One-dimensional Gratings Research Articles

Metal-assisted miniaturized refractive index sensor based on quasi-bound states in the continuum

A miniaturized optical refractive index sensor based on quasi-bound states in the continuum (quasi-BICs) is proposed in this work. By utilizing one-dimensional compound gratings with varying heights to break the symmetry of the grating, the structure supports the transition from BIC to quasi-BIC. Additionally, metallic mirrors are positioned at the edges of the grating to facilitate effective in-plane light confinement, thereby reducing light leakage and significantly enhancing peak efficiency within constrained dimensions. A comprehensive investigation was conducted to analyze the impact of metal height, edge spacing, and the number of periods on the resonance modes in finite structures, with the aim of optimizing structural parameters. An assessment of refractive index sensing performance was performed under TE and TM polarizations. The results indicate that the sensitivities for TM and TE polarizations are 427 nm/RIU and 434 nm/RIU, with a small footprint of approximately 18.51 µm and 18.48 µm, respectively. This study has the potential to enhance the design and application of miniaturized optical refractive index sensors.

Diffraction efficiency management by complex binary gratings.

The diffraction efficiencies of a complex binary diffraction grating with a rectangular profile are controlled through the steps' phases, amplitudes, and duty cycle, based on analytical expressions. It is demonstrated that the zeroth-diffraction order can be canceled for any arbitrary value of the duty cycle, provided that a π-phase difference is imposed, along with a specific ratio of the steps' amplitudes. This feature is not feasible for separated amplitude-only and phase-only rectangular binary gratings in the context of one-dimensional gratings. In this framework, a key analytic relationship between the duty cycle and the steps' amplitude ratio is derived, allowing the design of such gratings with this desired feature across a wide range of conditions, not limited to a duty cycle of 0.5. Concerning the higher diffraction orders, it is proved that their intensities cancel or maximize for fixed duty cycle no matter the amplitude and phase values of the steps. The intensity of the m-th diffraction order possesses m maxima and m - 1 zeros on the full range of the duty cycle. All these features were corroborated experimentally. The broad insight of such a grating allows the design of gratings with diffraction efficiencies tailored for specific applications.

Microfluidic channel of dual grating structures for directional fluorescence emission enhancement.

One-dimensional (1D) gratings can control the intensity and direction of fluorescence emission, which are widely applied in biological detection. However, they are limited in bio-detection due to the small region for light-matter interaction. In this work, we propose a microfluidic channel with a dual-grating structure, which, as shown by numerical simulations, has excellent directional fluorescence enhancement, with an enhancement of more than 100-fold. This enhancement is due to the resonance modes of the metal groove grating (MGG) and the dielectric grating (DG), along with the F-P cavity mode between the upper and lower gratings of the hybrid structure. High E-field achieved within the grooves of the MGG and within the microfluidic channel can greatly facilitates fluorescence excitation when the fluorophores are in the channel. Additionally, this hybrid structure offers the capability of dual-layer, multi-wavelength, high sensitivity parallel detection for multiple analyte. This work opens up vast prospects for its application in the field of high-sensitivity microfluidic fluorescence biochemical detection, environmental analysis, and other luminescent devices.

From high- to low-contrast: the role of asymmetries in dielectric gratings supporting bound states in the continuum.

One-dimensional sub-wavelength gratings are versatile photonic platforms supporting diverse resonances, including symmetry-protected bound states in the continuum. However, practical access to these bound modes relies on their quasi-bound form, which necessitates the introduction of perturbations in either geometry or material properties. Despite having a large, finite quality factor, quasi-bound modes retain their characteristically strong field confinement. Gaining control over field localization and leakage of quasi-bound modes requires an investigation not limited to studying the degree of asymmetry and the incoming polarization. Here, we demonstrate that by carefully combining specific types of asymmetries and refractive index contrast between the grating and its surrounding environment, one can tailor field localization and Q-factor almost at will. Our findings reveal a strategic roadmap for optimizing quasi-bound mode implementation, dramatically improving their use in applications such as optical communication, sensing, and nonlinear optical processes.

