Physics of near-wavelength high contrast gratings

Mechanism and tuning sensitivity of symmetry-protected resonances in high-contrast gratings

We develop a theory of refractive index tuning for symmetry-protected optical bound states (SP-BICs) in high-contrast gratings (HCGs). A compact analytical formula for tuning sensitivity is derived and verified numerically. We also discover a new type of SP-BIC in HCGs that has an accidental nature with a spectral singularity, which is explained in terms of hybridization and strong coupling among the odd- and even-symmetric waveguide-array modes. Our work elucidates the physics of tuning SP-BICs in HCGs and significantly simplifies their design and optimization for dynamic applications in light modulation, tunable filtering, and sensing.

Double-layer symmetric gratings with bound states in the continuum for dual-band high-Q optical sensing

Herein, we theoretically demonstrate that a double-layer symmetric gratings (DLSG) resonator consisting of a low-refractive-index layer sandwiched between two high-contrast gratings (HCG) layers, can host dual-band high-quality (Q) factor resonance. We find that the artificial bound states in the continuum (BIC) and Fabry-Pérot BIC (FP-BIC) can be induced by optimizing structural parameters of DLSG. Interestingly, the artificial BIC is governed by the spacing between the two rectangular dielectric gratings, while the FP-BIC is achieved by controlling the cavity length of the structure. Further, the two types of BIC can be converted into quasi-BIC (QBIC) by either changing the spacing between adjacent gratings or changing the distance between the upper and lower gratings. The simulation results show that the dual-band high-performance sensor is achieved with the highest sensitivity of 453 nm/RIU and a maximum figure of merit (FOM) of 9808. Such dual-band high-Q resonator is expected to have promising applications in multi-wavelength sensing and nonlinear optics.

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.

Bound States in the Continuum Empower Subwavelength Gratings for Refractometers in Visible

This paper describes a compact refractometer in visible with optical bounds states in the continuum (BICs) using silicon nitride (Si3N4) based sub-wavelength medium contrast gratings (MCGs). The proposed device is highly sensitive to different polarization states of light and allows a wide dynamic range from 1.330 (aqueous environment) to 1.420 (biomolecules) monitoring, apart from its being thermally stable. The proposed sensor has a sensitivity of 363 nm/RIU for X polarized light and 137 nm/RIU for Y polarized light. The spectral characteristics have been obtained with a high angular resolution for the smaller angle of incidence, which confirms the BIC hybrid modes with good quality factors and enhanced field confinement. The device is based on a normal-to-the-surface optical launching strategy to achieve exceptional interrogation stability and alignment-free performance. This system can also be used in the CMOS photodetectors for on-chip label-free biosensing.

Bound states in the continuum based on the total internal reflection of Bloch waves

A photonic-crystal slab can support bound states in the continuum (BICs) that have infinite lifetimes but are embedded into the continuous spectrum of optical modes in free space. The formation of BICs requires a total internal reflection (TIR) condition at both interfaces between the slab and the free space. Here, we show that the TIR of Bloch waves can be directly obtained based on the generalized Fresnel equations proposed. If each of these Bloch waves picks up a phase with integer multiples of 2π for traveling a round trip, light can be perfectly guided in the slab, namely forming a BIC. A BIC solver with low computational complexity and fast convergence speed is developed, which can also work efficiently at high frequencies beyond the diffraction limit where multiple radiation channels exist. Two examples of multi-channel BICs are shown and their topological nature in momentum space is also revealed. Both can be attributed to the coincidence of the topological charges of far-field radiations from different radiation channels. The concept of the generalized TIR and the TIR-based BIC solver developed offer highly effective approaches for explorations of BICs that could have many potential applications in guided-wave optics and enhanced light-matter interactions.

Graphene perfect absorber with loss adaptive Q-factor control function enabled by quasi-bound states in the continuum

Numerous device structures have been proposed for perfect absorption in monolayer graphene under single-sided illumination, all of which requires the critical coupling condition, i.e., the balance between the loss of graphene and the leakage rate of the device. However, due to the difficulty of the precise control of the quality of synthesized graphene and unwanted doping in graphene transferred to the substrate, the loss of graphene is rather unpredictable, so that the perfect absorption is quite difficult to achieve in practice. To solve this problem, we designed a novel perfect absorber structure with a loss adaptive leakage rate control function enabled by the quasi-bound states in the continuum (BIC) and numerically demonstrated its performance. Our designed device is based on a slab-waveguide grating supporting both the quasi-BIC and the guided-mode resonance (GMR); the quasi-BIC with an adjustable leakage rate controlled by an incident angle is responsible for absorption, while the GMR works as an internal mirror. Since the proposed device scheme can have an arbitrarily small leakage rate, it can be used to implement a perfect absorber for any kind of ultrathin absorbing media. Due to the simple structure avoiding an external reflector, the device is easy to fabricate.

