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Optical bound states in the continuum (BICs) lies in side the continuum and coexists with extended waves, but it remains perfectly confined without any radiation. This unique property of BICs has led to numerous applications, such as highly surface‐sensitive and spectrally sharp resonances for photonic biosensors. However, it remains challenging to experimentally realize the BICs in a single‐particle system, especially for subwavelength structures. This study presents the existence of optical BICs in a subwavelength metallic microstructure, and quasi‐BICs are observed experimentally in a waveguide system with only a single optimized aluminum meta‐particle. This plasmonic BICs is resulting from the destructive interference of two localized surface plasmon modes. Benefiting from its strong localized field confinement and substrate‐free merit of BICs, the experimentally measured quality factor (Q‐factor) of this transmission dip reach to 273. Additionally, this meta‐particle is experimentally verified to show a good sensitivity for both solids and liquids through the spectral shift of the BICs caused transmission dip. This finding extends the optical BICs to a subwavelength scale and opens practical application opportunities for ultrasmall‐quantity detection of biochemical substances. [ABSTRACT FROM AUTHOR]

The following study involved the utilization of dispersion polymerization to synthesize micron/nano-sized polystyrene (PS) spheres, which were then deposited onto a silicon substrate using the floating assembly method to form a long-range monolayer. Subsequently, dry etching techniques were utilized to create subwavelength structures. The adjustment of the stabilizer polyvinylpyrrolidone (PVP), together with changes in the monomer concentration, yielded PS spheres ranging from 500 nm to 5.6 μm in diameter. These PS spheres were suspended in a mixture of alcohol and deionized water before being arranged using the floating assembly method. The resulting tightly packed particle arrangement is attributed to van der Waals forces, Coulomb electrostatic forces between the PS spheres, and surface tension effects. The interplay of these forces was analyzed to comprehend the resulting structure. Dry etching, utilizing the PS spheres as masks, enabled the exploration of the effects of etching parameters on the resultant structures. Unlike traditional dry etching methods controlling RF power and etching gases, in the present study, we focused on adjusting the oxygen flow rate to achieve cylindrical, conical, and parabolic etched structures. [ABSTRACT FROM AUTHOR]

Front glass structured with random subwavelength structures provides photovoltaic (PV) modules with a spectrally broadband omnidirectional antireflective property, enhancing also the incident angle modifier factor and boosting energy production in the field. However, long‐term outdoor monitoring allowing a monthly quantification of the electrical improvement of PV devices laminated with nanostructured glasses has been missing so far. Herein, two mini‐modules, one laminated with flat glass and the other laminated with nanostructured glass, are installed in a single‐axis solar tracker, and the solar irradiance and the short‐circuit current (Isc) are monitored for over 4 months, from September to December. This monitoring provides the quantification of the impact of a broad range of sunlight's angle of incidence (AOI) on the electrical enhancement of the device. The relative Isc improvement between two samples averaged for each month is calculated, ranging from 3.0% in September up to 4.0% in December. Besides, the influence of the solar AOI on the Isc improvement is investigated for sunny days, analyzing separately the Isc performance in the morning/afternoon hours, which corresponds to a normal incidence, and at noon, when the AOI is higher, obtaining more than 5.0% with AOI higher than 60°. [ABSTRACT FROM AUTHOR]

The development of renewable energy sources is nowadays of enormous importance, not only for the climate change fight but also for the security of the energy supply. The latest geopolitical unfortunate events in Europe have highlighted our energy dependency on foreign fossil fuels. In this context, solar technologies are already playing an essential role in shifting towards neutral carbon economies, ensuring a reliable energy supply. In this regard, dispatchability provided by CSP plants is key to pave the way towards energy transition. It is worth noting that the long-term durability and performance of solar components in arid regions are crucial to increase the reliability and performance of CSP plants while reducing O&M costs. In this work, an innovative approach based on nano-structuring the solar receiver tube glass is presented, which provides improved anti-reflective (AR) and anti-soiling (AS) properties, also showing good durability with respect to abrasion. Spectral transmittance improvement, soiling rate decrease, and durability measurements are presented for nano-structured glasses, comparing with current state of the art glass performance. The achieved experimental results suggest that the new structured glasses would be good candidates for CSP applications

