Guided-mode Resonance Research Articles

Mid‐Infrared Off‐Axis Resonant Mode Hybridization in Amorphous Silicon Sub‐Wavelength Grating Structures

AbstractIn this study, the off‐axis excited guided‐mode resonance (GMR) hybridization is experimentally investigated in one dimensional (1D) partially etched amorphous silicon (a‐Si) subwavelength grating structures operating in the mid‐infrared wavelength range. The 1D dielectric photonic lattice is a promising platform for studying resonance hybridization effects owing to its simple working principle and ease of electromagnetic design, fabrication, and experimental characterization. It is observed that with increasing duty‐cycle of the gratings, the avoided crossing between the coupled GMR branches underwent band‐closure and subsequent band‐flip, resulting in the Friedrich–Wintgen type bound‐states‐in‐the‐continuum (FW‐BICs) transitioning from the upper to the lower GMR branch. On either side of the band closure, the transmission spectra show an interesting electromagnetically induced transparency (EIT)‐like resonance, which manifests as a transmission peak within a broad transmission dip with increased electric field concentration near the peak. As a conceptual application of coupled GMRs, differential enhancement of infrared absorption from a polymethyl methacrylate (PMMA) layer coated on the grating structure with one of the optical resonances aligned to the absorption feature and the other acting as a built‐in reference is demonstrated. The differential transmission contrast is enhanced by more than two orders of magnitude when compared to that of the native PMMA layer.

Self-hybridized exciton-polaritons in thin films of transition metal dichalcogenides for narrowband perfect absorption.

Monolayer direct-band gap transition metal dichalcogenides (TMDCs) have been extensively investigated in the context of light-matter interactions. To reach strong coupling, these studies make use of external optical cavities supporting well-defined resonant modes. However, use of an external cavity might limit the scope of possible applications of such systems. Here, we demonstrate that thin films of TMDCs can themselves serve as high-quality-factor cavities due to the guided optical modes they sustain in the visible and near-infrared ranges. Making use of the prism coupling, we achieve the strong coupling between excitons and guided-mode resonances lying below the light line, and show that the thickness of TMDC membranes can be used to tune and promote photon-exciton interactions within the strong-coupling regime. Additionally, we demonstrate narrowband perfect absorption in thin TMDC films through critical coupling with guided-mode resonances. Our work not only provides a simple and intuitive picture to tame interaction of light and matter in thin TMDC films, but also suggests that these simple systems are a promising platform for realizing polaritonic and optoelectronic devices.

THz transmissive all-dielectric guided-mode resonance filter with high Q and tunability using suspended gratings.

Most currently available THz narrowband filters employ metal that introduces loss, or work in reflection mode, which limits their scope of application. Here, a transmissive all-dielectric guided-mode resonance filter in the THz region is presented. It contains a suspended grating layer and a waveguide layer, separated by an air layer. A fabrication process of the filter is proposed. Simulation results show that the designed filter exhibits excellent transmittance of ∼97.5% with a high Q value of ∼1500 at 1.64THz. Furthermore, this transmission peak is surrounded by a wide and flat sideband with width of ∼0.75T H z and transmission below 10%. Moreover, tunability of the filter is realized by geometric scaling and by varying the thickness of the air layer. Using geometric scaling, the filtering frequency can be widely tuned from 0.54 to 1.64THz, covering the 625-725 and 780-910GHz wireless communication windows. Additionally, fine tuning achieved by varying the air layer thickness could be used to compensate for a tiny shift of the designed filtering frequency caused by errors introduced in the fabrication process. The Q value can be further boosted to ∼11,500 by adding another layer of waveguide. Due to its transmissive nature and high-Q resonant mode with a wide sideband and tunability, the presented filter exhibits great potential in THz applications such as spectroscopy, imaging, and communication.

