Guided-mode Resonance Research Articles

Enhancement and Sensing Application of Ultra-Narrowband Circular Dichroism in the Chiral Nanostructures Based on Monolayer MoS2 and a Distributed Bragg Reflector.

Narrowband circular dichroism (CD) has aroused wide concerns in high-sensitivity detections of chiral molecular and chiral catalysis. Nevertheless, the dynamical adjustment of ultra-narrowband (UNB) CD signals is hard to achieve. In this work, single-layer molybdenum disulfide (MoS2), vanadium dioxide (VO2), and a distributed Bragg reflector (DBR) are introduced into X-shaped chiral nanostructures (XCNs) for overcoming the above challenge. The simulation results show that XCNs can generate four strong UNB CD signals in the near-infrared band, and XCNs/MoS2 can further enhance the UNB CD signals. The full width at half-minimum of UNB CD signals can reach 0.14 nm. The electric field distributions of XCNs/MoS2 show that the four CD signals originate from the coupling between the guided mode resonances along the x and y axes in the VO2 layer and the Tamm plasmon polaritons along the x and y axes in the DBR layer. Four UNB CD peaks can be actively tuned by varying the structural parameters, the number of MoS2 layers, and the environmental temperature. The FOM of XCNs/MoS2 can reach 1487 by changing the refractive index of the DRB layers. These findings contribute to the design of UNB chiral devices and provide new possibilities for environmental monitoring and ultrasensitive detection of chiral molecules.

Multiparametric Guided-Mode Resonance Biosensor Monitoring Bulk and Surface-Film Variations

A guided-mode resonance (GMR) sensor with multiple resonant modes is used to measure the collection of biomolecules on the sensor surface and the index of refraction of the sensor environment (bulk). The number of sensor variables that can be monitored (biolayer index of refraction, biolayer thickness, and bulk, or background, index of refraction) is determined by the number of supported resonant modes that are sensitive to changes in these variable values. The sensor we use has a grating and homogeneous layer, both of which are made of silicon nitride (Si3N4), on a quartz substrate. In this work, we simulate the sensor reflection response as a biolayer grows on the sensor surface at thicknesses from 0 to 20 nm and biolayer indices of refraction from 1.334 to 1.43 RIU; simultaneously, we vary the bulk index of refraction from 1.334 to 1.43 RIU. In the specified span of sensor variable values, the resonance wavelength shifts for 2023 permutations of the biolayer index of refraction, biolayer thickness, and bulk index of refraction are calculated and accurately inverted. Inversion is the process of taking resonant wavelength shifts, for resonant modes of a sensor, as input, and finding a quantitative variation of sensor variables as output. Analysis of the spectral data is performed programmatically with MATLAB. Using experimentally measured resonant wavelength shifts, changes in the values of biolayer index of refraction, biolayer thickness, and bulk index of refraction are determined. In a model experiment, we deposit Concanavalin A (Con A) on our sensor and subsequently deposit yeast, which preferentially bonds to Con A. A unique contribution of our work is that biolayer index and biolayer thickness are simultaneously determined.

All-optical AZO-based modulator topped with Si metasurfaces

All-optical communication systems are under continuous development to address different core elements of inconvenience. Here, we numerically investigate an all-optical modulator, realizing a highly efficient modulation depth of 22 dB and a low insertion loss of 0.32 dB. The tunable optical element of the proposed modulator is a layer of Al-doped Zinc Oxide (AZO), also known as an epsilon-near-zero transparent conductive oxide. Sandwiching the AZO layer between a carefully designed distributed Bragg reflector and a dielectric metasurface—i.e., composed of a two-dimensional periodic array of cubic Si—provides a guided-mode resonance at the OFF state of the modulator, preventing the incident signal reflection at λ = 1310 nm. We demonstrate the required pump fluence for switching between the ON/OFF states of the designed modulator is about a few milli-Joules per cm2. The unique properties of the AZO layer, along with the engineered dielectric metasurface above it, change the reflection from 1 to 93%, helping design better experimental configurations for the next-generation all-optical communication systems.

