Polarization independence of a one-dimensional grating in conical mounting

High-Sensitivity Phase Detection of Reflection-Type Guided-Mode Resonance Sensor Based on Rotating Azimuth Angle Using a Heterodyne Interferometer

A high-sensitivity phase-detection system is proposed for a reflection-type guided-mode resonance (GMR) sensor, which achieves the resonance condition by rotating the azimuth angle and utilizes an electro-optic (EO) heterodyne interferometer. By rotating the GMR sensor azimuthally, the direction of the reflected light can be maintained in reflection-type detection, and the optical system can be compactly constructed because the light-tracking rotation stage is not required. The phase-detection sensitivity can be enhanced in this common-path EO heterodyne interferometer by rotating the analyzer in front of the photodetector; therefore, this system can achieve both a high sensitivity and low limit of detection. Numerical and experimental results of the reflectivity and phase response curves versus the azimuth angle were compared. The proposed system was used to perform gas sensing, and its detection sensitivity and limit were 3.73 × 104 deg/RIU and 2.68 × 10−7 RIU, respectively.

Polarization-independent all-dielectric guided-mode resonance filter according to binary grating and slab waveguide dimensions

All-dielectric binary gratings, with and without slab waveguides, are designed to generate polarization-independent guided-mode resonance filters (GMRFs) operating in the THz frequency region using the rigorous coupled-wave analysis (RCWA) method. The filling factor and thickness of the grating were adjusted to have equal resonance frequencies of transverse electric (TE)- and transverse magnetic (TM)-polarized THz beams. The single polarization-independent resonance for a binary grating without a slab waveguide was obtained at 0.459 THz with full width at half maximum (FWHM) values of 8.3 and 8.5 GHz for the TE and TM modes, respectively. Moreover, double-layered polarization-independent resonances for binary gratings with slab waveguides were obtained at 0.369 and 0.442 THz with very high Q-factors of up to 284. This is the first study to propose a polarization-independent GMRF with two resonant frequencies.

THz guided-mode resonance notch filter with variable filtering strength

In this paper, we propose a terahertz (THz) guided-mode resonance (GMR) notch filter made of a monolithic polyethylene terephthalate (PET) film, which has a monolayer grating structure. The proposed configuration shows both polarization-dependent and polarization-independent notch filter characteristics for the incident THz wave depending on the rotation angle of the second grating film. When the rotation angle is 0°, the filtering strength (transmittance) at resonance frequency changes from 0.4 (0.996) to 99.0% (0.010) according to the incident polarization. The transmittance continuously decreases with increasing rotation angle until 90°. When the rotation angle is 90°, the transmittance converges to 0.065 (± 0.015) independent of the incident wave polarization. When the incident polarization angle ranges from 90° to 180°, paradoxically, the transmittance through the two GMR grating films is greater than the transmittance through only the first GMR grating film due to the enhancement of the vertical component of the THz wave. These results agree well with a calculation using a polar coordinate system.

Guided-mode resonance-based bandpass filter operating at full conical mounting

We propose a cost-effective narrow bandpass filter based on a guided-mode resonance (GMR) structure, operating at full conical mounting configuration. Two additional layers above the GMR structure with designed geometry is considered to create a large angle of incidence (AOI) on the GMR structure with an almost normal AOI on the entire structure. In the visible wavelength range, the transmission spectrum contains three peaks: two peaks that are due to the GMR effect and one that is due to thin-film interference. The modal characteristic of the two GMR peaks is identified using a matrix method for the n-layer waveguide structure. The two GMR peaks with bandwidths of ∼1nm and ∼0.5nm correspond to the fundamental TE and TM modes, respectively. The GMR peaks can be pushed away from the thin-film interference peak by modulating the grating depth.

Resonant waveguide grating reflection filter with a quasi-rectangular spectrum under fully conical incidence

We present the design, analysis, and characterization of a filter with a quasi-rectangular spectrum. The spectral features of the filter are achieved by adjusting the incident angle under fully conical incidence. When the incident angle is 75°, the filter with a quasi-rectangular spectrum is presented at the central wavelength of 475 nm. The proposed filter has a bandwidth of 7.3 nm (R>90%), its corresponding Δλ/λ is approximately 1.5%, and the estimated rejection ratio is larger than 10 dB. Furthermore, the quasi-rectangular filtering feature is stable in the incident angle range of 75° to 85°. Our approach reveals the quasi-rectangular spectrum attributes of double resonance peaks merger under fully conical incidence and thus can be used to exploit filter devices.

