We demonstrate a method for fabricating arrays of plasmonic nanoparticles with separations on the order of 1 nm using an angle evaporation technique. Samples fabricated on thin SiN membranes are imaged with high-resolution transmission electron microscopy (HRTEM) to resolve the small separations achieved between nanoparticles. When irradiated with laser light, these nearly touching metal nanoparticles produce extremely high electric field intensities, which result in surface-enhanced Raman spectroscopy (SERS) signals. We quantify these enhancements by depositing a p-aminothiophenol dye molecule on the nanoparticle arrays and spatially mapping their Raman intensities using confocal micro-Raman spectroscopy. Our results show significant enhancement when the incident laser is polarized parallel to the axis of the nanoparticle pairs, whereas no enhancement is observed for the perpendicular polarization. These results demonstrate proof-of-principle of this fabrication technique. Finite difference time domain simulations based on HRTEM images predict an electric field intensity enhancement of 82400 at the center of the nanoparticle pair and an electromagnetic SERS enhancement factor of 10(9)-10(10).

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Abstract:We report the design, fabrication, and performance of monolithic, slot-antenna coupled Schottky-collector resonant tunnel diode (SRTD) oscillator arrays. A 64-element oscillator array oscillated at 650 GHz while a 16-element array produced 28 pt.W at 290 GHz. Introduction:Resonant tunnel diode (RTD) waveguide oscillators have been reported at frequencies as high as 712 GHz [1], well beyond the highest frequency transistor oscillators built to date [2]. Power levels achieved by discrete waveguide RTD oscillators are limited by constraints imposed on maximum device area for suppressing parasitic bias circuit oscillations [3]. Higher power levels can be obtained with monolithic RTD oscillators in which these constraints are eliminated by on-wafer bias stabilizers [4]. Power levels can then be further increased using quasi-optical array RTD oscillators [5]. Here, we report monolithic Schottky-collector RTD (SRTD) oscillator arrays with on-wafer Schottkydiode bias stabilizers. Different circuits operated at frequencies ranging from 100 GHz to 650 GHz. A 64-element array oscillated at 650 GHz. To our knowledge this is the highest oscillation frequency achieved by a monolithic oscillator. Additionally, a 16-element array produced 28 p,W at 290 GHz.

A transmission-type nonmechanical multiple-angle beam-steering device that uses liquid-crystal blazed grating has been developed. Sixteen steering angles with a contrast ratio of 18 has been demonstrated. A detailed analysis of the liquid-crystal and poly͑methyl methacrylate͒ blazed-grating deflector was carried out to provide guidance during the deflector's development. A manufacturing offset compensation technique is proposed to improve the device's performance greatly. A hybrid approach utilizing electrically generated blazed grating combined with the cascading approach described here yields in excess of 500 deflecting angles.

A novel velocity-matched distributed photodetector (VMDP) is proposed to simultaneously achieve high saturation photocurrent and broad bandwidth. Theoretical analysis on the tradeoff between saturation power and bandwidth shows that the VMDP offers fundamental advantages over conventional photodetectors. A comprehensive theoretical model has been developed for the design and simulation of the VMDP. Experimentally, the VMDP with very high saturation (56-mA) photocurrent and instrument-limited 3-dB bandwidth (49 GHz) has been demonstrated. The theoretical analysis and experimental results show that the VMDP is very attractive for high-performance microwave photonic links and high-power optical microwave applications.

We report the observation of steady-state photorefractive vortex-screening solitons. As a singly charged circular vortex nested on a broad beam propagates through a biased strontium barium niobate crystal, it self-traps in both transverse dimensions despite the inherent anisotropy of the photorefractive nonlinearity. When the vortex beam is a doughnut-shaped narrow beam, it breaks up into two elongated slices (with a selfdefocusing nonlinearity) or into two focused filaments (with a self-focusing nonlinearity). We demonstrate the optical guidance of a probe beam in a circular waveguide induced by the self-trapped vortex. © 1997 Optical Society of America Optical vortex solitons form when self-defocusing balances diffraction, leading to a vortexlike phase structure that undergoes stationary propagation in nonlinear media. Vortex solitons were predicted 1 and observed 2 previously in Kerr-type media and, more recently, in saturable self-defocusing nonlinear media. 3In all these studies, the nonlinearity that gives rise to the solitons is isotropic and therefore supports dark soliton stripes in any direction in the transverse plane. Thus it was possible to generate isotropic structures such as dark soliton crosses and grids 4 as well as circular vortex solitons. 2,3Following the first prediction of photorefractive spatial solitons, 5 self-trapping of an optical vortex was also demonstrated in these inherently anisotropic nonlinear media, including in biased strontium barium niobate 6 (SBN) and in unbiased photovoltaic LiNbO 3 . 7In a biased SBN crystal a vortex soliton was observed only in a transient state, corresponding to quasi-steady-state photorefractive solitons that arise from the nonlocal nature of the photorefractive effect and exist in a time window characterized by the slow screening process of an external field applied to the crystal. 6 In an unbiased LiNbO 3 crystal the steady-state self-trapping of the optical vortex was supported by the bulk photovoltaic effect, which gives rise to photovoltaic solitons. 8 Vortex solitons observed in the photorefractive materials 6,7 differ in their physical origin from those observed in the research reported in Refs. 2 and 3. A typical property of photorefractive nonlinearity is the inherent anisotropy with respect to the transverse dimensions of beam trapping, which permits formation of a one-dimensional (1D) dark soliton stripe only in specific directions in the transverse plane. -8Besides quasi-steady-state and photovoltaic solitons, another important type of photorefractive soliton is the screening soliton. Screening solitons are also generated in biased photorefractive media, but unlike quasi-steady-state solitons they exist in steady state when the applied f ield is completely screened nonuniformly. In fact, most of the recent research on photorefractive solitons has focused on screening solitons. There is substantial experimental evidence that two-dimensional (2D) circular bright screening solitons indeed exist in biased photorefractive media, 10 -12 despi...

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