Grating Structure Research Articles

Sidewall grating slot waveguide microring resonator biochemical sensor.

Integrated microring resonator structures based on silicon-on-insulator (SOI) platforms are promising candidates for high-performance on-chip sensing. In this work, a novel sidewall grating slot microring resonator (SG-SMRR) with a compact size (5 µm center radius) based on the SOI platform is proposed and demonstrated experimentally. The experiment results show that the refractive index (RI) sensitivity and the limit of detection value are 620 nm/RIU and 1.4 × 10-4 RIU, respectively. The concentration sensitivity and minimum concentration detection limit are 1120 pm/% and 0.05%, respectively. Moreover, the sidewall grating structure makes this sensor free of free spectral range (FSR) limitation. The detection range is significantly enlarged to 84.5 nm in lab measurement, four times that of the FSR of conventional SMRRs. The measured Q-factor is 3.1 × 103, and the straight slot waveguide transmission loss is 24.2 dB/cm under sensing conditions. These results combined with the small form factor associated with a silicon photonics sensor open up applications where high sensitivity and large measurement range are essential.

An improved curved grating demultiplexer based on silicon-on-insulator platform

This article proposes two basic principles to address the astigmatism problem of traditional Rowland gratings. Ray tracing method is employed to analyze the correlation between the optical performance of gratings and the structural parameters, and further, optimize the grating structure. Using the proposed principles, we design a 1×4 demultiplexer with an ultracompact footprint of 97 × 73 μm2, which is suitable of the coarse wavelength division multiplexing (CWDM) system, where the wavelength spacing is 10 nm. The numerical calculation results show that the channel crosstalk between the four channels of the device is less than −40 dB, the insertion loss is between −1.35 dB and −1.80 dB, and the channel uniformity is better than 0.45 dB. The structure proposed curved grating in this article offers broad applications in high-density integrated photonic circuits.

Theoretical simulated fabrication of nanostructure by interference of four-beam guided mode excited by 193 nm laser

This study proposed a lithography method for fabricating periodic nanostructures using the interference of four-beam TE0 guided modes excited by a 193 nm deep ultraviolet laser. The physical mechanism and normalized electric field intensity distribution of four-beam TE0 guided mode interference were theoretically analyzed and numerically simulated using the finite element method. The simulation results confirmed the ability of this method to fabricate periodic structures with a half-pitch resolution of 30.35 nm (approximately λ/6.36), an aspect ratio of 3.95, and a contrast ratio of 1. The theoretically calculated value of the resolution was consistent with the numerical simulation value. The resolution and aspect ratio of the fabricated nanostructures could be adjusted through changes to the thickness of the resist. Moreover, the shape of the fabricated nanostructures, such as the one-dimensional sub-wavelength grating structure and two-dimensional square array lattice structure, were adjustable via changes to the number and azimuth of the excited TE0 guided modes. The results obtained in this study provide valuable theoretical information for the practical manufacture of ultra-high-resolution lithography.

Preparation of Tilted Grating Structure by Nanoimprinting Material with High Refractive Index

The vigorous development of the semiconductor industry has made people’s demand for semiconductor devices increasingly high, but with the shrinking specifications of components, traditional lithography technology has been unable to adapt to the needs of pattern refinement. In optical lithography, it is difficult to produce patterns with feature dimensions of less than 7 nm. Nano-embossing lithography technology, as the next generation of lithography technology, has reached a feature size of less than 5 nm, and even according to relevant reports, the processing accuracy of the technology has reached 2 nm. In this paper, we experimentally illustrate the structural change trend of the inclined grating structure after continuous imprinting, so that the structural height will show a downward trend with the increase of the number of embossing, and the line width of the structural plane will show an upward trend. The best effect is obtained by obtaining high refractive index glue 133 repetitive imprinting two kinds of inclined grating structures. And the nature of the embossing adhesive is proportional to its refractive index, and the higher the refractive index of the embossing adhesive with better imprinting properties.

