Aperture In Metal Film Research Articles

(Invited) Optical Tweezers to Isolate Single Er Ion Emitter Containing Nanocrystals and in-Situ Perovskite Quantum Dot Analysis

We have used optical tweezers with shaped apertures in metal films to hold onto individual nanoparticles including erbium containing nanoparticles and perovskite quantum dots. In this talk, I will talk about our ability to isolate nanocrystals with just one erbium emitter, paving the way for single photon sources at low loss fiber wavelengths. I will also talk about our new work on in-situ characterization and assembly of perovskite quantum dots, allowing to relate size to emission wavelength and look at quantum hybridization.

Nano- and microscale apertures in metal films fabricated by colloidal lithography with perovskite nanocrystals

We demonstrate patterning of metal surfaces based on lift-off of perovskite nanocrystals that enables the fabrication of nanometer-size features without the use of resist-based nanolithography. The perovskite nanocrystals act as templates for defining the shape of the apertures in metal layers, and we exploit the variety of sizes and shapes that can be controlled in the colloidal synthesis to demonstrate the fabrication of nanoholes, nanogaps and guides with size smaller than the wavelength of light in the visible spectrum. The process can be readily integrated with standard lithography and etching techniques for the creation of more complex structures.

Amplification of resonant field enhancement by plasmonic lattice coupling in metallic slit arrays.

Nonlinear spectroscopic investigation in the terahertz (THz) range requires significant field strength of the light fields. It is still a challenge to obtain the required field strengths in free space from table-top laser systems at sufficiently high repetition rates to enable quantitative nonlinear spectroscopy. It is well known that local enhancement of the THz field can be obtained for instance in narrow apertures in metallic films. Here we show by simulation, analytical modelling and experiment that the achievable field enhancement in a two-dimensional array of slits with micrometer dimensions in a metallic film can be increased by at least 60% compared to the enhancement in an isolated slit. The additional enhancement is obtained by optimized plasmonic coupling between the lattice modes and the resonance of the individual slits. Our results indicate a viable route to sensitive schemes for THz spectroscopy with slit arrays manufactured by standard UV photolithography, with local field strengths in the multi-ten-MV/cm range at kHz repetition rates, and tens of kV/cm at oscillator repetition rates.

A Breaching of Electromagnetic Shielding by Narrow Aperture in Metal Film

We report a theoretical research for the condition of electromagnetic shield breaching when a narrow aperture is punctured in thin metal film. To calculate electromagnetic field transmission through a narrow slit, a Rayleigh wave expansion has been applied for free standing, thin metal film. We found that the DC electric field allows perfect transmission when the length of the slit is infinite, regardless of the other geometrical factors such as slitwidth and thickness. Slitwidth dependent transmission spectra as a function of frequency shows a cutoff frequency that decreases almost linearly to the slitwidth, giving that almost successful shielding is only possible when the slitwidth is smaller than 1 micron.

Localized and propagating surface plasmon resonances in aperture-based third harmonic generation.

We investigate the influence of localized and propagating surface plasmons on third harmonic generation from rectangular apertures in metal films. We designed optimal aperture array structures by using finite-difference time-domain simulations with nonlinear scattering theory. From this design space, we fabricated and measured the third harmonic in the region of maximal performance. We find the highest third harmonic conversion efficiency when the localized resonance is tuned to the fundamental wavelength and the propagating (Bragg) resonance is tuned to the third harmonic; this is 2.5 times larger than the case where the both localized and propagating are tuned to the fundamental wavelength. The two remaining configurations where also investigated with much lower conversion efficiency. When the Bragg resonance is tuned to the third harmonic, directivity improves the collection of third harmonic emission. On the other hand, due to the inherent absorption of gold at the third harmonic, tuning the localized surface plasmon resonance to the third harmonic is less beneficial. All cases showed quantitative agreement with the original theoretical analysis. This work points towards an optimal design criterion for harmonic generation from thin plasmonic metasurfaces.

Efficient vertical mode expansion method for scattering by arbitrary layered cylindrical structures.

