Formation and charge control of a quantum dot by etched trenches and multiple gates

We have fabricated a GaAs/InGaAs/AlGaAs-based single-electron transistor (SET) formed by etched trenches and multiple gates. Clear Coulomb-blockade oscillations have been observed when the gate biases are scanned. By self-consistently solving three-dimensional Schr?dinger and Poisson equations, we have studied the energy-band structure and the carrier distribution of our SET. General agreement between numerical simulation results and measurement data has been obtained, thus indicating the effectiveness of our SET-device design as well as the necessity of a complete three-dimensional quantum-mechanical simulation. Received: 18 October 2001 / Accepted: 6 January 2002 / Published online: 20 March 2002     

Self-similarity in highway traffic

Highway traffic as simulated with a simple cellular automata model has been analysed in a search for self-similarity in the behaviour of car density and car flow as a function of space. Fractal dimensions between 1.5 and 1.6, depending on the simulation characteristics, have been measured with a box counting algorithm. The self-similarity spans over about 2 orders of magnitude. A comparison with experimental data is suggested. Received 23 April 2001 and Received in final form 28 September 2001     

High-energy ion generation in interaction. of short laser pulse with high-density plasma

Multi-MeV ion production from the interaction of a short laser pulse with a high-density plasma, accompanied by an underdense preplasma, has been studied with a particle-in-cell simulation and good agreement is found with experiment. The mechanism primarily responsible for the acceleration of ions is identified. Comparison with experiments sheds light on the ion-energy dependence on laser intensity, preplasma scale length, and relative ion energies for a multi-species plasma. Two regimes of maximum ion-energy dependence on laser intensity, I, have been identified: subrelativistic, ∝I; and relativistic, ∝. Simulations show that the energy of the accelerated ions versus the preplasma scale length increases linearly and then saturates. In contrast, the ion energy decreases with the thickness of the solid-density plasma. Received: 13 December 2001 / Published online: 7 February 2002     

Squeezed light from microstructured fibres: towards free-space quantum cryptography

Amplitude-squeezed pulsed light has been produced using a microstructured silica fibre. By spectrally filtering after the non-linear propagation in the fibre a squeezing value of -1.7 dB has been measured. A quantum key distribution scheme based on squeezed light from such microstructured fibres is proposed. Received: 9 July 2001 / Published online: 23 November 2001     

Infiltration of colloidal crystal with nanoparticles using capillary forces: a simple technique for the fabrication of films with an ordered porous structure

A dipping method has been developed for the infiltration of nanoparticles into an opal template to fabricate high quality inverse opal. Titania and silica inverse opal films, with a uniform color over centimeter dimensions were derived. As there is no need for special substrates or equipment, a widespread application of this method is anticipated. Received: 23 August 2001 / Accepted: 27 August 2001 / Published online: 30 October 2001     

High Fe2+/3+ trap concentration in heavily compensated implanted InP

High Fe concentrations (up to 2×10¹⁹ cm^-3) have been implanted in n-doped InP to compensate the substrate donors. The resulting semi-insulating layers have been investigated by current–voltage (I-V) measurements and photo-induced current transient spectroscopy (PICTS) analyses to characterise the Fe activation process and to study the Fe related deep levels. The activation of the Fe^2+/3+ trap has been assessed by the identification of the deep level located at E_C-0.64 eV. The outcomes of the PICTS measurements have been correlated with the electrically active Fe concentration calculated from a numerical simulation of the I-V characteristics. We observe an increasing linear relation between the electrically active Fe concentration and the substrate doping density, with a maximum active Fe concentration as high as 2×10¹⁸ cm^-3, i.e. more than an order of magnitude above the equilibrium Fe solid solubility. These data are presented and their implications discussed. Received: 4 September 2000 / Accepted: 7 February 2001 / Published online: 23 May 2001     

Dynamic hysteresis in ferroelectric systems: experiment and Monte Carlo simulation

A Monte Carlo algorithm for dynamic hysteresis simulation in ferroelectric spin systems is developed based on a DIFFOUR model in which the local spontaneous polarization is defined by the double-well potential energy and the nearest-neighbor spin interaction as well as an external electrical field of variable amplitude and frequency. A direct measurement of the hysteresis loop for ferroelectric Pb(Zr_0.52Ti_0.48)O₃ thin film capacitors using the Sawyer–Tower technique is performed. Significant dependence of the hysteresis shape and pattern on the external field is revealed. Direct imaging of the simulated domain pattern indicates serious suppression of the domain switching over the high-frequency range. The evaluated scaling relations from the simulation for remanence, coercivity, and the area of the hysteresis over the low-frequency range are supported by theoretical predictions and experiments, but the high-frequency scaling behaviors as derived are different from one and another. Received: 23 January 2001 / Accepted: 17 August 2001 / Published online: 20 December 2001     