The following study involved the utilization of dispersion polymerization to synthesize micron/nano-sized polystyrene (PS) spheres, which were then deposited onto a silicon substrate using the floating assembly method to form a long-range monolayer. Subsequently, dry etching techniques were utilized to create subwavelength structures. The adjustment of the stabilizer polyvinylpyrrolidone (PVP), together with changes in the monomer concentration, yielded PS spheres ranging from 500 nm to 5.6 μm in diameter. These PS spheres were suspended in a mixture of alcohol and deionized water before being arranged using the floating assembly method. The resulting tightly packed particle arrangement is attributed to van der Waals forces, Coulomb electrostatic forces between the PS spheres, and surface tension effects. The interplay of these forces was analyzed to comprehend the resulting structure. Dry etching, utilizing the PS spheres as masks, enabled the exploration of the effects of etching parameters on the resultant structures. Unlike traditional dry etching methods controlling RF power and etching gases, in the present study, we focused on adjusting the oxygen flow rate to achieve cylindrical, conical, and parabolic etched structures.

Considerable effort is devoted to research on reconfigurable beam focusing and optical cloaking in the basis of metamaterials/metasurfaces. However, it remains a great challenge to achieve multifunctional wavefront control on curved platforms, especially in the reflective case, which is of paramount importance in the emerging fields of active optics and stealth research. This study proposes a novel strategy to control the versatile reflective structured wavefront on a conformal platform by curving a flexible metasurface. This metasurface is fabricated in a high‐throughput manner by using ultra‐violet nano‐imprinting lithography (UV‐NIL), and its efficacy is verified through consistent simulated and experimental results. The conformal metasurface is designed to realize multifunctional performance by employing all‐metallic plasmonic structures based on the Pancharatnam‐Berry (PB) phase. A general theoretical scheme is also proposed for the projection of a curved surface onto a reference plane by a judicious compensation, combining the geometrical PB phase with curvature‐induced propagation phase. Moreover, as a proof‐of‐concept, curved metasurfaces with cylindrical shapes are experimentally characterized by their multifunctionalities in generating a beam spot with a continuously tunable focal length and reflecting plane waves used for camouflage, thereby confirming the feasibility and effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Polarization-dependent dual-band THz metamaterial using a modified Trishul structure is proposed in this paper. The absorber has three layers namely, a conducting ground plane, a dielectric substrate, and a structure top metallic patch. The top and bottom metallic is made up of gold material and the dielectric substrate is polyimide. The characteristics are obtained using the finite element method. The structure has obtained two resonant peaks at 0.78 THz(f1) and 1.65 THz(f2) for the X-polarized wave and at 0.81 THz(f3) and 1.53 THz(f4) for the Y-polarized wave respectively. The simulation result reveals that the absorber has obtained ultra-narrow band absorption with a full width half maximum(FWHM) of 4.8 GHz, 10 GHz, 5 GHz, and 10 GHz for f1, f2, f3, and f4 respectively. In addition, the underlying physical mechanism of the multi-band characteristics is analyzed using electric field and surface current distributions. The proposed absorber design has importance in the emerging THz system for polarization imaging and sensing applications. Also, flexible metamaterials could be useful for soft robotics applications also.

Optical vortices carrying orbital angular momentum (OAM) have recently attracted increasing interest for providing an additional degree of freedom for capacity scaling in optical communications. The optical vortex generator is an essential component to facilitate OAM-enabled optical communications. Traditional devices face challenges of limited compactness, narrow bandwidth, and first-order OAM modes. Here, using the direct-binary search (DBS) optimization algorithm, we design, fabricate, and demonstrate a digitized subwavelength surface structure on silicon platform for the generation of wavelength-/polarization-/charge-diverse optical vortices. It features an ultra-compact footprint (∼3.6 × 3.6 μm2) and ultra-wide bandwidth (1480–1630 nm), supporting two polarizations (x-pol., y-pol.) and high-order OAM modes (OAM+1, OAM−1, OAM+2, OAM−2) with high purity of larger than 84%. The mode crosstalk matrix is measured in the experiment with favorable performance. When generating x-pol. OAM+1, x-pol. OAM−1, y-pol. OAM+1, and y-pol. OAM−1 mode, the crosstalk of the worst case is less than −14 dB. When generating OAM+1, OAM−1, OAM+2, and OAM−2 mode, the crosstalk between any two OAM modes is less than −10 dB, and the lowest crosstalk is about −17 dB. In addition, we also show the possibility for generating much higher-order OAM modes (e.g. OAM+3, OAM−3, OAM+4, and OAM−4) with the digitized subwavelength surface structure. The wavelength-/polarization-/charge-diverse optical vortex generator enables the full access of multiple physical dimensions (wavelength, polarization, space) of lightwaves. The demonstrations may open up new perspectives for chip-scale solutions to multi-dimensional multiplexing optical communications.

