to be done in this area. Face recognition is a problem that scarcely clamoured for attention before the computer age but, having surfaced, has involved a wide range of techniques and has attracted the attention of some fine minds (David Mumford was a Fields Medallist in 1974). This singular application of mathematical modelling to a messy applied problem of obvious utility and importance but with no unique solution is a pretty one to share with students: perhaps, returning to the source of our opening quotation, we may invert Duncan's earlier observation, 'There is an art to find the mind's construction in the face!'.

Linear and nonlinear waves, by G. B. Whitham. Pp.636. £50. 1999. ISBN 0 471 35942 4 (Wiley).

http://ltl.tkk.fi/ice/Kevo/noisereview.pdf

Shot noise in mesoscopic conductors

Optical Studies of Dynamics in Noble Metal Nanostructures

This review describes recent groundbreaking results in Si, Si/SiGe, and dopant-based quantum dots, and it highlights the remarkable advances in Si-based quantum physics that have occurred in the past few years. This progress has been possible thanks to materials development of Si quantum devices, and the physical understanding of quantum effects in silicon. Recent critical steps include the isolation of single electrons, the observation of spin blockade, and single-shot readout of individual electron spins in both dopants and gated quantum dots in Si. Each of these results has come with physics that was not anticipated from previous work in other material systems. These advances underline the significant progress toward the realization of spin quantum bits in a material with a long spin coherence time, crucial for quantum computation and spintronics.

/pdf/silicon-quantum-electronics-4bxpe9czeo.pdf

Silicon quantum electronics

https://hal.archives-ouvertes.fr/hal-00120432/document

Raman Spectroscopy of Nanomaterials: How Spectra Relate to Disorder, Particle Size and Mechanical Properties

We present the first experimental investigation of ultrafast optical switching in a three-dimensional photonic crystal made of a Si-opal composite. Ultrafast (30 fs) changes in reflectivity around the photonic stop band up to 1% were measured for moderate pump power $(70\text{ }\text{ }\ensuremath{\mu}\mathrm{J}/{\mathrm{c}\mathrm{m}}^{2})$. Short-lived photoexcited carriers in silicon induce changes in the dielectric constant of Si and diminish the constructive interference inside the photonic crystal. The results are analyzed within a model based on a two-band mixing formalism.

/pdf/ultrafast-optical-switching-in-three-dimensional-photonic-1l1ld4wv52.pdf

Ultrafast optical switching in three-dimensional photonic crystals.

At carrier densities above the Mott density, Coulomb screening destroys the exciton resonance. This, together with band-gap renormalization and band filling, severely affects the optical spectra. We have experimentally studied these effects by ultrafast pump-probe reflectivity measurements on a ZnO single crystal at various wavelengths around the exciton resonance and in a broad carrier-density range. Theoretically, we determined the Mott density in ZnO to be $1.5\ifmmode\times\else\texttimes\fi{}{10}^{24}$ m${}^{\ensuremath{-}3}$ at 300 K. Taking a field-theoretical approach, we derived and solved the Bethe-Salpeter ladder equation and we computed the density-dependent reflectivity and absorption spectra. A carrier dynamics model has been developed, containing three-photon absorption, carrier cooling, and carrier trapping near the surface. The agreement between the theoretical reflectivity based on our model and the experimental data is excellent.

/pdf/ultrafast-screening-and-carrier-dynamics-in-zno-theory-and-4orrakqrnk.pdf

Ultrafast screening and carrier dynamics in ZnO: Theory and experiment

Are excitons involved in lasing in ZnO nanowires or not? Our recently developed and experimentally tested quantum many-body theory sheds new light on this question. We measured the laser thresholds and Fabry-Perot laser modes for three radically different excitation schemes. The thresholds, photon energies, and mode spacings can all be explained by our theory, without invoking enhanced light-matter interaction, as is needed in an earlier excitonic model. Our conclusion is that lasing in ZnO nanowires at room temperature is not of excitonic nature, as is often thought, but instead is electron-hole plasma lasing. more accurate result than was obtained by others. This value was confirmed in a pump-probe reflectivity experi- ment by the gradual disappearance of the exciton reso- nance when passing this electron-hole density (18). In this Letter, we use the same theory to determine the laser mechanism in ZnO nanowires at room temperature. We compare the theory with experimental results, where we excited ZnO nanowires in three radically different ways. Our results consistently show that room-temperature lasing in ZnO nanowires occurs in the electron-hole plasma re- gime. Moreover, many-body theory can excellently explain the observed laser threshold, the photon energy of the laser emission, and the spectral spacings between the laser peaks.

/pdf/room-temperature-laser-emission-of-zno-nanowires-explained-77l7ks4lu3.pdf

Room-temperature laser emission of ZnO nanowires explained by many-body theory.

We demonstrate the development of high-amplitude picosecond strain pulses in a sapphire single crystal into an ultrafast compressional soliton train. For this purpose, large-intensity light pulses were used to excite a metal film, yielding a 2 orders of magnitude higher strain than that achieved in earlier studies. Propagation of the packets is monitored over a distance of several millimeters by means of Brillouin light scattering. A one-parameter model, based on the Korteweg-de Vries-Burgers equation, simultaneously explains the observed behavior at all strains and temperatures under study. We predict up to 11 solitons in the train, reaching pressures as high as 40 kbar and 0.5 ps temporal widths.

/pdf/high-amplitude-ultrashort-longitudinal-strain-solitons-in-489d14v5ri.pdf

High amplitude, ultrashort, longitudinal strain solitons in sapphire.

We demonstrate that a diffusive narrow wire of a length much longer than the elastic scattering length and the phase-coherence length, but of the order of the energy relaxation length, exhibits shot noise with an intensity lower than the full shot-noise level. At 4.2 K, the reduction factor is shown to vary between 0.2 and 0.45, depending on the width of the wire. The reduction is consistent with recent theoretical predictions.

Jaap I. Dijkhuis

Papers

Ultrafast optical switching in three-dimensional photonic crystals.

Ultrafast screening and carrier dynamics in ZnO: Theory and experiment

Room-temperature laser emission of ZnO nanowires explained by many-body theory.

High amplitude, ultrashort, longitudinal strain solitons in sapphire.

Experimental study of reduced shot noise in a diffusive mesoscopic conductor.