Optical Field Enhancement Research Articles

Silver Nanoparticle-Based Arrays into Mesoporous Thin Films Structures for Photoelectronic Circuits

Background: Silver nanosystems have attracted considerable attention for numerous applications in optoelectronics. The localized surface plasmon of silver nanoparticles embedded into mesoporous titania gives rise to an enhancement of local optical field in the vicinity of Ag nanoparticles which act as efficient light-trapping components, resulting in a visible wavelength-dependent photocurrent. Objective: In this paper, we synthetized patterned nanocomposites formed by titania mesoporous thin films loaded with alkanethiol functionalized Ag nanoparticles and we demonstrated that these stable and accessible nanostructures possess a photocurrent response. Method: Mesoporous thin films are created by combining sol-gel synthesis and template selfassembly. Based on a photolithography technique, silver nanoparticles were selectively photodeposited and then stabilized with octanethiols. Current vs. voltage curves with and without light were compared, where selective light wavelength measurements were achieved by using visible bandpass filters. The optofluidic behavior was evaluated by placing a drop of solutions on the mesoporous film. Results: We demonstrate photocurrent in these mesoporous thin film structures decorated with chemistabilized Ag nanoparticle-based conductive arrays, with significantly enhanced photocurrent peak at the plasmon resonant wavelength around 540 nm. Our findings offer a possibility to perform improved fluid detection with silver-mesoporous titania electronic devices. Conclusion: We showed that an optofluidic sensitive nanocomposite circuit consisting of alkanethiol- functionalized metal nanoparticles embedded in a mesoporous oxide thin film matrix can be produced.

Plasmonic Optical and Chiroptical Response of Self-Assembled Au Nanorod Equilateral Trimers.

Assembling metamolecules from anisotropic, shape-engineered nanocrystals provides the opportunity to orchestrate distinct optical responses one nanocrystal at a time. The Au nanorod has long been a structural archetype in plasmonics, but nanorod assemblies have largely been limited to end-to-end or side-to-side arrangements, accessing only a subset of potential metamolecule structures. Here, we employ triangular templates to direct the assembly of Au nanorods along the edges of an equilateral triangle. Using spatially resolved, dark-field scattering spectroscopy in concert with numerical simulation of individual metamolecules, we map the evolution in surface plasmon resonances as we add one, two, and three nanorods to construct triangular nanorod assemblies. The assemblies exhibit rotation- and polarization-dependent hybridized plasmon modes, which are sensitive to variations in nanorod size, position, and orientation that lead to geometrical symmetry breaking. The triangular arrangement of nanorods supports magnetic plasmon modes where electric fields are directed along the perimeter of the triangle, and the magnetic field intensity within the triangle's open interior is enhanced. Circumferential displacements of the nanorods within the templates impart either a left- or right-handed sense of rotation to the structure, which generates a chiroptical response under unidirectional oblique illumination. Our results represent an important step in realizing and characterizing metamaterial assemblies with "open" structures utilizing anisotropic plasmonic building blocks, with implications for optical magnetic field enhancement and chiral plasmonics.

Fano Resonance on Nanostructured Lithium Niobate for Highly Efficient and Tunable Second Harmonic Generation.

Second harmonic generation (SHG) is an important nonlinear process which is critical for applications, such as optical integrated circuit, nonlinear microscopy, laser, etc. Many challenges remain in the improvement of nonlinear conversion efficiency, since the typical value is of only 10−5 in nanostructures. Here, we theoretically demonstrate a periodic structure consisting of a lithium niobate (LN) bar and an LN disk, on a nanoscale (~300 nm) thin-film platform, which is proposed for a highly efficient SHG. By breaking the structure symmetry, a Fano resonance with a high Q, up to 2350 and a strong optical field enhancement reaching forty-two folds is achieved, which yields a high conversion efficiency, up to 3.165 × 10−4. In addition to its strong second harmonic (SH) signal, we also demonstrate that by applying only 0.444 V on the planar electrode configurations of the nanostructured LN, the wavelength of SH can be tuned within a 1 nm range, while keeping its relatively high conversion efficiency. The proposed structure with the high nonlinear conversion efficiency can be potentially applied for a single-molecule fluorescence imaging, high-resolution nonlinear microscopy and active compact optical device.