(Invited) In Situ Spectroscopy of Electrocatalytic and Photocatalytic Interfaces

We report various aspects of electrochemistry and photoelectrochemistry using in situ spectroscopy of electrode (metal) and photoelectrode (semiconductor) interfaces in situ under electrochemical working conditions. These spectroscopies include sum frequency generation (SFG), transient reflectance/absorption spectroscopy (TAS/TRS), and surface enhanced Raman spectroscopy (SERS). Using surface enhanced Raman scattering (SERS) spectroscopy, we monitor local electric fields using Stark-shifts of nitrile-functionalized silicon photoelectrodes.6 Using Graphene-enhanced Raman spectroscopy (GERS)-based Stark-shifts, we measure local electric fields and local charge densities at monolayer graphene electrode surfaces.1 We also measured the stacking dependence and Resonant interlayer excitation of monolayer WSe2/MoSe2 heterostructures for photocatalytic energy conversion.2 Using sum frequency generation (SFG) spectroscopy, we measure the voltage dependence of the orientation of D2O molecules at a graphene electrode surface, which is related back to the “stiffness of the ensemble”.3 In particular, we measured the “free OD” feature in the spectra, which corresponds to the topmost water molecule that is rotated up out of the bulk water solution and is, therefore, not hydrogen bonded. Using transient absorption spectroscopy (TAS), we measure the lifetime of hot electrons photoexcited in plasmon resonant nanostructures.5 Using transient reflectance spectroscopy (TRS), we measure the photoexcited carrier dynamics in a GaP/TiO2 photoelectrode, as well as the electrostatic field dynamics at this semiconductor-liquid interfaces in situ under various electrochemical potentials.4 Here, the electrostatic fields at the surface of the semiconductor are measured via Franz−Keldysh oscillations (FKO). These spectra reveal that the nanoscale TiO2 protection layer enhances the built-in field and charge separation performance of GaP photoelectrodes. Shi, H.T., B.F. Zhao, J. Ma, M.J. Bronson, Z. Cai, J.H. Chen, Y. Wang, M. Cronin, L. Jensen and S.B. Cronin, Measuring Local Electric Fields and Local Charge Densities at Electrode Surfaces Using Graphene-Enhanced Raman Spectroscopy (GERS)-Based Stark-Shifts. ACS Applied Materials & Interfaces, 11, 36252-36258 (2019).Chen, J., C.S. Bailey, D. Cui, Y. Wang, B. Wang, H. Shi, Z. Cai, E. Pop, C. Zhou and S.B. Cronin, Stacking Independence and Resonant Interlayer Excitation of Monolayer WSe2/MoSe2 Heterostructures for Photocatalytic Energy Conversion. ACS Applied Nano Materials, DOI:10.1021/acsanm.9b01898 (2020).Montenegro, A., C. Dutta, M. Mammetkuliev, H.T. Shi, B.Y. Hou, D. Bhattacharyya, B.F. Zhao, S.B. Cronin and A.V. Benderskii, Asymmetric response of interfacial water to applied electric fields. Nature, 594, 62 (2021).Xu, Z.H., B.Y. Hou, F.Y. Zhao, Z. Cai, H.T. Shi, Y.W. Liu, C.L. Hill, D.G. Musaev, M. Mecklenburg, S.B. Cronin and T.Q. Lian, Nanoscale TiO2 Protection Layer Enhances the Built-In Field and Charge Separation Performance of GaP Photoelectrodes. Nano Letters, 21, 8017-8024 (2021).Yu Wang, Yi Wang, Indu Aravind, Zhi Cai, Lang Shen, Boxin Zhang, Bo Wang, Jihan Chen, Bofan Zhao, Haotian Shi, Jahan M. Dawlaty, and Stephen B. Cronin. In Situ Investigation of Ultrafast Dynamics of Hot Electron-Driven Photocatalysis in Plasmon-Resonant Grating Structures. Journal of the American Chemical Society. DOI: 10.1021/jacs.1c12069 (2022).Haotian Shi, Ryan T. Pekarek, Ran Chen, Boxin Zhang, Yu Wang, Indu Aravind, Zhi Cai, Lasse Jensen, Nathan R. Neale, and Stephen B. Cronin. Monitoring Local Electric Fields using Stark Shifts on Napthyl Nitrile-Functionalized Silicon Photoelectrodes. The Journal of Physical Chemistry C, 124, 17000-17005 (2020).

Microbeam X-ray Reanalysis on Periodically Assembled Poly(β-Hydroxybutyric acid-Co-β-hydroxyvaleric acid) Tailored with Diluents.

Self-assembly of 3D interiors and iridescence properties of poly(β-hydroxybutyric acid-co-β-hydroxyvaleric acid) (PHBV) periodic crystals are examined using microcopy techniques and microbeam X-ray diffraction. Morphology of PHBV can be tailored by crystallizing in presence of poly(vinyl acetate) (PVAc) or poly(trimethylene adipate) (PTA) for displaying desired periodicity patterns. The regular alternate-layered lamellae of banded PHBV crystal aggregates, resembling the structures the natural mineral moonstone or nacre, are examined to elaborate the origin of light interference and formation mechanisms of periodic lamellar aggregation of PHBV spherulites. By using PHBV as a convenient model and the crystal diffraction data, this continuing work demonstrates unique methodology for effectively studying the periodic assembly in widely varying polymers with similar aggregates. Grating structures in periodically assembled polymer crystals can be tailored for microstructure with orderly periodicity.