We report on subwavelength reflective gratings for hyperspectral applications operating in a very large spectral band (340–1040 nm). Our study concerns a blazed-binary grating having a period of 30 μm and composed of 2D subwavelength structures with size from 120 nm to 350 nm. We demonstrate the manufacturing of the gratings on 3″ wafers by two lithography technologies (e-beam and nanoimprint) followed by classical dry etching process. Optical measurements show that the subwavelength grating approach enables a broadband efficiency, polarization behaviour and wavefront quality improvement with respect to the requirements for the next generation of spectro-imagers for Earth observation missions. An outlook towards spherical substrate based on nanoimprint lithography is also reported with the results of mixed features replication (holes and pillars in the range of 160–330 nm) on a 540 mm concave substrate which demonstrate uniformity and accuracy capabilities over 3″ surface. [ABSTRACT FROM AUTHOR]

Metalens has emerged as an important optical block in free-space optical systems, which shows excellent performance. Even the metalens based on gradient index (GRIN) profiles can be implemented for on-chip beam focusing behavior. However, for most previous schemes, the GRIN metalenses can only achieve on-chip beam focusing behavior in one dimension, which limits their applications in low-loss waveguide interconnecting or fiber-to-chip coupling. In this paper, an on-chip half Maxwell’s fisheye lens based on GRIN profiles with subwavelength features, integrated with silicon waveguides, is experimentally demonstrated. Benefitting from the index distribution and beam focusing characteristics of the half Maxwell’s fisheye lens, an on-chip beam transforming can be achieved for transverse electric (TE) fundamental mode in two waveguides with different heights and widths. The simulated 1 dB bandwidth can reach 1100 nm, which exhibits great prospects in integrated photonic circuits. The measured insertion loss of an on-chip 5.4 μm-length lens is less than 1 dB to connect a 220 nm-height, 8 μm-wide waveguide, and a 60 nm-height, 0.5 μm-wide waveguide in the wavelength range of 1280–1620 nm.

At nano-scale new phenomena have been discovered, allowing devices’ miniaturisation, energy harvesting, and power reduction. The Extraordinary Optical Transmission (EOT) phenomenon was reported in 1998 by Ebessen, who stated that the resonant peaks in the response of metallic nano antennas were more intense than predicted by classical diffraction theories. Years later, the main reason for this behaviour was attributed to the Surface Plasmon Polaritons (SPP), at least in ultraviolet and visible regions. In this article, a new method to model the radiation-matter interaction on a dielectric-metal interface is proposed, based on Maxwell’s equations and Fresnel Coefficients in absorbing media. Transmission percentage, angle and propagation length are obtained for a rigorous sweep on incident angle and wavelength. The results taken using some noble metals allow us to observe the presence of surface phenomena at expected SPP resonances. First, it is noticeable huge values of transmission in ultraviolet and visible regions, meaning that the metal does not reflect all the radiation. Also, the transmission angle tends to be higher, meaning huge surface components. Furthermore, the transmission length is on orders of 50-100 nm, meaning that phenomena as EOT only occurs at the nano-scale, since waves should arrive at another interface before being absorbed.

Multi-wavelength imaging ability is important for bio-imaging, to obtain multi-modal information of biological activities. We propose a multi-wavelength super-resolution imaging method, employing the Bloch surface waves (BSWs) supported by one-dimensional photonic crystal as subwavelength structured illumination fields. Here 488 nm (blue), 532 nm (green), and 632.8 nm (red) are chosen as the illumination wavelengths, and a two-dimensional equilateral triangular grating is designed to couple the incident plane waves to the BSWs in the all-dielectric photonic crystal. The triangular grating ensures a more uniform resolution improvement along with whole lateral directions and provides better imaging quality compared with lattice grating. In our demonstration, the resolution improvements are above 2.4 fold for the three wavelengths, and lateral resolutions are below 120 nm in the Rayleigh criterion. Moreover, as the illumination device is composed of a periodic planar structure, the larger field of view is achievable. The proposed method could achieve multi-wavelength wide-field super-resolution surface imaging while maintaining high speed, good imaging quality and bio-compatibility.

