Guided-mode resonance Brewster filter

Quantitative phase contrast imaging with a nonlocal angle-selective metasurface

Phase contrast microscopy has played a central role in the development of modern biology, geology, and nanotechnology. It can visualize the structure of translucent objects that remains hidden in regular optical microscopes. The optical layout of a phase contrast microscope is based on a 4 f image processing setup and has essentially remained unchanged since its invention by Zernike in the early 1930s. Here, we propose a conceptually new approach to phase contrast imaging that harnesses the non-local optical response of a guided-mode-resonator metasurface. We highlight its benefits and demonstrate the imaging of various phase objects, including biological cells, polymeric nanostructures, and transparent metasurfaces. Our results showcase that the addition of this non-local metasurface to a conventional microscope enables quantitative phase contrast imaging with a 0.02π phase accuracy. At a high level, this work adds to the growing body of research aimed at the use of metasurfaces for analog optical computing.

Non-local metasurfaces for spectrally decoupled wavefront manipulation and eye tracking

Metasurface-based optical elements typically manipulate light waves by imparting space-variant changes in the amplitude and phase with a dense array of scattering nanostructures. The highly localized and low optical-quality-factor (Q) modes of nanostructures are beneficial for wavefront shaping as they afford quasi-local control over the electromagnetic fields. However, many emerging imaging, sensing, communication, display and nonlinear optics applications instead require flat, high-Q optical elements that provide substantial energy storage and a much higher degree of spectral control over the wavefront. Here, we demonstrate high-Q, non-local metasurfaces with atomically thin metasurface elements that offer notably enhanced light-matter interaction and fully decoupled optical functions at different wavelengths. We illustrate a possible use of such a flat optic in eye tracking for eyewear. Here, a metasurface patterned on a regular pair of eye glasses provides an unperturbed view of the world across the visible spectrum and redirects near-infrared light to a camera to allow imaging of the eye.

Optimization of tunable guided-mode resonance filter based on refractive index modulation of graphene

To fabricate a tunable optical filter with a fast response in the near infrared region, a tunable guided-mode resonance (GMR) filter using graphene was proposed and its performance was optimized. In this study, a rigorous coupled wave analysis method was employed to systematically investigate the effects of geometrical configuration of graphene-integrated GMR filters and the optical properties of constituent materials including graphene on their spectral response in terms of tunability and extinction ratio. It was found that as the graphene is located close to the waveguide and the evanescent-field strength at the interface increases, the GMR filter exhibits better tunability. The bandwidth of the filter could be drastically reduced by adopting a low-index contrast grating layer, so that the extinction ratio of an optical signal could be greatly improved from 0.91 dB to 27.99 dB as the index contrast decreased from 0.99 to 0.47, respectively. Furthermore, new practical device designs, that is easy to fabricate and effectively implement the electric-field doping of graphene at low gate voltage, were also suggested and theoretically validated. These results demonstrate not only the excellent potential of a graphene-based tunable GMR filter but also provide practical design guidelines for optimizing the device performance.

Enhancement of THz resonance using a multilayer slab waveguide for a guided-mode resonance filter

We propose a multilayer slab waveguide (SWG) to enhance the resonance of the transmittance with a guided-mode resonance (GMR) filter. The resonance characteristics of the GMR filter were studied in three types according to the method of attaching the grating film to a SWG, which consists of 25 µm thick polyethylene terephthalate (PET) film layers separated by 25 µm air layers. The resonance depth with the multilayer SWG was improved over that of the monolayer SWG because the refractive index and absorption of the multilayer SWG were reduced. However, because resonance with a high Q-factor in the monolayer SWG has a large attenuation loss due to material absorption, the resonance enhancement was insufficient even for the multilayer SWG. We were able to make the resonance depth up to 5.2 times larger than the monolayer SWG in the TE_1,1 mode using a five-layer SWG. We verified the enhancements with the multilayer SWG using a finite-difference frequency-domain (FDFD) simulation.

