Luigi Sirleto

Transverse probe optical lifetime measurement as a tool for in-line characterization of the fabrication process of a silicon solar cell

Abstract In this paper an all-optical measurement procedure for the characterization of minority carrier recombination lifetime and surface recombination velocity is presented as a reliable tool to monitor the fabrication process of a standard crystalline silicon solar cell. In the methodology presented here, there are no stringent requirements concerning the state of wafer surface. The IMEC (Interuniversity Microelectronics Centre, Leuven, Belgium) fabrication process is taken as an example of the capability of this method to monitor the whole process from the silicon wafer to the finished cell. It is shown that the cell process does not degrade the bulk recombination lifetime and that the effect of the external surfaces is effectively screened.

Optoelectronic router in glass waveguide with a liquid crystal cladding

An optoelectronic router integrated in a sol-gel waveguide with a liquid crystal over-layer, working at the wavelength of 1.55 μm, is theoretically discussed. Mode-mixing principle and electro-optic effect of a smectic A* liquid crystal have been employed to reach the optical shift between two output waveguides. The device is composed by: a single mode input optical channel waveguide; a bimodal active region, having a liquid crystal as over-layer; an output Y branch. The active region is designed to allow a π shift between the two supported modes, by means of electro-optic effect of smectic A*. By doing so it is possible to steer light from one output channel to the other one of output Y.

Feasibility of an all-optical switch based on cylindrical microresonators and liquid crystals

Recently an ever-increasing activity in the area of coupling between optical waveguide and ring or micro resonator has been developed. Devices based on this coupling held the promise of a new modality of light switching, amplifications and modulation. In this paper, the feasibility of an all optical switch based on the integration of the potentiality of microcavity resonator and organic materials having large nonlinearities, i.e. liquid crystals (LCs), is discussed. The device is based on silicon technology with hybrid integration of liquid crystals as a nonlinear material.

Optoelectronic switch and continuously tunable filter based on a liquid crystal waveguide

In this paper the possibility of using ferroelectric liquid crystals in active waveguide devices is explored through the analysis of an integrated electro-optic switch and a continuously tunable filter. The design and the analysis of tow electro-optical devices, based on a Bragg grating integrated in a glass waveguide having liquid crystal as cover, are presented. The integrated optics structure allows to change the reflectivity of the Bragg mirror by means of electro-optic effects of smectic liquid crystals. The integrated fast electro-optic switch is based on electro- optic properties of smectic C* in the Surface Stabilized liquid crystal structure and on the selective properties of integrated Bragg grating. It presents the output directly in the frequency domain, overcoming the typical problems of intensity dependent devices, without requiring external electronic circuit. Moreover the possibility to realize a novel continuously tunable integrated filter, combining the linear electro-optic effect of smectic A* and the selective property of Bragg grating, has been explored. The proposed filter is characterized by a narrow bandwidth, desired feature for WDM technique. The principal advantages of such device include fast tuning speed, wide tuning range, low power consumption and low cost.

Optical characterization of the recombination process in silicon wafers, epilayers and devices

In this paper we present various approaches to the measurement of recombinative parameters in silicon wafers, epitaxial layers and solar cells. In particular our techniques are able to measure the bulk lifetime and the surface recombination velocity at low injection levels. We also show that the techniques presented are a reliable tool to monitor the fabrication process of a standard crystalline silicon solar cell. In all the methodologies presented there are no stringent requirements concerning the state of wafer surface.

Label-free imaging of lipid droplets in cells by stimulated Raman microscopy

Recent technological developments in ultrafast laser physics have permitted to make new kind of nonlinear microscopies, as microscopy based on Stimulated Raman scattering. These techniques are based on vibrational contrast mechanism for imaging with high sensitivity, high spatial and spectral resolution and 3D sectioning capability. The interest in the study of lipids and the possibility to image lipid droplets, thanks to their isolated Raman peaks associated with vibrational C-H bond, have encouraged investigation and identification of lipid structures inside cells, taking advantage of Stimulated Raman Scattering (SRS) imaging. In this work, we report and discuss label free images on biological environmental and structural analysis, to detect lipid microstructures inside adipocyte cells.

Implementation of stimulated Raman losses and stimulated Raman gain microscopy using three femtosecond laser sources

Stimulated Raman scattering microscopy allows vibrational contrast mechanism for imaging with high spectral and spatial resolution along with three-dimensional sectioning. In this paper, the implementation of a Stimulated Raman Scattering microscope (SRSM), obtained by the integration of a femtosecond SRS spectroscopy set-up with an optical microscope equipped with a scanning unit, is described. Femtosecond Stimulated Raman Scattering microscope is equipped with three femtosecond laser sources: a Ti:Sapphire (Ti:Sa), a synchronized optical parametric oscillator (SOPO) and a frequency converters for ultrafast lasers, i.e. a second harmonic generator optimized for the SOPO. The proposed implementation allows to cover all the regions of Raman spectra, taking advantage of two different laser combinations. The first, Ti:Sa and SHG laser combination can cover in SRL modality the fingerprint region (500 − 1700 cm−1 ) and the silent region. The second, Ti:Sa and OPO, can cover the C-H region or O−H region (2800 − 3200 cm−1 ) in SRG modality. In order to demonstrate its successful realization Stimulated Raman Gain (SRG) and Stimulated Raman Losses (SRL) images of polystyrene beads are reported and discussed.

Micro and nanophotonics in silicon: new perspectives and applications

In the last few years, silicon photonics has been characterized by a wide range of applications in several fields, from communications to sensing, from biophotonics to the development of new artificial materials. In this communication, we report a review of the main results obtained in our laboratories in design, fabrication and characterization of new silicon-based optical structures and devices, including metamaterials, photodetectors, raman light amplifiers, and porous silicon based bio-chemical sensors and biochips. Future perspectives in integration of silicon based MEMS and MOEMS are also presented.

Raman approach in silicon nanostructure at 1.5 micron

In the last three years, the possibility of light generation and/or amplification in silicon, based on Raman emission, has achieved significant results. However, limitations inherent to the physics of silicon have been pointed out, too. In order to overcome these limitations, a possible option is to consider low dimensional silicon. n nIn this paper, an approach based on Raman scattering in porous silicon is theoretically and experimentally investigated. We prove two significant advantages with respect to silicon: the broadening of the spontaneous Raman emission and the tuning of the Stokes shift. Finally, we discuss about the prospect of Raman amplifier in porous silicon.

Electro-optical switch based on a Bragg grating in a liquid crystal waveguide

In this paper an integrated electro-optical switch, designed to operate at the infrared communications wavelength of 1550 nm, is presented. The device is based on a Bragg grating in planar waveguide with a bistable smectic C* liquid crystal overlayer, which allows to change the spectral behavior of the device. The principal advantage of this device is its spectral signature, which allows overcoming the problems of intensity dependent switches.