Plasmonic nanoheaters, have attracted great attention in nanomedicine, due to their ability to efficiently generate and control heat delivery at the nanoscale. There is a wide offer of nanostructures that provide with strong photothermal responses and heat delivery, however most of them deliver heat in a symmetric way or require a certain orientation with respect to the excitation source to perform adequately. Here, we will first present an overview of some novel designs, fully plasmonic or hybrid dielectric/plasmonic, capable of offering not only enhanced photothermal response but also directional heat delivery. The thermal enhancing mechanism rely on the excitation of either dipolar or anapolar modes, depending on the design [4]. In the rest of the talk, we will show some of our recent findings on the thermal performance of DNA origami-based structures, offering theoretical insights into their potential for photothermal therapy applications.
From plasmonics with pure coinage metallic materials to the current advances in phase-change materials (PCM), research in nanophotonics has rapidly evolved, pushed by its wide gamut of applications. In this contribution, we will present three phase-change material examples: MoOx, Ga2S3 and GaS as promising candidates for reconfigurable plasmonic applications with fast and low-loss response. The first because modification of its oxygen stoichiometry (2< ×<3) induce a phase change in the VIS region, suitable for developing reflective pixels for display applications. The other two, because they show phase transitions due to changes in their lattice configurations, making them attractive for new broadband devices for switching and photodetection applications. They will be characterized and studied by resorting to exact DFT calculations. Also, their plasmonic response as well as their possible plasmonic coupling effects will be analyzed by considering the different envisaged applications.
An overview on our latest research on UV plasmonics with Rh and Ga metal nanostructures is presented. We will pay attention to their plasmonic performance and UV tunability. For Ga, its polymorphism will be analyzed and for Rh three characteristic geometries will be studied: tripod star, nanocube and tetrahedron. As an alternative to metals, low heat generation materials for bio applications will be analyzed. A numerical analysis of several candidate low loss dielectric materials that show HRI properties in the UV will be presented. In particular, this analysis will focus on the near-field enhancement and scattering directionality above 3 eV.
Nanoparticles made of High Refractive Index (HRI) dielectric materials, such as Si, GaP, Ge or other semiconductor compounds have been proposed recently as an alternative to metals, driven by their low-losses and presence of magnetic response in spite of being non-magnetic materials. However, they are known to suffer relatively large losses and absence of magnetic response at optical frequencies. Here, we intend to show a brief overview of our recent research in light scattering by HRI dielectric nanostructures. In particular, we will show how the strong confinement of electromagnetic energy and the outstanding scattering efficiencies of these HRI dielectric structures make them promising candidates to act as basic units for the design of the next generation of nanoantennas that may be able to boost applications such as sensing, light directivity, optical switching, surface enhanced spectroscopies, all-dielectric metamaterials, or non linear phenomena, such as third harmonic generation
Subwavelength metallic nanoparticles have been proposed for optimizing efficiency of current solar cells. However, their inherent ohmic losses limit their performance. High Refractive Index dielectric particles have been suggested as an alternative to metallic ones due to their low-losses in the visible and near infrared spectral regions and also to their magnetic response. The directionality properties that arise from the coherence effects between electric and magnetic resonances, make them very attractive for redirecting the incident radiation. In this work, we analyze the Scattering Directionality Conditions of a symmetric dimer made of High Refractive Index dielectric particles as a function of the gap. We demonstrate that, by using a dimer, it is possible to find, in the dipolar regime, two spectral regions where the incident radiation is redirected in the forward direction. They correspond to the Zero-Backward condition (also observed for isolated particles) and to a “near Zero-Backward” condition. The last would correspond to a “rotation” of the near Zero-Forward condition as a consequence of the interaction effects between the dimer components. The proposed scattering unit could constitute a new block for building more complex systems for applications in optical communications, light guiding and solar energy harvesting devices.
Plasmonics in the UV-range constitutes a new focus of research due to new challenges arising in fields such as biology, chemistry or spectroscopy. Very recent studies point out gallium and rhodium as good candidates for these purposes because of their low oxidation tendency and at the same time, having a good plasmonic response in the UV and excellent photocatalytic properties. Here we present an overview of the current state of UV-plasmonics with our latest findings in the plasmonic activity of materials like gallium and rhodium.
The use of nanoparticles is spreading in many fields and a frequent way of preparing them is in the form of colloids, whose characterization becomes increasingly important. The spectral reflectance and transmittance curves of such colloids exhibit a strong dependence with the main parameters of the system. By means of a two-flux model we have performed a colorimetric study of gold colloids varying several parameters of the system, including the radius of the particles, the particle number density, the thickness of the system and the refractive index of the surrounding medium. In all cases, trajectories in the L*a*b* color space have been obtained, as well as the evolution of the luminosity, chroma and hue, either for reflectance or transmittance. The observed colors agree well with typical colors found in the literature for colloidal gold, and could allow for a fast assessment of the parameters involved, e.g., the radius of the nanoparticle during the fabrication process.
