Effects of modulation strength in guided-mode resonant subwavelength gratings at normal incidence

Miniature Spectroscopes with Two-Dimensional Guided-Mode Resonant Metal Grating Filters Integrated on a Photodiode Array

A small spectroscope with 25 color sensors was fabricated by combining metamaterial color filters and Si photodiodes. The metamaterial color filters consisted of guided-mode resonant metal gratings with subwavelength two-dimensional periodic structures. Transmittance characteristics of the color filters were designed to obtain peak wavelengths proportional to grating periods. For each color sensor, a peak wavelength of the spectral sensitivity could be tuned in the range of visible wavelengths by adjusting each grating period. By performing spectrum reconstruction using Tikhonov regularization, the spectrum of an incident light was obtained from the signal of photodiodes. Several monochromatic lights were made incident on the fabricated device and the spectral characteristics of the incident light were reconstructed from the output signals obtained from the respective color sensors. The peak wavelengths of the reconstructed spectra were in good agreement with the center wavelengths of the monochromatic lights.

Optical notch filter with tunable bandwidth based on guided-mode resonant polarization-sensitive spectral feature

A novel bandwidth-tunable notch filter is proposed based on the guided-mode resonance effect. The notch is created due to the superposition spectra response of two guided-mode resonant filters. The compact, bandwidth tuning capability is realized by taking advantage the effect of spectra-to-polarization sensitivity in one-dimensional classical guided-mode resonance filter, and using a liquid crystal polarization rotator for precise and simple polarization control. The operation principle and the design of the device are presented, and we demonstrate it experimentally. The central wavelength is fixed at 766.4 nm with a relatively symmetric profile. The full width at half maximum bandwidth could be tuned from 8.6 nm to 18.2 nm by controlling the applied voltage in electrically-driving polarization rotator.

Multi-periodic nanostructures for photon control

We propose multi-periodic nanostructures yielded by superposition of multiple binary gratings for wide control over photon emission in thin-film devices. We present wavelength- and angle-resolved photoluminescence measurements of multi-periodically nanostructured organic light-emitting layers. The spectral resonances are determined by the periodicities of the individual gratings. By varying component duty cycles we tune the relative intensity of the main resonance from 12% to 82%. Thus, we achieve simultaneous control over the spectral resonance positions and relative intensities.

Triple-layer guided-mode resonance Brewster filter consisting of a homogenous layer and coupled gratings with equal refractive index

A triple-layer guided-mode resonance Brewster filter consisting of a homogeneous layer and two identical gratings with their refractive indices equal to that of the homogeneous layer is presented. The spectral properties of this filter are analyzed based on the coupling modulation of two identical binary gratings at Brewster angle for a TM-polarized wave. The grating layer between substrate and homogeneous layers can significantly change the linewidth and resonant mode position, which are due to the asymmetric field distribution inside the grating layers. The tunability of the resonance can be altered on different resonant channels and a practical filter can be obtained in TM2 waveguide mode. Variation of filling factor can alter the field localization in the grating structure and significantly adjust the linewidth of the filter.

Enhancing the resonance quality factor in membrane-type resonant grating waveguides

In this Letter, we present a method of reducing the spectral width of guided-mode resonance (GMR) in air-bridged resonant grating-waveguide structures to enhance the Q factor. The posttreatment of adding a dielectric film to the bottom of the membrane to manipulate the resonance behavior is practicable. The introduced underlayer is shown to be capable of effectively reducing the coupling and enhancing the resonant Q factor. The proposed method provides an effective means of adjusting the resonance property without varying the original GMR structure. The results also imply that TM resonance is more feasible for achieving narrow resonance and potentially in sensing applications, because it has higher sensitivity than TE resonance.

Ultracompact silicon/polymer laser with an absorption-insensitive nanophotonic resonator

A planar nanophotonic Fabry-Perot-like resonator that can defy strong absorption of about 20 000 cm(-1) in the cavity material is demonstrated. Visible laser emission is observed from two silicon subwavelength-sized high index contrast gratings with embedded polymer gain material. The size of the laser is reduced by an order of magnitude compared to established designs based on photonic bandgap structures. As silicon constitutes the most common carrier for electronics, the cost-efficient integration of compact laser sources for visible wavelengths comes within reach.

Large incident angle tolerance of guided-mode resonant gratings by light coupling via waveguide end faces

By designing and fabricating appropriate structures, guided-mode resonant gratings can be used as optical filters to attain extremely narrow bandwidths. This high performance makes it difficult to couple light into a waveguide via the grating, which demands extremely high mechanical accuracy to adjust the incident conditions. This paper shows both numerically and experimentally that the incident angle tolerance is significantly wider when the incident light is coupled into the waveguide through an end face rather than via the grating.

Integrated 2D photonic crystal stack filter fabricated using nanoreplica molding

The design, fabrication, and characterization of an integrated 2D photonic crystal stack are described for application as optical filters with improved optical density and angle tolerance compared to single photonic crystal slabs. The 2D photonic crystals are designed as polarization independent reflectance filters with a narrow spectral bandwidth centered at lambda=532 nm by utilizing the guided mode resonance effect. Up to three photonic crystal layers are vertically stacked upon a single plastic substrate by using repeated nanoreplica molding process steps, with no alignment required between stacked layers. The photonic crystal stack filters achieve optical density of 2.24 with an angular tolerance of 14.8 degrees.