An aligned octahedral core in a nanocage: synthesis, plasmonic, and catalytic properties.

Plasmonic metal nanostructures with complex morphologies provide an important route to tunable optical responses and local electric field enhancement at the nanoscale for a variety of applications including sensing, imaging, and catalysis. Here we report a high-concentration synthesis of gold core-cage nanoparticles with a tethered and structurally aligned octahedral core and examine their plasmonic and catalytic properties. The obtained nanostructures exhibit a double band extinction in the visible-near infrared range and a large area electric field enhancement due to the unique structural features, as demonstrated using finite difference time domain (FDTD) simulations and confirmed experimentally using surface enhanced Raman scattering (SERS) tests. In addition, the obtained structures had a photoelectrochemical response useful for catalyzing the CO2 electroreduction reaction. Our work demonstrates the next generation of complex plasmonic nanostructures attainable via bottom-up synthesis and offers a variety of potential applications ranging from sensing to catalysis.

Optical properties of aluminum intercalated magnesium nanoparticle square array: a computational study.

Magnesium nanostructures have recently emerged as a vivid and amazing plasmonic material. Using computationally efficient time-dependent density-functional theory calculations, we investigate the optical response of a highly ordered magnesium nanoparticle array with interparticle gap distances varying in the range of 0.5 nm to 2.0 nm. We reveal that depending on the interparticle gap distance (d), significant spectral modulations are possible. More specifically, for d = 1.0 nm, we have observed a huge optical field enhancement due to the interparticle plasmon coupling. In contrast, we obtained highly quenched spectra for d = 0.5 nm, 1.5 nm and 2.0 nm. While for d = 0.5 nm, strong electron tunnelling causes spectral reduction, whereas, for d = 1.5 nm and 2.0 nm, the nanoparticles are plasmonically decoupled to a large extent. Further, we study the impact of aluminum intercalation on the optical response, finding vivid spectral modulations. We believe that our findings will be helpful in the design of ultra-sensitive plasmonic devices based on magnesium nanoparticle arrays.

Characteristics and control of fiber ring resonator

Optical resonators play an active role in fundamental research and applications in atomic fine spectra, laser generation, precision measurements, and quantum information processing because of their high-resolution spectra and strong optical field enhancement. The fiber ring resonators, as a derivative of the resonant resonators, have the advantages of simple structure, small size, stable performance and easy integration. The fiber ring resonators are widely used in fiber lasers, optical communication devices, optical fiber sensing, etc. In this paper, we demonstrate the characteristics of a fiber ring resonator based on a tunable fiber beam splitter experimentally. Control of under-coupling, critical coupling and over-coupling state of the fiber ring resonator can be achieved by adjusting the splitting ratio of the tunable fiber beam splitter. The relationship between the minimum resonator reflectance and resonator loss is given. The intrinsic decay rate of the fiber ring resonator is <inline-formula><tex-math id="M92">\begin{document}${\kappa _0}{\rm{ = }}2{\text{π}} \times \left( {1.60 \pm 0.03} \right)\;{\rm{ MHz}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182296_M92.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182296_M92.png"/></alternatives></inline-formula>, and the quality factor is <inline-formula><tex-math id="M93">\begin{document}$Q = \left( {1.10 \pm 0.02} \right) \times {10.8}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182296_M93.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182296_M93.png"/></alternatives></inline-formula>. The resonance frequency of the fiber ring resonator is controlled by stretching the fiber. The fiber resonator is kept straight and fixed on a self-made U-shaped holder by gluing two points. A piezoelectric transducer is used to change the distance between the two glued points. The fiber ring resonator length is changed and controlled when the fiber is stretched. The Pound-Drever-Hall technique is used to lock the resonator to resonance with the laser. The phase of the laser beam is modulated by using an electro-optical modulator, and two sidebands of the laser frequency are generated. Due to the phase sensitivity of the fiber resonator, the reflected light of the fiber resonator with an intensity modulation is observed when the fiber ring resonator is locked. The intensity modulation is caused by the interference between the resonance frequency and the sidebands of the fiber ring resonator. The reflected spectrum of the fiber ring resonator carries the same-frequency modulation as the phase modulation. This is a disadvantage for the usage of the fiber ring resonator. Thus, we reduce the phase modulation power to reduce the intensity modulation of the resonator reflectance. The linear relationship between the phase modulation power and the intensity modulation of the resonator reflectance caused by the phase modulation is obtained. The fiber ring resonator can be locked when the phase modulation power decreases to –9 dBm. The fiber ring resonator has laid a solid experimental foundation for experimental research on the interaction between the fiber ring resonator and quantum emitters such as atoms and color centers in diamond.

