M. Labardi

Artifact-free Near Field Optical Imaging by Apertureless Microscopy

A method for optical near field discrimination, leading to drastic artifact reduction in superresolved imaging by scanning interference apertureless microscopy is presented. The method relies on second harmonic detection of the modulated optical signal scattered by a vibrating silicon tip. An edge resolution of 15 nm, or 7 nm Rayleigh-type resolution, with optical contrast as high as 50%, has been obtained on aluminum projection pattern samples in the constant gap width mode. Our method has been determined not to be affected by topographical artifacts by constant height mode scans.

Near-field second-harmonic generation in single gold nanoparticles

Second-harmonic generation from single gold elliptical nanoparticles is experimentally investigated by a nonlinear scanning near-field optical microscope (SNOM). The near-field nonlinear response is found to be directly related to local surface plasmon resonances and to particle morphology. The combined analysis of linear and second-harmonic SNOM images provides discrimination among different light extinction particle behaviors, not achievable just with linear techniques. The polarization state of the emitted second harmonic is also investigated, providing experimental evidence of second-harmonic particle emission modes peculiar to near-field excitation.

Dynamical friction coefficient maps using a scanning force and friction microscope

A novel method to analyze and to distinguish the non-dissipative component from the dissipative frictional component of the lateral force in a Scanning Force and Friction Microscope (SFFM) is worked out and applied to images acquired on various samples. We have determined the twisting spring constants of the cantilevers by computer statical analysis, since they are essential for the quantitative determination but are not provided by the manufacturer. Quantitative results for the μD dynamical friction coefficient images are reported. Comparison with the forward-backward lateral force image subtraction model is made.

Unexpected polarization behavior at the aperture of hollow-pyramid near-field probes

Polarization in the proximity zone beyond the illuminated aperture of a near-field optical microscope is determined by means of a thin dichroic layer of fluorescent molecules used as a near-field polarization analyzer. Near-field probes of the hollow-pyramid type, with a metal coating and about 100 nm apertures, are used. Surprisingly, it is found that the input polarization is always maintained in the near field, independently of the aperture geometry, in spite of the behavior of the transmitted far field, which may result either isotropic or strongly dichroic depending on the ellipticity of the aperture.

Dynamical studies of the ferroelectric domain structure in triglycine sulfate by voltage-modulated scanning force microscopy

Voltage-modulated scanning force microscopy has been employed to investigate the dynamics of ferroelectric domains as a function of time and temperature in triglycine sulfate (TGS) single crystals. Branching of the domain structure and nucleation of fine domain patterns by internal fields of thermal origin have been directly observed on the submicron scale. Domain coarsening after quenching TGS samples from the paraelectric phase into the ferroelectric one has been studied, revealing a nonlinear time dependence of the characteristic correlation domain length and supporting the validity of dynamical scaling law and the conservation of the total surface charge. Substantial differences in the evolution of the domain structure have been detected during cooling or heating treatment in the ferroelectric phase. Domain contrast is shown to monitor the ferroelectric phase transition through its temperature dependence, reflecting the competition between electrostatic and piezoelectric effects.

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.

VERSATILE SCANNING NEAR-FIELD OPTICAL MICROSCOPE FOR MATERIAL SCIENCE APPLICATIONS

We describe an aperture emission mode scanning near-field optical microscope (SNOM), optimized for material surface science applications. This instrument can be operated in both transmission and reflection configurations, in order to investigate transparent as well as opaque samples. It employs optical shear-force detection for tip/sample distance control, designed to minimize interference with the probe light. The SNOM head has been fully integrated on a homemade atomic force microscope platform and is placed in a controlled atmosphere chamber for reduction of surface contaminants. Within the compactness and the versatility obtained in our instrument, we have been able to optically discriminate different materials with a λ/20 lateral resolution, and to distinguish polymeric aggregates, without damaging the surface, in spite of their rather poor optical contrast.

Optical nanowriting on azobenzene side-chain polymethacrylate thin films by near-field scanning optical microscopy

Optical writing and subsequent reading of information on thin films of azobenzene side-chain polymethacrylates on the 100-nm scale are demonstrated by near-field scanning optical microscopy (NSOM) with polarization control. Polarized blue light at 488 nm coupled to the NSOM aperture probe activates trans–cis–trans isomerization cycles of the side chains, causing their alignment and thus locally inducing optical birefringence. Red light at 690 nm with modulated polarization is coupled to the same aperture and used to detect optical anisotropy on the local scale. Lines of width on the 100-nm scale were optically inscribed and detected even with no concurrent topographic modification.

Two-Photon Lithography of 3D Nanocomposite Piezoelectric Scaffolds for Cell Stimulation

In this letter, we report on the fabrication, the characterization, and the in vitro testing of structures suitable for cell culturing, prepared through two-photon polymerization of a nanocomposite resist. More in details, commercially available Ormocomp has been doped with piezoelectric barium titanate nanoparticles, and bioinspired 3D structures resembling trabeculae of sponge bone have been fabricated. After an extensive characterization, preliminary in vitro testing demonstrated that both the topographical and the piezoelectric cues of these scaffolds are able to enhance the differentiation process of human SaOS-2 cells.

Highly efficient second-harmonic nanosource for near-field optics and microscopy

A nanometric source of second-harmonic (SH) light with unprecedented efficiency is demonstrated; it exploits the grazing-incidence illumination of a metal tip, which is conventionally used for atomic force microscopy, by 25-fs laser pulses of a high-energy Ti:sapphire oscillator. Tip scanning around the beam focus shows that the SH generation is strongly localized at its apex. The polarization dependence of the SH light complies with the model of an on-axis nonlinear oscillating dipole.