A new method for calculating the effective dielectric function of a metal-oxide core-shell nanoparticle is presented and compared with existing theories. This new approach can be helpful for predicting the reflectance, transmittance and absorbance spectra of core-shell colloids and nanocomposites which are widely used in photocatalysis or solar energy harvesting.
We analyze the effect of contaminants on the quadrupolar magnetic, dipolar electric and dipolar magnetic resonances of silicon nanoparticles (NPs) by considering the spectral evolution of the linear polarization degree at right angle scattering configuration, PL(90°). From an optical point of view, a decrease in the purity of silicon nanoparticles due to the presence of contaminants impacts the NP effective refractive index. We study this effect for a silicon nanosphere of radius 200 nm embedded in different media. The weakness of the resonances induced on the PL(90°) spectrum because of the lack of purity can be used to quantify the contamination of the material. In addition, it is shown that Kerker conditions also suffer from a spectral shift, which is quantified as a function of material purity.
X. Zhang, Y. Gutiérrez, P. Li, Á. Barreda, A. Watson, R. Alcaraz de la Osa, G. Finkelstein, F. González, D. Ortiz, J. Saiz, J. Sanz, H. Everitt, J. Liu, F. Moreno
Plasmonics in the UV-range constitutes a new challenge due to the increasing demand to detect, identify and destroy biological toxins, enhance biological imaging, and characterize semiconductor devices at the nanometer scale. Silver and aluminum have an efficient plasmonic performance in the near UV region, but oxidation reduces its performance in this range. Recent studies point out rhodium as one of the most promising metals for this purpose: it has a good plasmonic response in the UV and, as gold in the visible, it presents a low tendency to oxidation. Moreover, its easy fabrication through chemical means and its potential for photocatalytic applications, makes this material very attractive for building plasmonic tools in the UV. In this work, we will show an overview of our recent collaborative research with rhodium nanocubes (NC) for Plasmonics in the UV.
The spectral evolution of the degree of linear polarization (PL) at a scattering angle of 90° is studied numerically for high refractive index (HRI) dielectric spherical nanoparticles. The behaviour of PL(90°) is analyzed as a function of the refractive index of the surrounding medium and the particle radius. We focus on the spectral region where both electric and magnetic resonances of order not higher than two are located for various semiconductor materials with low absorption. The spectral behavior of PL(90°) has only a small, linear dependence on nanoparticle size R. This weak dependence makes it experimentally feasible to perform real-time retrievals of both the refractive index of the external medium and the NP size R. From an industrial point of view, pure materials are nonrealistic, since they can only be provided under certain conditions. For this reason, we also study the effect of contaminants on the resonances of silicon NPs by considering the spectral evolution of PL(90°).
Recent studies show that the spectral behaviour of localized surface plasmon resonances (LPSRs) in metallic nanoparticles suffer from both a redshift and a broadening in the transition from the far- to the near-field regimes. An interpretation of this effect was given in terms of the evanescent and propagating components of the angular spectrum representation of the radiated field. Due to the increasing interest awakened by magnetodielectric materials as a both low-loss material option for nanotechnology applications, and also for their particular scattering properties, here we study the spectral response of a magnetodielectric nanoparticle as a basic element of a dielectric nano-antenna. This study is made by analyzing the changes suffered by the scattered electromagnetic field when propagating from the surface of this dielectric nanostructure to the far-zone in terms of propagating and evanescent plane wave components of the radiated fields.
Laser transmission welding (LTW) of thermoplastics is a direct bonding technique already used in different industrial applications sectors such as automobiles, microfluidics, electronics, and biomedicine. LTW evolves localized heating at the interface of two pieces of plastic to be joined. One of the plastic pieces needs to be optically transparent to the laser radiation whereas the other part has to be absorbent, being that the radiation produced by high power diode lasers is a good alternative for this process. As consequence, a tailored laser system has been designed and developed to obtain high quality weld seams with weld widths between 0.7 and 1.4 mm. The developed laser system consists of two diode laser bars (50 W per bar) coupled into an optical fiber using a nonimaging solution: equalization of the beam parameter product (BPP) in the slow and fast axes by a pair of step-mirrors. The power scaling was carried out by means of a multiplexing polarization technique. The analysis of energy balance and beam quality was performed considering ray tracing simulation (ZEMAX®) and experimental validation. The welding experiments were conducted on acrylonitrile/butadiene/styrene (ABS), a thermoplastic frequently used in automotive, electronics and aircraft applications, doped with two different concentrations of carbon nanotubes (0.01% and 0.05% CNTs). Quality of the weld seams on ABS was analyzed in terms of the process parameters (welding speed, laser power and clamping pressure) by visual and optical microscope inspections. Mechanical properties of weld seams were analyzed by mechanical shear tests. High quality weld seams were produced in ABS, revealing the potential of the laser developed in this work for a wide range of plastic welding applications.
