Event
Ph.D. Dissertation Defense: Trisha Chakraborty
Thursday, April 3, 2025
2:30 p.m.
IREAP Large Conference Room (ERF 1207)
Maria Hoo
301 405 3681
mch@umd.edu
ANNOUNCEMENT: Ph.D. Dissertation Defense
Name: Trisha Chakraborty
Committee:
Professor Thomas E. Murphy (Chair)
Professor Karen E. Grutter (Co-chair)
Professor Kevin M. DanielsProfessor Edo Waks
Professor Miao Yu (Dean’s Representative)
Date/Time: Thursday, April 3, 2025 at 2:30 PM
Location: IREAP Large Conference Room (ERF 1207)
Title: Integrated Polymer Photonics: Thermo-Optic Properties and Low-Loss Polymer Fiber-to-Chip Couplers for Cryogenic and Broadband Applications
Abstract:
Integrated photonics consolidates multiple photonic functions onto a compact platform, enabling high-speed data transmission, advanced sensing technologies, and energy-efficient optical computing, making it a transformative innovation across diverse industries. 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. By bridging these two aspects, this work advances fiber-to-chip coupling techniques while providing critical insights into SU-8’s low-temperature optical properties, paving the way for its integration into quantum and superconducting photonic applications.
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-built fiber receptacles provide stable and repeatable fiber positioning, eliminating the need for high-precision alignment. The design is optimized using eigenmode expansion (EME) and finite-difference time-domain (FDTD) simulations, while direct laser writing (DLW) ensures sub-micron precision, followed by UV curing for structural stability. Experimental validation using a tunable laser (1520–1620 nm) and an optical spectrum analyzer confirms a coupling loss of approximately 0.42 dB per facet, outperforming conventional techniques. These results establish 3D-fabricated polymer couplers with integrated fiber receptacles as a robust solution for fiber-to-chip integration, enabling advancements in high-speed data transfer, optical interconnects, biomedical sensing, and quantum photonics.
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 ultra-low 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 temperature conditions. The successful deployment of these couplers in both room-temperature and cryogenic optical experiments highlights their versatility, reliability, and long-term stability, solidifying their value as a key enabling technology in integrated photonics research.
The third and final part of this dissertation focuses on the development of 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, operating at around 780 nm, the coupler requires careful optical design adaptation. The proposed design is optimized 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 to mitigate reflection losses and facilitate robust coupler integration. While experimental validation is ongoing, the findings provide a foundation for improving coupling efficiency in visible-wavelength III-V PICs, advancing fiber-to-chip coupling for single-photon sources, and supporting scalable quantum photonic technologies.
Integrated photonics consolidates multiple photonic functions onto a compact platform, enabling high-speed data transmission, advanced sensing technologies, and energy-efficient optical computing, making it a transformative innovation across diverse industries. 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. By bridging these two aspects, this work advances fiber-to-chip coupling techniques while providing critical insights into SU-8’s low-temperature optical properties, paving the way for its integration into quantum and superconducting photonic applications.
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-built fiber receptacles provide stable and repeatable fiber positioning, eliminating the need for high-precision alignment. The design is optimized using eigenmode expansion (EME) and finite-difference time-domain (FDTD) simulations, while direct laser writing (DLW) ensures sub-micron precision, followed by UV curing for structural stability. Experimental validation using a tunable laser (1520–1620 nm) and an optical spectrum analyzer confirms a coupling loss of approximately 0.42 dB per facet, outperforming conventional techniques. These results establish 3D-fabricated polymer couplers with integrated fiber receptacles as a robust solution for fiber-to-chip integration, enabling advancements in high-speed data transfer, optical interconnects, biomedical sensing, and quantum photonics.
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 ultra-low 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 temperature conditions. The successful deployment of these couplers in both room-temperature and cryogenic optical experiments highlights their versatility, reliability, and long-term stability, solidifying their value as a key enabling technology in integrated photonics research.
The third and final part of this dissertation focuses on the development of 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, operating at around 780 nm, the coupler requires careful optical design adaptation. The proposed design is optimized 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 to mitigate reflection losses and facilitate robust coupler integration. While experimental validation is ongoing, the findings provide a foundation for improving coupling efficiency in visible-wavelength III-V PICs, advancing fiber-to-chip coupling for single-photon sources, and supporting scalable quantum photonic technologies.
