Resonant Multiphoton Transitions Research Articles

Effects of tunneling and multiphoton transitions on squeezed-state generation in bistable driven systems

Bistability of nonlinear resonantly driven oscillator in the presence of external noise is analyzed by means of classical Fokker-Planck equation in quasienergy space with account for tunneling effects and by quantum master equation in quasienergy states representation. Two time scales responsible for different stages of bistable system relaxation have been obtained. We found out that the slow relaxation rate caused by fluctuation--induced transitions between different stable states can be enhanced by several orders due to tunneling effects. It was also revealed that tunneling between nearly degenerate quasienergy states and resonant multiphoton transitions between the genuine eigenstates of the nonlinear oscillator are just the similar effects. It was demonstrated that the quasienergy states in the bistability region corresponding to higher amplitude are squeezed. The degree of squeezing is determined by the ratio between nonlinearity and detuning, so the uncertainty of one quadrature can be considerably smaller than the quantum limit. It was found out that tunneling effects can enhance the generation of output oscillator squeezed states. It is shown that 1D Fokker--Planck equation is a quasiclassical limit of a quantum master equation.

Quantum tunneling effect on switching rates of bistable driven system

The influence of quantum tunneling effects on the bistability of nonlinear resonantly driven oscillators in the presence of external noise is analyzed by means of the generalized Fokker–Planck equation in quasi-energy space. Two time scales responsible for different stages of bistable system relaxation have been revealed. It was found that quantum tunneling effects could strongly change the switching rates between different stable states and considerably enhance occupation of the stable state with higher amplitude. It was also revealed that tunneling between nearly degenerate quasi-energy states and resonant multi-photon transitions between the genuine eigenstates of the nonlinear oscillator are just similar effects.

LASER MULTIPHOTON SPECTROSCOPY OF ATOM EMBEDDED IN DEBYE PLASMAS: MULTIPHOTON RESONANCES AND TRANSITIONS

The consistent relativistic energy approach to atom in a realistic laser field, based on the Gell-Mann and Low S-matrix formalism, is applied to studying the resonant multiphoton transitions in atoms embedded in the Debye plasmas. There is considered a new scheme to calculating the multiphoton transitions characteristics, shifts and widths of multiphoton resonances. An approach is used for treating the three-photon transitions in krypton embedded in the Debye plasmas.

Bistability effect in the extreme strong coupling regime of the Jaynes-Cummings model

We study the nonlinear response of a driven cavity QED system in the extreme strong coupling regime where the saturation photon number is below one by many orders of magnitude. In this regime, multi-photon resonances within the Jaynes--Cummings spectrum up to high order can be resolved. We identify an intensity and frequency range of the external coherent drive for which the system exhibits bistability instead of resonant multi-photon transitions. The cavity field evolves into a mixture of the vacuum and another quasi-classical state well separated in phase space. The corresponding time evolution of the outgoing intensity is a telegraph signal alternating between two attractors.

Floquet formulation for the investigation of multiphoton quantum interference in a superconducting qubit driven by a strong ac field

We present a Floquet treatment of multiphoton quantum interference in a strongly driven superconducting flux qubit. The periodically time-dependent Schr\"odinger equation can be reduced to an equivalent time-independent infinite-dimensional Floquet matrix eigenvalue problem. For resonant or nearly resonant multiphoton transitions, we extend the generalized Van Vleck (GVV) nearly degenerate high-order perturbation theory for the treatment of the Floquet Hamiltonian, allowing the reduction of the infinite-dimensional Floquet matrix to an $N\ifmmode\times\else\texttimes\fi{}N$ effective Hamiltonian, where $N$ is the number of eigenstates under consideration. The GVV approach allows accurate treatment of ac Stark shift, power broadening, time-dependent and time-averaged transition probability, etc., well beyond the rotating wave approximation. We extend the Floquet and GVV approaches for numerical and analytical studies of the multiphoton resonance processes and quantum interference phenomena for the superconducting flux qubit system $(N=2)$ driven by intense ac fields.

Anomalous transitions in two-level systems driven by the ac Stark effect

An unusual type of nonresonant absorption signal produced by the ac Stark effect has been observed in a two-level avoided-crossing system. The theory for these anomalous transitions has been developed. The nonresonant signals have been shown to be caused by the perturbation by the oscillating field of the dephasing of the two-level system at the avoided crossing. A series of measurements of these anomalous transitions has been carried out using the avoided-crossing molecular-beam electric-resonance technique. In addition, different types of resonant multiphoton transitions have been investigated. Results are reported for the AE-barrier anticrossing with J = 1 in CH3SiH3. The experimental findings are in good agreement with the theory developed. PACS Nos.: 33.20Bx, 33.80Be, 42.50Hz

Transform-Limited Pulses Are Not Optimal for Resonant Multiphoton Transitions.

Maximizing nonlinear light-matter interactions is a primary motive for compressing laser pulses to achieve ultrashort transform limited pulses. Here we show how, by appropriately shaping the pulses, resonant multiphoton transitions can be enhanced significantly beyond the level achieved by maximizing the pulse's peak intensity. We demonstrate the counterintuitive nature of this effect with an experiment in a resonant two-photon absorption, in which, by selectively removing certain spectral bands, the peak intensity of the pulse is reduced by a factor of 40, yet the absorption rate is doubled. Furthermore, by suitably designing the spectral phase of the pulse, we increase the absorption rate by a factor of 7.

Anomalous transitions in two-level systems driven by the ac Stark effect

An unusual type of nonresonant absorption signal produced by the ac Stark effect has been observed in a two-level avoided-crossing system. The theory for these anomalous transitions has been developed. The nonresonant signals have been shown to be caused by the perturbation by the oscillating field of the dephasing of the two-level system at the avoided crossing. A series of measurements of these anomalous transitions has been carried out using the avoided-crossing molecular-beam electric-resonance technique. In addition, different types of resonant multiphoton transitions have been investigated. Results are reported for the AE-barrier anticrossing with J = 1 in CH3SiH3. The experimental findings are in good agreement with the theory developed. PACS Nos.: 33.20Bx, 33.80Be, 42.50Hz

Pressure effects in multiple resonant multiphoton transitions

Although the rotational structure of a multiphoton process generally remains unaltered over a large range of gas pressure, this is not the case when multiple resonances are present. The rotational structure observed through intermediate rotational levels in a multiphoton process depends strongly on the resonance conditions. We show, for the NO molecule, that this structure changes drastically when the resonance conditions are modified by intermolecular collisions.

Summation over the intermediate vibrational states of a diatomic molecule under nonadiabatic conditions

The Green's function for the nuclear motion of a diatomic molecule in the quasi-classical approximation has been found taking into account the nonadiabatic coupling of two-electron terms. The Green's function obtained permits the resonant multiphoton transitions to be calculated effectively in molecules under nonadiabatic conditions.

Investigation of multiphoton absorption in SF<inf>6</inf>by third harmonic generation

In a series of third harmonic generation experiments, we are able to probe the excitation of SF 6 molecules through the discrete states below 3000 cm-1. The nonlinear susceptibility of SF 6 is strongly affected by multiphoton resonances and by population redistribution and depletion. The third harmonic generation spectrum for SF 6 was measured for intensities above and below the molecular dissociation threshold. Experimental evidence indicates the presence of several resonant multiphoton transitions in the discrete states and one and two-photon transitions from discrete states to the quasi-continuum, depending on the frequency of laser excitation.