Ranojoy Bose

Carrier dynamics in GaAs photonic crystal cavities near the material band edge

We measure fast carrier decay rates (6 ps) in GaAs photonic crystal cavities with resonances near the GaAs bandgap energy at room temperature using a pump-probe measurement. Carriers generated via photoexcitation using an above-band femtosecond pulse cause a substantial blue-shift of three time the cavity linewidth for the cavity peak. The experimental results are compared to theoretical models based on free carrier effects near the GaAs band edge. The probe transmission is modified by nearly 30% for an estimated above-band pump energy of 4.2 fJ absorbed in the GaAs slab.

Information processing with large-scale optical integrated circuits

Photonic integrated circuits (PICs) offer an enticing platform for further advances in computation. Photonic communications hardware is already widely used within datacenters and is now reaching into the board and chip level. This trend is driving the development of more complex PICs that are more tightly integrated into computing systems. This PIC technology could be attractive for building photonic computational accelerators and for incorporating all-optical signal processing tasks into photonic communications and sensing. At Hewlett Packard Labs, we are using a silicon photonics platform to build complex PICs with many hundreds of components, including nonlinear components. We use these PICs to test various approaches to photonic computation, including neuromorphic approaches as well as traditional logic circuits. For example, we are currently fabricating a circuit to solve the so-called Ising problem, a classic problem of solid-state physics that turns out to be equivalent to a number of combinatorial optimization problems. The circuit is closely related to Hopfield neural networks. In parallel, we are investigating PICs based on photonic crystals in an InGaAs platform. These PICs offer radically reduced power consumption compared to CMOS circuits, potentially consuming less than 1 fJ per elementary operation.

Silicon Mach-Zehnder Interferometer modulator with PAM-4 data modulation at 64 Gb/s

This paper outlines the co-design and simulation of the photonics and electronics circuits for an optical modulator implementing a 4-level phase amplitude modulation scheme at 64 Gb/s. The photonics circuit is designed for an SOI process with a 300 nm top Si layer and is based on a Mach-Zehnder Interferometer architecture modulator using carrier depletion. The CMOS driver circuit employs two on-chip PRBS sources and tunable delay pre-driver segments. This driver is flip-chip bonded to the photonics die to complete the packaging. Details of the model are outlined below and a simulated optical eye diagram is presented.

Integrated all-optical phase-sensitive amplifier using the thermal nonlinearity

We demonstrate an all-optical phase-sensitive amplifier, a critical component in integrated circuits for all-optical computing. The amplifier is fabricated in amorphous silicon-on-insulator and relies on thermo-optic self-heating in a ring-loaded Mach-Zehnder interferometer. changing the power and phase of the bias input tunes the gain.

Thermally tunable III-V photonic crystals for coherent nonlinear optical circuits

We present a hybrid photonic architecture using gallium arsenide photonic crystals coupled to silicon nitride waveguides. Chrome microheaters are integrated in the system for tuning the cavities. The combination of low-energy switching elements, combined with low loss photonic waveguides provides an ideal architecture for applications in dedicated optical computing and machine learning applications.

Design automation for integrated nonlinear logic circuits (Conference Presentation)

A key enabler of the IT revolution of the late 20th century was the development of electronic design automation (EDA) tools allowing engineers to manage the complexity of electronic circuits with transistor counts now reaching into the billions. Recently, we have been developing large-scale nonlinear photonic integrated logic circuits for next generation all-optical information processing. At this time a sufficiently powerful EDA-style software tool chain to design this type of complex circuits does not yet exist. Here we describe a hierarchical approach to automating the design and validation of photonic integrated circuits, which can scale to several orders of magnitude higher complexity than the state of the art. Most photonic integrated circuits developed today consist of a small number of components, and only limited hierarchy. For example, a simple photonic transceiver may contain on the order of 10 building-block components, consisting of grating couplers for photonic I/O, modulators, and signal splitters/combiners. Because this is relatively easy to lay out by hand (or simple script) existing photonic design tools have relatively little automation in comparison to electronics tools. But demonstrating all-optical logic will require significantly more complex photonic circuits containing up to 1,000 components, hence becoming infeasible to design manually. Our design framework is based off Python-based software from Luceda Photonics which provides an environment to describe components, simulate their behavior, and export design files (GDS) to foundries for fabrication. At a fundamental level, a photonic component is described as a parametric cell (PCell) similarly to electronics design. PCells are described by geometric characteristics of their layout. A critical part of the design framework is the implementation of PCells as Python objects. PCell objects can then use inheritance to simplify design, and hierarchical designs can be made by creating composite PCells (modules) which consist of primitive building-block PCells (components). To automatically produce layouts, we built on a construct provided by Luceda called a PlaceAndAutoRoute cell: we create a module component by supplying a list of child cells, and a list of the desired connections between the cells (e.g. the out0 port of a microring is connected to a grating coupler). This functionality allowed us to write algorithms to automatically lay out the components: for instance, by laying out the first component and walking through the list of connections to check to see if the next component is already placed or not. The placement and orientation of the new component is determined by minimizing the length of a connecting waveguide. Our photonic circuits also utilize electrical signals to tune the photonic elements (setting propagation phases or microring resonant frequencies via thermo-optical tuning): the algorithm also routes the contacts for the metal heaters to contact pads at the edge of the circuit being designed where it can be contacted by electrical probes. We are currently validating a test run fabricated over the summer, and will use detailed characterization results to prepare our final design cycle in which we aim to demonstrate complex operational logic circuits containing ~50-100 nonlinear resonators.

Femtojoule optical switching in hydrogenated amorphous silicon photonic crystal nanocavities

We demonstrated 1.5-μm-band optical switching using the Kerr effect in a-Si:H photonic crystal nanocavities. Switching with pulse energies down to 18 fJ using a degenerate pump-probe technique was observed in cavities with Q-factors up to 30,000.

Low power all-optical switching in a gallium arsenide photonic molecule

We demonstrate low power all-optical switching in a gallium arsenide (GaAs) photonic molecule with resonances near the material band edge. The enhanced nonlinear effects in this spectral region result in a low estimated switching energy of a few femtojoules with 45% transmission contrast.

Amorphous Silicon Nanophotonic Circuits for All-Optical Logic

Advances in nanofabrication are enabling efficient chip-scale nonlinear optics in highly manufacturable platforms. We describe characterization of optical nonlinearities in a-Si:H photonic devices and applications for all-optical logic.

Gallium arsenide photonic crystal devices for fast integrated optical networks

We will present our experimental measurements of fast carrier-based dynamics in GaAs photonic crystal cavities at room temperature, and discuss how these devices can be integrated on chip for optical networking.