Tunable narrowband and broadband coexisting absorber enabled by a simple all-metal grating for sensing applications

We realize a tunable narrowband and broadband coexisting absorber based on a simple step-shaped all-metal grating structure. The absorber presents an ultra-narrow absorption band of 1.5 nm and a relatively broad absorption band of 29.8 nm, both with nearly 100% absorption in the infrared region. The mechanism underlying the dual-band perfect absorption is the interaction between two diffraction coupled surface plasmon polariton (SPP) modes with one of them modulated by a cavity resonance. Influences of structure parameters on the absorption performance are numerically investigated. It is found that the positions of the two perfect absorption peaks can be easily tuned both independently and together by changing the structural parameters. In addition, the designed grating structure presents excellent sensing performance with sensitivity and figure of merit as high as 2,514 nm/RIU and 1,600 RIU−1, respectively. The excellent sensing performance, flexible tenability and simple structure design endow this grating absorber with great potential in high-precision biochemical sensing applications.

Nested micro-ring refractive index sensor based on a subwavelength grating waveguide and theVernier effect.

In this paper, a nested micro-ring refractive index sensor based on a subwavelength grating waveguide and the Vernier effect is proposed. In this scheme, the nested micro-ring structure is combined with a subwavelength grating structure to enhance the contact area between the optical field and the analyte, and the wavelength offset is doubled through the Vernier effect. The proposed sensor can effectively increase sensing sensitivity, taking into account the improvement of the free spectral range. This structure enables the device to reach a sensitivity of 8030nm/RIU near 1550nm wavelength in a deionized water environment, with a detection limit of 5.659×10-5RIU and free spectral range of 41.956nm. The device suggested in this study has a greater reduced footprint than the conventional micro-ring resonant sensor, measuring just 35µm×25µm. Due to its high integration, high sensitivity, and large free spectral range compared to conventional micro-ring resonant sensors, such structures are of great value in biosensing and environmental monitoring.

Circular dichroism enhancement induced by surface plasmons in the structure of a metallic grating coated by chiral TDBCs

We investigate the enhancement of the circular dichroism (CD) spectrum in a metallic grating by coating it with chiral TDBCs. Using the finite difference time domain method, we obtain reflection spectra and CD spectra. The amplification of TDBCs chirality is achieved through the excitation of both surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) in the Ag grating structure. Our results indicate that, compared to chiral TDBCs alone, the enhancement factor of chiral TDBCs coated on the metallic grating can reach up to 200 times. The coupling between chiral TDBCs and electromagnetic fields induced by SPPs and LSPs is tuned by both the period and groove thickness of the grating structure. Under certain conditions, strong coupling phenomena are observed, demonstrating a competitive relationship between the dissipation of our proposed structure and the coupling of electromagnetic fields and chiral TDBCs. The substantial amplification of the CD spectra suggests that our proposed structure provides a novel method to enhance the chirality of TDBCs experimentally.

Unidirectional manipulation of Smith-Purcell radiation by phase-gradient metasurfaces.

Here, we present a new, to the best of our knowledge, approach to control Smith-Purcell radiation (SPR) via phase-gradient metasurfaces (PGMs), i.e., periodic grating structures with gradient phase modulation. We show that the phase gradient and the parity design of the PGM can efficiently manipulate higher order diffraction to achieve perfect unidirectional SPR, which significantly alters the SPR in the spectrum and the spatial distribution beyond traditional understanding. Specifically, the even-parity PGM results in incidence-free unidirectional radiation, while the odd-parity PGM enables incidence-locking unidirectional radiation. This unidirectional SPR is very robust, ensured by the parity-dependent diffraction rule in PGMs. A modified formula is presented to reveal the relationship between the radiation wavelength and the radiation angle. Our findings offer a new way to control the electromagnetic radiation of moving charged particles (CPs) with structured materials, which may lead to novel applications in tunable, efficient light sources and particle detectors.

Optimizing Stability and Performance of Silver-Based Grating Structures for Surface Plasmon Resonance Sensors.

