Ruggero Micheletto

An original planar multireflection system for sensing using the local surface plasmon resonance of gold nanospheres

We realized a planar multireflection glass system to investigate the localized surface plasmon resonance (LSPR) of gold nanoparticles. These nanospheres were realized in situ using an original and simple chemical growth method that is described. We were able to observe resonance phenomena as reflected by variations in the spectrum and as enhancements in the refractive index sensing ability. The system was able to clearly discern 2% sucrose solution and demonstrated outstanding linearity and reproducibility. We believe this study could be useful for elucidating the fundamental processes in nanoparticle LSPR and contribute to the realization of new and more efficient sensors.

Shape dependent thermal effects in apertured fiber probes for scanning near-field optical microscopy

Metal-coated, “pulled,” and conically shaped fiber probes used in scanning near-field optical microscopy (SNOM) typically undergo a thermal expansion when injected with laser light, due to partial energy absorption by the metallic film. Here, we report investigations into the thermal behavior of fiber probes produced by selective chemical etching that in our experience provide high light throughputs (10−3–10−4 vs 10−6 for the pulled fibers). Unexpectedly, we find a shortening of such probes in response to “high-power” laser injection (>1mW). Thermal stress due to prolonged high-power laser injection (∼9mW at 325nm; compared to powers <1mW often used in SNOM experiments) determines permanent alterations of the probes, after which their thermomechanical behavior reverts to the commonly observed elongation in response to laser injection. Scanning electron microscopy after high-power irradiation on such probes shows partial detachment of the metallic coating near the fiber termination. This, however, does not...

Observation of optical instabilities in the photoluminescence of InGaN single quantum well

We investigate a peculiar optical instability (blinking) phenomena associated with spatial inhomogeneity in InxGa(1−x)N single quantum well systems. We studied the time dependence of this dynamic phenomenon and tested a “quantum jump” single exponential model on the system. A comparative analysis of the behavior of different samples suggests that indium-rich localized centers participate in the mechanism of blinking and that the instability behavior differs with the excitation wavelength. Our study indicates that the trapping and de-trapping process between the localized-luminescent centers and surrounding less luminous regions plays important roles in the carrier recombination mechanism.

A simple method for the fabrication of low cost scanning near field optical microscope probes with acrylic paint as coating element

Abstract We derived a simple method to fabricate scanning near field optical microscopes (SNOMs) optical probes from commercial cheap communication optical fibers. The tips are etched in one single step at the oil–acid interface of a HF solution. The resulting probes are then coated with acrylic paint in order to screen the light and a small optical aperture is spontaneously formed at the apex. The method is straightforward and requires a few hours for a number of ready to use probes. The original use of paint simplifies strongly the procedure of screening of the fiber that usually has been performed by costly and time consuming metal coating technique. We show that the tips are fully operational and we demonstrate a good optical resolving power (100 nm). As far as we know, our method makes the cheapest SNOM probes within this resolution range.

Tenfold improved sensitivity using high refractive-index substrates for surface plasmon sensing

Surface plasmon resonance sensors exploit the high sensitivity to local perturbations of plasma waves in a thin metal layer. These devices have a wide range of applications as biomedical, environmental, industrial, and homeland security. We concentrate on the theoretical aspects of the sensing principle. By calculations at various indexes of refraction we proved that using substrate material of higher index, sensitivity and dynamics range improve conspicuously. Finally, we show experimental data taken using a special transparent ceramic material of exceptionally high index of refraction n=2.04. Tests demonstrate sensitivity about one order of magnitude better than those obtained with conventional BK7 glass.

Acoustical nanometre-scale vibrations of live cells detected by a near-field optical setup.

The Scanning Near-field Optical Microscope (SNOM) is able to detect tiny vertical movement on the cell membrane in the range of only 1 nanometer or less, about 3 orders of magnitude better than conventional optical microscopes. Here we show intriguing data of cell membrane nanometer-scale dynamics associated to different phenomena of the cells The Scanning Near-field Optical Microscope (SNOM) is able to detect tiny vertical movement on the cell membrane in the range of only 1 nanometer or less, about 3 orders of magnitude better than conventional optical microscopes. Here we show intriguing data of cell membrane nanometer-scale dynamics associated to different phenomena of the cells life, such as cell cycle and cell death, on rat pheochromocytoma line PC12. Working in culture medium with alive and unperturbed samples, we could detect nanometer-sized movements; Fourier components revealed a clear distinct behavior associated to regulation of neurite outgrowth and changes on morphology after necrotic stimulus.

Development of laser assisted nanometric resolution scanning tunneling microscopy time-of-flight mass analyzer system

This study describes a ground-breaking process that provides a direct highly localized measurement of the atomic mass on surfaces at room temperature. Employing an original system that joins a scanning tunneling microscopy (STM) device and a time-of-flight (TOF) mass analyzer, we could locally ionize surface atoms by the combination of an optical laser pulse and an appropriate voltage variation between the sample and the STM tip. Desorbed ions were accelerated and detected by a TOF chamber. Detection and discrimination of single atomic species from nanolocalized area have been demonstrated for the first time.

Near-field optical imaging of flagellar filaments of salmonella in water with optical feedback control

We report on the high-resolution observation of biological samples in water with a collection-mode near-field optical microscope (c-mode NOM) operating under optical feedback control. With rapidly decreasing evanescent field power used as the feedback signal, for the first time to our knowledge, an image of straight-type flagellar filaments of salmonella in water has been obtained. The estimated diameter of a single filament is around 55 nm with a pixel size of 10 nm. A comparison with its nominal value of 25 nm obtained from electron microscope observations under high vacuum confirms that our c-mode NOM performs high-resolution imaging in water.

Index-of-refraction sensors: virtually unlimited sensing power at the critical angle

In an effort to improve and simplify refractive index sensors, we identified a basic operation mode at the critical angle. Sensitivity to the refractive index is higher than in standard surface plasmon resonance sensors, and we have been able to demonstrate analytically that it is virtually an unbounded value. We describe this approach and submit a complete analytical study demonstrating its unlimited sensing power. To test the approach, we constructed an economical and basic sensor. Despite its simplicity, we demonstrated the discrimination capability to be of the order of 10−6, as far as we know close to the best sensitivity ever recorded. This detection method is generally applicable to any optical system and may pave the way for the next generation of optical sensing devices.

Optical study of a polymeric LED with a nano-sized electrode realized by a modified SNOM setup

Abstract Semiconductive polythiophene derivate polymers show electroluminescence (EL) properties if excited under opportune conditions and can be used as active element in novel efficient light-emitting diodes (LEDs) or in photovoltaic applications as detectors or solar cells. These intriguing and innovative materials are so far studied in bulk condition, under the excitation of two macroscopic electrodes, averaging the optical phenomena over a large region. To realize a better understanding of the spatial distribution of the EL on the films and its optical properties, we modified a scanning near-field optical microscope (SNOM) setup in order to measure the EL signal of poly-3-(2-(5-chlorobenzotriazolo)ethyl)thiophene. The film were deposited over a macroscopic electrode substrate (a transparent indium tin oxide (ITO) treated glass), while a sharp aluminum coated glass probe worked as a nano-sized Al electrode, realizing an Al/polymer/ITO light emitting device in a highly confined region. We describe in detail the methodology we used for the novel EL measurements and we will show the first resulting high-resolution EL emission mapping, as well as other spectroscopic and morphological characterization of the film used.