Yi Zou

Mid-infrared materials and devices on a Si platform for optical sensing

Abstract In this article, we review our recent work on mid-infrared (mid-IR) photonic materials and devices fabricated on silicon for on-chip sensing applications. Pedestal waveguides based on silicon are demonstrated as broadband mid-IR sensors. Our low-loss mid-IR directional couplers demonstrated in SiNx waveguides are useful in differential sensing applications. Photonic crystal cavities and microdisk resonators based on chalcogenide glasses for high sensitivity are also demonstrated as effective mid-IR sensors. Polymer-based functionalization layers, to enhance the sensitivity and selectivity of our sensor devices, are also presented. We discuss the design of mid-IR chalcogenide waveguides integrated with polycrystalline PbTe detectors on a monolithic silicon platform for optical sensing, wherein the use of a low-index spacer layer enables the evanescent coupling of mid-IR light from the waveguides to the detector. Finally, we show the successful fabrication processing of our first prototype mid-IR waveguide-integrated detectors.

Demonstration of high-Q mid-infrared chalcogenide glass-on-silicon resonators

We demonstrated high-index-contrast, waveguide-coupled As2Se3 chalcogenide glass resonators monolithically integrated on silicon fabricated using optical lithography and a lift-off process. The resonators exhibited a high intrinsic quality factor of 2×10(5) at 5.2 μm wavelength, which is among the highest values reported in on-chip mid-infrared (mid-IR) photonic devices. The resonator can serve as a key building block for mid-IR planar photonic circuits.

Effect of annealing conditions on the physio-chemical properties of spin-coated As 2 Se 3 chalcogenide glass films

Thin film selenide glasses have emerged as an important material for integrated photonics due to its high refractive index, mid-IR transparency and high non-linear optical indices. We prepared high-quality As2Se3 glass films using spin coating from ethylenediamine solutions. The physio-chemical properties of the films are characterized as a function of annealing conditions. Compared to bulk glasses, as-deposited films possess a distinctively different network structure due to presence of Se-Se homo-polar bonds and residual solvent. Annealing partially recovers the As-Se3 pyramid structure and brings the film refractive indices close to the bulk value. Optical loss in the films measured at 1550 nm wavelength is 9 dB/cm, which was attributed to N-H bond absorption from residual solvent.

A Fully-Integrated Flexible Photonic Platform for Chip-to-Chip Optical Interconnects

We analyze a chip-to-chip optical interconnect platform based on our recently developed flexible substrate integration technology. We show that the architecture achieves high bandwidth density (100 Tbs/cm2), and does not require optical alignment during packaging. These advantages make the flexible photonics platform a promising solution for chip-to-chip optical interconnects. We further report initial experimental characterizations of the flexible photonics platform fabricated using thermal nanoimprint patterning of glass waveguides and III-V die bonding.

Chip-to-chip optical interconnects based on flexible integrated photonics

A high bandwidth density chip-to-chip optical interconnect architecture is analyzed. The interconnect design leverages our recently developed flexible substrate integration technology to circumvent the optical alignment requirement during packaging. Initial experimental results on fabrication and characterization of the flexible photonic platform are also presented.

Nanoscale optical features via hot-stamping of As2Se3 glass

Here we show our ability to fabricate two-dimensional (2D) gratings on chalcogenide glasses with peak-to-valley amplitude of ~200 nm. The fabrication method relies on the thermal nano-imprinting of the glass substrate or film in direct contact with a patterned stamp. Stamping experiments are carried out using a bench-top precision glass-molding machine, both on As2Se3 optically-polished bulk samples and thermally-evaporated thin films. The stamps consist of silicon wafers patterned with sub-micron lithographically defined features. We demonstrate that the fabrication method described here enables precise control of the glass’ viscosity, mitigates risks associated with internal structural damages such as dewetting, or parasitic crystallization. The stamping fidelity as a function of the Time-Force-Temperature regime is discussed, and further developments and potential applications are presented.

Parasitic loss suppression in photonic and plasmonic photovoltaic light trapping structures

In this paper, we examine the optical loss mechanisms and mitigation strategies in classical photovoltaic light trapping structures consisting of diffractive gratings integrated with a backside reflector, which couple normal incident solar radiation into guided modes in solar cells to enhance optical absorption. Parasitic absorption from metal or dielectric backside reflectors is identified to be a major loss contributor in such light trapping structures. We elucidate the optical loss mechanism based on the classical coupled mode theory. Further, a spacer design is proposed and validated through numerical simulations to significantly suppress the parasitic loss and improve solar cell performance.

Chalcogenide glass planar photonics: from mid-IR sensing to 3-D flexible substrate integration

Chalcogenide glasses, namely the amorphous compounds containing sulfur, selenium, and/or tellurium, have emerged as a promising material candidate for integrated photonics given their wide infrared transparency window, low processing temperature, almost infinite capacity for composition alloying, as well as high linear and nonlinear indices. Here we present the fabrication and characterization of chalcogenide glass based photonic devices integrated on silicon as well as on flexible polymer substrates for mid-IR sensing, optical interconnect and nonlinear optics applications.

Demonstration of high-performance, sub-micron chalcogenide glass photonic devices by thermal nanoimprint

High-index-contrast optical devices form the backbone of densely integrated photonic circuits. While these devices are traditionally fabricated using lithography and etching, their performance is often limited by defects and sidewall roughness arising from fabrication imperfections. This paper reports a versatile, roll-to-roll and backend compatible technique for the fabrication of high-performance, high-index-contrast photonic structures in composition-engineered chalcogenide glass (ChG) thin films. Thin film ChG have emerged as important materials for photonic applications due to their high refractive index, excellent transparency in the infrared and large Kerr non-linearity. Both thermally evaporated and solution processed As-Se thin films are successfully employed to imprint waveguides and micro-ring resonators with high replicability and low surface roughness (0.9 nm). The micro-ring resonators exhibit an ultra-high quality-factor of 4 × 105 near 1550 nm wavelength, which represents the highest value reported in ChG micro-ring resonators. Furthermore, sub-micron nanoimprint of ChG films on non-planar plastic substrates is demonstrated, which establishes the method as a facile route for monolithic fabrication of high-index-contrast devices on a wide array of unconventional substrates.

Substrate-blind photonic integration based on high-index glass materials

Conventional photonic integration technologies are inevitably substrate-dependent, as different substrate platforms stipulate vastly different device fabrication methods and processing compatibility requirements. Here we capitalize on the unique monolithic integration capacity of composition-engineered non-silicate glass materials (amorphous chalcogenides and transition metal oxides) to enable multifunctional, multi-layer photonic integration on virtually any technically important substrate platforms. We show that high-index glass film deposition and device fabrication can be performed at low temperatures (< 250 °C) without compromising their low loss characteristics, and is thus fully compatible with monolithic integration on a broad range of substrates including semiconductors, plastics, textiles, and metals. Application of the technology is highlighted through three examples: demonstration of high-performance mid-IR photonic sensors on fluoride crystals, direct fabrication of photonic structures on graphene, and 3-D photonic integration on flexible plastic substrates.