Grating couplers (GCs) are important for light to couple between optical fibers and photonic integrated circuits (PICs). The Bound State in Continuum (BIC) waveguides offer new functionalities for PIC. High-Q resonators, electro-optical modulator, and photodetectors can be built on it without etching steps. Due to the special waveguide geometry of BIC waveguides, a high efficiency GC is still missing. In this work, we demonstrated a subwavelength grating (SWG) GC for BIC waveguide with a coupling efficiency of 8 dB and a 1 dB bandwidth >70 nm. The device shows ultra-broadband behavior which is extremely useful for BIC-based PICs.

With the traditional multilayers coating, it is difficult to achieve broadband and wide-angle antireflection optical properties simultaneously. As one of the most promising candidates for omnidirectional and broadband antireflection coating, the moth-eye structure has gained considerable attention in the past several decades because of its excellent optical performance and self-cleaning function. The challenges in the use of this technology, namely, its manufacturing cost and mechanical structural stability, limit its popularization. In this paper, a moth-eye structure surface combined with a residual layer of an identical material is proposed; the proposed composite structure can be formed by a one-step process of half-etched technology. For the design and optimization of the composite moth-eye structure, composite structures were compared by performing effective medium approximation (EMA) and rigorous coupled– wave analysis (RCWA). The structural parameters of the micro–nano surface were optimized to increase the mechanical stability considerably. The results showed that the optimized composite moth-eye structure has high transmittance for incident angles from 0° to 80° in the infrared spectrum. Furthermore, the transmission diffraction at high orders was suppressed efficiently at super-wide incident angles.

We propose a 1310/1550 nm wavelength diplexer on a 220-nm silicon-on-insulator (SOI) platform. The device is based on a compact two-mode interference (TMI) coupler enabled by a subwavelength grating (SWG) slot. The ideal beat length ratio of 2:1 is achieved with the transverse magnetic (TM) mode by fine-tuning the SWG slot parameters, resulting in a TMI length of only 37 µm. We reveal that the key to high extinction ratio (ER) is the careful design of the tapers, and the device achieves high ERs of 28.05/42.54 dB at 1310/1550 nm with simulation. The measured bandwidths for ER $>$ 15 dB are 82 nm and 56 nm at O- and C-band. Moreover, the design guarantees large calculated 1-dB-insertion-loss (IL) bandwidths of 192/123 nm at 1310/1550 nm. To the best of our knowledge, this is the first experimental demonstration of a high-performance compact silicon 1310/1550 nm diplexer based on a TMI coupler.

We propose a graphene plasmonic structure by applying two graphene layers mingled with a thin gold layer in a silicon grating. By utilizing the finite-difference time-domain (FDTD) method, we investigate the optical response of the system, and observe that the design achieves dual tunable electromagnetically induced transparency (EIT)-like effect at terahertz frequencies. The EIT-like effect arises from the destructive interference between the grapheme-layer bright modes and the gold-layer dark mode. The EIT-like phenomenon can be adjusted by the Fermi level, which is related to the applied voltage. The results show that the group delay of the present structure reaches 0.62 ps in the terahertz band, the group refractive index exceeds 1200, the maximum delay-bandwidth product is 0.972, and the EIT-like peak frequency transmittance is up to 0.89. This indicates that the device has good slow light performance. The proposed structure might enable promising applications in slow-light devices. [ABSTRACT FROM AUTHOR]

Optical sensors have been used to detect refractive index variations in several areas, such as medicine or the environment, due to their excellent characteristics, mainly in reproducibility, sensitivity, and response time. Sensors based on surface plasmon polaritons (SPPs), such as Kretschmann’s, are a promising alternative since they allow miniaturization of the devices, useful for “in vivo” detection. In this article, a more accurate method is proposed to obtain the reflectance at the interfaces of the Kretschmann structure. The successive steps of the new method allow a simple physical interpretation of the whole process, unlike previous methods. Experimental results and others obtained from the previous methodologies allow the validation of the proposed method. The method validation from experimental results is performed in BK7–Au–air and BK7–Cr–Au–air structures, revealing an excellent agreement of the reflectance curves. Also, the new method is applied to three simulated Kretschmann sensors already presented in the literature, BK7–Au–analyte, SF10–Au–analyte, and SF10–Au–graphene–analyte structures, for refractive index variation of the analyte between 1.33 and 1.35, leading to a good agreement between both methodologies results. The operating point of SF10–Au–analyte and SF10–Au–graphene–analyte structures is analyzed and optimized. The analyte refractive index changes lead to reflectance variations higher than 92% in the infrared region, meaning the reflectance almost varies from the highest to the lowest possible value. [ABSTRACT FROM AUTHOR]