Strong resonance response with ultrahigh quality factor in grating-multilayer systems based on quasi-bound states in the continuum

Optical bound states in the continuum (BICs) exist in many photonic crystals and periodic structures with a strong resonance and ultrahigh Q factor. Such phenomena can be used in the designs of narrowband transmission filters, lasers, and sensors. In this paper, we consider the energy bands of a complex structure consisting of a grating and a multilayer substructure to obtain the position of the BIC in the structure. Hence, the higher Q factor can be obtained in the grating-multilayer structure than can be realized in the simple grating geometry. We analyze the wave propagation process in the complex structure and the change in the Q value via the use of transmission matrix theory. In addition, the reflectance spectrum is found to exhibit a series of asymmetric line-shapes with different values of the asymmetry parameter, δ, due to the interference between the two channels. One of these channels is the broadband channel, induced by the Fabry–Perot resonance, and the other channel is the narrowband channel, induced by guided mode resonance. Quasi-BICs are seen to transform into BICs as the value of δ is decreased gradually to zero. Therefore, a large Goos–Hänchen shift can be achieved as a result of the high Q factor and quasi-BIC. This work designs a complex structure with ultrahigh Q factor and strong resonance properties, which has significant implications for exploring the phenomenon of BICs.

Topological beaming of light.

Nanophotonic light emitters are key components in numerous application areas because of their compactness and versatility. Here, we propose a topological beam emitter structure that takes advantage of submicrometer footprint size, small divergence angle, high efficiency, and adaptable beam shaping capability. The proposed structure consists of a topological junction of two guided-mode resonance gratings inducing a leaky Jackiw-Rebbi state resonance. The leaky Jackiw-Rebbi state leads to in-plane optical confinement with funnel-like energy flow and enhanced emission probability, resulting in highly efficient optical beam emission. In addition, the structure allows adaptable beam shaping for any desired positive definite profiles by means of Dirac mass distribution control, which can be directly encoded in lattice geometry parameters. Therefore, the proposed approach provides highly desirable properties for efficient micro-light emitters and detectors in various applications including display, solid-state light detection and ranging, laser machining, label-free sensors, optical interconnects, and telecommunications.

Giant Photonic Spin Hall Effect by Anisotropic Band in Photonic Crystal Slabs

When a paraxial light beam is reflected from/ transmitted through an optical element with rotation symmetry, a tiny small incident angle can arise wave-vector-varying Pancharatnam-Berry (PB) phases, resulting in high-performance photonic spin Hall effect (PSHE). Differently, here we investigate the PSHE in photonic crystal slabs of square lattices constructed with air holes, and demonstrate that the C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4v</sub> symmetry of the slabs enables giant PSHE in both isotropic and anisotropic band structures with a small incident angle. The guided-mode resonances of the subwavelength-thickness slab enhance the PSHE, allowing the spin separation to exceed the incident beam waist. Via changing the incident angle, wavelength and even the beam waist, the spin separation can be adjusted in a wide range, which makes our proposal more flexible in manipulation of spin photons.

A novel method for fabricating guided-mode resonance filters by dip-coating colloidal nanospheres

A guided-mode resonance filter (GMRF) with nanosphere grating layer fabricated by a modified dip-coating method is designed and demonstrated through experiment. The influence of waveguide thickness, radius of nanosphere, incident angle and polarization characteristics on the spectral reflectance are investigated, and optimized structure parameters are obtained. Field distributions of the designed GMRF at resonance and non-resonance wavelength are simulated, and proved that GMR effect will generate when strong coupling occurs between the waveguide layer and the grating layer. In the end, GMRF with 260 nm diameter nanospheres grating layer is experimentally fabricated, neat hexagonal close row structure and a maximum reflectance of 82 % at around 402.3 nm wavelength are obtained.