Optically tunable guide-mode resonance grating based on VO2 phase transition material

In this paper, highly efficient tunable guide-mode resonance (GMR) grating patterned on top of a film of the phase transition material, vanadium dioxide (VO2), is demonstrated. The optimal optical transmission and tuning can be achieved by optimizing the device structure. Based on the large insulator-to-metal transition (IMT) characteristic tuned by thermally driven of VO2 in the near-infrared range, we show that the full width at half maximum (FWHM) of about 100 nm of the transmittance resonance spectra of the dielectric GMR in VO2 grating offers remarkable extinction modulation depths (68%) at 1535 nm for tunability. The tunability is accompanied by a hysteresis-like behavior that can be exploited for versatile memory effects. In addition, we experimentally demonstrate tuning of the reflection phase over 20°. This work further improves the research topic of GMR and expands the design of novel reconfigurable devices for photonics.

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 1nm. The fill factor apodization narrows the resonance band.

Strong nonreciprocal radiation for extreme small incident angle

The perfect nonreciprocal radiation effect for extreme small incident angle is studied. The proposed scheme consists of a patterned magneto-optical (MO) material on a continuous MO film backed with a metallic mirror and coated with a metallic film. Perfect emission and extreme small absorption are achieved at resonance, which results in the realization of near-complete nonreciprocal radiation for an incident angle of only 1o. Such phenomenon is attributed to the excitation of guided mode resonance (GMR) in the MO material, which is revealed by investing the magnetic field intensity distribution and confirmed by the impedance matching theory. In addition, the near-complete nonreciprocal radiation performance remains excellent in a wide range of structure parameters, which lowers the costs of production. Last, the scheme is also employed to investigate the possibility for realizing enhanced nonreciprocity with small applied magnetic field and dual-band nonreciprocal radiation. It is believed the proposed scheme is promising in improving the energy conversion in solar cells, thermophotovoltaics as well as thermal routers and thermal diodes.

High-Q guided-mode resonance of a crossed grating with near-flat dispersion

Guided-mode resonances in diffraction gratings are manifested as peaks (dips) in reflection (transmission) spectra. Resonances with smaller line widths, i.e., with higher Q-factors, ensure stronger light–matter interactions and are beneficial for field-dependent physical processes. However, these high-Q resonances often suffer from strong angular and spectral dispersions. We demonstrate that a class of resonant modes with extraordinarily weak dispersion and Q-factor ∼1000 can be excited in crossed gratings simultaneously with the modes with well-known linear dispersion. Furthermore, the polarization of the incoming light can be adjusted to engineer the dispersion of these modes, and strong to near-flat dispersion or vice versa can be achieved by switching between two mutually orthogonal linear polarization states. We introduce a semi-analytical model to explain the underlying physics behind these observations and perform full-wave numerical simulations and experiments to support our theoretical conjecture. The results presented here will benefit all applications that rely on resonances in free-space-coupled geometries.

All-dielectric metasurface-based color filter in CMOS image sensor

The development of all-dielectric metasurfaces vigorously prompts the applications of optical metasurfaces for a CMOS (Complementary Metal-oxide-semiconductor) image sensor. Dielectric nanostructures, due to their resonant behavior and its tunability, offer the possibility to be assembled into flexible and miniature spectral filters, which could potentially replace conventional pigmented and dye-based color filters. In this paper, a transmission filter based on a GMR (Guided Mode Resonance) mode of all-dielectric metasurface is adopted as an example to study the effect of application scenario of integrated CMOS image sensor chip and manufacturing mistakes, such as the small size of pixel, special dielectric environment around, surface roughness and structural defects. Numerical calculations based on the FDTD (finite difference time domain) method are used to study the relationship between the robustness of the metasurfaces and the optical resonant modes. More important, a method simplifying different manufacturing mistakes as basic geometry structure during the FDTD simulation is also proposed and demonstrated. This work deepens our understanding of the working mechanism of all-dielectric metasurfaces and paves the way for their use in CMOS image sensor.

