Paolo Bettotti

Rewritable photonic circuits

The authors present a technique that allows to modify the local characteristics of two-dimensional photonic crystals by controlled microinfiltration of liquids. They demonstrate experimentally that by addressing and infiltrating each pore with a simple liquid, e.g., water, it is possible to write pixel by pixel optical devices of any geometry and shape. Calculations confirm that the obtained structures indeed constitute the desired resonators and waveguide structures.

P-type macroporous silicon for two-dimensional photonic crystals

Macroporous silicon with two-dimensional periodicity has been produced by electrochemical etching, using a p-type doped silicon substrate. The structure shows photonic energy gaps in the infrared region, as demonstrated by variable angle reflectance measurements. The agreement between measurement and band calculations confirms the high quality of the samples. The use of an optimized electrolyte allows the fabrication of very high quality samples, with high aspect ratio and low roughness both at the surface and on the pore walls. The best results are obtained with aprotic and protophilic solvents.

Porous silicon free-standing coupled microcavities

We report the experimental characterization of porous silicon free-standing coupled microcavities. We have grown free-standing structures of up to 109 stacked layers. Free-standing structures are interesting because reflectance spectra can be measured on both sides of the samples. The comparison of reflectance spectra from the front and back side indicates that the porous silicon anodization process has a natural drift along the growth direction. However, we demonstrate that this drift can be compensated, showing a homogeneous structure of ten coupled microcavities, in which all ten resonance peaks are resolved in both transmission and reflection measurements.

Si nanocrystals obtained through polymer pyrolysis

In this letter, we report the formation of bulk samples of silica-based glass containing Si nanocrystals (Si-ncs) by pyrolysis of a preceramic precursor. The starting precursor is a sol–gel-derived polysiloxane containing only Si–H groups which leads, after annealing in a controlled atmosphere in the range 1000–1200u200a°C, to the precipitation of Si-ncs. Characterization of the nanostructure was performed by x-ray diffraction and Raman scattering analyses. Room-temperature luminescence experiments show the interesting optical properties of the Si-ncs/SiO2 material.

Band gap characterization and slow light effects in one dimensional photonic crystals based on silicon slot-waveguides

We investigate the photonic properties of one dimensional photonic crystals realized on Silicon On Insulator channel slot-waveguide to engineer slow light effects. Various geometries of the photonic pattern have been characterized and their photonic band-gap structure analyzed. The optimal geometry has been further used to realize a coupled resonator optical waveguide (CROW). A first optimization of these CROW devices shows a group velocity of more than c/10 at 1.55 mum. Full three dimensional calculations based on the planar wave expansion method have been used to compute the band diagram while full three dimensional calculations based on finite difference time domain methods have been used to study light propagation.

Dry adhesive bonding of nanoporous inorganic membranes to microfluidic devices using the OSTE(+) dual-cure polymer

We present two transfer bonding schemes for incorporating fragile nanoporous inorganic membranes into microdevices. Such membranes are finding increasing use in microfluidics, due to their precisel ...

Interference lithography by a soft x-ray laser beam: Nanopatterning on photoresists

We have studied the feasibility of high-resolution laser interference lithography using a tabletop 46.9nm, 1.5ns Ar laser, combined with two different optical configurations based on a Lloyd’s mirror interferometer. Using one of these schemes we have encoded periodic grating structures with a half pitch of 42nm and a vertical modulation of 5nm on a commercial PMMA photoresist. Experiments performed with larger half-pitch structures and detailed theoretical calculations demonstrate the potentiality of producing periodic structures with a half-pitch resolution down to 20nm and a height of up to 60nm. The results can be of considerable interest for the development of a complete high-resolution lithographic process operating with the 47nm laser wavelength.

Role of microstructure in porous silicon gas sensors for NO2

Electrical conductivity of porous silicon fabricated from heavily doped p-type silicon is very sensitive to NO2, even at concentrations below 100ppb. However, sensitivity depends strongly on the porous microstructure. The structural difference between sensitive and insensitive samples is independently confirmed by microscopy images and by light scattering behavior. A way to change the structure is by modifying the composition of the electrochemical solution. We have found that best results are achieved using ethanoic solutions with HF concentration levels between 12% and 13%.

Scattering rings in optically anisotropic porous silicon

We report the observation of strongly anisotropic scattering of laser light at oblique incidence on a (100)-oriented porous silicon layer. The scattered light forms cones tangent to the incident and reflected beams. The conical pattern is caused by scattering on the vertical walls of pores, which are straight along the layer thickness. The light cone defines structured light rings onto a screen normal to the cone axis. We explain the various structures by optical anisotropy of porous silicon. For the sample under analysis, we directly measure from the ring patterns a value of Δn/nord=8% of positive birefringence.

Nanostructured silicon as a photonic material

Photonics applications of silicon are presented. In particular it is demonstrated that silicon when rendered low dimensional, e.g. in form of nanocrystals or quantum wires, can be turned into an active photonic materials which shows light amplification characteristics, non-linear optical effects, photon confinement in both one and two dimensions, photon trapping with evidences of light localization, and gas sensing properties.