Demonstration of a compact double-reflection transmissive beam scanner operating at 1550 nm wavelength.

A compact forward-directed transmissive beam scanner operating at a wavelength of 1550 nm was constructed and characterized. The scanner consists of two wire-grid polarizers (WGPs) surrounding a 45° Faraday rotator, causing incident light to reflect once from each WGP before transmitting through the second polarizer. Scanning is achieved by tilting one of the WGPs. Measured efficiency remained above 73% over a 90° forward scan range (-45∘ to +45∘) for vertically polarized incident light. Additionally, we measured the efficiency versus beam deflection for four different incident linear polarization configurations, three of which maintained >70% efficiency for deflection angles up to -60∘.

Physical Properties of Ultrafine Bubbles Generated Using a Generator System.

BACKGROUND/AIM: Ultrafine bubbles (UFBs) have been extensively researched owing to their promising physical and biological properties. However, determining the lifespan or ideal concentration of UFBs for various biological events is challenging. This study aimed to determine the maximum concentration and longest lifespan of UFBs and to verify the validity of UFBs for assessing cell properties. MATERIALS AND METHODS: A generator system (HMB-H0150+P001, TOSSLEC Corporation Limited, Kyoto, Japan) generated UFBs using various gases. The size and concentration of UFBs in ultrapure water and cell culture medium were measured through a nanoparticle tracking analysis method. RESULTS: The UFB concentration increased when the generator operated in a time dependent manner. The mean size of UFBs was approximately 120 nm. In the UFB lifespan, the concentration decreased by approximately 30% within the first two weeks of generation and was stable for up to 6 months. The UFB size increased by approximately 20% within the first two weeks of generation and demonstrated minor changes until the 6th month. The number of cells differed significantly with various concentrations of nitrogen gas UFBs. CONCLUSION: The generator system can generate UFBs with multiple concentrations within a suitable temperature. Consequently, the solution containing UFBs could be widely acceptable in cell culture systems.

Subwavelength-size solid immersion lens.

We report on the fabrication and characterization of nanoscale solid immersion lenses (nano-SILs) with sizes down to a subwavelength range. Submicrometer-scale cylinders fabricated by electron-beam lithography are thermally reflowed to form a spherical shape. Subsequent soft lithography leads to nano-SILs on transparent substrates for optical characterization. The optical characterization is performed using a high-resolution interference microscope with illumination at 642 nm wavelength. The focal spots produced by the nano-SILs show both spot-size reduction and enhanced optical intensity, which are consistent with the immersion effect.

Near-field imaging of Bloch surface waves on silicon nitride one-dimensional photonic crystals.

We perform a near-field mapping of Bloch Surface Waves excited at the truncation interface of a planar silicon nitride multilayer. We directly determine the field distribution of Bloch Surface Waves along the propagation direction and normally to the surface. Furthermore, we present a direct measurement of a near-field enhancement effect under particular coupling conditions. Experimental evidence demonstrates that a approximately 10(2) near-field intensity enhancement can be realistically attained, thus confirming predictions from rigorous calculations.

Analysis of mode coupling due to spherical defects in ideal fully metal-coated scanning near-field optical microscopy probes.

We investigate the effect of defects in the metal-coating layer of a scanning near-field optical microscopy (SNOM) probe on the coupling of polarization modes using rigorous electromagnetic modeling tools. Because of practical limitations, we study an ensemble of simple defects to identify important trends and then extrapolate these results to more realistic structures. We find that a probe with many random defects will produce a small but significant coupling of energy between a linearly polarized input mode and a radial/longitudinal polarization mode, which is known to produce a strongly localized emitted optical field and is desirable for SNOM applications.

Near-field characterization of propagating optical modes in photonic crystal waveguides.

We analyze the propagating optical modes in a Silicon membrane photonic crystal waveguide, based on subwavelength-resolution amplitude and phase measurements of the optical fields using a heterodyne near-field scanning optical microscope (H-NSOM). Fourier analysis of the experimentally obtained optical amplitude and phase data permits identification of the propagating waveguide modes, including the direction of propagation (in contrast to intensity-only measurement techniques). This analysis reveals the presence of two superposed propagating modes in the waveguide. The characteristics of each mode are determined and found to be consistent with theoretical predictions within the limits of fabrication tolerances. An analysis of the relative amplitudes of these two modes as a function of wavelength show periodic oscillation with a period of approximately 3.3 nm. The coupling efficiency between the ridge waveguide and the photonic crystal waveguide is also estimated and found to be consistent with the internal propagating mode characteristics. The combination of high-sensitivity amplitude and phase measurements, subwavelength spatial resolution, and appropriate interpretive techniques permits the in-situ observation of the optical properties of the device with an unprecedented level of detail, and facilitates the characterization and optimization of nanostructure-based photonic devices and systems.

