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Ethylene (C2H4) has many commercial usage and in agriculture it plays a key role in climacteric fruit ripening. This paper reports on a copper complex-coated nanostructures patterned on the cleaved facet of an optical fiber to realize a high-performance ethylene gas sensor. The novelty of this paper lies in fiber-optic based sensitive and selective monitoring of gaseous ethylene with excellent performance. A periodic array of polymer nanoposts are formed at the fiber tip and coated with titanium dioxide to serve as a guided mode resonant (GMR) device. A direct method of selectivity is developed for recognizing ethylene, by applying a copper (I) complex coating atop the GMR fiber-tip. The nanopatterned fiber-tip GMR sensor exhibits resonance sensitivity to variations in refractive index at the surface. For repeated use, a simple and cost-effective controllable heater is also integrated at the fiber tip to achieve a complete desorption of the analyte molecules from the sensor surface. This is a first-of-its-kind heater-integrated fiber optic sensor utilizing guided mode resonance and coated with a copper (I) complex coating, that is demonstrated for tracking the ethylene-promoted ripening and senescence of banana. The proposed sensor provides a maximum sensitivity of 60 pm/ppm to ethylene gas with a limit of detection of ~4.7 ppm. Compared to our earlier non-specific fiber-tip gas sensor, we improved the limit of detection for ethylene by 170-fold and sensitivity by 60-fold.

We are reporting the design procedure for a narrow-band ultra-low-power reflective optical modulator that operates at the critical coupling of an appropriately designed two-dimensional grating with the center frequency of a quarter wavelength distributed Bragg reflector (DBR). By sandwiching a three-layer stack of gated ITO/HfO2/ITO between the DBR and the grating, we can make the optical modulator operational, taking advantage of the tunable property of the ultrathin layer at the ITO/HfO2 interface accumulated by electrons under an ultra-low applied voltage (−0.1 V). The corresponding energy consumption is ∼5.5 fJ/bit. Moreover, our simulations show that the capacitance limited modulation speed is more than 80 Mbps. Our numerical results also predict the influence of possible fabrication errors on the guided-mode resonance wavelength. This investigation shows that a ±3 nm deviation in the grating pitch affects the modulator performance profoundly. Furthermore, the numerical results demonstrate the modulation depth of ∼24 dB is achievable for an appropriately designed modulator with an acceptable insertion loss of ∼0.05 dB. This paper paves the way for developing next-generation optical modulators with high modulation depth, low insertion loss, and ultra-low energy consumption.

In this study, we demonstrated a hydrogen (H2) sensor based on a guided-mode resonance (GMR) filter coated with a layer of palladium (Pd). The sensor consists of three key layers, namely the substrate (replicated grating structure on an optical adhesive), waveguiding layer of TiO2, and active-sensing layer of Pd. The sensor functions as an optical resonator. On absorption of H2, the change of the refractive index in Pd changes the optical response of the sensor. To monitor the output optical response, a transmission setup was used, and two detection modalities—the change in transmission efficiency at a specific wavelength and the shift in the resonant wavelength—were demonstrated. Upon injection of a mixture of 4% H2 and 96% argon, the resonant wavelength shifted 6.38 nm, and transmission efficiency increased by 45% in 24 s.

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.

Metalens is a planar optical component that uses nanostructures with a thickness on the order of the wavelength to manipulate the wavefront of the incident light. A key problem, especially in color imaging and display applications, is the correction of chromatic aberration, which is an inherent effect caused by the dispersion of periodic lattices and resonance modes. However, the current achromatic metalenses either use the PB phase method that is only valid for circularly polarized light or nanostructures with complex cross sections that are difficult to manufacture. Here, we designed a broadband achromatic metalens for linearly polarized light from 450 to 800 nm. Rectangular titanium dioxide nanofins of various lengths and widths were applied to modulate the phase and dispersion of the incident light. The metalens can fulfill three target phases simultaneously by using an optimization method. The designed metalens has a stable focus from 450 to 800 nm with an average focusing efficiency of 64%. It can be potentially applied in microscopes, lithography machines, sensors, and displays.

Active waveband selective switches for the telecommunication wavebands are fabricated and demonstrated. This type of active waveband selective switch is composed of a guided-mode resonance filter, a twisted nematic liquid crystal (TN-LC) layer, and a linear polariser. In voltage-off status, a fabricated active waveband selective switch with grating period of 893.86 nm exhibits high resonance peak with wavelength around 1311.2 nm. In voltage-on status, this switch exhibits a high resonance peak with wavelength around 1551.3 nm. The measured reflection peaks of another active waveband selective switch with grating period of 872.84 nm are located at 1309.1 and 1549.4 nm in voltage-on status and voltage-off status, respectively. Thus, light waveband can be tuned between O-band and C-band by electrically driving the TN-LC layer of this type of active waveband selective switch in real time.

Aptamer biosensor for label-free thrombin detection based on a tapered fiber-optic interferometer has been proposed and demonstrated. The biosensor probe can be easily fabricated by tapering a commercial double-cladding fiber down to 5 μm in diameter and it provides a high refractive index sensitivity of 1660 nm/RIU. The changes are determined by tracing the wavelength shift of the dip in the transmission spectrum with variable surrounding refractive index. With the help of the surface functionalization of thrombin-binding aptamers, the proposed fiber-optic biosensor proves its capability in label-free detection of thrombin with a lowest detectable concentration of 0.1 μm, together with high specificity in various concentrations of pure and mixed thrombin solutions.

