L. P. Yatsenko

Efficient adiabatic population transfer by two-photon excitation assisted by a laser-induced Stark shift

We demonstrate and analyze a novel scheme for complete transfer of atomic or molecular population between two bound states, by means of Stark-chirped rapid adiabatic passage (SCRAP). In this two-laser technique a delayed-pulse laser-induced Stark shift sweeps the transition frequency between two coupled states twice through resonance with the frequency of the population-transferring coupling laser. The delay of the Stark-shifting pulse with respect to the pulse of the coupling-laser Rabi frequency guarantees adiabatic passage of population at one of the two resonances while the evolution is diabatic at the other. The SCRAP method can give a population-transfer efficiency approaching unity. We discuss the general requirements on the intensity and timing of the pulses that produce the Rabi frequency and, independently, the Stark shift. We particularly stress extension to a double-SCRAP technique, a coherent variant of stimulated emission pumping in the limit of strong saturation. We demonstrate the success ...

Power broadening revisited : theory and experiment

The spectral width of an atomic absorption line, observed with a steady light source, typically increases as the light intensity increases, an effect known as power broadening. In this paper, we point out classes of pulsed-light observations where power broadening does not always occur. We present analytical and numerical results, supported by experimental data of coherent pulsed excitation probed by photoionization, which show that the extent of power broadening depends crucially upon the nature of excitation and the type of measurement. In particular, we show that a spectral line obtained from measurement performed after pulsed excitation exhibits no power broadening. For pulsed excitation and continuous measurement, the spectral line contains two components: a power-broadened signal collected during the excitation and an unbroadened signal collected after the excitation.

Laser-induced adiabatic atomic reorientation with control of diabatic losses

Abstract We discuss theoretical procedures for using a single laser pulse, of varying linear polarization, to transfer population between sublevels of a degenerate atomic level. The method is based on application of the procedure of stimulated Raman adiabatic passage (STIRAP). We also show how, in principle, a pulsed quasistatic magnetic field can be used to improve the usual adiabatic constraints on achieving complete population transfer by means of STIRAP. The technique allows use of shorter pulses, so that population transfer can be achieved more rapidly. It diminishes, or even eliminates, diabatic loss during the population transfer.

Creation of coherent superpositions using Stark-chirped rapid adiabatic passage

We show that the technique of Stark-chirped rapid adiabatic passage (SCRAP), hitherto used for complete population transfer between two quantum states, offers a simple and robust method for creating coherent superpositions of states. SCRAP uses two laser pulses: a strong far off-resonant pulse modifies the transition frequency by inducing ac Stark shifts in the energies of the two states, and an appropriately offset in time, near-resonant and moderately strong pump pulse drives the population between the states via one of the induced diabatic level crossings. The populations in the created superposition are controlled by the detuning of the pump laser from the transition frequency and are insensitive to variations in the intensities of the pump and Stark lasers, as long as these are sufficiently large to allow adiabatic evolution.

Photoionization Suppression by Continuum Coherence: Experiment and Theory

Abstract: We present experimental and theoretical results of a detailed study of laser-induced continuum structures (LICS) in the photoionization continuum of helium out of the metastable state 2s^1 S_0. The continuum dressing with a 1064 nm laser, couples the same region of the continuum to the 4s^1 S_0 state. The experimental data, presented for a range of intensities, show pronounced ionization suppression (by asmuch as 70% with respect to the far-from-resonance value) as well as enhancement, in a Beutler-Fano resonance profile. This ionization suppression is a clear indication of population trapping mediated by coupling to a contiuum. We present experimental results demonstrating the effect of pulse delay upon the LICS, and for the behavior of LICS for both weak and strong probe pulses. Simulations based upon numerical solution of the Schrodinger equation model the experimental results. The atomic parameters (Rabi frequencies and Stark shifts) are calculated using a simple model-potential method for the computation of the needed wavefunctions. The simulations of the LICS profiles are in excellent agreement with experiment. We also present an analytic formulation of pulsed LICS. We show that in the case of a probe pulse shorter than the dressing one the LICS profile is the convolution of the power spectra of the probe pulse with the usual Fano profile of stationary LICS. We discuss some consequences of deviation from steady-state theory.

Population transfer through the continuum using laser-controlled Stark shifts

Abstract We show, through analysis and numerical modeling of hydrogen and sodium, that it should be possible to transfer atomic population, with high probability, using a delayed-pulse two-photon Raman transition in which intermediate states are in the ionization continuum. We propose using auxiliary laser pulses to overcome dynamic Stark shifts that would otherwise prevent population transfer. We find that, even without such auxiliary pulses, population transfer can be improved by tuning the carrier frequencies away from the two-photon Raman resonance condition. Our computations for hydrogen predict nearly 85 percent population transfer.

Laser control of atomic and molecular motion by sequences of counterpropagating light pulses

The analysis of atomic motion in the field formed by sequences of counterpropagating light pulses reveals the conditions when the field creates the trap in which the temperature of trapped atoms drops to the Doppler limit. The atomic state is described by the wave function using the Monte Carlo wave function method, whereas the atomic motion is considered in the framework of classical mechanics. Laser cooling and trapping is achieved only for non-resonant atom–field interaction. The pulse area does not matter for this effect, in contrast to the repetition period. When the motion of a trapped atom is slowed down, it oscillates around the anti-nodes of a non-stationary standing wave formed by the counterpropagating light pulses at the point where they ‘collide’. The discussed trap is also applicable for trapping and cooling of the molecules for which the matrix of Frank–Condon factors is almost diagonal.

Steering population flow in coherently driven lossy quantum ladders.

We present a detailed theory of a technique for the adiabatic control of the population flow through a preselected decaying excited level in a three-level ladder quantum system, as was experimentally demonstrated recently by Garcia-Fernandez et al. [Phys. Rev. Lett. 95, 043001 (2005)]. Specifically, we consider a three-state excitation chain of bound states, 1-2-3, of successively increasing excitation energy, in which probability loss via fluorescence occurs from states 2 and 3. We describe a laser excitation scheme that can, by adjustment of laser parameters, alter at will the relative fraction of population that, starting from state 1, is ultimately lost through states 2 and 3. We present analytical results for the conditions under which quasiadiabatic passage can take place.

Adiabatic creation of entangled states by a bichromatic field designed from the topology of the dressed eigenenergies

Preparation of entangled pairs of coupled two-state systems driven by a bichromatic external field is studied. We use a system of two coupled spin-1/2 that can be translated into a three-state ladder model whose intermediate state represents the entangled state. We show that this entangled state can be prepared in a robust way with appropriate fields. Their frequencies and envelopes are derived from the topological properties of the model.

Population inversion using laser and quasistatic magnetic field pulses

Abstract We show how, using a combination of a radiative pulse and a pulsed quasistatic magnetic field, it is possible to obtain complete population inversion between two states in a multistate system. The population transfer occurs between states of opposite parity, and does not require either careful control of pulse area nor frequency chirp. When the pulses are applied in counter-intuitive order the method is dynamically equivalent to stimulated Raman adiabatic passage (STIRAP). Complete inversion is also possible for intuitively ordered pulses, with robustness equivalent to that of STIRAP.