Solar cells with porous silicon: modification of surface-recombination velocity

Laser-induced transient-grating measurements were performed to monitor the influence of porous silicon on the surface recombination of a highly doped n⁺-silicon emitter of solar cells. With this technique, photocarrier diffusion and recombination with a time resolution of some tens of picoseconds can be studied. Using pulses of the second- and third-harmonic radiation from an Nd³⁺:YAG laser (quantum energy 2.34 and 3.51 eV, respectively), two different-depth regions of the emitter were excited. Using a kinetic model, which includes carrier diffusion and recombination at the surface and in the bulk of the emitter, surface-recombination velocities in a series of samples typical for each successive operation of solar-cell technology with different surface-doping level and surface preparation were evaluated. From the analysis, we conclude that porous silicon formed on the emitter passivates the surface of the silicon layer, i.e. reduces the rate of surface recombination at the porous silicon–crystalline silicon interface. Ytterbium as a co-dopant of the emitter increases the surface-recombination velocity. Received: 26 June 2000 / Accepted: 4 December 2000 / Published online: 26 April 2001     

Nano-islands on a composite substrate with misfit dislocations

Spatial arrangements of nano-islands (quantum dots) on the free surface of a composite two-layer substrate containing misfit dislocations of edge type are theoretically examined. It is shown that the elastic interaction between misfit dislocations and nano-islands is capable of causing coagulation of nano-islands. The coagulation of nano-islands is shown to be favourable when the upper-layer thickness is smaller than a critical thickness, H₀. An analytical form of H₀ is presented for the partial case with four-to-one correspondence between nano-islands and cells of the misfit dislocation network. Received: 5 December 2000 / Accepted: 29 March 2001 / Published online: 20 June 2001     

Size dependence of non-linear optical properties of SiO2 thin films containing InP nanocrystals

SiO₂ composite thin films containing InP nanocrystals were fabricated by radio-frequency magnetron co-sputtering technique. The microstructure of the composite thin films was characterized by X-ray diffraction and Raman spectrum. The optical absorption band edges exhibit marked blueshift with respect to bulk InP due to strong quantum confinement effect. Non-linear optical absorption and non-linear optical refraction were studied by a Z-scan technique using a single Gaussian beam of a He-Ne laser (632.8 nm). We observed the saturation absorption and two-photon absorption in the composite films. An enhanced third-order non-linear optical absorption coefficient and non-linear optical refractive index were achieved in the composite films. The nonlinear optical properties of the films display the dependence on InP nanocrystals size. Received: 27 June 2000 / Accepted: 27 June 2000 / Published online: 13 September 2000     

Threshold of femtosecond laser-induced damage in transparent materials

The rate at which conduction-band electrons (CBE) absorb laser energy is calculated by both the quantum mechanical and the classical methods. Here fused silica irradiated with a 780-nm femtosecond-pulse laser is used as an example. It is found that the rate obtained by the quantum mechanical method is about one-third of that by the classical method, and it is much less than the direct-current limit. In the flux-doubling model, the avalanche rate in fused silica is 4 I  ps^-1 obtained by the quantum mechanical method, while it is about 13.7 I  ps^-1 by the classical method, where the laser intensity I is in units of TW cm^-2. The differential equation of the evolution of CBE density is solved numerically, and it is found that the combination of CBE–hole recombination, CBE diffusion and initial CBE density (<10¹³ cm^-3) is not important. The dependence of avalanche breakdown threshold on laser-pulse duration is presented. The threshold calculated by the quantum mechanical method agrees well with experimental results, while the threshold obtained by the classical method differs greatly from the experiments. Received: 18 December 2000 / Accepted: 27 April 2001 / Published online: 27 June 2001     

Measurement of gas jet flow velocities using laser-induced electrostrictive gratings

We used time-resolved light scattering of cw probe laser radiation from laser-induced electrostrictive gratings for the determination of flow velocities in air at room temperature. Some possibilities of the technique have been experimentally demonstrated with submerged planar air jets in atmosphere, both for accumulated and single-shot measurements. The range of investigated flow velocities was 5–200 m/s. The method of data treatment and of the estimate of the experimental parameters is described. Received: 8 Febuary 2000 / Revised version: 2 May 2000 / Published online: 2 August 2000     

