Fabrication of disk droplets and evaluation of their lasing action.

Disk resonators are difficult to create with droplets, since they self-form spheres due to the surface tension. In this study, disk (cylindrical) droplets were created by enclosing a dye (rhodamine 6G) solution in silicone rubber. Lasing actions of these droplets were examined by pulsed green laser excitation. In a large droplet (2 mm diameter), the whispering gallery mode emission was difficult to attain, since it competed with the radial or axial modes that made a round trip in the droplet. A disk droplet of 150 µm diameter exhibited a comb-like spectrum of the whispering gallery mode resonant emission.

Self-enhancement and suppression of optical pulses by use of photochromism in elastomer.

When violet or green pulses were launched into an elastomer containing photochromic diarylethene, two competitive absorption bands emerged at around 400 and 520 nm. The violet pulses suppressed the former and enhanced the latter, whereas the green pulses induced the opposite reaction. Consequently, these signals self-formed their optical path in the elastomer (self-enhancement). By contrast, blue pulses exhibited either a self-enhancement or suppression characteristic depending on whether the elastomer had been irradiated by the violet or green signal before the blue signal transmission. Measurement of the transient spectra during the irradiation process revealed that the blue photons were absorbed by both 400 and 520 nm bands inducing two competitive photochromic isomerizations simultaneously.

A spatial light modulator that uses scattering in a cholesteric liquid crystal.

When a cholesteric liquid crystal (helical pitch: 5 µm) was sandwiched between two glass plates with no alignment coating (gap: 20 µm), a random-domain texture appeared and a strong light scattering took place. This translucent texture turned to a transparent homeotropic phase when an electric voltage of 20 V was applied to the liquid crystal layer. This phase transition was used for constructing a spatial light modulator that needed no polarizers. Indium-tin-oxide electrodes (0.8 mm square) were arranged on a glass substrate to create a 20 × 20 pixel array (20 mm square). The liquid crystal was injected into a gap (20 µm thickness) between this substrate and another glass plate with a uniform electrode (ground). The transmittance of the pixels was originally below 10% and decreased to 0% by 7 V application because of increase in the scattering loss. As the voltage was raised, the transmittance increased gradually in the 7-17 V range and then rapidly in the 17-20 V range, attaining 40% at 27 V. Various transmittance distributions or gray-scale images were attainable by applying a suitable voltage (7-27 V) to each pixel. The transmission range of this spatial light modulator extended from ultraviolet (350 nm) to infrared wavelengths (>800 nm). Owing to this wide transmission range as well as capability of the polarizer-free operation, this spatial light modulator is useful to control a lamp spectrum in spectroscopic measurements.

Fade-resistant photochromic reactions in a self-healable polymer.

Molecular diffusion in a polymer matrix was studied to prevent degradation of photochromic reactions during repeated coloration-decoloration processes. Photochromic diarylethene was dispersed in polydimethylsiloxane (cured polymer), since it promoted exchange of damaged and fresh molecules owing to high diffusivity. The diffusion coefficient was evaluated by measuring a distribution of dye molecules that were colored within a narrow laser beam path. Temporal change of the distribution fitted well to theoretical curves that were drawn according to the 2-D solutions of Fick's equation. The experimental results indicated a fifteen-fold enhancement of the diffusion coefficient (0.0015 mm(2)/s) when the polymer was swollen with toluene. Fading of this photochromic polymer was examined by repeating alternative irradiation of violet and green laser beams. Although a non-swollen polymer faded seriously within 1,000 photochromic cycles, a swollen polymer exhibited an excellent photochromic function even after 30,000 cycles.

Deformable silicone grating fabricated with a photo-imprinted polymer mold.

A tunable transmission grating was fabricated by molding a silicone elastomer (polydimethylsiloxane). Its optical characteristics were then evaluated during compression. For fabrication, a glass plate with a photoimprinted polymer grating film was used as a mold. Both the grating period and diffraction transmittance of the molded elastomer were functions of the compressive stress. The grating period changed from 3.02 to 2.86 µm during compressing the elastomer in the direction perpendicular to the grooves.

Stabilization of photochromic isomers by copper nanoparticles in a high-diffusivity solid matrix.

