Subwavelength Focusing Of Light Research Articles

Mirror-coupled toroidal dipole bound states in the continuum for tunable narrowband perfect absorption

Metasurfaces have received a significant amount of attention in recent years as they offer novel ways for wave front control, subwavelength light focusing, and flat optics. Here, we employ sharp resonances in metasurfaces originated from the physics of bound states in the continuum (BICs) for both engineering and enhancing the absorption of light. Specifically, using temporal coupled-mode theory and numerical simulations, we demonstrate that an all-dielectric metasurface, of which the meta-atom consists of narrow-gap Si dimers with a monolayer graphene, can perfectly absorb the incident near-infrared light through critical coupling to the mirror-coupled toroidal dipole BIC. Due to unique loss engineering capabilities of the mode, multiple tuning knobs, including the narrow gap, meta-atom–mirror distance, and geometrical scaling factor, can be utilized for control over the coupling condition and the electric-field enhancement at the critical coupling condition. These results suggest that our platform could promise in on-chip optoelectronic applications.

Numerical study of an efficient light focusing nano-coupler based on C-shaped waveguides

We present an FDTD simulation study of a new design of light focusing nano-coupler consisting of a tapered succession of C-shaped silver waveguides. Its operation is based on the use of the fundamental guided mode \(TE_{10}\). Thanks to the optimization of the structural design parameters, we have demonstrated numerically that this compact coupler enables a sub-wavelength light focusing with a transmission coefficient of 65% and a coupling efficiency of 77% that have higher values than the optimized designs reported in the literature. We also show that our structure is an efficient functional coupler at multiple wavelengths when used in a periodic grating as well as an isolated device. This could pave the way for the development of a new generation of metamaterials for guidance and detection applications.

Nonlinear plasmonic response in atomically thin metal films.

Nanoscale nonlinear optics is limited by the inherently weak nonlinear response of conventional materials and the small light-matter interaction volumes available in nanostructures. Plasmonic excitations can alleviate these limitations through subwavelength light focusing, boosting optical near fields that drive the nonlinear response, but also suffering from large inelastic losses that are further aggravated by fabrication imperfections. Here, we theoretically explore the enhanced nonlinear response arising from extremely confined plasmon polaritons in few-atom-thick crystalline noble metal films. Our results are based on quantum-mechanical simulations of the nonlinear optical response in atomically thin metal films that incorporate crucial electronic band structure features associated with vertical quantum confinement, electron spill-out, and surface states. We predict an overall enhancement in plasmon-mediated nonlinear optical phenomena with decreasing film thickness, underscoring the importance of surface and electronic structure in the response of ultrathin metal films.

Segmented Bayesian optimization of meta-gratings for sub-wavelength light focusing

Using inverse design tools to engineer functional photonic nanostructures has been a subject of great interest over the past several years. We report combining a segmented Bayesian optimization algorithm with the rigorous coupled wave analysis to design meta-gratings for sub-wavelength light focusing. Specifically, the meta-gratings comprise one-dimensional periodic arrays of supercells, each of which consists of dozens of dielectric bars. By optimizing geometry of the structure, we demonstrate two kinds of meta-gratings operating at single and double wavelengths, respectively. Both of them can focus the incoming light into periodic sub-wavelength spots with high energy density. The full width at half-maximum (FWHM) of the focusing spots for the single wavelength ( λ = 633 n m ) case can be as small as 0.36 λ , while FWHMs of the focusing spots at double wavelengths ( λ = 533 n m and 633 nm) are about 0.4 λ . This proposed approach provides an affordable method to tackle the problem of complex photonic structure design.

Subwavelength Focusing of Light Reflected from a Silver Surface with a Periodic Structure

It is demonstrated that the reflection of light from the surface of silver with a periodic structure in the form of a one-dimensional grating can lead to subwavelength focusing of light near the surface. Under these conditions, the diffraction limit on spatial resolution is given by the expression Δx = a0/2, where a0 is the grating period. For short grating periods (about 10 nm), this limit is much smaller than the classical diffraction limit Δz = λ/2n (where n is the refractive index of an infinite optical medium and λ is the wavelength). It is shown that, with an increase in the depth of the grating profile, the field strength of the diffracted waves near the silver surface becomes significantly higher than that of the specularly reflected wave.

