Laser-driven Systems Research Articles

Alternative representation of time-dependent Hamiltonians with application to laser-driven systems

A representation of time-dependent Hamiltonians that describe laser-driven systems is presented. Unlike the well-known time-independent dressed potentials that are functions of the characteristic parameter ${\ensuremath{\alpha}}_{0}=\sqrt{I}/{\ensuremath{\omega}}^{2},$ where $\ensuremath{\omega}$ and I are the laser frequency and intensity, this approach provides a time-averaged potential that depends explicitly on the field parameters; e.g., I, $\ensuremath{\omega},$ and shape of the laser pulse. The modified dressed potential is $\ensuremath{\Elzxh}$ independent and adds a classical time-independent potential barrier to the Kramers-Henneberger dressed potential. We show that this dynamical potential barrier is identical to the Kapitza effective classical potential energy obtained for the motion of a particle in a rapidly oscillating field. As an illustrative numerical example, a simple one-electron effective model Hamiltonian of xenon atom in strong laser field is studied. We show that the zero-order quasienergies obtained by our representation are reasonably accurate and the second order high-frequency perturbation calculations provide quite accurately the lifetime of the photoionized electron for a broad range of laser frequencies.

Nontunnellinghigh-order harmonics from ultra-intense laser-driven tightlybound systems

High-order harmonic emission is investigatedby numerical solution of the weakly relativistic, two-dimensionalSchrödinger equation for the case of ultra-intense laser-driventightly bound systems (for example, multiply charged ionssuch as O7+ exposed to laser fields of the order of1018 W cm-2 at 248 nm).In contrast to their usual substantial decrease, thelow-order harmonics having an energy less than the ionization potentialexhibit a high-efficiency (i.e. intense) plateau with awell defined cutoff. The shape of this plateau is found to dependon the shape of the binding potential.A classical `surfing' mechanism for the generation of theseharmonics is proposed that does not involve tunnelling and thatnevertheless explains the observed cutoff.Thus we call them `nontunnelling harmonics'. The significance of relativisticeffects for these harmonics is investigated and found to be small,despite the high laser intensity, because of the absence of tunnelling.

Narrow high-frequency spectral features via laser-induced slow spin flips

The process of high harmonic generation in the tunneling regime of laser-driven atomic systems is investigated numerically with the Pauli equation. For an ion with electron wave packet in the ground state with spin-up orientation, a small fraction of the tunneled wave packet will be transformed into the spin-down orientation prior to the recollision with the ionic core due to the magnetic field component of the laser field. We show that the high-frequency harmonics associated with the wave packet in spin-down orientation, though weak in intensity, have a substantially reduced line width with respect to those without spin flip.

Restoring dark lines in spontaneous emission via Fano interference

We investigate the effects of spontaneous emission on two different laser-driven autoionization systems. We find that, under specific conditions, the relaxation decay processes can interfere destructively and lead to dark lines and spectral narrowing in the spontaneous emission spectra of these systems.

The asymptotic behaviour of the spectrum of resonance fluorescence

Abstract The spectrum of resonance fluorescence from laser-driven multi-level systems is investigated and especially its asymptotic frequency dependence in the wings. Analytical results are presented and the transitions within multilevel systems are classified into three categories according to the asymptotic frequency dependence of their spectra. Under certain conditions an arbitrarily fast asymptotic fall-off behaviour of the spectrum is found which, for intermediately strong driving fields, leads to subnatural linewidths. Numerical examples are presented to illustrate the analytical results.

Spectral structures induced by electron shelving.

We investigate the spectrum of resonance fluorescence of laser-driven three-level systems in the V or $\ensuremath{\Lambda}$ configuration where one transition is metastable. If the laser driving the weak transition has a sufficiently small Rabi frequency we find an extremely narrow structure in the resonance fluorescence spectrum of the strong transition. Analytical expressions approximating the spectrum are given and the properties of the spectrum are discussed. With the help of a simplified model of resonance fluorescence emitted on the strong transition we are able to show that the origin of the narrow peak can be traced back to electron shelving, i.e., to the existence of light and dark periods in the resonance fluorescence emitted on the strong transition.