Nanoparticles that are dispersed in a solid matrix have no effect on co-dispersed molecules unless they happen to be located close to one another. A polymer with a high diffusion coefficient allows molecules to circulate in the matrix to find a particle that enhances their function. A polymer composite containing copper nanoparticles was created by a low-temperature, nonvacuum process. That is, laser ablation was conducted in polydimethylsiloxane oil for both anti-oxidation and efficient particle collection, and then this suspension was cured to create a metal-polymer composite. This composite enhanced optical functions of a photochromic dye owing to its high diffusivity.

Integration of liquid crystal elements for creating an infrared Lyot filter.

An infrared Lyot filter was fabricated by integrating a polarization beam splitter and two retarders into a single device. A liquid crystal layer was constructed between two silicon pentaprisms that were designed suitably so that light was incident on this layer at 28°. At this angle, the liquid crystal transmitted p-polarized light (Brewster's angle) and reflected s-polarized light (total internal reflection). The p- or s-polarized light was directed to another liquid crystal layer (retarder) between the prism and a mirror, which induced a wavelength-dependent retardation in the reflection process. Consequently, the light that returned to the beam splitter was transmitted or reflected depending on wavelength.

Design and fabrication of an achromatic infrared wave plate with Sb-Ge-Sn-S system chalcogenide glass.

We designed and fabricated an achromatic infrared wave plate. To examine its phase retardation characteristics, the birefringence was calculated using the effective medium theory. A wave plate with a subwavelength grating was fabricated by direct imprint lithography on a low toxic chalcogenide glass (Sb-Ge-Sn-S system) based on calculated results. As a result of imprinting onto chalcogenide glass by a glassy carbon mold, a grating with 1.63 µm depth, a fill factor of 0.7, and a 3 µm period was obtained. The phase retardation of the elements reached around 30° in the 8.5-10.5 µm wavelength range. The fabrication of the infrared wave plate is less costly compared with conventional crystalline wave plates.

Fabrication of a mid-IR wire-grid polarizer by direct imprinting on chalcogenide glass.

A mid-IR wire-grid polarizer with a 500 nm pitch was fabricated on a low toxic chalcogenide glass (Sb-Ge-Sn-S system) by the thermal imprinting of periodic grating followed by the thermal evaporation of Al metal. After imprinting, deposition of Al on the grating at an oblique angle produced a wire-grid polarizer. The fabricated polarizer showed polarization with TM transmittance greater than 60% at 5-9 µm wavelengths and an extinction ratio greater than 20 dB at 3.5-11 µm wavelengths. This polarizer with a high extinction ratio can be fabricated more simply and less expensively than conventional IR polarizers.

Silicon-based liquid-crystal cell for self-branching of optical packets.

Self-controlled photonic switching was achieved by combining the photoconductivity of a semiconductor and the electrical tunability of a liquid crystal (LC). Pulse packets of 1.06 µm wavelength created free carriers in a silicon electrode of an LC cell, which triggered voltage application for LC reorientation. Consequently, polarization direction of the succeeding packets became perpendicular to that of the preceding packets, and they progressed in different directions after passing through a polarization beam splitter. The cascade LC cells divided a series of packets one by one in a self-controlled manner.

Infrared wire-grid polarizer with Y2O3 ceramic substrate.

Using two-beam interference lithography and dry etching, we fabricated a mid-IR wire-grid polarizer consisting of a 350 nm pitch WSi grating on an Y(2)O(3) ceramic substrate, which has wider transparency than sapphire. The transmittance of TM polarization was greater than 70% in the 3-7 µm wavelength range without antireflection films, and the extinction ratio was over 20 dB in the 2.5-5 µm wavelength range. The wire-grid polarizer with the Y(2)O(3) ceramic substrate provides high durability and good IR transparency.

Terahertz wire-grid polarizers with micrometer-pitch Al gratings.

We fabricated a terahertz wire-grid polarizer consisting of a micrometer-pitch Al grating on a Si substrate by photolithography and wet etching. The ratio of TM and TE transmittances (extinction ratio) was over 35 dB at 0.5 THz. At the Brewster angle of the Si substrate, the polarization transmittance of a TM wave through the fabricated polarizer exceeded 95% and the extinction ratio was over 45 dB at approximately 1 THz. The fabricated polarizer has a higher extinction ratio than conventional free-standing terahertz wire-grid polarizers.

Transmittance enhancement of a wire-grid polarizer by antireflection coating.

