Coherent Population Return Research Articles

Optical analog to coherent population return in coupled nonlinear waveguides

In this paper, we investigate the optical analog to coherent population return (CPR) in coupled nonlinear waveguides. The nonlinearity sometimes plays a negative role in classical simulations of quantum adiabatic passages in waveguides. Nevertheless, it is interesting to note that the nonlinearity here helps simulating the adiabatic CPR technique. The CPR can still occur when the detuning (difference of the propagation constants) between the two nonlinear waveguides is moderate, which is a typical requirement of the technique. Moreover, the adiabatic process can appear when the detuning is small or zero. This is done by simply adjusting the incident light power, which is easy and of high accuracy compared with modulating the propagation constants. Therefore, the requirements towards the propagation constants of the waveguides are further relaxed in the nonlinear case. Finally, we point out that the CPR in nonlinear waveguides can be extended to design achromatic beam splitters.

Classical simulation of coherent population return in coupled optical waveguides

An optical analog to coherent population return (CPR) is carried out in two evanescently coupled waveguides. Thepropagation constants of the two waveguides are different and the coupling between them is slowly varied for the adiabatic condition.The analog to the CPR corresponds to complete light return. Furthermore, we simulate two extensions of the CPR. One is that thecoupling is unchanged after it reaches its maximum, while the another one is that the coupling is replaced by a sudden decrease fromthe maximum to zero (half-CPR). We can both observe nearly 50:50 beam splitting in the two extensions. However, the half-CPRperforms better, which implies its application for achromatic beam splitters. Finally, we note that the waveguide structure proposedhere for beam splitters is simpler than those based on the stimulated Raman adiabatic passage and the rapid adiabatic passage.

Frequency up-conversion of intense, ultrashort laser pulses at maximal atomic coherence

We present experimental data on adiabatically driven frequency conversion with ultrashort (picosecond) laser pulses towards the vacuum-ultraviolet regime, enhanced by coherent population return and preparation of maximal atomic coherences. We generate the sum frequency of an intense pump pulse and a probe pulse via a two-photon resonance in xenon. When we slightly detune the pump laser from two-photon resonance, the atomic populations are adiabatically driven forth and back from the atomic ground state to an excited state. This coherent population return (CPR) prepares the medium in a transient state of maximal atomic coherence, which enhances frequency mixing with a probe laser. We thoroughly study the variation of CPR-enhanced frequency conversion with experimental parameters and compare CPR with conventional, resonantly enhanced frequency conversion. In particular, we investigate the temporal evolution of the atomic coherence and the effect of inevitable ac Stark shifts. We find that ac Stark shifts induce an asymmetry in the spectral characteristics of CPR, which nevertheless permits enhanced frequency conversion. For the case of resonantly enhanced frequency conversion, we show that the atomic coherences are maintained for several tens of picoseconds after the pump pulse, which permits time-delayed frequency conversion. Moreover, we analyze the pressure dependence of the atomic coherence' lifetime and observe a free induction decay at the two-photon coherence with the generation of a second harmonic field. These findings shall push applications of adiabatic light-matter interactions also to the regime of nonlinear optics, driven by intense laser pulses.

A novel technique to achieve atomic macro-coherence as a tool to determine the nature of neutrinos

The photon spectrum in macro-coherent atomic deexcitation via radiative emission of neutrino pairs has been proposed as a sensitive probe of the neutrino mass spectrum, capable of competing with conventional neutrino experiments. In this paper, we revisit this intriguing possibility, presenting an alternative method for inducing large coherence in a target based on adiabatic techniques. More concretely, we propose the use of a modified version of coherent population return (CPR), namely two-photon CPR, that turns out to be extremely robust with respect to the experimental parameters and capable of inducing a coherence close to 100 % in the target.

High-order harmonic generation enhanced by coherent population return

We present computations of high-order harmonic generation in atoms by using the coherent population return technique. This approach consists in two steps: firstly, the active medium is prepared in a coherent superposition of two bound states using a relatively long laser pulse which has a photon energy close to the energy gap between the states and then, the system is driven by an ultrashort infrared laser pulse. We demonstrate, by employing the numerical solution of the time-dependent Schrodinger equation in one and three dimensions, that using this scheme it is possible to enhance the harmonic conversion efficiency by several orders of magnitude. Our numerical results show that, on one hand the increase in ionization efficiency is responsible of an enhancement in the harmonic signal and on the other hand, the coherent preparation of the medium produces a much richer structure of the harmonic emission.

