Here we discuss the light transmission modulation by periodic and disordered one dimensional (1D) photonic structures. In particular, we will present some theoretical and experimental findings highlighting the peculiar optical properties of: (i) 1D periodic and disordered photonic structures made with two or more materials<sup>1,2</sup>; (ii) 1D photonic structures in which the homogeneity<sup>3</sup> or the aggregation<sup>4</sup> of the high refractive index layers is controlled. We will focus also on the fabrication aspects of these structures.
We present the possibility to tailor the optical properties of 1D photonic structures by using more than two materials and by clustering the high refractive index (hRI) layer in the structures. In particular, we show that: i) with a photonic crystal made of i different materials, the photonic band gap splits in i-1 bands; ii) with a proper choice of the layer thickness, disordered photonic structures made with a high number of layers show periodic transmission peaks; iii) when the size of the hRI layer clusters, randomly distributed within the low refractive index layers, follows a power law distribution, the total light transmission follows a sigmoidal function. Furthermore, we discuss the fabrication aspects to realize the above mentioned photonic structures.
We report the fabrication and validation of a microfluidic chip for fluorescence detection, which incorporates in the same glass substrate the microfluidic network, the excitation, the filtering, and the collection elements. The device is fabricated in a hybrid approach combining different technologies, such as femtosecond laser micromachining and RF sputtering, to increase their individual capabilities. The validation of the chip demonstrates a good wavelength selective light filtering and a limit of detection of a 600-nM concentration of Oxazine 720 perchlorate dye.
We report the realization and characterization of porous nanostructures where a periodic refractive index modulation is
achieved by stacking layers with different nano-architectures. One multilayer photonic crystal has been fabricated
starting from colloidal dispersion of silicon dioxide and zirconium dioxide using spin coating technique. Improved
efficiency of Bragg reflectivity (up to 85%) has been obtained by a new bottom-up fabrication technique of photonic
hierarchical nanostructures based on self-assembly from the gas-phase at low temperature whit a very thin (≈ 1 μm)
photonic crystal devices. Due to the high porosity, these systems can be infiltrated with nematic liquid crystals leading to
tuning of the Bragg reflection band by applying low voltages to the structure.
The investigation of the differences between ordered and disordered materials (in the hundreds of nanometer lengthscale) is a crucial topic for a better understanding of light transport in photonic media. Here we study the light transmission properties of 1D photonic structures in which disorder is introduced in two different ways. In the first study, we have grouped the high refractive index layers in layer clusters, randomly distributed among layers of low refractive index. We have controlled the maximum size of such clusters and the ratio of the high-low refractive index layers (here called dilution). We studied the total transmission of the disordered structure within the photonic band gap of the ordered structure as a function of the maximum cluster size, and we have observed a valley in trend of the total transmission for a specific maximum cluster size. This value increases with increasing dilution. Furthermore, within one dilution we observe oscillations of the total transmission with increasing cluster size. In the second study, we have realized photonic structures with a random variation of the layer thickness. The structures were fabricated by radio-frequency (RF) sputtering technique. The transmission spectrum of the disordered structure was simulated by taking into account the refractive index dispersion of the materials, resulting in a good agreement between the experimental data and the simulations. We found that the transmission of the photonic structure in the range 300– 1200 nm is lower with respect the corresponding periodic photonic crystal. The studied disordered 1D photonic structures are very interesting for the modelization and realization of broad band filters and light harvesting devices.
We report the fabrication of micro-Fresnel lenses by femtosecond laser surface ablation on one-dimensional (1-D) polymer photonic crystals. This device is designed to focus the transmitted wavelength (520 nm) of the photonic crystal and filter the wavelengths corresponding to the photonic band-gap region (centered at 630 nm, ranging from 530 to 700 nm). Integration of such devices in a wavelength selective light harvesting and filtering microchip is envisaged.
In this study we demonstrate the fabrication of one-dimensional porous multilayer photonic crystals made by metal oxide nanoparticles. We show the infiltration of these porous structures with a liquid crystal via a very simple method, resulting in a red shift of the photonic band gap due to increase of the effective refractive index of the medium. Taking advantage of structure thickness of only few micrometers, we have observed a blue shift of the photonic band gap owing the non-linear response of the liquid crystals by applying a very low external electric voltage, i.e. 8 V. The experimental observation of electric voltage tuning on the transmission spectrum has been corroborated by transfer matrix method simulations, by taking into account the non-linear optical properties of the liquid crystal. In this framework, we propose how the optical properties of these structure can be accurately predicted by our simulation software in terms of diffraction efficiency, of photonic band gap position when the porous photonic crystals is doped with a liquid crystal, of modulation of the photonic band gap position (electro-optic tuning) in the presence of applied voltage. According with results carried out by the custom simulation software it is possible to control the optical proprieties of the photonics crystal in very thin structures. Furthermore, the presented device could be very interesting for applications where high sensitivity sensor and selective color tunability is needed with the use of cheap and low voltage power supplies.
Charge generation at donor/acceptor interface is a highly debated topic in the organic photovoltaics (OPV)
community. The primary photoexcited state evolution happens in few femtosecond timescale, thus making very
intriguing their full understanding. In particular charge generation is believed to occur in < 200 fs, but no clear picture
emerged so far. In this work we reveal for the first time the actual charge generation mechanism following in real time
the exciton dissociation mechanism by means of sub-22 fs pump-probe spectroscopy. We study a low-band-gap polymer:
fullerene interface as an ideal system for OPV. We demonstrate that excitons dissociation leads, on a timescale of 20-50
fs, to two byproducts: bound interfacial charge transfer states (CTS) and free charges. The branching ratio of their
formation depends on the excess photon energy provided. When high energy singlet polymer states are excited, well
above the optical band gap, an ultrafast hot electron transfer happens between the polymer singlet state and the
interfacial hot CTS* due to the high electronic coupling between them. Hot exciton dissociation prevails then on internal
energy dissipation that occurs within few hundreds of fs. By measuring the internal quantum efficiency of a prototypical
device a rising trend with energy is observed, thus indicating that hot exciton dissociation effectively leads to a higher
fraction of free charges.