Perturbation approach to improve the angular tolerance of high-Q resonances in metasurfaces.

The interest in highqualityfactor (high-Q) resonances in metasurfaces has been rekindled with the rise of the bound states in the continuum (BIC) paradigm, which describes resonances with apparently limitlessly high quality-factors (Q-factors). The application of BICs in realistic systems requires the consideration of the angular tolerance of resonances, however, which is an issue that has not yet been addressed. Here, we develop an ab-initio model, based on temporal coupled mode theory, to describe the angular tolerance of distributed resonances in metasurfaces that support both BICs and guided mode resonances (GMRs). We then discuss the idea of a metasurface with a perturbed unit cell, similar to a supercell, as an alternative approach for achieving high-Q resonances and we use the model to compare the two. We find that, while sharing the high-Q advantage of BIC resonances, perturbed structures feature higher angular tolerance due to band planarization. This observation suggests that such structures offer a route toward high-Q resonances that are more suitable for applications.

Extremely wide-angle nonreciprocal thermal emitters based on Weyl semimetals with dielectric grating structure

Although various nonreciprocal thermal emitters have been suggested to break the balance between absorption and emission, the majority can only achieve nonreciprocal effect at one certain angle, and the angular range in which these structures exhibit nonreciprocal effect is heavily restricted. In this work, the scheme to realize extremely wide-angle nonreciprocal thermal radiation effect is proposed and investigated. It is achieved by placing a dielectric grating atop a Weyl semimetal film backed with a metal mirror. The results show that the emitter exhibits strong nonreciprocal radiation at the wavelength of 15.98 μm when the incident angle is 30°. What's more, the strong nonreciprocal radiation can be achieved in a wide angular range. Such behavior results from the guided mode resonance, which is revealed by investigating the distribution of electromagnetic field at the resonant wavelength and is also confirmed by the dispersion relation of guided mode. This work provides a new approach to the design of novel nonreciprocal thermal emitters.

Single-band to multi-band perfect absorption in monolayer-graphene-based dielectric metasurfaces

Perfect absorption in graphene-based nanostructures has attracted much attention due to its potential applications in novel photodectors and light modulators with high integrability. Based on critical coupling theory, we propose a graphene-based dielectric metasurface, demonstrating graphene-assisted single-band to multi-band perfect absorption with independent band selectivity in the near-infrared range. The proposed graphene-metasurface absorber can support a delocalized guided mode resonance and a series of localized cavity modes with different order, which can be selectively excited at will by simply tuning the geometry of the system, thus realizing an independent control of single-band to multi-band perfect absorption at critical coupling condition. The proposed hybrid structure possesses simple structural configuration and is easily accessible by standard nano-fabrication technique. Our results may find potential applications in the design of novel photoelectronic devices such as graphene-based photodectors or light modulators with flexible multi-band selectivity for desired wavelength.

Directional control of absorptivity with quasi-localized guided modes

We present a guided mode resonance grating that exhibits high-Q resonances with highly selective directional absorption around 3.7 μm. The grating is designed to create slow guided mode resonances with near-flat dispersion relation. Weakly coupled resonators enable such resonances with small dispersion characteristics. We study the dynamic control of directional absorption using the design presented. We show that weaker dispersion of resonance frequency allows a strong sensitivity concerning the directionality of resonance. Our results indicate that the directionality of absorption can be tuned from normal incidence (0°) to approximately 35° upon small index perturbation. The design presents a pathway toward tunable directional absorption and emission.

Generalized homogenization method for subwavelength periodic lattices.