Laser transmission welding (LTW) of polymers is a direct bonding technique which is already used in different
industrial applications sectors such as automobile, microfluidic, electronic and biomedicine. This technique offers
several advantages over conventional methods, especially when a local deposition of energy and minimum thermal
distortions are required. In LTW one of the polymeric materials needs to be transparent to the laser wavelength and the
second part needs to be designed to be absorbed in IR spectrum. This report presents a study of laser weldability of ABS
(acrylonitrile/butadiene/styrene) filled with two different concentrations of carbon nanotubes (0.01% and 0.05% CNTs).
These additives are used as infrared absorbing components in the laser welding process, affecting the thermal and optical
properties of the material and, hence, the final quality of the weld seam.
A tailored laser system has been designed to obtain high quality weld seams with widths between 0.4 and 1.0mm. It
consists of two diode laser bars (50W per bar) coupled into an optical fiber using a non-imaging solution: equalization of
the beam quality factor (M2) in the slow and fast axes by a pair of micro step-mirrors. The beam quality factor has been
analyzed at different laser powers with the aim to guarantee a coupling efficiency to the multimode optical fiber. The
power scaling is carried out by means of multiplexing polarization technique. The analysis of energy balance and beam
quality is performed in two linked steps: first by means ray tracing simulations (ZEMAX®) and second, by validation.
Quality of the weld seams is analyzed in terms of the process parameters (welding speed, laser power and clamping
pressure) by visual and optical microscope inspections. The optimum laser power range for three different welding
speeds is determinate meanwhile the clamping pressure is held constant. Additionally, the corresponding mechanical
shear tests were carried out to analyze the mechanical properties of the weld seams. This work provides a detailed study
concerning the effect of the material microstructure and laser beam quality on the final weld formation and surface
integrity.
The coherent combination of electric and magnetic responses is the basis of the electromagnetic behavior of new engineered metamaterials. The basic constituents of their meta-atoms usually have metallic character and consequently high absorption losses. Based on standard "Mie" scattering theory, we found that there is a wide window in the near-infrared (wavelengths 1 to 3 μm), where light scattering by lossless submicrometer Ge spherical particles is fully described by their induced electric and magnetic dipoles. The interference between electric and magnetic dipolar fields is shown to lead to anisotropic angular distributions of scattered intensity, including zero backward and almost zero forward scattered intensities at specific wavelengths, which until recently was theoretically established only for hypothetically postulated magnetodielectric spheres. Although the scattering cross section at zero backward or forward scattering is exactly the same, radiation pressure forces are a factor of 3 higher in the zero forward condition.
A set of light-scattering results is presented in the form of Mueller Matrices (MM) and their corresponding Polar
Decomposition (PD) parameters. The system under analysis is a square microstructure on a flat substrate, in the form of
either a rib or a groove (or several equally spaced, depending on the experiment). As it is well known, MM contains all
information, and many works have been carried out trying to connect its properties with those of the scattering system.
However, this is not as intuitive as the analysis allowed by other presentations of the results, based on the decomposition
of MM matrix in a set of matrices, each representing the action of a particular (non-real) element, and acting sequentially
on the incident beam. Our analysis is a quite conventional application of the Polar Decomposition. The resulting
parameters reveal, for instance, that the substrate plays an important role in the origin of the depolarization. Concerning
the polar components the main analysis is performed by means of the conventional diattenuation and retardance
parameters. The number and position of the discontinuities in the retardation parameter is associated to the size of the
defect. This, of course, can be also concluded from the observation of m00 element oscillations, but in the case of the PD
retardation parameter it is possible to connect the geometrical shape of the scattering element (rib or groove) to a single
condition established for the PD parameters.
This paper presents the design of an efficient collimating optical system for an extended light source, namely a highbrightness
high divergence light emitting diode (LED), sized 1x1mm, and viewing angle of 130°. The design lies in a
catadioptric rotationally symmetrical system, which modeling and optimization has been done by specific optical design
software, ZEMAX®, and its development was based on geometrical principles. The device consists of two optical
systems, one for the rays emerging from the source with low numerical apertures (NA<0.26) and another one for those
emerging with NA>0.26. The system for rays with low NA consists of an aspherical lens system which parameters are
optimized by means of standard criterion for collimation. The system for high NA rays is a combination of a hyperbolic
and a parabolic mirror, being the first one the only surface shared by both system (refractive near-axis, reflective offaxis).