UV-NIR polarization sensitive metamaterial absorber based on two-dimensional titanium grating

A UV-NIR metamaterial absorber (MMA) based on a two-dimensional titanium grating that is sensitive to polarization is proposed. The absorber is composed of a two-dimensional titanium grating shaped as a four-edge platform, a one-dimensional SiO2 grating, and an Au substrate. When the incident light is polarized in the X direction, the absorber achieves ultra-broadband absorption of 95.72 % in the range of 300–2400 nm. When the incident light is polarized in the Y direction, the absorber exhibits near-perfect ultra-narrowband absorption at 351 nm, and the Q factor and the sensitivity (S) of the narrowband absorption are 38.14 and 123.21 nm/RIU, respectively. The absorber also achieves polarization angle selective absorption at 400–2400 nm with an average extinction ratio of 25.73. Both the impedance matching theory and the energy distribution of the electromagnetic field explain the polarization-sensitive MMA well. The proposed absorber is resistant to changes in incidence angle and may be useful in polarization imaging, photoelectric detection, and optical sensors.

Transverse-electric surface plasmon polaritons in periodically modulated graphene

Transverse-electric (TE) surface plasmon polaritons are unique eigenmodes of a homogeneous graphene layer that are tunable with the chemical potential and temperature. However, as their dispersion curve spectrally lies below the light line, they cannot be resonantly excited by an externally incident wave. Here, we propose a way of exciting the TE modes and tuning their peaks in the transmission by introducing a one-dimensional graphene grating. Using the scattering-matrix formalism, we show that periodic modulation of graphene makes transmission more pronounced, potentially allowing for experimental observation of the TE modes. Furthermore, we propose the use of turbostratic graphene to enhance the role of the surface plasmon polaritons in optical spectra. Published by the American Physical Society 2024

Inverse optical scatterometry using sketch-guided deep learning

Optical scatterometry, also referred to as optical critical dimension (OCD) metrology, is a widely used technique for characterizing nanostructures in semiconductor industry. As a model-based optical metrology, the measurement in optical scatterometry is not straightforward but involves solving a complicated inverse problem. So far, the methods for solving the inverse scattering problem, whether traditional or deep-learning-based, necessitate a predefined geometric model, but they are also constrained by this model with poor applicability. Here, we demonstrate a sketch-guided neural network (SGNN) for nanostructure reconstruction in optical scatterometry. By learning from training data based on the designed generic profile model, the neural network acquires not only scattering knowledge but also sketching techniques, that allows it to draw the profiles corresponding to the input optical signature, regardless of whether the sample structure is the same as the generic profile model or not. The accuracy and strong generalizability of proposed approach is validated by using a series of one-dimensional gratings. Experiments have also demonstrated that it is comparable to nonlinear regression methods and outperforms traditional deep learning methods. To our best knowledge, this is the first time that the concept of sketching has been introduced into deep learning for solving the inverse scattering problem. We believe that our method will provide a novel solution for semiconductor metrology, enabling fast and accurate reconstruction of nanostructures.

Design and fabrication of two-dimensional holographic grating waveguide with large field of view and high uniformity

Augmented Reality (AR) technology is a technique that integrates virtual im- ages with the real environment. With the continuous improvement of comput- ing power in mobile electronic products, mobile devices with AR capabilities are gradually becoming the next generation computing platform. When eval- uating the performance of holographic waveguides, key indicators include the field of view (FOV), eye relief, and eye box uniformity. This study proposes a method for designing holographic waveguides with a large field of view and high uniformity based on a two-dimensional holographic grating (2DHG). By lever- aging the multiplexing characteristics of photopolymer, we superimposed two incoherent one-dimensional gratings on a holographic material layer, achieving dual-channel propagation of the field of view, with each channel responsible for half of the FOV. In the end, these two channels are stitched together to form a complete FOV. This method not only enlarges the eye box but also helps im- prove the system's FOV. By controlling the transmittance of the exposure area, the uniformity of the brightness of the wide-field eye box is improved. Finally, we conducted a performance analysis of the prepared holographic waveguide, verifying the feasibility and correctness of the proposed 2DHG waveguide in two-dimensional eye box expansion and field of view extension. Through eye box expansion, the eye box reached 14 mm × 17 mm, the diagonal field of view reached 58◦, the eye relief is 15 mm, and the eye box brightness uniformity reached 79.7 %.