Electrically switchable and tunable infrared light modulator based on functional graphene metasurface

Abstract Graphene is emerging as an ideal material for new-generation optoelectronic devices. In this paper, a novel graphene metasurface-based electrically switchable and tunable infrared light modulator has been proposed and theoretically studied. The functional modulator comprises a monolayer graphene sheet sandwiched in a Fabry–Perot (FP) like nanostructure consisting of a metal reflector, a dielectric spacer, and an ellipse patterned anisotropy antenna layer. As a result of the photon localization effect of the guided-mode resonance (GMR) in the FP structure, the graphene electroabsorption can be significantly enhanced to enable a high-performance light modulator. By fine-tuning the Fermi energy (E f) of graphene via controlling its bias-gate voltage, the proposed modulator can switch between a perfect absorber and a reflective polarization converter of high conversion efficiency (i.e., >90%) at 1550 nm. The conversion mechanism and the geometric dependences of the infrared light modulator have been investigated. We further demonstrated the tunability of the highly-efficient polarization converter over a broad spectrum by adjusting the real dispersion of E f. Our design concept provides an effective strategy for customizing novel optoelectronic devices by combining an electrically-tunable 2D material with a functional metasurface.

Thermal metasurface with tunable narrowband absorption from a hybrid graphene/silicon photonic crystal resonance.

We report the design of a tunable, narrowband, thermal metasurface that employs a hybrid resonance generated by coupling a tunable permittivity graphene ribbon to a silicon photonic crystal. The gated graphene ribbon array, proximitized to a high quality factor Si photonic crystal supporting a guided mode resonance, exhibits tunable narrowband absorbance lineshapes (Q > 10,000). Actively tuned Fermi level modulation in graphene with applied gate voltage between high absorptivity and low absorptivity states gives rise to absorbance on/off ratios exceeding 60. We employ coupled-mode theory as a computationally efficient approach to elements of the metasurface design, demonstrating an orders of magnitude speedup over typical finite element computational methods.

Improving transmittance of long-wave infrared guided-mode resonant filters

The long-wave infrared (LWIR) spectral region spanning from 8 to 12 μm is useful for many scientific and industrial applications. Many of these applications require use of either a bandpass or a bandstop filter that can be realized by the guided-mode resonance (GMR) effect with subwavelength periodic features in layered dielectric materials transparent in the LWIR. The GMR filters operating in the LWIR region are fabricated by depositing an amorphous germanium (Ge) film to form a zero-contrast (ZC) waveguide-grating (WGG) on a polished zinc selenide (ZnSe) substrate. In general, the backside of a ZnSe substrate with refractive index 2.41 is uncoated causing a 17% Fresnel-reflection loss in the light transmitted through the filter due to a large impedance mismatch at the ZnSe/air interface. Because we use such filters in the LWIR laser experiments for notch filtering, to improve the filter transmittance we used ZnSe substrates coated on one-side with broadband antireflection coating (ARC) covering the 7 to 12 μm spectral range to fabricate GMRFs with one-dimensional (1D) Ge ZC WGG. We employed high-spatial resolution e-beam lithography and reactive-ion etching nanofabrication techniques to achieve high-performance large-area (12 × 12 mm2) 1D notch filters with subwavelength periods. We characterized polarization dependent spectral performance of the prototype filters with both coherent and incoherent incident light using a tunable quantum cascade laser system that spans the 7 to 12 μm region, and a Fourier transform infrared spectrometer with collimated incident beam to achieve close to 15% improvement in the peak transmittance as well as significant reduction in coherent noise compared to our earlier results with GMRFs without ARC. Here, we present the filter design simulation and measurement results.

Photochemical Reduction of Silver Nanoparticles on Diatoms.

In this work, the photochemical reduction method was used at 440 or 540 nm excitation wavelengths to optimize the deposition of silver nanoparticles on the diatom surface as a potential DNA biosensor. The as-synthesized nanocomposites were characterized by ultraviolet-visible spectroscopy (UV-Vis), Fourier transforms infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), fluorescence microscopy, and Raman spectroscopy. Our results revealed a 5.5-fold enhancement in the fluorescence response of the nanocomposite irradiated at 440 nm with DNA. The enhanced sensitivity comes from the optical coupling of the guided-mode resonance of the diatoms and the localized surface plasmon of the silver nanoparticles interacting with the DNA. The advantage of this work involves the use of a low-cost green method to optimize the deposition of plasmonic nanoparticles on diatoms as an alternative fabrication method for fluorescent biosensors.

