Plasmonic Lens Research Articles

Solid and hollow plasmonic nanoresonators for carrier envelope phase read-out

The geometry of gold plasmonic nanoantennae was numerically optimized to maximize their sensitivity to the carrier envelope phase (CEP) of the exciting ultra-short laser pulses. Three structure types, triangular, teardrop-shaped and plasmonic lens, were optimized in solid and hollow compositions as well. Hollow / solid singlets results in the largest/intermediate CEP dependent (Q1) – to – CEP independent (Q0) integrated current components’ ratio, while their Q1 was the smallest / intermediate. The largest / intermediate Q12/Q0 CEP sensitivity was achieved via solid / hollow plasmonic lenses due to their large near-field enhancement and Q1, while the Q1/Q0 ratio was smaller than for counterpart singlets.

Harnessing surface plasmon polaritons from nanogroove plasmonic lenses to enhance the EM fields around plasmonic nanoantennas

In this paper, we present a device—consisting of a bowtie nanoantenna surrounded by either an asymmetric plasmonic nanogroove or a symmetric plasmonic nanogroove—that allows improved excitation of the bowtie nanoantenna via interaction between the bowtie nanoantenna and the surface plasmon polaritons (SPPs) generated from the nanogrooves. We study the effect of both a linear plasmonic nanogroove as well as a circular plasmonic nanogroove (i.e., a circular nanogroove plasmonic lens) on the EM fields around a bowtie nanoantenna placed at a certain distance from the nanogroove. We show that the electric field enhancement of a bowtie nanoantenna can be significantly improved by the interaction between the bowtie nanoantenna and the SPPs generated from the nanogrooves. We employed FDTD simulations to calculate quantities such as the electric field enhancements and power coupling into SPPs as well as a finite difference eigenmode solution to obtain dispersion relation of the nanogrooves. The geometrical parameters of the symmetric nanogroove and asymmetric nanogroove were optimized to couple maximum light into SPPs. We show that the bowtie nanoantenna surrounded by a single asymmetric nanogroove plasmonic lens produces a SERS electromagnetic enhancement factor (EMEF) of 1010—even when the gap between the arms of the bowtie nanoantenna is as large as 10 nm—which is three orders of magnitude higher than SERS EMEF of a stand-alone bowtie nanoantenna and one order of magnitude higher than the SERS EMEF of a bowtie nanoantenna surrounded by a single symmetric nanogroove plasmonic lens. In addition, the effect of the radius of nanogroove plasmonic lenses is studied. The calculation of collection efficiencies of Raman signal from the proposed nanostructures shows that ∼62% Raman signal can be collected from the bowtie nanoantenna surrounded by a symmetric nanogroove plasmonic lens or an asymmetric nanogroove plasmonic lens (PL) compared to 10% Raman signal collected from only a bowtie nanoantenna on a silica substrate.

A design of compact plasmonic lens consisting of high index dielectric gratings and metal nano-film

In this paper, a hybrid ultra-thin planar subwavelength focusing structure consisting of a high refractive index dielectric grating and a nano metal film was designed. The thickness of this structure is only 200 nm. Surface plasmon polaritons (SPPs) were excited from the nano metal film, propagated along the metal-air interface, and were then converted into a radiation field by the dielectric grating, forming a focused spot in free space. By adjusting the grating position and width parameters, the shape of the focused optical field could be controlled. The simulation results showed that, under 532 nm light irradiation, the lens could produce a 270 nm (full width at half-maximum, FWHM) spot size at a focal length of 2.46λ. Moreover, under the illumination of 633 nm and 780 nm light, the designed lenses were found to produce focal spot sizes of 304 nm (0.48λ) and 364 nm (0.47λ), respectively, which were smaller than the diffraction limit. The simplicity of this plasmonic lens design, coupled with its reduced thickness and minimal absorption loss, offered significant advantages in terms of cost-effectiveness and ease of fabrication.

