Subwavelength Dielectric Gratings Research Articles

Enhanced four-band absorption in a tunable graphene subwavelength grating structure for photodetection

This paper investigates a sub-wavelength multilayer dielectric grating structure based on a simple metasurface, achieving dynamically switchable four-band absorption enhancement. This enhancement spans the visible to near-infrared (NIR) range, with absorption exceeding 90%, representing nearly a 40-fold increase over the intrinsic absorption of a single graphene layer. The contributions of guided-mode resonance (GMR), optical Tamm state (OTS), and photonic crystal defect cavity resonance to the absorption spectra are analyzed separately, offering a method for independent multi-channel signal extraction. Additionally, we explore the impact of structural parameters on the absorption response and examine the broad-spectrum multiband detection mechanism. This mechanism, explained through the integration of resonant cavity perturbation theory and the dynamic tuning of absorption efficiency via graphene's Fermi energy levels, results in an independently tunable optical system for four key operating bands. These properties make the structure suitable for applications such as multiband biomedical photodetectors or components in optoelectronic devices with multiple operating bands.

Coexistence of large photonic spin Hall effect and high efficiency in a dielectric grating structure

Large reflected photonic spin Hall effects have been discovered in various nanostructures under either horizontal (H) or vertical (V) polarized wave. However, their corresponding reflectance efficiencies are always quite low and they generally cannot be enhanced simultaneously for both H and V polarized waves. How to achieve the large photonic spin Hall effects and high efficiencies for both H and V polarized waves at the same wavelength, which are essential for their efficient spin photonic applications, remains acute. To overcome this difficulty, we design one dimensional subwavelength dielectric grating structure and find that it supports the remarkable reflected photonic spin Hall effects under both H and V polarized waves at the same incident angle via well-designed geometrical parameters, which are also accompanied with high reflectances in the visible frequency. We also elucidate that the reflected spin shifts in this structure are related to the incident angle and thus can be engineered by it. Our findings about the dielectric grating structures contribute to an alternative approach to achieve the decent reflected spin shifts and efficiencies for both polarized waves, which enables the next-generation spin photonic devices, such as, precision metrology and quantum information processing.

Infrared bound states in the continuum: random forest method.

In this Letter, we consider optical bound states in the continuum (BICs) in the infrared range supported by an all-dielectric metasurface in the form of subwavelength dielectric grating. We apply the random forest machine learning method to predict the frequency of the BICs as dependent on the optical and geometric parameters of the metasurface. It is found that the machine learning approach outperforms the standard least square method at the size of the dataset of ≈4000 specimens. It is shown that the random forest approach can be applied for predicting the subband in the infrared spectrum into which the BIC falls. The important feature parameters that affect the BIC wavelength are identified.

Realization of large transmitted Goos-Hänchen shifts with high (near 100%) transmittance based on a coupled double-layer grating system.

Achieving Goos-Hänchen shift enhancement with high transmittance or reflectance based on the resonance effect is challenging due to the drop in the resonance region. This Letter demonstrates the realization of large transmitted Goos-Hänchen shifts with high (near 100%) transmittance based on a coupled double-layer grating system. The double-layer grating is composed of two parallel and misaligned subwavelength dielectric gratings. By changing the distance and the relative dislocation between the two dielectric gratings, the coupling of the double-layer grating can be flexibly tuned. The transmittance of the double-layer grating can be close to 1 in the entire resonance angle region, and the gradient of the transmissive phase is also preserved. The Goos-Hänchen shift of the double-layer grating reaches ∼30 times the wavelength, approaching 1.3 times the radius of the beam waist, which can be observed directly.

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.

Subwavelength dielectric grating structures with tunable higher order resonance for achromatic augmented reality display.

Color non-uniformities caused by a dispersion effect can seriously affect the image quality for a diffractive waveguide display system. In this work, we propose a subwavelength multilayered dielectric grating structure by a rigorous coupled wave analysis as a novel coupling grating, to the best of our knowledge, for waveguide-based near-eye displays to overcome the "rainbow" effect. Such a grating structure exhibits a tunable high-efficiency resonance in first-order diffraction due to resonant coupling of incident light with the grating structure. A further analysis of the resonant behaviors helps us get a clear understanding of the underlying physics for the mode excitation and resonant coupling process. The first-order resonance with a diffraction efficiency of more than 60% can be achieved with the resonant angle continuously shifted to get a large field of view. The resonant angle, diffraction efficiency, and spectral linewidth can be easily tuned by the geometrical parameters of the grating structure.

