Formation of Ge nanocrystals with sharp size distribution: structural and optical characterization

We have studied the formation of Ge nanocrystals in a SiO_x matrix using high-resolution electron microscopy, Raman- and luminescence spectroscopy. The room temperature cathodoluminescence centered at 2.2-2.4 eV and the photoluminescence at 2.4 eV are correlated with the crystallization of the as-prepared Ge cluster. However, no peak shift was found in spite of the sharp size distribution of the particles. The data suggest that quantum confinement is not the origin of the luminescence.     

Nonlinear optical studies in semiconductor-doped glasses under femtosecond pulse excitation

Nonlinear optical studies in semiconductor-doped glasses (SDGs) are performed under femtosecond laser pulse excitation. Z-scan experiments with 800 nm wavelength pulses are used to excite SDG samples in the resonance and non-resonance regimes. Schott colour glass filter OG 515 shows stronger two-photon absorption than GG 420 and both the samples exhibit positive nonlinearity. However, in resonantly excited RG 850 the intensity-dependent Z-scan shows transition from saturable to reverse saturable absorption behaviour with the increase in intensity.     

Dynamics of optical sine-Gordon solitons under double coherent pulse excitation

On the basis of a numerical simulation of the sine-Gordon equation the conclusion is made that the number of solitary waves is invariant with respect to the input pulse separation interval. But the value of this interval affects the form of the pulses: either 2π and a breather or 2π and a (± 2π) pair.     

Spectral dependent kinetics of the EHD luminescence in As-doped Ge

Following pulsed laser excitation of As-doped Ge with impurity concentrations between 10¹⁵-10¹⁷cm^-3, we observe the electron-hole drop (EHD) and excitonic luminescence decay. The spectrum resolved no-phonon (NP) EHD luminescence kinetics are found to depend on its spectral position. “Plateaus” on the kinetic curves for the high energy side of NP-spectrum are observed at high excitation. The data suggest this luminescence is due to the states which are in dynamical equilibrium during some time after excitation pulse.     

Nonlinear optical response of Ge nanocrystals in silica matrix with excitation of femtosecond pulses

We report an investigation of third-order optical nonlinearities in Ge nanocrystals (∼6 nm radius) embedded in silica matrix using the Z-scan and pump-probe techniques with femtosecond laser pulses at 780-nm wavelength. The nanocrystallite Ge samples were prepared using magnetron co-sputtering and post-thermal annealing at 800 °C. The nonlinear absorption coefficient and refractive index of the Ge nanocrystals were determined to be in the range from 1.8×10^-7 to 6.8×10^-7 cm/W and 1.5×10^-12 to 8.0×10^-12 cm²/W, respectively, which are proportional to the Ge atomic fraction in the matrix. Relaxation of the nonlinear response was found to have two characteristic time constants, 1.8 ps and 65 ps. The mechanisms responsible for the observed nonlinear response are discussed. Received: 21 August 2000 / Revised version: 17 January 2001 / Published online: 30 March 2001     

Formation and droplet size of EHD drippings induced by superimposing an electric pulse to background voltage

The effect of electric voltages on droplet size was investigated. Results show that both background voltage and amplitude of electric pulse could affect the process of droplet formation and droplet size while the width of pulsed voltage had a slight effect on droplet size. Pulsed voltages introduced a tangential electric stress that led to relatively greater droplet formation while greater background voltage was associated with smaller droplets. Due to different components of the normal and tangential electric stress three dripping modes were created. Based on above observations, a simplified discussion is presented on the relationships between droplet size and above parameters.     

Size distribution and optical properties of self-assembled Ge on Si

The distribution of Ge islands is analysed in order to understand their optical behaviour. The Ge islands described in this paper were deposited by low-pressure chemical vapour deposition at relatively high temperature (700 °C), therefore the diffusion length of adatoms is high (∼100 μm) and thus, not the limiting factor for nucleation. By changing the deposition time and the coverage, square-based pyramids, domes and relaxed domes are nucleated. Mainly domes emit light, the emission being in the wavelength range 1.38–1.55 μm. When pyramids or relaxed domes are present, the photoluminescence broadens and decreases in intensity. The electroluminescence of vertically correlated islands increases with the number of layers, i.e. with the number of islands. The nucleation of islands on patterned (001) Si is changed when the deposition is performed on Si mesas with high index facets. The size distribution becomes narrower when the mesa size is decreased. An intermixing of up to 40% Si in the 2D layer was determined from photoluminescence data. PIN diodes fabricated on patterned wafers show an area-dependent electroluminecence related to a different microstructure of islands on large and small mesas. Finally, the lateral ordering on {hkl} facets is discussed. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

Cluster size distribution at criticality

Monte Carlo studies of the cluster size distribution for the site percolation problem on the triangular lattice are extended to lattices with up to 4 × 10¹¹ sites. Agreement with the predictions of scaling theory at p_c is excellent over a range of cluster sizes spanning five orders of magnitude.     

Magnetic-field dependent intensity oscillations of the EHD luminescence in pure germanium

The magnetic-field dependence of the EHD luminescence in pure germanium is investigated using pulsed laser excitation. Intensity oscillations are found which vary with the delay time of the detection. A fit to a reaction-kinetic model yields a quantum efficiency of Q ≈ 25%. The main nonradiative process in the electron-hole liquid is found to be the Auger-recombination.     

170 fs pulse generation by optical pulse compression at 308 nm

Pulses of 170 fs duration have been obtained by compression of 220–250 fs pulses of a hybrid excimer-dye laser setup.     