Efficient Coherent Population Transfer of D2 Molecules by Stark-Induced Adiabatic Raman Passage

Preparation of a high flux of hydrogen molecules in a specific vibrationally excited state is the major prerequisite and challenge in scattering experiments that use vibrationally excited hydrogen molecules as the target. The widely used scheme of stimulated Raman pumping suffers from coherent population return which severely limits the excitation efficiency. Recently we successfully transferred D2 molecules in the molecular beam from (υ=0, J=0) to ( υ=1, J=0) level, with the scheme of Stark-induced adiabatic Raman passage. As high as 75% of the excitation efficiency was achieved. This excitation technique promise to be a unique tool for crossed beam and beam-surface scattering experiments which aim to reveal the role of vibrational excitation of hydrogen molecules in the chemical reaction.

Power narrowing in coherent atomic excitation by smoothly shaped pulsed fields

When atoms are excited by laser radiation the conventional wisdom dictates that the spectral excitation profile – the excited-state population vs the detuning – broadens as the radiation intensity increases. This common expectation has been shown to be incorrect in the fluorescence signal following two-photon excitation of xenon atoms by a Gaussian laser pulse, which showed almost no power broadening [T. Halfmann, T. Rickes, N.V. Vitanov, K. Bergmann, Optics Communications, 220 (2003) 353] Here we go a step further and show that for a class of pulse shapes, the falling edges of which vanish as inverse powers of time, ∝t−n (with n>1), the spectral excitation profile must exhibit power narrowing, i.e. the spectral linewidth scales as Ω0−1/(n−1), where Ω0 is the Rabi frequency of the transition. We provide a simple physical explanation based upon the notions of adiabatic evolution and coherent population return and discuss important experimental applications.

Femtosecond evolution of the pyrrole molecule excited in the near part of its UV spectrum

The evolution of the isolated pyrrole molecule has been followed after excitation in the 265-217 nm range by using femtosecond time delayed ionization. The transients collected in the whole excitation range show the vanishing of the ionization signal in the femtosecond time scale, caused by the relaxation along a πσ(∗) type state (3s a(1)←π 1a(2)), which is the lowest excited electronic state of the molecule. This surface is dissociative along the NH bond, yielding a 15 ± 3 fs lifetime that reflects the loss of the ionization cross-section induced by the ultrafast wavepacket motion. Although a weak πσ(∗) absorption is detected, the state is mainly reached through internal conversion of the higher bright ππ(∗) transitions, which occurs with a 19 ± 3 fs lifetime. In addition to its resonant excitation, the intense ππ(∗) absorption extending in the 220-190 nm interval is also out-of-resonance populated at energies far to the red from its absorption onset. This coherent adiabatic excitation of the ππ(∗) transition should follow the excitation pulse (coherent population return effect), but instead the system relaxes toward the lower πσ(∗) surface through a conical intersection during the interaction time, leading to the population of πσ(∗) state at wavelengths as long as 265 nm. According to the observed behavior, the time evolution of the system in the full excitation range studied is modeled by a coherent treatment that provides key insights on the photophysical properties of the molecule.

Can stimulated Raman pumping cause large population transfers in isolated molecules?

When stimulated Raman pumping (SRP) is applied to a stream of isolated molecules, such as found in a supersonic molecular beam expansion, we show that SRP can neither saturate nor power broaden a molecular transition connecting two metastable levels that is resonant with the energy difference between the pump and Stokes laser pulses. Using the optical Bloch-Feynman equations, we discuss the pumping of the hydrogen molecule from H(2) (v = 0, J = 0, M = 0) to H(2) (v = 1, J = 2, M = 0) as an illustration of how coherent population return severely reduces the SRP pumping efficiency unless the pump and Stokes laser pulses are applied with an appropriate relative delay and ratio of intensities.

Adiabatically driven frequency conversion towards short extreme-ultraviolet radiation pulses

We present an experimental demonstration of sum-frequency generation toward tunable picosecond, broadband radiation pulses in the extreme-ultraviolet regime, supported by adiabatically prepared atomic coherences. We drive a two-photon transition in a dense medium of xenon atoms with a long (nanosecond) pump laser pulse at the Fourier-transform-limited bandwidth and with a wavelength of 225 nm. The frequency of the pump pulse is slightly detuned from exact two-photon resonance. In this configuration, the medium is adiabatically driven by a process of coherent population return. The adiabatic passage process generates a maximal coherent superposition of two quantum states with large energy spacing. An additional, short (picosecond) probe laser pulse at a wavelength of 540 nm beats with the maximal atomic coherence and generates a short (picosecond) signal radiation pulse at 93 nm. As compared to conventional (diabatic) frequency conversion, the adiabatically driven maximal atomic coherence yields enhanced conversion efficiency and significantly enhanced stability.