A relatively simple and efficient numerical method is developed for analyzing the scattering of light by a layered cylindrical structure of arbitrary cross section surrounded by a layered background. The method significantly extends an existing vertical mode expansion method (VMEM) for circular or elliptic cylindrical structures. The original VMEM and its extension give rise to effective two-dimensional formulations for the three-dimensional scattering problems of layered cylindrical structures. The extended VMEM developed in this paper uses boundary integral equations to handle the two-dimensional Helmholtz equations that appear in the vertical mode expansion process. The method is applied to analyze the transmission of light through subwavelength apertures in metallic films and the scattering of light by metallic nanoparticles.

Self-assembled large-area annular cavity arrays with tunable cylindrical surface plasmons for sensing.

Surface plasmons that propagate along cylindrical metal/dielectric interfaces in annular apertures in metal films, called cylindrical surface plasmons (CSPs), exhibit attractive optical characteristics. However, it is challenging to fabricate these nanocoaxial structures. Here, we demonstrate a practical low-cost route to manufacture highly ordered, large-area annular cavity arrays (ACAs) that can support CSPs with great tunability. By employing a sol-gel coassembly method, reactive ion etching and metal sputtering techniques, regular, highly ordered ACAs in square-centimeter-scale with a gap width tunable in the range of several to hundreds of nanometers have been produced with good reproducibility. Ag ACAs with a gap width of 12 nm and a gap height of 635 nm are demonstrated. By finite-difference time-domain simulation, we confirm that the pronounced dips in the reflectance spectra of ACAs are attributable to CSP resonances excited in the annular gaps. By adjusting etching time and Ag film thickness, the CSP dips can be tuned to sweep the entire optical range of 360 to 1800 nm without changing sphere size, which makes them a promising candidate for forming integrated plasmonic sensing arrays. The high tunability of the CSP resonant frequencies together with strong electric field enhancement in the cavities make the ACAs promising candidates for surface plasmon sensors and SERS substrates, as, for example, they have been used in liquid refractive index (RI) sensing, demonstrating a sensitivity of 1505 nm/RIU and a figure of merit of 9. One of the CSP dips of ACAs with a certain geometry size is angle- (0-70 degrees) and polarization-independent and can be used as a narrow-band absorber. Furthermore, the nano annular cavity arrays can be used to construct solar cells, nanolasers and nanoparticle plasmonic tweezers.

Anomalous light absorption around subwavelength apertures in metal films.

In this Letter, we study the heat dissipated at metal surfaces by the electromagnetic field scattered by isolated subwavelength apertures in metal screens. In contrast to the common belief that the intensity of waves created by local sources should decrease with the distance from the sources, we reveal that the dissipated heat at the surface remains constant over a broad spatial interval. This behavior that occurs for noble metals at near infrared wavelengths is observed with nonintrusive thermoreflectance measurements and is explained with an analytical model, which underlines the intricate role played by quasicylindrical waves in the phenomenon. Additionally, we show that, by monitoring the phase of the quasicylindrical waves, the total heat dissipated at the metal surface can be rendered substantially smaller than the heat dissipated by the launched plasmon. This interesting property offers an alternative to amplification for overcoming the loss issue in miniaturized plasmonic devices.

Optical magnetic field mapping using a subwavelength aperture

Local distribution of the optical magnetic field is a critical parameter in developing materials with artificially engineered optical properties. Optical magnetic field characterization in nano-scale remains a challenge, because of the weak matter-optical magnetic field interactions. Here, we demonstrate an experimental visualization of the optical magnetic field profiles by raster scanning circular apertures in metal film or in a conical probe. Optical magnetic fields of surface plasmon polaritons and radially polarized beam are visualized by measuring the transmission through metallic apertures, in excellent agreements with theoretical predictions. Our results show that Bethe-Bouwkamp aperture can be used in visualizing optical magnetic field profiles.