Scribing and cutting a blue LED wafer using a Q-switched. Nd:YAG laser

Chemical doping of single-wall carbon nanohorns (SWNHs) with K and Br was examined by vapor-phase reaction and studied by Raman scattering. Electro-chemical Li-ion doping using an electrolyte of LiAsF₆ in a mixture (1:1 by volume) of ethylene carbonate and diethyl carbonate was also carried out. All these experiments indicate that an anomalously small charge transfer occurs in saturation-doped SWNHs using these reagents, in sharp contrast to the behavior observed for single-wall carbon nanotube bundles and graphite. This rather remarkable result is not understood at present. Received: 13 July 2000 / Accepted: 14 July 2000 / Published online: 5 October 2000     

Laser manipulation of the size and shape of supported nanoparticles

Laser manipulation of the size and shape of metal nanoparticles prepared by self assembly of atoms on dielectric surfaces is discussed. The technique relies on the optical properties of the aggregates and the ability to remove atoms from their surfaces by laser induced thermal evaporation. A theoretical model which allows one to understand the basic mechanisms of the process is presented. Furthermore, experiments are reviewed which demonstrate that laser irradiation can be exploited for strong narrowing of initially broad size distributions yielding almost monodispersed samples and generation of aggregates with predetermined shape irrespective of their size. This makes possible preparation of very special surfaces with novel physical and chemical properties. Optical spectroscopy of the supported particles is demonstrated to be a very sensitive tool for characterization of such adsorbate/substrate systems, in particular for detection of laser induced modifications of the nanostructured surfaces. Finally, prospects for future experiments in this field and possible applications of the monodispersed systems are outlined. Received: 31 March 2000 / Accepted: 21 June 2000 / Published online: 7 March 2001     

Sub-picosecond photo-induced melting of a charge-ordered state in a perovskite manganite

Ultrafast melting of a charge-ordered state has been observed in the photo-irradiated colossal magnetoresistive compound Pr_0.7Ca_0.3MnO₃. Pump-and-probe spectroscopy experiments reveal the formation of a conducting phase with typical features of an insulator–metal transition (IMT) after less than 1 ps. This phase is metastable and can be maintained for about 1 μs unless it is stabilized persistently into a pathlike metallic region by an electric field. Although laser-induced lattice heating may play a role in the initial excitation, electronic correlations are the dominant effect which leads to the formation of the metallic state upon the breakdown of the charge-ordered state. Received: 26 January 2000 / Published online: 16 June 2000     

Ultra-fast spectroscopy and extreme nonlinear optics by few-optical-cycle laser pulses

Many fundamental processes of radiation–matter interaction, which take place on the ultra-short time scale, can now be directly investigated by using few-optical-cycles light pulses with duration <10 fs. We discuss two techniques for generating such pulses: broad-band parametric amplification, which allows the generation of pulses in the visible range suitable for spectroscopy, and compression of high-energy light pulses in a hollow fiber. As an example of application in time-resolved spectroscopy we report results of pump–probe experiments in a prototypical conjugated molecule, namely sexithiophene. The new laser sources, due to their characteristics of peak power and coherence, also allow the exploration of new fields of experimental physics, such as extreme nonlinear optics. We focus on high-order harmonics, showing that a high-energy bunch of photons, up to the X-ray-energy region, with coherence typical of laser radiation and time duration comparable to or shorter than the exciting pulses, can be generated. Received: 31 July 2000 / Revised version: 19 September 2000 / Published online: 8 November 2000     

Quantum Groupoids

We introduce a general notion of quantum universal enveloping algebroids (QUE algebroids), or quantum groupoids, as a unification of quantum groups and star-products. Some basic properties are studied including the twist construction and the classical limits. In particular, we show that a quantum groupoid naturally gives rise to a Lie bialgebroid as a classical limit. Conversely, we formulate a conjecture on the existence of a quantization for any Lie bialgebroid, and prove this conjecture for the special case of regular triangular Lie bialgebroids. As an application of this theory, we study the dynamical quantum groupoid , which gives an interpretation of the quantum dynamical Yang–Baxter equation in terms of Hopf algebroids. Received: 6 April 2000 / Accepted: 15 August 2000     

