Grating Substrate Research Articles

Incoherent phenomena in anisotropic periodic structures: from modeling to experimental demonstration.

The periodic structures are widely studied in numerous optical applications and there is a number of good tools for numerical modeling of such a structures (for example rigorous coupled-wave analysis, finite-difference time-domain, finite element method etc.). However, when it comes to the modeling of incoherent effects in many cases of practical interest, the current methods are not rigorous enough or depend on computationally demanding averaging of coherent response. In this paper, we present a novel approach to modeling of incoherent effects in structures with lateral periodicity based on scattering matrix formalism, as a way to describe optical response of a structure, and on application of incoherent wave summation in the form of infinite geometric series and generalized Mueller matrix calculus. This method can be combined with any of the existing coherent methods of modeling periodic structures and it offers significantly faster computational performance than partially coherent/incoherent methods based on averaging. It is compared with other methods for modeling of incoherent effects and also with experimental spectroscopic data. This method is then used to explain phenomena emerging from the complex interaction between diffraction grating and thick substrate.

Ultrasensitive Upconversion Nanoparticle Immunoassay for Human Serum Cardiac Troponin I Detection Achieved with Resonant Waveguide Grating.

Selective detection of biomarkers at low concentrations in blood is crucial for the clinical diagnosis of many diseases but remains challenging. In this work, we aimed to develop an ultrasensitive immunoassay that can detect biomarkers in serum with an attomolar limit of detection (LOD). We proposed a sandwich-type heterogeneous immunosensor in a 3 × 3 well array format by integrating a resonant waveguide grating (RWG) substrate with upconversion nanoparticles (UCNPs). UCNPs were used to label a target biomarker captured by capture antibody molecules immobilized on the surface of the RWG substrate, and the RWG substrate was used to enhance the upconversion luminescence (UCL) of UCNPs through excitation resonance. The LOD of the immunosensor was greatly reduced due to the increased UCL of UCNPs and the reduction of nonspecific adsorption of detection antibody-conjugated UCNPs on the RWG substrate surface by coating the RWG substrate surface with a carboxymethyl dextran layer. The immunosensor exhibited an extremely low LOD [0.24 fg/mL (9.1 aM)] and wide detection range (1 fg/mL to 100 pg/mL) in the detection of cardiac troponin I (cTnI). The cTnI concentrations in human serum samples collected at different times during cyclophosphamide, epirubicin, and 5-fluorouracil (CEF) chemotherapy in a breast cancer patient were measured by an immunosensor, and the results showed that the CEF chemotherapy did cause cardiotoxicity in the patient. Having a higher number of wells in such an array-based biosensor, the sensor can be developed as a high-throughput diagnostic tool for clinically important biomarkers.

Systematic analysis of the Rayleigh–Wood anomalies and the symmetry relations of 2D‐periodic nanostructures by imaging spectroscopic ellipsometry

AbstractPeriodic arrays of metallic nanostructures were fabricated by electron beam lithography and studied by means of spectroscopic imaging ellipsometry in the near infrared to ultraviolet spectral ranges. The sample consists of gold nanodisc and nanorod gratings on a silicon substrate. Spectroscopic imaging ellipsometry, allowing for the simultaneous observation of all gratings with microscopic resolution, was used to systematically analyze the varying grating and nanostructure parameters. The ellipsometric spectra obtained for a full in‐plane sample rotation proved to be a highly sensitive measure of the gratings’ Rayleigh–Wood anomalies. Their dependence on the sample azimuth was in excellent agreement with the Rayleigh lines calculated from the grating parameters and could be tracked for both the ambient‐to‐grating and the grating–substrate interfaces. Contrary to other studies on similarly sized nanoparticle arrays, we found no indication of localized plasmon resonance in the visible range of the spectrum, but only a weak NIR response truncated by the Rayleigh lines. Finally, the symmetry of the structures was investigated by imaging Mueller‐matrix measurements.

Low line density blazed gratings with low blaze angles

Gratings installed in synchrotron beamlines and instruments have standard line densities ranging from ~300 L/mm up to 2400 L/mm with blaze angles from 0.3° to 2°. For FEL beamlines lower line densities and lower angles are required. To manufacture these in the required high quality poses some challenges with regard to the ruling process and the ion etching process. Our investigation and process results will be described here. Additionally, first results of the process on a grating substrate are demonstrated.

