In this paper, we review the principal theoretical models through which the dielectric function of metals can be described. Starting from the Drude assumptions for intraband transitions, we show how this model can be improved by including interband absorption and temperature effect in the damping coefficients. Electronic scattering processes are described and included in the dielectric function, showing their role in determining plasmon lifetime at resonance. Relationships among permittivity, electric conductivity and refractive index are examined. Finally, a temperature dependent permittivity model is presented and is employed to predict temperature and non-linear field intensity dependence on commonly used plasmonic geometries, such as nanospheres.

Terahertz spectroscopy has vast potentialities in sensing a broad range of elementary excitations (e.g., collective vibrations of molecules, phonons, excitons, etc.). However, the large wavelength associated with terahertz radiation (about 300 μm at 1 THz) severely hinders its interaction with nano-objects, such as nanoparticles, nanorods, nanotubes, and large molecules of biological relevance, practically limiting terahertz studies to macroscopic ensembles of these compounds, in the form of thick pellets of crystallized molecules or highly concentrated solutions of nanomaterials. Here we show that chains of terahertz dipole nanoantennas spaced by nanogaps of 20 nm allow retrieving the spectroscopic signature of a monolayer of cadmium selenide quantum dots, a significant portion of the signal arising from the dots located within the antenna nanocavities. A Fano-like interference between the fundamental antenna mode and the phonon resonance of the quantum dots is observed, accompanied by an absorption enhancement factor greater than one million. NETS can find immediate applications in terahertz spectroscopic studies of nanocrystals and molecules at extremely low concentrations. Furthermore, it shows a practicable route toward the characterization of individual nano-objects at these frequencies.

A combined system of gold nanorods and NaGdF 4 :Er 3+ /Yb 3+ upconverting nanoparticles with double functionality, luminescence enhancement, and monitored heating is introduced. The paired nanostructures could become an excellent optical heater with thermal probe incorporated. To study their interaction, the longitudinal surface plasmon resonance of the gold nanorods is tuned to 980 nm, in resonance with the Yb 3+ absorption wavelength, so they can be simultaneously excited. Gold nanorods create a localized electromagnetic fi eld that enhances the emission intensity from upconverting nanoparticles. This luminescence enhancement is shown to depend on the interparticle distance and excitation power and, in this system, reaches a maximum enhancement of 9 for the green emission of Er 3+ ions. At the same time, evidence of strong collective heating from the gold nanorods is demonstrated. The temperature can be controlled by changing the excitation power and measured in situ via the Er 3+ thermally sensitive luminescence. At high excitation powers, the heating can trigger a deformation of the gold nanorods, which limits the maximum temperature achievable in the system to 160 °C. Combining these nanostructures provides an all-optical heating system with improved emission intensity that can monitor the temperature achieved.

A cw laser using a 2.5×10−4 M solution of rhodamine 6G in water containing a deaggregating agent (1.5% Triton-X100) is described. The laser consists of a 4.5-mm transverse-flow hemispherical cavity excited longitudinally by the 5145-Å output of a 1-W argon ion laser. With mirror reflectances of 97.5% and 99.5%, a 200-mW threshold excitation was measured. Power output was approximately 30 mW at 597 nm with 960 mW excitation power.

Rod photoreceptors consist of an outer segment (OS) and an inner segment. Inside the OS a biochemical machinery transforms the rhodopsin photoisomerization into electrical signal. This machinery has been treated as and is thought to be homogenous with marginal inhomogeneities. To verify this assumption, we developed a methodology based on special tapered optical fibers (TOFs) to deliver highly localized light stimulations. By using these TOFs, specific regions of the rod OS could be stimulated with spots of light highly confined in space. As the TOF is moved from the OS base toward its tip, the amplitude of saturating and single photon responses decreases, demonstrating that the efficacy of the transduction machinery is not uniform and is 5-10 times higher at the base than at the tip. This gradient of efficacy of the transduction machinery is attributed to a progressive depletion of the phosphodiesterase along the rod OS. Moreover we demonstrate that, using restricted spots of light, the duration of the photoresponse along the OS does not increase linearly with the light intensity as with diffuse light.V ertebrate photoreceptors, rods and cones, are morphologically specialized light sensing neurons and consist of four parts: an outer segment (OS), an inner segment (IS), the nuclear region, and the synapse (1). The OS of rod photoreceptors is stacked with thousands of lipid discs containing rhodopsin molecules that absorb photons (2-4). They are surrounded by a plasma membrane and differ in their lipid and protein composition.It is known that within 1 s, each excited rhodopsin, densely packed in the disc membrane, activates tens of G proteins (named transducin), each of which activates one phosphodiesterase (PDE) molecule (5-7). Activated PDEs rapidly hydrolyze cytoplasmic cyclic guanosine monophosphate (cGMP) thereby closing cyclic nucleotide-gated (CNG) channels (8, 9). In darkness, a current carried by Na + , K + , and Ca 2+ ions, which is known as the photocurrent, enters via the CNG channels into the OS and is pumped out by Na + /K + ATPase, located in the IS (1). This current can be recorded using suction electrodes (3, 10).The existence of distinct compartments on photoreceptor OSs is a consequence of their function. The OS is a highly modified nonmotile cilium developed for the absorption of light that is translated into the electrical signal. The IS and the nuclear region-containing the organelles and the nucleus-are dedicated to metabolism, homeostasis, and synthesis of the membrane and transportation of proteins and lipids supplied to the OS by an extensive trafficking through the tight restriction connecting the OS to the IS (11). Considering that, unlike typical cilia, the OS is continuously renewed throughout its entire life and given the varying density and asymmetry of the molecules involved, it is not surprising to find an efficacy gradient of phototransduction between the base and the tip. Indeed, the cholesterol (12, 13), the phospholipids (13-15), the CNG channels, the PDE (16), and the rhodop...

We present a simple method that is able to predict the resonant frequencies of a metallic conical nanoantenna. The calculation is based on an integral relation that takes into account the dependence of the effective refractive index of the plasmonic mode on the cone radius. Numerical simulations retrieving the near field properties of nanocones with different lengths are also performed for comparison. The fine agreement between the two approaches demonstrates the validity of our method.

Small misalignments between two standard telecom single-mode fibers in a physical contact connection can lead to large optical losses. It is known that by expanding the mode field diameter of the fiber, the misalignment tolerances can be relaxed. One of the approaches to obtain this beam expansion is to use tapers. We propose an air-clad taper structure to transmit the fundamental mode of a single-mode fiber adiabatically to a 3 times larger mode field area in physical contact expanded beam connectors. This results in a 241.4 µm long linear taper. The taper itself is fabricated on top of a cleaved fiber facet by means of the two-photon polymerization direct laser writing technique. Experimental results for lateral misalignment show excellent agreement with simulated values and give an increase in lateral misalignment tolerance of 1 µm (-1 dB) and 1.8 µm (-3 dB). Total insertion losses down to 0.76 dB are measured, showing the trade-off between achievable insertion loss and misalignment tolerance relaxation. Finally, we show that the use of additive manufacturing techniques in fiber beam expansion applications make it possible to fabricate taper structures with full 3D design freedom and to upscale the process to multi-fiber components.

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