Periodic photonic lattices based on Guided-Mode Resonance (GMR) enable the manipulation of the incident light, making them essential components in a plethora of optical elements including filters, sensors, lasers, and polarizers. The GMR is regarded as a resonance phenomenon in the resonant-subwavelength regime of periodic lattices. We present a method that homogenizes these periodic structures in the subwavelength regime and provides an appropriate analytical interpretation of the resonance effect. Here, we propose a technique based on utilizing the dispersion relation for homogenization, which can be applied to multi-part period lattices under oblique incidence. The effect of asymmetry and emergence of the odd/even modes, not considered in previous methods, will also be taken into account and discussed. As a result of this analytical procedure, resonance lines are obtained, which are useful in designing optical elements such as wideband/narrowband reflectors and polarizers.

Ultra-wideband transmission filter based on guided-mode resonances in two terahertz metasurfaces.

This paper reports on a broadband transmission filter that employs the guided mode resonances pertaining to a terahertz metasurface composed of metallic gold disks with a quartz slab. Unlike structures involving conventional metasurfaces, two identical metasurfaces are placed on the upper and lower sides of a thick quartz slab. This structure can excite both even and odd guided mode resonances. The interaction of the two resonances at similar frequencies produces a broadband transmission peak. The sharp spectral feature of each resonance leads to the abrupt degradation of the transmission at the spectral edge, which can enable the development of the filter application. The proposed scheme can facilitate practical applications such as those of broadband filters at a terahertz frequency.

Computational spectropolarimetry with a tunable liquid crystal metasurface

While conventional photodetectors can only measure light intensity, the vectorial light field contains much richer information, including polarization and spectrum, that are essential for numerous applications ranging from imaging to telecommunication. However, the simultaneous measurement of multi-dimensional light field information typically requires the multiplexing of dispersive or polarization-selective elements, leading to excessive system complexity. Here, we demonstrate a near-infrared spectropolarimeter based on an electrically-tunable liquid crystal metasurface. The tunable metasurface, which acts as an encoder of the vectorial light field, is tailored to support high-quality-factor guided-mode resonances with diverse and anisotropic spectral features, thus allowing the full Stokes parameters and the spectrum of the incident light to be computationally reconstructed with high fidelity. The concept of using a tunable metasurface for multi-dimensional light field encoding may open up new horizons for developing vectorial light field sensors with minimized size, weight, cost, and complexity.

Switchable strong coupling between dual hyperbolic phonon polaritons and photons in hybrid structure of metasurfaces and h-BN slab

We propose an all-dielectric hybrid structure combined with hexagonal boron nitride slab and strontium titanate (STO) metasurfaces to excite dual hyperbolic phonon polaritons (HPhPs) and an additional optical (TO) phonon, and achieve their strong coupling with photons. The metasurfaces, supporting tunable guided-mode resonance via adjusting the external temperature, consists of STO two-dimensional grating and STO layer. Thus, the strong coupling can be switched and tuned actively between the dual HPhPs and TO phonon via adjusting the external temperature of metasurfaces. This work has numerous potential applications on multi-channel biosensors, filters and tunable source and detectors.

Dynamically switchable triple-band absorption enhancement of graphene by a subwavelength grating coupled hybrid structure

The dynamically switchable triple-band absorption enhancement over 96% of monolayer graphene in the visible to the near-infrared range is achieved by a simple configuration consisting of subwavelength grating and periodically distributed Bragg reflectors. The isolated optical field distribution enabled by the optical Tamm state (OTS), guided mode resonance (GMR) and Fabry–Perot (FP) mode allows each graphene layer to selectively absorb the incident light in different wavelengths at different parts of the structure, creating favorable conditions for the independent extraction of multi-channel signals. Moreover, the continuous regulation of the absorption wavelength is futher realized by varying the geometrical parameters and dielectric permittivity. Also, the electrical tunability of graphene allows the independent step modulation of the absorption efficiency of three eigenmodes, resulting in the real-time dynamic switching of the system's triple-band/dual-band/single-band operation modes. The proposed structure can be integrated as a dynamic component into other graphene-based multifunctional and switchable optoelectronic devices.

Multiple resonant modes coupling enabled strong CD response in a chiral metasurface.