Design and analysis of all-dielectric subwavelength focusing flat lens

In this letter, we numerically designed and experimentally demonstrated a compact photonic structure for the subwavelength focusing of light using all-dielectric absorption-free and nonmagnetic scattering objects distributed in an air medium. In order to design the subwavelength focusing flat lens, an evolutionary algorithm is combined with the finite-difference time-domain method for determining the locations of cylindrical scatterers. During the multi-objective optimization process, a specific objective function is defined to reduce the full width at half maximum (FWHM) and diminish side lobe level (SLL) values of light at the focal point. The time-domain response of the optimized flat lens exhibits subwavelength light focusing with an FWHM value of 0.19λ and an SLL value of 0.23, where λ denotes the operating wavelength of light. Experimental analysis of the proposed flat lens is conducted in a microwave regime and findings exactly verify the numerical results with an FWHM of 0.192λ and an SLL value of 0.311 at the operating frequency of 5.42 GHz. Moreover, the designed flat lens provides a broadband subwavelength focusing effect with a 9% bandwidth covering frequency range of 5.10 GHz–5.58 GHz, where corresponding FWHM values remain under 0.21λ. Also, it is important to note that the designed flat lens structure performs a line focusing effect. Possible applications of the designed structure in telecom wavelengths are speculated upon for future perspectives. Namely, the designed structure can perform well in photonic integrated circuits for different fields of applications such as high efficiency light coupling, imaging and optical microscopy, with its compact size and ability for strong focusing.

Focusing of light beyond the diffraction limit by randomly distributed graded index photonic medium

Sub-wavelength focusing of light holds great potential in various applications of science and engineering, including nanolithography, optical microscopy, optical measurements, and data storage. In the present paper, we propose a new concept to obtain sub-wavelength focusing of light by using structures composed of all-dielectric materials. The approach utilizes the design of an inhomogeneous refractive index profile with random distributions of individual elements occupying the unit cells of two-dimensional photonic crystals (PCs). Light focusing phenomenon is both systematically and quantitatively analyzed at different selected frequencies and we show that the randomly generated graded index (GRIN)-like photonic medium provides light focusing in air with a spot size below λ/3, where λ is the wavelength of light. The numerically obtained minimum spot size is equal to 0.260λ. Gaussian probability function is used to implement numerous random designs to investigate the optical characteristics of the photonic medium. Specific attention is paid to the sub-wavelength focusing properties of the designed random PC structures. The random ingredient of GRIN PC provides not only narrow focusing of light but also off-centered or asymmetric light focusing at the back side of the structure. Experimental verification conducted at the microwave region shows good agreement with the numerical results.

Microlens-aided focusing of linearly and azimuthally polarized laser light.

We have investigated a four-sector transmission polarization converter (4-SPC) for a wavelength of 633 nm, that enables the conversion of a linearly polarized incident beam into a mixture of linearly and azimuthally polarized beams. It was numerically shown that by placing a Fresnel zone plate of focal length 532 nm immediately after the 4-SPC, the incident light can be focused into an oblong subwavelength focal spot whose size is smaller than the diffraction limit (with width and breadth, respectively, measuring FWHM = 0.28λ and FWHM = 0.45λ, where λ is the incident wavelength and FWHM stands for full-width at half maximum of the intensity). After passing through the 4-SPC, light propagates in free space over a distance of 300 μm before being focused by a Fresnel zone plate (ZP), resulting in focal spot measuring 0.42λ and 0.81λ. The focal spot was measured by a near-field microscope SNOM, and the transverse E-field component of the focal spot was calculated to be 0.42λ and 0.59λ. This numerical result was verified experimentally, giving a focal spot of smaller and larger size, respectively, measuring 0.46λ and 0.57λ. To our knowledge, this is the first implementation of polarization conversion and subwavelength focusing of light using a pair of transmission micro-optic elements.