We report an inverse-designed, high numerical aperture (∼0.44), extended depth of focus (EDOF) meta-optic, which exhibits a lens-like point spread function (PSF). The EDOF meta-optic maintains a focusing efficiency comparable to that of a hyperboloid metalens throughout its depth of focus. Exploiting the extended depth of focus and computational post processing, we demonstrate broadband imaging across the full visible spectrum using a 1 mm, f/1 meta-optic. Unlike other canonical EDOF meta-optics, characterized by phase masks such as a log-asphere or cubic function, our design exhibits a highly invariant PSF across ∼290 nm optical bandwidth, which leads to significantly improved image quality, as quantified by structural similarity metrics.

Grating couplers (GCs) are important for light to couple between optical fibers and photonic integrated circuits (PICs). The Bound State in Continuum (BIC) waveguides offer new functionalities for PIC. High-Q resonators, electro-optical modulator, and photodetectors can be built on it without etching steps. Due to the special waveguide geometry of BIC waveguides, a high efficiency GC is still missing. In this work, we demonstrated a subwavelength grating (SWG) GC for BIC waveguide with a coupling efficiency of 8 dB and a 1 dB bandwidth >70 nm. The device shows ultra-broadband behavior which is extremely useful for BIC-based PICs. [ABSTRACT FROM AUTHOR]

This paper reports on the formation of moth-eye nanopillar structures on surfaces of alkali-aluminosilicate Gorilla glass substrates using a self-masking plasma etching method. Surface and cross-section chemical compositions studies were carried out to study the formation of the nanostructures. CF x induced polymers were shown to be the self-masking material during plasma etching. The nanostructures enhance transmission at wavelengths over 525 nm may be utilized for fluid-induced switchable haze. Additional functionalities associated with nanostructures may be realized such as self-cleaning, anti-fogging, and stain-resistance. [ABSTRACT FROM AUTHOR]

For example, in Ref. [[19]] the reported achromatic metalenses have a diameter of HT ht and an NA of HT ht . [Extracted from the article]

Amid the rising demand for high-performance computing, photonic integrated circuits are increasingly overcoming the conventional two-dimensional barriers, transitioning toward more flexible multilayer structures. To fulfill this aim, we have engineered a wide-bandwidth, multilayer tunable power splitter that enables the transmission of information along the vertical direction while allowing for flexible allocation of optical power. The power splitter is constructed with an asymmetric coupler, complemented by a grating structure. The design of the coupler has been refined through optimization employing the particle swarm algorithm, while the grating structure has undergone optimization via the direct binary search algorithm. The simulation results indicate that the power divider can achieve proportional regulation from 0.285 to 3.5 across the 1400 to 1700 nm wavelength spectrum, with insertion losses (ILs) consistently below 0.34 dB. Significantly, the IL is <0.21 dB at a 1:1 power ratio. This compact, low-loss, high-bandwidth tunable power splitter was designed to offer a new idea for the multilayer integration of microchips. [ABSTRACT FROM AUTHOR]

In-plane birefringent materials present an effective modulation of the optical properties and more degrees of freedom for the signal detection in low dimension, and thus remain a hot topic in realizing the integrated, miniature, and flexible devices for multiple applications. Here, the artificial in-plane birefringence properties have been successfully achieved on a graphene oxide film by a novel femtosecond laser lithography method, which provides a high-speed, large-area, and regular subwavelength gratings (∼ 380 nm) fabrication and photoreduction. The obtained sample manifests an evident optical birefringence (∼ 0.18) and anisotropic photoresponse (∼ 1.21) in the visible range, both of which can be significantly modulated by either the structural morphology or the degree of oxide reduction. Based on the analysis of effective-medium theory and measurements of angle-resolved polarized Raman spectroscopy, the artificial in-plane birefringence is originated from various optical responses of the periodic subwavelength structures for the incident light with different polarization states. This technique shows great advantages for the fabrication of integrated in-plane polarization-dependent devices, which is expected to solve the problems in this field, such as the deficient selection of materials, complex design of micro/nanostructure, and inflexible processing technology. [ABSTRACT FROM AUTHOR]

