Rajat Sharma

Characterizing the effects of free carriers in fully etched, dielectric-clad silicon waveguides

We theoretically characterize the free-carrier plasma dispersion effect in fully etched silicon waveguides, with various dielectric material claddings, due to fixed interface charges and trap states at the silicon-dielectric interfaces. The values used for these charges are obtained from the measured capacitance-voltage characteristics of SiO2, SiNx, and Al2O3 thin films deposited on silicon substrates. The effect of the charges on the properties of silicon waveguides is then calculated using the semiconductor physics tool Silvaco in combination with the finite-difference time-domain method solver Lumerical. Our results show that, in addition to being a critical factor in the analysis of such active devices as capacitively driven silicon modulators, this effect should also be taken into account when considering the propagation losses of passive silicon waveguides.

Dispersion-Free SOI Interleaver for DWDM Applications

Integrated optical asymmetric Mach-Zehnder interferometer-based dense wavelength division multiplexing (DWDM) channel interleaver structures (2 × 2) have been theoretically studied using various single-mode waveguide geometries in silicon-on-insulator platform. Subsequently, a dispersion-free interleaver design has been proposed, fabricated, and characterized. It was designed to separate alternate DWDM channels with 100 GHz spacings. Some of the fabricated prototype devices have been fiber pigtailed and packaged in a suitably designed metal housing. The transmitted spectra (1520 nm <; λ <; 1600 nm) at each of the output ports reveal that alternate DWDM channel wavelengths are separated with a uniform inter-channel extinction of ~ 18 dB. Each of the channel passbands shows their 3-dB bandwidth of ~ 40 GHz.

Effect of dielectric claddings on the electro-optic behavior of silicon waveguides.

We fabricate silicon waveguides in silicon-on-insulator (SOI) wafers clad with either silicon dioxide, silicon nitride, or aluminum oxide and, by measuring their electro-optic behavior, we characterize the capacitively induced free-carrier effect. By comparing our results with simulations, we confirm that the observed voltage dependences of the transmission spectra are due to changes in the concentrations of holes and electrons within the semiconductor waveguides and show how strongly these effects depend on the cladding material that comes into contact with the waveguide. Waveguide loss is additionally found to have a high sensitivity to the applied voltage, suggesting that these effects may find use in applications that require low- or high-loss propagation. These phenomena, which are present in all semiconductor waveguides, may be incorporated into more complex waveguide designs in the future to create high-efficiency electro-optic modulators and wavemixers.

Observation of second-harmonic generation in silicon nitride waveguides through bulk nonlinearities

We present experimental results on the observation of a bulk second-order nonlinear susceptibility, derived from both free-space and integrated measurements, in silicon nitride. Phase-matching is achieved through dispersion engineering of the waveguide cross-section, independently revealing multiple components of the nonlinear susceptibility, namely χ(2) yyy = 0.14 ± 0.08 pm/V and χ(2) xxy = 0.30 ± 0.18 pm/V. Additionally, we show how the second-harmonic signal may be tuned through the application of bias voltages across silicon nitride. The material properties measured here are anticipated to allow for the realization of new nanophotonic devices in CMOS-compatible silicon nitride waveguides, adding to their viability for telecommunication, data communication, and optical signal processing applications.

Silicon nanoridge array waveguides for nonlinear and sensing applications.

We fabricate and characterize waveguides composed of closely spaced and longitudinally oriented silicon ridges etched into silicon-on-insulator wafers. Through both guided mode and bulk measurements, we demonstrate that the patterning of silicon waveguides on such a deeply subwavelength scale is desirable for nonlinear and sensing applications alike. The proposed waveguide geometry simultaneously exhibits comparable propagation losses to similar schemes proposed in literature, an enhanced effective third-order nonlinear susceptibility, and high sensitivity to perturbations in its environment.

Electronic Metamaterials with Tunable Second-order Optical Nonlinearities

The ability to engineer metamaterials with tunable nonlinear optical properties is crucial for nonlinear optics. Traditionally, metals have been employed to enhance nonlinear optical interactions through field localization. Here, inspired by the electronic properties of materials, we introduce and demonstrate experimentally an asymmetric metal-semiconductor-metal (MSM) metamaterial that exhibits a large and electronically tunable effective second-order optical susceptibility (χ(2)). The induced χ(2) originates from the interaction between the third-order optical susceptibility of the semiconductor (χ(3)) with the engineered internal electric field resulting from the two metals possessing dissimilar work function at its interfaces. We demonstrate a five times larger second-harmonic intensity from the MSM metamaterial, compared to contributions from its constituents with electrically tunable nonlinear coefficient ranging from 2.8 to 15.6 pm/V. Spatial patterning of one of the metals on the semiconductor demonstrates tunable nonlinear diffraction, paving the way for all-optical spatial signal processing with space-invariant and -variant nonlinear impulse response.

Synthesis of second-order nonlinearities in dielectric-semiconductor-dielectric metamaterials

We demonstrate a large effective second-order nonlinear optical susceptibility in electronic optical metamaterials based on sputtered dielectric-semiconductor-dielectric multilayers of silicon dioxide/amorphous silicon (a-Si)/aluminum oxide. The interfacial fixed charges (Qf) with opposite signs on either side of dielectric-semiconductor interfaces result in a non-zero built-in electric field within the a-Si layer, which couples to the large third-order nonlinear susceptibility tensor of a-Si and induces an effective second-order nonlinear susceptibility tensor χeff(2). The value of the largest components of the effective χeff(2) tensor, i.e., χ(2)zzz, is determined experimentally to be 2 pm/V for the as-fabricated metamaterials and increases to 8.5 pm/V after the post-thermal annealing process. The constituents and fabrication methods make these metamaterials CMOS compatible, enabling efficient nonlinear devices for chip-scale silicon photonic integrated circuits.

Nanoridge Arrays for Integrated and Free-Space Nonlinear Optical Applications

We report on the characterization of nanoridge array waveguides consisting of silicon, Al2O3, and SiO2. We determine the loss of these devices using ring resonators and measure enhanced third-harmonic generation from similarly patterned silicon surfaces.

Engineering of a second-order nonlinearity in silicon-dielectric multilayers

We demonstrate a way to engineer a second-order nonlinearity (χ⁽²⁾) in silicon-dielectric multilayers via the electric-field induced second-harmonic effect. The value of χ⁽²⁾ measured using the Maker fringe method is 1.2 pm/V.

Realization of a SOI-like III-V platform based on the integration of GaAs with silicon

We demonstrate the integration of gallium arsenide with silicon to create a SOI-like platform capable of exploiting the optical properties of III-V materials. We fabricate nanoscale waveguides and design Bragg gratings on this new platform.