Differential evolution algorithm based photonic structure design: numerical and experimental verification of subwavelength λ/5 focusing of light.

Photonic structure designs based on optimization algorithms provide superior properties compared to those using intuition-based approaches. In the present study, we numerically and experimentally demonstrate subwavelength focusing of light using wavelength scale absorption-free dielectric scattering objects embedded in an air background. An optimization algorithm based on differential evolution integrated into the finite-difference time-domain method was applied to determine the locations of each circular dielectric object with a constant radius and refractive index. The multiobjective cost function defined inside the algorithm ensures strong focusing of light with low intensity side lobes. The temporal and spectral responses of the designed compact photonic structure provided a beam spot size in air with a full width at half maximum value of 0.19λ, where λ is the wavelength of light. The experiments were carried out in the microwave region to verify numerical findings, and very good agreement between the two approaches was found. The subwavelength light focusing is associated with a strong interference effect due to nonuniformly arranged scatterers and an irregular index gradient. Improving the focusing capability of optical elements by surpassing the diffraction limit of light is of paramount importance in optical imaging, lithography, data storage, and strong light-matter interaction.

Subwavelength light focusing using quadric cylinder surface plasmonic lens with gold film slits filled with dielectric

A novel plasmonic lens (PL) with simple nano-structure is proposed to realize the subwavelength focusing. The proposed PL is composed of the gold film with only five dielectric-filled nanoslits. The exit surface of the gold film is processed into quadric shape that can be parabolic, elliptical or hyperbolic cylinders. The film is fabricated to form five uniformly spaced nanoslits with different widths and depths. All five slits are symmetrically arranged with respect to the center of lens and filled with a dielectric medium (i.e., SiO2). Under the illumination of TM polarized beams, the surface plasmon polaritons (SPPs) are excited at the entrance surface of the PL, then pass through the SiO2-filled slits while acquiring specific phase retardations, and are finally coupled to the light waves in the free space. Each light wave originating from the slit can be regarded as an individual point source, and the constructive interference of light waves from slits gives rise to the beam focusing at the focal plane of the PL. We investigate the phase modulation mechanism of the PL and find that the focusing performance relies on the shape of exit surface, filling medium and geometric parameters of the slits. A suitable phase modulation can be achieved by adjusting the structure parameters of the PL with a specific exit surface shape. Three kinds of quadratic cylindrical PLs, i.e., parabolic, elliptical and hyperbolic cylindrical ones with continuous or stepped exit surface are designed to realize the focusing of TM polarized subwavelength beams in visible spectrum. The finite difference time domain (FDTD) method is employed to compute the light field and to investigate the focusing characteristics of the proposed PL. The performance measurements include the focal length, depth of focus (DOF) and full-width half-maximum (FWHM). The simulation results confirm that the proposed PL with a 2-m-diameter aperture can achieve the subwavelength focusing at a focal length of micron scale. The attainable smallest FWHM of the focal spot is 0.4050 (0 denoting the wavelength of the incident light) which is well beyond the diffraction limit. It is also worth mentioning that the step-like cylindrical PL can yield a sharper focal spot than the continuous cylindrical PL. For example, the FWHM of focal spot produced by the stepped elliptical cylindrical PL is about 92% of that produced by the continuous elliptical cylindrical PL. The proposed PL has the advantages of simple and compact structure with much smaller lateral dimension and easy integration with other photonic devices. Our study helps design the easy-to-fabricate PLs and facilitates applications of plasmonic devices in the fields such as optical micro manipulation, super-resolution imaging, optical storage and biochemical sensing.

Direct laser writing of pyramidal plasmonic structures with apertures and asymmetric gratings towards efficient subwavelength light focusing.