Optical trapping and manipulation of micrometric silica particles dispersed in a nematic liquid crystal is reported.
Several kind of samples are considered: homeotropic and planar undoped cells and homeotropic and planar cells doped
by a small amount of the azo-dye Methyl-Red. The incident light intensity is over the threshold for optical reorientation
of the molecular director. The refractive index of the dispersed particles is lower than the ones of the liquid crystal
therefore the usual conditions for laser trapping and manipulation are not fulfilled. Nevertheless optical trapping is
possible and is closely related to the optical nonlinearity of the hosting liquid crystal<sup>1</sup>. Trapping in doped and undoped
cells are compared and it is shown that in the first case intensity lower by more than one order of magnitude is required
as compared to the one needed in undoped samples. The effect is faster and the structural forces are of longer range. The
formation of bubble-gum like defects in doped samples under certain experimental conditions is also reported and
We report the properties of a new polymer-guest photosensitive mixture based on two monomers with different
properties as basic components. The material has been characterized by recording holographic gratings using a low
power blue laser (405 nm) and a spectro-photometric technique to get the optical properties of the grating during and
after recording. The detected grating shows a very high sensitivity (of the order of 10<sup>3</sup> cm/J) and a surprising "antishrinkage" phenomenon (red-shift of the Bragg reflection grating wavelength).
We present a novel optical sensor able to measure the distance between the tip of an endoscopic probe and the anatomical object under examination. In medical endoscopy, knowledge of the real distance from the endoscope to the anatomical wall provides the actual dimensions and areas of the anatomical objects. Currently, endoscopic examination is limited to a direct and qualitative observation of anatomical cavities. The major obstacle to quantitative imaging is the inability to calibrate the acquired images because of the magnification system. However, the possibility of monitoring the actual size of anatomical objects is a powerful tool both in research and in clinical investigation. To solve this problem in a satisfactory way we study and realize an absolute distance sensor based on fiber optic low-coherence interferometry (FOLCI). Until now the sensor has been tested on pig trachea, simulating the real humidity and temperature (37°C) conditions. It showed high sensitivity, providing correct and repeatable distance measurements on biological samples even in case of very low reflected power (down to 2 to 3 nW), with an error lower than 0.1 mm.
In the last three decades several kinds of organic mixtures for holographic recording were developed in order to
achieve a new class of DVD-like optical memories for high-density optical data storage. The holographic materials
should satisfy the following requirements: high sensitivity to blue light, low losses, high spatial resolution and long term
stability. To this aim we developed new organic photosensitive mixtures based on only three components. We recorded
high spatial frequency reflection gratings up to 7400 lines/mm with blue laser light (405 nm) by using a conventional
holographic setup. We obtained a macro grating diffraction efficiency up to 67%, refractive index modulation over 0.01,
optical shrinkage < 2 % and overall losses ~5%. In order to characterize data-storage materials independently on the
experimental conditions, the sensitivity has been evaluated through the S parameter which takes into account the
diffraction efficiency, recording light intensity, exposure time and sample thickness. The amazing obtained values of <i>S</i>
>10<sup>5</sup> cm/J evidences a very fast recording process with a very low writing intensity (less than 20 mW/cm<sup>2</sup>) corresponding
to a recording energy density of few mJ/cm<sup>2</sup>. The performance of these materials have been also tested in the microholographic
The aim of this work is to understand the effects of the shrinkage phenomenon in H-PDLC reflection gratings through the realtime analysis of their transmission spectra during the recording process. The realtime spectroscopic analysis of the samples showed a light intensity dependent free-radical polymerization indicated by a quick growth of the reflection peak whose corresponding diffraction efficiency, measured at normal incidence, is in the range of 40%-45% depending on the photopolymerization conditions. An optical shrinkage corresponding to a 4.3% displacement of the reflected wavelength from the expected value has been detected. This value of the shrinkage is in agreement to that mesaured in a similar system.
Holographic gratings recorded in polymer-dispersed liquid crystals (H-PDLC) are very interesting as systems for electrically switchable diffractive devices. From the early 1990s, experiments, Bragg and Raman-Nath gratings are recorded in PDLC. A very thorough review of these investigations is published recently. In this paper we report our first experiments for H-PDLC gratings, recorded in two different optical arrangements using total internal reflection (TIR): 1) Stetson's scheme - when low and high spatial frequencies gratings are simultaneously recorded in the PDLC's volume, and 2) Nassenstein's scheme - when low or high spatial frequency grating is recorded with evanescent waves in very thin layer of the PDLC. A polarization grating recorded by the Stetson's scheme is also reported. The realtime diffraction efficiency dependence during the recording and the polarization characteristics are investigated.
Reflection gratings have been recorded and investigated in H-PDLC materials by means of real-time spectroscopy during the polymerization process in view of possible application in optical data storage. High spatial frequency gratings (>6000lines/mm) with diffraction efficiency up to 45% and index modulation over 0.01 were obtained. The effects of the shrinkage of the reflection gratings has been detected showing a displacement of the reflected wavelength from the expected value of about 3.8%. Finally recording of microgratings has been carried out observing some optimal agreement between the microscopic and the macroscopic parameters.