Tian Gu

Multiscale free-space optical interconnects for intrachip global communication: motivation, analysis, and experimental validation.

The use of optical interconnects for communication between points on a microchip is motivated by system-level interconnect modeling showing the saturation of metal wire capacity at the global layer. Free-space optical solutions are analyzed for intrachip communication at the global layer. A multiscale solution comprising microlenses, etched compound slope microprisms, and a curved mirror is shown to outperform a single-scale alternative. Microprisms are designed and fabricated and inserted into an optical setup apparatus to experimentally validate the concept. The multiscale free-space system is shown to have the potential to provide the bandwidth density and configuration flexibility required for global communication in future generations of microchips.

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.

Chalcogenide glass-on-graphene photonics

Two-dimensional (2D) materials are of tremendous interest to integrated photonics, given their singular optical characteristics spanning light emission, modulation, saturable absorption and nonlinear optics. To harness their optical properties, these atomically thin materials are usually attached onto prefabricated devices via a transfer process. Here, we present a new route for 2D material integration with planar photonics. Central to this approach is the use of chalcogenide glass, a multifunctional material that can be directly deposited and patterned on a wide variety of 2D materials and can simultaneously function as the light-guiding medium, a gate dielectric and a passivation layer for 2D materials. Besides achieving improved fabrication yield and throughput compared with the traditional transfer process, our technique also enables unconventional multilayer device geometries optimally designed for enhancing light–matter interactions in the 2D layers. Capitalizing on this facile integration method, we demonstrate a series of high-performance glass-on-graphene devices including ultra-broadband on-chip polarizers, energy-efficient thermo-optic switches, as well as graphene-based mid-infrared waveguide-integrated photodetectors and modulators.Exploiting the peculiar properties of graphene, a series of high-performance glass-on-graphene devices, such as polarizers, thermo-optic switches and mid-infrared waveguide-integrated photodetectors and modulators are realized.

Hybrid chip-scale optical interconnects using multiple quantum well devices bonded to silicon

A hybrid MQW-device-based chip-scale optical interconnect is demonstrated. Novel small-footprint couplers are fabricated with gray-scale lithography to enable high-density fabrics and low-capacitance devices, with sub-pJ/b link performance. Contrast ratio and refined coupler results are presented.

Prismatic Coupling Structure for Intrachip Global Communication

Metal wires for global communication on integrated circuits have become problematic as device integration densities scale with rapid advancements in CMOS technology. They may not be able to deliver the growing bandwidth requirements of future microprocessors. Optical interconnect technologies may provide a solution to meet this challenge and extend Moores law. In this paper, a novel guided-wave optical interconnect fabric aiming to replace the slow global metal interconnections is proposed and analyzed. The reflection-mode multiple-quantum-well-modulator-based optical interconnection approach is projected to achieve high coupling efficiency and be compatible with standard CMOS processes. The key notion is a prismatic coupling structure that is embedded in the optical waveguide and therefore has a very small footprint in the circuit. Ray-trace and finite-difference time-domain simulation results predict high coupling efficiency of this structure.

On-Chip Infrared Spectroscopic Sensing: Redefining the Benefits of Scaling

Summary form only given. Infrared (IR) spectroscopy is widely recognized as a gold standard technique for chemical analysis. Traditional IR spectroscopy relies on fragile bench-top instruments located in dedicated laboratory settings, and is thus not suitable for emerging field-deployed applications such as in-line industrial process control, environmental monitoring, and point-of-care diagnosis. Recent strides in photonic integration technologies provide a promising route towards enabling miniaturized, rugged platforms for IR spectroscopic analysis. It is therefore attempting to simply replace the bulky discrete optical elements used in conventional IR spectroscopy with their on-chip counterparts. This size down-scaling approach, however, cripples the system performance as both the sensitivity of spectroscopic sensors and spectral resolution of spectrometers scale with optical path length. In light of this challenge, we will discuss two novel photonic device designs uniquely capable of reaping performance benefits from microphotonic scaling. We leverage strong optical and thermal confinement in judiciously designed micro-cavities to circumvent the thermal diffusion and optical diffraction limits in conventional photothermal sensors and achieve a record 104 photothermal sensitivity enhancement [1-3]. In the second example, an on-chip spectrometer design with the Fellgetts advantage is analyzed. The design enables sub-nm spectral resolution on a millimeter-sized, fully packaged chip without moving parts.

Hybrid micro-scale CPV/PV architecture

A novel hybrid PV solar cell architecture is presented that integrates high-performance micro-optics-based CPV array technology with a 1-sun PV cell within a low-profile panel structure. The approach simultaneously captures the direct solar radiation components with arrayed high-efficiency CPV cells and the diffuse solar components with an underlying wide-area PV cell. Performance analyses predict that the hybrid approach will significantly enhance the average collected energy for the full range of diffuse/direct radiation patterns across the USA. Furthermore, cost analyses indicate that the hybrid concept may be beneficial for a wide range of locations. Preliminary out-door experimental evaluation of a micro-optical system for use in a hybrid architecture verified that a large proportion of the direct radiation component was concentrated onto emulated micro-PV cell regions while the photons collected in the 1-sun area would considerably boost the overall performance of such a CPV system.

Chip-Level Multiple Quantum Well Modulator-Based Optical Interconnects

High performance computing systems are becoming increasingly limited by the capacity of interconnects due to the continued scaling down of CMOS critical dimensions, resulting in the implementation of optical interconnects at ever decreasing distances. At the chip level, the communication bottleneck and energy consumption per bit are now major limitations to the continued performance scaling of microprocessors. In this paper, a novel integrated photonic approach is presented that uses polymer waveguides and surface-normal GaAs/AlAs multiple quantum well devices integrated directly onto a silicon chip. The concept provides sub-pJ/b performance and seamless interfacing between the on- and off-chip domains. This is the first demonstrated waveguide-coupled surface-normal MQW-based approach to be fully integrated within a photonic layer and to a large extent mitigates packaging issues for future photonics systems integrated with Si chips. Key aspects of the architecture are efficient and minimum-footprint optical fabrics and low-power-consuming optical transceivers. Gray-scale lithography is used to fabricate the 3-D coupling structures directly in the waveguide polymer layer. Analyses and experimental results show that the optical fabric concept provides the necessary bandwidth density and low power consumption for future chip-scale interconnections.

Mid-infrared integrated photonics on silicon: a perspective

Abstract The emergence of silicon photonics over the past two decades has established silicon as a preferred substrate platform for photonic integration. While most silicon-based photonic components have so far been realized in the near-infrared (near-IR) telecommunication bands, the mid-infrared (mid-IR, 2–20-μm wavelength) band presents a significant growth opportunity for integrated photonics. In this review, we offer our perspective on the burgeoning field of mid-IR integrated photonics on silicon. A comprehensive survey on the state-of-the-art of key photonic devices such as waveguides, light sources, modulators, and detectors is presented. Furthermore, on-chip spectroscopic chemical sensing is quantitatively analyzed as an example of mid-IR photonic system integration based on these basic building blocks, and the constituent component choices are discussed and contrasted in the context of system performance and integration technologies.

Integrated free-space optical interconnects: All optical communications on- and off-chip

Compact free-space optical interconnects using MQW modulators heterogeneously integrated on silicon with novel waveguide-coupled free-space optical fabrics are presented. The goal is to provide seamless ultra-high density links spanning the intra- and inter-chip levels.