Neural network assisted design of plasmonic nanostructures on superconducting transition-edge-sensors for single photon detectors.

Transition edge sensors (TESs) are extremely sensitive thermometers made of superconducting materials operating at their transition temperature, where small variations in temperature give rise to a measurable increase in electrical resistance. Coupled to suitable absorbers, they are used as radiation detectors with very good energy resolution in several experiments. Particularly interesting are the applications that TESs may bring to single photon detection in the visible and infrared regimes. In this work, we propose a method to enhance absorption efficiency at these wavelengths. The operation principle exploits the generation of highly absorbing plasmons on the metallic surface. Following this approach, we report nanostructures featuring theoretical values of absorption reaching 98%, at the telecom design frequency (λ = 1550 nm). The optimization process takes into account the TES requirements in terms of heat capacity, critical temperature and energy resolution leading to a promising design for an operating device. Neural networks were first trained and then used as solvers of the optical properties of the nanostructures. The neural network topology takes the geometrical parameters, the properties of materials and the wavelength of light as input, predicting the absorption spectrum at single wavelength as output. The incorporation of the material properties and the dependence with frequency was crucial to reduce the number of required spectra for training. The results are almost indistinguishable from those calculated with a commonly used numerical method in computational electromagnetism, the finite-difference time-domain algorithm, but up to 106 times faster than the numerical simulation.

Amplification of stimulated light emission in arrays of nanoholes by plasmonic absorption-induced transparency.

Absorption induced transparency is an optical phenomenon that occurs in plasmonic nanostructures when materials featuring narrow lines in their absorption spectra are deposited on top of it. First reported in the visible range for metallic arrays of nanoholes, using dye lasers as covering, it has been described as transmission peaks unexpectedly close to the absorption energies of the dye. In this work, amplification of stimulated light emission is numerically demonstrated in the active regime of absorption induced transparency. Amplification can be achieved in the regime where the dye laser behaves as a gain material. Intense illumination can modify the dielectric constant of the gain material in a short span of time and thus the propagation properties of the plasmonic modes excited in the hole arrays, providing both less damping to light and further optical feedback that enhances the stimulated emission process.

Attomolar SERS detection of organophosphorous pesticides using silver mirror-like micro-pyramids as active substrate.

Surface-enhanced Raman spectroscopy (SERS) is gaining importance as an ultrasensitive analytical tool for routine high-throughput analysis of a variety of molecular compounds. One of the main challenges is the development of robust, reproducible and cost-effective SERS substrates. In this work, we study the SERS activity of 3D silver mirror-like micro-pyramid structures extended in the z-direction up to 3.7 µm (G0 type substrate) or 7.7 µm (G1 type substrate), prepared by Si-based microfabrication technologies, for trace detection of organophosphorous pesticides, using paraoxon-methyl as probe molecule. The average relative standard deviation (RSD) for the SERS intensity of the peak displayed at 1338 cm-1 recorded over a centimetre scale area of the substrate is below 13% for pesticide concentrations in the range 10-6 to 10-15 mol L-1. This data underlies the spatial uniformity of the SERS response provided by the microfabrication approach. According to finite-difference time-domain (FDTD) simulations, such remarkable feature is mainly due to the contribution on electromagnetic field enhancement of edge plasmon polaritons (EPPs), propagating along the pyramid edges where the pesticide molecules are preferentially adsorbed. Graphical abstract.

Absorption-induced transparency metamaterials in the terahertz regime.

Contrary to what might be expected, when an organic dye is sputtered onto an opaque holey metal film, transmission bands can be observed at the absorption energies of the molecules. This phenomenon, known as absorption-induced transparency, is aided by a strong modification of the propagation properties of light inside the holes when filled by the molecules. Despite having been initially observed in metallic structures in the optical regime, new routes for investigation and applications at different spectral regimes can be devised. Here, to illustrate the potential use of absorption-induced transparency at terahertz, a method for molecular detection is presented and supported by a theoretical analysis.

Coherent control of light scattering from nanostructured materials by second-harmonic generation.