The chiral structures with strong circular dichroism (CD) response and narrow linewidth are desirable in chiral sensing, circularly-polarized light detection, and polarization imaging. Here, we theoretically proposed a hybrid chiral metasurface for differential absorption of circularly polarized light. Based on the multiple resonant modes coupling effect in a two-dimensional dielectric slab, it is realizable then to achieve a nearly perfect absorption for right circularly polarized light and simultaneously reflects 90% of left circularly polarized light, suggesting the generation of strong CD of 0.886 within a narrowly spectral linewidth of 4.53 nm. The multipole analysis reveals that the electric dipole, the magnetic dipole, and the electric quadrupole make dominant contributions to chiral absorption and the high CD response in this metsurface. The excitation of guided mode resonance enhances the ability of this metasurface to absorb electric field. Moreover, the optical chirality response can be further manipulated through the geometry features. These findings pave a powerful way to realize the narrowing and strong CD platform for single-band and multiband chirality behaviors.

High Resolution and Stability Self-Reference Plasmonic Sensor With Metallic Grating on Multilayered Dielectric Cavity Substrate

In order to improve the sensing performance of surface plasmon resonance (SPR) refractive index (RI) sensor and avoid the negative effect of various environmental factors, a self-reference plasmonic sensor with high resolution and stability based on the combination of metallic grating and multilayered dielectric cavity is proposed in this article. The sensing mode, based on SPR in the metallic grating, is sensitive to the RI variations of the analyte. The guided mode resonances (GMRs), excited by the multilayered dielectric cavity, can be served as self-references due to the insensitivity to the analyte’s RI variations. It is verified that the sensitivity of 500 nm/RIU is achieved, and the smallest sensitivity of self-reference resonance is 0.83 nm/RIU; thus, the sensitivity ratio (stability) is up to 602. Also, a large figure of merit (FOM) of 238 RIU−1 in the sensor is realized and a high-quality factor of 4668 of self-reference mode with high resolution is achieved, which is much better than that of the previous works. The results provide a theoretical basis to design a highly sensitive self-reference plasmonic sensor and have a potential application in the industrial measuring instruments such as the sensor to measure water content in heavy oil.

Enhanced terahertz third harmonic generation with high-order guided-mode resonance of graphene plasmonic gratings

We present a unique scheme to efficiently enhance terahertz third harmonic generation (THG). By exploiting the high-order guided-mode resonance of graphene plasmonic grating, the off-plane transverse localized field can be suppressed, which enables the electromagnetic energy to be tightly concentrated and significantly enhanced in the monolayer graphene. Meanwhile, high density of in-plane hot spots can be excited to form a quasi-continuous enhanced-field profile called as hot surface. This can bring about the giant nonlinearity enhancement of terahertz field-graphene interaction, greatly pushing forward the enhanced THG in monolayer graphene to breakout the limitation of various patterned metamaterials. It is anticipated that the graphene plasmonic grating has promising application for nonlinear terahertz spectroscopy, imaging and communication.

Polarization-independent long-wave infrared transmission bandpass filters based on guided mode resonance.

Subwavelength dielectric gratings based on guided mode resonances (GMRs) provide a compact, low-cost, and readily fabricable platform for narrowband spectral filters applied in numerous fields, such as sensors, spectrometers, and hyperspectral imaging. We numerically designed two long-wave infrared transmission bandpass filters based on Ge-ZnS-Ge thin film stacks and 2D zero-contrast gratings. The first filter is attributed to double GMR mode coupling, exhibiting a broadband high reflectance between 8 and 12 µm and a narrow transmission peak with a spectral linewidth of 53 nm and transmittance efficiency of 98% at the resonance. The other filter adopts a low-refractive-index material BaF2 as the substrate, which displays a lower-transmittance sideband from 8.6 to 9.4 µm, and a higher transmission efficiency up to 100% at the resonance. The physical mechanisms of two spectral responses are all relevant to guided mode coupling and Fabry-Perot resonance.