Because silicon photonic crystal (PC) high-quality factor (high- Q ) nanocavities are useful for the development of novel optical devices with small sizes, they have been subject to intense research. In many of these research works, continuous-wave (cw) lasers were used as excitation light sources. However, cw lasers can be disadvantageous for certain applications. In this study, we report on the excitation of high- Q resonance modes of PC nanocavities by a superluminescent diode (SLD). Since the emission from the SLD has a broad spectrum that extends over several tens of nanometers, it is able to simultaneously excite many nanocavities with significantly different resonance wavelengths. Furthermore, the SLD is able to excite the high- Q resonance modes in a stable manner; SLD excitation achieves cavity emission intensities that are hardly influenced by resonance-wavelength shifts due to temperature changes or natural surface oxidization. We demonstrate that by using the SLD as excitation light source, the emission from a nanocavity with a Q of several tens of thousands can be clearly observed by a near-infrared InGaAs camera. In addition, we demonstrate the refractive-index change sensing using the SLD excitation of a nanocavity array as an application example.

In this paper, the general characteristics of the polarization-dependent tuning property and sensitivity optimization of the graphene integrated tilted fiber Bragg grating (TFBG) are investigated theoretically. The tuning mechanism of the graphene on mode characteristics and transmission spectrum is thoroughly explored. The results reveal that the graphene induces a stronger influence on the s-polarized mode than its p-polarized counterpart, but the TFBGs with or without graphene in all the polarized states have nearly the same wavelength sensitivity ( $\sim \text{540}\; \text{nm/RIU}$ in average). In contrast, the resonance strength is highly dependent on the tunable state of the graphene, especially for the s-polarized case. In consequence, the normalized power response is increased in the low refractive index region. More importantly, the s-polarized leaky mode resonance that is ignored for regular TFBGs is significantly enhanced (by increasing field content in the fiber region and decreasing the confinement loss) whereas the guided counterpart is suppressed greatly by the graphene with lower chemical potential. The sensitivity of up to $ \text{6595.8 dB/RIU}$ (9.17 times higher than that of the regular counterpart) is achieved in response to an extremely small analyte perturbation ( $ \text{10}^{-\text{4}}$ close to 1.333) based on the graphene enhanced leaky mode resonance. This paper provides a novel technique for significantly optimizing the sensing performance of the TFBG, which has great potential in biological and biochemical sensing applications.

A polarization-independent perfect graphene absorber is numerically demonstrated. The novel absorber is composed of an array of dielectric square resonators and a graphene monolayer on a Bragg reflector. An absorption peak at a wavelength of 1550 nm with an absorbance of up to ∼ 99 % is achieved, owing to the critical coupling effect. Moreover, the designed graphene absorber exhibits an ultra-narrow spectrum bandwidth with an ultra-high quality factor, owing to the combined effects of the guided mode resonance achieved with the dielectric square resonators and photonic band gap with the Bragg reflector. In addition to the numerical results, an analysis model based on the coupled-mode theory is presented. Moreover, the electromagnetic field intensity distributions are plotted to provide a physical interpretation of the complete absorption. This study paves the way for the development of a perfect ultra-narrow spectrum-selective graphene absorber.

Photonic crystal slabs are the state of the art in studies for the light confinement, optical wave modulating and guiding, as well as nonlinear optical response. Previous studies have shown abundant real-world implementations of photonic crystals in planar optics, metamaterials, sensors, and lasers. Here, we report a novel full 2{\pi} phase control method in the reflected light beam over the interaction with a photonic crystal resonant mode, verified by the temporal coupled-mode analysis and S-parameter simulations. Enhanced by the asymmetric coupling with the output ports, the 2{\pi} phase shift can be achieved with the silicon photonics platforms such as Silicon-on-Silica and Silicon-on-Insulator heterostructures. Such photonic crystal phase control method provides a general guide in the design of phase-shift metamaterials, suggesting a wide range of applications in the field of sensing, spatial light modulation, and beam steering., Comment: Including supplementary

Guided-mode resonance (GMR) displays a wide range of optical properties that could prove useful for many applications in photonics. We report the enhancement of laser emission from the GMR sparse grating-based dye laser. The enhancement is due to an introduction of the low-refractive-index (low-n) separation layer, which causes a strong interaction between the local electric field and dye molecules. Finite-difference time-domain method is used to study how the structure’s geometry, including the thickness of the separation layer and the fill factor of the grating, affects the electric field and laser intensity. The results show that the emission can be enhanced by a factor of up to 8 compared with high-refractive-index SU-8 as the separation layer. The laser intensity reaches to the maximum value when the fill factor of the grating is 0.2, i.e., the sparse grating is used. The threshold of the low-n GMR dye laser is also examined. This design can effectively improve the performance of pumped photonic band-edge lasers.