Multi-band optical bandpass filter with picometer bandwidth in visible spectrum formed by prism pair coupled planar optical waveguide

An optical bandpass filter structure of multiple bands in the visible spectrum with bandwidths in picometer scale (full width at half maximum, FWHM) is theoretically analyzed. The filter is in a form of prism pair coupled planar optical waveguide. For each individual guided mode, multiple pass-bands are obtainable in violet, green, and red regions of the visible spectrum, respectively. High efficiencies and symmetric band shape with flat null sideband can be obtained with practical material dispersion and loss taken into account. The central wavelength of each band can also be angularly tuned within a certain range with little influence on the efficiency or bandwidth. This kind of ultra-narrow bandpass optical filter can be used in high resolution spectroscopies, laser technologies, optical communications, and optical sensing.

Enhanced absorption of graphene monolayer with a single-layer resonant grating at the Brewster angle in the visible range

One method for enhancement and manipulation of light absorption with monolayer graphene covered on a single-layer guided mode resonant Brewster filter surface is demonstrated. By means of the rigorous coupled-wave analysis method, the effect of geometrical parameters on the optical response of the structure is investigated. It is possible to achieve a maximum absorption of 60% at the Brewster angle; dual-band optical absorption can also be realized when the depth of the grating is increased. The situation of oblique incidence for TM polarization is studied as well; the absorption property can be controlled by adjusting the incident angle without changing the structural parameters. The proposed structure has the advantage of more free geometry parameters compared to the graphene disk and ribbon, so the absorption could be tuned more flexibly.

Multiple-channel guided mode resonance Brewster filter with controllable spectral separation

In this work, a single-layer, multiple-channel guided mode resonance (GMR) Brewster filter with controllable spectral separation is proposed using the plane waveguide method and rigorous coupled-wave analysis. Based on the normalized eigenvalue equation, the controllability of the spectral separation is analyzed when the fill ratio of the grating layer is changed while its effective index is identical to that of the substrate. The location and the separation between resonances can be specifically controlled by modifying the fill ratio of the grating layer. In contrast to the ordinary GMR filter, where the location of the resonances is material dependent, it is demonstrated that the spectral separation for the first and second resonances can be linearly controlled by altering the fill ratio of the grating layer. In addition, the maximal shift of the second resonance is up to 5% of the first resonant wavelength using the single-layer Brewster filter.

Colored image produced with guided-mode resonance filter array

A method to reproduce colored images with a guided-mode resonance filter (GMRF) array is presented in this Letter. Because of their excellent characteristics, monochromatic light of the three primary colors with high purity can be achieved by using GMRF structures. Moreover, the primary colors are obtained without changing other GMRF parameters except the period, which could be realized easily with laser direct writing technology. The result shows that a colored image with high resolution and verisimilitude can be reproduced.

Temporal differentiation of optical signals using resonant gratings

We study theoretically the possibility of performing temporal differentiation of optical signals using a resonant diffraction grating. We demonstrate that the resonant grating allows the calculation of the first-order derivative of an optical signal envelope in the vicinity of waveguide resonant frequencies in the zeroth transmitted diffraction order. The grating is shown to allow the calculation of the fractional derivative of order 1/2 in the vicinity of Rayleigh-Wood anomalies. Numerical simulations based on the rigorous coupled-wave analysis of Maxwell's equations demonstrate the high-quality differentiation of optical signals with temporal features in the picosecond range.

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.