Giant distributed optical-field enhancements from Mie-resonant lattice surface modes in dielectric metasurfaces

High-index dielectric nanoparticles can support strong localized Mie scattering resonances of both electric- and magnetic-dipole character. Compared to the plasmonic excitations of similar metallic nanostructures, these resonances have the distinct advantage of negligible absorption losses at the expense, however, of significantly reduced optical-field-intensity enhancements. To address this limitation, here we investigate their coherent superposition across neighboring nanoparticles in suitably designed periodic arrays via coupling to lattice surface modes. Numerical simulations show that the resulting excitations can provide giant spatially-averaged field enhancements (distributed across the entire surface of the array) with ultranarrow spectral linewidths, particularly in arrays embedded within a symmetric dielectric environment. These systems are therefore ideally suited for applications involving tailored light-matter interactions with extended-area active layers, such as surface-enhanced sensing, spectroscopy, fluorescence, and photochemistry.

Enhanced optical transmission of composite subwavelength rectangular-hole array based on metal silver thin-film

Metal nanostructures can effectively couple the optical radiation energy in free space to a highly confined surface mode due to the excitation of surface plasmons (SPs), resulting in a greatly enhanced local field in the nanoscale range of the metal surface, it’s great significance for the collection and excitation of broadband light. In order to achieve the broadband and high transmittance of metal nanostructures, a composite rectangular-hole array metal micro/nano structure was designed in this paper, and the transmission characteristics of the structure were studied by using the finite difference time domain (FDTD). The results showed that compared with the single hole array, the compound hole structure has many advantages, such as optical field enhancement, tenability and so on, and there are multiple transmission peaks in the transmission spectrum. In addition, we discussed the effects of the length and width of the rectangular holes on the light transmission characteristics of the structure array. For example, with the increase of the length b of the rectangular hole, the maximum transmittance of the structure increases from 79.7% to 88.3%, and corresponding to the central peak increases from 526 nm to 611 nm. Both the transmission bandwidth and transmittance are improved. While the change of the length a of the rectangular hole produces a peculiar multi-peak phenomenon, the analysis shows that when the length a of the rectangular hole is 250 nm, the maximum transmittance of the full symmetric composite rectangular hole array can reach up to 93% and the FWHM of the transmission peak is 354 nm, much larger than the 80 nm proposed in ref. [27] , so the frequency selectivity will be enhanced in the design of the filter. At the same time, the effect of the side length D of the center square hole on the transmission characteristics of the structure array is also discussed. The results of our research have certain guiding significance for the development of enhanced optical transmission theory and application value in the fields of novel optical sensors, t filter and optical transparent electrode.

Giant enhancement of the effective Raman susceptibility in metasurfaces made of silicon photonic crystal nanocavities.

We demonstrate that stimulated Raman amplification can be enhanced by more than four orders of magnitude in a silicon metasurface consisting of a periodic distribution of specially engineered photonic crystal (PhC) cavities in a silicon PhC slab waveguide. In particular, by designing the PhC cavities so as they possess two optical modes separated by the Raman frequency of silicon, one can achieve large optical field enhancement at both the pump and Stokes frequencies. As a consequence, the effective Raman susceptibility of the nonlinear metasurface, calculated using a novel homogenization technique, is significantly larger than the intrinsic Raman susceptibility of silicon. Implications to technological applications of our theoretical study are discussed, too.