Integrated photonic sensors can provide large scale, flexible detection schemes. Photonic crystal slabs (PCSs) offer a miniaturized platform for wideband, sensitive ultrasound detection by exploiting the photoelastic effect in water. However, poor modal overlap with the sensing medium and non-negligible absorption loss of the aqueous medium have previously limited PCS sensor performance. In this study, we detail the development and optimization of a PCS-based acoustic sensor by adding to it a low-loss high-index polymer cladding layer. Exploiting a mode-optimized TM-like optical resonance present in a PCS, with high bulk index sensitivity ( > 600 n m / R I U ) and quality factor Q ( > 8000 ), we demonstrate real-time ultrasound sensing at a noise equivalent pressure of 170 Pa ( 1.9 P a / H z ). The PCS sensor is backside-coupled to an optical fiber, which, along with its intensity-based ultrasound-sensing architecture, will allow us to scale up easily to a 2D array. This work paves the way to a sensitive compact ultrasound detector for photoacoustic-based diagnostics and monitoring of tissue.

The long-wave infrared (LWIR) spectroscopy has emerged as a promising technique for applications ranging from medical diagnosis to satellite imaging and terrestrial imaging. However, traditional optical elements are not realized well for the LWIR region. In this work, notch filters for LWIR spectral range (8 ~ 12 μm) based on the guided mode resonance (GMR) effect were designed, fabricated, and characterized with Germanium (Ge) thin film on Zinc Selenide (ZnSe) substrates. In contrast to the typical photolithography process, which faces challenge of resolution limit, we used e-beam lithography process to pattern the grating. By using a reactive ion etching process with a mixture of etching and passivation gases, we fabricated grating with well-defined vertical sidewalls. Finite-difference time-domain (FDTD) method was used to calculate the optical responses and model the geometry of the notch filters, and the optical transmittance of fabricated filters agrees well with the calculations. Moreover, we demonstrate the improved notch filtering response with reduced Fabry-Perot noise by introducing anti-reflection layer on the bottom of the ZnSe substrate. Therefore, findings of this work will be useful for various filter fabrications that prefer high spatial resolution in the LWIR spectral region.

All-dielectric metasurfaces have attracted extensive attention because of their ability to control electromagnetic waves with low loss. However, compared with the research on the physical basis of a single nanoparticle controlling electromagnetic waves, foundational research on the influence of the periodicity of all-dielectric metasurfaces on their control of electromagnetic waves is still insufficient. Thus, the aim of this article is to further explore and emphasize the effect of the periodicity of metasurfaces on their optical performance and their electromagnetic response and coupling, and to describe how the above theoretical exploration can be employed for designing devices. We first explore some optical performance caused by the periodicity of cube silicon metasurfaces in the near-infrared band and reveal that these phenomena cannot be explained by the Mie resonances of the single nanoparticles composing the metasurfaces but by the lattice resonance theory and guided-mode resonance filter theory. Then, the influence of the periodicity on the electromagnetic response and coupling in the metasurfaces are analyzed via the multipole expansion method. We emphasize that the electromagnetic coupling between array nanoparticles enhances dipole and octupole radiation in the metasurfaces and analyze how the radiation is enhanced. We also study some phenomena and applications caused by the guided-mode resonances for future design. The results of this study may have potential applications in lasers, reflectors, absorbers, detectors, and other fields, and provide theoretical basis for the design of specific performance devices by adjusting the size of metasurfaces related to periodicity.

We propose a tunable dual-wavelength absorption (TDWA) switch based on an asymmetric guided mode resonance (AGMR) structure. A TDWA switch consists of a graphene layer and an AGMR structure sandwiched by cap and slab layers on a buffer/silicon substrate. The AGMR structure adds a smaller grating unit cell next to a larger one, exciting a second resonance close to but distinct from the first resonance. For switching, the TDWA between an absorptive or reflective mode with each on-/off-state, the chemical potential of graphene is tuned from 0.0 eV to 0.6 eV. For the absorptive mode, two absorption peaks of ≥ 96.2% are separated by 23 nm, both having an on-off ratio of ∼15.52. For the reflective mode, two reflectance peaks of ≥ 93.8% are separated by 23 nm, having on-off ratios of 15.56 dB and 18.95 dB. The maximum on-off ratios of 39.98 dB and 34.55 dB are achieved near the reflectance peaks. Both the period of the AGMR and the cap thickness alters the two peak wavelengths linearly, while the grating width of the AGMR varies nonlinearly from 17 nm to 28 nm. The buffer excites a weak Fabry-Perot resonance, which interacts with the TDWA structure, the result of which is the two absorption peaks are varied. Finally, as the incidence angle of light increases up to 5.3°, the distance of the two peak wavelengths is tuned from ∼22 nm to ∼77 nm with ≥ 96% absorption or ≥ 93% reflectance in each mode.

We describe recent progress in the development of spectrally tunable micro-engineered notch filters operating in the longwave infrared (LWIR) region from 8 to 12 µm based on using the guided-mode resonance (GMR) effect. The device structure consists of a subwavelength dielectric grating on top of a homogeneous waveguide using high-index dielectric transparent materials, i.e., germanium (Ge) with a refractive index of 4.0 and zinc selenide (ZnSe) with a refractive index of 2.4. We design the filters to reflect the incident broadband light at one (or more) narrow spectral band while fully transmitting the rest of the light. Filters based on one-dimensional (1-D) gratings are polarization-dependent and those based on two-dimensional (2-D) gratings are less polarization-dependent. We designed and characterized both 1-D and 2- D filters. Anti-reflection coatings (ARCs) were applied on the backside of some of the filter substrates to improve transmission over the entire spectral region. We carried out transmission measurements of these filters using two separate experimental setups—an automated tunable room-temperature quantum cascade laser (QCL) system as well as a modified Fourier Transform Infrared (FTIR) spectrometer with normal incidence of light on the sample. We will present filter designs, theoretical simulation, characterization experiments and results.