Efficient confining of photons into subwavelength scale is of great importance in both fundamental researches and engineering applications, of which one major challenge lies in the lack of effective and reliable on-chip nanofabrication techniques. Here we demonstrate the efficient subwavelength light focusing with carefully engineered pyramidal structures fabricated by direct laser writing and surface metallization. The important effects of the geometry and symmetry are investigated. Apertures with various sizes are flexibly introduced at the apex of the pyramids, the focusing spot size and center-to-sidelobe ratio of which could be improved a factor of ~4 and ~3, respectively, compared with the conical counterparts of identical size. Moreover, two pairs of asymmetric through-nanogratings are conceptually introduced onto the top end of the pyramids, showing significantly improved focusing characteristics. The studies provide a novel methodology for the design and realization of 3D plasmonic focusing with low-noise background and high energy transfer.

On- and off-optical-resonance dynamics of dielectric microcylinders under plane wave illumination

We explore the on-resonance and off-resonance optical response of dielectric cylinders excited by normal incident plane waves. Both the analytical method, based on Mie theory, and the numerical method, implemented with the spectral element method, are undertaken in the study. We demonstrate that the whispering gallery mode characteristic of resonance behavior is strongly dependent on the refractive index and radius changes. Detuning of either parameter deteriorates the resonance action and creates yet another exciting phenomenon known as photonic nanojets. The subwavelength light focusing property can be associated with nanojets, and engineering the parameters yields strong field confinement and slowly diffracting beam propagation. The current work investigates the optical properties of the dielectric microcylinders at close proximity to the resonance condition. Both strong field focusing associated with photonic nanojets and enhanced field localization linked with the resonance condition are desired for photon manipulation scenarios in nanophotonics.

Subwavelength focusing of light with orbital angular momentum.

The spatial structure of light with Orbital Angular Momentum, or "twisted light", closely resembles the shape of atomic wave functions. It could therefore make symmetry-forbidden transitions possible in quantum dots, or "artificial atoms". However, the vanishing intensity in the center of an OAM beam usually makes this effect weak. Here we show a plasmonic approach to focus OAM light to subwavelength dimensions using metallic nanoscale resonant optical antennas. This allows to increase the field intensity of OAM light at the typical dimensions of quantum dots to an intensity larger than a regular Gaussian beam, which corresponds to increasing the interaction strength by 3 orders of magnitude.

Subwavelength light focusing of plasmonic lens with dielectric filled nanoslits structures

A plasmonic lens composed of a dielectric-filled nanoslits structure on an aluminum film is proposed and experimentally demonstrated. The slits' structure is designed with equal distance, length, and width, but filled with variant thickness SiO2 dielectric for specific phase retardations. A dual focused ion beam instrument is employed to mill the slits and deposit SiO2 into the slits. The phase modulation by SiO2-filled slits is illustrated by a double slits interference experiment. The light focusing behavior of the fabricated plasmonic lens is experimentally characterized by a scanning near-field optical microscope. Experimental results show good agreement with the simulations. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE).

Development of new classes of plasmon active nano-structures and their application in bio-sensing and energy guiding

Metal nanostructures exhibit special optical resonance modes originating from the subwavelength confinement of conductive electrons in the material. These resonance modes represent a strong research focus due to their application potential in optics and sensing application. In this short review recent achievements of our group in this field are highlighted. A wet-chemistry approach synthesis of advanced metallic nanostructures will be introduced and their exact positioning and manipulation by electric field is shown. Next, the application of these nanostructures for a detection of small molecules will be described in several examples. Also, it will be shown that metal nanostructures can be used for sub-wavelength light focusing and for efficient energy coupling into polymer chains.

Subwavelength light focusing with a single slit lens based on the spatial multiplexing of chirped surface gratings

A lens consisting of a subwavelength slit engraved into a metal film and surrounded by periodic surface gratings and spatial multiplexing chirped surface gratings (SMCSGs) for focusing light is proposed. The focal length of the lens can be accurately designed and tuned by controlling the periods of the local gratings of the SMCSGs. Simulation results show that a subwavelength beam spot can be produced by the lens at a distance of several times the incident wavelength from the slit and the difference between the simulated and designed focal lengths can be reduced to below 3% of the designed focal length.

