8 microring resonator packaging for characterization inside the cryostat, which lacks real-time active fiber alignment capabilities. Their robust design ensures consistent optical coupling throughout multiple thermal cycles, demonstrating exceptional resilience in extreme temperatures. This dissertation’s third and final part focuses on developing a fiber-to-chip coupler for III-V photonic integrated circuits (PICs) operating in the visible wavelength range. This work addresses fiber-to-chip coupling challenges in AlGaAs waveguides that contain embedded single-photon sources for quantum applications. Efficient optical pumping of these emitters is expected to generate a high flux of single photons propagating through the waveguides. However, existing grating-based coupling schemes suffer from extremely low collection efficiency, limiting the practical viability of these quantum photonic devices. To overcome this limitation, a novel coupler design is proposed to enhance photon extraction and fiber-to-chip coupling. A key challenge in this work is developing a fabrication process for suspended AlGaAs devices, which differ structurally from conventional planar waveguides. Moreover, the coupler operating at around 780 nm requires careful photonic design adaptation. While experimental validation is ongoing, the proposed design is designed to improve mode conversion between waveguide and fiber modes, with simulations predicting a coupling efficiency exceeding 80%. This dissertation also discusses the initial steps toward experimental realization, addressing fabrication constraints due to AlGaAs’s high reflectivity. Additional parameter tuning and fabrication steps, particularly related to the release of suspended structures after 3D nanoscale printing of couplers on AlGaAs are explored.