The use of surface plasmon resonance sensors allows for the fabrication of highly sensitive, label-free analytical devices. This contribution reports on a grating coupler to enable surface plasmon resonance studies using silver on silicon oxide technology to build long-term stable plasmonic structures for biological molecule sensing. The structural parameters were simulated and the corresponding simulation model was optimized based on the experimental results to improve its reliability. Based on the model, optimized grating nanostructures were fabricated on an oxidized silicon wafer with different structural parameters and characterized using a dedicated optical setup and scanning electron microscopy. The combined theoretical and experimental results show that the most relevant refractive index range for biological samples from 1.32-1.46 may conveniently be covered with a highest sensitivity of 128.85°/RIU.

Perfect Talbot self-imaging effect of aperiodic gratings.

We propose and investigate a class of aperiodic grating structure which can achieve perfect Talbot effect under certain conditions. The aperiodic grating structure is obtained by the superposition of two or more sine terms. In the case of two sine terms, the Talbot effect can be realized when the period ratio of two terms is arbitrary. While in the case of more than two sine terms, the period ratios of each term must meet certain extra conditions. The theory has been further verified by numerical simulations. It expands the field of Talbot effect and is of potential significance for subsequent research applications such as optical imaging and measurement.

Highly efficient second harmonic generation assisted by the quasi-bound states in the continuum from AlGaAs meta-gratings

Due to the design freedom of metasurfaces in tailoring wavefronts, they have become a general platform for miniaturized functional nonlinear optics. Recently, with the help of the quasi-bound states in the continuum (Q-BIC), the all-dielectric resonance metasurfaces with high quality factor (Q) boost the interaction between pump light and dielectric structure, which have become an effective way to achieve high nonlinear efficiency. Here, an asymmetric grating structure composed of AlGaAs is utilized to enhance the second harmonic generation (SHG) near Q-BIC. By breaking the symmetry of the grating structure, a simple normal incidence linearly polarized plane wave can be treated to excite stronger localized fields within the metasurfaces, allowing strong light–matter​ interactions in nonlinear metasurfaces. Numerical investigations depict that the Q-factor and resonant field enhancement noticed on the metasurface are intimately related to the asymmetric structure of the system. In addition, the high Q-factor resonance facilitates the SHG conversion process in AlGaAs metasurfaces. Especially, for the AlGaAs metasurface with Q-factor of 4.0314 × 104, the conversion efficiency of SHG can reach 10.5% when the pumping intensity is 5 MW/cm 2. This study is of great importance for analyzing the efficient nonlinear metasurfaces with high Q-factors.

Low-loss grating coupler based on inter-layer mode interference in a hybrid silicon nitride platform.

Surface grating couplers are an important component for interfacing photonic integrated circuits with optical fibers. However, conventional coupler designs typically provide limited performance due to low directionality and poor fiber-to-grating field overlap. The efficiency can be improved by using non-uniform grating structures at the expense of small critical dimensions complicating the fabrication process. While uniform gratings can alleviate this constraint, they produce an exponentially decaying near-field with the Gaussian fiber mode overlap limited to a theoretical maximum of 80%. In this work, we propose a uniform grating coupler that circumvents this field overlap limitation. This is achieved by leveraging inter-layer mode interference through a virtual directional coupler effect in a hybrid amorphous-silicon (α-Si) on silicon nitride (Si3N4) platform. By optimizing the inter-layer gap and grating geometry, a near-Gaussian profile of the out-radiated beam is achieved, resulting in an unprecedented grating-to-fiber overlap of 96%. The full three-dimensional (3D) finite-difference time-domain (FDTD) simulations show a high directionality of 84% and a record coupling loss of -1.27 dB with a 1-dB bandwidth of 20 nm for the uniform grating coupler design. Our device is designed for a wavelength of 950 nm aimed for use in hybrid quantum photonic integrated circuits using III-V quantum dot single photon sources.

Optical gratings fabricated using the capillary-assisted self-assembly of nanoparticles on a flexible substrate.

We demonstrate a cost-effective and high-throughput fabrication technique to deposit colloidal nanoparticles on a patterned polymer substrate using a capillary-assisted self-assembly method over a large area. In particular, we fabricate optical gratings using gold nanoparticles and a polymer substrate. We show the versatility of the technique over different nanoparticle diameters and grating periodicities. Through both experiments and simulations, we show enhanced transmission in the first-order diffraction of the gold-polymer grating as compared to the air-polymer grating. Our fabrication technique also enables the transfer of the nanoparticle pattern from the polymer substrate to any desired surface. Here we demonstrate the transfer of the nanoparticle grating structure to the tip of optical fibers.

