Session F3: AMO2: Atomic, Molecular and Optical Physics

Tunable femtosecond dispersive waves generation in PCF

Tunable femtosecond optical pulses are widely needed in various applications. Recently, the largest tuning range is generally achieved by adding the desired filter after white light generation, which causes a complicated system and a low efficiency. Due to the most recent development of photonic crystal fibers (PCF), the optical dispersion behavior can be modulated a lot. With ultrashort pulses propagating along the fiber, this modulated dispersion can resolve generation of new wavelengths. We will show that more efficient tunibility can be achieved by simply using a femtosecond oscillator and a piece of PCF, and the tuning range will cover more than octave spectrum. The autocorrelation shows that with a femtosecond pulse propagating in a reasonable short PCF, the shifted pulses are femtosecond pulses as well. This phenomenon may expand applications in the fields that the laser wavelengths are not easily obtained.   

Dynamical evolution of correlated spontaneous emission of a single photon from a N atoms cloud

We study the correlated spontaneous emission from a dense spherical cloud of $N$ atoms uniformly excited by absorption of a single photon. We find that the decay of such a state depends on the relation between an effective Rabi frequency $\Omega \propto \sqrt{N}$ and the time of photon flight through the cloud $R/c$. If $\Omega R/c<1$ the state exponentially decays with rate $\Omega ^{2}R/c$ and the state life time is greater then $R/c$. In the opposite limit $\Omega R/c\gg 1$, the coupled atom-radiation system oscillates between the collective Dicke state (with no-photons) and the atomic ground state (with one photon) with frequency $\Omega $ while decaying at a rate $c/R$.   

Femtosecond pulses propagation through pure water

Recently, considerable attention has been dedicated to the field of optical precursors, which can possibly be applied to long-distance underwater communications. Input beam intensities have been carefully adjusted to keep experiments in the linear regime, and some experiments have shown violation of the Beer-Lambert law. We are presently carrying out experiments using femtosecond laser pulses propagating through pure water strictly in the linear regime to study this interesting and important behavior. We are also employing several new and innovative schemes to more clearly define the phenomena.   

The Ultrashort laser pulses in water that violates the Lambert-Beer Law

Recent experiments have opened the possibility that by using ultrashort Laser Pulse in H2O, it may be possible to propagate light (signal) over much further distance than predicted by the familiar Beer-Lambert Law. To explain it, the complete femtosecond-width pulse propagation process will be modeled and simulated by FDTD method in visible frequency range. We will show how the FDTD method can be used to accuately model the propagation of Ultrashort pulses in water. We will also show the development of the both the Sommerfeld and Brillouin optical precursors. We will, for the first time, use the actual absorption spectrum of water in these calculations and compare the results with experimental data.   

Quantum control of electromagnetically induced transparency by optical phase

Coherence in a three-level atomic medium can be created by two resonant optical fields. In this case the optical and atomic phases are locked and absorption of the medium decreases. This effect is well known as electromagnetically induced transparency (EIT). Recently F. A. Narducci with co-workers has suggested a quantum control of the transparency by changing phase of the optical field. In this paper we report results of experimental study of EIT dynamic in Rubidium atomic vapor. The optical phase of one of the optical fields is changed as a step function by electro-optical phase modulator. We have observed very fast variation of the transmission with a rise time of the order of inverse Rabi frequency at nanosecond scale. Variation of the transmission is proportional to the phase change and it can be comparable with amplitude of the EIT resonance. The transmission restores to the original EIT level slowly with the ground state relaxation rate(microsecond scale). We have confirmed that it is possible to control EIT by the optical phase. These results open a way to build up fast optical modulates and switches based on EIT.   

Two-photon excitation by chirped and optimally shaped pulses

Two-photon fluorescence of different dyes was optimized by using a feedback control femtosecond pulse shaping technique. For optimization we implemented a liquid crystal pulse shaper in a folded 4f set-up with an evolutionary algorithm. The optimization procedure that started with a near transform-limited pulse noticeably improved the two-photon fluorescence. Several signal ratios involving two-photon fluorescence, second harmonic generation and the incident laser power were successfully optimized. The two-photon fluorescence was also optimized by varying the chirp of the laser pulse with a liquid crystal and acousto- optical modulators. The correlation between the two-photon fluorescence and the second harmonic generation was studied, and it was found to decrease when the pulse shape was close to the optimum.   

Towards Superfluorescence in Cesium Vapor

Cesium atoms have been excited by two photon process into coherent superpostion of the excited 8S$_{1/2}$ and ground 6S$_{1/2}$ states via ultrashort laser pulse ($\sim $50fs). A superfluorescent blue light (456nm) at the lower transtion of 7P$_{3/2}$-- 6S$_{1/2}$ has been studied. By the use of high resolution (2.5ps) streak camera (Hamamatsu) the delay of the blue pulse has been measured as a function of the input IR beam power. In cases of the different vapor temperature, it has been shown that, this dependence can be scaled by the ratio of the number of interacting Cs atoms. The delayed ultrashort pulses provide temporal characteristics of the superfluorescence. In doing so, we have observed the autocorrelation of the superlfuorescent light which shows interference much later, eventhough, there no interference exhibits between two input pulses.   

Visualizing Molecular Wavefunctions Using Monte Carlo Methods

Using explicitly correlated wavefunctions and variational Monte Carlo we calculate the electron density, the electron density difference, the intracule density, the extracule density, two forms of the kinetic energy density, the Laplacian of the electron density, the Laplacian of the intracule density and the Laplacian of the extracule density on a dense grid of points for the ground state of the hydrogen molecule at three internuclear distances (0.6,1.4,8.0). With these values we construct a series of contour plots and describe how each function can be used to visualize the distribution of electrons in this molecule. We also examine the effect of electron correlation on each expectation value by calculating each function with a Hartree-Fock wavefunction and then comparing these values with our explicitly correlated values.   

Loading Dynamics and Characteristics of a Far Off-Resonance Optical Dipole Trap

We implement an optical dipole trap in a crossed beam configuration for experiments with ultracold strontium. Strontium atoms cooled to nearly 1 $\mu$K are loaded into the optical dipole trap from a magneto-optical trap operating on the 689 nm intercombination line. Loading dynamics and characteristics of the far off-resonance dipole trap are explored as part of our group's study of ultracold collisions in strontium.