Session Z1: General Atomic, Molecular and Optical Physics

The Timing of Sonoluminescence

We measured the timing of the sonoluminescence flash by scattering laser light from the bubble. We performed this measurement on 17.8 kHz, 13.28 kHz and 7920 Hz systems and found that the flash typically occurs 100 nanoseconds before the minimum radius, contrary to previous claims that the flash always occurs within a nanosecond of the minimum radius. These results are important because they imply that previous \emph{hot} models of sonoluminescence are wrong. We propose a new model: that the flash results from the discharge of an excited cold condensate, formed during the adiabatic expansion of the bubble.   

Coherence Enhanced Transient Lasing

We study the effect of a coherent drive on transient lasing with inversion in three-level $\Lambda $ and $\Xi$ configurations ($c\leftrightarrow a\leftrightarrow b$). We show that the presence of a resonant coherent drive on the $a\leftrightarrow c$ optical transition can yield substantial enhancement of the output laser energy on a $a\rightarrow b$ XUV or X-ray transition. We demonstrate the crucial role of coherence $\varrho _{ac}$ for this laser power enhancement. Contrary to the forward direction, where forward gain can be enhanced for some choice of $\Omega_{c}$, coherent drive on the $ac$ transition always suppresses the backward gain. Thus, the use of a coherent drive at optical frequency could be a useful tool for increasing power of lasers in XUV and X-ray regions.   

Variable Atomic Radius of Hydrogen Due to Vibrating Nucleus

The H-atomic radius is variable because the H-nucleus is vibrating and the electric force field upon the electron is repeatedly changing due to the changing distance from the positive nucleus to the negatively charged electron. If the the distance from the nucleus to the electron is $d=r + Acos2\pi ft$ where $r=5.29x10^{-11}m$, the calculated Bohr radius, and $d=2.5x10^{-11}m$, the measured atomic radius of the H-atom, then the equation for the variable atomic radius of the H-atom is $5.29x10{-11}m + Acos2\pi ft= 2.5x10{-11}m$. If the RMS value for the average cosine is $0.707$, solving for A, the average amplitude of nuclear vibration, $A=3.95x10^{-11}m$. Therefore, the oscillating orbit of the electron in an H-atom has an average amplitude of $A=3.95x10^{-11}$.   

Strong Field Control of Atomic and Molecular Dynamics: An Attosecond Resolved Study

Strong laser fields are routinely used in attosecond pump-probe studies of atomic and molecular phenomena. However, even a moderately strong field ($\sim $ 10$^{12}$ W/cm$^{2})$ can significantly alter the electronic structure. By understanding and quantifying the effect of strong fields, one can obtain a high degree of control over the photo-absorption and photo-fragmentation processes. Here, we study the atomic and molecular response to the simultaneous presence of XUV attosecond pulse trains and strong IR fields. We describe an IR laser-dressed atom using Floquet picture. We observe quantum interference between XUV excitation paths to the Fourier components of a given Floquet state, which leads to oscillations in the ion-yield. By measuring the phase of the ion-yield oscillations, we extract the quantum phase difference between the Fourier components of that Floquet state. We obtain a quantitative understanding of how Floquet ionization channels change with intensity and what is the phase associated with each channel. We also extend our studies to molecular fragmentation processes. Our work represents real-time measurement and control of dynamics using strong-field modification of the atomic and molecular structure. This work was support by NSF grant PHY-0955274.   

Steady State Ab Initio Laser Theory: Generalizations

We show that Steady-state Ab initio Laser Theory (SALT)\footnote{L. Ge, Y. D. Chong, and A. D. Stone, Phys. Rev. A \textbf{82}, 063824 (2010).} can be generalized to find the stationary multimode lasing properties of gain systems with $N$ levels, and to include carrier diffusion. The former result is achieved by mapping the $N$-level rate equations to an effective two-level inversion equation of the type typically quoted in the Maxwell-Bloch equations.\footnote{A. Cerjan, Y. D. Chong, L. Ge, and A. D. Stone, Opt. Express \textit{in press}, arXiv: 1111.2279v1 [physics.optics]} The latter result is found by rewriting the SALT algorithm to non-linearly solve two coupled non-linear equations in the steady state, with one equation determining the modal field intensities, given the inversion and the other equation determining the inversion given the field intensities. In both cases we find excellent agreement with more computationally demanding Finite-Difference Time-Domain (FDTD) simulations for the steady state. These results generalize the SALT algorithm to handle more realistic lasing systems, including semiconductor lasers.   

Dynamical mean-field theory for transition metal dioxide molecules

The utility of the dynamical mean-field approximation in quantum chemistry is investigated in the context of transition metal dioxide molecules including $TiO_2$ and $CrO_2$. The choice of correlated orbitals and correlations to treat dynamically is discussed. The dynamical mean field solutions are compared to state of the art quantum chemical calculations. The dynamical mean-field method is found to capture about 50\% of the total correlation energy, and to produce very good results for the d-level occupancies and magnetic moments. We also present the excitation spectrum in these molecules which is inaccessible in many wave-function based methods. Conceptual and technical difficulties will be outlined and discussed.   

