Plasmonic Nanogratings Research Articles

Hybrid graphene-high-aspect ratio plasmonic nanograting systems

One-dimensional plasmonic nanogratings (1D-PNGs) with high aspect ratios and narrow grooves promise enhanced coupling for hybrid graphene systems with the localized surface plasmon of the metallic grating and graphene surface plasmons. However, both the fabrication of the 1D-PNG and the application of graphene to it are difficult. We developed 1D-PNGs with a high aspect ratio of 15 and narrow grooves of 100 nm in width using the tapered mold method and a dry graphene-transfer procedure. Raman spectroscopy measurements showed that monolayer graphene was successfully transferred onto the 1D-PNGs, and the graphene was strongly doped with Au in the 1D-PNGs. Graphene on narrow grooves (free-standing graphene) demonstrated an almost identical p-doping level to graphene on Au because the narrow groove width allowed sufficient doping by Au for graphene on grooves. Reflectance measurements showed that the 1D-PNGs exhibited polarization- and wavelength-selective absorption at infrared (IR) wavelengths, and the effect of graphene blue-shifted the absorption peak wavelength induced by the surface plasmon resonance of 1D-PNGs. Numerical calculations agree well with these experimental results and indicate that the electric field strongly localizes on graphene in the grooves. Moreover, the doping level tunes the absorption wavelength owing to the coupling with graphene plasmons and the surface plasmon resonance of 1D-PNGs. This could provide electrical tunability to the graphene plasmons. Our fabrication procedure produced hybrid graphene-1D-PNGs with high aspect ratios and narrow groove systems for IR wavelengths. This system can contribute to developing high-performance electrically tunable graphene-based IR photodetectors, tunable IR emitters/absorbers, and biological sensors.

Holographic silver nanograting formation driven by TE0, TE1, and TE2 mode propagation in an AgCl slab Waveguide: Optical properties and analysis

Holographic silver nanogratings on the surface and within the volume, which have significant potential for data storage applications, are generated in silver chloride (AgCl) waveguides using a single laser beam exposure. This formation process leverages the interference between the polarized incident wave and the TEm modes propagating inside the AgCl waveguide. These plasmonic nanogratings are anisotropic nanostructures, demonstrating intriguing optical traits such as wavelength-specific linear dichroism and birefringence. The observed linear dichroism and birefringence in the holographic silver nanograting can modify the optical rotation and ellipticity of a probe beam traversing this anisotropic medium. Notably, volume holographic nanogratings function as complex nanogratings with a periodicity exceeding that of the incident wavelength. As a result, upon exposure, these complex nanogratings exhibit discernible light diffraction, while the surface holographic nanogratings do not show any diffraction pattern. Additionally, as the AgCl film thickness is increased and more complex nanogratings are formed within the AgCl waveguides, there is a noted reduction in the resulting birefringence.

Design of a binary metal micron grating and its application in near-infrared hot-electron photodetectors.

Metal plasmonic nano-gratings possess a high absorption ability and exhibit potential applications in sensing, hot-electron photodetection, metasurfaces, etc. However, the fabrication techniques of high-quality nano-gratings are challenging. In this article, a binary metal micron grating for near-infrared hot-electron photodetectors (HEPDs) is designed in which the surface plasmons are excited by high-diffraction-order modes. The high-diffraction-order micron grating can be fabricated by conventional lithography and has a significantly higher tolerance in the grating parameters than a nano-grating. The range of absorption greater than 70% is ∼3 times that of a nano-grating. Moreover, an interesting relationship between the resonant wavelength and the grating duty cycle is found. When the high-diffraction-order micron grating is applied in metal-insulator-metal HEPDs, a high zero-biased responsivity of 0.533 mA/W is achieved.