We present new, to the best of our knowledge, large-scale, high-quality spectral filters operating in the long-wave infrared (LWIR) spectral region. We employ high-spatial resolution nanofabrication techniques to achieve large-area ( 12 m m × 12 m m ) spectrally tunable notch filters. Filter operation is based on the guided-mode resonance effect. The device structure consists of a germanium waveguide grating on top of a zinc selenide substrate. The filters reflect the incident broadband light at one (or more) narrow spectral bands while fully transmitting the rest. We tune the reflected wavelength by tilting the filter. Filters based on one-dimensional gratings are polarization sensitive. We fabricate prototype filters and characterize their polarization dependence and spectral tuning performance using a tunable quantum cascade laser system that spans the ∼ 7 − 13 µ m spectral band. We obtain an excellent agreement between the theoretical and experimental results.

We developed hybrid graphene-Si3N4 cross-bar photonic crystal cavity devices, strengthening light interaction with the coupled carbon nanotube leading to generation of an enhanced electroluminescent signal in telecom wavelength band.

We propose a cost-effective narrow bandpass filter based on a guided-mode resonance (GMR) structure, operating at full conical mounting configuration. Two additional layers above the GMR structure with designed geometry is considered to create a large angle of incidence (AOI) on the GMR structure with an almost normal AOI on the entire structure. In the visible wavelength range, the transmission spectrum contains three peaks: two peaks that are due to the GMR effect and one that is due to thin-film interference. The modal characteristic of the two GMR peaks is identified using a matrix method for the n-layer waveguide structure. The two GMR peaks with bandwidths of ∼ 1 n m and ∼ 0.5 n m correspond to the fundamental TE and TM modes, respectively. The GMR peaks can be pushed away from the thin-film interference peak by modulating the grating depth.

Optical super-oscillatory lenses based on planar micro/nano structures have been demonstrated as promising alternatives for shaping wavefronts of light and realizing super-resolution images in a NA-limited optical system. However, as the super-oscillatory foci originated from the delicate interference of the light, the change of the parameters might destroy the hotspots, such as the incident wavelength. Here, a multiwavelength achromatic super-oscillatory metasurface (ASOM) is proposed through simultaneously controlling distinct wavelength-dependent wavefronts. The constructed multiwavelength ASOM is then verified numerically, and the foci are precisely formed at the same axial plane for the design wavelengths with resolution beyond the diffraction limit. We expect that our proposed multiwavelength controllable method will give more freedom for the designs of planar and lightweight components, which would be useful in optical applications, such as data storage, super-resolution imaging, holography, etc.

Resonant biosensors are attractive for diagnostics because they can detect clinically relevant biomarkers with high sensitivity and in a label-free fashion. Most of the current solutions determine their detection limits in a highly stabilised laboratory environment, which does, however, not apply to real point-of-care applications. Here, we consider the more realistic scenario of low-cost components and an unstabilised environment and consider the related design implications. We find that sensors with lower quality-factor resonances are more fault tolerant, that a filtered LED lightsource is advantageous compared to a diode laser, and that a CMOS camera is preferable to a CCD camera for detection. We exemplify these findings with a guided mode resonance sensor and experimentally determine a limit of detection of 5.8 ± 1.7×10−5 refractive index units (RIU), which is backed up by a model identifying the various noise sources. Our findings will inform the design of high performance, low cost biosensors capable of operating in a real-world environment.

Greatly improving the light absorption efficiency of graphene and simultaneously manipulating the corresponding absorption bandwidth (broadband or narrowband) is practically important to design graphene-based optoelectronic devices. In this work, we will theoretically show how to largely enhance the absorption in graphene and efficiently control the absorption bandwidth in the visible region, by the excitation of the waveguide mode for the graphene monolayer to be sandwiched between the gold sphere array and dielectric waveguide structure composed of indium tin oxide (ITO) film on a quartz substrate. It is found that the maximum absorption efficiency can reach as high as about 45% and the full-width at half-maximum (FWHM) of the absorption peak can be tuned from about 1 to 10 nanometers, when the array period of gold spheres or the thickness of ITO film is changed.

In optical nanostructures used as artificial resonance-based color filters, there is unfortunate universal trade-off between spectral sharpness and angular tolerance as well as maximum extinction. We rigorously derive the maximum performance bounds of wavelength-rejection filters realized by single-layer plasmonic metasurfaces with a dominant resonance and weak near-field coupling, and propose a multi-layer approach to overcome these single-layer limits and trade-offs. We also present a realistic example that has a narrow full-width-at-half-maximum bandwidth of 24 nm with 10 dB extinction at 532 nm with good angular tolerance up to 60°. The performance of the proposed metasurface is close to the general theoretical bound.

A comparison of optical sensors&rsquo, characteristics based on guided-mode resonance has been carried out. It was considered a prism structure with a metal film, a metal grating on a metal substrate and a dielectric grating on a dielectric substrate. It is shown that the main characteristics are determined by the sensitivity of the constant propagation of the respective waveguides on a change in wavelength and a change in the refractive index of the tested medium. In addition, they depend on the full width at half maximum of the spectral or angular reflectance dependence. The corresponding analytical relationships obtained for the three types of sensors are almost the same. It is demonstrated that the ratio of the sensor spectral sensitivity on the resonance curve spectral width is equal to the ratio of the angular sensitivity on the angular width of the corresponding resonance curve for all three types of sensors.