High-efficiency unidirectional vertical emitter achieved by an aperture-coupling nanoslot antenna array

Coupling light from in-plane guided light into free space or optical fibers is crucial for many photonic integrated circuits and vice versa. However, traditional grating couplers or waveguide grating antennas suffer from low upward coupling efficiency due to the light radiating in both upward and downward directions simultaneously. In this paper, a compact aperture-coupling nanoslot antenna array is proposed for high-efficiency unidirectional radiation, where a two-dimensional high-contrast grating (HCG) is employed as a mirror to reflect the undesired downward radiation. Upon the HCG separated by a low-index spacing layer, a thin silver layer is deposited. Finally, a series of H-shaped slots are patterned on the silver thin film to arrange the aperture fields and radiate the in-plane guided light into free space. The proposed nanoslot antenna array features a front-to-back ratio (F/B) over 10 dB within the wavelength range of 1500 ∼ 1600 nm. At the same time, a high radiation efficiency of over 75% and a maximum radiation efficiency of 87.6% are achieved within the 100 nm bandwidth. The high-efficiency unidirectional antenna array is promising for the integrated photonic applications including wireless optical communications, light detection and ranging, and fiber input/output couplers.

Mechanism and sensitivity of Fano resonance tuning in high-contrast gratings

We develop a theory for Fano resonance tuning in dual-mode high-contrast gratings (HCGs). Compact analytical formulas of tuning sensitivity are derived and verified numerically, and are in good agreement with reported experiments. We show that the resonance tuning in HCGs, containing cooperative contribution from two propagating modes, is fundamentally different from that in single-mode microresonators. Our theory reveals the important role of the higher-order mode, which can possess large modal dispersion, especially in the long-wavelength limit beyond the cutoff of slab waveguides, to enable large tuning sensitivity. Our findings will simplify the design and optimization of active and passive tuning in HCG resonators.

Broadband mirrors for surface plasmon polaritons using integrated high-contrast diffraction gratings

We propose and numerically investigate integrated high-contrast gratings (HCGs) for surface plasmon polaritons (SPPs) propagating along metal-dielectric interfaces, which consist of periodically arranged silicon pillars located on the gold surface. We demonstrate that such on-chip HCGs can be used as broadband plasmonic mirrors, which have subwavelength footprint in the SPP propagation direction and mean reflectance exceeding 85% in a 200-nm-wide spectral range for both the cases of normal and oblique SPP incidence. In order to increase the HCG efficiency and design practically feasible structures, we utilize a parasitic scattering suppression technique based on the use of two-layer grating pillars. The presented results may find application in two-dimensional optical circuits for steering the SPP propagation.

Low-loss optical waveguides made with a high-loss material

For guiding light on a chip, it has been pivotal to use materials and process flows that allow low absorption and scattering. Based on subwavelength gratings, here, we show that it is possible to create broadband, multimode waveguides with very low propagation losses despite using a strongly absorbing material. We perform rigorous coupled-wave analysis and finite-difference time-domain simulations of integrated waveguides that consist of pairs of integrated high-index-contrast gratings. To showcase this concept, we demonstrate guiding of visible light in the wavelength range of 550-650 nm with losses down to 6 dB/cm using silicon gratings that have a material absorption of 13,000 dB/cm at this wavelength and are fabricated with standard silicon photonics technology. This approach allows us to overcome traditional limits of the various established photonics technology platforms with respect to their suitable spectral range and, furthermore, to mitigate situations where absorbing materials, such as highly doped semiconductors, cannot be avoided because of the need for electrical driving, for example, for amplifiers, lasers and modulators.

Grating lobe suppression in optical phased arrays by loading near-wavelength grating

Optical phased arrays based on optical waveguides are compelling components enabling efficient and accurate beam steering. However, to avoid crosstalk between the waveguides, the element pitch is typically larger than one wavelength, which gives rise to grating lobes in real space. In this Letter, we report that near-wavelength gratings can be employed to suppress the grating lobes by utilizing the angular low-pass-filter characteristics. The properly designed near-wavelength grating acts as an angle-sensitive transmission structure. Nearly 100% transmissivity can be realized at small incident angles. However, it quickly declines to a low level when the incident angle is over the critical one. Then, a simple line current array is utilized to demonstrate the grating lobe suppression effect with the grating designed for TE-polarized incidence. Finally, we demonstrate that by loading the proposed grating designed for TM-polarized incidence upon a waveguide grating array with a 2.4 µm pitch, a grating lobe suppression of 10 dB can be achieved when scanning up to ±14^∘.

GaN-Based High-Contrast Grating for Refractive Index Sensor Operating Blue-Violet Wavelength Region

Owing to its versatility, optical refractive index (RI) sensors with compact size and high chemical stability are very suitable for a wide range of the applications in the internet of things (IoT), such as immunosensor, disease detection, and blood mapping. In this study, a RI sensor with very simple system and high chemical stability was developed using GaN-based high-contrast grating (HCG). The designed HCG pattern was fabricated on GaN-film grown on c-plane sapphire substrate. The fabricated GaN-HCG sensor can detect minuscule RI change of 1.71 × 10-3 with extreme simple surface normal irradiation system. The light behavior inside the GaN-HCG was discussed using numerical electromagnetic field calculation, and the deep understand of the sensing mechanism was provided. The simple system and very high chemical stability of our sensor exploit RI sensing applications in IoT society.