Design of an agile optical switcher based on a 1D silver plasmonic nanograting filled with a nonlinear Kerr material

In this paper, we investigate the nonlinear optical response characteristics of a metallic nanograting with nonlinear Kerr media within its slits using the finite element method. The proposed nanograting system is illuminated by a nanosecond Gaussian pulse laser under normal incidence and the electric filed pointing across the slits. The results show a perfect linear absorption at resonance wavelength thanks to the coupling of the surface plasmon resonance (SPR) mode and photon cavity mode. We simulate the transient nonlinear absorption variation of the system when the pulse laser is set up at either resonance or off-resonance wavelengths. The results indicate that the unit linear absorption drastically decreases by increasing the laser fluence around the center of the pulse. Interestingly, one can also enhance the weak linear off-resonance absorption to the value of unit by increasing the pulse laser fluence. The higher the laser fluence, the higher the maximum absorption contrast between linear and nonlinear regimes occurs owing to the nonlinear Kerr effect. Indeed, when the laser fluence reaches a critical value, it can excite the Kerr nonlinearity, which changes the coupling strength of SPR mode and the photon cavity mode leading to the absorption adjustment in the nanograting. These properties indicate the possibility of utilizing the proposed nanograting in dual functional absorber and nonabsorber systems, which make it an appropriate candidate for agile optical switching devices.

Dopamine-induced neural activity detection onto a cell-cultured plasmonic nanograting platform

Dopamine-induced neural activity detection onto a cell-cultured plasmonic nanograting platform

Three orders of magnitude enhancement of second and third harmonic generation in the visible and ultraviolet ranges from plasmonic gold nanogratings

We report experimental observations and numerical simulations of second and third harmonic generation from a gold nanograting, which exhibits a plasmonic resonance in the near infrared. The resonance is tunable, with a spectral position that depends on the angle of incidence. All things being equal, the enhancement of nonlinear optical processes produced by the field localization in the nanograting when compared with a flat gold mirror manifests itself dramatically from the ultraviolet to the visible range: second harmonic generation conversion efficiencies increase by more than three orders of magnitude, while we report a third harmonic generation conversion efficiency enhancement factor of 3200, both in excellent agreement with our theoretical predictions. The clear inferences one may draw from our results are that our model describes the dynamics with unprecedented accuracy and that much remains to be revealed in the development of nonlinear optics of metals at the nanoscale.

Electrically switchable multicolored filter using plasmonic nanograting integrated with liquid crystal for optical storage and encryption.

This study proposed the synergistic merging of twisted-nematic liquid crystals (LCs) and nanograting embedded etalon structures for plasmonic structure color generation, realizing dynamic multifunctional metadevices. Metallic nanogratings and dielectric cavities were designed to provide color selectivity at visible wavelengths. Meanwhile, the polarization for the transmission of light could be actively manipulated by electrically modulating these integrated LCs. Moreover, manufacturing independent metadevices as single storage units with electrically controlled programmability and addressability facilitated secure information encoding and secretive transfer by dynamic high-contrast images. The approaches will pave the way for the development of customized optical storage devices and information encryption.

Ultrathin Film Amorphous Silicon Solar Cell Performance using Rigorous Coupled Wave Analysis Method

The issues related to global energy needs and environmental safeties as well as health crisis are some of the major challenges faced by the human, which make us to generate new pollution-free and sustainable energy sources. For that the optical functional nanostructures can be manipulated the confined light at the nanoscale level. These characteristics are emerging and leading candidate for the solar energy conversion. The combination of photonic (dielectric) and plasmonic (metallic) nanostructures are responsible for the development of better optical performance in solar cells. Here, the enhancement of light trapping within the thin active region is the primary goal. In this work, we have studied the influence of front-ITO (rectangular) and back-Ag (triangular) nanogratings were incorporated with ultrathin film amorphous silicon (a-Si) solar cell by using rigorous coupled wave analysis (RCWA) method. The improvement of light absorption, scattering (large angle), diffraction and field distributions (TE/TM) were demonstrated by the addition of single and dual nanogratings structures. Significantly, the plasmonic (noble metal) nanogratings are located at the bottom of the cell structure as a backside reflector which is helpful for the omni-directional reflection and increased the path length (life time) of the photons due to that the collection of the charge carriers were enhanced. Further, the proposed solar cell structure has optimized and compared to a back-Ag, front-ITO and dual nanogratings based ultrathin film amorphous silicon solar cell. Finally, the obtained results were evidenced for the assistance of photonic and plasmonic modes and achieved the highest current density (Jsc) of 23.82 mA/cm2(TE) and 22.75 mA/cm2 (TM) with in 50 nm thin active layers by integration of (dual) cell structures.

