Despite fluorescent sensing is a reference method for the detection of a plethora of different compounds, the exploitation of this class of sensors is still limited to a few application scenarios as a result of the restricted availability of miniaturized, portable, and user-friendly devices.
Here, the smart combination of an organic photodiode (OPD), a Distributed Bragg Filter (DBR), and an organic light-emitting diode (OLED) is proven to provide a stacked device architecture capable of detecting fluorescent signals for a wide range of concentrations of “Rhodamine 700” ranging from 10-3 M to 10-6 M.
Despite fluorescent sensing is a reference method for the detection of a plethora of different compounds, the exploitation of this class of sensors is still limited to a few application scenarios as a result of the restricted availability of miniaturized, portable, and user-friendly devices.
Here, the smart combination of an organic photodiode (OPD), a Distributed Bragg Filter (DBR), and an organic light-emitting diode (OLED) is proven to provide a stacked device architecture capable of detecting fluorescent signals for a wide range of concentrations of “Rhodamine 700” ranging from 10-3 M to 10-5 M.
Here we show the way to easily tune and engineer the optical response of hybrid structures composed by self ordered dielectric nanospheres partially covered by anisotropic plasmonic structures. The overall structure is a hybrid plasmonic-photonics nanostructure acting as a meta-surface witch morphology allows efficient and versatile light manipulation both for linear polarized and circular polarized fields in the visible and near infrared frequencies.
Here we present the measurements of the second harmonic generation (SHG) signal raised by self ordered dielectric nanospheres partially covered by thin (10nm) Au layer. The measurement were performed by studying the SHG efficiency in different polarization states of the light. In particular measurement performed with circular polarized light show the presence of chiral response of the nanospheres that is induced by the particular geometry of the metasurface.
Artificial opals are a simple and cheap playground to manipulate the propagation of light. The interest in these kind of
photonic crystals is further increased by the possibility to be infiltrated with highly polarisable media like organic
semiconductors, i.e. conjugated polymers, push-pull molecules and multipolar chromophores.
In this work, we report on the optical properties of polystyrene opals infiltrated with a heteroaromatic quadrupolar
derivative endowed with strong nonlinear optical properties (two-photon absorption) in solution. The insertion of
tris(ethylene glycol)monomethyl ether chains on the conjugated skeleton allows the molecule to be soluble in water, a
non-solvent for polystyrene. This condition is fundamental in order to attempt opal infiltration.
Variable angle transmittance and photoluminescence spectroscopy are used to characterize the system. The bathochromic
shift of the opal stop band upon immersion in the chromophore solution confirms that the infiltration process easily takes
place preserving a dielectric contrast suitable for further investigations.
Photoluminescence spectra recorded at different emission angle with respect to the normal of the sample for both the
chromophore solution and opals infiltrated with such solution show interesting characteristics. The presence of opal
modifies the chromophore emission spectrum by filtering the light for wavelengths corresponding to those of the stop
band and according to its dispersion.
Polystyrene 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.
We report on the anisotropic photoluminescence (PL) properties of stretch-oriented free standing films of poly(p-phenylene-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.
Absorption, photoluminescence and photoinduced absorption spectra of polycarbazolyldiacetylenes (polyCzDAs) carrying selected acyl (polyDPCHD) or alkyl (polyDCHD-HS) groups are reported. The absorption spectrum of polyDCHD-HS films, which is almost independent of the temperature, shows an excitonic absorption followed by an inhomogeneously broadened vibronic progression. Very different is instead the case of polyDPCHD whose thin films show a very sharp excitonic transition and a vibronic progression with a lower electron-phonon (el-ph) coupling with respect to that of polyDCHD-HS. Unlike other polydiacetylenes (PDAs), the polyCzDAs which exhibit narrow spectral features and reduced el-ph coupling, such as polyDPCHD films and polyDCHD-HS in benzene solutions, are luminescent thus suggesting a different ordering of the dipole forbidden and allowed states. This interpretation is confirmed by two-photon spectroscopic data. The ordering of the Ag and Bu excited states also affects the generation of triplet excitons, which are the main long-living excited states of PDAs. The rise and decay of the triplet signal for polyDCHD-HS are accounted for through a dynamical model, based on a monomolecular decay regime including saturation effects. A more complex kinetics is instead observed for polyDPCHD. From the study of the kinetics of the PIA spectra both the triplet generation efficiency and the density of traps are evaluated.
Bulk polystyrene opals have been grown. Variable incidence angle reflectance spectroscopy is used to probe their photonic band structures. Several different structures are observed and accounted for by theoretical calculations of photonic bands and density of states. The results yield a clear distinction between diffraction in the direction of propagation by the (111) family planes (leading to the formation of the stop band) and diffraction in other directions by higher-order planes (corresponding to excitation of photonic modes in the crystal).
We report results of steady-state photoinduced absorption experiments on PDA-4BCMU grown and oriented on silica substrate. A high degree of axial order, checked by linear and nonlinear optical dichroism and correlated with X-ray measurements was found. Polycrystalline films were obtained with a preferential orientation of crystallites. Unlike the solution cast films exhibiting the so called `red form' only, the deposited films are blue thus showing a better (pi) -electron conjugation than in the former case due to a better planarity of polymer chains. The films were used to perform steady-state photoinduced absorption experiments. Three photoinduced absorption bands at 0.82, 0.95, and 1.39 eV were observed. The 0.82 and 0.95 eV absorption features are assigned to charged states while the 1.39 eV peak is originated by triplet excitons. The photogeneration in PDA- 4BCMU of both charged and neutral states is discussed and compared with the results obtained in polydiacetylenes having carbazolyl groups as substituents. The relative weight of one excited species over the other one is dictated by the molecular and supramolecular organization of the polymer.
We report on the photoinduced absorption spectra of polycarbazolydiacetylenes polyDCHD and its soluble analogous polyDCHD-S in a broad time domain ranging from femtosecond to millisecond. Both the blue and red forms of polyDCHD-S are investigated and the results are compared with those of blue polyDCHD. In the blue form of the two polymers charged states are detected in the ms domain in addition to triplet excitons. In the red form of polyDCHD-S only triplets are detected. Femtosecond pump and probe measurements allow us to interpret the generation mechanism of the triplet states as due to singlet excitons fission.
We present an extensive photoinduced absorption study of alkyl substituted poly (2,5- thienylenevinylene)s in both the solid state and in solution. The optical and conformational properties of the samples are monitored with UV-Vis absorption and emission, IR absorption and Raman scattering. Three photoinduced states are detected under steady state condition which display lifetimes of the order of 100 ms in the solid state. The two low-energy bands are assigned to bipolarons, while a third band peaked near the band edge has a different origin. In solution very lone-lived photoexcitated states are observed and they are assumed to recombine via a solvent-assisted photo-doping mechanism.
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