Bound states in the continuum for optomechanical light control with dielectric metasurfaces

We investigate a reconfigurable dielectric metasurface merging optomechanical interaction and quasi-bound states in the continuum promising for all-optical light control light. The surface consists of a dimerized high-contrast grating with a compliant bilayer structure. The optical forces induced by a control light field lead to structural deformations changing the optical response. We discuss requirements for the geometry and optical force distribution to enable an efficient optomechanical coupling, which can be exploited to tune reflectivity, phase and polarization of a beam impinging on the metasurface. Numerical results explore some tunable devices as mirrors, saturable output couplers, phase modulators and retarder plates.

Near-field analysis of bound states in the continuum in photonic crystal slabs

Bound states in the continuum (BICs) can be derived from a generalized waveguide condition in which the total internal reflection is substituted by coherent perfect reflection. Coherent perfect reflection can occur in the truncated photonic crystal (PhC) due to the interference of different Bloch modes. Based on the coherent reflection, BICs can be constructed by the bulk Bloch modes of PhC slabs. In contrast to the determination of BICs from the topological vortices of far-field radiation, this interpretation from coherent reflection can give the spatial field profile in detail in the near field. We show that the BICs can be characterized by the indices (or number of nodes) of their constituent Bloch modes. Moreover, all the guided resonances in addition to BICs can also be labelled by these mode indices. It is found that for the guided resonances the mode indices can change suddenly on the same frequency band. Our results may have potential applications in guided-wave optics and enhanced light-matter interaction.

Nonlinear Optics in Dielectric Guided-Mode Resonant Structures and Resonant Metasurfaces

Nonlinear optics is an important area of photonics research for realizing active optical functionalities such as light emission, frequency conversion, and ultrafast optical switching for applications in optical communication, material processing, precision measurements, spectroscopic sensing and label-free biological imaging. An emerging topic in nonlinear optics research is to realize high efficiency optical functionalities in ultra-small, sub-wavelength length scale structures by leveraging interesting optical resonances in surface relief metasurfaces. Such artificial surfaces can be engineered to support high quality factor resonances for enhanced nonlinear optical interaction by leveraging interesting physical mechanisms. The aim of this review article is to give an overview of the emerging field of nonlinear optics in dielectric based sub-wavelength periodic structures to realize efficient harmonic generators, wavelength mixers, optical switches etc. Dielectric metasurfaces support the realization of high quality-factor resonances with electric field concentrated either inside or in the vicinity of the dielectric media, while at the same time operate at high optical intensities without damage. The periodic dielectric structures considered here are broadly classified into guided-mode resonant structures and resonant metasurfaces. The basic physical mechanisms behind guided-mode resonances, electromagnetically-induced transparency like resonances and bound-states in continuum resonances in periodic photonic structures are discussed. Various nonlinear optical processes studied in such structures with example implementations are also reviewed. Finally, some future directions of interest in terms of realizing large-area metasurfaces, techniques for enhancing the efficiency of the nonlinear processes, heterogenous integration, and extension to non-conventional wavelength ranges in the ultra-violet and infrared region are discussed.

Spin-preserving chiral photonic crystal mirror

Chirality refers to a geometric phenomenon in which objects are not superimposable on their mirror image. Structures made of nanoscale chiral elements can exhibit chiroptical effects, such as dichroism for left- and right-handed circularly polarized light, which makes these structures highly suitable for applications ranging from quantum information processing and quantum optics to circular dichroism spectroscopy and molecular recognition. At the same time, strong chiroptical effects have been challenging to achieve even in synthetic optical media, and chiroptical effects for light with normal incidence have been speculated to be prohibited in thin, lossless quasi-two-dimensional structures. Here, we report an experimental realization of a giant chiroptical effect in a thin monolithic photonic crystal mirror. Unlike conventional mirrors, our mirror selectively reflects only one spin state of light while preserving its handedness, with a near-unity level of circular dichroism. The operational principle of the photonic crystal mirror relies on guided-mode resonance (GMR) with a simultaneous excitation of leaky transverse electric (TE-like) and transverse magnetic (TM-like) Bloch modes in the photonic crystal slab. Such modes are not reliant on the suppression of radiative losses through long-range destructive interference, and even small areas of the photonic crystal exhibit robust circular dichroism. Despite its simplicity, the mirror strongly outperforms earlier reported structures and, contrary to a prevailing notion, demonstrates that near-unity reflectivity contrast for opposite helicities is achievable in a quasi-two-dimensional structure.

Experiment and Simulation of a Selective Subwavelength Filter with a Low Index Contrast

Subwavelength gratings have been of great interest recently due to their ability to eliminate multiple orders. However, high index contrast ( Δ n ∼ 3 ) is typically achieved using metals or high-index dielectrics surrounded by vacuum in order to maintain good optical selectivity. Here, we theoretically propose and experimentally realize a selective subwavelength grating using an index contrast of Δ n ∼ 1.2 without vacuum. Despite its low index contrast, our simulation and experiments show that good optical selectivity is achieved using the same physics as subwavelength gratings made of high-index contrast. Such polymer-based encapsulated gratings are easier to scale up for use in large-area applications such as photovoltaics and lighting.

