A resonance, induced by a DC field in a two-photon transition of an atom between states with opposite parity, results in a number of new effects related to the polarization of photons. These effects are caused by the interference of the non-resonant second-order dipole-forbidden amplitude and of a resonant part of the DC field-induced third-order amplitude. The linear and circular dichroism for the cross section of the process is revealed and analysed in detail for a two-photon excitation of the state from the ground state with a resonance on an state, induced by a DC electric field. The numerical data for alkali atoms demonstrate that the effects have quantitative characteristics of sufficiently large magnitudes and may be observed in experiments.
We have discovered an interference which results in the vanishing of some electric dipole transition amplitudes. A transition-matrix element
T between certain
S and
P sublevels with the same value of
MF vanishes when an applied magnetic field satisfies ${X}_{s}+{X}_{p}=\ensuremath{-}2{M}_{F}$, where $X={\ensuremath{\mu}}_{B}{g}_{J}\frac{B}{A}$, and
A is the hfs constant. There must be an avoided crossing between the two states of the same
MF which can only occur when one of the hyperfine manifolds has an inverted Zeeman effect. These conditions imply
MF<0 and
I>J. We observed the vanishing of
T in
23Na for one of the $\ensuremath{\Delta}{M}_{F}=0$ optical transitions of the $3{S}_{\frac{1}{2}}(F=1)\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}(F=1)$ manifold. Absorption of cw laser light was monitored by observing total fluorescence perpendicular to both the atomic and laser beams, and absorption vanished near an applied field of 155 G.
T vanishes for a large number of cases including rf and microwave frequencies and is a rather general consequence of angular momentum selection rules and perturbation theory. This phenomenon may have application in optical pumping, Lamb-shift measurements, and atomic weak neutral-current experiments.
We study how the synergetic effect of spin-orbit coupling (SOC) and Zeeman splitting (ZS) affects the optical conductivity in the one-dimensional Hubbard model using the Kubo formula. We focus on two phenomena: (1) the electric dipole spin resonance (EDSR) in the metallic regime and (2) the optical conductivity in the Mott-insulating phase above the optical gap. In both cases, we calculate qualitatively the effects of SOC and ZS and how they depend on the relative angle between the SOC vector and the magnetic field direction. First, we investigate the spin resonance without electron correlation (the Hubbard parameter
U=0). Although, neither SOC nor ZS causes any resonance by itself in the optical conductivity, the EDSR becomes possible when both of them exist. The resulting contribution to the optical conductivity is analyzed analytically. The effect of
U on the spin resonance is also studied with a numerical method. It is found that at half filling, the resonance is first enhanced for small
U and then suppressed when the optical gap is large enough. In the strong coupling limit $U\ensuremath{\rightarrow}\ensuremath{\infty}$ at half filling, we also refer to the resonance between the lower and upper Hubbard bands appearing at $\ensuremath{\omega}\ensuremath{\sim}U$, above the optical gap. A large magnetic field tends to suppress the signal while it is recovered thanks to SOC depending on the relative angle of the magnetic field.
Single-spin state evolution induced by the Landau-Zener-St\"uckelberg-Majorana (LZSM) interference in a Zeeman-spit four-level system in a periodically driven double quantum dot is studied theoretically by the Floquet stroboscopic method. An interplay between spin-conserving and spin-flip tunneling processes with the electric dipole spin resonance (EDSR) that is induced in an individual dot and enhanced by the LZSM multiple level crossings with the neighboring quantum dot is investigated as a function of the microwave (MW) frequency, driving amplitude, interdot detuning, and magnetic field. A number of special points in the parameter space are identified, out of which all three features are merged. Under this triple-crossing resonance condition, the interdot tunneling is combined with a fast spin evolution in each dot at the EDSR frequency. Harmonics of the EDSR are revealed in the spin-dependent tunneling maps versus variable magnetic field and MW frequency. The results are applicable for both electron and hole systems with strong spin-orbit interaction and may be useful for developing new time-efficient schemes of the spin control and readout in qubit devices.
Two-photon transitions between atomic states of total electronic angular-momentum
Ja=0 and
Jb=1 are forbidden when the photons are of the same energy. This selection rule is analogous to the Landau-Yang theorem in particle physics that forbids decays of vector particle into two photons. It arises because it is impossible to construct a total angular-momentum ${J}_{2\ensuremath{\gamma}}=1$ quantum-mechanical state of two photons that is permutation symmetric, as required by Bose-Einstein statistics. In atoms with nonzero nuclear spin, the selection rule can be violated due to hyperfine interactions. Two distinct mechanisms responsible for the hyperfine-induced two-photon transitions are identified, and the hyperfine structure of the induced transitions is evaluated. The selection rule is also relaxed, even for zero-nuclear-spin atoms, by application of an external magnetic field. Once again, there are two similar mechanisms at play: Zeeman splitting of the intermediate-state sublevels, and off-diagonal mixing of states with different total electronic angular momentum in the final state. The present theoretical treatment is relevant to the ongoing experimental search for a possible Bose-Einstein-statistics violation using two-photon transitions in barium, where the hyperfine-induced transitions have been recently observed, and the magnetic-field-induced transitions are being considered both as a possible systematic effect, and as a way to calibrate the measurement.