S. P. A. Osorio

Localized surface plasmon resonance interaction with Er3+-doped tellurite glass

We show the annealing effect on silver and Erbium-doped tellurite glasses in the formation of nanoparticles (NPs) of silver, produced by the reduction of silver (Ag+ → Ag0), aiming to an fluorescence enhancement. The absorption spectra show typical Localized Surface Plasmon Resonance (LSPR) band of Ag0 NP in addition to the distinctive absorption peaks of Er3+ ions. Both observations demonstrate that the photoluminescence enhancement is due to the coupling of dipoles formed by NPs with the Er3+ 4I(13/2) → 4I(15/2) transition. This plasmon energy transfer to the Er3+ ions was observed in the fluorescence spectrum with a blue-shift of the peaks.

Influence of film thickness on the optical transmission through subwavelength single slits in metallic thin films

Silver and gold films with thicknesses in the range of 120-450 nm were evaporated onto glass substrates. A sequence of slits with widths varying between 70 and 270 nm was milled in the films using a focused gallium ion beam. We have undertaken high-resolution measurements of the optical transmission through the single slits with 488.0 nm (for Ag) and 632.8 nm (for Au) laser sources aligned to the optical axis of a microscope. Based on the present experimental results, it was possible to observe that (1) the slit transmission is notably affected by the film thickness, which presents a damped oscillatory behavior as the thickness is augmented, and (2) the transmission increases linearly with increasing slit width for a fixed film thickness.

Optical gain medium for plasmonic devices

Metallic nanostructures upon resonant excitation can enhance the local electric field, which could be increased, if these nanostructures are embedded in a gain medium. In this sense, we propose a gain medium as a candidate for the development of nanowaveguide amplifier based on Er3+-doped tellurite glass with embedded silver nanoparticles (NPs). Those glasses are characteristic for their amplifying response in the telecommunication window, and when we embedded metallic NPs modified the crystalline potential surrounding to the Er3+ ions, due to an electric coupling between NP (received/emitter) and Er3+ ion (emitter) enhancement luminescence intensity from the 4I13/2→4I15/2 transition radiative of the Er3+ ions. Besides, the presence these NPs changes the refraction index these glass modifying the complex parameter β(neff) increasing the polarizability of the samples f(Ɛd). Therefore, a gain medium – Er3+ ions (amplification function in the telecommunication band) with silver NPs (luminescence enhancement) – can increment the propagation length of light into nanowaveguide.

Focusing surface plasmons on Er3+ions with convex/concave plasmonic lenses

Plasmonic lenses consisting of convex/concave concentric rings with different periods were milled with a Focused Gallium Ion Beam on a gold thin film deposited onto an Er3+-doped tellurite glass. The plasmonic lenses were vertically illuminated with an Argon Ion laser (488 nm) highly focused by means of a 20x objective lens. The focusing mechanism of the plasmonic lenses is explained by using a simple coherent interference model of surface plasmon-polariton generation on the circular grating as a result of the incident field. Particularly, this beam focusing structure has a modulated groove depth (concave/convex). As a result, phase modulation can be accomplished by the groove depth profile, similarly to a nano-slit array with different thicknesses. This focusing allows a high confinement of SPPs which excited the Er3+ ions of the substrate. The luminescence spectrum of Er3+ ions was then measured in the far-field, where we could verify the excitation yield of the plasmonic lens on the Er3+ ions. We analyze the influence of physical and geometrical parameters on the emission spectra, such as the periodicity and depth profile of the rings. The variation of these parameters resulted in considerable changes of the luminescence spectra.

Luminescence enhancement of Er3+ ions from electric multipole nanostructure arrays

Periodic nanostructure arrays forming electric dipoles or quadrupoles were fabricated with a Focused Gallium Ion Beam on a gold thin film deposited onto an Er3+-doped tellurite glass. The nanostructures were vertically illuminated with an Argon Ion laser (488 nm). The Er3+ luminescence spectrum was then measured in the far-field. The observed luminescence is elucidated considering the following effects: (i) excitation of the Er3+ ions by means of the localized surface plasmon resonance from the electric dipole/quadrupole nanostructures, that produce an improvement of the local field, resulting in an enhanced luminescence, and (ii) the Er3+ luminescence spectrum depends on the albedo of the system (electric dipole/quadrupole arrays), for which its resonant properties is strongly affected. In this way, the emission of the Er3+ is achieved through the metallic nanostructures. Additional contributions for the observed emission spectrum regarding the influence of physical and geometrical parameters, period of the electric multipole and lattice symmetry have been investigated. The variation of these parameters resulted in a significantly improvement of luminescence spectra.