Slow light structure implementation to improve Fano response of optical waveguide as a refractive index sensing

Optical sensor based on the coupled cavity is an interesting topic because these types of sensors have higher Figure of merit (FOM) and sensitivity in comparison to other types of sensors. In this paper, a novel refractive sensor based on the coupled cavity by using T-stub and grating transmission line is suggested. The T-stub interaction with the cavity and coupled line make two capacitances and the Fano-shape of the transmission appeared. The grating as a slow light structure enhanced the transmission and Q-factor of the proposed coupled waveguide sensor by controlling the phase in the transmission line. The proposed sensor is modified to have dual-band characteristics as a Fano form at 1286 and 1629 nm (233 and 184 THz respectively) and for this aim, the T-Stub and the grating structure play the main role. This sensor is used to check the materials with refractive index in the range of 1–1.5 and the maximum sensitivity and FOM are obtained at 1730 nm/RIU and 728 RIU−1, respectively. The full wave method of FIT (Finite Integrated Technique) as a time domain method is utilized for analyzing this sensor.

The strange case of negative reflection

In this paper, we show the phenomenon of negative reflection occurring in a mechanical phononic structure, consisting of a grating of fixed inclusions embedded in a linear elastic matrix. The negative reflection is not due to the introduction of a subwavelength metastructure or materials with negative mechanical properties. Numerical analyses for out-of-plane shear waves demonstrate that there exist frequencies at which most of the incident energy is reflected at negative angles. The effect is symmetric with respect to a line that is not parallel to the normal direction to the grating structure. Simulations at different angles of incidence and computations of the energy fluxes show that negative reflection is achievable in a wide range of loading conditions.

Efficient Blue-Green Light Phased Array Based on High-Contrast Grating as a Demultiplexer

Wavelength division multiplexing (WDM) technology has been proven to effectively increase the capacity of optical communications in the infrared light band. However, to date, the WDM systems in the visible band only support limited quantity of wavelength channels, with coarse channel spacing of over 20 nm. This is because there are no available multi/demultiplexers (MUX/DeMUX) with fine wavelength spacing for visible light. In this work, we use a high-contrast grating (HCG) structure to effectively separate the blue-green wavelengths with fine spacing. An efficient blue-green light phased array based on HCG aperture is demonstrated to work as the DeMUX in a WDM system. The HCG aperture presents more efficient main-lobe radiation than typical one-dimensional waveguide surface grating arrays with fishbone antennas. The measured main-lobe loss of optical phased array (OPA) with HCG is over 15 dB lower than that of OPA with fishbone antennas, because HCG provides maximum scatterings for blue-green light. A wavelength spacing as small as 4 nm is validated in a blue-green WDM system, which is the finest channel spacing for WDM in visible band to the best of our knowledge.

A Novel BioNEMS Sensor Based on Tunable Plasmonic Modes of a Grating MIM Structure

In this paper, a novel optical BioNEMS (Bio Nano-Electro-Mechanical-Systems) sensor using a nanomechanical plasmonic structure is proposed to develop a high performance label-free biosensor. The present device is composed of a functionalized BioNEMS cantilever coated by a gold layer as a part of the proposed plasmonic grating waveguide. This plasmonic system is based on the light interaction with a metallic nanostructure as a metal-insulator-metal (MIM) Bragg grating where an air gap acts as the insulator. Passing patient samples through the functionalized surface of the nanocantilever, the biological interactions release a stress on the surface because of binding events. The cantilever can be thus deflected by inducing a differential surface stress. Therefore, the air gap distance (the insulator width) between two parts of the plasmonic grating structure is changed to excite surface plasmon polaritons in a different wavelength. Furthermore, the present biosensor is analyzed by numerical and analytical approaches and its functional characteristics are obtained as follows: optical sensitivity of 7872 nm/RIU, figure of merit (FOM) of 394 RIU^-1, mechanical sensitivity of 1.2 µm/Nm^-1. According to the results, the proposed label-free identification method can be used as a sensitive platform for detection of biomarkers in point of care testing (POCT) applications.

Fabrication of curved MLA-grating based on 3D printing mold and vacuum-assisted deformation replication process

The combination of Microlens Array (MLA) with a grating structure is an effective means of improving light shaping efficiency due to the light dispersion effects of both elements. This paper proposes an innovative method for fabricating a curved MLA-grating composite element using a 3D printing mold and vacuum-assisted deformation replication process. In addition to the cost advantages of the process, the optical parameters such as type and line-density of grating, curvature and aperture size of the microlens can be effectively controlled, so as to bring different diffraction and focusing effects. Test results demonstrate that the curved MLA-grating element has superior light dispersion efficiency, making it useful in improving the illumination range of light sources. Additionally, due to the focusing effect of the curved substrate, this element holds promise for enhancing the light energy utilization rate in the photovoltaic field.