We propose a generalized circuit model for accurate analysis of the extraordinary transmission phenomenon in multilevel stepped slit structures. The multilevel stepped slit consists of some continuous sub-wavelength metal slits with different widths. By applying the proposed circuit model, we study the extraordinary transmission property of this structure for the two cases of ideal and real metal structures. The accuracy of the achieved results is validated by a full wave analysis based on the finite element method. Additionally, since the multilevel slit structure is adjustable by simply manipulating slit parameters such as widths and heights, various kinds of transmission spectra and a different number of resonance points are investigated by the circuit model and the numerical approach. Multilevel stepped slits can be used potentially as bandpass filters or optical attenuators in photonics integrated circuits.

We provide the experimental research on a novel type of all-dielectric magnetic structure designed to achieve an enhanced magneto-optical response. 1D grating fabricated via etching of bismuth substituted iron garnet film supports the excitation of optical guided modes, which are highly sensitive to the external magnetic field. A unique feature of proposed structure is the synergetic combination of high transparency, tunability, high Q-factor of the resonances and superior magneto-optical response that is two orders higher in magnitude than in the non-structured smooth iron-garnet film. The considered all-dielectric magnetic garnet structures have great potential in various fields including the magneto-optical modulation of light, biosensing and magnetometry.

By doping a laser dye into two polyurethane layers in a guided-mode resonance (GMR) structure, we observed that the lasing emission can be enhanced by approximately fivefold. The structure comprises, from top to bottom, a grating layer, four alternating layers of polyurethane and T a 2 O 5 , and a bottom substrate. Two different GMR wavelengths can be generated because of the two films of T a 2 O 5 serving as waveguide layers. The enhancement of the lasing emission is achieved by matching both the absorption and emission wavelength of the laser dye with the two GMR wavelengths. When the absorption wavelength matches the GMR wavelength, the formation of high intensity near the polyurethane layer serves to efficiently excite the laser dye. Additionally, as the emission wavelength overlaps with the GMR wavelength, the extraction of lasing intensity can be further increased in the preferred directions owing to the high reflection efficiency and directivity of the GMR. Moreover, we found that the linewidth is reduced to approximately 1.06 nm, and the estimated threshold is approximately 0.92 m J / c m 2 when both excitation and extraction resonances occur in the waveguide structure.

Spectrometers are usually composed of diffraction gratings and prisms, and become large in size for high wavelength resolution. High-density integration of narrowband-pass filters of different wavelengths on a substrate can be expected to produce an ultra-compact spectrometer. We focused on guided-mode resonance (GMR) filters as such filters. The GMR filter consists of a thin-film waveguide and a sub-wavelength grating. GMR filters of different filtering wavelengths can be integrated in the same substrate because their periodic structure in refractive index spread along in-plane direction. On the other hand, GMR filter normally needs an aperture size and beam diameter of hundreds of microns and is not suitable for high-density integration. In this study, we proposed and investigated a GMR filter of a single-layer cross grating for a compact narrowband-pass filter. Different guided modes contributing to the broadband and narrowband reflection propagate orthogonally each other. By using simultaneously these guided modes, both reflections are cancelled at the competing wavelengths, resulting in high transmittance. In this paper, design examples with numerical simulation results are presented. It was predicted that a passband of 15 nm was obtained in the stopband of 200 nm for an incident beam of a 10-micron diameter.

Tilted fiber Bragg grating (TFBG) presents many unique spectral characteristics for sensing. The widespread approaches to date are based on the cut-off mode resonance and surface plasmon resonance (SPR), whereas the leaky mode resonance is ignored in the literature. Herein, we theoretically demonstrate that the s-polarized (or TE/HE) leaky mode resonance (and the guided mode resonance) can be efficiently enhanced (and suppressed) by integrating graphene on the TFBG for a highly sensitive sensor. In contrast, the p-polarized (or TM/EH) mode (both leaky mode and guided counterpart) presents slight variation. The enhancement principle is discussed based on the variation in the mode characteristics induced by the graphene. The results show that the graphene enhanced leaky mode resonance presents ultra-sensitive intensity response but insensitive wavelength response to the extremely small analyte perturbation. The sensitivities are achieved up to 14108dB/RIU and 5232.6dB/RIU for the first two leaky modes in gaseous media respectively, which are 121 and 13 times higher than that of bare TFBG. This novel sensing platform provides a promising technique that can compete with the widespread SPR approach in fiber optic sensing fields, which opens up new opportunities for industrial, environmental, and biochemical sensing applications.

A dual-channel two-dimensional (2D) reflection grating filter operating around the 1.55 µm wavelength region is demonstrated, exhibiting dual-channel reflection peaks at 1.492 µm and 1.647 µm. The sidebands intrinsic to this kind of grating are suppressed by appropriately designed antireflective thin films, and this can be proved by equivalent medium theory. Using the modal analysis method, the excitation modes of the dual-channel reflection peaks are determined to be the TM0 (1.490 µm) and TE0 (1.638 µm) modes. The estimated relative errors in the wavelength determination of these modes are less than 1%. This is found to be in accord with analyses of the reflectivity spectra and electromagnetic fields. The dual-channel reflection peaks are sensitive to the background refractive index and may be useful in biosensing applications.