We introduce an active, all-optical method for controlling the intensity and directionality of light scattering from single nanostructures. The method is based on the coherent interplay between linear light scattering and second-harmonic generation. The intensity and directionality of scattered light can be controlled by the phase delay and the relative angle between excitation beams. We discuss the principle of this coherent control technique and perform numerical model calculations.

Mechanisms for extraordinary optical transmission through bull's eye structures.

We analyze both experimentally and theoretically the physical mechanisms that determine the optical transmission through deep sub-wavelength bull's eye structures (concentric annular grooves surrounding a circular hole). Our analysis focus on the transmission resonance as a function of the distance between the central hole and its nearest groove. We find that, for that resonance, each groove behaves almost independently, acting as an optical cavity that couples to incident radiation, and reflecting the surface plasmons radiated by the other side of the same cavity. It is the constructive contribution at the central hole of these standing waves emitted by independent grooves which ends up enhancing transmission. Also for each groove the coupling and reflection coefficients for surface plasmons are incorporated into a phenomenological Huygens-Fresnel model that gathers the main mechanisms to enhance transmission. Additionally, it is shown that the system presents a collective resonance in the electric field that does not lead to resonant transmission, because the fields radiated by the grooves do not interfere constructively at the central hole.

Optimization of bull's eye structures for transmission enhancement.

We present an exhaustive exploration of the parameter space defining the optical properties of a bull's eye structure, both experimentally and theoretically. By studying the resonance intensity variations associated with the different geometrical features, several parameters are seen to be interlinked and scale laws emerge. From the results it is possible to give a simple recipe to design a bull's eye structure with optimal transmission properties.

Holes with very acute angles: a new paradigm of extraordinary optical transmission through strongly localized modes.

It is shown that submicrometer holes with very acute angles present extraordinary optical transmission peaks associated to strongly localized modes. The positions of these peaks are: (i) strongly redshifted with respect to the peak position that could be expected if the considered hole were in a film made of perfect electric conductor, (ii) independent on the angle of incidence for a large range of angles and (iii) strongly dependent on the direction of the incident electric field. In addition, it is demonstrated that these properties are linked to the mechanisms leading to the existence of channel-plasmon-polaritons.

Optical switching in metal-slit arrays on nonlinear dielectric substrates.

We propose a scheme for an optical limiter and switch of the transmitted light intensity in an array of subwavelength metallic slits placed on a nonlinear Kerr-type dielectric substrate of finite thickness, where the geometrical parameters are designed for operation at telecom wavelengths. Our approach is based on the abrupt changes of the output light intensity observed in these systems near transmission minima.

Extraordinary transmission through metal-coated monolayers of microspheres.

The spectral dependence of the extraordinary transmission through monolayers of close-packed silica or polystyrene microspheres on a quartz support, covered with different thin metal films (Ag, Au and Ni) was investigated. The measured spectra were compared with modeled transmission spectra using finite difference time domain (FDTD) calculations. Measured and modeled spectra show good overall agreement. The supported modes in the sphere array were found to be of utmost importance for the transmission mechanism and the results also suggest that the presence of guided modes in the photonic crystal may further enhance the extraordinary transmission through the metal film.

Triangular metal wedges for subwavelength plasmon-polariton guiding at telecom wavelengths.

We report on subwavelength plasmon-polariton guiding by triangular metal wedges at telecom wavelengths. A high-quality fabrication procedure for making gold wedge waveguides, which is also mass-production compatible offering large-scale parallel fabrication of plasmonic components, is developed. Using scanning near-field optical imaging at the wavelengths in the range of 1.43-1.52 microm, we demonstrate low-loss (propagation length approximately 120 microm) and well-confined (mode width congruent with 1.3 microm) wedge plasmon-polariton guiding along triangular 6-microm-high and 70.5 degree-angle gold wedges. Experimental observations are consistent with numerical simulations performed with the multiple multipole and finite difference time domain methods.

Waveguide and Plasmonic Absorption-Induced Transparency.