Resonant photonic biosensors with polarization-based multiparametric discrimination in each channel

In this paper, we describe guided-mode resonance biochemical sensor technology. We briefly discuss sensor fabrication and show measured binding dynamics for example biomaterials in use in our laboratories. We then turn our attention to a particularly powerful attribute of this technology not possessed by competing methods. This attribute is the facile generation of multiple resonance peaks at an identical physical location on the sensor surface. These peaks respond uniquely to the biomolecular event, thereby enriching the data set available for event quantification. The peaks result from individual, polarization-dependent resonant leaky modes that are the foundation of this technology. Thus, by modeling the binding event and fitting to a rigorous electromagnetic formalism, we can determine individual attributes of the biolayer and its surroundings and avoid a separate reference site for background monitoring. Examples provide dual-polarization quantification of biotin binding to a silane-coated sensor as well as binding of the cancer biomarker protein calreticulin to its monoclonal IgG capture antibody. Finally, we present dual-polarization resonance response for poly (allylamine hydrochloride) binding to the sensor with corresponding results of backfitting to a simple model; this differentiates the contributions from biolayer adhesion and background changes.

Pressure-tunable guided-mode resonance sensor for single-wavelength characterization

A method of tuning a one-dimensional guided-mode resonance grating through the use of an air-pressure-responsive membrane is demonstrated here using finite-element method simulation. The device consists of a titanium dioxide (TiO(2)) grating structure embedded in a flexible polydimethylsiloxane membrane. This grating has a resonant response to TM-polarized light at a wavelength dependent upon the refractive index of the surrounding medium. By varying the pressure by 5500 Pa, lateral strain may be applied to the grating; this allows resonances to be produced for medium refractive indices ranging from 1.33 to 1.50 for a fixed-wavelength 850 nm light source.

Narrowband midinfrared reflectance filters using guided mode resonance

There is a need to develop mid-infrared (IR) spectrometers for applications in which the absorbance of only a few vibrational mode (optical) frequencies needs to be recorded; unfortunately, there are limited alternatives for the same. The key requirement is the development of a means to discretely access a small set of spectral positions from the wideband thermal sources commonly used for spectroscopy. We present here the theory, design, and practical realization of a new class of filters in the mid-infrared (IR) spectral regions based on using guided mode resonances (GMR) for narrowband optical reflection. A simple, periodic surface-relief configuration is chosen to enable both a spectral response and facile fabrication. A theoretical model based on rigorous coupled wave analysis is developed, incorporating anomalous dispersion of filter materials in the mid-IR spectral region. As a proof-of-principle demonstration, a set of four filters for a spectral region around the C-H stretching mode (2600-3000 cm(-1)) are fabricated and responses compared to theory. The reflectance spectra were well-predicted by the developed theory and results were found to be sensitive to the angle of incidence and dispersion characteristics of the material. In summary, the work reported here forms the basis for a rational design of filters that can prove useful for IR absorption spectroscopy.

Type of tunable guided-mode resonance filter based on electro-optic characteristic of polymer-dispersed liquid crystal

A narrowband guided-mode resonance filter (GMRF) incorporating polymer-dispersed liquid crystal (PDLC) is designed. Simulating the characteristics of the filter with rigorous coupled-wave analysis, we find that the resonance wavelength of the new kind of GMRF can be tuned from 672.4 to 698.4 nm by varying the refractive index of the PDLC layer with the applied voltage. Furthermore, the resonance wavelengths vary in a linear fashion with respect to the refractive index of the PDLC layer. Therefore, the desired resonance wavelength can be conveniently selected and tuned in a tuning range of 26 nm by using the applied voltage.

Nonpolarizing guided-mode resonance filter

A normal-incidence nonpolarizing guided-mode resonance filter is designed. There are two waveguide layers and one grating layer in the filter. By adjusting the distance between the two waveguide layers, the same resonance wavelength for both TE and TM polarization can be achieved. An antireflection design method is also used to decrease the sideband reflection of the filter. The results show that the filter has high reflection, more than 99.9% at 500 nm, and the FWHMs of TE- and TM-polarized light are 2.16 and 0.15 nm, respectively.

