Bloch-surface-wave resonance in a cavity resonator for focusing retroreflection.

A wavelength-selective retroreflector for a diverging wave will be an attractive external mirror for compact wavelength-stabilized semiconductor lasers and consists of a focusing grating coupler and a cavity resonator. In this Letter, the retroreflector utilizing a Bloch surface wave (BSW) resonance was theoretically investigated for improving the retro-reflectance in comparison to a previously reported structure utilizing guided-mode resonance (GMR). A retroreflector with an aperture size of 31 µm and focal length of 67 µm was designed for an operation wavelength of 1550 nm. The maximum retro-reflectance is predicted by numerical simulation to be higher by 11% than that of the GMR-based retroreflector.

Reflection characteristics of a cavity-resonator-integrated guided-mode resonance mirror for a microdiameter Gaussian beam.

A guided-mode resonance mirror was designed for reflecting a vertically incident Gaussian beam of 3.6-µm beam waist to a backpropagating Gaussian beam. A grating coupler (GC) is integrated in a waveguide resonance cavity consisting of a pair of distributed Bragg reflectors (DBRs) on a reflection substrate. An incident free-space wave is coupled by the GC into the waveguide, and the guided wave is resonated in the waveguide cavity and coupled out by the same GC to a free-space wave simultaneously in resonance condition. The reflection phase can vary by 2π rad, according to wavelength in a wavelength band of resonance. The grating fill factors of the GC were apodized to have a Gaussian profile in its coupling strength and resultantly maximize a Gaussian reflectance defined by the power ratio of backpropagating Gaussian beam to the incident Gaussian beam. The fill factors of the DBR were also apodized in the boundary zone to the GC in order to avoid discontinuity in equivalent refractive index distribution and resultant scattering loss. Guided-mode resonance mirrors were fabricated and characterized. The Gaussian reflectance of the mirror with the grating apodization was measured to be 90%, higher by 10% than that of the mirror without apodization. It is also demonstrated that the reflection phase changes more than π rad within wavelength band of 1 nm. The fill factor apodization narrows the resonance band.

Wavelength division multiplexer based on cavity-resonator-integrated guided-mode resonance filters for a compact multi-wavelength light source.

Cavity-resonator-integrated guided-mode resonance filters (CRIGF) consisting of a grating coupler in a waveguide resonator formed by two distributed Bragg reflectors of different reflectances can act as a wavelength-selective reflector and an input waveguide coupler for an incident free-space wave. Integration of CRIGFs in a waveguide is proposed to give an array of external mirrors and a wavelength division multiplexer for constructing a compact multi-wavelength light source. Four CRIGFs of 10-µm-size aperture with a wavelength spacing of 15 nm were designed and fabricated. The reflectance of 62% and output efficiency of higher than 18% were theoretically predicted. Multiplexing of four wavelengths was confirmed experimentally.

Flat-top narrowband filters enabled by guided-mode resonance in two-level waveguides.

Resonant nanogratings and periodic metasurfaces express diverse spectral and polarization properties on broadside illumination by incident light. Cooperative resonance interactions may yield shaped spectra for particular applications, in contrast to a multilayer dielectric mirror. Here, we provide guided-mode resonance filters with flat-top spectra suitable for wavelength division multiplexing systems. Applying a single one-dimensional grating layer sandwiched by two waveguides, we theoretically achieve high-efficiency flat-top spectra in the near-infrared region. This result is obtained by inducing simultaneous nearly degenerate resonant modes. The resonance separation under this condition controls the width of the flat-top spectrum. This means we can implement spectral widths ranging from a sub-nanometer to several nanometers applying fundamentally the same device architecture.

Cavity-resonator-integrated guided-mode resonance band-stop reflector.

A cavity-resonator-integrated guided-mode resonance filter (CRIGF) consists of a grating coupler inside a pair of distributed Bragg reflectors. A combination of a CRIGF with a high-reflection substrate can provide a new type of a band-stop reflector with a small aperture for a vertically incident wave from air. A narrow stopband was theoretically predicted and experimentally demonstrated. It was quantitatively shown that reflection spectra depended on optical-buffer-layer thickness. The reflector of 10-µm aperture was fabricated and characterized. The extinction ratio in reflectance was measured to be lower than -20 dB at a resonance wavelength. The bandwidth at -3 dB was 0.15 nm.

Determination of cavity length of cavity-resonator-integrated guided-mode resonance filter.