Optical functionality simulation through traceable characterization of optical components

The production of high-value components containing functional micro- and submicron-scale features requires the implementation of high-quality in-process inspection technologies to guarantee zero-defect manufacturing, boosting the transition to digital manufacturing. In the present study, the impact of the measurement uncertainty assigned to surface micro- and submicron-structures on the optical performance of a one-dimensional diffraction grating was determined. Thus, 3D confocal microscopy was selected as an inspection technology, introducing measuring data and uncertainty values on an optical simulation model to evaluate the influence of those magnitudes on the irradiance profile of a light beam at a target plane after passing through the diffraction grating. To achieve this goal, the calibrated areal standards have been used in addition to good practice guides for instrument calibration combined with optical modeling to simulate the functional behavior of the optical device. Results proved that changes of hundreds of nanometers in the lateral dimensions of the grating profile lead to drastic deviations in the irradiance profile and, thus, deviation in the optical performance. Hence a change in the period and the structure width of the step grating to the limits of the calculated uncertainty (Nominal Period ± ux,y) supposes a change of down to a 50 % of decrease in the maximum peak of the irradiance profile detected. Thanks to these results, it is possible to define a range in the grating device's performance depending on the dimensional surface characterization and its uncertainty. Moreover, an in-situ measurement approach has been designed for further product quality control.

Fabrication and sensing properties of a molecularly imprinted polymer on a photonic PDMS substrate for the optical detection of C-reactive protein

This work presents the development of a novel photonic-based sensor on a nanoscale patterned polydimethylsiloxane (PDMS) substrate acting as transducer, combined with a molecularly imprinted polymer for the rapid and sensitive detection of C-reactive protein (CRP). A silicon wafer was patterned using electron-beam lithography with a one-dimensional (1D) reflective grating (periodicity of about 400 nm) and then replicated using nanoimprint lithography. The latter silicon mould was used for replica moulding to generate a patterned PDMS. The unique 1D design gave PDMS photonic properties that were suitable as a substrate to assemble the biorecognition layer. The surface of the photonic PDMS was modified to become hydrophilic and additionally functionalized with vinyl silane groups to bind the imprinted polymer, resulting in a photonic molecularly imprinted polymer substrate (PMIPS). The developed method was successfully used to detect CRP in serum samples. The reflectance intensity decreased with increasing CRP concentrations, and the sensing PMIPS showed a linear response between 0.005 and 1.215 µg mL−1, a limit of detection of 0.003 µg mL−1, and a selective response to CRP when tested against serum calprotectin. Overall, this novel optical sensor can be used to detect CRP, a serum biomarker of inflammation, particularly in inflammatory bowel diseases.

Exploiting Thin-Film Properties and Guided-Mode Resonance for Designing Ultrahigh-Figure-of-Merit Refractive Index Sensors

Guided-mode resonance (GMR) gratings have emerged as a promising sensing technology, with a growing number of applications in diverse fields. This study aimed to identify the optimal design parameters of a simple-to-fabricate and high-performance one-dimensional GMR grating. The structural parameters of the GMR grating were optimized, and a high-refractive-index thin film was simulated on the grating surface, resulting in efficient confinement of the electric field energy within the waveguide. Numerical simulations demonstrated that the optimized GMR grating exhibited remarkable sensitivity (252 nm/RIU) and an extremely narrow full width at half maximum (2 × 10−4 nm), resulting in an ultra-high figure of merit (839,666) at an incident angle of 50°. This performance is several orders of magnitude higher than that of conventional GMR sensors. To broaden the scope of the study and to make it more relevant to practical applications, simulations were also conducted at incident angles of 60° and 70°. This holistic approach sought to develop a comprehensive understanding of the performance of the GMR-based sensor under diverse operational conditions.