Optical super-oscillations: sub-wavelength light focusing and super-resolution imaging

Optical super-oscillations, first predicted in 1952 and observed in 2007, offer a promising route to optical super-resolution imaging and show potential for manufacturing with light and data-storage applications such as direct optical recording and heat assisted magnetic recording. We review the history and basic physics behind the phenomenon of super-oscillation and its application in optics. We overview recent results in creating optical super-oscillations using binary masks, spatial light modulators and planar metamaterial masks. We also investigate the limits and competitiveness of super-oscillatory imaging.

Hyperbolic secant lens with a slit for subwavelength light focusing

A modeling by FDTD method shown, that binary photonic crystal secant hyperbolic micro lens made of Si (n0 = 3,47), with size of 2×5 mm with a slit width of 50 nm and length of 250 nm focuses TM-polarized flat wave in a focal spot near exit surface of lens. Focal spot FWHM size is λ/23, diffractive efficiency of the lens is 44%, and the focal intensity is 10 times more then the intensity of the incident light.

Silicon Nanopillars for Field-Enhanced Surface Spectroscopy

Silicon nanowire and nanopillar structures have drawn increased attention in recent years due in part to their unique optical properties. Herein, electron beam lithography combined with reactive-ion etching is used to reproducibly create individual silicon nanopillars of various sizes, shapes, and heights. Finite difference time domain analysis predicts local field intensity enhancements in the vicinity of appropriately sized and coaxially illuminated silicon nanopillars of approximately 2 orders of magnitude. While this level of enhancement is modest when compared to plasmonic systems, the unique advantage of the silicon nanopillar resonators is that they enhance optical fields in substantially larger volumes. By analyzing experimentally measured strength of the silicon Raman phonon line (500 cm(-1)), it was determined that nanopillars produced local field enhancements that are consistent with these predictions. Additionally, we demonstrate that a thin layer of Zn phthalocyanine on the nanopillar surface with a total amount of <30 attomoles produced prominent Raman spectra, yielding enhancement factors (EFs) better than 2 orders of magnitude. Finally, silicon nanopillars of cylindrical and elliptical shapes were labeled with different fluorophors and evaluated for their surface-enhanced fluorescence (SEF) capability. The EFs derived from analysis of the acquired fluorescence microscopy images indicate that silicon nanopillar structures can provide enhancements comparable or even stronger than those typically achieved using plasmonic SEF structures without the limitations of the metal-based substrates, such as fluorescence quenching and an insufficiently large probe volume. It is anticipated that dense arrays of silicon nanopillars will enable SEF assays with extremely high sensitivity, while a broader impact of the reported phenomena is anticipated in photovoltaics, subwavelength light focusing, and fundamental nanophotonics.

Design of a monopole-antenna-based resonant nanocavity for detection of optical power from hybrid plasmonic waveguides

A novel plasmonic waveguide-coupled nanocavity with a monopole antenna is proposed to localize the optical power from a hybrid plasmonic waveguide and subsequently convert it into electrical current. The nanocavity is designed as a Fabry-Pérot waveguide resonator, while the monopole antenna is made of a metallic nanorod directly mounted onto the metallic part of the waveguide terminal which acts as the conducting ground. The nanocavity coincides with the antenna feed sandwiched in between the antenna and the ground. Maximum power from the waveguide can be coupled into, and absorbed in the nanocavity by means of the field resonance in the antenna as well as in the nanocavity. Simulation results show that 42% optical power from the waveguide can be absorbed in a germanium filled nanocavity with a nanoscale volume of 220 × 150 × 60 nm3. The design may find applications in nanoscale photo-detection, subwavelength light focusing and manipulating, as well as sensing.