Electron dynamics on gold surfaces driven by short laser pulses

This paper describes the application of time-dependent density functional theory (TDDFT) to explore dynamics in condensed matter driven by strong short laser pulses. As a specific example, the calculation of the optical response on a gold surface is presented. The real-time TDDFT treatment of metallic systems is difficult due to the presence of a surface of the Fermi level (EF ). Depending on EF , the occupation numbers of the states at each of the k points in the wedge of the first Brillouin zone should be assigned to obtain self-consistent charge-density and Kohn–Sham wavefunctions, which are set as the initial condition for the real-time propagation based on TDDFT. Furthermore, a computational scheme for exploring the optical field enhancement by the Au surface and numerical problems arising from the periodic boundary conditions are discussed.

Enantiomer-Selective Molecular Sensing Using Racemic Nanoplasmonic Arrays.

Building blocks of life show well-defined chiral symmetry which has a direct influence on their properties and role in Nature. Chiral molecules are typically characterized by optical techniques such as circular dichroism (CD) where they exhibit signatures in the ultraviolet frequency region. Plasmonic nanostructures have the potential to enhance the sensitivity of chiral detection and translate the molecular chirality to the visible spectral range. Despite recent progress, to date, it remains unclear which properties plasmonic sensors should exhibit to maximize this effect and apply it to reliable enantiomer discrimination. Here, we bring further insight into this complex problem and present a chiral plasmonic sensor composed of a racemic mixture of gammadions with no intrinsic CD, but high optical chirality and electric field enhancements in the near-fields. Owing to its unique set of properties, this configuration enables us to directly differentiate phenylalanine enantiomers in the visible frequency range.

A scatterer-assisted whispering-gallery-mode microprobe

Abstract A fiber-based whispering-gallery-mode (WGM) microprobe, combining both the high optical field enhancement of the WGMs and the compact structure of the optical fiber, is highly desired for sensing and imaging. Here we report a WGM microsphere resonator coupled to a single-mode fiber interfaced by a graded-index lens. By scattering a focused laser beam through a nano-scatterer, with the help of a two-step focusing technique as well as Purcell effects, the efficient far-field coupling of WGMs with an efficiency as high as 16.8% has been demonstrated in our system. With the feature of both input and output of the probe light propagating along the same fiber, such a scatterer-assisted WGM microprobe will serve as a convenient tool for sensing/imaging applications.

Optical fibre-tip probes for SERS: numerical study for design considerations.

Enhancement of sub-wavelength optical fields using sub-micron plasmonic probes has found many applications in chemical, material, biological and medical sciences. The enhancement is via localised surface-plasmon resonance (LSPR) which enables the highly sensitive vibrational-spectroscopy technique of surface-enhanced Raman scattering (SERS). Combining SERS with optical fibres can allow the monitoring of biochemical reactions in situ with high resolution. Here, we study the electromagnetic-field enhancement of a tapered optical fibre-tip coated with gold nanoparticles (AuNPs) using finite-element simulations. We investigate the electric-field enhancement associated with metallic NPs and study the effect of parameters such as tip-aperture radius, cone angle, nanoparticle size and gaps between them. Our study provides an understanding of the design and application of metal-nanoparticle-coated optical-fibre-tip probes for SERS. The approach of using fibre-coupled delivery adds flexibility and simplifies the system requirements in SERS, making it suitable for cellular imaging and mapping bio-interfaces.

Roll-to-Roll Nanoforming of Metals Using Laser-Induced Superplasticity.

