Three-photon absorption spectroscopy: the L(1Φ3) and m(3II1) states of HCl and DCl

Analysis of (3+1) REMPI room temperature spectra by use of three-photon absorption modelling allowed, for the first time, identification and characterization of Ω = 3, Φ states (L(¹Φ₃)) in HCl and DCl and of the m(³II₁) state in DCl. Simulation analyses and determination of isotope shifts allowed evaluation of vibrational and rotational spectroscopic parameters for both states and both molecules. The mechanism of three-photon absorption in the m(³II₁) ← X(¹Σ⁺) transition is discussed.     

Comparison of all-optical co- and counter-propagating high-speed signal processing in SOA-based Mach–Zehnder interferometers

The all-optical signal processing performance of a Mach–Zehnder interferometer (MZI) is investigated. Calculated switching windows are used to investigate and understand the physical mechanisms limiting the high speed performance. Especially, the co- and counter-propagating operation of the MZI is discussed and important differences in the performance for the two schemes are addressed. The non-regenerative all-optical clear and drop functionality is investigated for a 2, 4 and 8 × 40 Gbit/s signal, showing good performance in the co-propagating case. Regenerative simultaneous clear and drop functionality in a single MZI is demonstrated experimentally and compared to the large signal model predictions.     

Rabi oscillations of excitons in single quantum dots

Transient nonlinear optical spectroscopy, performed on excitons confined to single GaAs quantum dots, shows oscillations that are analogous to Rabi oscillations in two-level atomic systems. This demonstration corresponds to a one-qubit rotation in a single quantum dot which is important for proposals using quantum dot excitons for quantum computing. The dipole moment inferred from the data is consistent with that directly obtained from linear absorption studies. The measurement extends the artificial atom model of quantum dot excitonic transitions into the strong-field limit, and makes possible full coherent optical control of the quantum state of single excitons using optical pi pulses.     

EVALUATION OF VIBRATION AND SHOCK ATTENUATION PERFORMANCE OF A SUSPENSION SEAT WITH A SEMI-ACTIVE MAGNETORHEOLOGICAL FLUID DAMPER

The potential benefits of a semi-active magnetorheological (MR) damper in reducing the incidence and severity of end-stop impacts of a low natural frequency suspension seat are investigated. The MR damper considered is a commercially developed product, referred to as “Motion Master semi-active damping system” and manufactured by Lord Corporation. The end-stop impact and vibration attenuation performance of a seat equipped with such a damper are evaluated and compared with those of the same seat incorporating a conventional damper. The evaluation is performed on a servo-hydraulic vibration exciter by subjecting the seat-damper combinations to a transient excitation with dominant frequency close to that of the seat and continuous random excitation class EM1 applicable to earth-moving machinery, and a more severe excitation realized by amplifying the EM1 excitation by 150%. Tests are performed for medium and firm settings of the MR damper and for seat height positions corresponding to mid-ride and ±2·54 and ±5·08 cm relative to mid-ride. The results indicate that significantly higher levels of transient excitation are necessary to induce end-stop impacts for the seat equipped with the MR damper, particularly when set for firm damping, the difference with the conventional damper being more pronounced for seat positions closer to the end-stops. Under the EM1 excitation, the results indicate that under conditions which would otherwise favour the occurrence of end-stop impacts for a seat equipped with a conventional damper, the use of the MR damper can result in considerably less severe impacts and correspondingly lower vibration exposure levels, particularly when positioned closer to its compression or rebound limit stop.     

Optical pumping of the electronic and nuclear spin of single charge-tunable quantum dots

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the net charge from positive to neutral to negative with a charge-tunable heterostructure. Negative photoluminescence polarization memory is enhanced by optical pumping of ground state electron spins, which we prove with the first measurements of the Hanle effect on an individual quantum dot. We use the Overhauser effect in a high longitudinal magnetic field to demonstrate efficient optical pumping of nuclear spins for all three charge states of the quantum dot.     

Raman scattering and photoluminescence studies of two-dimensional electron systems in Ge/GaAs heterostructures

Resonant Raman scattering experiments on n-Ge/n-GaAs (100) heterostructures reveal transitions involving quasi-two-dimensional electron states in Ge-accumulation layers. The experiments were performed in the range of the E₁-gap of Ge. The electronic scattering transforms into E₁-luminescence as the laser energy is tuned above the resonance. Spectra obtained at higher excitation energies show luminescence bands associated with the E₁ and E₁ + Δ₁-gap of Ge. The latter results are compared with recent data for bulk heavily-doped Ge.     

Multiplicity of solutions for a class of nonsymmetric eigenvalue hemivariational inequalities

The aim of this paper is to establish the influence of a non-symmetric perturbation for a symmetric hemivariational eigenvalue inequality with constraints. The original problem was studied by Motreanu and Panagiotopoulos who deduced the existence of infinitely many solutions for the symmetric case. In this paper it is shown that the number of solutions of the perturbed problem becomes larger and larger if the perturbation tends to zero with respect to a natural topology. Results of this type in the case of semilinear equations have been obtained in [1] Ambrosetti, A. (1974), A perturbation theorem for superlinear boundary value problems, Math. Res. Center, Univ. Wisconsin-Madison, Tech. Sum. Report 1446; and [2] Bahri, A. and Berestycki, H. (1981), A perturbation method in critical point theory and applications, Trans. Am. Math. Soc. 267, 1–32; for perturbations depending only on the argument.