Two-dimensional materials have emerged as new type of smart materials that may impact advanced photonic devices. Here, to increase the light absorption, a black phosphorus-based nanostructure is proposed. The presented nanostructure has a grating-shaped structure based on monolayer/multilayer black phosphorus and silica. To access reasonable absorption, the structure is numerically simulated by the finite difference time domain (FDTD) method. To benchmark this nanostructure, the black phosphorus permittivity in the wavelength range of 5 to 15 μm was calculated, to achieve the transfer spectrum based on the lateral length changes of black phosphorus (i.e., L = 100, 150, 170 nm) and the silica substrate which is extracted from Palick experimental results; the proposed nanostructure is simulated using the FDTD method. Also, changes in the refractive index of the surroundings have been used to compute significant parameters in the nanosensors, such as sensitivity, FWHM, and FOM. The proposed nanostructure can be used in tunable absorbers in the range of infrared wavelengths. [ABSTRACT FROM AUTHOR]

Silicon photonic integrated circuits with the abilities of large-scale integration and low-cost fabrication are one of the most promising technologies for various emerging applications. For silicon photonic integrated circuits, which are usually polarization-sensitive, polarization-handling is of critical importance as a key manipulation of light. Basically, ultracompact polarization-handling devices with high performances in a broad band are highly desired. In this paper, a review is given for recent progresses of high-performance silicon polarization-handling devices assisted with subwavelength structures (SWSs), which introduce more degrees of freedom (DoFs) in design. In Section II, the fundamental of both regular and disordered SWSs with different DoFs is introduced and discussed. Different kinds of polarization-handling devices are presented in Section III, including polarizers, polarization beam splitters, polarization rotators/polarization-splitter-rotators, dual-polarization devices. Finally, the challenges and the opportunities of silicon polarization-handling devices are discussed in Section IV. [ABSTRACT FROM AUTHOR]

Structured illumination microscopy (SIM) is one of the most powerful and versatile super-resolution methods due to its live-cell imaging ability of subcellular structures with high speed. In this paper, we propose an alternative SIM assisted by Bloch surface wave (BSW). Through replacing the conventional laser interference fringes with sub-diffraction BSW counterparts with higher spatial frequency, the imaging resolution of SIM could be enhanced greatly and the super-resolution imaging capability down to 80 nm could be achieved. Compared with traditional wide-field fluorescence microscopy, this BSW SIM demonstrates 2.78 times enhancement in imaging resolution, which surpasses general SIM with only 2 times improvement. Moreover, the structured illumination intensity could be boosted drastically, which is beneficial to nonlinear super-resolution imaging techniques, such as saturated SIM. This BSW SIM would provide a super-resolution, wide field of view approach for surface super-resolution fluorescent imaging while maintaining high imaging speed and good bio-compatibility.

The applicability of resonant waveguide grating (RWG)-based structures as filters to control the spectral response in an optical communication system is investigated. A new physical model for the structure is established on the basis of the Fabry–Pérot (FP) etalon model and coupled leaky mode theory (CLMT). It is found that the flat-top spectral response of the filter is achieved by the combined effect of the guided-mode resonance of an RWG and its Fabry–Pérot resonance (FPR). The bandwidth-tunable spectral response of the filter can be varied according to the change in the eigenvalues of the RWG by changing the structural parameters such as strip width of the grating and incident angle. The flat-top and bandwidth-tunable RWG-based resonant filter is a promising application for high-performance optical communication systems.