Spallation of Isolated Aluminum Nanoparticles by Rapid Photothermal Heating.

The spallation of isolated aluminum (Al) nanoparticles (NPs) is initiated using rapid photothermal heating. The Al NPs exhibited a nominal diameter of 120 nm, with an average oxide shell thickness of 3.8 nm. Photothermal heating was achieved by coupling a focused laser (446 nm wavelength) to an optical grating substrate and to the plasmonic resonance of the Al NPs themselves. These factors enhanced the absorption cross section by a factor of 8-18 compared to no substrate and generated an Al NP heating rate on the order of 107-108 K/s. Observations indicate that molten Al is ejected from the heated NP, indicating that melting of the Al core is required for spallation. A graphene layer atop the grating substrate encouraged the formation of discrete particles of ejected Al, while irregular elongated filament products were observed without the graphene layer. Numerical simulations indicate that laser-heated Al NPs reach temperatures between approximately 1000 and 1500 K. These observations and experimental conditions are consistent with those anticipated for the melt dispersion mechanism, a thermomechanical reaction mechanism that has not previously been clearly demonstrated. Activating and controlling this mechanism is anticipated to enhance applications ranging from biological phototherapy to energetic materials.

High sensitivity micro-displacement sensor based on fiber Bragg grating and amplification substrate

High sensitivity micro-displacement sensor based on fiber Bragg grating and amplification substrate

Optical amplification of surface plasmon polaritons in a graphene single layer integrated with a random grating

In this paper, we design and simulate a terahertz (THz) controllable active plasmonic waveguide structure based on a single graphene layer that is placed on a random SiC grating substrate whose grooves are filled with CaF2. Optical gain in the proposed THz active plasmonic waveguide structure is provided by the stimulated emission process in the photoexcited graphene monolayer that leads to the amplification of surface plasmon polariton (SPP) waves. We use a random grating substrate to introduce Anderson localization of the SPP waves propagating through the graphene monolayer to enhance their optical amplification at resonance frequencies in which more amplification occurs due to multi-scattering and interference effects. It is shown that the input SPP wave can be amplified by 130 times at resonance frequencies of the random system. Significant amplification occurs only for frequencies that are equal to the resonance frequencies corresponding to the passive structure and are located within the gain bandwidth of graphene. The effects of depth and width of filled grooves on the amplification characteristics are also investigated. Furthermore, we show that increasing the ambient temperature by 30 K can reduce the output intensity by a factor of 12. This property of the proposed graphene-based THz plasmonic waveguide structure makes it useful in temperature sensing applications and on/off switchable laser devices.

Tunable surface plasmon resonance enhanced fluorescence via the stretching of a gold quantum dot-coated aluminum-coated elastomeric grating substrate.

In this study, the surface plasmon resonance (SPR)-enhanced fluorescence properties of gold quantum dots (AuQDs) on an aluminum (Al)-coated polydimethylsiloxane (PDMS) grating substrate were investigated by changing the grating pitch via mechanical stretching. The SPR-excitation wavelength of the AuQDs/Al-coated PDMS-grating substrate was tuned by changing the incident light angle from 5° to 60° and stretching it from 0 to 1.0 mm. In addition, the SPR-enhanced fluorescence tuning ability was studied using an AuQD/Al-coated PDMS-grating film by stretching the substrate. The SPR-enhanced fluorescence (SPF) of the AuQDs on the Al-grating was observed using a violet laser as the excitation source at 405 nm with p-polarization. The wavelengths of the SPR excitation, corresponding to the SP-dispersion mode of +1, were shifted to a longer wavelength upon stretching the grating substrate from 0 to 1.0 mm. By stretching the AuQDs/Al-grating PDMS substrate, the SPR-enhanced fluorescence intensity increased at fixed incident angles of 15° and 35°, whereas the SPR-enhanced fluorescence intensity decreased at 40°. Moreover, the SPF could be tuned to exhibit different properties in tunable optical sensors.

Novel Traveling Wave Amplifier Design with Interdigital Grating and 2DEG Substrate for High Gain at 300GHz

We propose a novel traveling wave device operating at sub-THz frequencies. The device structure employs metastuctures for wave slow down to increase gain by 10dB or more. The excitation of the device channel is done using a CPW line with an inserted interdigitated grating. The underlying 2DEG silicon substrate supports an electron beam injected from the drain after source biasing in HEMTs. The proposed device is simulated starting with a 3-D full wave prediction of the field structure to be supported by the interdigital grating within the CPW. This field structure is then used in a 2-D particle-in-cell (PIC) code to predict the device gain. Results from these simulations show more than 10 dB gain at 300 GHz.