Prediction of drop-on-demand (DOD) pattern size in pulse voltage-applied electrohydrodynamic (EHD) jet printing of Ag colloid ink

Drop-on-demand printing is receiving a great deal of interest in industrial applications; however, the desired pattern sizes are realized by trial and error, through repeated printing experiments with varied materials (ink and suspended particles), operating conditions (voltage, flow rate, nozzle-to-plate distance, etc.), and substrate wettability. Since this approach requires a great deal of time, cost, and effort, a more convenient and efficient method that will predict pattern sizes with a minimal number of experiments is needed. In this study, we patterned a series of Ag dots and lines using a pulsed voltage-applied electrohydrodynamic jet printing system and measured their sizes with an optical microscope. We then applied a model suggested by Stringer and Derby (J Eur Ceram Soc 29:913–918, 2009) and Gao and Sonin (Proc R Soc Lond Ser A 444:533–554, 1994) to predict the pattern sizes, comparing these predictions with the measured sizes. Finally, we demonstrated our methodology on disconnected line repairing.     

Improved scatterer size estimation using backscatter coefficient measurements with coded excitation and pulse compression

Scatterer size estimates from ultrasonic backscatter coefficient measurements have been used to differentiate diseased tissue from normal. A low echo signal-to-noise ratio (eSNR) leads to increased bias and variance in scatterer size estimates. One way to improve the eSNR is to use coded excitation (CE). The normalized backscatter coefficient was measured from three tissue-mimicking phantoms by using CE and conventional pulsing (CP) techniques. The three phantoms contained randomly spaced glass beads with median diameters of 30, 45, and 82 mum, respectively. Measurements were made with two weakly focused, single-element transducers (f(0)=5 MHz and f(0)=10 MHz). For CE, a linear frequency modulated chirp with a time bandwidth product of 40 was used and pulse compression was accomplished by the use of a Wiener filter. Preliminary results indicated that improved estimation bias versus penetration depth was obtained by using CE compared to CP. The depth of penetration, where the accuracy of scatterer diameter estimates (absolute divergence <25%) were obtained with the 10 MHz transducer, was increased up to 50% by using CE versus CP techniques. In addition, for a majority of the phantoms, the increase in eSNR from CE resulted in a modest reduction in estimate variance versus depth of penetration.     

Luminescence line shape of free excitons in pure Ge

We report luminescence experiments performed in pure Ge at low temperature. Taking into account the splitting of the ground state of free excitons in this material, we show that their emission line shape is subject to a Gaussian broadening which seems to be due to the phonon lifetime. From this study, we deduce new values of the binding energy of electron-hole drops in Ge and Si which are respectively -2 and -5.6meV.     

Low-lying states and isospin excitation in the Ge isotopes

The level structure of 64-70Ge isotopes has been studied within the framework of the interacting boson model-3 (IBM-3). The symmetry character in the proton and neutron degrees of freedom of the energy levels has been investigated. The isospin excitation states (T>Tz) have been assigned for the 64Ge (N=Z) nucleus. Some intruder states in these nuclei have been suggested. The calculated energy levels and transition probabilities are in good agreement with recent experimental data. The study indicates that the Ge isotopes are in transition from γ-unstable to vibrational.     

Low-lying states and isospin excitation in the Ge isotopes

The level structure of ^64-70Ge isotopes has been studied within the framework of the interacting boson model-3（IBM-3） . The symmetry character in the proton and neutron degrees of freedom of the energy levels has been investigated. The isospin excitation states（T 〉 Tz） have been assigned for the ^64Ge（N = Z） nucleus. Some intruder states in these nuclei have been suggested. The calculated energy levels and transition probabilities are in good agreement with recent experimental data. The study indicates that the Ge isotopes are in transition from γ-unstable to vibrational.     

Cooperative shortening and modulation of the resonance optical pulse stimulated by an off-resonance stepwise excitation

We present the first direct experimental evidence of the stimulated inelastic resonance fluorescence (SIRF) in a dilute, optically thick gas of 2-level systems. In particular, the shortening and modulation of the SIRF pulse and the rise time effects predicted by an analytical calculation are clearly pointed out. Previously computed shapes of the SIRF pulse are also interpreted.     

Anisotropy of photoconductivity and nonlinear effects in GaSe monocrystals at high optical excitation

In this work, the photoelectric properties of gallium selenide (GaSe) monocrystals in the edge absorption region, with various configurations of current contacts, at low and high optical excitation levels are investigated. The photoconductivity spectrum behavior is determined by localized electronic and excitonic states along c-axis. It is shown that the localization of electronic and excitonic states in one-dimensional fluctuation potential along c-axis results to an anisotropy in photoconductivity spectrum at various current contacts configurations. At E⊥c the photoconductivity is observed in the hν < E_g and hν > E_g regions. In the case of hv < E_g, the maximum photoconductivity, in the impurity and exciton absorption region are observed at 1.975 eV and 2.102 eV, respectively. With rising of excitation energy level, suppression of photoconductivity in the exciton absorption region and increases in impurity absorption region is observed. At E||c contact configuration, the considerable photoconductivity is observed only in the impurity absorption region, which also increases with rising of excitation level. It is supposed that, suppression of photoconductivity in the exciton absorption region at high excitation levels is connected with exciton-exciton interaction, which results to a nonlinear light absorption. The results are compared with the absorption and photoluminescence measurements.     

Complete pulse characterization at 1.5 mum by cross-phase modulation in optical fibers

Cross-phase modulation in optical fibers has been used for complete characterization of ultrashort pulses by a modified frequency-resolved optical gating (FROG) measurement technique. This technique has been used for characterization of picosecond pulses at 1.5mum with energy as low as 24 pJ, and the results are in excellent agreement with second-harmonic generation (SHG)-FROG characterization. The use of an optical waveguide gives measurement sensitivity comparable with that of SHG-FROG but without any temporal ambiguity in the retrieved pulse.