Determination of the dielectric function of porous silicon by high-order laser-harmonic radiation

Porous-silicon reflectance has been determined over a large energy range, from 1 eV to 16 eV, by combining a NIR/visible/UV spectrometer with a new VUV light source as laser-harmonic radiation. The porous-silicon dielectric function was deduced from reflectance measurements by Kramers–Kronig analysis. We point out that, for the first time, laser harmonics have been applied in the optical characterization of materials as a new and suitable alternative to synchrotron radiation. Received: 9 January 2001 / Accepted: 28 April 2001 / Published online: 20 June 2001     

Sampling high-frequency signals using a poled-polymer asymmetric Fabry–Perot microcavity as an electro-optic probe tip

The sampling of high-frequency signals using poled electro-optic polymer films as electro-optic probe tips has been demonstrated for the first time. The electro-optic polymer, which was spin coated onto a high-reflectivity glass substrate, was corona poled; thus an asymmetric Fabry–Perot microcavity was formed, based on the difference between the polymer reflectivity and that of high-reflectivity glass. This converts phase modulation to amplitude modulation, so only one laser beam is needed in this system. The sampling technique has been analyzed by multiple reflection and index ellipsoid methods. A 1.2 GHz microwave signal propagating on a coplanar waveguide transmission line has been sampled, and a voltage sensitivity of about 0.5 mV/ was obtained. Received: 16 August 2001 / Revised version: 20 November 2001 / Published online: 14 March 2002     

Anisotropic self-diffraction under Bragg mismatching

Anisotropic self-diffraction (ASD) under Bragg mismatch has been studied. We derive a solution that can describe well the diffraction characteristics of the anisotropic self-diffraction under Bragg mismatch. The solution is useful for estimating the Bragg constraint when the ASD is applied to optical information processing. Both the theory and the experiment are presented. Received: 11 July 2000 / Revised version: 2 January 2001 / Published online: 30 March 2001     

Impact of the cell geometry on the spectral properties of photonic crystal structures

Photonic-crystal-based devices are expected to perform many of the functionalities of standard integrated optics devices used in optical communication systems. Their reliability and the reliability of fabrication technologies in achieving acceptable geometrical tolerance are still to be demonstrated. In this work an analysis of the effects of the cell geometry is presented, showing how small variations can cause hundreds of GHz shifts in the spectral response. A wide-band, finite-element time-domain approach and a frequency-domain formulation have been used for this kind of analysis. The ability of the finite-element method in coping with any kind of geometry has been successfully exploited to investigate the effect of the cell shape. Received: 16 May 2001 / Revised version: 8 August 2001 / Published online: 30 October 2001     

Second-harmonic generation in Zn-diffused periodically poled LiNbO3 channel waveguides

In this work second-harmonic generation by quasi-phase matching (QPM) in Zn-diffused periodically poled lithium niobate channel waveguides is presented. A stable TM?TE conversion by QPM has been found. The results are in good accordance with theoretical estimations obtained by the phase-matching condition, either for the polarisation character of the second-harmonic wave as well as for the spectral range, taking into account the periodicity of the domains. Received: 16 May 2001 / Revised version: 7 September 2001 / Published online: 30 October 2001     

High efficiency lasing of a dye-doped polymer laser with 1.06 μm pumping

For the first time, tunable lasing from a dye laser with an active polymer medium has been obtained using 1.06 μm pumping. The conversion efficiency of 43% and the tunable range of Δλ=63 nm have been reached with the use of polymethine dye in a polyurethane matrix. Received: 20 April 2001 / Revised version: 16 July 2001 / Published online: 19 September 2001     

Structure and morphology of chemical bath-deposited CdS films and clusters

Thin films of cadmium sulfide have been deposited on glass substrates and the structural properties of films have been investigated using scanning electron microscopy and X-ray diffraction techniques. The films consist of domains (groups of grains) and weakly bound grain clusters. The structural parameters of grains, domains and clusters and the effect of film thickness on these parameters are reported. From the measurement of lattice constants in CdS films and in free CdS clusters, it has become evident that the films on glass substrates have a tensile strain along their planes. The effect of thermal annealing on the partial relaxation of the strain is discussed. Received: 29 January 2001 / Accepted: 30 January 2001 / Published online: 3 May 2001     