Transmission filter controlled by incident conditions in single-layer waveguide grating structures

The classical single-layer waveguide grating structure is often designed as a reflection filter under 0° azimuthal angle conditions. A new method for achieving a transmission filter by modulating incident conditions is proposed rather than adjusting structural parameters. A 90° azimuthal angle ensures single resonance peaks, and large incident angles provide a low-transmittance background. When the proposed single-layer waveguide grating structure is irradiated with an incident light azimuthal angle of 90° and incident angle of 86°, numerical results show that a high transmission peak emerges at 1550 nm with linewidth $\Delta \lambda = 7.2\,\,{\rm nm}$Δλ=7.2nm. Moreover, the transmission peak has nearly 100% transmittance. These findings show that modulating the incident conditions is also an effective means apart from optimizing the structural parameters. The classical reflection filter can be reversed into a transmission filter using this method.

Improving the sensitivity of guided-mode resonance sensors under oblique incidence condition

We present an investigation on the use of oblique incidence condition to enhance the sensitivity of guided-mode resonance (GMR) sensors. By adjusting the incident angle, the enhancement of GMR sensitivity in non-subwavelength regime can be obtained. The measured results show that the bulk sensitivity of the GMR sensors with period of 809 nm climbs to 177% or 292% as the incident angle increases from 15° to 25° or 35°, respectively. The same trend is also obtained for the grating period of 994 nm. Simulations based on the rigorous coupled wave analysis (RCWA) method were performed, and we also built a new slab waveguide model to describe the relationship between bulk sensitivity and the incident angle. The present investigation demonstrates a new method for enhancing the bulk sensitivity of GMR sensor. Moreover, simple fabrication techniques can be utilized since a large grating period was used.

A Spectrally Tunable Dielectric Subwavelength Grating based Broadband Planar Light Concentrator

Energy consumption of buildings is increasing at a rapid pace due to urbanization, while net-zero energy buildings offer a green and sustainable solution. However, limited rooftop availability on multi-story buildings poses a challenge for large-scale integration of photovoltaics. Conventional silicon solar panels block visible light making them unfeasible to cover all the surfaces of a building. Here, we demonstrate a novel dielectric grating based planar light concentrator. We integrate this functional device onto a window glass transmitting visible light while simultaneously guiding near infrared (NIR) portion of sunlight to edges of the glass window where it is converted to electricity by a photovoltaic cell. Gratings are designed to guide NIR region and realize polarization independent performance. Experimentally, we observe 0.72% optical guiding efficiency in the NIR region (700–1000 nm), transmitting majority of the visible portion for natural room lighting. Integrating solar cell at the window edge, we find an electrical conversion efficiency of about 0.65% of NIR light with a 25 mm² prototype. Major losses are coupling and guiding losses arising from non-uniformity in fabrication over a large area. Such a functional window combining energy generation, natural room lighting and heat load reduction could mitigate urban heat island effect in modern cities.

Spectral characteristics of a guided mode resonant filter with planes of incidence

Spectral responses of the guided mode resonant filter under different planes of incidence are comprehensively investigated in this paper. Commercially available software based on the rigorous coupled wave analysis method is used to perform the reflectance spectra. For normal incidence, there exists a pair of resonance bands, whose diffraction efficiencies can be tuned from 3.5% to 100% and 3.2% to 100% without shifting the resonant location as the plane of incidence (POI) angle ranges from 0° to 90°, whereas the resonances split into two branches as the POI angle decreases from 90° to 0° for oblique incidence. There exists a crossing point between the split bands that is proved to be polarization independent. Moreover, the physical mechanism of the above spectral characteristics is analyzed in detail by means of the wave vectors. The intensity-tunable feature under normal incidence can be used in the field of bioactive fluorescent protein dyes, displays, and signal processing. And the split characteristic under oblique incidence is applied to seek the polarization-independent wavelength, which has potential applications in the field of dense wavelength division multiplexing and laser devices.

Spectral responses of linear grating filters under full-conical incidence

To completely clarify the spectral responses of linear grating filters (LGFs) under full-conical incidence (where the incident plane is parallel to the linear grating bars), a bandstop LGF is implemented on an HfO₂-on-silicon platform, and its spectral responses are comprehensively investigated. The measured spectra agree well with the simulated outcomes. For the TM- (or TE-) polarized wave under full-conical incidence, there exists a pair of resonance bands, whose spectral features differ significantly from each other. One resonance band has a high angular tolerance and is capable of accommodating divergent waves, whereas the other band presents a tunable spectral linewidth and can be used to achieve an ultra-high Q-factor. In particular, it is demonstrated that all of the resonance bands under full-conical incidence are degenerate regardless of what the value of the incident angle is. Our investigations reveal interesting spectral attributes of LGFs under full-conical incidence, which are highly beneficial for developing new filtering devices.

