Kenneth Diest

PLASMOSTOR: A-METAL-OXIDE-SI FIELD EFFECT PLASMONIC MODULATOR

Realization of chip-based all-optical and optoelectronic computational networks will require ultracompact Si-compatible modulators, ideally comprising dimensions, materials, and functionality similar to electronic complementary metal-oxide-semiconductor (CMOS) components. Here we demonstrate such a modulator, based on field-effect modulation of plasmon waveguide modes in a MOS geometry. Near-infrared transmission between an optical source and drain is controlled by a gate voltage that drives the MOS into accumulation. Using the gate oxide as an optical channel, electro-optic modulation is achieved in device volumes of half of a cubic wavelength with femtojoule switching energies and the potential for gigahertz modulation frequencies.

Unity-order index change in transparent conducting oxides at visible frequencies.

We report a method for obtaining unity-order refractive index changes in the accumulation layer of a metal-oxide-semiconductor heterostructure with conducting oxide as the active material. Under applied field, carrier concentrations at the dielectric/conducting oxide interface increase from 1 x 10(21)/cm(3) to 2.8 x 10(22)/cm(3), resulting in a local refractive index change of 1.39 at 800 nm. When this structure is modeled as a plasmonic waveguide, the change corresponds to a modal index change of 0.08 for the plasmonic mode.

Tunable Color Filters Based on Metal-Insulator-Metal Resonators

We report a method for filtering white light into individual colors using metal-insulator-metal resonators. The resonators are designed to support photonic modes at visible frequencies, and dispersion relations are developed for realistic experimental configurations. Experimental results indicate that passive Ag/Si(3)N(4)/Au resonators exhibit color filtering across the entire visible spectrum. Full field electromagnetic simulations were performed on active resonators for which the resonator length was varied from 1-3 microm and the output slit depth was systematically varied throughout the thickness of the dielectric layer. These resonators are shown to filter colors based on interference between the optical modes within the dielectric layer. By careful design of the output coupling, the resonator can selectively couple to intensity maxima of different photonic modes and, as a result, preferentially select any of the primary colors. We also illustrate how refractive index modulation in metal-insulator-metal resonators can yield actively tunable color filters. Simulations using lithium niobate as the dielectric layer and the top and bottom Ag layers as electrodes, indicate that the output color can be tuned over the visible spectrum with an applied field.

Electrically Pumped Hybrid Evanescent Si/InGaAsP Lasers

Hybrid Si/III-V, Fabry-Perot evanescent lasers are demonstrated, utilizing InGaAsP as the III-V gain material for the first time to our knowledge. The lasing threshold current of 300-mum-long devices was as low as 24 mA, with a maximal single facet output power of 4.2 mW at 15 degrees C. Longer devices achieved a maximal single facet output power as high as 12.7 mW, a single facet slope efficiency of 8.4%, and a lasing threshold current density of 1 kA/cm2. Continuous wave laser operation was obtained up to 45 degrees C. The threshold current density, output power, and efficiency obtained improve upon those of previously reported devices having a similar geometry. Facet images indicate that the output light is largely confined to the Si waveguide.

Silver diffusion bonding and layer transfer of lithium niobate to silicon

A diffusion bonding method has been developed that enables layer transfer of single crystal lithium niobate thin films to silicon substrates. A silver film was deposited onto both the silicon and lithium niobate surfaces prior to bonding, and upon heating, a diffusion bond was formed. Transmission electron microscopy confirms the interface evolution via diffusion bonding which combines interfacial diffusion, power law creep, and growth of (111) silver grains to replace the as-bonded interface by a single polycrystalline silver film. The transferred film composition was the same as bulk lithium niobate.

Single crystalline BaTiO3 thin films synthesized using ion implantation induced layer transfer

Layer transfer of BaTiO3 thin films onto silicon-based substrates has been investigated. Hydrogen and helium ions were co-implanted to facilitate ion-implantation-induced layer transfer of films from BaTiO3 single crystals. From thermodynamic equilibrium calculations, we suggest that the dominant species during cavity nucleation and growth are H2, H+, H2O, Ba2+ and Ba–OH, and that the addition of hydrogen to the Ba–Ti–O system can effectively suppress volatile oxide formation during layer transfer and subsequent annealing. After ion implantation, BaTiO3 layers contain microstructural defects and hydrogen precipitates in the lattice, but after layer transfer, the single crystal is found to be stoichiometric. Using direct wafer bonding and layer splitting, single crystal BaTiO3 thin films were transferred onto amorphous Si3N4 and Pt substrates. Micro-Raman spectroscopy indicated that the density of defects generated by ion implantation in BaTiO3 can be significantly reduced during post-transfer annealing, returning the transferred layer to its single crystal state. Characterization using piezoresponse force microscopy shows that the layer transferred thin films are ferroelectric, with domain structures and piezoresponse characteristics similar to that of bulk crystals.

Competing failure mechanisms in thin films : Application to layer transfer

We investigate the origin of transverse cracks often observed in thin films obtained by the layer transfer technique. During this process, two crystals bonded to each other containing a weak plane produced by ion implantation are heated to let a thin layer of one of the material on the other. The level of stress imposed on the film during the heating phase due to the mismatch of thermal expansion coefficients of the substrate and the film is shown to be the dominent factor in determining the quality of the transferred layer. In particular, it is shown that if the film is submitted to a tensile stress, the microcracks produced by ion implantation are not stable and deviate from the plane of implantation making the layer transfer process impossible. However, if the compressive stress exceeds a threshold value, after layer transfer, the film can buckle and delaminate, leading to transverse cracks induced by bending. As a result, we show that the imposed stress σ_m —- or equivalently the heating temperature -— must be within the range −σ_c<σ_m<0 to produce an intact thin film where σ_c depends on the interfacial fracture energy and the size of defects at the interface between film and substrate.

Advanced Silicon Processing for Active Planar Photonic Devices

Using high quality, anisotropically etched Si waveguides bonded to InGaAsP, the authors demonstrate a hybrid laser, whose optical profile overlaps both Si and III-V regions. Continuous wave laser operation was obtained up to 45 °C, with single facet power as high as 12.7 mW at 15 °C. Planar Si optical resonators with Q = 4.8 × 10^6 are also demonstrated. By using a SF_6/C_(4)F_8 reactive ion etch, followed by H_(2)SO_4/HF surface treatment and oxygen plasma oxide, the optical losses due to the waveguide and the bonding interface are minimized. Changes of optical confinement in the silicon are observed due to waveguide width variation.

Active plasmonic devices and optical metamaterials

We studied active near-infrared metamaterials based on phase transition of vanadium oxide thin films, asymmetrically coupled split-ring resonators for narrowing resonance line-widths, field effect modulation of plasmon propagation and 3D single layer, plasmonic negative-index metamaterials.