A feasibility study on scanning imaging of tip-enhanced Raman spectroscopy using a spiral plasmonic lens

A feasibility study on scanning imaging of tip-enhanced Raman spectroscopy using a spiral plasmonic lens

Compound spin Hall plasmonic lens

The propagation phase and the Pancharatnam–Berry phase are two primary strategies to control surface plasmon polaritons (SPPs) wave, however, hampered by the spatial conjugation and the spin conjugation, respectively. Combining these two approaches together, the conjugations are broken and a compound spin Hall plasmonic lens (SHPL) is proposed and demonstrated numerically and experimentally in the visible frequency range. The proposed strategy can be utilized to realize both transversal and longitudinal deviations of SPP focus. The positions of the SPP focus can be dynamically modulated by switching the spin states of excitation light and adjusting the phase gradient encoded onto the lens. Promising applications of the proposed SHPL include on-chip communication and polarization detection.

Spatiotemporal modulation of ultrafast plasmonic vortices with spin–orbit coupling

The vortex field of surface plasmon polariton with orbital angular momentum (OAM), called plasmonic vortex, has played an important role in various research fields. However, the spatiotemporal properties of plasmonic vortex pulses excited by ultrafast laser, especially the dynamics of spin‒orbit coupling in the ultrafast plasmonic vortex field, have yet to be investigated deeply. Here, we study the spatiotemporal modulation of ultrafast plasmonic vortices with spin‒orbit coupling, using both analytical and simulation methods. The ultrafast plasmonic vortices are excited by a ring-shaped plasmonic lens, with an incident light composed of two time-delayed femtosecond sub-pulses carrying the same OAM but orthogonal circular polarizations. The dynamics of time-varying electric field, energy flow and angular momentum distributions of the plasmonic vortices are demonstrated, revealing details of the spin‒orbit coupling in spatiotemporal domain, such as the merging of multiple phase singularities with energy flow loops, and the variation of spin/orbital angular momentum per photon over time. This work could deepen the understanding of spin‒orbit coupling in plasmonic field and provide new ideas for ultrafast on-chip optical information processing.

Vertical coupling to photonic crystal waveguide using chiral plasmonic lenses

Manipulating surface plasmon polariton waves for the development of micro-nano devices has been widely studied in recent years. Two-dimensional artificial photonic crystals have bandstructure characteristics like semiconductors. However, the requirement for light to be incident along the structural periodic direction poses a challenge in coupling light into the photonic crystal, thereby impeding its integrations and applications. In this work, we proposed coupling vertically incident left-circularly polarized light into a photonic crystal waveguide using a chiral plasmonic lens. Linearly-polarized light can also generate surface plasmon polariton waves and couple them into photonic crystal waveguides, but the intensity is lower. In contrast, right-circularly polarized light propagates in the opposite direction and exhibits minimal propagation into the photonic crystal waveguide. The results indicate that the proposed structure can operate broadband within the wavelength range of 620–670 nm. This method provides a simple and easily integrated coupling method for photonic crystal devices.

Electrically tunable virtual image Luneburg lens using graphene.

Virtual image lenses play essential roles in various optical devices and applications, including vision correction, photography, and scientific instruments. Here, we introduce an approach for creating virtual image Luneburg lenses (LL) on graphene. Remarkably, the graphene plasmonic lens (GPL) exhibits electrically tunable virtual focusing capabilities. The design principle of the tunability is based on the nonlinear relationship between surface plasmon polariton (SPP) wave mode index and chemical potential of graphene. By controlling the gate voltage of graphene, we can achieve continuous tuning of virtual focus. A ray-tracing technique is employed to determine the required gate voltages for various virtual focal lengths. The proposed GPL facilitates adjustable virtual focusing, promising advancements in highly adaptive and transformative nanophotonic devices.

Plasmonic Contact Lenses Based on Silver Nanoparticles for Blue Light Protection.

