M. A. Ferrara

Optical properties of diatom nanostructured biosilica in Arachnoidiscus sp: micro-optics from mother nature.

Some natural structures show three-dimensional morphologies on the micro- and nano- scale, characterized by levels of symmetry and complexity well far beyond those fabricated by best technologies available. This is the case of diatoms, unicellular microalgae, whose protoplasm is enclosed in a nanoporous microshell, made of hydrogenated amorphous silica, called frustule. We have studied the optical properties of Arachnoidiscus sp. single valves both in visible and ultraviolet range. We found photonic effects due to diffraction by ordered pattern of pores and slits, accordingly to an elaborated theoretical model. For the first time, we experimentally revealed spatial separation of focused light in different spots, which could be the basis of a micro-bio-spectrometer. Characterization of such intricate structures can be of great inspiration for photonic devices of next generation.

Giant Raman gain in silicon nanocrystals

Nanostructured silicon has generated a lot of interest in the past decades as a key material for silicon-based photonics. The low absorption coefficient makes silicon nanocrystals attractive as an active medium in waveguide structures, and their third-order nonlinear optical properties are crucial for the development of next generation nonlinear photonic devices. Here we report the first observation of stimulated Raman scattering in silicon nanocrystals embedded in a silica matrix under non-resonant excitation at infrared wavelengths (~1.5 μm). Raman gain is directly measured as a function of the silicon content. A giant Raman gain from the silicon nanocrystals is obtained that is up to four orders of magnitude greater than in crystalline silicon. These results demonstrate the first Raman amplifier based on silicon nanocrystals in a silica matrix, thus opening new perspectives for the realization of more efficient Raman lasers with ultra-small sizes, which would increase the synergy between electronic and photonic devices.

Holographic imaging of unlabelled sperm cells for semen analysis: a review

Male reproductive health in both humans and animals is an important research field in biological study. In order to characterize the morphology, the motility and the concentration of the sperm cells, which are the most important parameters to feature them, digital holography demonstrated to be an attractive technique. Indeed, it is a label-free, non-invasive and high-resolution method that enables the characterization of live specimen. The review is intended both for summarizing the state-of-art on the semen analysis and recent achievement obtained by means of digital holography and for exploring new possible applications of digital holography in this field. Quantitative phase maps of living swimming spermatozoa.

Broadening and tuning of spontaneous Raman emission in porous silicon at 1.5μm

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 this letter, an approach based on Raman scattering in porous silicon is investigated. Two significant advantages with respect to silicon are proved: the broadening of spontaneous Raman emission and the tuning of the Stokes shift. Finally, we discuss about the prospect of Raman amplifier in porous silicon.

Non-invasive sex assessment in bovine semen by Raman spectroscopy

X- and Y-chromosome-bearing sperm cell sorting is of great interest, especially for animal production management systems and genetic improvement programs. Here, we demonstrate an optical method based on Raman spectroscopy to separate X- and Y-chromosome-bearing sperm cells, overcoming many of the limitations associated with current sex-sorting protocols. A priori Raman imaging of bull spermatozoa was utilized to select the sampling points (head-neck region), which were then used to discriminate cells based on a spectral classification model. Main variations of Raman peaks associated with the DNA content were observed together with a variation due to the sex membrane proteins. Next, we used principal component analysis to determine the efficiency of our device as a cell sorting method. The results (>90% accuracy) demonstrated that Raman spectroscopy is a powerful candidate for the development of a highly efficient, non-invasive, and non-destructive tool for sperm sexing.

Label-Free Imaging and Biochemical Characterization of Bovine Sperm Cells

A full label-free morphological and biochemical characterization is desirable to select spermatozoa during preparation for artificial insemination. In order to study these fundamental parameters, we take advantage of two attractive techniques: digital holography (DH) and Raman spectroscopy (RS). DH presents new opportunities for studying morphological aspect of cells and tissues non-invasively, quantitatively and without the need for staining or tagging, while RS is a very specific technique allowing the biochemical analysis of cellular components with a spatial resolution in the sub-micrometer range. In this paper, morphological and biochemical bovine sperm cell alterations were studied using these techniques. In addition, a complementary DH and RS study was performed to identify X- and Y-chromosome-bearing sperm cells. We demonstrate that the two techniques together are a powerful and highly efficient tool elucidating some important criterions for sperm morphological selection and sex-identification, overcoming many of the limitations associated with existing protocols.

Observation of stimulated Raman scattering in silicon nanocomposites

In this paper, we report on the observation of stimulated Raman scattering in silicon nanocomposites, consisting of silicon nanoparticles dispersed in SiO2 matrix prepared by sol-gel method. Using a 1427nm cw pump laser, amplification of Stokes signal, at 1542.2nm, up to 1.4dB∕cm is demonstrated. A preliminary valuation of approximately a fivefold enhancement of the gain coefficient in Raman amplifier based on silicon nanocomposites with respect to silicon and a significant reduction of threshold power are also reported. These results have a potential interest for silicon-based Raman lasers.

Enhanced stimulated Raman scattering in silicon nanocrystals embedded in silicon-rich nitride/silicon superlattice structures

In the last few years several strategies have been developed to engineer efficient light sources and amplifiers in Si-based materials, with the aim to demonstrate a convenient path to monolithic integration of optical and electronic devices within the mainstream Si technology In particular, light amplification by Stimulated Raman Scattering (SRS) in silicon waveguides has been recently demonstrated despite intrinsic limitations related to the nature of the bulk Si materials have been pointed out . The narrow-band (105 GHz) of stimulated Raman gain in Si limits its applicability in the context of Si photonics, and makes it unsuitable for its use in broad band division multiplexing (WDM) applications, unless expensive multi-pump schemes are implemented. Additionally, Raman amplification in Si is a small effect. Therefore, in order to build a laser based on stimulated Raman effects in Si, very high power intensity and very low absorption losses are required. Finally, Raman gain in Si is further reduced by the competing nonlinear effect of two-photon absorption. This effect generates electron-hole pairs, which remain excited in the sample for a long time (micro to milliseconds) and lead to strong absorption at both the pump and signal frequencies.

Raman Amplifier Based on Amorphous Silicon Nanoparticles

The observation of stimulated Raman scattering in amorphous silicon nanoparticles embedded in Si-rich nitride/silicon superlattice structures (SRN/Si-SLs) is reported. Using a 1427 nm continuous-wavelength pump laser, an amplification of Stokes signal up to 0.9 dB/cm at 1540.6 nm and a significant reduction in threshold power of about 40% with respect to silicon are experimentally demonstrated. Our results indicate that amorphous silicon nanoparticles are a great promise for Si-based Raman lasers.

Optics with diatoms: towards efficient, bioinspired photonic devices at the micro-scale

Diatoms are monocellular algae responsible of 20-25% of the global oxygen produced by photosynthetic processes. The protoplasm of every single cell is enclosed in an external wall made of porous hydrogenated silica, the frustule. In recent times, many effects related to photonic properties of diatom frustules have been discovered and exploited in applications: light confinement induced by multiple diffraction, frustule photoluminescence applied to chemical and biochemical sensing, photonic-crystal-like behavior of valves and girdles. In present work we show how several techniques (e.g. digital holography) allowed us to retrieve information on light manipulation by diatom single valves in terms of amplitude, phase and polarization, both in air and in a cytoplasmatic environment. Possible applications in optical microsystems of diatom frustules and frustule-inspired devices as active photonic elements are finally envisaged.