Absorption-induced transparency (AIT) is one of the family of induced transparencies that has emerged in recent decades in the fields of plasmonics and metamaterials. It is a seemingly paradoxical phenomenon in which transmission through nanoholes in gold and silver is dramatically enhanced at wavelengths where a physisorbed dye layer absorbs strongly. The origin of AIT remains controversial, with both experimental and theoretical work pointing to either surface (plasmonic) or in-hole (waveguide) mechanisms. Here, we resolve this controversy by carefully filling nanoholes in a silver film with dielectric material before depositing dye on the surface. Our experiments and modeling show that not only do plasmonic and waveguide contributions to AIT both exist, but they are spectrally identical, operating in concert when the dye is both in the holes and on the surface.

Highly reproducible near-field optical imaging with sub-20-nm resolution based on template-stripped gold pyramids.

With a template-stripping fabrication technique, we demonstrate the mass fabrication of high-quality, uniform, ultrasharp (10 nm) metallic probes suitable for single-molecule fluorescence imaging, tip-enhanced Raman spectroscopy (TERS), and other near-field imaging techniques. We achieve reproducible single-molecule imaging with sub-20-nm spatial resolution and an enhancement in the detected fluorescence signal of up to 200. Similar results are obtained for TERS imaging of carbon nanotubes. We show that the large apex angle (70.5°) of our pyramidal tip is well suited to scatter the near-field optical signal into the far-field, leading to larger emission enhancement and hence to a larger quantum yield. Each gold or silver pyramidal probe is used on-demand, one at a time, and the unused tips can be stored for extended times without degradation or contamination. The high yield (>95%), reproducibility, durability, and massively parallel fabrication (1.5 million identical probes over a wafer) of the probes hold promise for reliable optical sensing and detection and for cementing near-field optical imaging and spectroscopy as a routine characterization technique.

Extraordinary optical transmission through hole arrays in optically thin metal films.

A theoretical study is presented on the optical transmission through square hole arrays drilled in optically thin films, where transmission may occur through both the holes and the metal layer. It is shown that, as the thickness of the metal film decreases, the coupling of light with short-range surface plasmons redshifts the extraordinary optical transmission peak to longer wavelengths. At the same time, the maximum-to-minimum transmittance ratio is kept high even for metal thicknesses as small as one skin depth.

Nanofocusing with channel plasmon polaritons.

We investigate radiation nanofocusing with channel plasmon polaritons (CPPs) propagating along subwavelength metal grooves that are tapered synchronously in depth and in width. Efficient CPP nanofocusing at telecom wavelengths with the estimated field intensity enhancement of up to approximately 90 is directly demonstrated using near-field microscopy. Experimental observations are concurred with electromagnetic simulations, predicting the possibility of reaching the intensity enhancements of approximately 1200 and opening thereby exciting perspectives for practical applications of CPP nanofocusing.

Guiding and focusing of electromagnetic fields with wedge plasmon polaritons.

We study theoretically electromagnetic modes guided by metallic wedges at telecom wavelengths. These modes are found to exhibit superior confinement while showing similar propagation loss as compared to other subwavelength guiding configurations. It is also shown that mode focusing can be realized by gradual modification of the wedge geometry along the mode propagation direction.

Theory of negative-refractive-index response of double-fishnet structures.

A theory is presented of the negative refractive index observed in the so-called double-fishnet structures. We find that the electrical response of these structures is dominated by the cutoff frequency of the hole waveguide whereas the resonant magnetic response is due to the excitation of gap surface plasmon polaritons propagating along the dielectric slab. Associated with this origin, we show how the negative refractive index in these metamaterials presents strong dispersion with the parallel momentum of the incident light.

Channel plasmon-polaritons: modal shape, dispersion, and losses.

We theoretically study channel plasmon-polaritons (CPPs) with a geometry similar to that in recent experiments at telecommunication wavelengths [Bozhevolnyi et al., Nature 440, 508 (2006)]. The CPP modal shape, dispersion relation, and losses are simulated by using the multiple multipole method and the finite difference time domain technique. It is shown that, with an increase of the wavelength, the fundamental CPP mode shifts progressively toward the groove opening, ceasing to be guided at the groove bottom and becoming hybridized with wedge plasmon-polaritons running along the groove edges.