Coherent excitation phenomena in time-resolved experiments

We report on the influence of coherent phenomena on femtosecond pump-probe experiments in molecular systems. The signature of Coherent Population Return (CPR) has been observed, and satisfactorily simulated, by following the dynamics of the on and off resonance excitation of the S(0)-S(1) transition of aniline. While for the on resonance experiments the system dynamics can be described in terms of the simple incoherent rate equations, for the off resonance experiments a more detailed analysis based on the time-dependent Schrödinger equation that takes into account coherent effects is needed. This fact points out the necessity of considering coherent effects during and after the interaction process, even in systems where ordinarily they have been neglected, for a rigorous description of laser mater interaction.

On the population dynamics induced by an attosecond train interacting coherently with an atomic system within the electric dipole approximation

The population dynamics induced in an atomic system by XUV-attosecond radiation have been investigated within the electric dipole approximation. The main characteristics of the dynamics are explained in simple terms, extending our previous work on coherent laser-atom interactions in the XUV spectral region. Attosecond beating appearing in a second-order autocorrelation of a coherent superposition of high order harmonics, is investigated from the perspective of a superposition of Rabi oscillations, as well as of coherent population return (CPR). The work reveals the strength of attosecond radiation as a tool for time domain studies of coherent manipulations of quantum systems.

Enhanced four-wave mixing by atomic coherence in a Pr3+:Y2SiO5 crystal

We experimentally demonstrate an enhanced four-wave mixing (FWM) based on atomic coherence in a Pr3+:Y2SiO5 crystal. By employing coherent population return and fractional stimulated Raman adiabatic passage to prepare maximum atomic coherence in the crystal, an efficient FWM signal can be generated. By measuring the generated FWM signal, the time-dependent atomic coherence is monitored. Such an enhanced FWM in solids may have practical applications in nonlinear optics and laser spectroscopy.

Time-resolved probing of the ground state coherence in rubidium

We demonstrate the preparation and probing of the coherence between the hyperfine ground states |S(1/2),F=1> and |5S(1/2),F=2> of the Rb87 isotope. The effects of various coherence control techniques, i.e., fractional stimulated Raman adiabatic passage and coherent population return, on the coherence are investigated. These techniques are implemented using nearly degenerate pump and Stokes lasers at 795 nm (Rb D1 transition), which couple the two hyperfine ground states via the excited state |5P(1/2),F=1> through a resonant two-photon process in which a coherent superposition of the two hyperfine ground states is established. The medium is probed by an additional weak laser, which generates a four-wave mixing signal proportional to the ground state coherence and allows us to monitor its evolution in time. The experimental data are compared with numerical simulations.

Rapid adiabatic passage in a Pr3+:Y2SiO5 crystal

We report on rapid adiabatic passage (RAP) in a Pr3+:Y2SiO5 crystal, cooled to cryogenic temperatures. The medium is prepared by optical pumping and spectral hole burning, creating a spectrally isolated two-level system within the inhomogeneous bandwidth of the 3H4 → 1D2 transition of the Pr3+ ions. A chirped laser pulse drives a RAP process in the medium, i.e. inverts the initial population distribution. We study the properties and dynamics of RAP by means of fluorescence detection, absorption spectroscopy and amplified spontaneous emission. Time-resolved absorption measurements serve to monitor the adiabatic population dynamics during the excitation process. In addition, we compare the results with coherent excitation at fixed laser frequency detuned from resonance, i.e. coherent population return (CPR).

Influence of laser bandwidth and transit time on the excitation probability of a rovibrational two-level system

Practical considerations arising from Doppler distributions and beam divergence typically limit the amount of internal excitation that can be produced as a molecule passes across a cw laser beam. For coherent excitation this population transfer is furthermore limited by coherent population return (CPR), in which transiently excited molecules, off resonance, adiabatically evolve so as to return to the initial state at the end of a smooth pulse. We illustrate this effect with simulations, and show that in principle the presence of incoherence in the laser radiation (i.e., a finite bandwidth) can help suppress CPR, thereby substantially improving the averaged population transfer. For cases of practical relevance, the velocity-averaged population transfer can be optimized by a proper choice of the laser beam waist along the molecular beam direction.

Lineshapes in coherent two-photon excitation

We compare lineshapes obtained in laser-induced fluorescence and resonantly enhanced multi-photon ionisation, resulting from coherent pulsed excitation of a two-photon transition in xenon atoms. We find that excitation profiles probed by ionisation show power broadening as expected. Here ionisation occurs simultaneously with the coherent excitation. The profiles observed by fluorescence show almost no broadening, because of coherent population return, since the fluorescence occurs predominantly after the excitation pulse is over. Thus the observed lineshape depends characteristically upon the nature of the probing process.