Femtosecond Laser Direct‐Write Plasmonic Nanolithography in Dielectrics

Plasmon‐based devices have founded numerous applications in photonics based on optically excited strong near‐field effect at nanoscale. So far, the large‐area fabrication of periodic plasmonic nanostructures is still challenging due to a small write‐field limitation of lithography‐based techniques, especially for metallic substrates. A novel strategy is proposed to fabricate millimeter‐sized patterns of periodic plasmonic nanostructures inside dielectric materials (glass) through a femtosecond laser direct‐write plasmonic nanolithography approach. Noble metal nanoparticles are formed by the ion implantation in glass and reshaped into a nanowire‐like nanoparticles’ assembly by direct writing with femtosecond laser harnessing plasmonic interaction. As‐designed patterns at nanoscale are inscribed by this type of plasmonic lithography to form wires composed of nanoparticles. Examples for applications, the linear dichroism response, and structural color have been achieved by the plasmonic nanogratings buried inside glass (i.e., at subsurface regions). The work opens a new avenue to manipulate metallic nanoparticles in solids by direct plasmonic nanolithography and offers a reliable implementation of large‐area fabrication of plasmonic nanostructures for diverse range of photonic applications.

Exploring near-field sensing efficiency of complementary plasmonic metasurfaces for immunodetection of tumor markers

Plasmonic metasurface biosensors have great potential on label-free high-throughput clinical detection of human tumor markers. In the past decades, nanopillar and nanohole metasurfaces have become the common choices for plasmonic biosensing, because they typically enable universal simple large-area nanopatterns via a low-cost reproducible fabrication manner. The two kinds of metasurfaces have the complementary shapes and are used to be assumed as the same type of two-dimensional plasmonic nanograting for biosensing. Up to date, there is still a lack of comparison study on their biosensing performance, which is critical to guide their better applications on tumor marker detection. In this study, we compare the bulk/surface refractive index and sensitivity of plasmonic nanopillar (PNP) and plasmonic nanohole (PNH) metasurfaces in order to evaluate their biosensing capabilities. The sensing physics about their space near-field utilization is systematically revealed. The PNH metasurface demonstrates a higher biomolecule sensitivity versus the complementary PNP metasurface, and its limit of detection for bovine serum albumin reaches ∼0.078 ng/mL, which implies a greater potential of detecting cancer biomarkers. We further adopt the PNH metasurfaces for immunoassay of three typical tumor markers by testing clinical human serum samples. The results imply that the immunodetection of alpha-fetoprotein has the most optimal sensing efficiency with the lowest detection concentration (<5 IU/mL), which is much lower than its clinical diagnosis threshold of ∼16.5 IU/mL for medical examination. Our work has not only illuminated the distinct biosensing properties of complementary metasurfaces, but also provided a promising way to boost plasmonic biosensing for point-of-care testing.

Colorimetric Hydrogen Gas Sensor Based on One-Dimensional Plasmonic Nano-Grating

Colorimetric Hydrogen Gas Sensor Based on One-Dimensional Plasmonic Nano-Grating

Performance evaluation of metal photocathodes based on plasmonic nano-grating

An important class of photocathodes is metal photocathodes, which are used in various systems due to their rapid response time. However, they have low quantum efficiencies due to electron scattering and high photon reflectance losses. Nanoscale surface engineering and plasmonic capability are used to improve the performance and increase the quantum efficiency of metal photocathodes. Plasmonic technology is a good choice due to optical trapping and increased electric field at the surface of the structure. In this study, metal photocathodes Au, Ag, Al, and Cu with similar nano-grating surfaces are designed and exposed to photon radiation and reflected spectra are analyzed and compared using FDTD method. In the reflectance spectrum, surface plasmon polaritons are excited at specific wavelengths and reduce the light reflection from metal nano-grating surfaces. The resonance wavelength adjustment is investigated by changing the height of the nano-grating grooves. The quantum efficiencies of metal photocathodes are calculated and compared with each other based on the relations and equations. The quantum efficiencies of nano-grating metal photocathodes relative to the flat surfaces show that the quantum efficiencies of gold, silver, copper, and aluminum nano-grating increase by 56-fold, 35-fold, 32-fold, and 14-fold, respectively, higher than their flat surfaces during resonant wavelengths.

