L. Pardi

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.

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.

Micromechanical detection of magnetic resonance by angular momentum absorption

Observations of electron paramagnetic resonance by means of the angular momentum absorbed by a small sample are reported. The method is based on the detection of the torque induced in a microcantilever, like the ones employed in atomic force microscopy, integral with the sample. The results obtained with this experimental scheme are compared with the ones of the Sidles–Rugar method exploiting a force detection of the magnetic resonance in presence of strong magnetic field gradients. In particular, the signal drawn by angular momentum absorption shows opposite trends when the static magnetic field is swept in opposite directions, contrary to the case of experiments performed with force detection. The shape of the signal is characterized at different values of the static field and of radiation intensity.

Polarization-modulation near-field optical microscope for quantitative local dichroism mapping

A couple of experimental techniques have been implemented to an aperture near-field scanning optical microscopy (NSOM) to obtain reliable measurement of sample dichroism on the local scale. First, a method to test NSOM tapered fiber probes toward polarization conservation into the near optical field is reported. The probes are characterized in terms of the in-plane polarization of the near field emerging from their aperture, by using a thin dichroic layer of chromophore molecules, structured along stretched polymeric chains, to probe such polarization when approached in the near-field region of the probe. Second, to assure that the light intensity coupled in the fiber is polarization independent, an active system operating in real time has been realized. Such combination of techniques allowed quantitative imaging of local dichroism degree and average orientation by means of dual-phase lock-in demodulation of the optical signal. Translation of the coupled light polarization state in the near field has been...

Light emitting porous silicon diode based on a silicon/porous silicon heterojunction

A new structure is proposed to improve the external quantum efficiency of porous silicon (PS) light emitting diodes (LED). It is based on a heterojunction between n-type doped silicon and PS. The heterojunction is formed due to the doping selectivity of the etching process used to form PS. The improvement of the proposed LED structure with respect to usual metal/PS LED is demonstrated. This is thought to be due to a different injection mechanism for which carriers are injected directly into conduction band states. Anodic oxidation experiments show further improvements in the LED efficiency.

Near-field optical microscopy of polymer-based films with dispersed terthiophene chromophores for polarizer applications

Linear dichroic properties of polyethylene films containing a dispersion of terthiophene-based chromophore molecules are investigated by using a scanning near-field optical microscope (SNOM). The polarization-modulation technique implemented in our SNOM provides quantitative information on the dichroic ratio of the samples with sub-wavelength space resolution. Optically active domains are identified and their morphology is analysed as a function of the film fabrication parameters, e.g., the drawing ratio and the kind of dispersed chromophore mixture. These investigations complement conventional polarimetry analysis by adding nanometre-scale information on the spatial distribution of the chromophore molecules and their mutual alignment with the host polymer chains.

Scanning force microscopy study of the ferroelectric phase transition in triglycine sulfate

Voltage-modulated scanning force microscopy is applied to study the temperature dependence of the ferroelectric domain structure of triglycine sulfate up to TC, the ferroelectric transition temperature. The polarization image contrast exhibits a power-law decrease as TC is approached, associated with the competition between Maxwell stress and the converse piezoelectric effect. Substantial differences in the static and dynamic domain patterns are detected for two samples of different aging conditions.

Longitudinally detected electron spin resonance: Recent developments

Multiple-quantum spectroscopies are reviewed in the frame of electron paramagnetic resonance. Some properties of different nonlinear techniques are discussed for both transverse and longitudinal detection. The connections of effects recently presented with the procedure of longitudinal detection of electron paramagnetic resonance (LODESR) in presence of double transverse irradiation are stressed. Peculiarities of LODESR spectroscopy and its capabilities in facing problems related to relaxations in presence of very slow dynamic processes are evidentiated. Recent results show the vitality of the LODESR technique, that in the future could be applied to new fields, owing to experimental updating.

Dependence on relaxation times of longitudinally detected paramagnetic resonance

Abstract The spin system longitudinal magnetization at resonance, irradiated by two nearly resonant transversal waves, oscillates. The oscillation for low signal intensities depends linearly on the product T1·T2. This effect allows to resolve overlapping spectra which are usually not detectable.

Surface deformation and ferroelectric domain switching induced by a force microscope tip on a La-modified PbTiO3 thin film

Surface deformation of a ferroelectric (111)-oriented thin film of La-modified PbTiO3 is induced by contact with the tip of a scanning force microscope (SFM). The deformation is accompanied by switching of the out-of-plane polarization of ferroelectric domains revealed by simultaneous piezoresponse force microscopy. The effect shows up in topographic SFM images as strokes in the fast scan direction due to surface deformation occurring below the scanning tip, and is critically dependent on the contact force for which a threshold value is deduced that allows proper SFM characterization of such thin films. At higher force, SFM might be used as a nanoscale tool for investigating fundamental properties like phase transitions under applied stress in such systems.