Terahertz All-Optical Modulation in a Silicon-Polymer Hybrid System

Although Gigahertz-scale free-carrier modulators have been previously demonstrated in silicon, intensity modulators operating at Terahertz speeds have not been reported because of silicon's weak ultrafast optical nonlinearity. We have demonstrated intensity modulation of light with light in a silicon-polymer integrated waveguide device, based on the all-optical Kerr effect - the same ultrafast effect used in four-wave mixing. Direct measurements of time-domain intensity modulation are made at speeds of 10 GHz. We showed experimentally that the ultrafast mechanism of this modulation functions at the optical frequency through spectral measurements, and that intensity modulation at frequencies in excess of 1 THz can be obtained in this device. By integrating optical polymers through evanescent coupling to high-mode-confinement silicon waveguides, we greatly increase the effective nonlinearity of the waveguide for cross-phase modulation. The combination of high mode confinement, multiple integrated optical components, and high nonlinearities produces all-optical ultrafast devices operating at continuous-wave power levels compatible with telecommunication systems. Although far from commercial radio frequency optical modulator standards in terms of extinction, these devices are a first step in development of large-scale integrated ultrafast optical logic in silicon, and are two orders of magnitude faster than previously reported silicon devices.Comment: Under consideration at Nature Material

Flow Simulation of a Controlled Airfoil With Synthetic Jet Actuators

An airfoil moving with two degrees of freedom (pitching and plunging) is simulated with a closed-loop flow control system. The simulation of the unsteady airfoil is computed using extended delayed detached eddy simulation (EDDES), a hybrid RANS-LES turbulence model based on the Spalart-Allmaras turbulence model. The control system controls the airfoil in two modes, first through direct application of forces and torques, and second, through the use of tangential synthetic jet actuators. The approach was designed for an investigation of flow control via synthetic jet actuators on a pitching and plunging airfoil in A. Glezer’s wind tunnel at Georgia Tech. The software definition of the controller used for the wind tunnel facility, which includes a robust servomechanism Linear Quadratic Regulator (LQR) and a neural network based adaptive controller, is coupled to a CFD model, which includes a model for the synthetic jet actuators. The coupled CFD/controller model is used to simulate the maneuvers of the airfoil as performed in the wind tunnel, and the coupled model is validated against experiment results. Nomenclature α angle of attac