Tunable high-quality-factor absorption in a graphene monolayer based on quasi-bound states in the continuum.

A tunable graphene absorber, composed of a graphene monolayer and a substrate spaced by a subwavelength dielectric grating, is proposed and investigated. Strong light absorption in the graphene monolayer is achieved due to the formation of embedded optical quasi-bound states in the continuum in the subwavelength dielectric grating. The physical origin of the absorption with high quality factor is examined by investigating the electromagnetic field distributions. Interestingly, we found that the proposed absorber possesses high spatial directivity and performs similar to an antenna, which can also be utilized as a thermal emitter. Besides, the spectral position of the absorption peak can not only be adjusted by changing the geometrical parameters of dielectric grating, but it is also tunable by a small change in the Fermi level of the graphene sheet. This novel scheme to tune the absorption of graphene may find potential applications for the realization of ultrasensitive biosensors, photodetectors, and narrow-band filters.

Tailoring electromagnetic responses in a coupled-grating system with combined modulation of near-field and far-field couplings

Herein, we establish a hybrid coupling model (HCM) containing both near- and far-field couplings to describe the electromagnetic response of the coupled-grating system composed of two parallelly aligned subwavelength dielectric gratings. The HCM shows that the near-field coupling strength only contributes to the frequency splitting of two resonant modes, while the far-field one contributes to the frequency splitting and the linewidths of two resonant modes simultaneously. By changing the distance between two dielectric gratings, both the near- and far-field coupling strengths can be flexibly tuned, giving rise to rich electromagnetic responses. In addition, the formation of Fabry-Perot bound states in the continuum in coupled-grating systems can be clearly explained by the HCM. In this paper, we not only provide an all-dielectric platform for simultaneously manipulating near- and far-field couplings but also offer a viable approach to achieve reflectance/transmittance spectra with diverse shapes.

High throughput composite sensor based on sub-wavelength dielectric grating/MDM waveguide/periodic photonic crystal

Based on surface plasmon resonance (SPR) and coupled mode theory (CMT), the formation mechanism of double Fano resonances is studied in a hybrid sensing structure composed of subwavelength dielectric grating/metal-dielectric-metal (MDM) waveguide/periodic photonic crystal. Studies show that the dynamically adjustable double discrete states generated in the subwavelength dielectric grating and MDM waveguide are coupled with the continuous state formed in the periodic photonic crystal respectively to achieve double Fano resonances. And the double Fano resonances can be dynamically modulated under the angular modulation by the structure parameters. Surprisingly, the figure of merit (FOM) value of is also achieved, which demonstrates the proposed structure has potential applications in sensing. In addition, the low and high refractive index sensing regions are set up. Simultaneous detection of refractive index ranges 1.00–1.29 and 3.79–4.14 is achieved. Thus, the proposed structure provides an effective reference for studying the dynamic modulation of double Fano resonances and high throughput sensors.

Exciton-polaritons in a laterally patterned semiconductor microcavity

We present a new kind of optical cavity called laterally patterned microcavity bounded by two parallel subwavelength dielectric gratings (SDGs) on one side and by distributed Bragg reflectors (DBRs) on the other side. In addition to its diffractive properties, the SDG is characterized by a large reflection spectrum and a very high reflectivity. In this case, the cavity eigenmodes will be diffracted modes, so making possible to excite not only one cavity mode as in the case in the planar DBR-DBR microcavity but several modes of different diffraction orders. Diffraction induced by the DSGs is consequently used to study the coupling of these cavity modes with an exciton confined in a quantum well (QW) localized in an anti-node of the cavity. With the semi-classical theory and the scattering matrix method, the dispersion equation of a confined bidimensional exciton-polariton in this microcavity was formulated bringing out a coupling of more than two damped harmonic oscillators. The number of DBRs required to realize the spontaneous emission and the strong coupling regime exciton-cavity mode is significantly lower than those in the planar or DBR-DBR microcavities. The presence of SDGs as mirrors has induced in the cavity and near the exciton resonance energy, Rabi splittings at specific incidence angles of the light beam. The computed time resolved transmission spectra have shown an increase in the exciton-polariton lifetime inside the microcavity.