We examined the effect of an antireflection (AR) coating to enhance the TM transmittance of the wire-grid polarizer. The polarization transmission spectra were calculated using the rigorous coupled-wave analysis. As a result, we verified that an AR film should be inserted between a wire-grid and a Si substrate as regards the TM transmittance and the polarization function. Based on the simulation results, we fabricated a tungsten silicide (WSi) wire-grid polarizer with SiO films on both sides of the Si substrate. The transmittance exceeded 80% at a 4-5 microm wavelength range, although the theoretical transmittance of Si substrate is 54% and the ratio of the TM and TE transmittances reached 24 dB at a 3 microm wavelength when the WSi grating has a 300 nm thickness, a 400 nm period, and a fill factor of 0.6. Wire-grid polarizers with higher transmittance and larger extinction ratio can be obtained by adjusting the AR film thickness, the fill factor, and the thickness of the WSi grating.

Modeling, fabrication, and characterization of tungsten silicide wire-grid polarizer in infrared region.

We designed and fabricated a tungsten silicide wire-grid polarizer. To examine its polarization characteristics, the transmission spectra of the polarizer were simulated using the effective medium theory. The polarizer was fabricated based on the simulation results. The transverse magnetic (TM) polarization transmittance of the fabricated polarizer was greater than 50% over the 5 mum wavelength, and the ratio of TM and transverse electric transmittance was greater than 100 (20 dB) in the infrared range. This fabricated polarizer has higher durability and better compatibility with microfabrication processes than conventional infrared polarizers.

Photochromism induced by infrared two-photon absorption.

An IR laser of 940 nm wavelength induced photochromic reaction in acrylate that contained both spirobenzopyran and rare-earth doped oxide (Gd(2)O(2)S:YbEr). The rare-earth elements were excited by two 940 nm photons and emitted a 550 nm photon, which caused photochromic isomerization of spirobenzopyran. This acrylate turned to its original orange color by either thermal relaxation or ultraviolet irradiation, and was bleached again by IR laser irradiation.

Tunable whispering gallery mode emission from a microdroplet in elastomer.

A fluorescent microdroplet was formed in elastomer to facilitate handling and wavelength tuning. A methanol solution of rhodamine 6G was put into a solidifying polysiloxane resin with a needle. Addition of surfactant was effective to stabilize the droplet. Being excited by a laser pulse (532 nm, > 50 microJ/mm(2), 5 ns), the droplet exhibited a whispering gallery mode emission in the 570-610 nm wavelength range. The resonance peaks shifted as the droplet diameter expanded by elastomer deformation.

Mid-infrared wire-grid polarizer with silicides.

An infrared (IR) polarizer with tungsten silicide (WSi) wire grid was fabricated by two-beam interference exposure and reactive ion etching. To enhance TM transmittance, silicon monoxide was deposited between the WSi wire grid (400 nm period) and a Si substrate. The transmittance was over 80% in the 4-5 microm wavelength range. The ratio of TM and TE transmittances was over 100 (20 dB) in the 2.5-6 microm wavelength range. The fabricated polarizer has higher durability and better compatibility with microfabrication processes compared with conventional IR polarizers.

Time-resolved infrared spectrometry with a focal plane array and a galvano-mirror.

Time-resolved spectrometry was conducted in the mid-infrared region (2.8-4.6 microm). A galvano-mirror causes a spectrally dispersed beam to repeatedly sweep up and down a PtSi focal plane array so that each element of the array is struck by the beam only once at some point within a time frame. Transient spectra of flowing gases (hydrocarbon and carbon dioxide) were measured at 80 micros intervals.

Full-color illumination that needs no electric power.

Red, green, and blue phosphorescent resins were developed to realize full-color illuminators that shine without the need for electric power. Red phosphorescence, which used to be weak with conventional red phosphors, was enhanced notably with dye-doped resins in which green or blue phosphors were dispersed uniformly; i.e., brightness increased by seven times and afterglow duration extended by two times. Full-color phosphorescence from violet to red was attained by selecting suitable mixing ratios of the phosphors and dyes.

Photochromic isomerization of spirobenzopyran in nanoholes of anodic alumina.

A notable change in the photochromic characteristics was observed when the benzene solution of spirobenzopyran was put in nanoholes of anodic alumina. The absorption peak that appeared in the ultraviolet irradiation process shifted to a shorter wavelength, and the decay time of the decoloration process became approximately 200 times longer than that of the original solution. After a preservation period of several days, however, both the absorption wavelength and the decay time recovered to those of the original solution. These experimental results suggest that the photochromic isomerization in the alumina nanoholes is affected by the large surface area of the matrix rather than the limited free volume.