Method for exposure dose monitoring and control in scanning beam interference lithography.

To improve grating manufacturing process controllability in scanning beam interference lithography (SBIL), a novel method for exposure dose monitoring and control is proposed. Several zones in a narrow monitoring region are fabricated on a grating substrate by piecewise uniform scanning. Two monitoring modes are given based on the different widths of the monitoring region. The monitoring curve of the latent image diffraction efficiency to scanning velocity is calculated by rigorous coupled wave analysis. The calculation results show that the exposure dose in SBIL can be monitored by the shape change of the monitoring curve, and an optimized scanning velocity can be selected in the monitoring curve to control the exposure dose.

A dynamically tunable and wide-angle terahertz absorber based on graphene-dielectric grating

We theoretically and numerically demonstrate a tunable and wide-angle terahertz absorber, which is composed of multilayer graphene-dielectric grating and bottom metal substrate. Numerical simulation shows that the proposed absorber has the advantage of dynamically tunable range from 1.015 THz to 1.165 THz when the chemical potential of graphene increases from 10 meV to 150 meV. The absorption efficiency can reach a high value of 99%. To show the working mechanism of absorption, the near field distributions of magnetic components are presented at the absorption wavelength. We also demonstrate that the tunable range of absorption can be engineered by designing the geometry parameters. In addition, it is shown that the designed absorber can maintain the good performance of absorption over a wide incident angle from [Formula: see text] to [Formula: see text] under TM-polarization.

Parasitic‐coupled high‐gain graphene antenna employed on PBG dielectric grating substrate for THz applications

AbstractIn THz frequency region, the high‐gain miniaturized patch antenna is essential to overcome the atmospheric losses. A novel dual‐band miniaturized graphene antenna array with parasitic‐coupled feed and mounted on PBG dielectric grating substrate is proposed in the operating frequency band from 0.85 to 1.04 THz. The analysis of graphene surface conductivity, surface wave polariton (SPP) of photonic band gap (PBG) substrate, and the dielectric grating substrate has been done for the verification of a projected design. Parametric analysis of PBG radius, periodic perturbation of dielectric grating substrate, and mutual coupling distance has been verified to achieve the enhanced radiation characteristics of the antenna. The minimum return loss of −52.58 dB, VSWR of 1.005, impedance bandwidth of 33.34 GHz, the peak gain of 16.4 dB, and peak directivity of 17 dBi has been observed for the projected design. The parasitic‐coupled method is suitable to reduce the complexity of the antenna array and proposed design is useful in imaging, security using spectroscopy, and wireless communication applications.

Controlling Surface Plasmon Optical Transmission by Stretching a Silver-Coated Elastomeric Grating Substrate

In this study, transmission surface plasmon resonance (TSPR) of a silver-coated elastomeric grating substrate was tuned via mechanical stretching. The wavelengths of TSPR excitation peaks, corresponding to surface plasmon dispersion modes of + 1 and − 1, were simultaneously observed. The TSPR excitation wavelength was dramatically shifted up to 80 nm when the strain of the substrate reached 13.3%. The fine control of TSPR properties of the silver-coated elastomeric grating substrate has potential applications in mechanically tunable band pass filters, optical sensors, etc.

Reducing machine-ruling grating wavefront value by compensating grating substrate surface error in real time

To improve the grating wavefront quality especially the grating size, which is more than 1 m, we provide a method based on conical diffraction theory and grating groove active control technology to correct the grating substrate surface error. We apply the grating groove active control technology to carry out an echelle grating ruling experiment whose line density is 79 lines/mm in 36th order. Compared with the grating substrate surface error, the grating wavefront value reduces from 0.105 λ to 0.056λ i.e., decreases by 47%. The experimental result proves that the grating substrate surface error compensation method can be used to improve the grating wavefront quality effectively.

Reducing Rowland ghosts in diffraction gratings by dynamic exposure near-field holography.