Constant exposure to blue light emanating from screens, lamps, digital devices, or other artificial sources at night can suppress melatonin secretion, potentially compromising both sleep quality and overall health. Daytime exposure to elevated levels of blue light can also lead to permanent damage to the eyes. Here, we have developed blue light protective plasmonic contact lenses (PCLs) to mitigate blue light exposure. Crafted from poly(hydroxyethyl methacrylate) (pHEMA) and infused with silver nanoparticles, these contact lenses serve as a protective barrier to filter blue light. Leveraging the plasmonic properties of silver nanoparticles, the lenses effectively filtered out the undesirable blue light (400-510 nm), demonstrating substantial protection (22-71%) while maintaining high transparency (80-96%) for the desirable light (511-780 nm). The maximum protection level reaches a peak of 79% at 455 nm, aligned with the emission peak for the blue light sourced from LEDs in consumer displays. The presence of silver nanoparticles was found to have an insignificant impact on the water content of the developed contact lenses. The lenses maintained high water retention levels within the range of 50-70 wt %, comparable to commercial contact lenses. The optical performance of the developed lenses remains unaffected in both artificial tears and contact lens storage solution over a month with no detected leakage of the nanoparticles. Additionally, the MTT assay confirmed that the lenses were biocompatible and noncytotoxic, maintaining cell viability at over 85% after 24 h of incubation. These lenses could be a potential solution to protect against the most intense wavelengths emitted by consumer displays and offer a remedy to counteract the deleterious effects of prolonged blue light exposure.

Optimization of a plasmonic lens structure for maximum optical vortices induced on Weyl semimetal surface states.

Optical vortices have a topologically charged phase singularity and zero intensity distribution in the centre. Optical vortex creation is regarded as a significant means for information transmission for applications in quantum computing, encryption, optical communication, etc. In this study, using finite-difference time-domain (FDTD) simulation, we calculated the electric field intensity and phase distribution of 2D lattices of optical vortices generated from various polygonal plasmonic lens structures using surface states of a Weyl semimetal (MoTe2). It was shown that a hexagonal lens is the best performing plasmonic lens. Further, we posited here a unified mathematical formulation for optical electrical field and phase distribution in the near field for any polygonal plasmonic lens. Our theoretical calculation corroborated well with FDTD results, validating the proposed generalized formula. Such plasmonic lens structures demonstrating scaling behavior offer great potential for designing next-generation optical memories.

Manipulating plasmonic vortex based on meta-atoms with four rectangular slits.

In this paper, four rectangular slits with the same size and regular rotation angle are regarded as the meta-atom, arranged on circular contours, to create plasmonic vortex lenses (PVLs) solely based on the geometric phase. These PVLs can achieve the same purpose of exciting surface plasmon polariton (SPP) vortices with arbitrary combinations of topological charge (TC) when illuminated by circularly polarized (CP) light with different handedness as the traditional PVLs. Furthermore, they can generate SPP vortices with different TCs and specific constant or varying electric-field intensities when excited by linearly polarized (LP) light, which marks the first instance of this phenomenon solely through geometric phase manipulation. The TC can be dynamically altered by controlling the polarization order of the incident vector beam. These PVLs not only possess advantages in terms of device miniaturization and the creation of a more uniform vortex field, as compared to PVLs based on the transmission phase, but also offer a more straightforward design process in comparison to traditional structures that rely solely on the geometric phase.

Forty-Nanometer Plasmonic Lithography Resolution with Two-Stage Bowtie Lens.

Optical imaging and photolithography hold the promise of extensive applications in the branch of nano-electronics, metrology, and the intricate domain of single-molecule biology. Nonetheless, the phenomenon of light diffraction imposes a foundational constraint upon optical resolution, thus presenting a significant barrier to the downscaling aspirations of nanoscale fabrication. The strategic utilization of surface plasmons has emerged as an avenue to overcome this diffraction-limit problem, leveraging their inherent wavelengths. In this study, we designed a pioneering and two-staged resolution, by adeptly compressing optical energy at profound sub-wavelength dimensions, achieved through the combination of propagating surface plasmons (PSPs) and localized surface plasmons (LSPs). By synergistically combining this plasmonic lens with parallel patterning technology, this economic framework not only improves the throughput capabilities of prevalent photolithography but also serves as an innovative pathway towards the next generation of semiconductor fabrication.