Extremely large near-field enhancements in the vicinity of plasmonic nanoantennas on top of bull’s eye structures exhibiting the extraordinary transmission of light

In this paper, we demonstrate that the optical electromagnetic fields in the vicinity of plasmonic nanoantennas are substantially enhanced on combining the plasmonic nanoantennas with bull’s eye structures (BESs) as compared to those of isolated plasmonic nanoantennas or of isolated bull’s eye structures. The optical electromagnetic fields are transmitted with high field enhancement through the subwavelength apertures of the BESs at the resonance wavelength, owing to the extraordinary transmission (EOT) of light. By hybridization of the two plasmonic nanoresonators (nanoantennas and BESs), optical energy is coupled to the nanoantennas from the light transmitted through the BESs. This coupling of light energy to the nanoantennas in the transmission mode using EOT and strong near-field enhancement in the vicinity of the gap between the nanoantennas integrated with the BESs has been analyzed numerically using the finite difference time domain (FDTD) method. We optimized the geometrical parameters of both BESs and nanoantenna structures for achieving the highest possible near-field enhancement. The resonance wavelength of this combined plasmonic structure can easily be tuned from visible to the near-infrared range by varying the periodicity of the BESs. Employing the proposed hybrid nanostructure consisting of a BES coupled to a plasmonic nanoantenna, an electric field enhancement of more than 1000 (i.e. an intensity enhancement greater than 1 × 106) is achieved for a 10 nm gap between the two arms of the bowtie nanoantenna, of more than 4000 (i.e. an intensity enhancement greater than 1.6 × 107) for a 5 nm gap, and of more than 9000 (i.e. an intensity enhancement greater than 8.1 × 107) for a 2.5 nm gap. These values of electric field enhancement are substantially higher than what have been reported previously. In this paper, we also present a comparison of the near-field enhancements in the vicinity of plasmonic bowtie nanoantennas coupled to different types of plasmonic nanogratings—two-dimensional nanohole arrays in plasmonic thin films, nanoline arrays in plasmonic thin films, or subwavelength apertures in plasmonic thin films surrounded by concentric periodic grooves. Moreover, we provide a comparison of the electric field enhancements in the vicinity of different types of plasmonic nanoantennas coupled to plasmonic nanogratings. The proposed hybrid nanostructure can open new possibilities in different fields such as surface-enhanced Raman scattering (SERS), plasmon-enhanced fluorescence (PEF), optical trapping, and nonlinear optics.