The transmission spectrum of a photonic crystal slab features sharp dips created by guided mode resonances. The same photonic crystal slab placed between orthogonal polarizers shows peaks at the resonances, but the peak wavelength differs from the guided mode resonance wavelength by a few nanometres. We investigate the working principle of the orthogonal polarizer setup and the origin of the wavelength difference for the case of a TE resonance. We show that the peak in the orthogonal polarizer setup is formed by light from the non-resonant TM polarization. The wavelength difference is caused by the phase shift between the resonant TE and the non-resonant TM polarization. We compare our explanation to a temporal coupled-mode approach and the use of a time-domain window function in FDTD.

Graphene has originally low absorption at telecommunication wavelengths. Therefore, it is necessary to overcome this hurdle to use graphene as an absorption media in many optoelectronic devices. In this paper, a narrow-bandwidth graphene-based total absorber is numerically and theoretically investigated. The proposed structure consists of a graphene sheet on a Si rectangular grating, which is placed on $\mathrm{\text{SiO}}_{{\bf 2}}$ spacer backed with the gold substrate. Due to the absence of plasmonic response of graphene in near infrared wavelengths, the method of critical coupling with guided mode resonance is employed to realize highly efficient absorption of light in the graphene sheet. The simulation results show that the maximum magnitude of absorption reaches near 100% (43-fold enhancement compared to isolated monolayer graphene absorption of 2.3%). Also, based on calculated results, the absorption of the proposed structure can be controlled by altering the geometrical parameters and chemical potential of graphene. In particular, the absorption spectrum is modulated by the external gate voltage. Moreover, we find that the operating wavelength is adjusted flexibly by an incident source angle. As a result, the proposed absorber has high directivity and can act similar to an antenna. The control ability of total absorption in the proposed structure provides potential applications for the realization of ultra-compact and high-performance optoelectronic devices, such as sensors and narrow-band filters.

In this article, a novel refractive index sensor based on gradient waveguide thickness guided-mode resonance (GWT-GMR) is proposed. The GWT can convert an spectral information into an spatial information, which can be measured using a charged coupled device (CCD) instead of a bulky spectrometer. A GWT-GMR sensor of size only 1261 200m2 has a detection range of 0.109 refractive index units (RIU), sensitivity of 16239mRIU, and the limit of detection of 6.9 104RIU, which are sufficient for many practical applications. Multiplexing can be easily implemented because of sensor compactness and the simple readout of the CCD. Moreover, the system is suitable for handheld devices or integration with smartphones for potential biosensing applications.

Tunable guided-mode resonance filters (GMRFs) for multi-primary colors based on a twisted nematic liquid crystal (TN-LC) polarization rotator with high peak efficiencies, narrow linewidths, and low sidebands are experimentally demonstrated. The proposed tunable color filter consists of a linear polarizer, the TN-LC polarization rotator, and the GMRF from top to bottom. The polarization of the linear polarized beam generated by the linear polarizer is controlled using a TN-LC. The resonance wavelengths and resonance peaks of the GMRFs are tuned following the orientation of the TN-LC directors. Thus, 3 ~ 6 high saturated primary colors with tunable intensities can be obtained by controlling the applied voltages on the TN-LC of every filter separately.

An active optical switch based on polarization-tuned guided-mode resonance filter (GMRF) for the optical communication wave band is experimentally demonstrated. The proposed active optical switch consists of a linear polarizer, a twisted nematic liquid crystal (TN-LC) polarization rotator and a GMRF from top to bottom. Two fabricated optical switches exhibit high resonance peaks with wavelengths around 1310 and 1550 nm in the voltage-off status. In the voltage-on status, weak light signals without resonance peaks pass through the optical switches. Thus light signal can be tuned by controlling the applied voltage on the TN-LC of the optical switch.

In this study we report the first on the terahertz (THz) transmission characteristics of a guided-mode resonance (GMR) filter made of all-dielectric material. Two strong transverse electric (TE) resonance modes, TE0,1 and TE1,1, and one strong transverse magnetic (TM) resonance mode, TM0,1, were detected. The measured resonances can be explained by diffraction from the grating surface of the GMR filter, and by guiding along the inside of the filter (slab waveguide). Because two identical GMR filters were employed to overcome limited grating numbers, the measured Q-factors of the TM0,1, TE1,1, and TM0,1 modes were as high as 62.9, 71.0, and 74.4 respectively. Also, we obtained polarization efficiencies of up to 96.9, 96.3, and 92.9% for the TM0,1, TM1,1, and TM0,1 modes, respectively, when the GMR filter was rotated to 90°. By increasing the incident THz beam angle, one TE resonance can be divided into two TE resonances, and the resonant frequency can be adjusted like a THz tunable resonance filter. Furthermore, when the GMR filters were inserted between Teflon plates, only the TM1,1 mode was perfectly removed. The designed GMR filter has a high Q-factor, tunable filter, good polarizer, and good modulator characteristics. These experimental results were in good agreement with simulation results.