A Subwavelength Extraordinary‐Optical‐Transmission Channel in Babinet Metamaterials

W.-C. Chen , N. I. Landy , K. Kempa , and W. J. Padilla * IO N The study of light coupling to small apertures in metallic fi lms has a long and illustrious history. In 1897 Lord Rayleigh treated the problem by using the concept of effective dipoles. Most recently, researchers have made these apertures periodic, where they exhibited so-called extraordinary optical transmission (EOT) – transmission effi ciencies far in excess of unity at wavelengths greater than the lattice parameter of the surface. This behavior is not predicted by standard aperture theory as developed by Bethe in 1944, [ 1 ] nor was it noticed before fabricational and measurement techniques had advanced to the point that EOT could be observed at optical wavelengths. However, in 1998 Ebbesen et al. [ 2 ] showed EOT in the near-infrared and posited that this stemmed from plasmon coupling between surfaces. Since then, EOT [ 2–5 ] has been studied with increasing detail in both theoretical and experimental capacities. This result led to a fl urry of intense theoretical and experimental research to probe the nature of this extraordinary optical transmission. Babinet metamaterials [ 6–8 ] may also be described as periodic apertures in metal fi lms. Metamaterials are structured periodic metallic patterns which enable the construction of materials with specifi ed electromagnetic properties, some of which cannot be obtained via naturally occurring materials. [ 9,10 ] Metamaterials exhibit electromagnetic resonances where the resonant wavelength is signifi cantly larger than the physical dimensions of the individual elements. Thus metamaterials are subwavelength media and well described by the optical constants e and μ . [ 11 ] In transmission metamaterials (scatterers) yield an absorptive like minimum feature at resonance, but are otherwise highly transmissive outside of this band. In contrast Babinet metamaterials (apertures)—made by taking the ‘inverse’ metamaterial structure—yield opposite behavior and exhibit a transmission maxima near resonance. [ 12,13 ]

Plasmonic quarter-wave plate

Here we present a strategy for designing wave plates utilizing resonances of subwavelength apertures in metallic films. Specifically, we show that it is possible to tune the geometry in a periodic array of cross-shaped apertures in a silver film to produce a quarter-wave plate at a particular wavelength in the near-infrared. This is achieved by introducing an asymmetry into the lengths of the arms of the crosses.

Effect of film thickness and dielectric environment on optical transmission through subwavelength holes

We present a detailed theoretical study for the spectral position of transmission resonances appearing in isolated subwavelength apertures in metallic films. We provide analytical expressions for the resonant wavelength as a function of the film thickness and the dielectrics surrounding (and filling) the system that are valid for hole shapes supporting large-cutoff wavelengths and for both isolated and periodically arranged holes. Our results are quantitatively valid in the microwave and terahertz regimes, but they also have qualitative validity in the optical regime. Our results show that for unfilled holes, in the limiting case when the hole is in a very thin film (metal thickness much smaller than the wavelength), the transmission resonance is controlled by a length scale related to the vanishing of the effective admittance of vacuum, as seen from the hole. On the contrary, for metal thicknesses larger than half the wavelength, the transmission resonance is controlled by the cutoff of the fundamental waveguide mode inside the hole. When thin films and high-index dielectrics are combined, the spectral location of the maximum transmission can be strongly redshifted compared to the cutoff wavelength of the apertures, and transmission intensity is substantially enhanced.

Template-stripped smooth Ag nanohole arrays with silica shells for surface plasmon resonance biosensing.

Inexpensive, reproducible, and high-throughput fabrication of nanometric apertures in metallic films can benefit many applications in plasmonics, sensing, spectroscopy, lithography, and imaging. Here we use template-stripping to pattern periodic nanohole arrays in optically thick, smooth Ag films with a silicon template made via nanoimprint lithography. Ag is a low-cost material with good optical properties, but it suffers from poor chemical stability and biocompatibility. However, a thin silica shell encapsulating our template-stripped Ag nanoholes facilitates biosensing applications by protecting the Ag from oxidation as well as providing a robust surface that can be readily modified with a variety of biomolecules using well-established silane chemistry. The thickness of the conformal silica shell can be precisely tuned by atomic layer deposition, and a 15 nm thick silica shell can effectively prevent fluorophore quenching. The Ag nanohole arrays with silica shells can also be bonded to polydimethylsiloxane (PDMS) microfluidic channels for fluorescence imaging, formation of supported lipid bilayers, and real-time, label-free SPR sensing. Additionally, the smooth surfaces of the template-stripped Ag films enhance refractive index sensitivity compared with as-deposited, rough Ag films. Because nearly centimeter-sized nanohole arrays can be produced inexpensively without using any additional lithography, etching, or lift-off, this method can facilitate widespread applications of metallic nanohole arrays for plasmonics and biosensing.