Resonant Grating without a Planar Waveguide Layer as a Refractive Index Sensor

Dielectric grating-based sensors are usually based on the guided mode resonance (GMR) obtained using a thin planar waveguide layer (PWL) adjacent to a thin subwavelength grating layer. In this work, we present a detailed investigation of thick subwavelength dielectric grating structures that exhibit reflection resonances above a certain thickness without the need for the waveguide layer, showing great potential for applications in biosensing and tunable filtering. Analytic and numerical results are thoroughly discussed, as well as an experimental demonstration of the structure as a chemical sensor in the SWIR (short wave infrared) spectral range (1200-1800 nm). In comparison to the GMR structure with PWL, the thick grating structure has several unique properties: (i) It gives higher sensitivity when the spaces are filled, with the analyte peaking at certain space values due to an increase in the interaction volume between the analyte and the evanescent optical field between the grating lines; (ii) the TM (transverse magnetic) resonance, in certain cases, provides a better figure of merit; (iii) the sensitivity increases as the grating height increases; (iii) the prediction of the resonance locations based on the effective medium approximation does not give satisfactory results when the grating height is larger than a certain value, and the invalidity becomes more severe as the period increases; (iv) a sudden increase in the Q-factor of the resonance occurs at a specific height value accompanied by the high local field enhancement (~10³) characteristic of a nano-antenna type pattern. Rigorous numerical simulations of the field distribution are presented to explain the different observed phenomena.

Improving the performance of optical antenna for optical phased arrays through high-contrast grating structure on SOI substrate

A novel optical antenna for optical phased arrays is proposed and simulated. A high-contrast grating structure is used to achieve extremely efficient emission. The emission efficiency is as high as 93.94% at 1.55 μm, which exceeds 50% in a range of wavelength from 1.48 μm to 1.62 μm. The antenna can achieve a perfect grating lobe suppression with background suppression of 28.4 dB when the phase difference between adjacent waveguides is 0. A 16-wire optical phased array can easily achieve a scan range of ± 22.8° × 20.2° with a beam width of 2.4° × 2.5°, by employing the optical antenna proposed.

Mid-infrared optical sensing using sub-wavelength gratings

Optical sensing has shown great potential for both quantitative and qualitative analysis of compounds. In particular sensors which are capable of detecting changes in refractive index at a surface as well as in bulk material have received much attention. Much of the recent research has focused on developing technologies that enable such sensors to be deployed in an integrated photonic device. In this work we demonstrate experimentally, using a sub-wavelength grating the detection of ethanol in aqueous solution by interrogating its large absorption band at 9.54 μm. Theoretical investigation of the operating principle of our grating sensor shows that in general, as the total field interacting with the analyte is increased, the corresponding absorption is also increased. We also theoretically demonstrate how sub-wavelength gratings can detect changes in the real part of the refractive index, similar to conventional refractive index (RI) sensors.

Noninterleaved Metasurface for (26-1) Spin- and Wavelength-Encoded Holograms

Nanostructured metasurfaces demonstrate extraordinary capabilities to control light at the subwavelength scale, emerging as key optical components to physical realization of multitasked devices. Progress in multitasked metasurfaces has been witnessed in making a single metasurface multitasked by mainly resorting to extra spatial freedom, for example, interleaved subarrays, different angles. However, it imposes a challenge of suppressing the cross-talk among multiwavelength without the help of extra spatial freedom. Here, we introduce an entirely novel strategy of multitasked metasurfaces with noninterleaved single-size Si nanobrick arrays and minimalist spatial freedom demonstrating massive information on 6-bit encoded color holograms. The interference between electric dipole and magnetic dipole in individual Si nanobricks with in-plane orientation enables manipulating six bases of incident photons simultaneously to reconstructed 6-bit wavelength- and spin-dependent multicolor images. Those massively reconstructed images can be distinguished by pattern recognition. It opens an alternative route for integrated optics, data encoding, security encryption, and information engineering.

Resonant gratings with an etch-stop layer and a fabrication-error tolerant design

Sub-wavelength gratings (SWG) have shown much promise for applications such as lightweight high bandwidth reflectors, polarising filters and focusing lenses. Unfortunately, grating performance may be rapidly degraded through variability in grating dimensions. We demonstrate, in particular, how an error in depth of etch can be detrimental to the performance of zero contrast grating reflectors. We mitigate the impact of this fabrication error through the introduction of an etch stop layer and in so doing we experimentally realise a high bandwidth reflector based on this modified structure. Another common fabrication error is variation in the duty-cycle of fabricated gratings. This duty-cycle variation can weaken grating performance, however we demonstrate that grating designs that exhibit tolerance to duty-cycle fluctuation can be identified through simulation. Finally, we discuss the impact of lateral etching and the resulting sidewall concavity. We present our approach for numerically predicting the spectral response from such a grating and also for convenience we outline an approach for quickly approximating grating performance. Good agreement is observed between these numerical predictions and measurements made on a HCG with concave sidewalls.