Self-assembling of gold nanoparticles in one, two, and three dimensions

In the present work, we report the growth of ordered arrays of gold nanoparticles passivated with n-alkylthiol molecules using crystallization in toluene vapor. We kept constant the average particle size and the length of the passivating molecule (1-dodecanethiol). The temperature and time of growth were varied. We show that in the initial stages of the growth, nano-chains of particles are produced. These nano-chains aggregate to form two-dimensional arrays. In the initial stages the nano-chains form a two-dimensional square lattice which then relaxes to a close-packed structure. In latter stages of growth a three-dimensional supercrystal is produced. It is found that the packing of nanoparticles corresponds to an average FCC lattice. However, large variations on the local parameters of the lattice are observed. Near the edges of the supercrystal the anomalous packing reported by Zanchet et al. [20] and Fink et al. [21] was observed. The energy of the observed structures is analyzed using molecular dynamics simulation. It is concluded that using the vapor growth method, it is possible to produce controlled ordered structures from one to three dimensions. Received: 24 February 2000 / Accepted: 28 March 2000 / Published online: 13 July 2000     

Femtosecond dynamics of chemical reactions at surfaces

One of the major goals in physical chemistry is to obtain a microscopic understanding of chemical reactions. Recent developments in femtosecond laser techniques provide the opportunity to resolve the timescale of elementary steps of chemical reactions at surfaces. This is exemplified for the femtosecond laser-induced oxidation of CO on Ru(001). Among other adsorbate-specific probes vibrational sum-frequency generation spectroscopy offers the possibility to monitor adsorbates or reaction intermediates directly at the surface. Recently, we have employed this technique to investigate the dynamics of the CO-stretch vibration of CO adsorbed on Ru(001) after optical excitation leading to CO desorption. Received: 4 August 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Thermoelastic wave induced by pulsed laser heating

In this work, a generalized solution for the thermoelastic plane wave in a semi-infinite solid induced by pulsed laser heating is developed. The solution takes into account the non-Fourier effect in heat conduction and the coupling effect between temperature and strain rate, which play significant roles in ultrashort pulsed laser heating. Based on this solution, calculations are conducted to study stress waves induced by nano-, pico-, and femtosecond laser pulses. It is found that with the same maximum surface temperature increase, a shorter pulsed laser induces a much stronger stress wave. The non-Fourier effect causes a higher surface temperature increase, but a weaker stress wave. Also, for the first time, it is found that a second stress wave is formed and propagates with the same speed as the thermal wave. The surface displacement accompanying thermal expansion shows a substantial time delay to the femtosecond laser pulse. On the contrary, surface displacement and heating occur simultaneously in nano- and picosecond laser heating. In femtosecond laser heating, results show that the coupling effect strongly attenuates the stress wave and extends the duration of the stress wave. This may explain the minimal damage in ultrashort laser materials processing. Received: 23 May 2000 / Accepted: 26 May 2000 / Published online: 20 September 2000     

Optical investigation of CdS quantum dots in Langmuir–Blodgett films

Structures with CdS quantum dots produced by the Langmuir–Blodgett (LB) technique were investigated by Raman, IR, and UV spectroscopies. The confinement effect of longitudinal optical (LO) phonons in CdS quantum dots was investigated by Raman spectroscopy. Surface vibrational modes of CdS quantum dots were observed in IR spectra. It was shown experimentally that the frequency of the surface vibrational modes depends on the properties of the surrounding media. An average size of CdS quantum dots of about 3–6.4 nm was obtained from the analysis of UV measurements. Received: 1 February 1999 / Accepted: 1 April 1999 / Published online: 19 May 1999     