Non-Uniform Narrow Groove Plasmonic Nano-Gratings for SPR Sensing and Imaging

A surface plasmon resonance sensing and imaging platform based on plasmonic non-uniform nano-gratings with narrow groove (sub-10 nm) is presented. In these nanogratings, normally incident optical radiation is directly coupled to surface plasmons without the requirements of any other conventional surface plasmon coupling mechanisms such as prism-based or grating-based coupling. Theoretical analysis of practically realizable plasmonic non-uniform nano-gratings with rounded tops and slanted sidewalls is carried out to numerically to determine reflectance and differential reflectance signals when the localized refractive index of the medium around the gold layer present in these nano-gratings is changed. This change in the localized refractive index can occur due to the binding of biomolecules to the gold layer. Two kinds of plasmonic non-uniform nano-gratings are studied using finite difference time domain (FDTD) modelling: gold nano-gratings (GNGs) and gold-coated silicon nano-gratings (GSNGs). The plasmonic non-uniform nano-gratings being proposed, more specifically the GSNGs, can be easily fabricated with the presently existing nanofabrication and thin film deposition methods as opposed to uniform nano-gratings (with parallel sidewalls) that are very difficult to fabricate. The plasmonic non-uniform nano-gratings with narrow grooves eliminate the strict requirements on the angle of incidence for coupling of light into surface plasmons, which are needed in conventional prism-based coupling mechanisms. By employing FDTD calculations, we demonstrate that these plasmonic non-uniform nano-gratings provide very high differential reflectance amplitude values, which are indicative of high sensitivities of the SPR or SPRi sensors when the localized refractive index around the sensors is varied. Moreover, the sensors being proposed in this article provide a maximum sensitivity of localized refractive index sensing (i.e. surface sensitivity or $\text{S}_{\mathrm {S}}$ ) of 70 nm/nm with a figure of merit of the localized sensor (FOMS) of 1.5 nm $^{-1}$ . This sensitivity of localized refractive index sensing is the highest reported thus far in comparison with previously reported plasmonic sensors. Moreover, these plasmonic non-uniform nano-grating based sensors exhibit significantly better performance when compared with conventional SPR or SPRi sensors based on the Kretschmann configuration.

Design of a near-infrared plasmonic gas sensor based on graphene nanogratings

In this work, a gas sensor based on the plasmonic double-layer graphene nanograting (GNG) structure with an enhanced figure of merit (FoM) is presented in the near-infrared region. This structure includes double periodic graphene nanoribbon arrays, separated by a dielectric. The wavelength interrogation method is employed to accurately investigate the behavior of the proposed structure for various physical and geometrical parameters, including the array pitch, graphene nanoribbon width, refractive index of the intermediate dielectric between the GNGs, and the chemical potential of the graphene. A sharp dip is achieved by the guided-mode resonance between the two GNG layers, due to their near-field coupling. For the optimized design, obtained sensitivity and FoM are 430.91 nm/RIU and 174.68 R I U − 1 , respectively, when the finite-element method is used for the simulations. The high FoM is a result of the field enhancement at the edges of the graphene nanoribbons, as well as the narrow resonance linewidth achieved by the sharp transmission dip. In addition to the high performance and FoM, the structure is robust to the misalignment of two GNG layers, offering a solution for practical gas sensing applications. To the best of our knowledge, the proposed GNG-based structure enjoys a boosted FoM compared to the previously proposed integrated gas sensors, as well as a practically feasible design for fabrication.

Polarization Selective Color Filter Based on Plasmonic Nanograting Embedded Etalon Structures.

In this paper, we propose a polarization-selective color filter that can generate two different color informations simultaneously depending on the polarization direction. The proposed color filter is mainly composed of the etalon structure to generate the color by the structural resonance properties while the upper layer of the etalon is made of plasmonic nanogratings to promote polarization-dependent color properties. When the duty ratio of the silver nanogratings is fixed, the proposed color filter can maintain identical optical properties for orthogonal polarization, while the etalon structure of the proposed color filter can manipulate different color information depending on the cavity height for the horizontal polarization. Finally, we experimentally confirm that polarization-dependent security images can be generated using the proposed color filters with a fixed duty ratio of various nanograting arrays.

On-chip plasmonic spin-Hall nanograting for simultaneously detecting phase and polarization singularities

Phase and polarization singularities are important degrees of freedom for electromagnetic field manipulation. Detecting these singularities is essential for modern optics, but it is still a challenge, especially in integrated optical systems. In this paper, we propose an on-chip plasmonic spin-Hall nanograting structure that simultaneously detects both the polarization and phase singularities of the incident cylindrical vortex vector beam (CVVB). The nanograting is symmetry-breaking with different periods for the upper and lower parts, which enables the unidirectional excitation of the surface plasmon polariton depending on the topological charge of the incident optical vortex beam. Additionally, spin-Hall meta-slits are integrated onto the grating so that the structure has a chiral response for polarization detection. We demonstrate theoretically and experimentally that the designed structure fully discriminates both the topological charges and polarization states of the incident beam simultaneously. The proposed structure has great potential in compact integrated photonic circuits.

Palladium-coated narrow groove plasmonic nanogratings for highly sensitive hydrogen sensing.

