Two-mode Laser Field Research Articles

Inside Front Cover: Observation of Robust Polarization Squeezing via the Kerr Nonlinearity in an Optical Fiber (Adv. Quantum Technol. 3/2023)

The cover shows the evolution of the state of the light field represented on the Poincaré sphere (green) due to the nonlinear Kerr interaction described in article number 2200143 by Nikolay Kalinin, Gerd Leuchs, and co-workers. The quantum uncertainty of the initial coherent two-mode laser field is the small grey sphere on the larger Poincaré sphere which develops into the orange squeezed ellipsoid.

Nonlinear Bethe-Heitler Pair Creation in an Intense Two-Mode Laser Field

We investigate electron-positron pair creation in the interaction of a nuclear Coulomb field and a highly intense two-mode laser field. For bichromatic laser fields, we examine the differences arising for commensurable and incommensurable frequencies in a continuous variation of the laser frequency ratio and the quantum interference effects, which may occur in the commensurable case. We show that the interference manifests in the angular distributions and the total pair-production rates of the created particles. Additionally, by varying the amplitudes of the two modes we study pair creation in a monochromatic laser wave of arbitrarily elliptical polarization.

Exciton Recurrence Motion in Double-Ring Molecular Aggregates Induced by Two-Mode Circular-Polarized Laser Field

Exciton recurrence motions in double-ring molecular aggregates are investigated using the quantum master equation approach. The rotatory and oscillatory recurrence behaviors are found to be controlled by changing the relative direction of the circular polarization of an incident two-mode laser field. These dynamics are understood by the relative phases among off-diagonal exciton density matrices, the feature of which is determined by the interaction between the circular-polarized field and the degenerate states of the double-ring structure. The results of exciton relaxation effects caused by the exciton−phonon coupling and static disorder also demonstrate the robustness of the exciton recurrence motions in double-ring molecular aggregates.

Quantum Master Equation Approach to Exciton Recurrence Motion in Ring-Shaped Aggregate Complexes Induced by Linear- and Circular-Polarized Laser Fields

The coherent exciton dynamics in molecular complexes composed of ring-shaped aggregates induced by linear- and circular-polarized laser fields has been investigated by using the quantum master equation (QME) approach. As shown in previous studies, near-degenerate states create the superposition states after irradiation of linear-polarized laser fields and thus cause the oscillatory exciton recurrence motion. In contrast, the rotatory exciton recurrence motion is found to be induced by circular-polarized laser field in a C3-symmetry complex composed of identical three ring-shaped aggregates. This exciton dynamics is predicted to originate in the superposition states between the two pairs of degenerate states, which are coherently excited by a circular-polarized laser field. The rotatory exciton recurrence motion induced by a two-mode laser field with mutually opposite circular polarizations also has been examined in the complex composed of two different-sized groups of ring-shaped aggregates. It turns out that the two-mode laser field induces mutually counter-rotatory exciton recurrence motions concurrently, which are generated separately on the two different groups of ring-shaped aggregates. These results suggest the possibility of controlling rotatory exciton recurrence motions by using the circular-polarized laser fields and ring-shaped aggregate complexes.

Atomic motion in a two-mode laser field

A modified perturbation theory is developed for the calculation of the time-dependent wave function describing the propagation of an atom in a two-mode laser field. With the mode frequencies nearly equal, a first approximation is obtained that reproduces and extends known results, for both resonant and nonresonant transitions. This lowest-order solution sums terms that describe the absorption of a photon from one mode and emission into the other, leaving the total number of photons unchanged and shifting the momentum of the atom accordingly. Corrections are generated by expansion of a resolvent from which the nearly degenerate states have been projected out. These approximate solutions are used to construct asymptotic states in a formulation of atom-atom scattering in the two-mode field. In close analogy with the soft-photon approximation, a ``soft-pair'' approximation is developed based on the dominance of those asymptotic interactions in which the transfer of energy from the field to the atom is small, allowing one to relate the scattering amplitude in the presence of the field to the field-free amplitude. When the atomic ground state and an excited state are closely coupled by the field, the asymptotic wave function can have mixed parity allowing for long-range contributions to the effective potential. In addition to an enhancement of forward-scattering probabilities, this leads to a threshold anomaly that is analyzed here, in the resonant case, with the aid of a modified effective-range theory.

