Coaxial Nanoapertures Research Articles

Plasmon-induced 0.13 T optomagnetic field in a gold coaxial nanoaperture

We theoretically investigate a resonant inverse Faraday effect within individual coaxial nanoapertures. Upon illumination with circularly polarized light, resonant gold coaxes are shown to develop an optomagnetic field that is controllable by the helicity of the light. This magnetic field is found to reach 0.13 T upon excitation at an intensity of 0.5 · 1011W.cm−2 that is typical from sub-ps light pulses. At an intensity of 2.4 · 108W.cm−2 (consistent with the CW regime), we obtain a static magnetic field of about 1 mT, leading to a helicity-dependent magnetic force of 4.5 · 106 N onto a point-like magnetic dipole of unit moment. Given their submicron footprint, individual coaxial nanoapertures open new prospects towards ultrafast and polarization-controlled tunable magnetism on the nanoscale, thus potentially impacting a large panel of application and techniques including all optical magnetization switching, spin-wave excitation and optomagnetic tweezing of nano-objects.

Fast and efficient nanoparticle trapping using plasmonic connected nanoring apertures

The manipulation of microparticles using optical forces has led to many applications in the life and physical sciences. To extend optical trapping towards the nano-regime, in this work we demonstrate trapping of single nanoparticles in arrays of plasmonic coaxial nano-apertures with various inner disk sizes and theoretically estimate the associated forces. A high normalized experimental trap stiffness of 3.50 fN nm−1 mW−1 μm−2 for 20 nm polystyrene particles is observed for an optimum design of 149 nm for the nanodisk diameter at a trapping wavelength of 980 nm. Theoretical simulations are used to interpret the enhancement of the observed trap stiffness. A quick particle trapping time of less than 8 s is obtained at a concentration of 14 × 1011 particles ml−1 with low incident laser intensity of 0.59 mW μm−2. This good trapping performance with fast delivery of nanoparticles to multiple trapping sites emerges from a combination of the enhanced electromagnetic near-field and spatial temperature increase. This work has applications in nanoparticle delivery and trapping with high accuracy, and bridges the gap between optical manipulation and nanofluidics.

Impact of Surface Roughness in Nanogap Plasmonic Systems

Recent results have shown unprecedented control over separation distances between two metallic elements hundreds of nanometers in size, underlying the effects of free-electron nonlocal response also at mid-infrared wavelengths. Most of metallic systems however, still suffer from some degree of inhomogeneity due to fabrication-induced surface roughness. Nanoscale roughness in such systems might hinder the understanding of the role of microscopic interactions. Here we investigate the effect of surface roughness in coaxial nanoapertures resonating at mid-infrared frequencies. We show that although random roughness shifts the resonances in an unpredictable way, the impact of nonlocal effects can still be clearly observed. Roughness-induced perturbation on the peak resonance of the system shows a strong correlation with the effective gap size of the individual samples. Fluctuations due to fabrication imperfections then can be suppressed by performing measurements on structure ensembles in which averaging over a large number of samples provides a precise measure of the ideal system's optical properties.

Waveguide-integrated mid-infrared plasmonics with high-efficiency coupling for ultracompact surface-enhanced infrared absorption spectroscopy.

Waveguide-integrated plasmonics is a growing field with many innovative concepts and demonstrated devices in the visible and near-infrared. Here, we extend this body of work to the mid-infrared for the application of surface-enhanced infrared absorption (SEIRA), a spectroscopic method to probe molecular vibrations in small volumes and thin films. Built atop a silicon-on-insulator (SOI) waveguide platform, two key plasmonic structures useful for SEIRA are examined using computational modeling: gold nanorods and coaxial nanoapertures. We find resonance dips of 90% in near diffraction-limited areas due to arrays of our structures and up to 50% from a single resonator. Each of the structures is evaluated using the simulated SEIRA signal from poly(methyl methacrylate) and an octadecanethiol self-assembled monolayer. The platforms we present allow for a compact, on-chip SEIRA sensing system with highly efficient waveguide coupling in the mid-IR.

High-Contrast Infrared Absorption Spectroscopy via Mass-Produced Coaxial Zero-Mode Resonators with Sub-10 nm Gaps.

We present a wafer-scale array of resonant coaxial nanoapertures as a practical platform for surface-enhanced infrared absorption spectroscopy (SEIRA). Coaxial nanoapertures with sub-10 nm gaps are fabricated via photolithography, atomic layer deposition of a sacrificial Al2O3 layer to define the nanogaps, and planarization via glancing-angle ion milling. At the zeroth-order Fabry-Pérot resonance condition, our coaxial apertures act as a "zero-mode resonator (ZMR)", efficiently funneling as much as 34% of incident infrared (IR) light along 10 nm annular gaps. After removing Al2O3 in the gaps and inserting silk protein, we can couple the intense optical fields of the annular nanogap into the vibrational modes of protein molecules. From 7 nm gap ZMR devices coated with a 5 nm thick silk protein film, we observe high-contrast IR absorbance signals drastically suppressing 58% of the transmitted light and infer a strong IR absorption enhancement factor of 104∼105. These single nanometer gap ZMR devices can be mass-produced via batch processing and offer promising routes for broad applications of SEIRA.