Analysis of formation and evolution of double Fano resonances in sub-wavelength dielectric grating/MDM waveguide/periodic photonic crystal

Based on the diffraction principle and the mode coupling theory, a composite micro-nano structure of sub-wavelength dielectric grating/metal-dielectric-metal (MDM) waveguide/periodic photonic crystal is proposed. Combined with the angle spectrum of reflection, the transmission characteristics of the surface plasmon polaritons and the generation mechanism of double Fano resonances at different incident angles and fixed wavelength are analyzed. The studies show that the physical mechanism of double Fano resonances is that the surface plasmon resonance generated at the interface of sub-wavelength dielectric grating and upper metal Ag film, and the waveguide mode resonance occurring in the MDM waveguide, provide the independently tunable double discrete states, under the condition of satisfying wave vector matching, which can be respectively coupled in the near field with the continuous state formed by the photonic band gap effect in the photonic crystal, thereby achieving the double Fano resonances. Then the influence of the structural parameters on the double Fano characteristics is analyzed quantitatively, and the evolution law of the double Fano resonances is explored by the change of the reflection spectra of resonance curves. The results show that the tuning between double Fano resonance curves and the resonance angles can be realized by changing the structural parameters. And under optimal conditions, the figure of merit (FOM) values of FR a and FR b in resonance A region can be as high as 460.0 and <inline-formula><tex-math id="M3">\begin{document}$ 4.00 \times {10^4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211491_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211491_M3.png"/></alternatives></inline-formula>, and the FOM values of FR a and FR b in resonance B region can be as high as 269.2 and <inline-formula><tex-math id="M4">\begin{document}$ 2.22 \times {10^4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211491_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211491_M4.png"/></alternatives></inline-formula>. The structure can provide an effective theoretical reference for designing the refractive index sensors based on Fano resonances.

Bi-layered composite gratings with high diffraction efficiency enabled by near-field coupling.

In this paper, we present a design method for bi-layered composite gratings to achieve high diffraction efficiency. These composite gratings feature strong near-field coupling between their constituent dielectric subwavelength gratings, thus enabling high-efficiency first-order diffraction in the far-field. An intuitive explanation based on a wavevector matching condition for such high diffraction efficiency composite gratings is provided. According to theoretical analysis, a design strategy for the proposed composite gratings is developed and verified by numerical simulations with gratings working in both TE and TM modes. The proposed strategy could open door to develop bi-layered composite gratings for manipulating diffracted waves with high efficiency, thus may potentially enable new applications in photonic systems.

Fano resonance sensing based on coupling of a sub-wavelength grating and an all-dielectric multilayer film under angle modulation.

Based on the diffraction effect of the waveguide grating, a hybrid structure of an all-dielectric multilayer film and a sub-wavelength dielectric grating is proposed. The discrete state provided in the sub-wavelength waveguide grating couples with the continuous state provided in the Fabry-Perot cavity (F-P cavity) containing the photonic crystal to form a Fano resonance. The relationship between the structural parameter and the incident angle is analyzed by numerical simulation. The result shows that the figure of merit (FOM) value is 533.2RIU-1. This structure provides an effective theoretical basis for the realization of Fano resonance in the all-dielectric sensing structure.

Fano resonance sensing based on coupled sub-wavelength dielectric grating and periodic photonic crystal

Based on the diffraction effect of sub-wavelength dielectric grating and the optical property of periodic photonic crystal, a hybrid structure of sub-wavelength grating all-dielectric multilayer thin film containing periodic photonic crystal is proposed. The transmission property of the structure is simulated by finite element method (FEM). The result shows that the discrete state generated by the sub-wavelength waveguide grating will be coupled with the continuous state generated by the photonic crystal cavity and the Fano resonance can be formed. The Fano resonance sensing model based on structural parameters and resonance wavelength are established, the influence of structural parameters on the Fano resonance spectral curve is quantitatively analyzed by numerical simulations, and the dynamic detection of the refractive index of samples is realized. The above structure can realize the optical refractive index sensing with high figure of merit (FOM) value and provide an effective theoretical reference for the formation of Fano resonance in the all-dielectric hybrid structure.