2 × 1D crossed strongly modulated gratings for polarization independent tunable narrowband transmission filters

We design a narrowband polarization independent transmission guided mode resonance filter whose center wavelength is tunable with respect to the angle of incidence. The device is composed of two identical structures assembled back to back. Each half structure is a dielectric multilayer stack in which a grating is engraved. This so-called 2×1D crossed gratings component has already been demonstrated for reflection filtering [Opt. Lett.36, 1662 (2011)OPLEDP0146-959210.1364/OL.36.001662; Opt. Lett.39, 6038 (2014)OPLEDP0146-959210.1364/OL.39.006038]. The functioning in transmission requires the use of a high index material for the grating bumps. For the design, we resort to a clustering global optimization algorithm, used for the first time to our knowledge for grating structures. We demonstrated two filters with a quality factor of about 4000, tunable over more than 15 nm when the angle of incidence varies over a range of 4°, and with a transmittivity at resonance greater than 95% whatever the incident polarization.

Properties of wideband resonant reflectors under fully conical light incidence

Applying numerical modeling coupled with experiments, we investigate the properties of wideband resonant reflectors under fully conical light incidence. We show that the wave vectors pertinent to resonant first-order diffraction under fully conical mounting vary less with incident angle than those associated with reflectors in classical mounting. Therefore, as the evanescent diffracted waves drive the leaky modes responsible for the resonance effects, fully-conical mounting imbues reflectors with larger angular tolerance than their classical counterparts. We quantify the angular-spectral performance of representative resonant wideband reflectors in conic and classic mounts by numerical calculations with improved spectra found for fully conic incidence. Moreover, these predictions are verified experimentally for wideband reflectors fashioned in crystalline and amorphous silicon in distinct spectral regions spanning the 1200-1600-nm and 1600-2400-nm spectral bands. These results will be useful in various applications demanding wideband reflectors that are efficient and materially sparse.

Electrically driving bandwidth tunable guided-mode resonance filter based on a twisted nematic liquid crystal polarization rotator

A novel bandwidth-tunable filter is proposed based on nonpolarizing guided-mode resonance effect. The compact, electrically driving bandwidth-tunable optical filter is realized by taking advantage of the effect of bandwidth-to-polarization sensitivity and using a twisted nematic liquid crystal polarization rotator for simple and precise polarization control. The operation principle and the design of the device are presented. The center wavelength is fixed at 623.1 nm with a relatively symmetric line shape. The full-width at half-maximum bandwidth is tuned from 12 to 44.8 nm by controlling the voltage in the polarization rotator.

Design of dense transmission diffraction gratings for high efficiency

We propose a design method for dense surface-relief diffraction gratings with high efficiency in transmission mode. Closed-form analytical relations between diffraction efficiency, polarization, and grating parameters are derived and verified in the resonance domain of diffraction under general three-dimensional angles of incidence traditionally termed conical mounting. A powerful tool for rigorous design of computer-generated holograms and diffractive optical elements with spectroscopic scale periods is now enabled.

Experimental demonstration of 1D crossed gratings for polarization-independent high-Q filtering

We demonstrate experimentally a spectral filter with high Q-factor (≃3238), wide accordability range (1500-1600 nm) with respect to the angle of incidence, and record polarization independence. This work is an experimental validation of the theoretical work reported in [Opt. Lett. 36, 1662 (2011)]: the filter is composed of two 1D crossed gratings engraved on each side of a planar waveguide. We provide a good comparison with theory and physical interpretations of the features observed experimentally.

Numerical investigation of one-dimensional nonpolarizing guided-mode resonance gratings with conformal dielectric films

We present the nonpolarizing resonance properties of two types of one-dimensional (1D) guided-mode resonance (GMR) gratings consisting of the sinusoidal-profile grating substrate and the conformal dielectric thin films. The optimization with respect to the grating height and the phase of the conformal graded-index layer is important for the design of nonpolarizing type-I GMR gratings. The thin films design of the conformal step-index multilayer and the optimization with respect to the grating height are of two critical steps to obtain the nonpolarizing type-II GMR gratings. Both of the two types of nonpolarizing GMR gratings can be designed to support single-mode resonance and multimode resonance under normal incidence.

