Suspended gallium arsenide platform for building large scale photonic integrated circuits: passive devices

Abstract

The spectacular success of silicon-based photonic integrated circuits (PICs) in the past decade naturally begs the question of whether similar fabrication procedures can be applied to other material platforms with more desirable optical properties. In this work, we demonstrate the individual passive components (grating couplers, waveguides, multi-mode interferometers and ring resonators) necessary for building large scale integrated circuits in suspended gallium arsenide (GaAs). Implementing PICs in suspended GaAs is a viable route towards achieving optimal system performance in areas with stringent device constraints like energy efficient transceivers for exascale systems, integrated electro-optic comb lasers, integrated quantum photonics, cryogenic photonics and electromechanical guided wave acousto-optics.The scale, complexity and performance of silicon photonic integrated circuits (PICs) has revolutionized optical communications in the past decade 1 . Perhaps the most surprising aspect of this revolution is the fact that silicon does not possess many desirable optical properties (apart from a high refractive index) and the silicon photonics revolution was primarily driven by the availability of a foundry fabrication infrastructure, courtesy of the microelectronics industry, that could be applied to optics 2-4 . Over the past two decades, a wide variety of component designs have been optimized and their fabrication process perfected for silicon 5 and it is hard to foresee a similar investment of resources in any other material platform. On the other hand, there are a number of application areas in which silicon's lack of desirable optical properties proves a severe limitation to achieving system performance. These limitations include the absence of a direct bandgap, lack of a χ (2) nonlinearity to build fast electro-optic devices and zero piezoelectric response which makes it challenging to design acousto-optic devices. As a representative example, one of the key challenges facing transceivers for exascale systems 6 is avoiding the ∼ 3 dB penalty for coupling light from the III-V laser die to the silicon PIC. Electro-optic frequency comb 7 based coherent communication systems 8 will also benefit greatly from monolithic integration of lasers and modulators. On the quantum photonics side, one of the outstanding problems facing linear optic implementations of quantum computing is implementing feed-forward routines on a chip 9 , which requires (monolithically) interfacing fast, low loss modulators with efficient single photon detectors. Despite the outstanding performance improvements of carrier based depletion modulators, electro-optic modulators present the only near-term solution that can satisfy both the bandwidth (∼ 40 GHz) and loss requirements (< 3 dB) necessary for scalability 10 . Other application areas where alternative material platforms are worth exploring are: cryogenic photonic circuits for interfacing superconducting digital circuits with the outside world 11 and integrated acousto-optics, which re...

Piezo-optomechanical signal transduction using Lamb wave supermodes in a suspended Gallium Arsenide photonic integrated circuits platform

“…The key insight behind the mechanical supermode approach is to start from a mode that has intrinsically high g 0 and hybridize it with the Lamb wave resonance. In contrast to 1D nanobeam optomechanical crystals which have a higher g 0 , the rib waveguide geometry studied in this work provides two key advantages: straightforward integration into an integrated photonics platform [24] because of relaxed dimensional tolerances and side-on coupling; and access to higher (> 5 GHz) frequency mechanical modes with moderate g 0 . This becomes critical for engineering quantum transduction [8] in a material with strong acoustooptic interactions, but relatively low speed of sound like GaAs (v SAW < 3000 m/s).…”

“…In this work, we design and fabricate Lamb wave resonators in a suspended GaAs photonic integrated circuits platform [24] with a view towards improving η PIE for microwave to optical signal transduction, by hybridizing the Lamb wave resonance with the mechanical breathing mode of a rib waveguide. The GaAs platform supports strong acousto-optic interactions with g 0 > 1 MHz in 1D nanobeam photonic crystal cavities [23].…”

Piezoelectric optomechanical approaches for efficient quantum microwave-to-optical signal transduction: the need for co-design

“…For example, consider a microring resonator that is excited by an IDT. The actual acoustic energy inside the microring is very sensitive to both the relative position of the IDT with respect to the ring and the magnitude of the undercut, assuming the microring is suspended [56]. Such overlay errors will also affect the other geometries that are being considered in this work, but it is magnified for the rings because we are not working with a discrete mechanical resonance, but instead with broadband (limited by the IDT bandwidth) mechanical excitation of the ring.…”

CORNERSTONE’s Silicon Photonics Rapid Prototyping Platforms: Current Status and Future Outlook

“…The main challenge that waveguides present is the requirement for suspending long (>50 µm) beams while maintaining narrow gaps (200 nm) and avoiding beam buckling upon release. While this might seem at first sight very challenging, the high refractive index of Si enables us to tether rib waveguides without incurring significant additional scattering loss (see [82]) and suspend long tethered waveguide phase shifters without beam buckling (work in progress). We plan to use the new CORNERSTONE suspended Si platform to demonstrate MEMS devices in a foundry environment (see Section 3.3).…”