Thermophotovoltaic emitter design with a hyper-heuristic custom optimizer enabled by deep learning surrogates

Micro/nanostructures hold promise for use as emitters to boost the efficiency of thermophotovoltaic (TPV) systems. For example, periodic gratings may alter the spectrum of irradiation on the photovoltaic cell to better match the spectral response of the cell. Photons with energies slightly higher than the bandgap of the semiconductor are the most desired as they generate electron-hole pairs with minimal thermalization losses. This prompts the use of gratings as selective emitters, and the choice of grating geometry has been an active research topic. Even for a one-dimensional (1D) grating, millions of possible geometries exist, and each grating requires computationally intensive full-wave simulation, e.g., the rigorous coupled-wave analysis (RCWA), to calculate the spectral, directional emissivity. Since optimization algorithm performance is problem-dependent, in this work, a hyper-heuristic search enabled by a fully connected neural network surrogate of the native RCWA is employed to select an algorithm for a specific grating design problem. A comparison with existing untuned algorithms for an ideal emitter problem demonstrates that the hyper-heuristically generated algorithm yields superior performance. This algorithm is then employed for the optimization of the emitter for a full TPV system comprising a heated 1D tungsten binary grating paired with a 300 K InGaSb cell. The system is optimized for maximum power or efficiency at 2000 K and 1500 K, respectively, and the grating properties for the optimized cases are analyzed.

Ultrahigh Figure-of-Merit Optical Sensing Based on Guided-Mode Resonances by One-Dimensional Dielectric Grating

Ultrahigh Figure-of-Merit Optical Sensing Based on Guided-Mode Resonances by One-Dimensional Dielectric Grating

Transmission enhancement effect of distributed Bragg reflector structure covered with metal grating

In order to observe the extraordinary optical transmission (EOT) through a metal gratings, induced by Tamm plasmon polaritons (TPPs), a layered structure consisting of a distributed Bragg reflector covered with a one-dimensional metal grating is proposed in this work. When an incident light wave passes through DBR regime and impinges on the DBR-metal interface normally, the generation of TPPs and the resulting highly localized energy on the metal-DBR interface are simulated in detail by the finite element method. As a result, the surface plasmon polariton (SPPs) modes accommodated inside the slits of metal gratings can be excited more effectively by the enhanced electromagnetic field associated with TPPs located on the interface. Furthermore, the enhanced transmission of incident light waves in the structure can be achieved when the SPP mode inside the grating slits satisfies the Fabry-Perot (FP)-like resonance condition, which reveals that the EOT in this structure comes from a TPPs-FP hybrid resonance. This inference can be confirmed by the relationships between the central wavelength and the grating height for the two transmission peaks, and the magnetic field modal profiles associated with the two peaks. Quantitative effects of the slit width and duty cycle on the transmission peak of the metal grating are analyzed numerically, and the results demonstrate that when the period is determined, as the slits width increases, the two peak transmittances first increase and then decrease. On the other hand, when the slit widths are chosen to be 40 nm, 50 nm, and 60 nm respectively, the peak transmittance first increases and then decreases with the duty cycle increasing. Meanwhile, it is found that the center wavelengths of the transmission peaks are related to the duty cycle in a nearly linear manner for three slit widths, which can be used to flexibly adjust the center wavelength of extraordinary optical transmission.

Error investigation for SI traceable pitch calibration of one-dimensional grating by grazing-incidence small-angle X-ray scattering

Grazing-incidence small-angle X-ray scattering (GI-SAXS) is a promising technique for investigating nanostructured surfaces, particularly for the fine pitch calibration of one-dimensional gratings. However, only a few studies validated this technique from a metrological perspective. In this study, we developed a GI-SAXS system traceable to length and angle standards and used it to analyze the uncertainty of pitch calibrations in detail. Consequently, we found statistically significant systematic errors which have not been reported so far. We also found that the systematic error varied depending on the signal width of the diffracted X-rays. Through theoretical modeling and quantitative experiments, we successfully explained that the source of the systematic error is intensity modulation along the grating truncation rod. Since the systematic error decreases with smaller signal width, optimizing the optical system may efficiently reduce the uncertainty.

Quantitative Ultrafast Magnetoacoustics at Magnetic Metasurfaces.

Femtosecond (fs) time-resolved magneto-optics is applied to investigate laser-excited ultrafast dynamics of one-dimensional nickel gratings on fused silica and silicon substrates for a wide range of periodicities Λ = 400-1500 nm. Multiple surface acoustic modes with frequencies up to a few tens of GHz are generated. Nanoscale acoustic wavelengths Λ/n have been identified as nth-spatial harmonics of Rayleigh surface acoustic wave (SAW) and surface skimming longitudinal wave (SSLW), with acoustic frequencies and lifetimes being in agreement with theoretical calculations. Resonant magnetoelastic excitation of the ferromagnetic resonance (FMR) by SAW's third spatial harmonic, and, most interestingly fingerprints of the parametric resonance at 1/2 SAW frequency have been observed. Numerical solutions of Landau-Lifshitz-Gilbert (LLG) equation magnetoelastically driven by complex polychromatic acoustic fields quantitatively reproduce all resonances at once. Thus, our results provide a solid experimental and theoretical base for a quantitative understanding of ultrafast fs-laser-driven magnetoacoustics and tailoring the magnetic-grating-based metasurfaces at the nanoscale.

