David Lombardo

Low control-power wavelength conversion on a silicon chip

We demonstrate controlled wavelength conversion on a silicon chip based on four-wave mixing Bragg scattering (FWM-BS). A total conversion efficiency of 5% is achieved with strongly unbalanced pumps and a controlling peak power of 55 mW, while the efficiency is over 15% when using less asymmetric pumps. The numerical simulation agrees with the experimental results. Both time domain and spectral domain noise measurements show as low as 2 dB signal-to-noise ratio (SNR) penalty because of the strong pump noise, two-photon absorption, and free-carrier absorption in silicon. We discuss how the scheme can be used to implement an all-optically controlled high-speed switch.

All-optical switching via four-wave mixing Bragg scattering in a silicon platform

We employ the process of non-degenerate four-wave mixing Bragg scattering to demonstrate all-optical control in a silicon platform. In our configuration, a strong, non-information-carrying pump is mixed with a weak control pump and an input signal in a silicon-on-insulator waveguide. Through the optical nonlinearity of this highly confining waveguide, the weak pump controls the wavelength conversion process from the signal to an idler, leading to a controlled depletion of the signal. The strong pump, on the other hand, plays the role of a constant bias. In this work, we show experimentally that it is possible to implement this low-power switching technique as a first step towards universal optical logic gates, and test the performance with random binary data. Even at very low powers, where the signal and control pump levels are almost equal, the eye-diagrams remain open, indicating a successful operation of the logic gates.

Deep-UV interference lithography combined with masked contact lithography for pixel wiregrid patterns

Pixelated wiregrids are of great interest in polarimetric imagers, but there are no straightforward methods available for combining the uniform exposures of laser interference with a masking system to achieve pixels at different rotational angles. In this work we demonstrate a 266nm deep-UV interference lithography combined with a traditional i-line contact lithography to create such pixels. Aluminum wiregrids are first made, following by etching to create the pixels, and then a planarizing molybdenum film is used before patterning subsequent pixel arrays. The etch contrast between the molybdenum and the aluminum enables the release of the planarizing layer.

Nonlinear optical properties of single crystal cadmium magnesium telluride

The third-order nonlinear susceptibility of crystalline Cadmium Magnesium Telluride (CdMgTe) was studies using a spatially resolved Irradiance Scan method including picosecond and nanosecond laser pulse widths at 1064nm. The samples were placed in a loosely focused beam, and a series of individual laser pulses at different energies were collected. The transmitted beam was reimaged to a CCD with a microscope objective providing a detailed objective function for numerical simulations. The nonlinear transmission results were modeled by way of a split-step nonlinear beam propagation method including diffraction, nonlinear absorption, and refraction arising from bound electrons and light-generated free carriers. The angular dependence of the third order susceptibility with respect to the electric field is also represented along with laser-induced damage thresholds.

High efficiency geometric-phase polarization fan-out grating on silicon

We report the design, fabrication and characterization of a 1-by-5 geometric-phase polarization fan-out grating for coherent beam combining at 1550 nm. The phase profile of the grating is accurately controlled by the local orientation of the binary subwavelength structure instead of the etching depth and profile empowering the grating to be more tolerant to fabrication errors. Deep-UV interference lithography on silicon offers an inexpensive, highly efficient and high damage threshold solution to fabricating large-area fan-out gratings than electron beam lithography (EBL) and photoalignment liquid crystals. The theoretical and experimental diffraction efficiency of the grating is 87% and 85.7% respectively. Such a fan-out grating may find application to high-power beam combining in the infrared regime.

Vanadium dioxide switchable components based on wiregrids for mid-infrared applications

Vanadium dioxide (VO2) is a polycrystalline material that exhibits reversible transition from a monoclinic semiconducting phase to a tetragonal metallic phase at 68°C. During this phase transition, both the real part and the imaginary part of the refractive index undergoes a dramatic change. Furthermore, the finite conductivity of the VO2 in both phases allows for ohmic heating as the source of the temperature change, which circumvents the need for an external heat source. This provide an attractive means for designing switchable optical components such as sub-wavelength polarizers and beam steerers. In this work, VO2 films were grown by ion-assisted deposition (IAD) using electron beam evaporation on sapphire substrates. Since the VO2 thin film is difficult to etch, we designed a unique lift-off process to obtain the VO2 wire grids that is compatible with the high temperature deposition process. Rigorous Coupled Wave Analysis (RCWA) was applied to the wiregrid polarizer to study its polarization properties in the mid-infrared region. The polarizers extinction was modeled at room temperature when the polarizer is at the OFF state and the 68°C when it is at the ON state.

Low switching power low-noise wavelength conversion in silicon-on-insulator waveguides

We demonstrate low-noise low-power four-wave mixing Bragg scattering in a silicon-on-insulator platform as a possible controlled all-optical switch. Efficiency and quantum noise measurements are performed to verify the viability of the device.