Narrowband multichannel filters and integrated optical filter arrays

We propose and demonstrate three approaches to achieve narrowband multichannel filters. These are multiple heterostructures with defects, guided-mode resonance (GMR) Brewster filters with multiple channels, and integrated narrow bandpass filter arrays. Transmission studies for multiple heterostructures with defects are presented. We show that the enlargement of the forbidden band and multiple-channel filtering can be reached simultaneously with these configurations. GMR Brewster filters with multiple channels can be obtained with a single-layer grating. The same properties can be obtained by use of double-layer structures that consist of a homogeneous layer and a grating with equal refractive index. We developed a combinatorial etching technique that has 32 elements on a single substrate with which to fabricate integrated narrow bandpass filters. Single- and double-chamber integrated optical filter arrays were fabricated by use of this etching technique. These narrowband multichannel filters and narrow bandpass filter arrays show good filtering features and can be utilized in many optical applications.

Combined enhanced fluorescence and label-free biomolecular detection with a photonic crystal surface

A 2D photonic crystal surface with a different period in each lateral direction is demonstrated to detect biomolecules using two distinct sensing modalities. The sensing mechanisms both rely on the generation of a resonant reflection peak at one of two specific wavelengths, depending on the polarization of light that is incident on the photonic crystal. One polarization results in a resonant reflection peak in the visible spectrum to coincide with the excitation wavelength of a fluorophore, while the orthogonal polarization results in a resonant reflection peak at an infrared wavelength which is used for label-free detection of adsorbed biomolecules. The photonic crystal resonance for fluorescence excitation causes enhanced near fields at the structure surface, resulting in increased signal from fluorophores within 100 nm of the device surface. Label-free detection is performed by illuminating the photonic crystal with white light and monitoring shifts in the peak reflected wavelength of the infrared resonance with a high-resolution imaging detection instrument. Rigorous coupled-wave analysis was used to determine optimal dimensions for the photonic crystal structure, and devices were fabricated using a polymer-based nanoreplica molding approach. Fluorescence-based and label-free detection were demonstrated using arrays of spots of dye-conjugated streptavidin. Quantification of the fluorescent signal showed that the fluorescence output from protein spots on the photonic crystal was increased by up to a factor of 35, and deposited spots were also imaged in the label-free detection mode.

Computational design for guided-mode grating resonances

We are concerned with optimal design of guided-mode grating resonant structures (GMGRs). A typical structure is the integration of a zeroth-order grating and a planar waveguide. Our approach has two main steps. The first is to find the resonant wavelength. For any fixed grating structure the resonant wavelength is found by solving a nonlinear eigenvalue problem. The second step is to develop a Newton-type local optimization method. A crucial step is to determine an appropriate initial guess of the design parameters. Numerical design examples for both TE and TM polarization are presented. The design algorithm is expected to provide systematic guidance in engineering design of GMGRs.

Resonant Brewster filters with absentee layers

A method for lowering the sideband levels associated with the spectral response of resonant waveguide-grating filters is presented. With a TM-polarized incident wave near the Brewster angle, the filter sidebands are suppressed by application of a half-wavelength absentee waveguide layer and an arbitrary-thickness grating layer. Adjusting the thickness of the grating layer permits control of the filter linewidth without appreciably affecting the sideband features. To verify the theoretical prediction, we fabricated and tested a double-layer waveguide-grating filter. It exhibited a peak efficiency of 82.4%, with sideband reflection levels below 0.6%, over a 95-nm spectral range.

Guided-mode resonant grating filter with an antireflection structured surface

We describe a new structure of guided-mode resonant grating (GMRG) filters with low sideband reflectance. This GMRG filter consists of a high-index thin film on an antireflective structured surface called "moth-eye structure." Since the high-index film undulates along the surface structure, the film acts as a modulated optical waveguide. An incident light wave satisfying a resonant condition is reflected by the GMRG filter, and nonresonant light waves pass through the filter. This GMRG filter is valid for reducing reflection of nonresonant light waves in a wide spectral range. The resonant reflection of this new filter was investigated by numerical calculation based on an electromagnetic grating analysis. In the case of a triangular antireflective surface structure whose thickness is 2x greater than its period, the sideband reflectance for nonresonant light waves was lower than 0.5% for TM-polarized light in a wide range of wavelengths.

