Student Scholars: 2018

Breakthrough Starshot aims to complete interstellar flight in a given human lifespan by using laser radiation ressure to propel nano-crafts at one-fifth of the speed of light. The laser beam can be effectively focused on the nano-craft (the prescribed target) through an optical phased array (OPA) that consists of an array of emitters. To maximize the ower in the focal point and steer the beam, the phase of the light injected in each emitter is controlled by an array of integrated phase shifters. In this work, we designed and optimized a thermally tunable phase shifter made of AlxGa1-xAs that works at 1060 nm. The temperature of the waveguide, made of GaAs, is adjusted through the Joule effect by injecting a current in a 100 nm thick titanium-nitride strip on top of the structure. To operate in single-mode, we choose the width, the height, and the slab thickness of the waveguide equal to 2.0 µm, 2.1 µm, and 1.2 µm respectively. The top cladding (Al0.25Ga0.75As) is optimized to minimize the propagation loss induced by the metal and maximize the thermal heating. The phase change is linked to the temperature variation through the thermo-optic coefficient of the GaAs. We demonstrated that a minimum length of L= 21.0 µm is required for a 2π shift with a negligible insertion loss for the TE polarized optical mode.

The project aims to develop a high-throughput screening method for new optical materials that can be monolithically integrated on silicon photonic platforms. By combining combinatorial deposition and unique high-throughput optical measurement methods, the structural, thermal and optical properties of a large number of thin film compositions can be screened in a significantly reduced time frame to down select an optimal material for various photonic applications. The student will also learn through the project thin film processing and characterization methods as well as photonic testing techniques.

Coupling from fiber optic cables to III-V semiconductor waveguides is inefficient. Due to modal size difference and effective refractive index mismatch, up to 90% of the energy contained in the fiber is lost at the interface. To increase the coupling efficiency, a tapered structure is considered. The tapered waveguide aims to adiabatically reduce the effective index while increasing the mode size. Simulations of the structure were performed using RSoft BeamPROP. The design consisted of an amorphous Silicon channel waveguide connected to a GaAs channel waveguide. Both structures were tapered to reduce their effective indices. The system was optimized by considering the two components separately. For the GaAs tapered waveguide, the effective refractive index was found for various widths of the structure. For the a-Si channel waveguide, the effective refractive index and cutoff frequency were found at various widths and thicknesses. The objective was to find the maximum width of GaAs that will index match to a-Si waveguide. The two models were then merged with the a-Si structure placed on top of the GaAs waveguide. Power efficiency tests were then simulated for the combination of structures and the optimum configuration was selected.

A long-term goal is to make chemical sensing devices on a single chip. Integrated photonic sensors have the potential to be low cost, low maintenance, low energy, small, and accurate. Mid-infrared integrated absorption sensors are sensitive and selective, and show improved signal to noise ratios when sensors are waveguide-integrated with detectors. Simon Egner will work on Mid-infrared detectors for on-chip gas sensing.

Technology using light for sensing and communications has potential to be used in self-driving cars, space exploration, unmanned aircraft, and various other applications. Conventional technology requires the optical components to be constructed separately, increasing cost, size, weight and power (c-SWaP). Integrated Photonic technology, on the other hand, places all the optical components of a system on a single chip, alleviating these downsides. At the Integrated Photonics Lab, materials for use in Photonic Integrated Circuits are developed by growing III-V semiconductors via metal organic chemical vapor deposition (MOCVD), characterized using various techniques, including Photoluminescence Spectroscopy (PL) and finally processed into devices. A custom built experimental PL setup was developed for this purpose. The available hardware was configured, troubleshot, repaired or replaced. A control software was developed using object-oriented programming techniques to maximize hardware modularity for future use. While the startup of the setup is still work in progress, PL measurements of MOCVD grown InAs quantum dots using a similar instrument are shown to present the capability of PL.

As power consumption by data centers continues to increase, the demand for energy-efficient and high-speed communication must follow commensurately. Silicon photonics pose a solution that is low-loss, compact, and has a wide bandwidth. In this work, we characterize a microring-based wavelength sensitive switch fabricated through the AIM Photonics foundry. Our design includes electronically integrated thermal tuning to realize wavelength division multiplexing (WDM) switching and can select up to two wavelengths. We characterize two sets of switches from three sets of input/output channels by their transfer function, cross-talk, and switching time. Our results demonstrate simultaneous switching of two wavelengths, transmission at a bit error rate of 2.8 in a billion (at 15 dB attenuation), and has less than 20 µs of rise time/3 µs of fall time.