Electrically tunable graphene plasmonic lens: from Maxwell Fisheye Lens to Luneburg Lens

A graphene plasmonic lens with an electrically tunable focal length is proposed and numerically investigated. The design philosophy of the proposed tunable lens is based on the nonlinear relationship of surface plasmon polariton (SPP) wave index with respect to chemical potential of graphene. By controlling the gate voltage of graphene, the proposed lens can be continuously tuned from a Maxwell Fisheye lens to a Luneburg lens. A ray-tracing method is employed to find out the corresponding gate voltages for various focal lengths. Full-wave EM simulations using COMSOL show that excellent focusing performances can be achieved. This work offers a new way in exploiting active transformational plasmonic elements in the mid-infrared region.

Wavefront engineering based on hybrid plasmonic mode

Plasmonic lens provided great optical performance through the interaction between light and metallic nanostructure in the past years. However, the wavefront control on the metallic lens was still limited by the fabrication and geometrical design. Here, an unconventional plasmonic lens with sufficient phase modulation was developed as a focusing device; it was constructed using both circular and annular apertures in a silver film. The circular and annular apertures induced localized and cylindrical surface plasmon modes with the incident light, respectively. The propagation phase in these apertures was related to their geometric parameters. These apertures with lower aspect ratios were designed to have the artificial phase from zero to Pi and arranged in concentric arrays to manipulate the wavefront of the lens. The focal length and spot size were determined by the phase distribution on the plasmonic lens. The focal spot with a subwavelength size was obtained by the appropriate manipulation of the phase. We also demonstrated the focusing properties of this plasmonic lens experimentally to validate our design. Owing to the circular symmetry of the circular and annular apertures, this type of plasmonic lens has considerable potential for use as a polarization-insensitive device.

High‐Efficiency Metasurface‐Based Surface‐Plasmon Lenses

AbstractA device that can couple propagating light into surface plasmon polaritons (SPPs) focused into a small region is highly desired for on‐chip photonics applications (e.g., energy‐harvesting, sensing, etc.). However, current technologies suffer from large device footprint, low working efficiency, and insufficient light‐manipulation freedom. Here, a generic approach for designing plasmonic lenses to generate predesigned vector SPP vortices with high efficiencies is established. Constructed with a set of meta‐atoms exhibiting tailored reflection phases and polarization‐conversion capabilities, the devices can convert normally incident circularly polarized light into predesigned vector SPP vortices with high efficiencies, due to both phase and polarization matching. As the illustrations, this study experimentally demonstrates directional SPP conversion (coupling efficiency: 35%; utilization efficiency: 98%) and SPP focusing effect at the wavelength of 1064 nm, with two meta‐couplers in stripe and arc shapes, respectively. Finally, a ring‐shaped meta‐coupler is designed/fabricated, and the generation of a vector SPP vortex with significantly enhanced efficiency as compared to previous schemes is experimentally demonstrated. The results pave the way for realizing on‐chip plasmonic devices to efficiently utilize SPPs with minimal footprints.

Design studies of a far-field plasmonic superlens with an enlarged field of view

Based on our previous studies, we numerically demonstrate imaging capability of a cascaded plasmonic superlens with an enlarged field of view for directly imaging subwavelength objects with magnification in the far field at a visible wavelength. The superlens consists of two plasmonic slabs. One is a plasmonic cavity lens for near-field coupling, and the other one is a planar plasmonic lens for phase retardation and thus image magnification is enabled. First, we investigate the superlens with an inverted plasmonic cavity structure (with respect to our previous design), which simplifies the nano-fabrication process. Then we design the planar plasmonic lens with an increased aperture size. Simulation results show that the superlens with the inverted structure can still achieve subwavelength imaging. Furthermore, a lateral resolution of 180 nm at the wavelength of 640 nm is manifested by the superlens with a larger field of view. When the aperture size of the planar plasmonic lens is doubled from 2.1 μm to 4.2 μm, the field of view of the superlens is increased by a factor of 1.4. The results provide a profound physical insight into the design of a cascaded plasmonic superlens system and pave the way for further experimental investigations.

Dynamic single microparticle manipulation in the far-field region using plasmonic vortex lens multiple arms with a circular groove.

