1,721,055 research outputs found
POLARIZED OPTICAL SPECTROSCOPY OF HIGHLY ORIENTED CONJUGATED POLYMERS: INTRA- AND INTER-MOLECULAR EFFECTS
No full textTHE ELECTRONIC STRUCTURE OF YbCu4In AS OBSERVED THROUGH OPTICAL MEASUREMENTS
No full texthe intermetallic compound YbCu4In is an almost unique (except for metallic cerium) material exhibiting a first-order temperature-induced valence transition. At about 40-60 K all the physical properties show a sudden change, whereas no distortion of the crystal structure occurs.
The optical response does not make an exception to this trend and an abrupt change in the optical functions at low temperatures is presented here. The spectra are discussed by taking into account some recent band structure calculations
Optical probes of electronic states in highly anisotropic polymeric semiconductors
No full textWe report an extensive investigation of the optical properties of highly oriented poly(p-phenylene-vinylene), PPV, a reference material for polymeric semiconductors. This system provides an unique opportunity to study the anisotropic spectroscopic response (transmittance, reflectance and photoluminescence), thus allowing to address fundamental physical issues, which are the basic principle of operating devices based on plastic semiconductors
POLARIZED OPTICAL AND PHOTOLUMINESCENCE PROPERTIES OF HIGHLY ORIENTED POLY(P-PHENYLENE-VINYLENE)
No full textOptical effects in artificial opals infiltrated with gold nanoparticles
No full textPolystyrene artificial opals are directly grown with embedded gold nanoparticles (NpAu) in their interstices. Reflectance spectra of samples having different sphere diameters and nanoparticles load clearly show a red shift of the photonic band gap as well as a reduction of its width without showing direct evidence of NpAu absorption. The case of transmission spectra is instead more complicated: here, overlapped to a broad NpAu absorption, a structure having unusual lineshape is detected. The infiltration of opal with NpAu removes the polarization dependence of the photonic band structure observed in bare opals. The lineshape of the absorption spectra suggest a spatial localization of the electromagnetic field in the volume where NpAu are confined thus enhancing its local intensity. This effect seems to be effective to stimulate optical nonlinearities of NpAu. Nanosecond transient absorption measurements on NpAu infiltrated opals indicate that a variation of transmission of about 10% is observed. Since this effect takes place within the pump pulse and since NpAu photoluminescence has been subtracted to the signal, we attribute it to an optical switching process
GROWTH AND OPTICAL STUDIES OF OPALS AS THREE-DIMENSIONAL PHOTONIC CRYSTALS
No full textThree-dimensional artificial opals showing photonic crystals properties are grown by self-assembly of polystyrene nanospheres. Atomic
force microscopy (AFM) images of opal surfaces show domains with regular triangular and squared packing of the spheres separated by
cracks whose relative fraction depends on the sample quality. The energy position of an optical pseudo gap in transmittance spectra is
observed by varying the angle of incidence and is accounted for by theoretical calculations of the photonic band structure based on a planewave
expansion method
ANISOTROPIC PHOTOLUMINESCENCE PROPERTIES OF ORIENTED POLY(P-PHENYLENE-VINYLENE) FILMS: EFFECTS OF THE OPTICAL CONSTANTS DISPERSION
No full textWe report on the effects of dispersive optical constants on the anisotropic photoluminescence spectra of
highly oriented polyp-phenylene-vinylene films. Polarized transmittance, reflectance, and photoluminescence
PL spectra have been measured over a broad spectral range, at different temperatures. Due to the high degree
of chain orientation in the polymer films, the emission properties are highly anisotropic. The PL spectral shape
and external quantum efficiency are significantly affected by self-absorption inside the polymer film and by
refractive effects at the polymer-air interface. In order to elucidate these aspects we have determined the
dispersion of the parallel and perpendicular components of the complex dielectric constant 1+i2, by which
the PL spectra have been corrected according to Fresnel equations. After correction, the PL intensity is found
to be higher when the excitation is polarized perpendicular to the stretching direction, a fact that we attribute
to charge-induced PL quenching. We have also considered the influence of the refractive index on the PL
spontaneous emission rate and on the light extraction from the polymer film. The spectral dependence of the
emission anisotropy functions of the corrected PL spectra indicates the presence of two distinct emitting
species, which may originate from intramolecular and intermolecular states
ANISOTROPIC PHOTOLUMINESCENCE OF HIGHLY ORIENTED POLY(P-PHENYLENE-VINYLENE)
No full textWe report on the anisotropic photoluminescence (PL) properties of stretch-oriented free standing films of poly(pphenylene-
vinylene) (PPV) at different temperatures. The PL quantum efficiency is strongly dependent on the pump
polarization; it is higher when the pump is polarized perpendicularly to the polymer chain orientation. Independently of
the pump polarization, we find that the PL emission spectra are mainly polarized along the polymer chain axis. The PL
spectra show high-energy features, close to the onset of the HOMO-LUMO transition, that are significantly affected by self-absorption of the emitted light in the optically thick samples as well as by refractive effects at the polymer-air interface. In order to clarify the origin of these features, we have made a detailed characterization of the anisotropic optical constants of the PPV film. The optical constants have been derived from polarized reflectance and transmittance measurements and were used for the renormalisation of the PL spectra using the Fresnel equations. Frank-Condon analysis for the absorption oscillator strength and for the corrected emission spectra suggests that two different emitting
states contribute to the optical properties. The connection of these states with film morphology and intermolecular
interactions is described
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