Very high efficiency optical coupler for silicon nanophotonic waveguide and single mode optical fiber

Integrated optical circuits are poised to open up an array of novel applications. A vibrant field of research has emerged around the monolithic integration of optical components onto the silicon substrates. Typically, single mode optical fibers deliver the external light to the chip, and submicron single-mode waveguides then guide the light on-chip for further processing. For such technology to be viable, it is critically important to be able to efficiently couple light into and out of the chip platform, and between the different components, with low losses. Due to the large volume mismatch between a fiber and silicon waveguide (on the order of 600), it has been extremely challenging to obtain high coupling efficient with large tolerance. To date, demonstrated coupling has been relatively lossy and effective coupling requires impractical alignment of optical components. Here, we propose the use of a high contrast metastructure (HCM) that overcomes these issues, and effectively couples the off-chip, out-of-plane light waves into on-chip, in-plane waveguides. By harnessing the resonance properties of the metastructure, we show that it is possible to spatially confine the incoming free-space light into subwavelength dimensions with a near-unity (up to 98%) efficiency. The underlying coupling mechanism is analyzed and designs for practical on-chip coupler and reflector systems are presented. Furthermore, we explore the two-dimensional HCM as an ultra-compact wavelength multiplexer with superior efficiency (90%).

A proposal of a perfect graphene absorber with enhanced design and fabrication tolerance

We propose a novel device structure for the perfect absorption of a one-sided lightwavve illumination, which consists of a high-contrast grating (HCG) and an evanescently coupled slab with an absorbing medium (graphene). The operation principle and design process of the proposed structure are analyzed using the coupled mode theory (CMT), which is confirmed by the rigorous coupled wave analysis (RCWA). According to the CMT analysis, in the design of the proposed perfect absorber, the HCG, functioning as a broadband reflector, and the lossy slab structure can be optimized separately. In addition, we have more design parameters than conditions to satisfy; that is, we have more than enough degrees of freedom in the device design. This significantly relieves the complexity of the perfect absorber design. Moreover, in the proposed perfect absorber, most of the incident wave is confined in the slab region with strong field enhancement, so that the absorption performance is very tolerant to the variation of the design parameters near the optimal values for the perfect absorption. It has been demonstrated numerically that absorption spectrum tuning over a wider wavelength range of ~300 nm is possible, keeping significantly high maximum absorption (>95%). It is also shown that the proposed perfect absorber outperforms the previously proposed scheme in all aspects.

Design of angularly tolerant zero-contrast grating filters for pixelated filtering in the mid-IR range

Zero-contrast gratings (ZCG) can be used to implement narrow bandpass transmission filters. However, they suffer from poor angular tolerance, which hinders their use in pixelated applications. Combining ZCG with double-corrugation grating, we increase the resonance width and angular tolerance of the filter by more than 1 order of magnitude. Filters tunable around 4.6 μm with more than 90% transmission and compatibility with 140 μm pixel size are demonstrated.

High-finesse Fabry-Perot cavities with bidimensional Si3N4 photonic-crystal slabs

Light scattering by a two-dimensional photonic-crystal slab (PCS) can result in marked interference effects associated with Fano resonances. Such devices offer appealing alternatives to distributed Bragg reflectors and filters for various applications, such as optical wavelength and polarization filters, reflectors, semiconductor lasers, photodetectors, bio-sensors and non-linear optical components. Suspended PCS also have natural applications in the field of optomechanics, where the mechanical modes of a suspended slab interact via radiation pressure with the optical field of a high-finesse cavity. The reflectivity and transmission properties of a defect-free suspended PCS around normal incidence can be used to couple out-of-plane mechanical modes to an optical field by integrating it in a free-space cavity. Here we demonstrate the successful implementation of a PCS reflector on a high-tensile stress Si₃N₄ nanomembrane. We illustrate the physical process underlying the high reflectivity by measuring the photonic-crystal band diagram. Moreover, we introduce a clear theoretical description of the membrane scattering properties in the presence of optical losses. By embedding the PCS inside a high-finesse cavity, we fully characterize its optical properties. The spectrally, angular- and polarization-resolved measurements demonstrate the wide tunability of the membrane's reflectivity, from nearly 0 to 99.9470±0.0025%, and show that material absorption is not the main source of optical loss. Moreover, the cavity storage time demonstrated in this work exceeds the mechanical period of low-order mechanical drum modes. This so-called resolved-sideband condition is a prerequisite to achieve quantum control of the mechanical resonator with light.

Normal incidence filters using symmetry-protected modes in dielectric subwavelength gratings

We investigate narrowband transmission filters based on subwavelength-grating reflectors at normal incidence. Computational results show that the filtering is realized through symmetry-protected mode coupling. The guided mode resonances introduced by the slab layer allow flexible control of the filter frequencies. The quality factor of the filters could exceed 10⁶. Dielectric gratings can be used over the entire range of electromagnetic waves, owing to their scale-invariant operations. Owing to the high refraction index and low index dispersion of semiconductors in the infrared range, these filters can be applied over a broad range from near infrared to terahertz frequencies.