Single-layer one-dimensional nonpolarizing guided-mode resonance filters under normal incidence

We demonstrate that properly designed one-dimensional guided-mode resonance filters (GMRFs) with only one grating layer can exhibit a nonpolarizing resonant filtering effect under normal incidence. A sinusoidal profile nonpolarizing GMRF is realized by photoinduced surface-relief grating formation on thin films of polymer-azobenzene complexes and subsequent atomic layer deposition, showing the feasibility of fabrication of such compact GMRFs.

Tunable, polarization independent, narrow-band filtering with one-dimensional crossed resonant gratings

We propose an optical component for widely tunable, narrow-band filtering. It takes advantage of the tunability properties, with respect to the angle of incidence, of guided-mode resonance filters. The intrinsic polarization sensitivity of the resonances is suppressed by exciting the modes through two identical, differently oriented one-dimensional gratings flanking a thick substrate. An example is provided that theoretically shows a polarization independent peak at 1.6 μm with a Q factor of 13,000 and a reflectivity greater than 99% at resonance, which is tunable over 100 nm. Finally, we discuss the fabrication limitations and conclude that the proposed configuration is realistic.

Measurement and modeling of 2D hexagonal resonant-grating filter performance

We report the measurement of a polarization-independent guided-mode resonant filter with a Q factor of approximately 2200 functioning near normal incidence in the near infrared (850 nm). Besides this remarkable performance, we provide a detailed optical and structural characterization of the component, which points out the origins of the limitation of the experimental performance. We conclude that the defaults in question can be corrected by improving the lithography process, and we are confident that even greater performance will be obtained in future realizations.

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.

Role of photonic bandgaps in polarization-independent grating waveguide structures

Polarization independence in a one-dimensional resonant grating waveguide structure involves the simultaneous excitation of two guided modes propagating in different directions. Possible simultaneous excitations occur when the two excited guided modes have either the same polarization, i.e., TE-TE (transverse electric) or TM-TM (transverse magnetic), or different polarizations, i.e., TE-TM. Simultaneous excitations may result in bandgaps and singularities. We confirm and show that in order to achieve polarization independence, it is necessary to find the conditions that minimize the effects of such bandgaps and singularities and experimentally demonstrate tunable polarization independence for simultaneously excited TE-TM-guided modes.

Authentication labels based on guided-mode resonant filters

A guided-mode resonance (GMR) filter with wide angular tolerances is experimentally demonstrated as an authentication label illuminated with unpolarized white light. The proposed filter, based on a free-standing silicon nitride membrane suspended on a silicon substrate, is fabricated by using anisotropic wet etching to remove the substrate beneath the silicon nitride layer. Both grating and waveguide structures without a lower cladding layer, i.e., a substrate, are fabricated simultaneously on a silicon nitride membrane. Since the silicon nitride is transparent within the spectra of visible and infrared light, such suspended-membrane-type GMR filters are well suited for applications within the visible spectrum. Moreover, the high refractive index of silicon nitride allows the proposed filters to have strongly modulated gratings and an immunity to high angular deviation. The measured reflection resonance has an angular tolerance up to +/-5 degrees under normal incidence for the wavelength of 629.5 nm.

Electric field enhancement in guided-mode resonance filters

Electric fields inside guided-mode resonance filters (GMRFs) may be intensified by resonance effects. The electric field enhancement is investigated in two GMRFs: one is resonant at normal incidence, the other at oblique incidence. It is shown that the two GMRFs exhibit different behaviors in their electric enhancement. Differences between the electric field distributions of the two GMRFs arise because coupling between counterpropagating modes occurs in the first case. It is also shown that the order of the electric field of maximum amplitude can be controlled by modulation of the dielectric constant of the grating.

Tunable, oblique incidence resonant grating filter for telecommunications

We have designed a tunable, oblique-incidence resonant grating filter that covers the C band as an add-drop device for incident TE-polarized light. We tune the filter by tilting a microelectromechanical systems platform onto which the filter is attached. The fabrication tolerances as well as the role of finite incident-beam size and limited device size were addressed. The maximum achievable efficiency of a finite-area device as well as a scaling law that relates the resonance peak width and the minimum device size is derived. In good agreement with simulations, measurements indicate a negligible change in shape of the resonance peak from 1526 nm at a 45 degrees angle of incidence to 1573 nm at a 53 degrees angle with a full width at half-maximum of 0.4 nm. In this range the shift of the peak wavelength is linear with respect to changes in the angle of incidence.