Loss reduction in electromechanically tunable microring cavities.

Nanophotonic structures coupled with mechanics enable large effective index perturbation. To date, however, the relation between index tuning and induced optical loss has not been considered in detail. In this work we present an in-depth study of optical loss mechanisms in an electromechanically-tunable waveguide filter. Gradient electric forces modify the coupling between a microring optical cavity and a suspended micromechanical (MEMS) perturber resulting in a measured tuning greater than one free-spectral range (FSR) and an effective index tuning of 3×10-2. We examine various loss contributions and find, for certain conditions, a surprising reduction in loss with greater MEMS-induced mode perturbation. Modeling confirms the device behavior and loss mitigation is discussed.

Suspended photonic waveguide devices.

This article describes recent research at the U.S. Naval Research Laboratory that focuses on the use of micro- and nanomachining techniques for photonic waveguide devices. By selectively etching a sacrificial layer that the waveguide core is supported by, in whole or in part, the waveguide obtains enhanced properties and functionality, such as mechanical flexibility, index contrast, birefringence, and evanescent field depth. We describe how these properties enable unique waveguide applications in areas such as cavity optomechanics, displacement sensing, electro-optics, and nonlinear optics.

Mid-infrared difference-frequency generation in suspended GaAs waveguides.

We experimentally demonstrate mid-infrared difference-frequency generation in suspended 181 nm thick GaAs waveguides. Generation of the idler at wavelengths between 2800 and 3150 nm is enabled by form-birefringent phase-matching in ultrahigh index-contrast waveguides. Nonlinear mixing has a measured efficiency of 0.4 W⁻¹ in a 1.2 mm long waveguide using a CW signal tunable between 1490 and 1620 nm and a CW pump tunable between 1018 and 1032 nm at powers of a few mW.

Trace gas absorption spectroscopy using functionalized microring resonators.

We detect trace gases at parts-per-billion levels using evanescent-field absorption spectroscopy in silicon nitride microring resonators coated with a functionalized sorbent polymer. An analysis of the microring resonance line shapes enables a measurement of the differential absorption spectra for a number of vapor-phase analytes. The spectra are obtained at the near-infrared overtone of OH-stretch resonance, which provides information about the toxicity of the analyte vapor.

Leveraging crystal anisotropy for deterministic growth of InAs quantum dots with narrow optical linewidths.

Crystal growth anisotropy in molecular beam epitaxy usually prevents deterministic nucleation of individual quantum dots when a thick GaAs buffer is grown over a nanopatterned substrate. Here, we demonstrate how this anisotropy can actually be used to mold nucleation sites for single dots on a much thicker buffer than has been achieved by conventional techniques. This approach greatly suppresses the problem of defect-induced line broadening for single quantum dots in a charge-tunable device, giving state-of-the-art optical linewidths for a system widely studied as a spin qubit for quantum information.

Nanodimensionally driven analyte response reversal in gold nanocluster chemiresistor sensing.

The modulation of electron transport through an ensemble of ligand-stabilized gold nanoclusters by the sorption of vapors is made exceptionally sensitive and selective by terminal carboxylic acid functionalization of the alkanethiol ligand. Of further importance, the directionality of the response (conductance increase or decrease) is strongly dependent on the nanoscale dimensions of the gold core and ligand shell thickness. Films of gold nanoclusters composed of a 2 nm metal core with a 0.5 nm -S(CH(2))(5)COOH shell are compared to those based on an 8 nm core and a 1.5 nm -S(CH(2))(15)COOH shell with the finding of very strong and selective responses to amine vapors but with a reversal of response in the direction of the conductance transduction. This unexpected result cannot be accommodated by known vapor response transduction mechanisms based on a swelling expansion and a dielectric alteration of the ligand shell to modulate conductance in the ensemble. A speculative new mechanism is proposed on the basis of intercluster nanodomains of low and high dielectric character whose domain dimensions are determined by the ligand molecular structure and dielectric character that can range up to that associated with an ionic capacitance if generated by a vapor interaction.

Azimuthally and radially polarized light with a nematic SLM.

We demonstrate a technique for generating azimuthally and radially polarized beams using a nematic liquid crystal spatial light modulator and a pi phase step. The technique is similar in concept to prior techniques that interfere TEM(01) and TEM(10) laser modes, but the presented technique removes the requirement of interferometric stability. We calculate an overlap integral of >0.96 with >70% efficiency from an input Gaussian mode. The technique can easily switch between beams with azimuthal and radial polarization.

Efficient excitation of the TE(01) hollow metal waveguide mode for atom guiding.

We demonstrate excitation of the azimuthally-polarized TE(01) cylindrical waveguide mode in hollow glass and metal waveguides with 780 nm light. Experimentally, we demonstrate formation of the vectorial vortex beams, and measure attenuation lengths of the TE(01) mode in hollow optical fibers with diameters of 50-100 microns. By silver-coating the inner walls of the dielectric fibers, we demonstrate a approximately 200% increase in the attenuation length.