Semiclassical steady-state analysis of a degenerate two-photon laser

The steady-state analysis of a single-mode two-photon laser are treated semiclassically by using the Maxwell-Bloch equations. The theory is applied to a ring-laser model. We find similarities and significant differences between the one- and two-photon polarizations of the medium, population inversion and mode-pulling formula. The population inversion and the longitudinal variation of the steady-state modulus of the field are studied numerically. Received: 1 March 1999 / Received in final form: 8 April 1999     

Optical field enhancement effects in laser-assisted particle removal

We report on the role of local optical field enhancement in the neighborhood of particles during dry laser cleaning (DLC) of silicon wafer surfaces. Samples covered with spherical colloidal particles (PS, SiO₂) and arbitrarily shaped Al₂O₃ particles with diameters from 320–1700 nm were cleaned using laser pulses with durations from 150 fs to 6.5 ns and wavelengths ranging from 400–800 nm. Cleaned areas were investigated with scanning electron and atomic force microscopy. Holes in the substrate with diameters of 200–400 nm and depths of 10–80 nm, depending on the irradiation conditions, were found at the former positions of the particles. For all pulse durations analyzed (fs, ps, ns), holes are created at laser fluences as small as the threshold fluence. Calculations of the optical field intensities in the particles’ neighbourhood by applying Mie theory suggest that enhancement of the incident laser intensity in the near field of the particles is responsible for these effects. DLC for sub-ns pulses seems to be governed by the local ablation of the substrate rather than by surface acceleration. Received: 31 May 2000 / Accepted: 7 September 2000 / Published online: 22 November 2000     

Design and interpretation of laser pulses for the control of quantum systems

An extended theory of optimal control strategies for the design of feasible laser pulses is presented. The theoretical framework includes the control of population-transfer and expectation values. Selected studies for two-dimensional model systems providing aspects of uni- and bimolecular reactions are presented. Subsequent analysis of the obtained laser fields is performed to gain insight into the underlying reaction mechanisms. The effect of non-uniform spatial laser profiles and the orientation of the molecules versus the laser beam on controlling quantum dynamics is investigated with the main interest focussed on the achievable quantum yield and information on variable pathways stored in the laser field. Received: 10 November 1999 / Published online: 5 July 2000     

Magnetic field tomography

Neutral atoms may be trapped via the interaction of their magnetic dipole moment with magnetic field gradients. One of the possible schemes is the cloverleaf trap. It is often desirable to have at hand a fast and precise technique for measuring the magnetic field distribution. We use for instantaneous imaging the equipotential lines of the magnetic field a diagnostic tool which is based on spatially resolved observation of the fluorescence emitted by a hot beam of sodium atoms crossing a thin slice of resonant laser light within the magnetic field region to be investigated. The inhomogeneous magnetic field spatially modulates the resonance condition between the Zeeman-shifted hyperfine sublevels and the laser light and therefore the amount of scattered photons. We apply this technique for mapping the field of our cloverleaf trap in three dimensions under various conditions. Received 20 March 2001 and Received in final form 12 May 2001     

Effect of Laser Field and Mechanical Force on Deoxyribonucleic Acid Melting

We propose a physics method to study the effect of laser field and mechanical force on the melting process of double-stranded deoxyribonucleic acid （DNA）. A two-dimensional lattice model is established for DNA molecules stuck on the surface, and the stretching energy of the hydrogen bond and stacking energy for each DNA molecule are calculated by using a nonlinear potential. A real-time algorithm is employed to deal with the dynamics process of DNA melting. Numerical results explain the experimental observations. The spatial distribution of the laser field determines the sequences of DNA melting. The simulation has shown the dependence of the final number of melted DNA on the laser field and mechanical force.     

