E. De Tommasi

Frequency-comb-referenced singly-resonant OPO for sub-Doppler spectroscopy

We present a widely-tunable, singly-resonant optical parametric oscillator, emitting more than 1 W between 2.7 and 4.2 μm, which is phase locked to a self-referenced frequency comb. Both pump and signal frequencies are directly phase-locked to the frequency comb of a NIR-emitting fs mode-locked fibre laser, linked, in turn, to the caesium primary standard. We estimate for the idler frequency a fractional Allan deviation of ∼ 3 × 10⁻¹²τ⁻½ between 1 and 200 s. To test the spectroscopic performance of the OPO, we carried out saturation spectroscopy of several transitions belonging to the ν1 rovibrational band of CH₃I, resolving their electronic quadrupole hyperfine structure, estimating a linewidth better than 200 kHz FWHM for the idler, and determining the absolute frequency of the hyperfine components with a 50-kHz-uncertainty.

Hybrid polymer-porous silicon photonic crystals for optical sensing

Porous silicon based one-dimensional photonic crystals, such as Bragg mirror and optical microcavity, has been modified by infiltration of a new biocompatible polymer, an amino functionalized poly(e-caprolactone) (PCL-NH2), and characterized as optical biochemical sensors. The entrapped polymer adds strong chemical stability to the nanocrystalline matrix on exposure to alkaline solutions preserving the device sensing abilities in monitoring volatile substances and chemical compounds. On the basis of these results, these hybrid structures can be proposed as a high performance platform for biochemical applications.

Bioactive modification of silicon surface using self-assembled hydrophobins from Pleurotus ostreatus

A crystalline silicon surface can be made biocompatible and chemically stable by a self-assembled biofilm of proteins, the hydrophobins (HFBs) purified from the fungus Pleurotus ostreatus. The protein-modified silicon surface shows an improvement in wettability and is suitable for immobilization of other proteins. Two different proteins were successfully immobilized on the HFBs-coated chips: the bovine serum albumin and an enzyme, a laccase, which retains its catalytic activity even when bound on the chip. Variable-angle spectroscopic ellipsometry (VASE), water contact angle (WCA), and fluorescence measurements demonstrated that the proposed approach in silicon surface bioactivation is a feasible strategy for the fabrication of a new class of hybrid devices.

Biologically enabled sub-diffractive focusing

Evolution shows that photonic structures are a constituent part of many animals and flora. These elements produce structural color and are useful in predator-prey interactions between animals and in the exploitation of light for photosynthetic organisms. In particular, diatoms have evolved patterned hydrated silica external valves able to confine light with extraordinary efficiency. Their evolution was probably guided by the necessity to survive in harsh conditions of sunlight deprivation. Here, we exploit such diatom valves, in conjunction with structured illumination, to realize a biological super-resolving lens to achieve sub-diffractive focusing in the far field. More precisely, we consider a single diatom valve of Arachnoidiscus genus which shows symmetries and fine features. By characterizing and using the transmission properties of this valve using the optical eigenmode technique, we are able to confine light to a tiny spot with unprecedented precision in terms of resolution limit ratio, corresponding in this case to 0.21λ/NA.

SPECIAL issue: 22nd colloquium on high-resolution molecular spectroscopy HRMS Dijon 2011 (Part 2) A narrow-linewidth optical parametric oscillator for mid-infrared high-resolution spectroscopy

We present a narrow-linewidth, singly-resonant cw optical parametric oscillator, emitting more than 1 W in the 2.7–4.2 µm range. The OPO is pumped by a narrow linewidth (40 kHz) fibre-laser system and the signal frequency is locked to a high-finesse Fabry–Pérot cavity in order to increase the spectral resolution, thus obtaining a residual linewidth of 70 kHz for the signal. We tested the spectral performance of our OPO on several transitions in the ν1 rovibrational band of CH3I, measuring line intensities and showing sub-Doppler dip detection.

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.