Heat transport in a four-perpendicularity-bend quantum waveguide

Using the scattering matrix method, we investigate acoustic phonon transmission and thermal conductance in a four-perpendicularity-bend quantum waveguide at low temperatures. The transmission spectrum of the quantum waveguide displays a series of resonant peaks and dips; and when one of the bend heights is larger than or equal to the minimum of the dimensions of the phonon channel in the quantum waveguide, a stop-frequency gap will appear; and some single four-perpendicularity-bend quantum waveguides with larger bend heights exhibit narrower width or smaller number of the stop-frequency gaps than that with smaller bend heights. The thermal conductivity is much sensitive to the change of the smaller heights and longitudinal lengths of the bend section; and the thermal conductivity decreases with the increasing of the temperature first, then increases after it reaches a minimum. The investigations of multiple four-perpendicularity-bend waveguides connected in series indicate that the first additional waveguide suppresses the transmission coefficient and forms the stop-frequency gap; and two additional resonance peaks will be formed when each four-perpendicularity-bend waveguide is added in the series. The results could be useful for controlling thermal conductance artificially and the design of phonon devices.     

Micromanipulation of neutral atoms with nanofabricated structures

A large variety of trapping and guiding potentials can be designed by bringing cold atoms close to charged or current carrying material objects. We describe the basic principles of constructing microscopic traps and guides and how to load atoms into them. The simplicity and versatility of these methods will allow for miniaturization and integration of atom optical elements into matter-wave quantum circuits on Atom Chips. These could form the basis for robust and widespread applications in atom optics, ranging from fundamental studies in mesoscopic physics to possibly quantum information systems. Received: 20 December 1999 / Revised version: 7 March 2000 / Published online: 5 April 2000     

Ultrafast redistribution of vibrational excitation of CH-stretching modes probed via anti-Stokes Raman scattering

Resonant infrared femtosecond light pulses in the 3 μm region are used to excite specific CH-stretching modes of liquid 1,1,1-trichloroethane, cyclopentane, and cyclohexane. The ultrafast redistribution of vibrational excitation between different CH-stretching modes and the vibrational relaxation of these modes are monitored by spontaneous anti-Stokes Raman scattering of properly delayed and spectrally shaped probe pulses. The results are treated with a rate-equation system, yielding energy-redistribution and energy-relaxation times. Received: 16 December 1999 / Published online: 7 August 2000     

Atomistic processes and the strain distribution in the early stages of thin film growth

Structural relaxations in small Co islands on the Cu(001) surface are investigated performing atomistic calculations. We demonstrate that the strain relief at the metal interface in the early stages of heteroepitaxy is more complicated than suggested by simple considerations based on the small mismatch between the Co and Cu bulk metals. We found that the strain distribution in the surface region near the islands varies strongly on an atomic scale. The effect of strain on the shape of the Co islands is revealed. Diffusion on the top of strained islands and edge diffusion are considered. Received: 10 April 2000 / Accepted: 15 May 2000 / Published online: 7 March 2001     

Depth profiling the thermal reflection coefficient of an opaque solid via an inverse thermal wave scattering theory based on the Method of Images

A new formulation of the inverse problem of depth profiling the thermal properties of an opaque solid based on one-dimensional photo-generated thermal waves is presented. The inverse problem as posed is linear in a set of lumped thermal reflection coefficients which account for the return of energy to the surface by all significant heat conduction channels. An analysis based on the Method of Images relates these coefficients to individual values of the interface thermal reflection coefficients in the material. No weak backscattering assumption is invoked to linearize the problem. The method yields a unique solution subject to a given condition of regularization. Solutions recovered by the method are stable at experimentally feasible error levels. Received: 27 September 1999 / Published online: 16 June 2000     

Strongly Coupled Quantum Discrete Liouville Theory.¶I: Algebraic Approach and Duality

The quantum discrete Liouville model in the strongly coupled regime, 1 < c < 25, is formulated as a well defined quantum mechanical problem with unitary evolution operator. The theory is self-dual: there are two exponential fields related by Hermitian conjugation, satisfying two discrete quantum Liouville equations, and living in mutually commuting subalgebras of the quantum algebra of observables. Received: 26 May 2000 / Accepted: 28 May 2000