Trapping and cooling cesium atoms in a speckle field

We present the results of two experiments where cold cesium atoms are trapped in a speckle field. In the first experiment, a YAG laser creates the speckle pattern and induces a far-detuned dipole potential which is a nearly-conservative potential. Localization of atoms near the intensity maxima of the speckle field is observed. In a second experiment we use two counterpropagating laser beams tuned close to a resonance line of cesium and in the linlin configuration, one of them being modulated by a holographic diffuser that creates the speckle field. Three-dimensional cooling is observed. Variations of the temperature and of the spatial diffusion coefficient with the size of a speckle grain are presented. Received 16 December 1998 and Received in final form 16 April 1999     

Ionization of a hydrogen atom from its ground state by a coherent bichromatic laser field

We study the “coherent phase control” between the three-photon ionization by a fundamental laser field and the one-photon ionization by its third harmonic for a hydrogen atom in its ground state. The relative phase δ of the harmonic field with respect to the fundamental laser radiation “modulates” the interference between the two ionization channels, which is important near the crossing points between the ionization rates of the two individual processes. Numerical results for the total ionization rate and for the angular distribution of the photoelectrons as a function of the phase δ are presented for frequencies located in the vicinity of the atomic resonances corresponding to the absorption of two laser photons. Received 31 August 2000 and Received in final form 6 February 2001     

Casimir Force in a One-Dimensional Cavity with Quasimode

We calculate the Casimir force between a perfect reflective wall and a semitransparent wall in the laser cavity. Using the Fox-Li quasimode theory to describe the electromagnetic field in the laser cavity, the vacuum energy and the Casimir force are calculated. We compare our results to the force in the ideal situation and find it smaller in the dissipative cavity. We also find that the Casimir force decreases with the increase of the wall-wall distance and the decay rate of the quasimodes in the laser cavity.     

Pulsed standing wave deflection of sodium atoms

We study the deflection of sodium atoms by a resonantly tuned pulsed standing wave of high field intensity. The effects of the phase fluctuations of the pulsed laser field on the momentum distribution of the deflected atoms are experimentally determined. The results are explained using a theoretical model based on the generalized density matrix formalism of two-level atoms. Received 23 November 1998 and Received in final form 27 January 1999     

Saturation of the weak probe amplification in a strongly driven cold and dense atomic cloud

We present an experimental and theoretical investigation of the weak probe amplification in a cold and optically thick atomic cloud that is highly driven by a strong pump laser. We find that for high optical densities the probe amplification is strongly saturated. We compare our saturation measurements with a model based on dressed-atom population equalization due to re-scattering of spontaneous emission. Good agreement between theory and experiment is obtained only when corrections due to multiple scattering are included. Received 3 November 1998 and Received in final form 5 March 1999     

Triplet interactions in star polymer solutions

We analyze the effective triplet interactions between the centers of star polymers in a good solvent. Using an analytical short-distance expansion inspired by scaling theory, we deduce that the triplet part of the three-star force is attractive but only 11% of the pairwise part even for a close approach of three star polymers. We have also performed extensive computer simulations for different arm numbers f to extract the effective triplet force. The simulation data show good correspondence with the theoretical predictions. Our results justify the effective pair potential picture even beyond the star polymer overlap concentration. Received 1 September 1999 and Received in final form 18 January 2000     

Laser-induced hybrid trap for micro-bubbles

Micro-bubbles have been stably trapped in liquid ethanol by a focused argon laser beam. Two equilibrium points where bubbles can be trapped are observed to be at the center and the rim of the laser beam. The light force is also able to push a bubble into the liquid to a position well below the liquid surface. Both the light-pressure force and the fluid force induced by the convection of the liquid medium are calculated. The results clearly show that these two kinds of forces account for the formation of trapping potential. Received: 3 February 2000 / Revised version: 6 July 2000 / Published online: 8 November 2000     

Laser wake field acceleration: the highly non-linear broken-wave regime

We use three-dimensional particle-in-cell simulations to study laser wake field acceleration (LWFA) at highly relativistic laser intensities. We observe ultra-short electron bunches emerging from laser wake fields driven above the wave-breaking threshold by few-cycle laser pulses shorter than the plasma wavelength. We find a new regime in which the laser wake takes the shape of a solitary plasma cavity. It traps background electrons continuously and accelerates them. We show that 12-J, 33-fs laser pulses may produce bunches of 3×10¹⁰ electrons with energy sharply peaked around 300 MeV. These electrons emerge as low-emittance beams from plasma layers just 700-μm thick. We also address a regime intermediate between direct laser acceleration and LWFA, when the laser-pulse duration is comparable with the plasma period. Received: 12 December 2001 / Published online: 14 March 2002     