The result of this work is a system that reaches a collection efficiency of 80% of the LED emitted light. Moreover, the
beam collimation quality has been analyzed obtaining a residual divergence of less than 2°. Thus, the results achieved by
the proposed optical system improve those obtained with several commercially available devices and other previously
proposed systems.
Spherical particles on flat surfaces produce light scattering patterns with a characteristic peak in the backscattering direction, provided that the size polydispersity is sufficiently high. This is shown for metallic particles on conducting substrates by means of a double interaction model, i.e. an approximate solution of the scattering problem. The results contain useful information about the dependence of this peak with the degree of polydispersity, the mean size of the particles, or the angle of incidence.
We have developed an experimental light-scattering method to obtain information about particles with low polydispersities in size on flat substrates. It is based on the analysis of the visibility factor of the lobes in the light scattering patterns obtained from flat metallic substrates seeded with the particles. The visibility factor of a pattern is obtained for different minima. The solution of the scattering problem may be provided by a theoretical model, and analytical expressions for the visibility are derived. This relation between visibility and polydispersity is experimentally tested, and it is shown how the origin of the loss of visibility may be exploited to characterize the polydispersity.
A fast and accurate method is introduced and analyzed for size determination of metallic particles on flat substrates. It is based on the measurement of the minima angular positions of the S-polarized far field scattering patterns at normal incidence. The method has been theoretically and experimentally checked for both cylindrical and spherical protuberance geometry on conducting flat substrates.
An experimental analysis has been made of the light scattering from particulate surfaces as a function of the surface particle density. The samples were composed of a conducting flat substrate seeded with spherical metallic particles whose radius is on the order of the wavelength of the incident radiation (λ = 0.633 μm). We analyze how the multiple-scattering effect evolves as the separation between particles changes. From a theoretical point of view, we have applied a ray-tracing method to obtain the scattered intensities. These were compared with the experimental results. Good basic agreement was found, especially in the backscattering region.
The copolarized backscattered intensity from surfaces composed of metallic particles on conducting flat substrates is analyzed experimentally as a function of the incidence angle. The analysis is done for particle sizes smaller than, comparable to, and larger than the incident wavelength (0.633 μm) and for low particle surface densities. Numerical calculations based on the extinction theorem for a onedimensional surface model consisting of an infinitely long cylinder located on a flat substrate for the same optical constants used in the experiment are also presented for qualitative comparison with the experimental results. This serves to analyze the effect of particle aggregation. For the surfaces with particles smaller than the incident wavelength, conclusions are drawn concerning the possible relevance of this study in radar wave scattering from the sea surface.
Light scattering from a set of samples constitued by spherical metallic particles on a conducting flat substrate is analyzed for the co- and cross-polarized case and for several surface particle densities. We show how the incident beam is depolarized due to multiple scattering effects arising from both the sphere-substrate and sphere-sphere interactions.
KEYWORDS: Luminescence, Lab on a chip, Computer simulations, Fourier transforms, Photon counting, Solid state physics, Information operations, Crystals, Solids, Dispersion
A simple experimental technique for measuring the lifetime of long-lived excited levels for low intensity monoexponential fluorescence signals is presented. It is based on the measurement of the imaginary part of the Fourier transform of the probability density function of the time of arrival of the first photon after the excitation. Owing to its theoretical and practical contents (photon counting techniques, lifetime measurement techniques), this experimental procedure may constitute a suitable practice for undergraduate students in Optics and Solid State Physics.
An alternative technique is provided to the measurement of second order correlation function g(2)(t) for low intensity scattering experiments It is based on the measurement of the Lap1e Trasform (Li'') of the time interval probability between successive photoelectrons(Q(s)). It is applied to the analysis ofboth sphere and rod solutions measuring the polarized and depolarized component of the scattered light respectively.
A simple Fourier transform method for processing the time intervals recorded in a single photon decay spectroscopy (SPDS) experiment has been developed for fluorescence signals containing more than one decay constant (multiexponential signals). The main advantage of the proposed method consists in showing clearly the existence of each of the components which constitute the decaying fluorescence. This advantage increases when we are measuring relatively long decay constants (> 1 μs) and the mean fluorescence intensity is low, since in these cases the conventional technique of SPDS requires rather long data acquisition times. A comparison between the conventional technique and the sine transform method has been made for a great variety of experimental situations by resorting to a computer simulation for fluorescence signals containing two or three decaying constants.
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