Xi-Yi Huang

Resonance fluorescence of a two‐level atom near a metal surface

A derivation of surface-dressed optical Bloch equations, involving a treatment of surface-reflected photons and a surface plasmon resonance, is presented for a collision-damped two-level atom near or adsorbed on a metal surface. Effects of the laser bandwidth are included by means of a phase-diffusion model for the driving field. In the weak-field or large detuning limit, the population inversion and resonance fluorescence spectrum are obtained analytically. These quantities along with the surface-induced phase-decay constant of the adatom show strong oscillatory behavior as a function of the adatom-surface distance.

Quantum statistical theory of vibrational dynamics in a laser-driven admolecule-surface system

A quantum statistical theory for a laser-driven anharmonic oscillator coupled to a system of many degrees of freedom by means of energy- and phase-dissipative mechanisms is presented. The theory is applied to the description of vibrational dynamics in an admolecule-surface system that are associated with desorption–dissociation processes. It is found that phase relaxation plays an important role in assisting the laser driving force to overcome the anharmonicity and/or detuning bottleneck. It is also shown that a shorter duration of the laser pulse is more efficient for depositing photon energy into the admolecule system.

Quantum dynamical model for laser excitation of a two-level adatom: Surface-dressed Bloch equations

Abstract Surface-dressed Bloch equations are derived from a microscopic Hamiltonian which includes the interactions among an adatom, the laser photon and the phonon modes. The influence of the surface on the population inversion of the adatom is analyzed in terms of: (1) the reflected-field-induced damping factor which depends on the orientation of the transition dipole and the adatom-surface separation and (2) the frequency shift and damping factor induced by the phonon modes.

Theory of laser-induced phenomena on conventional and phase-conjugated surfaces

Laser-induced processes on conventional and phase-conjugated surfaces are investigated theoretically. Resonance fluorescence of two-level atoms on smooth and rough surfaces are reviewed. The new phenomenon of a radiative dipole at a phase-conjugated surface (PCS), in which the lifetime of the dipole can be virtually infinite under certain conditions, is examined. A PCS originates from the interference of two laser beams incident upon an absorbing layer, and this effect is discussed in terms of a phenomenological model and a hydrodynamic theory in which laser-induced periodic structure and the PCS reflectivity are analyzed. Finally, practical applications of these new phenomena occurring on PCS are discussed.

Transient delayed spectrum of collisionally damped resonance fluorescence in a partially coherent field

The time-dependent spectrum of the light scattered by a collisionally damped two-level atom weakly excited off resonance by a partially coherent laser field is evaluated. Both the collisional and the laser noise are treated as classical Markovian, Gaussian processes while, in addition, the atom is assumed to experience damping and fluctuations that are due to its coupling to an unspecified exterior reservoir. The scattered-field spectra are determined both at steady state and at times following the abrupt turn off of the laser field (the delayed spectrum). The distinct contributions to these spectra arising from the various noise sources as well as the finite bandwidth of the detecting interferometer are investigated in each case. Where appropriate, comparisons are made with related work of Saari [ Light Scattering in Solids ( Plenum, New York, 1979)]. A general inequality is also found that must be satisfied by the longitudinal and the transverse decay rates induced by the exterior reservoir. Finally, the inverse detuning of the laser is found to provide a limit on the time in which the undressing of the atom could be observed to occur following the turn off of the laser.

Quantum model of laser excitation of isotopic species adsorbed on a solid surface

A quantum model of laser-excited interacting isotopic adspecies is presented. The bistability governed by a cubic equation of the active-mode-excitation is analyzed in terms of the nonlinear effects of the anharmonicity, where the transition point and critical laser intensity are exactly calculated. The isotopic selectivity governed by the frequency difference and the coupling strength of the active modes are investigated.

Transient excitation of anharmonic adspecies by pulsed laser radiation

The excitation of anharmonic adspecies by pulsed laser radiation is investigated theoretically in the transient regimes. New features include the optimum values of pulse duration and detuning and the oscillatory behavior of the time‐dependent excitation caused by the anharmonicity.

Quantum model of dephasing‐enhanced laser desorption: Master equation approach

A new model for laser‐induced desorption is proposed based on a master equation for the photon population in a closely coupled subsystem involving the relevant degrees of freedom. The validity of approximations such as the Born and Markovian approximations is discussed in terms of the laser pulse duration and multiple time scales of the adspecies‐surface system. The desorption rate is numerically calculated from the photon population and the threshold number of photons absorbed by the adspecies. The effects of anharmonicity, dephasing and laser detuning on the desorption rate are examined. The mechanism of dephasing and the possible direct and indirect desorption channels are discussed.

Energy and phase relaxation in laser‐induced admolecular processes

Vibrational excitation and relaxation of a molecule adsorbed on a surface is investigated. The phase relaxation is seen to assist the laser‐driving force in overcoming the anharmonic and/or detuning bottleneck. The incoherent thermalization processes in the initial state of the admolecular multiphoton excitation are evaluated. It is found that phase relaxation (dephasing) due to the surface and admolecular perturbation plays an important role in the thermalization processes.

Theory of Spectroscopy and Dynamics in Laser-Irradiated Adspecies-Surface Systems

Recent progress in the theory of spectroscopy and dynamics in laser-irradiated adspecies-surface systems is discussed. This article is divided into three main sections. Beginning with Section 2 following this Introduction, a first-principles approach for describing vibrational excitation and relaxation of a laser-excited adatom is presented. A tractable approximation, known as the isomnesic (constant-memory) approximation is introduced in order to numerically deal with the longtime behavior of the dynamics. In Section 3 surface-dressed optical Bloch equations are solved for the resonance fluorescence spectrum of a laser-driven two-level atom near a flat metal surface and also near a rough metal surface modeled by a hemispheroid on a perfectly conducting plane. In Section 4 the problem of laser-induced period chemical vapor deposition is addressed, and a theoretical analysis of selected aspects of an experiment on the deposition of cadmium onto a silica surface is carried out, where the cadmium is produced by laser-induced dissociation of dimethyl cadmium gas above the surface.