We propose a coupled-tolerant subwavelength photonic racetrack resonator (PRR) on the silicon-on-insulator (SOI) platform, in which a subwavelength grating (SWG) PRR is coupled to a symmetrical SWG bus waveguide. The material equivalent refractive index can be manipulated by the SWG characteristic and thus an additional fabrication degree of freedom can be achieved. The resonant characteristics can be improved significantly by controlling the pillar size and increasing the interaction length between SWG racetrack and bus waveguide (RBW). The simulation results show that the PRR has a high-quality factor and large extinction ratio around 1550nm. Two groups of SWG and strip waveguide (SW) PRRs are fabricated. The fabrication tolerance is tested when fabrication error is changed from 0 to 20 nm and the quality factor is more than 7000 in de-ionized water. The testing results show that the tolerance in SWG-PRR has a 2.0-fold improvement than that in the SW-PRR. Then a fabricated SWG-PRR is employed to monitor different concentration glycerol injected on the chip. An optimal quality factor of 9500 around 1550 nm and bulk sensitivity of 412± 0.5 nm/RIU are achieved. Therefore, the structure is a potential candidate for label-free sensing in medical diagnosis and environmental monitoring.

An independently tunable dual-resonance hybrid structure incorporating graphene is proposed to achieve selective light trapping and response tuning in the visible and near-infrared band. The incident light with different wavelengths are intensely localized in two separated resonant cavities via the synergetic effect of the guided mode resonances (GMRs) and optical Tamm states (OTSs), which leads to the strong enhancement of the light-matter interaction of graphene in different parts of the structure. The dual-band perfect absorption associated with critical coupling is achieved, whereas the two resonant modes can be independently tuned by adjusting the corresponding structural parameters. Furthermore, by dynamically changing the permittivity of the resonant cavities or the chemical potential of graphene, the absorption wavelength can be continuously adjusted and the absorption efficiency can be regulated stepwise.

In this paper, a double-layer approach is proposed to design a compact four states polarization-independent grating coupler (GC). The proposed polarization-independent GC is designed to couple a 700 nm polarized light propagated in a 150 nm Gallium Phosphide (GaP) waveguide to a polarization-maintaining fiber. The double-layer approach is based on the deposition of GaP gratings designed to couple the transverse magnetic (TM) light over the GaP gratings designed to couple the transverse electric (TE) light. The two layers are separated by a Hydrogen silsesquioxane (HSQ) with an optimum thickness of 20 nm. The grating periods and the fill factors of the two layers are optimized to achieve maximum possible coupling efficiency (CE). The proposed method resulted in relatively high CEs of 39.2, 31.1, and 23.3% for the TE, TM, and 45°/-45° linearly polarized light, respectively. The polarization-dependent loss (PDL) is 1 dB, 1.26 dB, and 2.26 dB corresponding to TE-TM, TM-45°/-45°, and TE-45°/-45°, respectively. The performance of the double-layer approach is numerically verified by the two-dimensional (2D) finite element algorithm (FEM) using COMSOL software. The proposed method suggests a novel and simple approach to design a compact four states polarization-independent GC that could be used in integrated (on-chip) photonic communication circuits. [ABSTRACT FROM AUTHOR]

The temperature sensors are remarkably required for highly sensitive temperature monitoring in advanced applications including nanobiosensing, healthcare, disease diagnosis, and so on. Therefore, this paper presents a fiber Bragg gating (FBG)–based sensor designed for demanding novel applications, such as temperature measurements in biotechnology. We propose a highly sensitive temperature sensor in the near-infrared range made from germanium-doped silica core optical fiber, which provides high-performance properties. To evaluate the structure, several practical parameters are considered including environmental temperature variations. The structure first is numerically simulated by the finite difference time domain method. Then, by using the PSO algorithm, appropriated results are obtained. Finally, a fabricated structure is presented. It is demonstrated that the sensitivity of the transmitted light can be tuned through temperature variations of FBG. Moreover, the effects of alteration of FBG period on the sensitivity have been analyzed. Results show that the sensitivity of the proposed temperature sensor can be controlled by tuning the temperature. In the optimum design of the proposed FBG-based temperature sensor, the maximum value of sensitivity is achieved as high as S = 717 1/°C as temperature change from 0 to 140 °C. This work may have significant prospects in tunable, highly sensitive temperature sensors in optical biotechnology. [ABSTRACT FROM AUTHOR]

An independently tunable dual-resonance hybrid structure incorporating graphene is proposed to achieve selective light trapping and response tuning in the visible and near-infrared band. The incident light with different wavelengths are intensely localized in two separated resonant cavities via the synergetic effect of the guided mode resonances (GMRs) and optical Tamm states (OTSs), which leads to the strong enhancement of the light-matter interaction of graphene in different parts of the structure. The dual-band perfect absorption associated with critical coupling is achieved, whereas the two resonant modes can be independently tuned by adjusting the corresponding structural parameters. Furthermore, by dynamically changing the permittivity of the resonant cavities or the chemical potential of graphene, the absorption wavelength can be continuously adjusted and the absorption efficiency can be regulated stepwise. [ABSTRACT FROM AUTHOR]

