Nanograting Structure Research Articles

Plasmonic Modes and Fluorescence Enhancement Coupling Mechanism: A Case with a Nanostructured Grating.

The recent development and technological improvement in dealing with plasmonic metasurfaces has triggered a series of interesting applications related to sensing challenges. Fluorescence has been one of the most studied tools within such a context. With this in mind, we used some well characterized structures supporting plasmonic resonances to study their influence on the emission efficiency of a fluorophore. An extended optical analysis and a complementary investigation through finite-difference time-domain (FDTD) simulations have been combined to understand the coupling mechanism between the excitation of plasmonic modes and the fluorescence absorption and emission processes. The results provide evidence of the spectral shape dependence of fluorescence on the plasmonic field distribution together with a further relationship connected with the enhancement of its signal. It has made evident that the spectral region characterized by the largest relative enhancement closely corresponds to the strongest signatures of the plasmonic modes, as described by both the optical measurements and the FDTD findings.

Picosecond Wide‐Angle Dynamic Beam Steering for Object Tracking

AbstractBeam steering plays a vital role in light detection and ranging (LiDAR). However, the scanning speed is below megahertz (MHz) in current LiDAR systems. Frequency‐gradient metasurface has been reported to achieve gigahertz (GHz) beam steering with 25° scanning angle in two dimensions. However, the fabrication of millions of form‐varying nanoscale meta‐atoms poses challenges in operation efficiency, massive production, and overall cost‐effectiveness when the size goes beyond millimeter scale. Here, a dynamic beam steering comprising periodic grating nanostructures fabricated through nanoimprint lithography with an ultrafast scanning rate up to GHz (≈81 ps) and ultra‐wide three‐dimensional (3D) fields of view up to 53° × 23° is reported, which shows the powerful acquisition of 3D space data. The detection distance of object reaches to ≈230 m. This work demonstrates a wide‐angle and high‐speed beam steering, and paves a robust way to realize cost‐efficient, high‐volume manufacturing, and powerful 3D all‐solid‐state LiDAR for next‐generation intelligent vision and self‐driving technologies.

Investigation of the optical properties of a deep-ultraviolet LED with an Al nanograting structure.

The optical properties of deep-ultraviolet (DUV) light-emitting diode (LED) with Al nanograting structure are investigated by three-dimensional (3D) finite-difference time-domain (FDTD) simulation. The peak intensity of TE and TM polarization radiation recombination rate of the grating structure is increased by 33.3% and 22.0% as compared to the control structure with Al plane. The light extraction efficiency (LEE) of the emitted light whose propagation direction is in the plane perpendicular to the Al-grating ridge is much higher than that in the plane parallel to the Al-grating ridge due to the scattering of the grating. Without considering the lateral surface extraction and packaging, the total LEE of the grating structure can be improved by 41.4% as compared to the control structure. The peak intensity of the output spectrum of the DUV LED with the grating structure can be increased by 70.3%.

Nanoimprint Lithography‐Dependent Vertical Composition Gradient in Pseudo‐Planar Heterojunction Organic Solar Cells Combined with Sequential Deposition

AbstractAlthough suitable vertical phase separation morphology in organic solar cells (OSCs) can be obtained by the donor/acceptor sequential deposition (SD) method, the lack of precisely adjusting vertical composition gradient and molecular crystallinity is a key limitation. Here, nanoimprint lithography (NIL) combined with SD dual‐functionalized regulation strategy is first used to fabricate high‐performance pseudo‐planar heterojunction (PPHJ) OSCs, which is conducive to constructing vertical bi‐continuous donor/acceptor network to provide sufficient charge separation interface area and orderly charge transport channels. PM6 donor with regular periodic nanograting structure and improved crystallinity is formed via NIL, effectively avoiding the erosion problem ascribed from the subsequent depositing of the Y6 acceptor. Furthermore, the finite‐different time‐domain (FDTD) measurement is employed to confirm the vertical composition gradient of the donor/acceptor, revealing a strong regular light absorption and reduced voltage loss. As a result, the best‐imprinted device enables a power conversion efficiency as high as 17.36%, which is higher than the control SD‐based device (15.46%). It is the first time to obtain high‐quality PM6 nanograting by NIL, which can provide an avenue to form favorable phase separation morphology and adjust the vertical composition gradient for the high‐performance PPHJ OSCs.