This Letter describes a low-cost, scalable nanomanufacturing process that enables the continuous forming of thin metallic layers with nanoscale accuracy using roll-to-roll, laser-induced superplasticity (R2RLIS). R2RLIS uses a laser shock to induce the ultrahigh-strain-rate deformation of metallic films at room temperature into low-cost polymeric nanomolds, independently of the original grain size of the metal. This simple and inexpensive nanoforming method does not require access to cleanrooms and associated facilities, and can be easily implemented on conventional CO2 lasers, enabling laser systems commonly used for rapid prototyping or industrial cutting and engraving to fabricate uniform and three-dimensional crystalline metallic nanostructures over large areas. Tuning the laser power during the R2RLIS process enables the control of the aspect ratio and the mechanical and optical properties of the fabricated nanostructures. This roll-to-roll technique successfully fabricates mechanically strengthened gold plasmonic nanostructures with aspect ratios as high as 5 that exhibit high oxidation resistance and strong optical field enhancements. The CO2 laser used in R2RLIS can also integrate the fabricated nanostructures on transparent flexible substrates with robust interfacial contact. The ability to fabricate ultrasmooth metallic nanostructures using roll-to-roll manufacturing enables the large scale production, at a relatively low-cost, of flexible plasmonic devices toward emerging applications.

Nanofocusing of Toroidal Dipole for Simultaneously Enhanced Electric and Magnetic Fields Using Plasmonic Waveguide

Optical toroidal dipole and electromagnetic field enhancement have attracted much interest owing to their interesting physical features and various potential nanophotonic applications. In this paper, novel antireflective nanofocusing methods to deliver and squeeze toroidal dipole moment with simultaneous enhancement of electric and magnetic fields are suggested. By coupling the two fundamental modes of a metal–insulator–metal waveguide with a partial mirror in a waveguide and a nanocavity, resonant squeezing of longitudinal and perpendicular toroidal dipoles in an ultracompact nanocavity (∼ λ 2 / 190) is achieved at the near-telecom wavelengths. Moreover, electric and magnetic field enhancements occur simultaneously in a designated nanocavity. We expect that the proposed scheme would pave a way to engineer nanophotonic waveguide based light-matter interactions by enhancing both electric and magnetic fields. This study would interest both nanophotonics and quantum optics communities.

Plasmonic metasurface cavity for simultaneous enhancement of optical electric and magnetic fields in deep subwavelength volume.

It has been hard to achieve simultaneous plasmonic enhancement of nanoscale light-matter interactions in terms of both electric and magnetic manners with easily reproducible fabrication method and systematic theoretical design rule. In this paper, a novel concept of a flat nanofocusing device is proposed for simultaneously squeezing both electric and magnetic fields in deep-subwavelength volume (~λ3/538) in a large area. Based on the funneled unit cell structures and surface plasmon-assisted coherent interactions between them, the array of rectangular nanocavity connected to a tapered nanoantenna, plasmonic metasurface cavity, is constructed by periodic arrangement of the unit cell. The average enhancement factors of electric and magnetic field intensities reach about 60 and 22 in nanocavities, respectively. The proposed outstanding performance of the device is verified numerically and experimentally. We expect that this work would expand methodologies involving optical near-field manipulations in large areas and related potential applications including nanophotonic sensors, nonlinear responses, and quantum interactions.

Strain induced plasmon tuning in planar square-shaped aluminum nanoparticles array

Metal nanoparticle aggregate is an exciting platform for manipulating light-matter interactions at the nanoscale, thanks to the optically driven free electrons couple electrically across the inter-particle gap region. We use time dependent density functional theory calculations to investigate the optical response modulations in planar square-shaped aluminum nanoparticles array via morphology deformation (varying the inter-particle gap distance in the range of 2–20 Å) separately along one and two directions. We report the surprising observation that irrespective of the different morphology deformations, there exists a unique inter-particle gap distance of 12 Å for which, a maximum optical field enhancement can be achieved. We remark that plasmonic interaction between metal nanoparticles in an aggregate is controlled to a large extent by the size of the inter-particle gap distance. We believe that our quantum mechanical calculations will inspire and contribute to the design, control, and exploitation of aluminum based plasmonic devices.

