Antigone Marino

Photonic band gaps analysis of Thue-Morse multilayers made of porous silicon

Dielectric aperiodic Thue-Morse structures up to 128 layers have been fabricated by using porous silicon technology. The photonic band gap properties of Thue-Morse multilayers have been theoretically investigated by means of the transfer matrix method and the integrated density of states. The theoretical approach has been compared and discussed with the reflectivity measurements at variable angles for both the transverse electric and transverse magnetic polarizations of light. The photonic band gap regions, wide 70 nm and 90 nm, included between 0 and 30 degrees , have been observed for the sixth and seventh orders, respectively.

Lasing Thresholds of Cholesteric Liquid Crystals Lasers

ABSTRACT Cholesteric liquid crystals (CLCs) are one dimensional photonic band-gap materials. Due to distributed feedback, low threshold mirrorless lasing can occur in dye-doped and pure CLCs. In order to optimize the lasing conditions, we have studied the dependence of the lasing threshold on dye concentration and sample thickness. In particular, we have studied dye concentrations in the range of 0.1–3.0 wt%, and cell thicknesses are in the range of 5–50 μm. We have found that the system has a shallow lasing threshold minimum and can operate efficiently in the range of dye concentrations of 0.3–2.4 wt% and sample thicknesses of 10–50 μm. We discuss the physical processes responsible for the observed behaviour.

Two-dimensional photonic quasicrystals by single beam computer-generated holography

Recently important efforts have been dedicated to the realization of a new kind of photonic crystals, known as photonic quasicrystals, in which the lack of the translational symmetry is compensated by rotational symmetries not achievable by the conventional periodic crystals. Here we show a novel approach to their fabrication based on the use of a programmable Spatial Light Modulator encoding Computer-Generated Holograms. Using this single beam technique we fabricated Penrose-tiled structures possessing rotational symmetry up to 23-fold, and a two-dimensional Thue-Morse structure, which is an aperiodic structure not achievable by multiple beam holography.

Optical characterization of liquid crystals by combined ellipsometry and half-leaky-guided-mode spectroscopy in the visible-near infrared range

In this paper, we present our results for the anisotropic refractive index measurements of commonly used liquid crystal (LC) materials (E7 and 5CB) in the whole visible–near infrared range. In order to achieve a high accuracy in the obtained values, we have employed a combination of two techniques, namely, Mueller matrix spectroscopic ellipsometry in transmission and half-leaky-guided-mode (HLGM). Measurements with the HLGM technique are usually performed at a single wavelength. In order to obtain a spectroscopic measurement based on the HLGM technique in a wide wavelength range, we modified our commercial VASE® ellipsometer. In particular, we designed a sample holder by means of which measurements in guiding structures are also possible. Thus, we take advantage of the wide-spectrum light, emitted from a standard Xe lamp of the ellipsometer, also in the HLGM setup. The complementary and in-one-setup utilization of both techniques has allowed us to overcome most of the problems previously encountered in ap...

FDTD analysis of photonic quasicrystals with different tiling geometries and fabrication by single-beam computer-generated holography

Multiple-beam holography has been widely used for the realization of photonic quasicrystals with high rotational symmetries not achievable by conventional periodic crystals. Accurate control of the properties of the interfering beams is necessary to provide photonic band-gap structures. Here we show, by finite difference time domain (FDTD) simulations of the transmission spectra of 8-fold quasiperiodic structures, how the geometric tiling of the structure affects the presence and properties of the photonic band-gap for low refractive index contrasts. Hence, we show an interesting approach to the fabrication of photonic quasicrystals based on the use of a programmable spatial light modulator encoding computer-generated holograms, that permits an accurate control of the writing pattern with almost no limitations in the pattern design. Using this single-beam technique we fabricated quasiperiodic structures with high rotational symmetries and different geometries of the tiling, demonstrating the great versatility of our technique.

Ellipsometric Study of Liquid Crystal Infiltrated Porous Silicon

Porous silicon (PS) based devices are nowadays under an intense and widespread investigation in the optoelectronic and sensing fields. Recently, the range of possible applications has been widened by the use of the liquid crystals (LCs), which can be infiltrated in the PS sponge-like structure. The large changes of the optical properties, as exhibited by LCs under the action of electrical or thermal fields, allow developing a new family of optical devices like tunable PS based multilayer mirrors, microcavities and optical filters. In this work, we have optically characterized the LC-PS heterogeneous composite as a guest-host system by means of the variable angle spectroscopic ellipsometry (VASE). A PS layer, 450 nm thick, has been infiltrated with the nematic LC 5CB, and the main optical parameters, anisotropic refractive indices and thickness of both materials, have been estimated below and above the isotropic transition temperature, at 27.0°C and 38.0°C respectively. We have found a clear indication that the LC molecules tend to align parallel to the direction of the pore columns.

High accuracy optical characterization of anisotropic liquids by merging standard techniques

The design and fabrication of active and passive hybrid photonic devices, with tunable optical properties, based on liquid crystals require a very accurate knowledge of their anisotropic refractive indices up to 100ppm in the wavelength region of interest. At this aim, the authors have integrated two standard optical techniques commonly used in liquid crystals characterization, the variable angle spectroscopic ellipsometry and the half leaky guided mode spectroscopy, exploiting their best performances and overcoming their limits. The dispersion curves of nematic liquid crystal E7 have been estimated in the 450–1700nm wavelength interval with both precision and accuracy of 10−4.

Measuring liquid crystal anchoring energy strength by spectroscopic ellipsometry

We describe an experimental procedure for accurate measurement of anchoring energy strength of liquid crystal cells. This technique is based on the possibility of gathering a large amount of very precise data about the linear optical response of the cell in different experimental conditions, using spectroscopic ellipsometry. Then, a careful data analysis exploiting data inversion method, supplemented by simulations from the elastic theory, is able to provide the searched information. The technique has been applied to vertical aligned nematic cells, chosen for its widespread use in the present display market. The results obtained in this particular case together with a thorough comparison with existing alternative techniques, suggest that our technique can be an optimum candidate for the industrial implementation of such measurement. A particular example is fully worked out, giving a result with a precision of 1.5% and an accuracy of 10%.

Ellipsometric study of vertically aligned nematic liquid crystals

The director tilt angle distribution in vertically aligned nematic liquid crystal displays has been investigated by means of variable angle spectroscopic ellipsometry. Liquid crystal vertical alignment has been realized by thermal evaporation of SiOx. By changing the deposition angle, it is possible to control the pretilt angle. The director profile inside the sample was inferred by reflection and transmission ellipsometric measurements. The tilt angle distribution inside the cell versus the applied voltage is reported and eventually, comparing it with the simulations from the elastic theory, the anchoring energy has been obtained.

Nematic Liquid Crystal Optical Dispersion in the Visible-Near Infrared Range

A commercial variable angle spectroscopic ellipsometer from J. A. Woollam Company is modified in order to make possible measurements in guiding structures. The half leaky guided mode technique is realised for liquid crystal optical dispersion measurements. Features and problems related to experimental data interpretation and error sources in the refractive index measurements are carefully analyzed, achieving a final accuracy of ±0.0002. We present the measured optical dispersion curves in the wavelength range from 0.5 to 1.7 μm for two widely studied liquid crystals, namely E7 and 5CB.