Nonlinear Chiral Metasurfaces Based on the Optical Kerr Effect

AbstractChiral nanostructures have immense potential for complete polarization control of light. Given the intrinsic 3D structure of circularly polarized light, nanoarchitectures with structural chirality in 3D space are usually required to achieve “full” chiroptical response. On the other hand, due to the lack of available active materials at optical wavelengths, examples of chiral photonic devices that can be tuned in an active manner remain rare. Here, based on the large Kerr nonlinearities of silicon, the nonlinear chiroptical response from a planar Si metasurface supporting high quality (Q)‐factor guided mode resonances (GMRs) at near‐infrared wavelengths, are numerically demonstrated. In the linear regime, the optical anisotropy of the C2‐symmetric structure associated with the observed GMRs leads to a pair of chiral resonance modes, enabling pronounced cross‐polarization transmission circular dichroism and a strong handedness‐dependent field enhancement effect. Owing to the nonlinear Kerr effect, such Si metasurfaces are seen to exhibit prominent chiral‐selective optical nonlinearity. Importantly, a pronounced co‐polarization transmission circular dichroism is observed, suggesting that the circularly polarized waves in the nonlinear regime possess asymmetric Jones matrices. Dielectric chiral metasurfaces with Kerr nonlinearities can be utilized to facilitate polarization‐state modulators with simple planar structures.

Design and Polarization Control of the Modal Splitting in Hybrid Anisotropic Nanocavities

AbstractThe strong coupling of optical resonators results in a mode splitting proportional to the coupling strength, which can be achieved with metal‐dielectric‐metal (MDM) cavities that have similar thickness and refractive index. However, an active control of the mode coupling is challenging. Here, an alternative configuration of an MDM cavity coupled with a Guided‐Mode‐Resonator (GMR) is explored. The GMR grating is fabricated on top of the MDM cavity, such that the coupling can be tuned by the thickness of the central metal. The typical modal anti‐crossing (with a splitting of ≈50–65 meV) detected in the angular dispersion of the GMR‐MDM is observed. Excellent agreement of the experimental reflectance with simulations allows to report the angular dispersion from −50° to 50°. The anisotropy of the GMR modes enables to switch in and out of the strong coupling by changing the polarization of the incident light. Moreover, the asymmetry of the GMR‐MDM architecture induces a side‐dependent response. The MDM cavity is only transparent at its resonances, which leads to a suppression of all modes outside the resonance bands when impinging from the MDM side. These features are highly interesting for optoelectronic applications such as optical switches and multi‐level optical multiplexing.

Ultrawide dynamic modulation of perfect absorption with a Friedrich–Wintgen BIC

Dynamical control of perfect absorption plays an indispensable role in optical switch and modulators. However, it always suffers from the limited modulation range, small depth, and susceptible absorption efficiencies. Here, we propose a new strategy based on Friedrich–Wintgen bound states in the continuum (F–W BICs) to realize a tunable perfect absorber with large dynamic modulation range. For proof of concept, we demonstrate a pentaband ultrahigh absorption system consisting of graphene gratings and graphene sheets through elaborately tuning F–W BIC. The nature of the F–W BIC arises from the destructive interference between Fabry–Perot resonance and guided mode resonance modes in the coherent phase-matching condition. The radiation channels are avoided from crossing. The BIC can be dynamically modulated by engineering the Fermi level of graphene gratings, which breaks the traditional modulation methods with an incidence angle. Remarkably, the perfect absorber with this F–W BIC approach achieves the largest modulation range of up to 3.5 THz. We believe that this work provides a new way to dynamically engineer perfect absorption and stimulates the development of multiband ultracompact devices.

Polarization-independent enhanced and tunable Goos-Hänchen shift in a metal-dielectric grating structure with monolayer graphene

The sensitivity of Goos-Hänchen (GH) shifts on the geometric parameters of the structures suggest their great application prospects in sensing and detection. However, most of the enhanced GH shifts are achieved under either the transverse electric (TE) wave or transverse magnetic (TM) polarized wave. Here, we theoretically demonstrate that the well-designed metal-dielectric grating structure with monolayer graphene has the potential for realizing the enhanced GH shifts under both the TE and TM polarized waves at the same specific wavelength, which is dramatically different from the previous works and suggests that the enhancement of the GH shift is polarization independent. In particular, the enhancement of the GH shift obtained in this structure under the TE polarized wave is caused by the excited guided mode resonance in the dielectric layers of the grating strip. Moreover, the enhancement of the GH shift under the TM polarized wave is mainly due to the excited surface plasmon polariton at the interface between the dielectric layer and the metal layer in the top of grating strip. We also find that the size and sign of the GH shift can be controlled by the chemical potential of monolayer graphene and the geometric parameters of this structure. The highly controllable and polarization independent GH shift in the metal-dielectric grating structure with monolayer graphene paves the way for the future applications in the polarization independent devices, such as, optical sensors, optical switches and so on.

