Guided-mode Resonance Filter Research Articles

Enhanced Terahertz Characterization of Multilayer Graphene on Guided‐Mode Resonance Filter: Boosting Sensitivity and Precision in Electrical and Optical Characteristics

AbstractIn this study, terahertz time‐domain spectroscopy (THz‐TDS) is employed for the first time to explore the characteristics of mono‐, bi‐, and tri‐layer graphene coated on guided‐mode resonance filters (GMRFs). Owing to high quality‐factor (Q‐factor) resonances of GMRF, the proposed method significantly enhances the resonance depth variation by up to 9.3, 5.1, and 4.2 times at 0.58 THz in TE mode for mono‐, bi‐, and tri‐layer graphene, respectively, in contrast to conventional THz‐TDS methods relying on amplitude variation at 0.50 THz in TE mode. Excellent agreement is observed between experimental results and theoretical simulations using the Kubo formula and Drude model, even accounting for variations in sidelobes at an incident angle of 0.6 degrees. Through meticulous fitting process between measurements and simulations for the resonances formed by the GMRF and graphene, the study accurately determines the electrical and optical properties of mono‐, bi‐, and tri‐layer graphene, including frequency‐dependent sheet conductivity (σs(ω)), mobility (μ), carrier density (N), and Fermi velocity (vF). Furthermore, in the THz high‐frequency region, the observation reveals that as the number of graphene layers increases, the decrease in σs(ω) occurs more rapidly than in single‐layer graphene, attributed to the screening effect arising from electronic interactions between each graphene layer.

Narrowband and high-transmissivity guided-mode resonance filter in the visible range

Narrowband and high-transmissivity guided-mode resonance filter in the visible range

Fabrication of Cu grating guided-mode resonance filter by electroplating

A polarization wavelength filter was fabricated using two-beam interference and electroplating methods. The device consists of a Cu grating with a pitch of 400 nm on an ITO film. For normal incidence, the transverse magnetic (TM) transmission spectrum had a sharp spectrum at a wavelength of 648 nm and a significant dip at 735 nm. Furthermore, a sharp reflection spectrum in TM-light was obtained at a 685 nm wavelength and incident angle of 40°, and the peak wavelength shifted to a longer wavelength as the incident angle increased. This fabrication method is less complex and inexpensive than conventional methods.

Design of Reflective Tunable Structural Color Metasurface Based on Guided-Mode Resonance Filter and Sb2S3

In recent years, dynamically tunable structural color has attracted great interest. Here, we introduce the guided-mode resonance (GMR) filter and the phase-change material Sb2S3 to design a reflective optical metasurface to produce tunable structural color, in which the combination of the GMR filter, with narrow resonant wavelength, and the Sb2S3, with a much larger bandgap and higher refractive index, helps to produce high-quality tunable structural color. The simulation results indicate that through the phase transition between the amorphous and crystalline states of Sb2S3, the proposed metasurface can generate tunable structural color that can be perceived by the naked eye. Furthermore, the metasurface can sensitively sense environmental changes through changes in structural color. This work provides a new method for realizing dynamically tunable structural color, and paves the way for the application of controllable structural color in dynamic displays, optical stealth, colorimetric sensing, and other fields.

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.

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.

Polarization independent electron-beam written 2-D longwave infrared guided-mode resonant filters

We fabricated guided mode resonance filters (GMRFs) with two-dimensional (2-D) gratings operating in the 8 to 12 µm long-wave infrared (LWIR) region by depositing amorphous germanium (Ge) film to form a zero-contrast (ZC) waveguide-grating (WGG) on polished zinc selenide (ZnSe) substrates with and without antireflection coating (ARC). We employed high-spatial resolution e-beam lithography and reactive-ion etching (RIE) nanofabrication techniques. We characterized the fabricated filters for their polarization independent spectral performance using a tunable quantum cascade laser (QCL) system and a modified Fourier transform infrared (FTIR) spectrometer. Here, we will present both theoretical and experimental results and their comparison.

Switchable optics based on guided mode resonance in lithographically patterned vanadium dioxide with integrated heating layer

Vanadium dioxide (VO2) has promising applications in smart windows and active micro-optical devices due to its thermochromic properties. However, the successful fabrication and patterning of VO2 thin films with the correct stoichiometry and phase are challenging. In this study, we investigated lithographically patterned and non-patterned VO2 thin films fabricated by reactive ion beam deposition, using variable angle spectroscopic ellipsometry, Raman spectroscopy, and transmission and reflection measurements. The results show that the refractive index and extinction coefficient exhibit significant changes for near-infrared wavelengths when heated above 68 °C, confirming its thermochromic properties. The Raman spectroscopy results indicate the formation of the monoclinic phase VO2(M) after annealing, which was not changed by reactive ion etching. Lithographically structured VO2-layers were successfully realized demonstrating the potential of VO2 as a material for active micro-optical devices, such as guided mode resonance filters with switchable reflectance. The results suggest that VO2 has great potential as a promising material for actively switched optical elements and micro-optical devices.