Simple and efficient photo-ionization loading of ions for precision ion-trapping experiments

We report a simple and efficient method to load a Paul trap with Ca⁺ ions. A beam of neutral atomic calcium is ionized in a two-step photo-ionization process using uv-diode lasers near 423 nm and 390 nm. Photo-ionization of a calcium beam for loading a Paul trap has first been demonstrated by Kjaergaard et al. The advantages of our method are the use of cheap and easily handled diode-laser systems and the large cross section for field ionization when exciting high-lying Rydberg states. Finally, we discuss the advantages of photo-ionization for ion generation compared to loading by electron bombardment. Received: 24 August 2001 / Revised version: 16 October 2001 / Published online: 23 November 2001     

Magnetic resonance imaging of Bose–Einstein condensates

As a new method for measuring the spatial distribution of Bose–Einstein condensates, the magnetic resonance imaging (MRI) method is proposed and studied in detail. The basic concepts, the resolution limit and the formalism of the MRI method are presented. It is expected that a resolution higher than that in optical imaging methods can be obtained by using the MRI method. Results of simulation of expected MRI signals for Bose–Einstein condensates containing dark solitons are also presented. Received: 27 September 2001 / Revised version: 24 October 2001 / Published online: 17 January 2002     

Observations of band structure and reduced group velocity in epitaxial GaN–sapphire 2D photonic crystals

We report experimental results for the band structure of 2-dimensional triangular photonic crystals of air holes in an epitaxial group III–nitride waveguide film. Surface coupling techniques enable the observation of sharp resonance dips in the transmission spectra due to a resonance phenomenon between the incident light and Bloch modes of the photonic crystal. The position of the dips has been measured as a function of angle of incidence and the photonic band structure has been successfully constructed by the measurement. Corresponding Bloch-mode group velocities have also been obtained. Received: 28 May 2001 / Published online: 23 October 2001     

Integrated all-optical nonlinear device for re- configurable add/drop and wavelength shifting of WDM signals

A novel configuration is proposed for an all-optical device performing two key functionalities in a communication network based on wavelength-division-multiplexing (WDM): reconfigurable add/drop and wavelength shifting of single channels. The device is based on guided second-order nonlinear interactions, such as sum-frequency generation and difference-frequency generation, between the WDM channels and a suitable pump beam. A directional coupler and two parallel waveguides allow the spatial separation between the main WDM signal and the dropped or wavelength-shifted channel for a subsequent routing in the desired path. A first numerical simulation provided a cross-talk level in the dropped channel lower than -41 dB and a wavelength-shifting range of more than 40 nm. Received: 18 May 2001 / Published online: 30 October 2001     

Energy dependence of the decay pathways of optically excited small gold clusters

The pathway competition between neutral monomer and neutral dimer evaporation from optically excited odd-size gold cluster ions Au⁺ _n, n=7–15, has been investigated as a function of cluster size and excitation energy. Gold cluster ions of these sizes are the only ones to show observable pathway competition while all other sizes exclusively evaporate either neutral monomers or neutral dimers. The investigation has been performed by photoexcitation of stored size-selected gold cluster ions with a single 10-ns laser pulse. Subsequent time-resolved observation of the delayed dissociation allows us to quantitatively determine the relative fragment yields of the respective decay channels as a function of excitation energy. Contrary to theoretical expectations, the dimer-to-monomer branching ratio of evaporated particles is found to decrease monotonously with increasing excitation energy for all cluster sizes under investigation. Possible explanations for this behaviour are discussed. Received: 9 July 2001 / Revised version: 24 September 2001 / Published online: 15 October 2001     

Dispersion of second-order nonlinear optical coefficient

A simple, approximate relation between the dispersion of the second-order nonlinear optical (SNLO) coefficient and first-order susceptibility has been derived using the well-known results of density-matrix calculation of quantum-mechanical theory. A new tensor of SNLO coefficient has been defined, which retains symmetries when the input beams are in the spectral range of transparency while the generated beam can also be in the spectral range of absorption. The validity of the relation has been checked for three ferroelectric crystals, KTP, LiNbO₃ and KNbO₃, which are transparent in the visible and near infrared, and for two semiconductors, GaP and GaAs, which absorb in the visible. Contrary to Miller’s law, the presented relation is in good agreement with measurements. Received: 8 February 2001 / Revised version: 2 July 2001 / Published online: 10 October 2001