Realization of high-contrast gratings operating at 10 μm

We present a new material pairing that can be used to realize high-contrast gratings at wavelengths of 10 μm and greater. Using only optical lithography, the material pair solves the absorption issue limiting the popular Si/SiO₂ pairing from operation above 6 μm. We describe the obstacles that exist with the currently used grating materials for this wavelength range and outline why our chosen materials overcome this obstacle. We numerically demonstrate that gratings utilizing these materials are capable of wideband high reflectivity. We experimentally show that the spectral response of gratings that are fabricated using such a process show good agreement with theoretically predicted performance.

Grating-coupled Otto configuration for hybridized surface phonon polariton excitation for local refractive index sensitivity enhancement

We demonstrate numerically through rigorous coupled wave analysis (RCWA) that replacing the prism in the Otto configuration with gratings enables us to excite and control different modes and field patterns of surface phonon polaritons (SPhPs) through the incident wavelength and height of the Otto spacing layer. This modified Otto configuration provides us the following multiple modes, namely, SPhP mode, Fabry-Pérot (FP) cavity resonance, dielectric waveguide grating resonance (DWGR) and hybridized between different combinations of the above mentioned modes. We show that this modified grating-coupled Otto configuration has a highly confined field pattern within the structure, making it more sensitive to local refractive index changes on the SiC surface. The hybridized surface phonon polariton modes also provide a stronger field enhancement compared to conventional pure mode excitation.

Evaluation and improvement of simplified modal method for designing dielectric gratings

Recently, the simplified modal method (SMM) has proved to be very successful to facilitate grating design by reducing the diffraction problem to the interference (and reflection at interfaces) of a very small number of grating modes In this work, an intuitive and fully-analytical matrix formalism is developed to evaluate and improve the SMM. The present method focuses on the coupling between the grating modes and the influence of evanescent modes, which have not been touched on in detail in previous formulations of the SMM. In particular, we show that when there are only two grating modes, their coupling is exactly zero only for Littrow mounting and the reflection coefficients also reduce to the familiar Fresnel's form as is commonly used by previous formulations. For other incidence angles, mode coupling can be significant, and our model shows greatly improved accuracy over the common SMM when compared with numerical results. A new parameter measuring the boundary condition mismatch and reflecting the accuracy of the method is proposed, which can serve as a criterion for choosing the number of evanescent modes in the model. The improved model will be of great value for grating designs.

High resolution on-chip optical filter array based on double subwavelength grating reflectors

An optical filter array consisting of vertical narrow-band Fabry-Pérot (FP) resonators formed by two highly reflective high contrast subwavelength grating mirrors is reported. The filters are designed to cover a wide range of operation wavelengths (Δλ/λ = 5%) just by changing the in-plane grating parameters while the device thickness is maintained constant. Operation in the telecom band with transmission efficiencies greater than 40% and quality factors greater than 1,000 are measured experimentally for filters fabricated on the same substrate.

Near-wavelength diffraction gratings for surface plasmon polaritons

In this Letter, we study numerically diffraction gratings for surface plasmon polaritons. Investigated plasmonic gratings consist of a periodic set of dielectric ridges located on the metal surface. The grating period is comparable with the wavelength of the incident wave. The simulation results obtained within the rigorous coupled-wave analysis framework have shown that the SPP diffraction on a plasmonic grating with parasitic scattering suppression is remarkably close to the diffraction of a TE-polarized plane wave on a conventional grating. A compact and efficient reflecting plasmonic grating is considered as an example. The presented results can be used for the design of high-efficiency two-dimensional optical elements for steering surface plasmon propagation.

Broadband high reflectivity in subwavelength-grating slab waveguides

We computationally study a subwavelength dielectric grating structure, show that slab waveguide modes can be used to obtain broadband high reflectivity, and analyze how slab waveguide modes influence reflection. A structure showing interference between Fabry-Perot modes, slab waveguide modes, and waveguide array modes is designed with ultra-broadband high reflectivity. Owing to the coupling of guided modes, the region with reflectivity R > 0.99 has an ultra-high bandwidth (Δf / ̅f > 30%). The incident-angle region with R > 0.99 extends over a range greater than 40°. Moreover, an asymmetric waveguide structure with a semiconductor substrate is studied.