Optical properties of microfabricated fully-metal-coated near-field probes in collection mode.

A study of the optical properties of microfabricated, fully-metal-coated quartz probes collecting longitudinal and transverse optical fields is presented. The measurements are performed by raster scanning the focal plane of an objective, focusing azimuthally and radially polarized beams by use of two metal-coated quartz probes with different metal coatings. A quantitative estimation of the collection efficiencies and spatial resolutions in imaging both longitudinal and transverse fields is made. Longitudinally polarized fields are collected with a resolution approximately 1.5 times higher as compared with transversely polarized fields, and this behavior is almost independent of the roughness of the probe's metal coating. Moreover, the coating roughness is a critical parameter in the relative collection efficiency of the two field orientations.

Wide-field-of-view GaAs/AlxOy one-dimensional photonic crystal filter.

The design, fabrication, and characterization of a one-dimensional photonic crystal optical filter that has a relatively narrow, flat-topped passband within a wide stop band and small angular sensitivity is presented. The filter is based on a one-dimensional photonic crystal structure that has multiple defects, facilitating simultaneous minimization of the angular sensitivity and optimization of the passband's characteristics. We use epitaxially grown and selectively oxidized GaAs/AlxOy multilayers to achieve a high-index-contrast material system and incorporate the experimentally determined optical and material properties into the design of the device. A flat-topped bandpass filter with a bandwidth of 65 nm and a wide field of view of 50 degrees is experimentally characterized and compared with the design predictions.

Design and characterization of a tunable polarization-independent resonant grating filter.

We present the design, analysis and characterization of a polarization-independent tunable resonant grating filter. Polarization independence is achieved by setting the plane of incidence parallel to the grating grooves and optimizing the fill factor to obtain a strong reflection peak for all incident polarization states. Experimental measurements show that approximate angular insensitivity to the input polarization orientation concurrent with tunability over a wavelength range of roughly 1530 nm to 1560 nm is achieved. Modulation of the reflectivity peak shape with variations in the orientation of the incidence plane are observed, and found to be in qualitative agreement with theoretical predictions.

Fabrication of two-dimensional photonic crystals with controlled defects by use of multiple exposures and direct write.

We have developed an approach for relatively rapid and easy fabrication of large-area two-dimensional (2-D) photonic crystal structures with controlled defects in the lattice. The technique is based on the combination of two lithographic steps in UV-sensitive SU-8 photoresist. First, multiple exposures of interference fringes are used in combination with precise rotation of the sample to define a 2-D lattice of holes. Second, a strongly focused UV laser beam is used to define line-defect waveguides by localized exposure in the recorded but not yet developed lattice from the first step. After development, the mask is transferred into a GaAs substrate with dry etching in chemically assisted ion-beam etching.

Analysis of enhanced second-harmonic generation in periodic nanostructures using modified rigorous coupled-wave analysis in the undepleted-pump approximation.

We present an extension of the rigorous coupled-wave analysis technique to analyze second-harmonic generation (SHG) in periodic optical nanostructures in the undepleted-pump approximation. We apply this method to analyze SHG in two example nanostructures for which we predict enhanced nonlinearity due to transverse near-field localization of the fundamental optical field in the nonlinear material. First, we examine a periodic nanostructure that yields up to twice the transmitted SHG intensity output compared with the bulk nonlinear material but only for small nanostructure depths because of mismatch of the fundamental and second-harmonic mode phase velocities. Second, we develop and analyze a modified nanostructure and find that this nanostructure concurrently achieves transverse localization and phase matching for SHG. In principle, this permits an arbitrary coherent interaction length, and for several specific nanostructure depths we predict a transmitted SHG intensity output more than two orders of magnitude greater than that of the bulk material.

Wide-field-of-view narrow-band spectral filters based on photonic crystal nanocavities.

We describe a novel approach to implementing wide-field-of-view narrow-band spectral filters, using an array of resonant nanocavities consisting of periodic defects in a two-dimensional three-material photonic-crystal nanostructure. We analyze the transmissivity of this type of filter for a range of wavelengths and in-plane incidence angles as a function of the defect's refractive index, the number of layers in the photonic-crystal reflectors, and the period of the defects and find that this structure diminishes the angular sensitivity of the resonance condition relative to that of a standard multilayer filter.