Wavelength sensitivity reconfigurable SPR photodetector with a blazed grating profile.

Surface plasmonic detectors based on one-dimensional half-wavelength gratings have attracted attention due to their wavelength- or polarization-specific photodetection. Although the effect of a grating period and a grating depth on the photoelectric conversion of 1D half-wavelength grating-based surface plasmon resonance (SPR) detectors has been discussed thoroughly in recent years, the effect of different grating profiles on device performance is still limited to the rectangular shape. In this article, we proposed a wavelength sensitivity reconfigurable photodetector enhanced by SPR with a blazed grating profile. The gold layer was fabricated on a silicon-based blazed grating to form a Schottky barrier and act as an SPR coupler. By measuring the photocurrent in the range of -58° to -48°of an incident angle, the peak shifts of a photocurrent signal waveform are found to depend on the wavelength over 800-1000 nm.

Transparent conducting oxides (TCOs) based gratings for light absorption in near infrared spectrum

The present work examines the absorption enhancement of Transparent Conducting Oxides (e.g., Aluminum doped Zinc Oxide (AZO), Gallium doped Zinc Oxide (GZO), and Indium doped Tin Oxide (ITO) for widely used telecom spectral window around λ = 1550 nm. This is by virtue of the fact that the silica-based optical fiber exhibits minimum loss in this window. It is well established that one-dimensional metallic lamellar grating can significantly enhance light absorption through diffraction and excitation of surface plasmon polaritons (SPPs) at the metal-dielectric interface. This results in subwavelength confinement and hence, enhanced optical absorption in the absorbing layer near the grating. The absorption enhancement can be tuned in different spectral regions by controlling grating parameters, such as grating period, grating height, and grating aspect ratio. The present work focused on establishing Transparent Conducting Oxides (TCOs) as effective plasmonic materials for metallic grating to optimize optical absorption in the near-infrared (NIR) spectral region. The role of TCOs and the grating parameters in fine-tuning the optical absorption enhancement is investigated. Moreover, TCO grating-based designs optimized for absorption enhancement in and around the telecommunication wavelength are proposed.

Optical rectification Hall effect in a one-dimensional grating

We observe a transverse optical rectification current enhanced by surface plasmon polaritons in a one-dimensional gold thin film with a grating whose unit cell is asymmetric. In the linear regime, momentum transfer from incident laser light in one plane of incidence cannot yield an electrical current in the transverse direction. We show that a nonlinear system can cross-couple coordinates and lead to a transverse photocurrent despite no linear momentum transfer in the transverse direction. Nonlinear effects can be described by higher order terms in the polarization density expansion, which relates the polarization to the electric fields in the system, via linear and higher-order susceptibility tensor coefficients. We modify this expression with a phenomenological term to explain the counterintuitive observations.

Surface-Emitting Lasers with Surface Metastructures

Vertical-cavity surface-emitting lasers (VCSELs) have been widely used in consumer electronics, light detection and ranging, optical interconnects, atomic sensors, and so on. In this paper, a VCSEL with the surface metastructure like one-dimensional high-contrast grating (HCG), based on the HCG-DBR vertical cavity, was first designed and fabricated. The polarization characteristic of the HCG-VCSEL were experimentally studied. The p-doped top 4-pair DBR for the current spreading and the direction shift between the HCG and the elliptical oxide aperture may result in a low orthogonal polarization suppression ratio in the HCG-VCSEL. Then, the Bloch surface wave surface-emitting laser (BSW-SEL), based on the HCG-DBR metastructure, is proposed for single-mode, high-efficiency, and high-power output with a low divergence angle. The mode field and the far field profile of the BSW-SEL are calculated for verification. The surface-emitting lasers with surface metastructures are useful for the sensing applications and optical interconnects.