Experimentally-implemented genetic algorithm (Exp-GA): toward fully optimal photovoltaics

The geometry and dimension design is the most critical part for the success in nano-photonic devices. The choices of the geometrical parameters dramatically affect the device performance. Most of the time, simulation is conducted to locate the suitable geometry, but in many cases simulation can be ineffective. The most pronounced examples are large-area randomized patterns for solar cells, light emitting diode (LED), and thermophtovoltaics (TPV). The large random pattern is nearly impossible to calculate and optimize due to the extended CPU runtime and the memory limitation. Other scenarios that numerical simulations become ineffective include three-dimensional complex structures with anisotropic dielectric response. This leads to extended simulation time especially for the repeated runs during its geometry optimization. In this paper, we show that by incorporating genetic algorithm (GA) into real-world experiments, shortened trial-and-error time can be achieved. More importantly, this scheme can be used for many photonic design problems that are unsuitable for simulation-based optimizations. Moreover, the experimentally implemented genetic algorithm (Exp-GA) has the additional advantage that the resultant objective value is a real one rather than a theoretical one. This prevents the gaps between the modeling and the fabrication due to the process variation or inaccurate numerical models. Using TPV emitters as an example, 22% enhancement in the mean objective value is achieved.

Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve the solar conversion efficiency of a concentrated photovoltaic system. The proposed system will also lower the solar cell cost in the concentrated photovoltaic system by replacing the expensive tandem solar cells with the cost-effective single junction solar cells. The structures and the parameters of high contrast gratings for the dispersive elements were numerically optimized. The large-area fabrication of high contrast gratings was experimentally demonstrated using nanoimprint lithography and dry etching. The quality of grating material and the performance of the fabricated device were both experimentally characterized. By analyzing the measurement results, the possible side effects from the fabrication processes are discussed and several methods that have the potential to improve the fabrication processes are proposed, which can help to increase the optical efficiency of the fabricated devices.

Ultracompact resonator with high quality-factor based on a hybrid grating structure

We numerically investigate the properties of a hybrid grating structure acting as a resonator with ultrahigh quality factor. This reveals that the physical mechanism responsible for the resonance is quite different from the conventional guided mode resonance (GMR). The hybrid grating consists of a subwavelength grating layer and an un-patterned high-refractive-index cap layer, being surrounded by low index materials. Since the cap layer may include a gain region, an ultracompact laser can be realized based on the hybrid grating resonator, featuring many advantages over high-contrast-grating resonator lasers. The effect of fabrication errors and finite size of the structure is investigated to understand the feasibility of fabricating the proposed resonator.

Tunable dark modes in one-dimensional "diatomic" dielectric gratings

Recently researchers have demonstrated ultra high quality factor (Q) resonances in one-dimensional (1D) dielectric gratings. Here we theoretically investigate a new class of subwavelength 1D gratings, namely "diatomic" gratings with two nonequivalent subcells in one period, and utilize their intrinsic dark modes to achieve robust ultra high Q resonances. Such "diatomic" gratings provide extra design flexibility, and enable high Q resonators using thinner geometry with smaller filling factors compared to conventional designs like the high contrast gratings (HCGs). More importantly, we show that these high Q resonances can be efficiently tuned in situ, making the design appealing in various applications including optical sensing, filtering and displays.

Dispersion engineering for vertical microcavities using subwavelength gratings

We show that the energy-momentum dispersion of a vertical semiconductor microcavity can be modified by design using a high-index-contrast subwavelength grating (SWG) as a cavity mirror. We analyze the angular dependence of the reflection phase of the SWG to illustrate the principles of dispersion engineering. We show examples of engineered dispersions such as ones with much reduced or increased energy density of states and one with a double-well-shaped dispersion. This method of dispersion engineering is compatible with maintaining a high cavity quality factor and incorporating fully protected active media inside the cavity, thus enabling the creation of new types of cavity quantum electrodynamics systems.

2D photonic-crystal optomechanical nanoresonator

We present the optical optimization of an optomechanical device based on a suspended InP membrane patterned with a 2D near-wavelength grating (NWG) based on a 2D photonic-crystal geometry. We first identify by numerical simulation a set of geometrical parameters providing a reflectivity higher than 99.8% over a 50-nm span. We then study the limitations induced by the finite value of the optical waist and lateral size of the NWG pattern using different numerical approaches. The NWG grating, pierced in a suspended InP 265-nm thick membrane, is used to form a compact microcavity involving the suspended nanomembrane as an end mirror. The resulting cavity has a waist size smaller than 10 μm and a finesse in the 200 range. It is used to probe the Brownian motion of the mechanical modes of the nanomembrane.

Flat grating lens utilizing widely variable transmission-phase via guided-modes

We experimentally demonstrate a polarization-independent flat grating lens in the near-infrared region. The grating lens consists of ridges in the square lattice arrangement, and the ridge dimensions are gradually changed to distribute a phase map with focusing ability. It is well known that guided modes in gratings offer unity-reflection at a resonance, and therefore the transmission phase is widely varied around the resonance. We employ such transmission phase behavior and show that high transmittance is obtained in each unit cell for wide variation range of the transmission phase at the operation wavelength by sharpening the resonance. This enables us to accomplish a highly efficient transmissive grating lens.