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.

Inverse design of hybrid metal-dielectric guided mode resonance optical filters with a deep learning neural network and Fano function matching

In this work, we propose and implement a machine learning method of using a forward deep learning neural network and Fano function inverse matching to design and optimize hybrid metal-dielectric guided mode resonance narrow linewidth optical filters. First, a forward deep learning neural network is trained with a small design sample set generated with finite difference time domain physical simulations. The trained forward neural network is then used to generate a large sample set of three million designs. In inverse matching process, filter peak wavelength and spectral linewidth are two matching parameters first used for down selecting designs from the large sample set to a small sample set. Because of the asymmetric nature of the guided mode resonance filter spectral line-shape, Fano functions are used to match against the spectra in the small sample set to find the design with narrow filter linewidth. Optical transmission filters with linewidth between 6.8 nm and 8.7 nm are designed in visible spectrum.

Two-dimensional guided-mode resonance gratings with an etch-stop layer and high tolerance to fabrication errors.

Guided-mode resonance (GMR) bandpass filters have many important applications. The tolerance of fabrication errors that easily cause the transmission wavelength to shift has been well studied for one-dimensional (1D) anisotropic GMR gratings. However, the tolerance of two-dimensional (2D) GMR gratings, especially for different design architectures, has rarely been explored, which prevents the achievement of a high-tolerance unpolarized design. Here, GMR filters with common 2D zero-contrast gratings (ZCGs) were first investigated to reveal their differences from 1D gratings in fabrication tolerance. We demonstrated that 2D ZCGs are highly sensitive to errors in the grating linewidth against the case of 1D gratings, and the linewidth orthogonal to a certain polarization direction has much more influence than that parallel to the polarization. By analyzing the electromagnetic fields, we found that there was an obvious field enhancement inside the gratings, which could have a strong effect on the modes in the waveguide layer through the field overlap. Therefore, we proposed the introduction of an etch-stop (ES) layer between the gratings and the waveguide-layer, which can effectively suppress the interaction between the gratings and modal evanescent fields, resulting in 4-fold increased tolerance to the errors in the grating linewidth. Finally, the proposed etch-stop ZCGs (ES-ZCGs) GMR filters were experimentally fabricated to verify the error robustness.

Switchable dual-functional guided-mode resonant filters based on VO2 phase transition

Here we propose a hybrid dielectric waveguide grating with an ultrathin vanadium dioxide (VO2) thin film for achieving switchable dual-functional GMR filters. We show that, when the VO2 thin film undergoes the insulator-to-metal phase transition, the hybrid dielectric waveguide gratings enable the switch between the reflection and transmission filters. This work expands the research topic of GMR and provides valuable strategy for the design of novel reconfigurable devices for photonics.

Cavity resonator-integrated guided-mode resonance filters with on-chip electro- and thermo-optic tuning.

Cavity resonator grating filters (CRIGFs) integrated on lithium niobate on insulator (LNOI) with electrical tuning elements are reported. The resonance wavelength of the filters is in the 780 nm range. Integrated thermo-optical tuning range of 2.5 nm is measured using integrated resistors, whilst a 0.7 nm electro-optical tuning range using capacitive metallic pads is achieved with ±400V drive voltage.

Longwave infrared polarization independent monolithic guided-mode resonance filters with double-sided orthogonal linear gratings

We present novel polarization independent, high-quality monolithic spectral filters based on the guided-mode resonance (GMR) effect with orthogonal linear gratings on either side of the substrate operating in the longwave infrared (LWIR) spectral region. We employ high-spatial resolution e-beam lithography and reactive-ion etching (RIE) nanofabrication techniques to achieve large-area (10×10 mm2) notch filters with subwavelength features. We fabricated prototype filters and characterized their polarization independent spectral performance with both coherent and incoherent incident light using a tunable quantum cascade laser (QCL) system that spans the ∼8–12 µm spectral band as well as a Fourier transform infrared (FTIR) spectrometer with collimated incident beam.

