P. G. Gucciardi

Thermal-expansion effects in near-field optical microscopy fiber probes induced by laser light absorption

We present a study on thermal-expansion effects induced by laser light coupling into metal-coated probes for near-field scanning optical microscopy. An expansion of the probe edge of 170 nm per mW of coupled power has been observed as an effect of the temperature raise due to light absorption in the metal coating. The phenomenon has been studied in both time and frequency domains by modulating the coupled laser power, in order to measure the typical time constants related to heat exchange processes, that turned out to be of the order of a few milliseconds. An analytical model, taking into account both the heat conduction through the coating and the convection losses, provides scales for the system parameters and fits the experimental data remarkably well.

Far-field background suppression in tip-modulated apertureless near-field optical microscopy

In apertureless near-field optical microscopy the vertical dithering of the tip, associated with demodulation at higher harmonics (n>1), allows us to suppress the far-field background, providing artifact free elastic scattering images. This paper analyzes, both theoretically and experimentally, the physical origin of the background signal at the different harmonics and the mechanisms underlying its rejection for the general case of propagative-field illumination. We show that Fourier components of the background must be expected at every harmonic, evidencing why demodulation at higher harmonics is not an inherently background-free technique, and assessing the experimental conditions in which it becomes like that. In particular, we put forward the fundamental roles of both the harmonic order and the tip oscillation amplitude in the background suppression mechanisms. Furthermore, we outline how the lock-in detection of the signals amplitude can enhance the nonlinear dependence of the background on the tip-s...

Nano-Raman imaging of Cu–TCNQ clusters in TCNQ thin films by scanning near-field optical microscopy

We have combined scanning near-field optical microscopy (SNOM) techniques with Raman scattering in order to measure the topography and the local optical properties of solid samples together with the chemical properties of molecular adsorbates, on subwavelength scales. The proposed experimental setup is simple, but very sensitive. In particular, we have focussed our attention on the optimization of the collection and detection of the scattered light, in order to circumvent the limited imaging capabilities of the SNOM due to the low cross-section of the Raman effect. The sample we will report on is a tetracyanoquinodimethane (TCNQ) thin film kept in contact with Cu powder in order to give rise to local complexes of Cu–TCNQ. A comparative investigation based on independent micro- and nano-Raman maps, allows us to chemically discriminate the aggregates and to assess the enhanced lateral resolution of the SNOM in the nanometre range.

Optical probing of sample heating in scanning near-field experiments with apertured probes

We have used the inherent thermochromism of conjugated polymers to investigate substrate heating effects in scanning near-field experiments with metal-coated “apertured” probes. Chemically etched and pulled fibers were used to provide near-field excitation of fully converted films of poly(p-phenylene vinylene), PPV, and of poly(4,4′-diphenylene diphenylvinylene). We detect no significant blueshift of the photoluminescence spectra generated with near-field excitation, in comparison to those collected with far-field excitation. We conclude that polymer heating in the region contributing to the luminescence is less than 40K. We also demonstrate that thermolithography of the PPV precursor is not significant by comparing UV (325nm) and red (670nm) illumination.

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 tip-to-sample heat transfer in near-field optical microscopy using metal-coated fiber probes

Metal-coated scanning near-field optical microscopy fiber probes can undergo significant heating due to partial absorption of the coupled light by the metallic film covering the apical zone. In this letter we report experimental evidence of tip-to-sample heat transfer on a 7,7′,8,8′-tetracyanoquinodimethane molecular crystal. Local melting is observed at nanometric tip–sample distances, when increasing the laser power injected into the fiber above a threshold of 8.8mW. Hole formation and material displacement are observed, as well as failure of the shear-force-based imaging process, due to partial sticking of the melted material to the probe.

Artifacts identification in apertureless near-field optical microscopy

The aim of this paper is to provide criteria for optical artifacts recognition in reflection-mode apertureless scanning near-field optical microscopy, implementing demodulation techniques at higher harmonics. We show that optical images acquired at different harmonics, although totally uncorrelated from the topography, can be entirely due to far-field artifacts. Such observations are interpreted by developing the dipole-dipole model for the detection scheme at higher harmonics. The model, confirmed by the experiment, predicts a lack of correlation between the topography and optical images even for structures a few tens of nanometers high, due to the rectification effect introduced by the lock-in amplifier used for signal demodulation. Analytical formulas deduced for the far-field background permit to simulate and identify all the different fictitious patterns to be expected from metallic nanowires or nanoparticles of a given shape. In particular, the background dependence on the tip-oscillation amplitude is put forward as the cause of the error-signal artifacts, suggesting, at the same time, specific fine-tuning configurations for background-free imaging. Finally a careful analysis of the phase signal is carried out. In particular, our model correctly interprets the steplike dependence observed experimentally of the background phase signal versus the tip-sample distance, and suggests to look for smooth variations of the phase signal for unambiguous near-field imaging assessment.

Interferometric measurement of the tip oscillation amplitude in apertureless near-field optical microscopy

We have implemented an optical homodyne interferometer to measure the tip oscillation amplitude in apertureless near-field optical microscopy. The setup is fully embedded in the microscope’s design, avoiding the presence of external arms. Our method is based on the synchronous detection of the interference between the fields reflected by the tip and a glass sample surface, while scanning the tip–sample distance over a few wavelengths. With the help of a simple model, we show how the different interference terms arising at frequencies multiple of the tip oscillation can be exploited to easily achieve sub-Angstrom resolution.

Optical near-field Raman imaging with subdiffraction resolution

We report optical near-field Raman imaging with subdiffraction resolution (approximately 120 nm) without field enhancement effects. Chemical discrimination on tetracyanoquinodimethane organic thin films showing localized salt complexes is accomplished by detailed Raman maps. Acquisition times that are much shorter than previously reported are due to the high Raman efficiency of the materials and to careful collection and detection of the optical signals in our near-field Raman spectrometer.

A versatile multipurpose scanning probe microscope.

A combined scanning probe microscope has been developed that allows simultaneous operation as a non‐contact/tapping mode atomic force microscope, a scattering near‐field optical microscope, and a scanning tunnelling microscope on conductive samples. The instrument is based on a commercial optical microscope. It operates with etched tungsten tips and exploits a tuning fork detection system for tip/sample distance control. The system has been tested on a p‐doped silicon substrate with aluminium depositions, being able to discriminate the two materials by the electrical and optical images with a lateral resolution of 130 nm.