Nanostructure arrays in free-space: optical properties and applications

Dielectric and metallic gratings have been studied for more than a century. Nevertheless, novel optical phenomena and fabrication techniques have emerged recently and have opened new perspectives for applications in the visible and infrared domains. Here, we review the design rules and the resonant mechanisms that can lead to very efficient light-matter interactions in sub-wavelength nanostructure arrays. We emphasize the role of symmetries and free-space coupling of resonant structures. We present the different scenarios for perfect optical absorption, transmission or reflection of plane waves in resonant nanostructures. We discuss the fabrication issues, experimental achievements and emerging applications of resonant nanostructure arrays.

Experimental investigation of double-groove grating satisfying total internal reflection condition

We experimentally demonstrate a TiO(2) double-groove grating coupler with two different groove widths on a SiO(2) substrate in the visible region. Tolerance investigations based on Bloch-mode profiles in the grating and coupling strengths between the Bloch modes and diffraction orders reveal that the transmission performance is robust when one of the paired ridges is narrow enough (60 nm and less) considering a typical nanofabrication accuracy. Moreover, the ridge shape affects weakly the transmission performance due to the non-resonance operation of our dielectric device. Such tolerance investigations together with current nanofabrication technology enable us to accomplish a 70% efficiency for coupling the normal incident light into the + 1st order transmission diffraction satisfying the total internal reflection condition at a 640 nm wavelength of operation.

Hybrid grating reflector with high reflectivity and broad bandwidth

We suggest a new type of grating reflector denoted hybrid grating (HG) which shows large reflectivity in a broad wavelength range and has a structure suitable for realizing a vertical cavity laser with ultra-small modal volume. The properties of the grating reflector are investigated numerically and explained. The HG consists of an un-patterned III-V layer and a Si grating. The III-V layer has a thickness comparable to the grating layer, introduces more guided mode resonances and significantly increases the bandwidth of the reflector compared to the well-known high-index-contrast grating (HCG). By using an active III-V layer, a laser can be realized where the gain region is integrated into the mirror itself.

Cavity-enhanced measurements for determining dielectric-membrane thickness and complex index of refraction

The material properties of silicon nitride (SiN) play an important role in the performance of SiN membranes used in optomechanical applications. An optimum design of a subwavelength high-contrast grating requires accurate knowledge of the membrane thickness and index of refraction, and its performance is ultimately limited by material absorption. Here we describe a cavity-enhanced method to measure the thickness and complex index of refraction of dielectric membranes with small, but nonzero, absorption coefficients. By determining Brewster's angle and an angle at which reflection is minimized by means of destructive interference, both the real part of the index of refraction and the sample thickness can be measured. A comparison of the losses in the empty cavity and the cavity containing the dielectric sample provides a measurement of the absorption.

Modelling of diffraction grating based optical filters for fluorescence detection of biomolecules

The detection of biomolecules based on fluorescence measurements is a powerful diagnostic tool for the acquisition of genetic, proteomic and cellular information. One key performance limiting factor remains the integrated optical filter, which is designed to reject strong excitation light while transmitting weak emission (fluorescent) light to the photodetector. Conventional filters have several disadvantages. For instance absorbing filters, like those made from amorphous silicon carbide, exhibit low rejection ratios, especially in the case of small Stokes' shift fluorophores (e.g. green fluorescent protein GFP with λ exc = 480 nm and λ em = 510 nm), whereas interference filters comprising many layers require complex fabrication. This paper describes an alternative solution based on dielectric diffraction gratings. These filters are not only highly efficient but require a smaller number of manufacturing steps. Using FEM-based optical modelling as a design optimization tool, three filtering concepts are explored: (i) a diffraction grating fabricated on the surface of an absorbing filter, (ii) a diffraction grating embedded in a host material with a low refractive index, and (iii) a combination of an embedded grating and an absorbing filter. Both concepts involving an embedded grating show high rejection ratios (over 100,000) for the case of GFP, but also high sensitivity to manufacturing errors and variations in the incident angle of the excitation light. Despite this, simulations show that a 60 times improvement in the rejection ratio relative to a conventional flat absorbing filter can be obtained using an optimized embedded diffraction grating fabricated on top of an absorbing filter.

Optical phased array using high contrast gratings for two dimensional beamforming and beamsteering

We have developed a microelectromechanical system (MEMS) optical phased array incorporating a high-index-contrast subwavelength grating (HCG) for beamforming and beamsteering in a range of ± 1.26° × 1.26°. Our approach needs only a thin single-layer HCG made of silicon, considerably improving its speed thanks to the low mass, and is suitable for high optical power applications. The measured resonant frequency of HCG is 0.32 MHz.

Experimental realization of a high-contrast grating based broadband quarter-wave plate

Fabrication and experimental characterization of a broadband quarter-wave plate, which is based on two-dimensional and binary silicon high-contrast gratings, are reported. The quarter-wave plate feature is achieved by the utilization of a regime, in which the proposed grating structure exhibits nearly total and approximately equal transmission of transverse electric and transverse magnetic waves with a phase difference of approximately π/2. The numerical and experimental results suggest a percent bandwidth of 42% and 33%, respectively, if the operation regime is defined as the range for which the conversion efficiency is higher than 0.9. A compact circular polarizer can be implemented by combining the grating with a linear polarizer.