Recent development of particle manipulation has led to high demand for dynamic optical tweezer structures. However, confining and rotating a single microparticle in the far-field region with a uniform potential distribution remains a complicated task. A plasmonic vortex lens (PVL) has been proven to easily rotate the dielectric particle owing to its effect on orbital angular momentum (OAM). Here we propose and demonstrate PVL multiple arms with a circular groove (CG). The device consists of a multiple arm spiral slit that generates a plasmonic vortex (PV) and a circular groove to bring the PV from the surface to the far-field region. Numerical simulations are performed to calculate the intensity distribution of the primary ring, the optical force and potential. The primary ring size can be adjusted using different polarization directions. PVL 2-arms with a CG has primary ring sizes of 1082 nm under right-handed circular polarization (RCP) and 517 nm under left-handed circular polarization (LCP). Based on these primary ring sizes, a 1 μm polystyrene (PS) bead can be rotated under RCP with a minimum required power of 7.45 mW and trapped under LCP with a minimum required power of 11.84 mW. For PVL 4-arms with a CG under RCP illumination, we optimize the uniform potential distribution by carefully selecting the radius of the groove. Using a groove radius of 1050 nm, we obtain the potential difference between the smallest and largest depth along the x- and y-directions of only 70 k B T/W with a minimum required power of 14.86 mW. The method and design discussed here offer an efficient way to manipulate microparticles for micro-rotors, cell dynamic analysis, etc.

Compact and Efficient Ring Resonator–Based Plasmonic Lens with Multiple Functionalities

A novel plasmonic lens based on a metal–insulator–metal bus waveguide coupled to a ring resonator is proposed and numerically investigated. The inner edge of the ring resonator was perforated with nanogrooves so that light emitted by these nanogrooves was sharply focused at the center of the ring. The structure relies on interference and resonance theory. The principle of the lens can be extended to a device 1 × N demultiplexer as well as a 1 × N power splitter. These functionalities were validated through finite element method simulations. Results show that the lens had a sharp focus, without any smearing, and a full width at half maximum (FWHM) intensity of around 240 nm. For the 1 × N demultiplexer, the selectivity was high with an FWHM bandwidth of less than 20 nm and crosstalk of less than − 10 dB, whereas the 1 × N power splitter had a narrow bandwidth and was able to split power into N equal parts with negligible imbalance. The proposed focusing structure is compact, and the simulation results show that the structure performs the various functionalities with high efficiency. Due to this, these structures will be of utmost utility in future all-optical signal processing systems.

Ultra-compact Archimedes spiral plasmonic lens with a circular groove for low power optical trapping in the far-field region.

Particle levitation is crucial in optical trapping considering contamination and alteration of the character of the particle due to physical contact with the structure. A strong field gradient along the optical axis is required in this case. To manipulate the particle at a distance from the surface, we propose an Archimedes spiral plasmonic lens with a circular groove (CG-ASPL). The optical properties and parameters influencing the trapping performance of CG-ASPL are fully analyzed and discussed. By illuminating the structure with circular polarization and structure optimization, we can reduce the required optical power down to 2.4 mW for trapping particle of 1 µm in diameter with groove width and height of 100 and 125 nm, respectively. The particle can be stably trapped with trapping potential of 4138 kBT/W in the far-field region (1.1λ) owing to constructive interference of the scattered SPP waves. Furthermore, this structure is ultra-compact with a size of about 6.7 µm in diameter. We believe the results demonstrated in this work would be very useful for lab-on-a-chip applications and many others.

Generation of Tunable Plasmonic Vortices by Varying Wavelength of Incident Light

Surfaces plasmon polaritons carrying orbital angular momentum (OAM), known as plasmonic vortex, hold potential applications for on-chip information multiplexing. However, a traditional plasmonic vortex lens was usually designed for monochromatic incident light and encountered challenges in generating multiple vortices. Here, we demonstrated a wavelength-tunable plasmonic vortex generator that ameliorates these limits, relying on the simultaneous design of a geometric metasurface on an Archimedean spiral. Through this design strategy, both the topological charges and the location of vortices can be controlled with different wavelengths of incident beams. This design and concept can preserve incident wavelength information and can be further applied to integrated and high-dimensional on-chip devices.