A cavity-resonator-integrated guided-mode resonance filter is a kind of narrowband filters, which uses a resonance effect of a waveguide cavity. Two experimental methods for determining the cavity length were investigated in order to estimate the response time of the filter. SiO(2)-based filters for operation at 1540-nm wavelength were fabricated and their cavity lengths were determined from measured resonance wavelengths. In the both of methods, the cavity length determined to be 65 µm and the response time was estimated to be 4 psec.

Reflection characteristics of guided-mode resonance filter combined with bottom mirror.

A new type of mirror, based on guided-mode resonance, was proposed and discussed to provide a mirror having high reflectance and large wavelength dependence of reflection phase variation. The proposed mirror consists of a surface grating integrated in a channel waveguide on a high-reflection layer. A SiO2-based device was fabricated for 0.85-µm wavelength operation, and reflection phase variation of almost π, with wavelength change of sub-nanometers, was confirmed experimentally.

Cavity-resonator-integrated guided-mode resonance filter for aperture miniaturization.

A guided-mode resonance filter integrated in a waveguide cavity resonator constructed by two distributed Bragg reflectors is designed and fabricated for miniaturization of aperture size. Reflection efficiency of >90% and wavelength selectivity of 0.4 nm are predicted in the designed SiO(2)-based filter with 50-µm aperture by a numerical calculation using the finite-difference time-domain method. A maximum reflectance of 67% with 0.5-nm bandwidth is experimentally demonstrated by the fabricated device at around 850-nm wavelength.

Ultra-small, self-holding, optical gate switch using Ge2Sb2Te5 with a multi-mode Si waveguide.

We report a multi-mode interference-based optical gate switch using a Ge(2)Sb(2)Te(5) thin film with a diameter of only 1 µm. The switching operation was demonstrated by laser pulse irradiation. This switch had a very wide operating wavelength range of 100 nm at around 1575 nm, with an average extinction ratio of 12.6 dB. Repetitive switching over 2,000 irradiation cycles was also successfully demonstrated. In addition, self-holding characteristics were confirmed by observing the dynamic responses, and the rise and fall times were 130 ns and 400 ns, respectively.

Polarization-independent guided-mode resonance filter with cross-integrated waveguide resonators.

A cavity-resonator-integrated guided-mode resonance filter (CRIGF) has been proposed and investigated in order to realize high-efficiency narrowband reflection with a small aperture. The CRIGF consists of a grating coupler integrated in a cavity resonator constructed by a pair of distributed Bragg reflectors on a thin-film waveguide. This time, orthogonally crossed integration of two CRIGFs was demonstrated in order to obtain polarization-independent reflection spectrum. An SiO2-based device with 10 µm aperture was designed and fabricated for around 850 nm wavelength operation, and narrowband polarization-independent reflection was confirmed experimentally.

Examination of structure and optical properties of Ce3+-doped strontium borate glass by regression analysis.

Amorphous materials with non-periodic structures are commonly evaluated based on their chemical composition, which is not always the best parameter to evaluate physical properties, and an alternative parameter more suitable for performance evaluation must be considered. Herein, we quantified various structural and physical properties of Ce-doped strontium borate glasses and studied their correlations by principal component analysis. We found that the density-driven molar volume is suitable for the evaluation of structural data, while chemical composition is better for the evaluation of optical and luminescent data. Furthermore, the borate-rich glasses exhibited a stronger luminescence due to Ce3+, indicating a higher fraction of BO3/2 ring and larger cavity. Moreover, the internal quantum efficiency was found to originate from the local coordination states of the Ce3+ centres, independent of composition or molar volume. The comparison of numerical data of the matrix is useful not only for ensuring the homogenous doping of amorphous materials by activators, but also for determining the origin of physical properties.

Potential characterization of free-space-wave drop demultiplexer using cavity-resonator-integrated grating input/output coupler.

A prototype free-space-wave drop demultiplexer consisting of a cavity-resonator-integrated grating input/output coupler (CRIGIC) and a different-guided-mode-coupling distributed Bragg reflector (DGM-DBR) was designed for constructing a high-density wavelength-division-multiplexing intra-board chip-to-chip optical interconnection. The CRIGIC consists of one grating coupler and two DBRs, and can vertically couple a guided wave and a free-space wave with high efficiency. A two-channel drop demultiplexer operating at around 850-nm wavelength with 5-nm channel spacing in wavelength was fabricated in a thin-film SiO2-based waveguide. The device performance was predicted theoretically, characterized experimentally, and discussed.

Low-crosstalk 2 x 2 thermo-optic switch with silicon wire waveguides.

We demonstrate a low-crosstalk 2 x 2 thermo-optic switch with silicon wire waveguides. The device is based on a 2 x 2 array of Mach-Zehnder interferometer (MZI) switches. Lowest crosstalk levels of -50 dB and -30 dB are obtained for 'bar' and 'cross' switching states, respectively. An intersection in the switch is important for low-crosstalk operation. The power consumption of one MZI element switch is 40 mW and the total power consumption of the device is at most 160 mW.