Whirling Plasmons: Angular Momentum Selection Rule

P lasmonic systems have been shown to be resonantly excited when the linear momentum selection rule is fulfi lled.1 However, conservation of total angular momentum (AM) in a closed physical system results in additional selection rules. e AM of an optical beam comprises the intrinsic component—the spin, associated with the handedness of the circular polarization—and the extrinsic component— orbital AM (OAM), associated with a spiral phase front.2 Here, we demonstrate a plasmonic nanostructure that exhibits a crucial role of an AM selection rule in a lightsurface plasmon scattering process. In our experiment, the intrinsic AM of the incident radiation was coupled to the extrinsic momentum of the surface plasmons via spin-orbit interaction, which was manifested by a geometric Berry phase.3 Due to this eff ect, we achieved a symmetry breaking that resulted in a spin-dependent enhanced transmission through coaxial nanoapertures, even in rotationally symmetric structures.4 In an optical paraxial beam with a spiral phase distribution (= –l, where  is the azimuthal angle in polar coordinates, and the integer number l is the topological charge), the total AM per photon, in units of h– (normalized AM), was shown to be j=(+l), where =1 is the right-handed circular polarization and =–1 is the left-handed circular polarization.2 In accordance with fundamental physical principles, resonant excitation of the nanoaperture eigenmode requires that the exciting wave match the excited mode, both with its linear and angular momentum. is matching imposes restrictions, or selection rules, on the excitation process. e coaxial nanoaperture was milled by a focused ion beam into a 200-nmthick gold fi lm evaporated onto a glass wafer. e inner and the outer radii of the ring slit were 250 and 350 nm, respectively. e aperture was designed to be a single mode system—in other words, to possess a single allowed excitation with OAM of lGM=±1. e aperture was surrounded by an annular coupling grating with a period of 500 nm. is element was illuminated by a green laser light (532 nm) whose phase was modulated by a spatial light modulator to achieve a spiral phase corresponding to an OAM of lext=0, ±2. e incident spin (in=±1) induced a spiral phase of the excited surface plasmons via spinorbit interaction, and, therefore, was converted to the OAM.5 e surface mode then acquired OAM of lSM= in+ lext. e best overlapping of the surface mode and guided modes was obtained when lSM= lGM, i.e. (a) Mechanism of the nanoaperture’s excitation controlled by AM selection rules. Incident beam bears the intrinsic AM of in and the extrinsic AM of lext. Excited surface mode acquires the OAM of lSM as a result of spin-orbit interaction. Guided mode with lGM is excited only if selection rule is satis ed. (Inset) Scanning electron microscope image of the structure. (b) Intensity distribution cross-sections for different lext. Blue dashed lines correspond to in=1 and red solid lines to in=–1. Intensity was normalized by the transmission measured via coaxial aperture without the surrounding corrugation. (Horizontal dimension was scaled according to the optical magni cation.) (a)

Plasmon Dispersion in Coaxial Waveguides from Single-Cavity Optical Transmission Measurements

We determine the plasmon dispersion relation in coaxial waveguides composed of a circular channel separating a metallic core and cladding. Optical transmission measurements are performed on isolated coaxial nanoapertures fabricated on a Ag film using focused ion-beam lithography. The dispersion depends strongly on the dielectric material and layer thickness. Our experimental results agree well with an analytical model for plasmon dispersion in coaxial waveguides. We observe large phase shifts at reflection from the end facets of the coaxial cavity, which strongly affect the waveguide resonances and can be tuned by changing the coax geometry, composition, and surrounding dielectric index, enabling coaxial cavities with ultrasmall mode volumes.

Extraordinary midinfrared transmission of rectangular coaxial nanoaperture arrays

We analyze the extraordinary transmission effect in the periodic and random nanoaperture arrays in optically thick metal films. Experimental studies are combined with simulations to show that extraordinary transmission effect at midinfrared wavelengths is enhanced when the decaying waveguide mode of a single nanoaperture couples through the collective surface plasmon excitations created through periodicity. Transmission characteristics of the individual nanoapertures are investigated by randomizing the apertures. By comparing rectangular to coaxial rectangular nanoapertures, both of which support TE01-like decaying waveguide modes, we study the effect of nanoaperture geometry on the efficiency of the transmission.

Enhanced transmission with coaxial nanoapertures: Role of cylindrical surface plasmons

We simulate the optical fields and optical transmission through nanoarrays of silica rings embedded in thin gold films using the finite-difference-time-domain method. By examining the optical transmission spectra for varying ring geometries we uncover large enhancements in the transmission at wavelengths much longer than the usual cutoffs for cylindrical apertures or where the usual planar surface plasmons or other periodic effects from the array could play a role. We attribute these enhancements to closely coupled cylindrical surface plasmons on the inner and outer surfaces of the rings, and this coupling is more efficient as the inner and outer ring radii approach each other. We confirm this hypothesis by comparing the transmission peaks of the simulation with cylindrical surface plasmon (CSP) dispersion curves calculated for the geometries of interest. One important result is that a transmission peak appears in the simulations close to the frequency where the longitudinal wave number ${k}_{\mathrm{z}}$ in the ring satisfies ${k}_{\mathrm{z}}=m\ensuremath{\pi}∕\mathrm{L}$, where $m$ is an integer and $L$ the length of the aperture, for a normal CSP ${\mathrm{TE}}_{1}$ or ${\mathrm{TM}}_{1}$ mode. The behavior of the CSP dispersion is such that propagating modes can be sent through the rings for ever longer wavelengths as the ring radii approach, whereas the transmission decreases only in proportion to the ring area.