A 7-W diode-pumped Nd:YVO4 cw laser at 1.34 μm

A Nd:YVO₄ laser, end-pumped by a fiber-coupled diode-laser array, generates 7.3 W of output power at 1342 nm, the highest so far reported for this host crystal. The slope efficiency is 40% and the output-beam divergence is close to the diffraction limit. An important point in attaining such results is the choice of crystals with low Nd concentration. Received: 16 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +39-050/844333, E-mail: dilieto@df.unipi.it RID="**" ID="**"Permanent address: Scuola Normale Superiore, Piazza dei Cavalieri 7, Pisa, Italy     

Optical characterization and manipulation of alkali metal nanoparticles in porous silica

Rubidium and cesium metal nanoparticles were grown in nanoporous silica samples placed in alkali vapor cells. Their size and shape were investigated by measuring the sample optical transmittance. Spectral changes due to photodesorption processes activated by weak light were also analyzed. Alkali atoms photoejected from the silica walls diffuse through and out of the nanopores, modifying both the nanoparticle distribution in the silica matrix and the atomic vapor pressure in the cell volume. The number of rubidium and cesium atoms burst out of the samples was measured as a function of photon energy and fluence. The optical absorption measurements together with the analysis of the photodesorption yield give a complete picture of the processes triggered by light inside the nanopores. We show that atomic photodesorption, upon proper choice of light frequency and intensity, induces either growth or evaporation of nanosized alkali metal clusters. Cluster size and shape are determined by the host-guest interaction.     

Self-assembling of In(Ga)As/GaAs quantum dots on (N11) substrates: the (311)A case

We have investigated the In(Ga)As island formation, in the Stranski-Krastanov growth mode, on (311)A GaAs substrates. The surface topography of InAs and InGaAs strained epilayers was studied by contact microscopies. The different substrate affects the overgrown island shape. In(Ga)As grown on (311)A gives rise to quantum wire-like islands. Quantum dots (QDs), but with highly anisotropic shapes, are the outcomes of InAs deposition. QD samples were also characterized by photoluminescence (PL) measurements. Correlation between optical and morphological properties was observed.     

Fast emission dynamics in droplet epitaxy GaAs ring-disk nanostructures integrated on Si

The emission dynamics in GaAs/AlGaAs coupled ring-disk (CRD) quantum structures fabricated on silicon substrates is presented. The CRD structures are self-assembled via droplet epitaxy, a growth technique which, due to its low thermal budget, is compatible with the monolithic integration of III-V devices on Si based electronic circuits. Continuous wave, time resolved photoluminescence and theoretical calculations in the effective mass approximations are presented for the assessment of the electronic and carrier properties of the CRDs. The CRDs show a fast carrier dynamics which is expected to be suitable for ultrafast optical switching applications integrated on silicon.     

Quantum Dots Luminescence Collection Enhancement and Nanoscopy by Dielectric Microspheres

In recent years, dielectric microspheres have been used in conjunction with optical microscopes to beat the diffraction limit and to obtain superresolution imaging. The use of microspheres on quantum dots (QDs) is investigated, for the first time, to enhance the light coupling efficiency. The enhancement of the QD luminescence collection in terms of extraction and directionality is demonstrated, as well as the enhancement of spatial resolution. In particular, it is found that a dielectric microsphere, placed on top of an epitaxial QD, increases the collected radiant energy by about a factor of 42, when a low numerical aperture objective is used. Moreover, if two or more QDs are present below the microsphere, the modification of the far field emission pattern allows selective collection of the luminescence from a single QD by simply changing the collection angle. Dielectric microspheres present a simple and efficient tool to improve the QD spectroscopy, and potentially QD-based devices.     

Carrier recombination kinetics in (3 1 1)A InGaAs sidewall quantum wires

We present a study of the optical properties and carrier dynamics in strained InGaAs sidewall quantum wires (QWR) on patterned GaAs (3 1 1)A substrates by means of picosecond time-resolved photoluminescence (PL). A pronounced dynamical red shift of the QWR-PL band when increasing the delay time after the pulse excitation is observed. In addition, time-resolved data show a significant shortening of the PL decay time from the wire at short delay and when high excitation power is used. The data are compared with theoretical predictions. The results, i.e. the dynamical red shift observed in the wire emission and the shortening of the PL decay with increasing the excitation density, are interpreted in terms of a dynamical screening effect of the piezoelectric field.     

Francium sources and traps for fundamental interaction studies

Francium is one of the best candidates for atomic parity nonconservation (APNC) and for the search of permanent electric dipole moments (EDMs). APNC measurements test the weak force between electrons and nucleons at very low momentum transfers. They also represent a unique way to detect weak nucleon-nucleon interactions. EDMs are instead related to the time-reversal symmetry. Preliminary to these fundamental measurements are precision studies in atomic spectroscopy and the development of magneto-optical traps (MOT), which partially compensate for the lack of stable Fr isotopes. At LNL Legnaro, francium is produced by fusion of 100-MeV ¹⁸O with ¹⁹⁷Au in a thick target, followed by evaporation of neutrons from the compound nucleus. Francium diffuses inside the hot target (1200 K) and is surface ionized for injection at 3 keV in an electrostatic beamline. Typically, we produce 1×10⁶ (²¹⁰Fr ions)/s for a primary flux of 1.5×10¹² particles/s. We have studied Fr yields as a function of primary beam energy, intensity, and target temperature. Information on the efficiency of bulk diffusion, surface desorption and ionization is deduced. The beam then enters a Dryfilm-coated cell, where it is neutralized on a heated yttrium plate. The escape time of neutral Fr (diffusion + desorption) is approximately 20 s at 950 K, as measured with a dedicated setup. In the MOT, we use 6 orthogonal Ti:sapphire laser beams for the main pumping transition and 6 beams from a stabilized diode repumper. Fluorescence from trapped atoms is observed with a cooled CCD camera, in order to reach noise levels from stray light equivalent to approximately 50 atoms. Systematic tests are being done to improve the trapping efficiency. We plan to further develop Fr traps at LNL; in parallel, we will study APNC and EDM techniques and systematics with stable alkalis at Pisa, Siena, and Ferrara.     

Correlations of secondaries in events with a forward negative hadron at the ISR

Data obtained from p-p collisions at centre-of-mass energies between 31 and 63 GeV are presented on correlations between momentum analysed forward π^?, K^? and $\text{p}$ and charged particles observed in an omnidirectional hodoscope. The data show that significant correlations are present over the whole rapidity range for all three types of negative particles. The dependence on various kinematic variables suggests a cluster mechanism for the production of particles. In this picture, pions would be produced in clusters emitted in the fragmentation region while K^? and $\text{p}$ emanate from non-leading clusters.