Elastic scattering of electrons by metastable hydrogen atoms in the presence of a resonant laser field

Within the framework of the rotating wave approximation the elastic scattering of electrons by metastable 2s state of hydrogen atoms is studied in the presence of a resonant laser field. The frequency of the circularly polarized laser field is chosen to match the 2s-3p transition frequency in the hydrogen atom. Variation of the cross section with laser intensity and with incident electron energy (50-150 eV) is investigated. Received: 18 July 1997 / Received in final form: 5 December 1997 / Accepted: 19 January 1998     

General theory of population transfer by adiabatic rapid passage with intense, chirped laser pulses

We present a general theory of adiabatic rapid passage (ARP) with intense, linearly chirped laser pulses. For pulses with a Gaussian profile and a fixed bandwidth, we derive a rigorous formula for the maximum temporal chirp rate that can be sustained by the pulse. A modified Landau-Zener formula displays clearly the relationships among the pulse parameters. This formula is used to derive the optimal conditions for efficient, robust population transfer. As illustrations of the theory, we present results for two- and four-level systems, and selective vibronic excitation in the I₂ molecule. We demonstrate that population transfer with chirped pulses is more robust and more selective than population transfer with transform-limited pulses. Received 6 September 2000 and Received in final form 25 September 2000     

Colored noise in a two-dimensional nonlinear system

A two-dimensional decoupling theory is developed when colored noise is included in a nonlinear dynamical system. By a functional analysis, the colored noise is transformed to an effective noise that includes the noise correlation time, the mean dynamical variable, and the original noise strength. When the two-dimensional decoupling theory is applied to single-mode and two-mode dye laser systems, the mean, variance, and effective eigenvalue of laser intensity are calculated. Excellent agreement between theoretical analysis, numerical simulations, and experimental measurements are obtained. It is seen that the increase of noise correlation time can reduce the fluctuations in the laser system. It is also shown that there is relatively large fluctuation in the phase when the laser undergoes from thermal light to coherent light when the theory is applied to a single mode dye laser. Received 20 August 2001 and Received in final form 4 December 2001     

Polarisation effect of laser field in inelastic electron - hydrogen collisions

We study the influence of the laser polarization on the electron impact excitation of atomic hydrogen. Our method takes into account the “dressing” of the target states by including the laser-atom interaction to first order time-dependent perturbation theory, while the interaction of the laser field with the incident electron is treated to all orders by using the non relativist Volkov function. The interaction of the fast projectile with the target atom is treated in the first Born approximation. The calculations are performed via two distinct computations. The first one is based on a direct calculation, the second based on a Sturmian approach. Important differences appear between the angular distributions depending on the polarization chosen. Received : 17 february 1998 / Revised : 20 july 1998 / Accepted : 2 september 1998     

Convective amplification of parametrically generated acoustic wave in a piezoelectric semiconductor plasma

Using hydrodynamic model of semiconductor plasmas and coupled-mode theory of interacting waves, we have analytically investigated parametric interaction in a magnetised piezoelectric semiconductor plasma in non-relativistic domain. The temperature dependence of momentum transfer collision frequency of electrons due to their heating by the pump is assumed to induce nonlinearity in the medium. We have derived a dispersion relation which finally gives four unstable acoustic modes; two forward amplifying modes and two backscattered attenuating modes. We have also obtained an expression for the critical pump amplitude ( ) at and around which gains and phase velocities of amplifying acoustic modes become least dependent on the pump amplitude and static magnetic field . The required can be readily obtained from a pulsed 10.6 μm CO₂ laser. The magnetic field is found to shift the critical point towards lower pump amplitudes. Received 5 September 2000 and Received in final form 5 March 2001