A flexible and stretchable photonic crystal sensor for biosensing and tactile sensing

Biosensing and tactile sensing are considered to be essential functions for intelligent diagnostic medical robot. In this paper, biosensing and tactile sensing had been demonstrated with a single photonic crystal structure. The flexible and stretchable photonic crystal structure consists of PDMS as the flexible substrate and TiO2 as the guided layer, and the nanograting structure was realized by nanoreplica molding. For biosensing experiment, a sensitivity of 93 nm/RIU is verified with ambient environment RI variance simulation results. For tactile sensing experiment, the highest resolution for strain sensing is 0.1%, and the minimum detected scale of the grating period variation is 0.1 nm. The TiO2/PDMS structure based flexible and stretchable photonic crystal sensor demonstrates highly sensitivity and potentially cost effective for biosensing and tactile sensing, and it is promising in the area of intelligent diagnostic medical robot.

MEMS-based metamaterial grating waveguide for tunable optical attenuator and modulator applications

We present a metamaterial grating waveguide (MGW) for tunable optical attenuator and modulator applications in the infrared (IR) wavelength range, which is composed of sixty periodically one-dimensional gold (Au) nanograting structures on silicon (Si) nanograting waveguide. The first thirty Au nanograting structures along the opposite propagation direction of the guided wave are divided equally into three groups. By elevating the first group (G1) of Au nanograting structures from the Si waveguide surface, the transmission intensity of MGW decreases from 1.0 to 0.3 at the wavelength of 1.278 μm. While elevating the second (G2) and third (G3) groups of Au nanograting structures from the Si waveguide surface, the transmission intensities can be attenuated from 1.0 to 0.3 and 1.0 to 0.4, respectively. For the elevation of G3, the transmission intensity at a specific wavelength can be switched between 0 and 0.8. Furthermore, MGW shows a linear blue-shifting relationship of resonant wavelengths and incident angles between 8° and 20° with a correction coefficient of 0.9991. The resonances of MGW also exhibit blue-shifting characteristics by increasing the environmental refraction index. This proposed design of MGW provides a useful approach for variable optical attenuating, optical switching, wavelength tuning, and sensing applications in the IR spectrum range.

All-Dielectric Structural Colors with Lithium Niobate Nanodisk Metasurface Resonators

Lithium niobate (LN) is a promising optical material, its micro–nano structures have been applied to fields such as photonic crystals, nonlinear optics, optical waveguides, and so on. At present, lithium niobate structural colors are rarely studied. Although the nanograting structure was researched, it has such large full width at half-maximum (fwhm) that it cannot achieve red, green, or blue pixels or other high-saturation structural colors, thus, its color printing quality is poor. In this paper, we design and simulate lithium niobate nanodisk metasurface resonators (LNNDMRs), which are based on Mie magnetic dipole (MD) and electric dipole (ED) resonances. In addition, the resonators yield very narrow reflection peaks and high reflection efficiencies with over 80%, especially the reflection peaks of red, green, and blue pixels with fwhm around 11 nm, 9 nm, and 6 nm, respectively. Moreover, output colors of different array cells composed of single nanodisk in finite size are displayed, which provides a theoretical basis for their practical applications. Therefore, LNNDMRs pave the way for high-efficiency, compact photonic display devices based on lithium niobate.

GST-VO2-based near-field multistage radiative thermal rectifier

A near-field multistage radiative thermal rectifier is proposed based on two different phase-change materials, which can achieve multistage thermal rectification with different rectification ratios. The phase-change materials vanadium dioxide (VO2) and Ge2Sb2Te5 (GST), with different metal-insulator transition temperatures, are utilized within the active terminal of thermal rectifier. Four types of active terminal structures, including multi-film and composite nanograting structures, are introduced to explore to multistage thermal rectification. Our calculations find that the active terminal composed of a one-dimensional VO2 grating atop a GST thin film is the most suitable for multistage thermal rectification due to its realization of well-distributed and flexible thermal rectification. Furthermore, it is found that the passive terminal temperature of thermal rectifier can significantly affect the multistage radiative thermal rectification by modifying the rectification ratio and adjusting the stage number of multistage thermal rectification. This work sheds light on the role of different phase-change materials within the design of promising radiative thermal rectifiers boasting multistage thermal rectification.