Tunable terahertz phase shifter based on dielectric artificial birefringence grating filled with polymer dispersed liquid crystal

An active terahertz (THz) anisotropic manipulation is based on a structure combined polymer dispersed liquid crystal (PDLC) with sub-wavelength dielectric gradient grating. In this structure, the PDLC works as an adjustable anisotropic material due to the change of the optical axis direction induced by applying a biased electric field, while the dielectric grating serves as an artificial high birefringence material. By using an appropriate design, the THz birefringence of this structure can be enhanced or offset under different biased voltages, and the phase shift curve of this structure becomes flatter than that of the pure PDLC cell due to the dispersion manipulation of the grating. Moreover, the experimental results fit with the simulative designing, demonstrating that the phase shift of the structure can vary from π to 0 near 0.8 THz when the electric field increases from 0 to 80V, and this device realizes the function of polarization conversion as a tunable THz half-wave plate. This work exhibits potential applications in THz functional devices, such as actively controlled phase shifters and polarization convertors combined LC with artificial microstructure.

Dynamically switchable dual-band mid-infrared absorber with phase-change material Ge2Sb2Te5

Spectral control of light absorption in mid-infrared (MIR) regime is of great importance from fundamental research to practical applications. A Ge2Sb2Te5 (GST)-based tunable MIR absorber by reversible phase transitions is theoretically investigated based on the rigorous coupled wave analysis (RCWA) method. The absorber is composed of subwavelength dielectric grating and GST layer sandwiched into zinc sulfide (ZnS) layer which sitting on optical glass (BK7) substrate. Dual absorption peaks can be observed and a huge contrast in the peak of absorption can be obtained by intermediate phase states of the GST layer. The presented structure acting as a perfect absorber is likely to be exploited in applications where require active control over light absorption.

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.

Terahertz wave asymmetric transmission based on double-layer subwavelength dielectric grating

A double-layer subwavelength dielectric grating has been designed, fabricated, and characterized for terahertz (THz) wave asymmetric transmission. The subwavelength grating resonance effect and the first-order diffraction effect of the two different grating structures lead to the different light paths of the forward and backward waves, and thus the one-way transmission can be realized. The transmission and resonance properties of this grating with different polarization and propagation directions have been demonstrated experimentally and theoretically, which show that this is a nonmagnetic one-way transmission device with an extinction ratio of over 15 dB and insertion loss of lower than 5 dB at 1.45 THz. We provide an effective way to nonmagnetic THz asymmetric transmission devices operating at room temperature without rotating or converting the polarization state of incident wave.

77‐4: Large Form Birefringence Realized by Dielectric Subwavelength Grating

We present a detailed investigation of form birefringence realized by subwavelength grating based on high refractive index dielectric materials, e.g. titanium dioxide. The anticipated degree of birefringence is evaluated using Finite‐Difference Time‐Domain simulation of designed optical structure and in good accordance with the theoretical prediction. The designed subwavelength grating was fabricated using electron beam lithography and subsequent ICP dry etching technique on fused silica substrate. The artificial grating was then characterized under SEM and AFM, and a large birefringence 0.33 was obtained which can be furtherly improved with deposition process optimization.

Light radiation from surface plasmon polaritons in a structure of nanometal film on a subwavelength dielectric grating

A nanoscale light radiation source based on the diffraction radiation mechanism is proposed. In this mechanism, tunable and highly directional light emission from visible to ultraviolet frequency regime can be generated from surface plasmon polaritons (SPPs) excited by a moving electron beam above a nanometal film embedded on a subwavelength dielectric grating. The light emits in both the free-space and the dielectric substrate in specific directions. The radiation frequencies can be widely tuned by adjusting the beam energy or structure parameters. A remarkable enhancement (up to three orders of magnitude) of the radiation power density is found. The origin of the enhancement is the strong local field. Furthermore, an interesting physical phenomenon of radiation interference in the dielectric substrate is found. The physical origin of the interference is uncovered and discussed. With advances of miniature size, compatible on-chip integration, high power density, well directionality and wavelength tunability, our result has potential applications in electronic-photonic and nanophotonic systems as on-chip light sources, or in electron-emitter displays building into large-area array.