The efficiency of the conduction of photocurrent in discotic liquid crystals is known to depend on the quality of the
columnar organization. Solvents have shown to be able to influence the formation of wire structures on substrates
promoting very long and ordered wired formations or bulkier structures depending on the affinity of the solvent with
parts of the molecular structure of discotics. Here we present a study on the effect of solvents when the liquid crystal is
confined between two substrates with the columns running perpendicular to them, geometry used in solar cells. We
focused on toluene and dodecane, solvents that have shown to promote on substrates the formation of aligned and long
nanowires and bulk large and isolated fibers, respectively. The phase transition behavior indicates that toluene does not
interfere with the columnar formation while dodecane strongly influence increasing the disorder in the structure.
Discotic liquid crystal (LC) can arrange in columnar structures along which electrical conduction occurs via π-π interaction between adjacent molecular cores. The efficiency of the conductivity is strongly dependent on the overlap of the orbitals of neighbor molecules and, in general, on the structural arrangements. The understanding of the factors that influence the organization is crucial for the optimization of the final conductive properties of the self-assembled columns. In this paper we present a study on the self-organization into molecular wires of a discotic LC using a solution based method. In particular, we focus on the effect of solvents used for preparing the LC solution. The resulting morphologies were investigated by atomic force microscopy (AFM) and optical microscopy, showing that diverse structures result from different solvents. With suitable conditions, we were able to induce very long fibers, with several tents of micrometer in length that, in turn, self-organize assuming a common orientation on a macroscopic scale.
We study nonlinear optical response of nanofabricated gold particles with sub-100-nm spatial resolution by
means of non-linear near-field scanning optical microscopy (NSOM). In our instrument, femtosecond pulses at 800 nm
wavelength are coupled to hollow-pyramid aperture sensors. Such probes show high throughput and preserve pulse
duration and polarization, enabling the achievement of sufficiently high peak power in the near field to perform
nonlinear optics on the nanoscale. We study second-harmonic generation (SHG) from gold nanoparticles of two different
kinds, namely, closely-packed gold triangles and nanoellipsoids. We find a strong dependence of SHG efficiency on the
shape and the fine structure of the nanoparticles. Near-field SHG is therefore a subwavelength resolution probe of local
field enhancements occurring at specific sites of the particles. This work is focused on the dependence of NSOM linear
and nonlinear images on the aperture size and linear polarization direction of light. Our measurements give strong
evidence that SHG is mainly excited by a high field concentration at the rims of the metal NSOM aperture. This
conclusion is supported by the high spatial resolution obtained for SHG even with apertures so large that FW imaging
shows much poorer resolution.
Proc. SPIE. 5927, Plasmonics: Metallic Nanostructures and Their Optical Properties III
KEYWORDS: Gold, Nanostructures, Optical microscopes, Second-harmonic generation, Femtosecond phenomena, Spectroscopy, Near field scanning optical microscopy, Near field, Nonlinear optics, Near field optics
An aperture-type near-field optical microscope based on hollow-pyramid cantilevered probes has been developed and optimized for ultrafast nonlinear nanospectroscopy applications. These probes have many advantages for near-field microscopy such as higher throughput, higher thermal damage threshold, and absence of pulse chirping. The input pulse duration (as short as 30 fs from a mode-locked, stretched cavity 26 MHz Ti:Sapphire oscillator) is maintained beyond the aperture. Such short pulses, combined with the high peak powers available at the output of hollow-pyramid probes, allow experiments of nonlinear microscopy and spectroscopy with higher spatial and temporal resolution as compared to similar experiments based on optical fiber tips. Results on second-harmonic generation by gold nanostructures and BBO nonlinear crystals are reported demonstrating a spatial resolution down to 100 nm with 40 fs pulses. Implications for local femtosecond time-resolved pump-probe spectroscopy are anticipated.
We have studied the influence of Si atomic force microscope (AFM) probes on fluorescence of ZnS overcoated CdSe quantum dots (QDs) in an apertureless near-field scanning optical microscope (ANSOM). In these ANSOM measurements, the excitation light polarization and probe preparation procedure strongly affect the QD fluorescence. When the excitation light polarization is orthogonal to the probe axis (and parallel to the substrate surface), we detect 50 to 80% fluorescence quenching, with a HF-etched Si probe scanning ~10 nm above the sample. With polarization of the excitation collinear with the probe axis, the optical field is amplified many times in the near-field zone, and the net result is a 2-4 times fluorescence enhancement. In this work we utilize a home-built, non-contact AFM and confocal ANSOM microscope under total internal reflection of Ar<sup>+</sup> laser beam excitation.