Near-infrared narrow plasmonic resonances for high-performance optical sensing in a sodium-based nanograting

Plasmonic resonances are attractive and promising for biosensing applications benefiting from their strong nanoscale light confinement and sensitivity to the refractive index of surrounding medium. Here, we numerically and theoretically investigate a metal–insulator-metal (MIM) nanograting structure, where gold nanostrip arrays are placed on a thin dielectric layer with a sodium film underlay. The structure is obliquely excited by a transverse magnetic (TM) wave from a prism supporting the whole MIM structure. We found that the plasmonic surface lattice resonance (SLR) mode and gap plasmon resonance (GPR) mode can be efficiently excited in our structure at the near infrared wavelength. Remarkably, the SLR mode can achieve ultranarrow linewidth less than 3 nm (quality factor > 500), sensitivity over 1000 nm/RIU and the corresponding figure of merit (FOM) larger than 300 RIU−1. Our study may pay a way to achieve high-performance sensing in a simple plasmonic nanostructure that could be vastly used in biological and chemical circumstances.

Deepening of nanograting structures on Si by a two-step laser spatial-selective amorphization strategy combined with chemical etching

Femtosecond laser induced periodic surface structures (LIPSSs) have been implemented on various materials. However, its further development in manipulation of optical properties is hindered by the shallow depth. Herein, the paper proposes a novel method of two-step spatial-selective amorphization for producing the deep regular nanogratings on silicon by femtosecond laser and chemical etching with HF acid. In the first step, the spatially periodic amorphization of silicon surface is fabricated by the surface plasmon excitation of the incident femtosecond laser, the chemical etching of which produces the shallow gratings with a period of 765 nm, or so-called prefabricated-LIPSSs (pre-LIPSSs). During the second step, the spatial-selective amorphization of sample takes place by exploiting the local field enhancement of laser irradiation within the grooves. After chemical etching, the deep local field enhancement assisted LIPSSs (LFE-LIPSSs) are formed, with a three-fold larger depth of 155 nm but an invariable period. Comparing with the pre-LIPSSs, the LFE-LIPSSs show 45% increase in anti-reflection performance at around 700 nm and surface-enhanced Raman scattering (SERS) anisotropy ratio is increased about four times with 4-MBA molecules as probe molecules. This work provides a promising approach to fabricate deeper LIPSSs for more effectively controlling the optical properties of materials by femtosecond laser.

Effective Approximation Method for Nanogratings-induced Near-Field Radiative Heat Transfer.

Nanoscale radiative thermal transport between a pair of metamaterial gratings is studied within this work. The effective medium theory (EMT), a traditional method to calculate the near-field radiative heat transfer (NFRHT) between nanograting structures, does not account for the surface pattern effects of nanostructures. Here, we introduce the effective approximation NFRHT method that considers the effects of surface patterns on the NFRHT. Meanwhile, we calculate the heat flux between a pair of silica (SiO2) nanogratings with various separation distances, lateral displacements, and grating heights with respect to one another. Numerical calculations show that when compared with the EMT method, here the effective approximation method is more suitable for analyzing the NFRHT between a pair of relatively displaced nanogratings. Furthermore, it is demonstrated that compared with the result based on the EMT method, it is possible to realize an inverse heat flux trend with respect to the nanograting height between nanogratings without modifying the vacuum gap calculated by this effective approximation NFRHT method, which verifies that the NFRHT between the side faces of gratings greatly affects the NFRHT between a pair of nanogratings. By taking advantage of this effective approximation NFRHT method, the NFRHT in complex micro/nano-electromechanical devices can be accurately predicted and analyzed.

Nano-grating Assisted Light Absorption Enhancement for MSM-PDs Performance Improvement: An Updated Review

The primary focus of this review article mainly emphasizes the light absorption enhancement for various nanostructured gratings assisted metal-semiconductor-metal photodetectors (MSM-PDs) that are so far proposed and developed for the improvement of light capturing performance. The MSM-PDs are considered as one of the key elements in the optical and high-speed communication systems for applications such as faster optical fiber communication systems, sensor networks, high-speed chip-to-chip interconnects, and high-speed sampling. The light absorption enhancement makes the MSM-PDs an ideal candidate due to their excellent performances in detection, especially in satisfying the high-speed or high-performance device requirements. The nano-grating assisted MSM-PDs are preordained to be decorous for many emerging and existing communication device applications. There have been a significant number of research works conducted on the implementation of nano-gratings, and still, more researches are ongoing to raise the performance of MSM-PDs particularly, in terms of enhancing the light absorption potentialities. This review article aims to provide the latest update on the exertion of nano-grating structures suitable for further developments in the light absorption enhancement of the MSM-PDs.

Thermal Characterization of Metal-Oxide Interfaces Using Time-Domain Thermoreflectance with Nanograting Transducers.