Near-field holography (NFH) combined with electron beam lithography (EBL)-written phase masks is a promising method for the rapid realization of diffraction gratings with high resolution and high accuracy in line density distribution. We demonstrate a dynamic exposure method in which the grating substrate is shifted during pattern transfer. This reduces the effects of stitching errors, resulting in the decreased intensity of the optical stray light (i.e., Rowland ghosts). We demonstrate the intensity suppression of ghosts by 60%. This illustrates the potential for dynamic NFH to suppress undesirable periodic patterns from phase masks and alleviate the stitching errors induced by EBL.

Long-range surface plasmon excitation and photoisomerization of Azobenzene dye

ABSTRACTEnhancement of photoisomerization of Azobenzene dye Chicago Sky Blue 6B (CSB) was attempted using long-range surface plasmon resonance (LRSPR). The CSB film was prepared on a grating substrate with metal film for LRSPR, and its transmission light property was investigated before and after white light irradiation for photoisomerization. The transmission light was markedly changed by irradiation of p-polarized white light, and the change was attributed to photoisomerization of CSB induced by LRSPR which accompanies an intense electric field. Furthermore, the effect of LRSPR was significant compared to standard SPR.

Polarization-insensitive and wide-incident-angle optical absorber with periodically patterned graphene-dielectric arrays.

A polarization-insensitive and angle-independent graphene absorber (GA) with periodically patterned grating is demonstrated. A periodic nanocavity composed of multilayer subwavelength grating and metal substrate supports a strongly localized mode inside the cavity, where the mode helps to absorb more electromagnetic waves. The proposed GA exhibits polarization-insensitive behavior and maintains the high absorption above 90% within a wide range of incident angle (more than 80°). We attribute the high absorption to the excitation of the cavity mode resonance and magnetic resonance for the transverse electric and transverse magnetic polarizations, respectively. The proposed GA has potential applications in the design of various devices, such as optical modulators or tunable absorption filters because of its remarkable angle-insensitive absorption performance.

An ultra-narrowband TE-polarization absorber with a dielectric grating and metal substrate

We report an ultra-narrowband absorber with a dielectric grating and metal substrate. The simulation results show that we can achieve ultra-narrowband absorption with the absorption bandwidth less than 0.6 nm and the absorption rate more than 0.99 for TE-polarization (electric field is parallel to grating grooves). The simulation results also show the guide-mode resonance in the grating region and low power loss at the absorption peak. In addition, the ultra-narrowband absorption peak can be tuned by shrinking or enlarging the structure parameters. The figure of merit (FOM) is larger than 760 if this absorber is applied as a refractive index sensor.

Spatially and spectrally resolved ultra-narrowband TE-polarization absorber based on the guide-mode resonance

A spatially and spectrally resolved ultra-narrowband absorber with a dielectric grating and metal substrate has been reported. The absorber shows that the absorption rate is more than 0.99 with the absorption bandwidth less than 1.5 nm at normal incidence for TE polarization (electric field is parallel to grating grooves). The angular width of the absorption is about 0.27[Formula: see text]. The wavelength-angle sensitivity and absorption-angle sensitivity are 13.4 nm per degree and 296.3% per degree, respectively. The simulation results also show the spatially and spectrally resolved ultra-narrowband absorption is originated from the guide-mode resonance. In addition, the wavelength-angle sensitivity can be improved by enlarging the grating period according to the guide-mode resonance mechanism. The proposed absorber has potential applications in optical filters, angle measurement and thermal emitters.

Tailored Fano resonance and localized electromagnetic field enhancement in Ag gratings

Metallic gratings can support Fano resonances when illuminated with EM radiation, and their characteristic reflectivity versus incident angle lineshape can be greatly affected by the surrounding dielectric environment and the grating geometry. By using conformal oblique incidence thin film deposition onto an optical grating substrate, it is possible to increase the grating amplitude due to shadowing effects, thereby enabling tailoring of the damping processes and electromagnetic field couplings of the Fano resonances, hence optimizing the associated localized electric field intensity. To investigate these effects we compare the optical reflectivity under resonance excitation in samples prepared by oblique angle deposition (OAD) and under normal deposition (ND) onto the same patterned surfaces. We observe that by applying OAD method, the sample exhibits a deeper and narrower reflectivity dip at resonance than that obtained under ND. This can be explained in terms of a lower damping of Fano resonance on obliquely deposited sample and leads to a stronger localized electric field. This approach opens a fabrication path for applications where tailoring the electromagnetic field induced by Fano resonance can improve the figure of merit of specific device characteristics, e.g. quantum efficiency (QE) in grating-based metallic photocathodes.