Session B2: Atomic, Molecular and Optical Physics I

Toward Magneto-optical Cooling of $^7$Li

Laser cooling, the standard approach for producing ultra-cold atoms for the past thirty years, has reached saturation in the number of ultra-cold atoms produced per second and atomic density. We report the development of an alternative method for producing ultra-cold atoms and predict that it will far-surpass laser cooling. Our approach is based on magnetic deceleration of supersonic beams. Once the atoms are brought to rest in the lab frame, we will use internal-state optical pumping and magnetic field gradients to further cool and compress the atomic cloud. This talk will discuss progress toward the physical implementation of these methods with $^7$Li atoms.   

Neutral Atom Lithography Using a Pulsed Magnetic Lens

We present the status of a method of neutral atom lithography that achieves sub-10nm resolution. This method is based on the nanoscale imaging of a beam of metastable atoms with an aberration-corrected hexapole lens. The lens creates a magnetic field gradient that increases with the distance from the center of the lens so as to focus divergent low field seeking atoms toward a single focal spot past the lens. The scheme takes advantage of the narrow velocity distribution of a pulsed supersonic beam as well as an optical pumping and cooling scheme that selects the magnetic state of the atoms and further reduces its velocity dispersion. This method can be used not only to pattern but to spectroscopically probe surfaces with spatial resolution below 10nm.   

High resolution neutral atom microscope

We are developing a high resolution neutral atom microscope based on the technique of metastable impact electron emission (MIEES). When an incoming metastable noble gas atom approaches the surface of our sample, the noble gas atom falls to the ground state and an electron is emitted. The emitted electrons carry information regarding the density of states of the surface without any information from the underlying layers. Furthermore, using a chromatic aberration corrected magnetic hexapole lens we expect to image our atomic beam to a spot with a diameter less than 10nm. Our primary goal is to investigate how local phenomena can give rise to macroscopic effects in materials that cannot be probed using a scanning tunneling microscope.   

Microscopic theory of Bose-Einstein condensation in an interacting gas

We find, for the first time, a microscopic theory of Bose-Einstein condensation in an interacting gas, which is valid both inside and outside a critical region. The derived exact fundamental equations for the condensate wave function and the Green's functions allow one to describe critical fluctuations and formation of an ordered condensate phase from a disordered phase across the entire critical region continuously. These equations asymptotically turn into the usual Gross-Pitaevskii and Beliaev-Popov equations in a low-temperature limit outside the critical region. The theory is readily extendable to other phase transitions.   

Theoretical Analysis of Noise Induced Quantum Coherence

Quantum coherence has recently been studied in quantum heat engines such as lasers, solar cells, and photosynthetic complexes. Noise-Induced Coherence (NIC) can be spontaneously generated and does not require external laser sources. We investigate perform theoretical analysis of the effects of NIC under various conditions and the dependence on various parameters such as sample geometry and dynamics. Our work may lead to better understanding of photosynthesis and to development of more efficient solar cells.   

Formation of trains of attosecond pulses via ionization switching of the resonant interaction between XUV radiation and IR-field-dressed atoms

Investigating processes unfolding on the attosecond time scale is one of the key aims of modern physics. Attosecond light pulses with carrier frequency in the vicinity of atomic resonances present themselves the major tool for study of such processes in atoms and molecules. Recently, we presented an analytical model describing formation of attosecond pulses from quasi-resonant quasi-monochromatic vacuum-ultraviolet (VUV) radiation in an atomic gas simultaneously irradiated by a moderately strong infrared (IR) laser field. Subcycle time-dependence of ionization rate of excited states of atoms is used to form attosecond pulses from VUV field. In this contribution we present the results of numerical solution of time-dependent Schrodinger equation for IR-dressed He atoms interacting with quasi-resonant XUV radiation. Our results demonstrate the possibility to form trains of pulses with pulse duration in the range of hundreds of attoseconds in atomic helium dressed by IR field. The results are in a rather good agreement with the analytical solution. Required parameters of IR dressing laser are achievable experimentally.   

Photoionization and photocurrents at sub-field-cycle temporal scale

The Keldysh theory of photoionization in solids is generalized to the case of arbitrarily short driving pulses of arbitrary shape and polarization. We derive a closed-form solution for the nonadiabatic ionization rate and field-driven currents in the solid-state electron-hole plasma. Our results indicate important role of ultrafast photoionization dynamics within the field cycle and link the ultrafast photoionization dynamics with experimentaly accessible quantities.   

Beam shaping and production of vortex beams in coherent Raman generation

Broadband coherent Raman generation provides one promising pathway toward production of ultrashort pulses and time-shaped laser fields. We explore another dimension for light shaping, and add the possibility of transverse beam shaping. Experimental results from the generation of Raman sidebands using optical vortices are presented. In particular, a series of experiments on the helicity and topological charge in each sideband order will be discussed. We also propose a new, improved setup that will incorporate spatial light modulators, allowing us to change the order of each beam quickly and easily, thereby extending our study of optical vortices to higher order beams.   

Maximizing the response of a specific vibrational mode to a laser pulse

In previous publications~[1,2] our group predicted the following for molecules and materials responding to femtosecond-scale optical laser pulses: The maximum relative excitation of a Raman-active vibrational mode with period $T$ will be attained when the pulse has a full-width-at-half-maximum duration $\tau \approx 0.42 \, T$. The analytical model used for this prediction involved averaging over the oscillations of the field within the pulse, and the absolute (rather than relative) response is maximized as $\tau \rightarrow \, 0$. Here we generalize the model to include the oscillations of the field, and we find that the absolute maximum is shifted to a nonzero value of the duration which depends on the other parameters of the laser pulse, as well as the period of the vibrational mode. This result is obtained analytically (using Mathematica) and is confirmed by numerical calculations. \\[4pt] [1] Xiang Zhou, Zhibin Lin, Chenwei Jiang, Meng Gao, and Roland E. Allen, Physical Review B 82, 075433 (2010); arXiv:1001.1016 [cond-mat]. \newline [2] Chenwei Jiang, Xiang Zhou, Ruihua Xie, Fuli Li, and Roland E. Allen, Chemical Physics Letters 515, 137 (2011).   

Vibrational Spectra, Theoretical Calculations, and the Two-Dimensional Potential Energy Surface of 2-Cyclopenten-1-one Ethylene Ketal

The bicyclic spiro molecule 2-cyclopenten-1-one ethylene ketal (CEK) was studied by infrared and Raman spectroscopy. Density functional theory (DFT) calculations were utilized to compute the theoretical spectra and excellent agreement with the experimental spectra was observed. The structures and conformational energies for the two pairs of conformational minima, which can be defined in terms of ring-bending (x) and ring-twisting ($\tau$) vibrational coordinates, were also calculated. Utilizing the results from \textit{ab initio} MP2/cc-PVTZ computations, a two-dimensional potential energy surface (PES) was established. The energy levels and wavefunctions of the PES were then calculated and their characteristics were analyzed. At lower energies all of the quantum states are doubly degenerate and correspond to either the lower energy conformation L or to conformation H which is 154 cm$^{-1}$ higher in energy. At energies above the 264 cm$^{-1}$ barrier, the wavefunctions show that the quantum levels have significant probabilities for both conformations.