A new method for coherent phase control of excitation of atoms in a discrete spectrum under the action of bichromatic laser radiation with the frequency ratio 1 : 2 is analysed. An important feature of this control method is the presence of a electrostatic field, which removes the parity selection rule for one of the control channels. It is shown that for the phase difference between the monochromatic radiation components, corresponding to the destructive interference between channels, there exists the electrostatic field strength at which the excited atomic transition is `bleached'. It is proposed to use luminescence at the adjacent atomic transition for detecting the phase dependence of optical excitation.
A new type of modulation of the photoluminescence intensity of atoms excited by a bichromatic laser radiation with the frequency ratio 1:2 is proposed and analysed. The modulation is produced by alternating electric field acting on atoms and occurs due to the quantum interference of the amplitudes of two excitation channels of an atom, which proves to be possible because the applied electric field removes the parity selection rule for one of the excitation channels. An important feature of this process is the dependence of photoluminescence on the phase difference of monochromatic components of exciting radiation. The calculation was performed for an alkali metal atom excited at the s — s transition taking the saturation effect into account.
Coherent phase control under the interaction between bichromatic radiation with multiple frequencies and quantum systems is the topic of the last time investigations both experimental and theoretical. This type of nonlinear optical control enables one to manipulate the characteristics of radiation processes by the change of the relative phase of bichromatic field. The present study is devoted to the theoretical analysis of the coherent control of two-level system behavior under bichromatic excitation in the frame of perturbation approach. The general expressions have been derived which are valid for any integer ratio n between the frequencies of monochromatic components of radiation. Obtained formulas describe the system characteristics as functions of relative phase of bichromatic field and other radiation parameters. Bichromatic excitation of Rubidium atom is considered as numerical example.
Phase sensitive coherent control on atomic Rydberg states and in the presence of a static electric field is theoretically studied using Harmin's frame transformation theory. It is shown that under the presence of the static field, the total excitation rate of an atomic target by a bichromatic laser field (consisting of a fundamental radiation frequency and of its second harmonic) depends on all the elements of the density-of-states matrix dictating the Stark effect. Some of these off-diagonal matrix elements are inaccessible by any other photoexcitation scheme and affect non-trivially the optimum achievable rate modulation contrast. This fact leads to important qualitative and quantitative differences with respect to static-field-free coherent control. The obtained formal expressions for the phase-controlled excitation rate are applied to the lithium target atom. It is demonstrated that the electric field strength can be used as an additional, externally adjustable, control and manipulation parameter. Finally, it is shown that above the saddle point energy, where quasi-discrete and continuum states coexist, the lineshapes of individual Fano-like resonances can be quite efficiently modified by applying this scheme.
A review of experimental and theoretical studies on radiative processes induced by a bichromatic (two-frequency) laser radiation in atoms, molecules, and solids is presented. The optical excitation of matter is considered at transitions of different types: bound—bound, bound—free, and free—free. A special attention is devoted to the possibility of the coherent control of the basic characteristics of radiative phenomena by varying the relative phase of monochromatic components of radiation. It is shown that this method opens up new possibilities for controlling photoinduced processes.
We experimentally demonstrate one- and two-photon phase sensitive coherent control over the excitation and ionization of Rydberg states of Sr atom in the presence of a static homogeneous electric field. Without the field this excitation scheme can only be employed for the manipulation of angular distributions of photoelectrons and molecular photofragmentation products. Total atomic excitation-ionization yields cannot be modulated because the final states excited by each pathway are orthogonal to each other. When, however, a static electric field is applied,
s,
p and
d character is admixed into the final Rydberg state, which has no definite parity anymore. Hence, the excitation of the target Stark state from the ground state is possible with either two (fundamental laser frequency) photons or one (second harmonic frequency) photon and its population and further ionization can be controlled by varying the relative phase between the two radiation fields. The concept is successfully tested below as well as above the classical saddle point and with either mutually crossed or parallel linear laser beam polarizations (while the second harmonic beam polarization vector is always parallel to the static field direction). We examine the behavior of the obtained photoionization signal modulation depth
V as a function of the static field strength
F for otherwise identical experimental conditions. The
V(F) curve exhibits a maximum (typically $\ensuremath{\sim}65%--85%$) at a field strength value that is dictated by the interplay between the employed laser power densities, one- and two-photon transition dipole moments, and relative amounts of field-dependent
s,
p and
d character. It is therefore shown that the static field strength may serve as an additional, experimentally adjustable, control parameter in a fashion complementary to both the intensities and relative phase of the two light beams.