In situ, real-time monitoring of wound-induced ethylene in cherry tomatoes by two infrared laser-driven systems

Abstract Two laser-based detection setups, one involving photothermal deflection and the other photoacoustics, have been used to follow ethylene release when ripening cherry tomatoes (cv. Favorita) were mechanically wounded. Removing the calyx caused a double peak in ethylene release, the first peak over 1–2 h and the second over the next 4–7 h. Wounding the stem-scar yields similar results. However, wounding part of the fruit skin led to modest and variable releases of ethylene, much less than given on calyx-removal or stem-scar wounding. This higher emission does not originate from ethylene accumulated in the tomato already before wounding. Laser systems are shown to be useful in quantifying ethylene-releasing systems.

Metastable quasienergy positions and widths for time-periodic Hamiltonians by the complex-coordinate method.

A formulation according to which the positions and widths of metastable quasienergy states can be obtained for any time-periodic field within the framework of the finite-basis-set (real or complex) approximation is presented. A numerical example is given to illustrate the potential use of the proposal method in the study of the dynamics of laser-driven systems and of the dynamics of time-periodic classical chaotic Hamiltonians.

Driven Morse oscillator: Classical chaos and quantum theory for two-frequency excitation.

We extend our previous comparison of classical and quantum theories for the excitation and dissociation of a sinusoidally driven Morse oscillator (Phys. Rev. A 37, 796 (1988)) to the case of two-frequency driving. In both the classical and quantum theories the total threshold intensity for dissociation can be considerably smaller in the two-frequency case. This is consistent with results obtained with more artificial models of laser-driven molecular systems, and we provide an approximate resonance-overlap analysis to explain this trend. We also compare results obtained with adiabatic and sudden turn-on of the applied field, and comment on the use of absorbing boundaries for the computational identification of dissociation or ionization.

Vibrational Control of a Laser Illuminated Thermochemical System

Vibrational control is a nonclassical open-loop control principle which proposes a utilization of zero mean parametric excitation of a dynamical system for achieving control objectives. In the present paper, excitations introduced into the incident power of a laser beam in a laser illuminated thermochemical reaction are shown to be capable of inducing asymptotically stable operating regimes with averages located at initially unstable steady states. This opens a possibility of alleviating measurement and actuator related difficulties associated with conventional methods in a stabilization of laser-driven systems.

Thermodynamics of laser systems

The “pump and decay” part of the equations of motion for lasers and laser-driven systems presently used in the literature violates the conservation of the system's entropy under symmetry operations commuting with the Hamiltonian. Thermodynamics yield alternative equations. The spectral lineshape predicted by thermodynamics is Gaussian.

Some safety considerations in laser-controlled thermonuclear reactors

The potential safety problems which might be encountered in laser-driven controlled thermonuclear reactors (LCTR) have been examined and compared with previous studies of magnetically-confined systems. In several significant ways, including tritium hazards and some blanket cooling functions, laser-driven systems appear to differ only in quantitative ways from magnetic systems. Several aspects of safety of laser-driven systems are distinctive to them. These include cooling means designed to handle the pulsetile energy deposition from pellet explosions, and various equipment and containment penetrations associated with the lasers. Specify safety analyses have been performed for a loss-of-flow accident (LOFA) in a wetted-wall LCTR, and failure modes for laser penetrations are considered in detail to ascertain appropriate component placement and design criteria.

Kinetic theory of vibrational relaxation in a radiation field: The optic-acoustic effect

A microscopic derivation is presented of the rate equations governing vibrational relaxation occurring in the optic-acoustic effect. Detailed expressions applicable to the spectrophone experiment are given both for an excitation source consisting of a broadband radiation field and for laserdriven systems. It is clear from the present treatment that no real advantage accrues from the use of laser excitation sources in the standard spectrophone experiment, due to the resultant strong dependence of the driving force itself on the mechanical chopper frequency. For broadband radiation field the dependence on the chopper frequency is removed and the standard result containing the Einstein coefficient of induced absorption is recovered. The spectrophone response for the simplest case of a two-level system is given explicitly and its similarity to phenomenologically derived expressions is pointed out.