In this paper, we propose novel plasmonic hydrogen sensors based on palladium coated narrow-groove plasmonic nanogratings for sensing of hydrogen gas at visible and near-infrared wavelengths. These narrow-groove plasmonic nanogratings allow the incident light to be coupled directly into plasmonic waveguide modes thereby alleviating the need for bulky coupling methods to be employed. We carried out numerical simulations of the palladium coated narrow-groove plasmonic nanogratings using rigorous coupled wave analysis (RCWA). When palladium is exposed to varying concentrations of hydrogen gas, palladium undergoes phase transition to palladium hydride (PdHx), such that there are different atomic ratios ‘x’ (H/Pd) of hydrogen present in the palladium hydride (PdHx) depending on the concentration of the hydrogen gas. RCWA simulations were performed to obtain the reflectance spectral response of the Pd coated nanogratings in both the absence and presence of hydrogen, for various atomic ratios ‘x’ (x ∼ 0.125 to 0.65) in palladium hydride (PdHx). The results of the RCWA simulations showed that as the dielectric permittivity of the palladium (Pd) thin film layers in between the adjacent walls of the plasmonic nanogratings changes upon exposure to hydrogen, significant shifts in the plasmon resonance wavelength (maximum Δλ being ∼80 nm for an increase in the value of the atomic ratio ‘x’ from 0 to 0.65) as well as changes in the differential reflection spectra are observed. The structural parameters of these Pd coated narrow groove nanogratings—such as the nanograting height, gap between the nanograting walls, thickness of the palladium layer, periodicity of the nanogratings—were varied to maximize the shift in the plasmon resonance wavelength as well as the differential reflectance when these nanostructures are exposed to different concentrations of hydrogen (i.e. for different atomic ratios ‘x’ in PdHx).

Thermal nanoimprint lithography based plasmonic nanogratings for refractive index sensing of polar solvents

This work illustrates a facile approach for the design of flexible plasmonic sensors using gold nanograting. The fabrication of surface plasmon resonance (SPR) coupled refractive index (RI) sensor is proposed to sense the various known RIs of polar solvents. Thermal nanoimprint lithography (T-NIL) is used to transfer the nanograting of the processed digital versatile discs (DVDs) through polydimethylsiloxane (PDMS) stamp to thermoplastic polymer coated flexible polyethylene terephthalate (PET) substrate. The gold nanograting sample was obtained after the subsequent deposition of the gold thin film of thickness 60 nm on top of polymer nanograting. The surface plasmon resonance (SPR) modes excited on gold-encrusted polymer nanograting appeared as a dip in the reflectance spectra measured at normal incidence under white light in air and liquid medium. We use refractive index calibration polar solvents [methanol (1.331), water (1.333), acetone (1.359), ethanol (1.361) and isopropanol (1.382)] to determine the refractive index sensitivity of our gold nanograting arrays. This produces a bulk refractive index sensitivity of 535 ± 47 nm/RIU. The real-time kinetic response demonstrates the stability of the sensor with respect to the time and reusability of the sensing chip for multiple measurements. Thus, we envisaged that our proposed sensor can be a potential candidate for the development of highly sensitive plamonic device.

Coherent Vibrational Spectroscopy of Electrochemical Interfaces with Plasmonic Nanogratings.

The fundamental understanding of electrochemistry urges accurate knowledge of all interfacial properties at the molecular level, but the retrieval of such information is a real challenge. Optical spectroscopies facilitated by surface plasmon enhancement can shed light on this field, yet past studies relied on either highly inhomogeneous "hot spots" or planar plasmon modes with limited enhancement. Here we report an in situ sum-frequency vibrational spectroscopy scheme using plasmonic nanogratings, which enable strong, coherent surface plasmon excitation even on planar electrodes. With two classical reactions, the gold oxidation and pyridine adsorption in water, we demonstrate the realization of coherent vibrational spectroscopy in the strong absorption region, revealing the polar orientation and ordering of interfacial species that are crucial toward the mechanistic understanding of electrochemical phenomena.