Dispersion relation of guided-mode resonances and Bragg peaks in dielectric diffraction gratings

We present the dispersion relation of guided-mode resonances in non-dissipative dielectric diffraction gratings, both for s-polarized (TE mode) and p-polarized (TM mode) incident waves. We present a simple approximate theory as well as a rigorous calculation within the so-called coupled-wave theory. We discuss the dependence of the positions and the lifetimes of the resonances on the thickness of the gratings and on the strength of its modulation. We find that the diffraction efficiency of the different orders show peaks at different Bragg orders.

Corrugated waveguides as resonance optical filters--advantages and limitations

The role of the excitation of guided waves propagating in a corrugated dielectric waveguide is discussed in view of the resonance anomalies in reflectivity. Narrow-wavelength filtering properties that are due to these sharp anomalies have been a topic of interest for some time, but a proper understanding of device performances requires an analysis of tolerances with respect to the incident-beam collimation and to waveguide losses. Such an analysis is proposed in this paper, and the conclusion is that the incident-beam divergence plays a critical role in reducing the maximum reflectivity for narrow-band filters.

Nonpolarizing guided-mode resonant grating filter for oblique incidence

A new type of guided-mode resonant grating filter is described. The filter is independent of polarization state for oblique incidence. The filter has a crossed grating structure, and the plane of incidence on the filter contains the symmetric axis of the grating structure. Theoretical considerations and numerical calculations using two-dimensional rigorous coupled-wave analysis show that a rhombic lattice structure is suitable to such filters. In this configuration an incident light wave is diffracted into the waveguide and is divided into two propagation modes whose directions are symmetric with respect to the plane of incidence. In particular, when the propagation directions of the two modes are perpendicular to each other, the fill factor of grating structure can be approximately 50%. The filter was designed for an incident angle of 45 degrees. Tolerances of setting errors and fabrication errors for this filter were estimated by numerical calculations.

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.

Low sideband guided-mode resonant filter

A guided-mode resonant filter with low sideband reflections is proposed. It is shown that, for serious reduction of out-of band reflectance, the combination of waveguide-grating filter design with conventional antireflective stack design methods is not adequate. To achieve symmetrical low sideband reflectances, independent control of various layer thicknesses is necessary. At a given illumination angle with appropriate control of the waveguide thickness, a specific resonant grating filter is designed whose out-of-band reflectance on both sides of the resonant peak is well below 10(-4). Even 50 nm away from the peak, on both sides, the out-of-band reflectance remains below 10(-3). Analysis of the variation in the main manufacturing parameters indicates that such filters can be reliably produced with present-day technologies.

Fabrication of high-spatial-frequency gratings through computer-generated near-field holography

We demonstrate an innovative method for fabrication of high-spatial-frequency grating structures. This technique makes use of the near-field diffraction patterns from computer-generated phase holograms for lithographic fabrication of grating structures with periods that are one half that of the phase hologram mask. Linear, rectilinear, and circular gratings were fabricated with this technique. Experimental results from gratings with periods to 0.5 mum and feature sizes to ~0.2 mum are presented.

High-efficiency guided-mode resonance filter

A high-efficiency guided-mode resonance reflection filter is reported. The device consists of a surface-relief photoresist grating and an underlying HfO (2) waveguide layer deposited on a fused-silica substrate. The spectral response measured with a dye-laser beam at normal incidence exhibited a peak reflectance of 98% at a wavelength of 860 nm with sideband reflectance below approximately 5% extending over the wavelength range provided by the dye (800-900 nm). At normal incidence the filter linewidth was 2.2 nm. High-efficiency double-peak resonances occurred at nonnormal incidence, with the spectral locations of the maxima vayring with the incidence angle. The filter response at various angles of incidence agreed well with the theoretically calculated reflectance curves.