The enhancement of upconversion luminescence (UCL) of rare earth doped upconversion nanoparticles (UCNPs) in aqueous solution is particularly important and urgently required for a broad range of biomedical applications. Herein, an effective approach to achieve highly enhanced UCL from NaYF4:Yb3+,Tm3+ UCNPs in aqueous solution is presented. We demonstrate that UCL of these UCNPs can be enhanced more than 104-fold by using a mesoporous silica low refractive index resonant waveguide grating (low-n RWG) in contact with aqueous solution, which makes it well-suited for biomedical applications. The structure parameters of the low-n RWG are tuned via rigorous coupled-wave analysis simulation to ensure strong local excitation field to form atop the TiO2 surface of the low-n RWG, where UCNPs are deposited. As the low-n RWG is excited by a near-infrared laser at 976 nm to match its guided mode resonance (GMR) condition, UCL emitted from UCNPs is greatly enhanced thanks to the strong interaction between excitation lo...

We present the design and realization of a graphene-based absorber with nearly 60% absorption efficiency using the encapsulated cascaded gratings (ECGs). The absorption locations can be estimated by using the dispersion relation of the ECGs, and it is confirmed that the absorption of the graphene-based ECGs is resulted from the excitation of the guided mode resonance (GMR). The absorption location is almost immune to the variation of the thickness of the top grating, and the absorption location is red-shifted with the increase of the fill factor. The shift of the absorption peak can be adjusted to the design value by tuning the incident angle as the grating period is altered. Multiple absorption bands can be achieved by increasing the thickness of the bottom grating. The absorption properties can be tailored based on the dispersion diagram of the ECGs structure.

Recently, based on the selective excitation of the guided mode, researchers realized quasi-bound states in the continuum (quasi-BICs) in all-dielectric compound grating waveguide structures. In this paper, we introduce a graphene layer into an all-dielectric compound grating waveguide layer supporting quasi-BIC to achieve near-infrared perfect absorption of graphene. The underlying physical mechanism of perfect absorption can be clearly explained by the critical coupling theory derived from temporal coupled-mode theory in a single-mode, one-port system. By changing the Fermi level and the layer number of the graphene, the absorption rate of the system can be flexibly tuned. In addition, by changing the geometric parameter of the compound grating waveguide structure, the radiation coupling rate of the quasi-BIC can also be flexibly tuned. Therefore, the critical coupling condition can be maintained in a broad range of the Fermi level and the layer number of the graphene. The full width at half maximum of the near-infrared perfect absorption peak can be flexibly tuned from 5.7 to 187.1 nm. This bandwidth-tunable perfect absorber would possess potential applications in the design of 2D material-based optical sensors, electrical switchers, and solar thermophotovoltaic devices.

In this letter, we present an ultra-broadband and wide-angle perfect absorber based on composite Ge–Ni grating. Near perfect absorption above 90% is achieved in a wide frequency range from 150 nm to 4200 nm, which covers almost the full spectrum of solar radiation. The absorption keeps robust in a wide range of incident angle from 0o to 60o. The upper triangle Ge grating works as an antireflection coating. The lower Ni grating works as a reflector and an effective energy trapper. The guided modes inside Ge grating are excited due to reflection of the lower Ni grating surface. In longer wavelength band, gap surface plasmons (GSPs) in the Ni grating are excited and couple with the guided modes inside the Ge grating. The coupled modes extend the perfect absorption band to the near-infrared region (150 nm–4200 nm). This design has potential application in photovoltaic devices and thermal emitters.

We propose and experimentally demonstrate high resolution angle sensors based on ultra-narrowband graphene perfect absorbers. Perfect absorption at wavelength of 1452.8 nm with absorption bandwidth of 0.8 nm is numerically demonstrated for a designed angle sensor based on single graphene absorber at normal incidence, and the angular width of the resonant absorption is only 0.05°. In the experiment, peak absorption over 95% with bandwidth about 2.8 nm is measured at normal incidence for a fabricated graphene sensor, and the device has a wavelength-angle sensitivity over 17 nm per degree which agrees well with the simulation result. Meanwhile, an optoelectronic angle sensor with high resolution and fast response by using an array of graphene absorbers is proposed. The demonstrated graphene angle sensors with ultra compact size and high resolution could be of valuable applications in many fields.

We propose a dual-layer split nanograting structure in crystalline silicon thin-film solar cells (TFSCs). The split nanograting is designed by introducing two partitioning factors and split times. By employing the finite-difference time-domain method, the light trapping performance and relevant parameters of TFSCs are analyzed and optimized. Numerical computation of optical and electrical simulation shows that the optimal dual-layer split nanograting structure has demonstrated great enhanced light absorption compared with the planar structure. Enhancement of the light trapping effect is associated with light coupling to waveguide modes. The short-circuit current density is reached at 21.66 m A / c m 2 with an improvement of 54.6% over the planar structure. All results provide a parting thought for the design of TFSC grating structures.

In this paper, we propose a high-performance refractive-index sensor at a near-infrared band based on a hybrid Tamm structure. The optical properties of this graphene-based hybrid Tamm plasmonic structure are analyzed and investigated by using the transfer matrix method (TMM). Due to the excitation of the guide mode resonance (GMR) and Tamm plasmon polariton (TPP) resonance, the structure can realize multi-channel perfect absorption. This structure can be utilized as a refractive index sensor because the position of the absorption peak is sensitive to the refractive index of the ambient layer. Therefore, we obtain the sensitivity to 950 nm per refractive index unit (nm/RIU) and figure of merit (FoM) of 161 RIU-1 after studying the performance under different structural parameters. We believe that the proposed configuration is expected to be used to manufacture high-performance biosensors or gas sensor devices and other related applications in the near-infrared band.