Band gaps and leaky-wave effects in resonant photonic-crystal waveguides

The paper addresses the leaky stop bands associated with resonant photonic crystal slabs and periodic waveguides. We apply a semianalytical model pertinent to the second band to compute the dispersion curves describing the leaky stop band and verify its correctness by rigorous band computations. This approximate model provides clear insights into the physical properties of the leaky stop band in terms of explicit analytical expressions found. In particular, it enables comparison of the structure of the bands computed in complex propagation constant, implying spatially decaying leaky modes, with the bands computed in complex frequency, implying temporally decaying modes. It is shown that coexisting Braggcoupling and energy-leakage mechanisms perturb the bands in complex propagation constant whereas these mechanisms are decoupled in complex frequency. As a result, the bands in complex frequency are well defined exhibiting a clear gap. These conclusions are verified by numerical diffraction computations for both weak and strong grating modulations where the resonance peaks induced by external illumination are shown to closely track the band profile computed in complex frequency. Thus, in general, phase matching to a resonant leaky mode occurs via real propagation constant that is found by dispersion computations employing complex frequency.

Angular tolerant resonant grating filters under oblique incidence

Resonant grating filters have been proposed as a promising alternative to multilayer stacks for narrowband free-space filtering. The efficiency of such filters under normal incidence has been demonstrated. Unfortunately, under oblique incidence, the limited angular tolerance of the resonance forbids any filtering applications with use of standard collimated incident beams. Using a multimode planar waveguide and a bi-atom grating, we show how to increase the angular tolerance up to the divergence of standard beams (0.2 deg) without modifying the spectral bandwidth (0.1 nm), under any oblique angle of incidence.

Gap maps and intrinsic diffraction losses in one-dimensional photonic crystal slabs

A theoretical study of photonic bands for one-dimensional (1D) lattices embedded in planar waveguides with strong refractive index contrast is presented. The approach relies on expanding the electromagnetic field on the basis of guided modes of an effective waveguide, and on treating the coupling to radiative modes by perturbation theory. Photonic mode dispersion, gap maps, and intrinsic diffraction losses of quasi guided modes are calculated for the case of self-standing membranes as well as for silicon-on-insulator structures. Photonic band gaps in a waveguide are found to depend strongly on the core thickness and on polarization, so that the gaps for transverse electric and transverse magnetic modes most often do not overlap. Radiative losses of quasiguided modes above the light line depend in a nontrivial way on structure parameters, mode index, and wave vector. The results of this study may be useful for the design of integrated 1D photonic structures with low radiative losses.

Polarization independence of a one-dimensional grating in conical mounting

We demonstrate theoretically a polarization-independent guided-mode resonant filter with only a one dimensional grating. A rigorous method, the modal method by Fourier expansion, is used to compute the diffracted efficiencies of the grating. Wave-vector analysis fails to correctly design a polarization-independent structure. We show that a rigorous analysis of the resonances must be employed to obtain such a device; using a pole approach, we study the effects of grating parameters on the resonances of both polarizations.

Evanescent resonator chips: a universal platform with superior sensitivity for fluorescence-based microarrays

In the present paper, we introduce for the first time a novel generation of a universal fluorescence transducer, the so-called evanescent resonator (ER) platform. The device comprises a transparent substrate and a thin dielectric surface layer containing sub-micron corrugated structures. The ER chip exhibits an inherent physical signal amplification due to confinement of excitation energy in the thin surface layer. Energy confinement is based on interference effects created by the abnormal reflection geometry and leads to efficient excitation of surface-bound fluorophores in the evanescent field of the chip. The evanescent resonator platform has the potential to increase the fluorescence yield of labelled biomolecules to more than 100-fold when compared with conventional microarray chips. The new ER device has been developed for analysis of nucleic acids from different species. However, it can be used with all kinds of biomolecular affinity systems. The platform combines superior sensitivity with exceptional reproducibility and ease of use. The chips are compatible with commercially available laser scanners, confocal microscopes, and portable or miniaturised CCD read-out equipment.

Normally incident resonant grating reflection filters for efficient narrow-band spectral filtering of finite beams

Resonant grating reflection filters operating at normal incidence are investigated. At normal incidence these structures are shown to have characteristics different from those of structures operated at oblique incidence. We show how higher-order diffraction within the structure laterally confines the incident energy about the point of incidence and results in a broadened angular selectivity. Multimode structures are shown to exhibit broader angular selectivities and narrower spectral linewidths than those of single-mode structures, achieving angular selectivities hundreds of times broader than what could be obtained at oblique incidence. When compared with that for oblique incidence, the increase in angular selectivity is shown to greatly improve the performance of these filters for spectrally filtering finite incident beams.

Guided-mode resonant subwavelength gratings: effects of finite beams and finite gratings

The effects of finite beams and finite gratings on the performance of guided-mode resonant subwavelength gratings are characterized by using the rigorous boundary element method. The gratings are strongly modulated, have a finite number of periods, and are illuminated by normally incident Gaussian beams. Quantitative results are presented for silicon-on-sapphire resonant gratings and gallium arsenide-aluminum arsenide resonant gratings.

Narrow-linewidth bandpass filters with diffractive thin-film layers

Bandpass filters based on guided-mode resonance effects in waveguide-grating structures are obtained by use of a genetic algorithm search-and-optimization routine. Calculated examples show that narrow linewidths, high peaks, and low sideband transmittances can be achieved in thin-film diffractive devices with few layers. A filter with a linewidth of 0.2 nm at a central wavelength of 0.55 microm is demonstrated in a two-layer-two-grating structure. At 10.6-microm wavelength, a filter consisting of a single binary grating is obtained that has a linewidth of 12.7 nm and extended, low sideband transmittance. A three-layer device with a surface relief Si grating and two underlying homogeneous layers of SiO(2) and Si yields a high-efficiency filter centered at 1.55 microm with a linewidth of 0.1 nm.