Intrinsic photoluminescence of diatom shells in sensing applications

Diatoms are monocellular micro-algae provided with external valves, the frustules, made of amorphous hydrated silica. Frustules present patterns of regular arrays of holes, the areolae, characterized by sub-micrometric dimensions. Frustules from centric diatoms are characterized by a radial disposition of areolae and exhibit several optical properties, such as photoluminescence, lens-like behavior and, in general, photonic-crystal-like behavior as long as confinement of electromagnetic field is concerned. In particular, intrinsic photoluminescence from frustules is strongly influenced by the surrounding atmosphere: on exposure to gases, the induced luminescence changes both in the optical intensity and peaks positions. To give specificity against a target analyte, a key feature for an optical sensor, a biomolecular probe, which naturally recognizes its ligand, can be covalently linked to the diatom surface. We explored the photoluminescence emission properties of frustules of Coscinodiscus wailesii centric species, characterized by a diameter of about 100-200 μm, on exposure to different vapours and in presence of specific bioprobes interacting with target analytes. Very high sensitivities have been observed due to the characteristic morphology of diatoms shells. Particular attention has been devoted to the emission properties of single frustules.

Light micro-lensing effect in biosilica shells of diatoms microalgae

Diatoms are monocellular micro-algae provided with external valves, the frustules, made of amorphous hydrated silica. Frustules present patterns of regular arrays of holes, the areolae, characterized by sub-micrometric dimensions. In particular, frustules from centric diatoms are characterized by a radial disposition of areolae and exhibit several optical properties, such as photoluminescence variations in presence of organic vapors and photonic-crystal-like behaviour as long as propagation of electromagnetic field is concerned. We have studied the transmission of coherent light, at different wavelengths, through single frustules of Coscinodiscus Walesii diatoms, a centric species characterized by a diameter of about 150 μm. The frustules showed the ability to focalize the light in a spot of a few μm2, the focal length depending on the wavelength of the incident radiation. This focusing effect takes place at the centre of the frustule, where no areolae are present and, as it is confirmed by numerical simulations, it is probably due to coherent superposition of unfocused wave fronts coming from the surrounding areolae. Diatoms-based micro-lenses could be used in the production of lensed optical fibers without modifying the glass core and, in general, they could be exploited with success in most of the optical micro-arrays.

Periodically-Poled Ferroelectric Crystals Based OPO—A Powerful Source for Precision Spectroscopy

Advances in material science and laser sources renewed the interest in optical parametric oscillators (OPOs). A major attraction of OPOs is the possibility to coherently generate new frequencies over extended tunability ranges. These characteristics are of great interest for spectroscopic applications, which require stable cw highly-coherent radiation sources in a wide range of frequencies, from visible-UV to infrared. We briefly present the operational principles of OPOs, with particular emphasis on the aspects related to spectroscopic applications. Then, we report on recent results achieved with a narrow-linewidth singly-resonant cw OPO, emitting in the MIR, used for high-resolution spectroscopy of molecular transitions.

Combining focusing properties of a single diatom valve with optical eigenmodes in ultra-shrinking of light

It is known that a properly arranged distribution of nanoholes on a metallic slab is able to produce, in far field conditions, light confinement at sub-diffraction and even sub-wavelength scale. The same effect can also be implemented by the use of Optical Eigenmode (OEi) technique. In this case, a spatial light modulator (SLM) encodes phase and amplitudes of N probe beams whose interference is able to lead to sub-wavelength confinement of light focused by an objective. The OEi technique has been already used in a wide range of applications, such as photoporation, confocal imaging, and coherent control of plasmonic nanoantennas. Here, we describe the application of OEi technique to a single valve of a marine diatom. Diatoms are ubiquitous monocellular algae provided with an external cell wall, the frustule, made of hydrated porous silica which play an active role in efficient light collection and confinement for photosynthesis. Every frustule is made of two valves interconnected by a lateral girdle band. We show that, applying OEi illumination to a single diatom valve, we can achieve unprecedented sub-diffractive focusing for the transmitted light.