Influence of Ag film on upconversion luminescence enhancement of a single β-NaYF4:Yb3+/Er3+ microcrystal particle

Rare-earth-doped upconversion (UC) is a nonlinear optical phenomenon in which the successive absorption of two or more low-energy photons via intermediate energy states leads to high-energy light emission (anti-Stokes emission). It endows their unique optical properties including low auto-fluorescence, high signal to noise ratio, a large anti-Stokes shift, and low toxicity. Therefore, rare-earth-doped UC materials have been widely used in photovoltaics, color displays, optical storages, biological imaging, and solar cells. However, the UC luminescence efficiency is much lower and largely limited the practical applications. The noble metal nanostructures as promised enhanced substrates could efficiently improve the intensity of UC luminescence and expand its applications. In this work, Yb3+and Er3+ co-doped β-NaYF4 microcrystals are synthesized via a simple and facile hydrothermal method with assistance of sodium citrate to control the crystal growth. Then, a Ag films/β-NaYF4:Yb3+/Er3+ hybrid structure that the β-NaYF4:Yb3+/Er3+ microcrystals were deposited on silver films with different thickness is constructed. The crystal phase and the morphology of the β-NaYF4:Yb3+/Er3+ microcrystals are analyzed with X-ray power diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscopy (TEM). According to the results of XRD, SEM, and TEM, the pure hexagonal phased NaYF4:Yb3+/Er3+ are arc-shaped hexagonal plate microcrystals with smooth surface. The average overall dimension of each microcrystal is about 5.0 µm in diameter. The UC luminescence property of Ag film/β-NaYF4:Yb3+/Er3+ hybrid structure is investigated carefully by a confocal setup equipped with an optical microscope. When 980 nm continuous wave (CW) laser is focused on the central of a single microcrystal particle deposited on silver films with different thickness, the intensity of blue (2H9/2→4I15/2), green (4S3/2→4I15/2 and 2H11/2→4I15/2), and red (4F9/2→4I15/2) emission bands increases with increasing the Ag film thickness. The maximum enhancement factor (EF) for each emission band occurs at the Ag film thickness of 100 nm. But we realize that the intensity EF of each emission band is different, the EF for the green emission is greater than the blue and red one. A comprehensive analysis, based on the rate equations and the numerical calculation | E |2, reveals that the optical reflection and local electric field enhancement effect of the Ag film play a role in the UC luminescence enhancement for the single β-NaYF4:Yb3+/Er3+ microcrystal. This finding breaks the limitation of conventional core-shell system, and it also provides a unique platform for micro-optoelectronic devices, solar energy conversion, optical sensors, and biological applications.

Field enhancement in plasmonic nanostructures

Efficient generation of charge carriers from a metallic surface is a critical challenge in a wide variety of applications including vacuum microelectronics and photo-electrochemical devices. Replacing semiconductors with vacuum/gas as the medium of electron transport offers superior speed, power, and robustness to radiation and temperature. We propose a metallic resonant surface combining optical and electrical excitations of electrons and significantly reducing powers required using plasmon-induced enhancement of confined electric field. The properties of the device are modeled using the exact solution of the time-dependent Schrödinger equation at the barrier. Measurement results exhibit strong agreement with an analytical solution, and allow us to extract the field enhancement factor at the surface. Significant photocurrents are observed using combination of optical power and 10 V DC excitation on the surface. The model suggests optical field enhancement of 3 orders of magnitude at the metal interface due to plasmonic resonance. This simple planar structure provides valuable evidence on the electron emission mechanisms involved and it can be used for implementation of semiconductor compatible vacuum devices.

Optical Magnetic Field Enhancement by Strong Coupling in Metamaterials

We study theoretically the interaction effects between optical waveguide modes and magnetic resonances in metamaterials, consisting of periodic arrays of paired metallic nanodisks lying on a dielectric waveguide substrate. The plasmon near-field coupling in individual metallic nanodisk pairs results into magnetic resonances. The periodic arrangement of the paired metallic nanodisks can excite optical waveguide modes propagating in the adjacent dielectric waveguide. When optical waveguide modes are close to magnetic resonances by varying the array period, they interact to form two hybridized modes. A coupling model of two oscillators is also proposed to predict well the positions of the hybridized modes. In the strong interaction regime, the hybridized modes have an obvious anticrossing, and thus exhibit an interesting phenomenon of Rabi splitting. Furthermore, the magnetic fields within metallic nanodisk pairs exhibit a huge enhancement, which could find promising potential applications in magnetic nonlinearity and sensors.