Quantum processes in a two-mode laser field

The probabilities of the emission of a photon by an electron and e + e −-pair photoproduction in a field which is a superposition of two electromagnetic plane waves with different frequencies and propagating in the same direction are obtained. The case where the frequencies of the two modes are commensurate is studied in detail. This case is interesting primarily because of the existence of effects due to the interference of amplitudes, corresponding to a different number of photons absorbed from different modes but having the same total 4-momentum. It is shown that the optimal field for observing interference effects is a field such that the ratio of the mode frequencies is 3. The probabilities of radiation and pair-photoproduction processes in the field of a monochromatic plane wave and in a two-mode field, obtained by splitting the initial wave into two waves, are compared. It is shown that the total probability of the emission of a photon by an electron in a two-mode field is lower than and the probability of pair photoproduction is higher than the probabilities of the same processes in the initial wave. The increase in the pair-photoproduction probability is explained by the fact that additional channels for reactions which are forbidden in the initial monochromatic field open up in a two-mode field.

Second-order coherence g (2) (τ) and its frequency-dependent characteristics of a two-longitudinal-mode laser

The degree of second-order coherence g(2)(τ) and its frequency-dependent formula of a two-longitudinal-mode laser (simply called two-mode laser) are derived from the quantum theory of light. The quasi-periodicity, frequency-dependent characteristics and photon statistics properties of g(2)(τ) are analysed theoretically. It is found that for a two-mode laser field there exists the effect of photon anticorrelation and the g(2)(τ) can be measured by the optical interference method. The frequency-tuning characteristics of g(2)(τ) in the two-mode laser field is also analysed. It is shown that the frequency-dependent characteristics of g(2)(τ) can be applied to frequency and power stabilization of the two-mode laser and the measurement of two-mode laser linewidth ΔνD and the frequency width δνh of each longitudinal-mode, respectively.

Effect of virtual Compton scattering on electron propagation in a laser field

Volkov wave functions describe the motion of an electron in a plane-wave laser field and are widely used in the treatment of multiphoton ionization and laser-assisted scattering. These functions are not applicable for fields of a more general type in which the various modes propagate in different directions. Recent work on the development of generalized Volkov solutions, applicable when the field is not a plane wave, is extended here to account for the effect of virtual Compton scattering involving photon pairs that transfer only a small amount of momentum. When such processes occur as initial- or final-state interactions in which the energy of the electron is nearly conserved, near degeneracies are introduced requiring special care in the solution of the wave equation. Methods for constructing approximate solutions that treat these effects accurately are described here for both nonrelativistic and relativistic wave equations. The solutions are used in an analysis of the Kapitza-Dirac effect---the scattering of an electron in a standing-wave laser field---and more general results are obtained than have been in earlier treatments of this problem. In a different class of applications, the generalized Volkov solutions are adopted in a study of the (relativistic) potential scattering of an electron in the presence of a two-mode laser field. A variational formulation, evaluated in the weak-field limit, is used to generate a low-frequency approximation for two-photon bremsstrahlung. A sum rule is derived that provides a simple relation between the true cross section and that obtained with the omission of virtual Compton scattering effects.

Two-photon absorption of a two-mode laser beam with phase diffusion

A study is made of the efficiency of two-photon absorption of a two-mode laser field with phase diffusion. Numerical computer calculations are reported.

Quantum and classical descriptions of the two-mode laser field

The coherent state representation of a two-mode laser fiels is derived using a signal plus noise model and compared with the corresponding classical result.

Time distribution of photons from coherent and Gaussian sources

The statistics of a radiation field is investigated by measuring the time distributions of photoelectrons from a single-photon counter. The statistics of a Gaussian field, a single-mode and a two-mode laser field are studied and compared.