Self-accelerating beams show considerable captivating phenomena and applications owing to their transverse acceleration, diffraction-free and self-healing properties in free space. Metasurfaces consisting of dielectric or metallic subwavelength structures attract enormous attention to acquire self-accelerating beams, owing to their extraordinary capabilities in the arbitrary control of electromagnetic waves. However, because the self-accelerating beam generator possesses a large phase gradient, traditional discrete metasurfaces suffer from insufficient phase sampling, leading to a low efficiency and narrow spectral band. To overcome this limitation, a versatile platform of catenary-inspired dielectric metasurfaces is proposed to endow arbitrary continuous wavefronts. A high diffraction efficiency approaching 100% is obtained in a wide spectral range from 9 to 13 μm. As a proof-of-concept demonstration, the broadband, high-efficiency and high-quality self-accelerating beam generation is experimentally verified in the infrared band. Furthermore, the chiral response of the proposed metasurfaces enables the spin-controlled beam acceleration. Considering these superior performances, this design methodology may find wide applications in particle manipulation, high-resolution imaging, optical vortex generation, and so forth.

Abstract The efficiency of optical power transferred between two bidimensional waveguides, continuous waveguide and periodic subwavelength waveguide has been designed and optimized by using a taper composed by variable length segments (0.03 − 0.27 µm). Waveguides with variable width (270 − 330 nm) have been considered and the number of segments on the taper region has been varied from 10 to 15. The optimized taper was efficiently designed using bio-inspired algorithms based on the genetic algorithms and the artificial immune system. The power coupled has been computed using the frequency domain finite element method. The best solution is 20% shorter than previously existent tapers with coupling light efficiency above 90% over a broadband interval of frequencies and it exhibits high fabrication error tolerances.

A mid-infrared (mid-IR) dual-band filter with ultra-high resolving power is designed and numerically investigated. It is composed of an Al2O3 layer between a gold grating and a CaF2 substrate. Each unit cell of the grating consists of two gold strips with different heights. The filter provides two high-extinction ultra-narrow stop bands. One is at a wavelength of ~3.8392 μm with a full-width at half-maximum (FWHM) of ~0.8 nm, and the other is at a wavelength of ~3.8435 μm with a FWHM of ~1.8 nm. The resolving power of the filter is >2100. The two stop bands result from the (+1) and (-1) waveguide modes, respectively. An interaction between the two resonance modes exists and affects the stop bands. We can control the interaction by adjusting the grating parameters, thus designing the FWHMs and lineshapes of the two stop bands flexibly.

This paper deals with the optimization of photonic structures, specifically waveguide couplers between a continuous waveguide (CWG) and a periodically segmented waveguide (PSW). For this purpose, a numerical technique called finite element method (FEM) is used in conjunction with an adapted optimization technique called Scatter Search (SS). In this work the possibilities of finding high coupling efficiencies are extended through the creation of adaptations to the Scatter Search algorithm and the enormous amount of solutions generated to be evaluated by such procedure. The results have showed that the adaptation of the technique is valid for optimization of tapers.

The strong light localization and long photon lifetimes in whispering gallery mode (WGM) microresonators, benefiting from a high-quality ( Q ) factor and a small mode volume ( V ), could significantly enhance light-matter interactions, enabling efficient nonlinear photon generation and paving the way for exploring novel on-chip optical functionalities. However, the leakage of energy from bending losses severely limits the improvement of the Q factor for subwavelength WGM microresonators. Here, we demonstrated an integrated self-suspended WGM microresonator that combines external rings and bridges with a microdisk on a platform of silicon on insulator, achieving about one-hundred-fold enhancement in the Q factor and an ultrasmall mode volume of 2.67(/λ n Si ) 3 as predicted by numerical simulations. We experimentally confirmed the improved performance of the subwavelength WGM resonator with the dramatic enhancement of third-harmonic generation and second-harmonic generation on this device. Our work is anticipated to enhance light-matter interactions on small-footprint microresonators and boost the development of efficient integrated nonlinear and quantum photonics.