Luca Dal Negro

Optical gain in silicon nanocrystals

Silicon nanocrystals, formed by ion implantation and subsequent thermal annealing, show positive optical gain under intense laser excitation. Gain has been measured by the variable strip length method where the amplified spontaneous emission intensity, which is emitted from the sample edge, is measured as a function of the excitation volume. Exponential increase, line narrowing and directionality of stimulated emission have been measured. In addition, by growing silicon nanocrystals in a quartz substrate, single pass gain in pump and probe transmission experiments has been measured. Material gain values as high as those typically found in III-V semiconductors quantum dots have been measured. We claim that population inversion is realized between the fundamental and the recently identified Si equals O interface state. This model explains the gain observations and could account for the lack of auger saturation, free carrier absorption and size dispersion. Critical issues to obtain sizable gain are (1) high oxide quality, (2) high areal density of silicon nanocrystals, and (3) nanocrystals placed in the core region of a waveguide.

Optical gain in PECVD silicon nanocrystals

Optical gain has been recently observed in ion implanted Si nanocrystals (nc). Critical issues to the observation of optical gain are the formation of a waveguide structure to improve the mode confinement and a large nanocrystal area den-sity in the samples. Here we confirm these results by measuring optical gain by the variable stripe length (VSL) method on a set of silicon nanocrystals (nc) formed by plasma enhanced chemical vapor deposition (PECVD) and annealing treatments. Time resolved VSL measurements with ns pulses at high pumping fluencies have revealed fast component in the recombination dynamics under gain conditions. Lifetime shortening and superlinear emission have been unambi-guously observed. The spectral shape of the fast luminescence is consistent with the amplified spontaneous emission lineshape (ASE) observed under CW pumping conditions and overlaps the gain spectral band. The observation of light amplification is critically dependent on a very delicate balance among the nc gain cross sections, the optical mode losses of the waveguide structure, and the fast non radiative Auger processes. Within a four levels model we quantify the strong competition among all these processes and we obtain a satisfactory agreement with the experiments.

Light transport through porous silicon coupled microcavities

We report the experimental characterization of porous silicon free-standing coupled microcavities and the investigation of light transport properties in such structures, when ultrashort laser pulses are introduced and analyzed.