Metal-oxide thermal boundary conductance (TBC) strongly influences the temperature rise in nanostructured systems, such as dense interconnects, when its value is comparable to the thermal conductance of the amorphous dielectric oxide. However, the thermal characterization of metal-amorphous oxide TBC is often hampered by the measurement insensitivity of techniques such as time-domain thermoreflectance (TDTR). Here, we use metal nanograting structures as opto-thermal transducers in TDTR to measure the TBC of metal-oxide interfaces. Combined with an ultrafast pump-probe laser measurement approach, the nanopatterned structures amplify the contribution of the thermal boundary resistance (TBR), the inverse of TBC, over the thermal resistance of the adjacent material, thereby enhancing measurement sensitivity. For demonstration purposes, we report the TBC between Al and SiO2 films. We then compare the impact of Al grating dimensions on the measured TBC values, sensitivities, and uncertainties. The grating periods L used in this study range from 150 to 300 nm, and the bridge widths w range from 72 to 205 nm. With the narrowest grating transducers (72 nm), the TBC of Al-SiO2 interfaces is measured to be 159-48+61 MW m-2 K-1, with the experimental sensitivity being 5× higher than that of a blanket Al film. This improvement is attributed to the reduced contribution of the SiO2 film thermal resistance to the temperature signal from TDTR response. The nanograting measurement approach described here is promising for the thermal characterization of a variety of nanostructured metal-amorphous passivation systems and interfaces common in semiconductor technology.

Effective light trapping in c-Si thin-film solar cells with a dual-layer split grating.

We propose a dual-layer split nanograting structure in crystalline silicon thin-film solar cells (TFSCs). The split nanograting is designed by introducing two partitioning factors and split times. By employing the finite-difference time-domain method, the light trapping performance and relevant parameters of TFSCs are analyzed and optimized. Numerical computation of optical and electrical simulation shows that the optimal dual-layer split nanograting structure has demonstrated great enhanced light absorption compared with the planar structure. Enhancement of the light trapping effect is associated with light coupling to waveguide modes. The short-circuit current density is reached at 21.66mA/cm2 with an improvement of 54.6% over the planar structure. All results provide a parting thought for the design of TFSC grating structures.

Broadband and Ultra-Low Threshold Optical Bistability in Guided-Mode Resonance Grating Nanostructures of Quasi-Bound States in the Continuum.

We model optical bistability in all-dielectric guide-mode resonance grating (GMR) nanostructures working at quasi-bound states in the continuum (BICs). The complementary metal-oxide-semiconductor (CMOS) compatible material silicon nitride (SiN) is used for the design of nanostructures and simulations. The ultra-low threshold of input intensity in the feasible nanostructure for nanofabrication is obtained at the level of ~100 W/cm2 driven by quasi-BICs. Additionally, the resonance wavelength in the GMR nanostructure can be widely tuned by incident angles with the slightly changed Q-factor that enables the optical bistable devices to work efficiently over a wide spectrum. The impact of the defects of grating that may be introduced in the fabrication process on the optical properties is discussed, and the tolerance of the defects to the optical performance of the device is confirmed. The results indicate that the GMR nanostructures of broadband and ultra-low threshold optical bistability driven by quasi-BICs are promising in the application of all-optical devices.

Side edge emission from a waveguide substrate.

In this study, we fabricated embedded titanium dioxide (TiO2) nanograting structures with slanted cross section on a silicon dioxide (SiO2) substrate using electron beam lithography (EBL) and reactive ion etching (RIE) methods, and we analyzed their optical signals. The surface morphologies of the embedded TiO2 nanograting structures were monitored by a scanning optical microscope (SEM). By focusing the transverse electric (TE) polarized beam with the wavelength λ=633nm at the incident angle θ=22.0∘ from the rear side of TiO2 face, we could observe +1st and -2nd diffraction orders with diffraction efficiencies (I/Itotal): I1=23% and I2=19%, respectively, on a SiO2/TiO2 interface. Diffraction beams were waveguided to both left and right directions in a SiO2 substrate via total internal reflection (TIR); next, two guided beams were emitted from the side edges of the substrate. This work is expected to develop a highly efficient two-way waveguide optical interconnector.

Tilted Nano-Grating Based Ultra-Compact Broadband Polarizing Beam Splitter for Silicon Photonics.

