The dynamic nature of the interaction between the electromagnetic field of radiation and a resonantly absorbing medium is manifested as Rabi oscillations and can substantially affect both stationary and transient optical processes. This effect was repeatedly demonstrated for nonstationary transient processes in gaseous media with homogeneously broadened absorption lines. A single approach to the coherent control of the transient processes in the media with essentially inhomogeneous broadening of the absorption line is proposed. The results of the study of the Doppler-broadened molecular transition are presented. The prospects for the study of alternative inhomogeneously broadened media (low-pressure atomic gas and the exciton transitions localized in quantum wells (nanosized semiconductor heterostructures)) are reviewed.
The dynamic nature of the interaction between the electromagnetic field of radiation and a resonantly absorbing medium is manifested as Rabi oscillations and can substantially affect both stationary and transient optical processes. This effect was repeatedly demonstrated for nonstationary transient processes in gaseous media with homogeneously broadened absorption lines. A single approach to the coherent control of the transient processes in the media with essentially inhomogeneous broadening of the absorption line is proposed. The results of the study of the Doppler-broadened molecular transition are presented. The prospects for the study of alternative inhomogeneously broadened media (low-pressure atomic gas and the exciton transitions localized in quantum wells (nanosized semiconductor heterostructures)) are reviewed.
Specific features of nonlinear interference processes at quantum transitions in near- and fully resonant Doppler-broadened optically dense media are studied. The possibility of all-optical switching of the medium to opaque or, alternatively, to absolutely transparent, or even to strongly amplifying states is explored, which is controlled by a small variation of two driving or probe radiations and does not require any change of the level populations. Optimum conditions for inversionless amplification of short-wavelength radiation above the oscillation threshold at the expense of the longer-wavelength control fields are investigated. The feasibility of overcoming the fundamental limitation on a velocity-interval of resonantly coupled molecules imposed by the Doppler effect is shown, based on quantum coherence.
The resonant absorption, emission, and scattering of light are the fundamental optical processes that have been used both to probe matter and to manipulate light itself. In the last decade there has been essential progress in coherent control of both linear and nonlinear optical responses based on resonant excitation of atomic coherence in multilevel quantum systems. Some interesting and useful phenomena, resulting from coherent control of absorption and the group index, such as electromagnetically induced transparency, lasing without inversion, and ultra-slow group velocity of light have been widely studied. This work is focused on coherent control of refractive index and resonant fluorescence in multilevel medium. \n \nWe suggest two promising schemes for resonant enhancement of the refractive index with eliminated absorption and propose their implementation in transition element doped crystals with excited state absorption and in a cell of Rb atoms at natural abundance. We show how to use one of these schemes for spatial variation of the refractive index via its periodic resonant increase/decrease, remarkably keeping at the same time zero absorption/gain. It opens the way to production of transparent photonic structures (such as distributed Bragg reflectors, holey fibers, or photonic crystals) in a homogeneous resonant atomic media such as dielectrics with homogeneously distributed impurities, atomic, or molecular gases. These optically produced photonic structures could easily be controlled (including switching on/off, changing amplitude and period of modulation) and would be highly selective in frequency, naturally limited by the width of the optical resonance. \n \nWe also derive the optical fluorescence spectra of a three-level medium driven by two coherent fields at the adjacent transitions in a general case when all three transitions are allowed. We show that coherent driving can efficiently control the distribution of intensities between the fluorescent channels. In particular, the total intensity of fluorescence at the transition which is not driven by the optical fields may essentially exceed the fluorescence intensity at the driven transitions under the condition of two-photon resonance. This counter-intuitive effect is due to depletion of the intermediate state via atomic interference.
We investigate theoretically coherent control of optical absorption and emission (optical gain) in semiconductor double wells by use of two coherent light sources. Our analysis is based on a microscopic theory of optical excitation in semiconductors which accounts for many-body effects. Specifically, we show that electron–hole pair (exciton) formation in an asymmetric double well can be controlled by intensity and phase of a dc microwave (MW) field if the pulse duration lies below the MW-induced Rabi period. Similarly, resonant MW fields are shown to allow control of intersubband absorption and emission spectra of semiconductor heterostructures.
The review deals with resonance phenomena resulting from saturation of low-pressure-gas absorption lines by a coherent optical field, when the spectral lines are inhomogeneously broadened by the Doppler effect. Principal attention is paid to narrow resonances of width much smaller than the Doppler widths, which are produced upon saturation of the absorption by light waves under various experimental conditions (inside and outside the laser resonator, in traveling and standing waves, in spatially separated waves, etc.), owing to different effects (absorption saturation, mode interaction in the laser). Each of the phenomena is considered from both the theoretical and the experimental points of view. The phenomena considered serve as a basis of a new branch of optical spectroscopy, namely nonlinear laser spectroscopy of ultrahigh resolution.
A class of molecular two-photon processes, occurring under transient or steady-state conditions, is analyzed exactly for certain cases using a density-matrix formalism. Our results are relevant to the recently observed phenomena of transient coherent Raman beats and steady-state two-photon absorption of oppositely directed laser beams. Lowest-order perturbation theories have shown that these processes are insensitive to the Doppler effect, and consequently the linewidth is unaffected by Doppler broadening and by elastic collisions that change molecular velocity. The solutions presented here are of the same form for the Raman and two-photon problems and reveal significant power-dependent frequency shifts and line broadening that correct the ideal Doppler-free solutions. For the transient case, we assume a molecular three-level quantum structure that can be switched in or out of optical resonance with cw laser radiation by means of a pulsed dc Stark field, but the solutions also apply when resonant optical pulses are introduced. During the resonant condition when the Stark pulse is on, the three levels are prepared in coherent superposition, and during the nonresonant condition following the Stark pulse, the laser field(s) stimulates the coherently prepared sample in a transient two-photon process. One of these processes, the Ramanbeat effect, has been observed in forward scattering, but the second one, transient two-photon emission, should be observable in backward scattering. Bloch-like equations are derived for this three-level problem that facilitate an exact pulse solution for state preparation. Following the pulse, transient decay is well-approximated by a perturbation calculation. For the steady-state case, an exact solution is also obtained and is of interest for continuous-wave spectroscopy or for transient experiments requiring an initial preparation of quantum states, prior to Stark switching. These solutions exhibit a power-dependent line broadening and a frequency shift of magnitude $\ensuremath{\sim}\frac{\ensuremath{\Delta}({\ensuremath{\alpha}}^{2}\ensuremath{-}{\ensuremath{\beta}}^{2})}{({\ensuremath{\Delta}}^{2}+\frac{1}{{T}_{2}^{2}})}$, in agreement with our earlier estimate, where $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ are the Rabi flopping frequencies for the two intermediate transitions and $\ensuremath{\Delta}$ is the off-resonance tuning behavior for one of them.
Summary form only given. Experiments in nonlinear optics and spectroscopy are often performed in atomic vapors, where the linewidths of the transitions of interest are dominated by Doppler broadening. In fact, several experiments on quantum coherent phenomena, such as lasing without inversion (LWI), and electromagnetically induced transparency (EIT), have been done in atomic vapors. Using the ideas of quantum coherence and interference, we propose a new scheme for obtaining subDoppler resolution at one transition of an inhomogeneously broadened, three-level atomic system, by use of an intense control field at the other transition.