A Theoretical Description of Node-Aligned Resonant Waveguide Gratings

Waveguide gratings are used for applications such as guided-mode resonance filters and fiber-to-chip couplers. A waveguide grating typically consists of a stack of a single-mode slab waveguide and a grating. The filling factor of the grating with respect to the mode intensity profile can be altered via changing the waveguide’s refractive index. As a result, the propagation length of the mode is slightly sensitive to refractive index changes. Here, we theoretically investigate whether this sensitivity can be increased by using alternative waveguide grating geometries. Using rigorous coupled-wave analysis (RCWA), the filling factors of the modes of waveguide gratings supporting more than one mode are simulated. It is observed that both long propagation lengths and large sensitivities with respect to refractive index changes can be achieved by using the intensity nodes of higher-order modes.

Weakly coupled hybrid guided mode resonance optical transmission filter

Hybrid metal-dielectric guided mode resonance devices have an advantage over the all-dielectric guided mode resonance device for having a thin metal grating conductive layer that can be used as an electrode for tunable applications. In this work, we investigate the coupling between the waveguide mode and surface plasmons of the gold nanoslits grating in the hybrid guided mode resonance filter. It is shown that the coupling between the waveguide mode and surface plasmons can be engineered by increasing either the thickness of the low index of the refraction spacing layer or the thickness of the high index of the refraction waveguide layer. Therefore, a narrow spectral linewidth and a high finesse of hybrid guided mode resonance filters can be obtained by increasing the thickness of the low index of the refraction spacing layer or the thickness of the high index of the refraction waveguide layer. A hybrid guided mode resonance transmission filter with a narrow spectral linewidth of 2.8 nm is designed at the 1660.2 nm center wavelength.

Broadband terahertz guided-mode resonance filter using cyclic olefin copolymer.

We propose an all-dielectric single-layer guided-mode resonance filter (GMRF) operating in the high-frequency terahertz (THz) region. For the fabrication of thin gratings to achieve strong resonance in the high-frequency region, the refractive index and absorption must be small, while the tensile strength must be high. Cyclic olefin copolymer (COC) films have a lower refractive index and absorption than polyethylene terephthalate (PET) films and a higher tensile yield strength than polytetrafluoroethylene (PTFE) films. Therefore, the COC film was found suitable to fabricate a GMRF operating in the high-frequency THz region. We fabricated COC-based single-layer GMRFs with a thickness of 50 µm and grating periods of 500, 400, 300, 200, and 100 µm; the resonance frequencies of the TE0,1 mode were 0.576, 0.712, 0.939, 1.329, and 2.759 THz, respectively. A shorter grating period caused a greater shift of the resonance to a higher frequency. In particular, the COC film enabled the fabrication of a 100-µm grating period with a ridge width of 32 µm and length of 2 mm, enabling the GMRF to operate up to 2.759 THz, which is very high frequency compared to the previous highest frequency of 0.7 THz. These results were in good agreement with a simulation using rigorous coupled-wave analysis.

Near-infrared spectroscopy using period-chirped Si/SiO/SiO2-based guided mode resonance filter.

We demonstrate a Si/SiO/SiO2-based period-chirped guided mode resonance (GMR) filter to discriminate telecom o-band wavelengths by spatially resolved horizontal movement. Continuously period-chirped silicon gratings were fabricated by using a Lloyd's laser interferometer with a convex mirror. Due to the large waveguide effective index, the GMR filter can be realized with a short grating period, thus enabling a slow grating period transition along the sample position and high optical resolution in wavelength discrimination. Depositing a SiO/SiO2 stack on top of silicon gratings enables a narrowband GMR filter with a linewidth of 1-1.5 nm over a wavelength range of 1260-1360 nm. By using the chirped GMR filter as a dispersive device, the optical spectra of a near-infrared broadband light source are reconstructed. An optimized aspheric mirror is proposed to further improve the linearity of chirped gratings. Such a period-chirped GMR filter is promising for compact on-chip spectroscopy and sensing applications.

Polarization-controlled chirped guided-mode resonance filter incorporating a hybrid splay-twist liquid crystal.

This work develops a tunable chirped guided-mode resonant (GMR) filter that has a hybrid splay-twist (HST) liquid crystal as a cladding layer. The GMR filter is a color reflector that strongly reflects light at the resonance wavelength, and its chirped grating structure supports tuning of the resonance peak over a wavelength range of over 50 nm. The HST-LC configuration serves as an achromatic polarization rotator that can rotate the axis of polarization of linearly polarized light by providing effective twist angles in the LC layer under an applied voltage. The HST-LC is used to change the direction of the polarization axis of the light that is reflected by the GMR filter; continuous angles of rotation of ∼90∘ are achieved and the linear polarization is retained under applied voltages. The proposed filter enables an ultrabroadband polarization rotation and still maintains a high degree of linear polarization, which allows more degrees of freedom in spectral and polarization controls.

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.