An ultra-compact broadband silicon polarizing beam splitter is proposed based on a tilted nano-grating structure. A light cross coupling can be realized for transverse-magnetic mode, while the transverse-electric light can almost completely output from the through port. The length of the coupling region is only 6.8 μm, while an extinction ratio of 23.76 dB can be realized at a wavelength of 1550 nm. As a proof of concept, the device was fabricated by a commercial silicon photonic foundry. It can realize a 19.84 dB extinction ratio and an 80 nm working bandwidth with an extinction ratio of larger than 10 dB. The presented device also shows a good fabrication tolerance to the structure deviations, which is favorable for its practical applications in silicon photonics.

Super-regular femtosecond laser nanolithography based on dual-interface plasmons coupling

Abstract Advances in femtosecond laser-material interaction facilitate the extension of maskless optical processing to the high efficiency and deep-subwavelength scale. Here, a hybrid plasmon lithography technique has been demonstrated by irradiating near-infrared femtosecond laser pulses onto the Si material coated with thin Cr films in a vacuum chamber, and superior nanograting structures are found to deeply penetrate through the thin Cr film into the underlying Si substrate. In stark contrast to the common ripple structures formed on the Si surface, the Cr-layer mediated Si nanograting structures not only exhibit the spatially super-regular arrangements with a deep-subwavelength period of 355 nm but also present the nonsinusoidal sharp-edged groove geometry with a large depth-to-width aspect ratio of 2.1. Theoretical analyses and calculations reveal that the anomalous structure characterizations are physically ascribed to the excitation of dual-interface coupled plasmons in the thin metal layer, which possess the squeezed spatial wavelength and the periodic columnar intensity distributions. Moreover, the further deepening of periodic nanostructures into the Si substrate is also elucidated by the simulation of electric field enhancements at the bottom of shallow grooves under irradiation of subsequent laser pulses. In combination with a wet etching process, the Si nanograting structures can be modified into the smooth and narrow-mouthed V-profiles, whose optical measurements show a near omnidirectional antireflection especially in the visible range of 565–750 nm, which is expected for the design of advanced photonic devices.

Critical-dimension scanning electron microscope characterization of smoothly varying wave structures with a Monte Carlo simulation

Scanning electron microscopy (SEM) characterization of a smoothly varying nanograting structure (a Pt-coated Cr grid on a Si substrate) with a sinusoidal waveform has been carried out by a Monte Carlo (MC) simulation technique. Previous studies with critical-dimension (CD) SEM (CD-SEM) have mostly concerned line structures with sharp edges so that there is an obvious edge bloom in the linescan profile of secondary electrons. In contrast, the present grating structures prepared by a laser-focused atomic deposition technique have a smoothly varying waveform in the cross-sectional profile, which pose greater difficulty for quantitative structural characterization by SEM. The grating structure, with a fixed pitch of λ/2 as the period of the standing-wave laser light field, where λ is the wavelength of the corresponding laser light, can be used as an ideal nanoscale metrological length tool; therefore, it is important to characterize its structural features over a large deposited area by SEM imaging for quality control, with the aim of mass reproduction. The present work extends the CD characterization of MC simulation methods to more complex structures. Taking into account different experimental factors, i.e. primary electron beam parameters, geometrical parameters and material properties, the unknown geometrical parameters (i.e. base height, peak height, linewidth shrinkage and peak tilt angle) of the grating lines have been successfully extracted from the experimental linescan profiles of SEM images.

Organic Light‐Emitting Transistors in a Smart‐Integrated System for Plasmonic‐Based Sensing

AbstractThe smart integration of multiple devices in a single functional unit is boosting the advent of compact optical sensors for on‐site analysis. Nevertheless, the development of miniaturized and cost‐effective plasmonic sensors is hampered by the strict angular constraints of the detection scheme, which are fulfilled through bulky optical components. Here, an ultracompact system for plasmonic‐sensing is demonstrated by the smart integration of an organic light‐emitting transistor (OLET), an organic photodiode (OPD), and a nanostructured plasmonic grating (NPG). The potential of OLETs, as planar multielectrode devices with inherent micrometer‐wide emission areas, offers the pioneer incorporation of an OPD onto the source electrode to obtain a monolithic photonic module endowed with light‐emitting and light‐detection characteristics at unprecedented lateral proximity of them. This approach enables the exploitation of the angle‐dependent sensing of the NPG in a miniaturized system based on low‐cost components, in which a reflective detection is enabled by the elegant fabrication of the NPG onto the encapsulation glass of the photonic module. The most effective layout of integration is unraveled by an advanced simulation tool, which allows obtaining an optics‐less plasmonic system able to perform a quantitative detection up to 10−2 RIU at a sensor size as low as 0.1 cm3.