Ultrafast nonlinear effects in hydrogenated amorphous silicon wire waveguide.

We, for the first time, present the ultrafast optical nonlinear response of a hydrogenated amorphous silicon (a-Si:H) wire waveguide using femtosecond pulses. We show cross-phase and cross-absorption modulations measured using the heterodyne pump-probe method and estimate the optical Kerr coefficient and two-photon absorption coefficient for the amorphous silicon waveguide. The pumping energy of 0.8 eV is slightly lower than that required to achieve two-photon excitation at the band gap of a-Si:H (approximately 1.7 eV). An ultrafast response of less than 100 fs is observed, which indicates that the free-carrier effect is suppressed by the localized states in the band gap.

Cavity-resonator-integrated grating input/output coupler for high-efficiency vertical coupling with a small aperture.

A cavity-resonator-integrated grating input/output coupler (CRIGIC) is designed to operate at about 850 nm wavelength for high-efficiency vertical coupling of a guided wave and a free-space wave with a small aperture. The CRIGIC consists of a grating coupler and a waveguide cavity resonator constructed by two distributed Bragg reflectors. A coupling efficiency of 96% with a 3 dB bandwidth of 1.2 nm is predicted by a theoretical calculation. An output coupling efficiency of about 60% is experimentally demonstrated on a 20 microm aperture device, fabricated in a thin-film SiO(2)-based waveguide on a substrate with an Au reflection layer, for what we believe to be the first time.

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.

Design of resonance grating coupler.

A new integrated-optic coupler was proposed for coupling a guided wave to a free-space wave propagating vertically from the waveguide surface. The coupler consists of a grating coupler in a cavity formed by two distributed Bragg reflectors, and has a small aperture. The cavity was designed to eliminate both transmission and reflection of the incident guided wave, resulting in 100% radiation by a several-micron aperture. Design consideration was theoretically discussed based on the coupled mode analysis. Predicted performance was simulated and confirmed by the finite difference time domain method.

Optical microscopic observation of fluorescence enhanced by grating-coupled surface plasmon resonance.

On the substrate carrying a sub-wavelength grating covered with a thin metal layer, a fluorescent dye-labeled cell was observed by fluorescence microscope. The fluorescence intensity was more than 20 times greater than that on an optically flat glass substrate. Such a great fluorescence enhancement from labeled cells bound to the grating substrate was due to the excitation by grating coupled surface plasmon resonance. The application of a grating substrate to two-dimensional detection and fluorescence microscopy appears to offer a promising method of taking highly sensitive fluorescence images.

Optimal Structure of a Plasmonic Chip for Sensitive Bio-Detection with the Grating-Coupled Surface Plasmon-Field Enhanced Fluorescence (GC-SPF).

Surface plasmon field-enhanced fluorescence (SPF) has been one of the powerful tools for biosensors and bioimaging. A wavelength-scale periodic structure coated with a thin metal film is called a plasmonic chip, and it can provide SPF. SPF of Cy5-streptavidin (Cy5-SA) was measured on a biotinylated plasmonic chip with a grating of 480 nm-pitch. The optimal structure of a plasmonic sensor-chip was designed for improving detection sensitivity. The silver film thickness dependence of the SPF intensity was measured under the irradiation of the top panel of a sensor chip. Furthermore, the dependence of the SPF intensity on the distance from the metal surface was also investigated. The optimal structure for the largest fluorescence enhancement factor was 150 nm-thick silver and 10 nm-thick SiO2 layers due to the enhanced electric field (excitation field), the surface plasmon coupled emission (SPCE), and the interference effect with reflected light. The largest enhancement factor was found to be 170-fold. Furthermore, not only the largest fluorescence intensity but also stable lower background noise were found to be essential for higher-sensitive detection.

Simultaneous interference exposure of different-period DBRs for intra-board WDM optical interconnection.

Integration of different-period distributed Bragg reflectors (DBRs) is required in constructing an intra-board optical interconnection device with wavelength division multiplexing (WDM). Interference exposure method with cylindrical Lloyd mirror optics and mask aligner was discussed for integrating those DBRs simultaneously. DBRs were designed to give coupling efficiency higher than 80% with coupling length of 0.6 mm and crosstalk noise less than -10 dB with 3 nm wavelength separation for optical interconnection using eight wavelengths around 850 nm. Interference exposure system was developed and integration of eight DBRs by two times exposure was demonstrated.