Optical transmission of periodic annular apertures in metal film on high-refractive index substrate: The role of the nanopillar shape

The influence of annular aperture parameters on the optical transmission through arrays of coaxial apertures in a metal film on high refractive index substrates has been investigated experimentally and numerically. It is shown that the transmission resonances are related to plasmonic crystal effects rather than frequency cutoff behavior associated with annular apertures. The role of deviations from ideal aperture shape occurring during the fabrication process has also been studied. Annular aperture arrays are often considered in many applications for achieving high optical transmission through metal films and understanding of nanofabrication tolerances are important.

Light passing through subwavelength apertures

This review provides a perspective on the recent developments in the transmission of light through subwavelength apertures in metal films. The main focus is on the phenomenon of extraordinary optical transmission in periodic hole arrays, discovered over a decade ago. It is shown that surface electromagnetic modes play a key role in the emergence of the resonant transmission. These modes are also shown to be at the root of both the enhanced transmission and beaming of light found in single apertures surrounded by periodic corrugations. This review describes both the theoretical and experimental aspects of the subject. For clarity, the physical mechanisms operating in the different structures considered are analyzed within a common theoretical framework. Several applications based on the transmission properties of subwavelength apertures are also addressed.

Light trapping cavity enhanced light transmission through a single sub-wavelength aperture in a metal film

We demonstrate that optical transmission of a normally incident, monochromatic plane wave through a single sub-wavelength aperture in an opaque metal film can be substantially enhanced by a thin, semitransparent metal film placed parallel to the opaque metal film in front of the aperture. When the semi-transparent and the opaque metal film are separated by a proper distance, a light trapping cavity is formed and the sub-wavelength aperture exhibits a transmission maximum. An enhancement factor of approximately 40 is demonstrated for a cylindrical 100 nm diameter hole in a silver film.

Accurate simulation of light transmission through subwavelength apertures in metal films

We use the finite-element method to simulate light transmission through a 2D periodic array of rectangular apertures in a film of highly conductive material. We report results with a relative error of transmissivity lower than 0.01%. This is an improvement of about one order of magnitude compared to previously reported results. Further, the influence of corner and edge roundings on light transmission through the subwavelength apertures is investigated.

Active control and spatial mapping of mid-infrared propagating surface plasmons

Periodic arrays of subwavelength apertures in metal films have been shown to exhibit strongly enhanced transmission at wavelengths determined by the periodicity of the film as well as the optical properties of the metal and surrounding dielectric material. Here we investigate the coupling between such a grating and a Quantum Cascade Laser. By actively tuning the optical properties of our grating, we control the coupling of laser light to the plasmonic structure, switching our grating from a predominantly transmitting state to a state that allows coupling to propagating surface waves, which can then be imaged on the metallic surface.

Loss mechanisms in mid-infrared extraordinary optical transmission gratings

The optical properties of periodic arrays of subwavelength apertures in metal films on GaAs substrates are studied. Specifically, geometric and material losses for these plasmonic structures are characterized using angular dependent transmission, normal incidence reflection, and angular dependent diffraction experiments, in addition to a crossed-polarizer transmission experiment. The optical properties of the samples as a function of engineered material losses are studied. Using this comprehensive approach to the characterization of the plasmonic structures, we are able to identify and isolate specific loss mechanisms, as well as identify the effect of free carriers on the optical properties of the structures.

Optical transmission through subwavelength slit apertures in metallic films

Transmission of polarized light through subwavelength slit apertures is studied in visible and near infrared range wavelengths. We examine the roles played by the slit apertures, such as length, depth, period and number of slits. The experiment results, including dispersion curves, demonstrate among other things that the surface plasmon polariton and Fabry–Perot waveguide modes collectively dictate the transmission properties of subwavelength slit arrays and that as they approach each other, not only large gaps are formed but also mode interconversion occurs. These findings are discussed and compared to theoretical predictions.