Structural color generation: from layered thin films to optical metasurfaces

AbstractRecent years have witnessed a rapid development in the field of structural coloration, colors generated from the interaction of nanostructures with light. Compared to conventional color generation based on pigments and dyes, structural color generation exhibits unique advantages in terms of spatial resolution, operational stability, environmental friendliness, and multiple functionality. Here, we discuss recent development in structural coloration based on layered thin films and optical metasurfaces. This review first presents fundamentals of color science and introduces a few popular color spaces used for color evaluation. Then, it elaborates on representative physical mechanisms for structural color generation, including Fabry–Pérot resonance, photonic crystal resonance, guided mode resonance, plasmon resonance, and Mie resonance. Optimization methods for efficient structure parameter searching, fabrication techniques for large-scale and low-cost manufacturing, as well as device designs for dynamic displaying are discussed subsequently. In the end, the review surveys diverse applications of structural colors in various areas such as printing, sensing, and advanced photovoltaics.

Theoretical Demonstration of Tunable Multichannel Midinfrared Optical Filters for Infrared Spectroscopic Gas Detection

A theoretical demonstration of the multichannel midinfrared (MIR) optical filters is proposed, and even-numbered multichannel spectral transmission is obtained by adding 1-D Ge2Sb2Te5 (GST) grating defect layers in a 1-D photonic crystal (1DPhC), which can be attributed to the excitations of defect modes (DMs) and guided mode resonance (GMR). Moreover, the channel numbers of the proposed optical filter can be designed by controlling the number of GST grating layers, and active tunability of the multichannel MIR optical filter can be realized by controlling the GST phase transition. In addition, two theoretically designed examples of optical filters for nondispersive infrared (NDIR) sensing are given for characteristic absorption wavelength acquisitions in single- (CO) and multicomponent (CH4, CH2O, CO2, and CO) gas detections. Our findings may have potential applications in infrared spectroscopic gas detection, material characterization, and biochemical sensing.

Thermally-Electrically Tunable Graphene-Based Guided-Mode Resonant Perfect Absorber

We propose a thermally-electrically tunable perfect absorber based on amorphous silicon (a-Si) and graphene. Numerical results reveal that a perfect absorption with narrow bandwidth can be induced owing to the guided-mode resonance. The thermal tuning of resonant wavelength, realized via the Joule heating, characters a high tuning efficiency and a linear controllability. The linear and thermal tunability is attributed to the linear relationship between refractive index of a-Si and temperature. Moreover, by adjusting the applied gate voltage, the chemical potential of graphene can be electrically modified, leading to a change of optical absorption and enabling a rapid switching of perfect-absorption and perfect-reflection. It is remarkable that the proposed absorber features a simple structure, perfect absorption, and efficient thermal-electric tunability, manifesting tremendous potential applications in modulator, optical switching, selective filter, etc.

Electromagnetic guided mode resonance in dielectric grating affected by transformation of refractive index periodicity

The present work studied effects of transformation of refractive index periodicity on electromagnetic wave propagation through grating waveguides. In lieu of the standard refractive index periodicity, although its unit cell consists of two kinds of materials, we consider few such unit cells as a new supercell, where the material parameters in a standard unit cell are changed. It has been shown how by changing parameters of the periodicity to control the wavelength and intensity of resonant optical mode (guided mode resonance) arising inside grating area. High quality factor calculated for the specific angle of incidence and periodicity parameter. Thus, we demonstrated that transformation of refractive index provides additional tools of controlling the GMR, and that means the sample can be designed more functional in terms of real application.