Surface plasmon propagation in novel multilayered metallic thin films

Multilayered Ag/Au/Ag/Au and Au/Ag/Au/Ag films with 200 nm of thickness (50 nm for each layer) were evaporated onto BK7 glass substrates. Sequences of slits (around 60-600 nm of width) were milled with a focused gallium ion beam in the films. We have undertaken a series of high-resolution measurements of the optical transmission through the slits. The transmission measurement setup consists of 488.0 nm (for the Ag/Au/Ag/Au film) and 632.8 nm (for the Au/Ag/Au/Ag sample) wavelength light beams from Ar ion and HeNe lasers, respectively, aligned to the optical axis of a microscope. The beam is focused onto the sample surface by a microscope objective in TM polarization (magnetic Hfield component parallel to the long axis of the slits). As well, theoretical estimates investigating the slits optical transmission were performed. The origin of the slits transmission is mainly attributed to plasmonic surface excitations. Based on the present results, it was possible to observe that (1) the transmission increases linearly with increasing slit width, and (2) the transmission of the multilayered structures is augmented in comparison with a single perforated metal film of equal thickness, for a fixed slit width. A very good correspondence between theory and experiment was observed.

Integrated hybrid plasmonic cavity with in-plane photon-plasmon coupling for luminescence enhancement

A device consisting of a disk-shaped, Moiré-type plasmonic cavity placed inside a plasmonic crystal cavity, with a 250 nm polymethyl-methacrylate (PMMA) film over the cavities is analyzed by 3D finite-difference time domain (FDTD). Both cavities can be fabricated by Focus Ion Beam, and the waveguide and the Moiré cavity contour can be defined by one-step lithographic process. The device is characterized by calculating the cavity spectrum, the reflection and the radiation spectra and the electric field intensity distribution. It was verified that the transverse-magnetic (TM) input mode generates surface plasmon polaritons (SPP) at the PMMA/gold interface that excites localized surface plasmon polariton on the Moiré cavity, that, in turn, generates reflected waves back to the waveguide and diffracted radiation. Also, the lack of plasmonic crystal bandgap permits the evanescent coupling of Bloch waves to the plasmonic crystal. The high electric field generated by the LSPP on the Moiré surface, and by the Bloch waves at the borders of the plasmonic crystal holes, contributes to the fluorescence of molecules dissolved in the PMMA film. The radiated fluorescence can be detected by a lensed fiber placed above the Moiré surface, and the reflected signal can be detected at the output.

Effect of V-shape on the light transmission of subwavelength slits in metallic thin films

Currently, the focused ion beam milling (FIB) technique is a commonly used approach to fabricate nanostructures because of its unique advantages of one-step fabrication, nanoscale resolution, and no material selectivity, etc. However, the FIB technique also has its own disadvantages. Regarding the process of fabrication of the corrugations and subwavelength apertures, nowadays, there is a major problem: the V-shaped structuring. In this work, we discuss the influence of V-shape on the optical transmission of subwavelength slits designed in silver (Ag) and gold (Au) thin films possessing different thicknesses. The effect of different cone angles (ratio between the widths at the incidence plane and at the exit plane) originated from the V-shaped slits was also considered. We had performed computational simulations carried out with COMSOL Multiphysics® to investigate the slits optical transmission. In most cases, the subwavelength slits were illuminated with 488 nm (for Ag) and 632.8 nm (for Au) wavelength light sources in TM polarization (magnetic H-field component parallel to the axis of the slits). The origin of the slits transmission is attributed to plasmonic surface excitations. Our simulation results demonstrated that different cone angles originated from the Vshaped subwavelength slits generate different influences on the beam propagation. The width variation affects the optical transmission intensity significantly. Hopefully, exploring the influence on the light propagation behaviour through subwavelength apertures via theoretical simulations can provide a better understanding of the beam propagation phenomena for future studies.

Integrated Plasmonic Moiré Cavity in Photonic Crystal Cavity for Luminescence Enhancement

We report the 3D simulation of a disk-shaped, Moiré-type plasmonic cavity inside a photonic crystal cavity. The simulation consider normal incidence of light over the sample to be analized with a confocal microscope in reflection mode. The plasmonic cavity is made of gold, 250 nm of thickness, whose surface is modulated by a sinusoidal function. The photonic crystal cavity is made in silicon nitride film (150 nm of thickness) over a SiO2 film (500 nm) on a silicon substrate, the overall structure being Si/SiO2/SiN/Au. The simulation results show a three-fold enhancement of the electric field intensity for the plasmonic cavity within the photonic cavity, in comparison with that for the plasmonic cavity without the photonic crystal cavity. The result indicates that the electric field intensities of the photonic crystal cavity modes add to the scattered field of the plasmonic cavity, thus enhancing the electric field just above the plasmonic cavity. A preliminary test of the structure was done with a 300 nm gold film over a silicon substrate, made by focus ion beam (FIB) milling, to show fluorescence enhancement of porphiryn molecules. The structure can be elaborated to serve either as fluorescence enhancement of molecules or as Surface-enhancement Raman scattering (SERS) sensor.

Fluorescence Enhancement in Er 3+ -doped Tellurite Glass from Gold Nanoparticles

It is well known that metallic structures have complex effects on fluorescence. In addition to an enhancement in fluorescence from the enhancement of the local electromagnetic field.