Perfect absorbers are of great importance in various applications such as photodetectors, optical sensors and optical modulators. Recently, perfect absorption metasurface based on monolayer graphene has attracted lots of research interest. In this paper, a graphene-lithium niobate (LN) perfect absorption metasurface is constructed, where graphene works as a thin absorptive layer as well as a conductive electrode. The proposed device achieves 99.99% absorption at 798.42 nm and 1.14 nm redshift of the absorption peak is realized at 300 V(from -150 V to 150 V) external bias voltage through the electro-optical effect of LN, which enables the proposed device work as a electrically tunable absorber in the visible and near infrared range. The switching ratio of reflected light R/R0 could reach -44.08 dB with an applied voltage tuning from -150 V to 0 V at 798.42 nm. Our work demonstrates the potential of LN integrated high-Q resonant metasurface in realizing electro-optic tunable nanophotonic devices in the visible and near infrared band. It will promote the research of graphene integrated optoelectronic devices as well as LN based tunable nanophotonic devices which have widespread applications such as modulators and optical phase arrays.

A novel metallodielectric grating coupled with graphene metamaterials (MGCGM) structure is proposed to achieve the tunable plasmon induced transparency (PIT) effect. Two different mechanisms are employed to explain the PIT effect in the reflection spectrum: one is originated from the destructive interference between graphene plasmonic modes and the hybrid mode in the dielectric layer, the other is related to the absorption of the graphene surface plasmons. The simulated results indicate high tunability in the amplitude and the operating wavelength of the PIT effect can be obtained by controlling the Fermi levels of graphene ribbons. Besides, double PITs are demonstrated in this scheme by altering the occupation ratio of the graphene-based ribbon grating. Compared with previous reports, the MGCGM structure is much easier to fabricate and the tunable PIT effect has significant applications in optical modulator, switching, absorber, sensor, slow light, etc.

This paper reports on an efficient and convenient method of patterning nanostructures on the cleaved facet of an optical fiber to realize a high-performance fiber-optic gas sensor. The fabrication method utilizes an ultraviolet assisted nanoimprint lithography to transfer nanoscale patterns from a pre-formed stamp to the fiber tip. The novelty of this paper lies in utilizing simpler fabrication steps with better control over angle of contact at the fiber tip, which leads to rapid and precise formation of nanostructures with well-defined features. A periodic array of polymer nanoposts are formed at the fiber tip and coated with titanium dioxide to serve as a guided mode resonant (GMR) device. A gas sensor is realized by coating the GMR structure with a thin layer of graphene oxide (GO) nanosheets. We have utilized the resonance sensitivity of the nanopatterned fiber-tip gas sensor to surrounding refractive index. The abundant functional groups available at GO provides an effective adsorption surface for gas molecules. Microscopic imaging and spectroscopic studies are conducted to illustrate the structural and optical properties, and gas-sensing performance of the sensor. Volatile organic compounds, such as ethylene and methanol, associated with crop plant health, are detected by the sensor. The sensor provides sensitivities of 0.92 and 1.37 pm/ppm for ethylene and methanol vapors, respectively, with a three-fold enhancement in sensitivity and 50% reduction in response time compared with the non-GO coated counterpart. In addition, the sensor demonstrates good stability and reproducibility, thus having a great potential in fiber-optic remote sensing applications.

In order to achieve the enhancement and manipulation of light absorption in graphene monolayer within the visible (Vis) and near infrared (NIR) regions, a design of absorber inspired by contact coupled gratings with an absentee layer is demonstrated. It is proved that the absorptance of monolayer graphene can be greatly enhanced to near unity through rigorous coupled-wave analysis (RCWA) numerical calculation. The thickness of grating and homogeneous absentee layers can significantly change the linewidth and resonant mode position in absorption spectrum. Furthermore, the lateral shift of the contact coupled gratings changes the magnetic field distributions in the grating cavity and the surface-confined mode at the cover/grating interface, thus facilitating the dynamic control of the spectral bandwidth of the graphene absorber. The proposed devices could be efficiently exploited as tunable and selective absorbers, allowing to realize other two-dimensional (2D) materials-based selective photo-detectors.

A reflective diffraction grating with a periodic square-wave profile will combine the effects of thin-film interference with conventional grating behavior when composed of features having a different refractive index than that of the substrate. A grating period of 700–1300 nm was modeled and compared for both silicon (Si) and silicon dioxide (SiO2) to determine the behavior of light interaction with the structures. Finite element analysis was used to study nanostructures having a multirefractive index grating and a conventional single material grating. A multimaterial grating has the same diffraction efficiency as that of a grating formed in a single material, but had the advantage of having an ordered relationship between the grating dimensions (thickness and period) and the intensity of reflected and diffracted optical wavelengths. We demonstrate a color-selective feature of the modeled SiO2 grating by fabricating samples with grating periods of 800 and 1000 nm, respectively. A high diffraction efficien...

Field distributions and power spectra of radiation modes in both two-layer and three-layer cross-sectional fibers are investigated. By mapping packets of radiation modes to discrete complex modes, the relationship between the radiation modes and the quasi-leaky modes obtained in an equivalent numerical model is revealed. Simulation results further show that quasi-leaky modes can accurately and intuitively describe packets of radiation modes, from the viewpoint of power distribution and attenuation in the mode propagating. The normalization and orthogonality of quasi-leaky modes as well as the model parameters of the equivalent numerical model are discussed.