Disalignment of the Ne$^{\ast }$(2p$_{10}$ [J=1]) atoms induced by Helium atom collisions from 10K to 3000K

Quantum close-coupling many-channel calculations using the new model potential for the interaction between Ne$^{\ast }$(2p$_{i}$ [J=1]) and He atoms proposed in [1] are performed in order to analyze the depolarization of Ne$^{\ast }$(2p$_{i}$ [J=1]) atoms in a gaseous mixture at thermal equilibrium. For temperatures above 77 K we successfully explain measurements of disalignment done with a laser-induced fluorescence spectroscopy method and destruction of alignment using a technique based on the Hanle effect for all four 2p$_{i}$ [J=1] states of the 2p$^{5}$3p configuration of neon [1, 2]. Our interpretation of the experimental data is based on the anisotropy between collisional channels which asymptotically converge toward the same 2p$_{i}$ [J=1] state [2]. Below 77 K our disalignment rate coefficients for the Ne$^{\ast }$(2p$_{10}$ [J=1]) atoms are much larger than the experimental data [3] after the radiation re-absorption is subtracted from the disalignment rates. The calculations indicate that for the 2p$_{10}$ state, at low collision energies, the nuclear rotation has a strong influence in the overall long-range interaction, while the experimental data suggests that below 16 meV, the intramultiplet transitions within the (2p$_{i}$ [J=1]) state of neon are completely negligible. The discrepancy between theory and experiment is carefully analyzed. \\[4pt] [1] Bahrim C and Khadilkar V 2009 \textit{Phys Rev A} \textbf{79} 042715. \\[0pt] [2] Khadilkar V and Bahrim C 2010 \textit{J Phys B }\textbf{43}235209. \\[0pt] [3] Matsukuma H, Shikama T, and Hasuo M 2011 \textit{J Phys B }\textbf{44 }075206.   

Cold collisions of complex polyatomic molecules

We introduce a method for classical trajectory calculations to simulate collisions between atoms and large rigid asymmetric-top molecules. Using this method, we investigate the formation of molecule-helium complexes in buffer-gas cooling experiments at the temperature of 6.5 K for molecules as large as naphthalene. Our calculations show that the mean lifetime of the quasi-bound collision complex is not long enough for the formation of stable clusters under the experimental conditions. Our results suggest that it may be possible to improve the efficiency of the production of cold molecules in buffer-gas cooling experiments by increasing the density of helium. In addition, we find that the shape of molecules is important for the collision dynamics where molecular vibrational motions are frozen. For some molecules, it is even more crucial than the number of accessible degrees of freedom. This indicates that by selecting molecules with suitable shape for buffer-gas cooling, one could cool molecules with a very large number of degrees of freedom.   

Spin-boson models for periodic $N$-site systems

The single/multi-mode spin-boson model provides a description for numerous two-level exciton-phonon and atom-cavity systems. Existing many-level extensions conserve symmetries but quickly become intractable due to the inclusion of multiple interacting modes. Other ad-hoc single-mode extensions contain arbitrary numbers of parameters and often ignore the symmetries of their respective systems. This work presents a simple model for the interaction of a periodic system of $N$ coupled sites with one or more non-interacting boson modes using a minimal number of parameters [1]. A group theoretic approach allows one to partially diagonalize the Hamiltonian, providing numerical advantages, physical insight, and a gateway to accurate approximations. The single-mode two-site system reduces to the single-mode spin-boson model, also known as the Rabi Hamiltonian. Two higher dimensional generalizations are reviewed in the exciton-phonon/atom-field interpretations and related to a new integrability criterion [2]. The model predicts that $2N$-level systems have parity symmetry and that the ground state of certain four-level atom-cavity systems will undergo parity change at large coupling. \\[4pt] [1] V. V. Albert, arXiv:1112.0849 \\[0pt] [2] D. Braak, PRL \textbf{107}, 100401 (2011)   

Thermal Effects on Quantum Sticking

Many-body effects on the threshold law of quantum sticking of a particle coupled to an ohmic bosonic bath are examined for finite temperature surfaces. Generalizing a variational mean-field method\footnote{Y. Zhang and D.P. Clougherty, arXiv:1012.4405} previously applied to zero temperature surfaces, we obtain an explicit expression for the sticking probability of a particle with incident energy $E$. We find that there is a critical particle energy below which the probability of its sticking to the surface discontinuously drops to zero. We show that this singularity, whose origin is rooted by analogy to the localization transition in the spin-boson model, is experimentally accessible for ultracold particles. We provide detailed numerical results for this effect.   

Theory of Non-Markovian dynamics in resonance fluorescence spectra

Robust quantum coherence is an important prerequisite for any system that may be used to perform quantum information processing tasks. For systems that can be probed optically, the resonance fluorescence spectrum may provide indirect evidence of coherence times when supplemented with an appropriate model of the decay process. A common approach is to assume a Markovian system, resulting in exponential decay of correlation functions and Lorentzian features in the associated spectrum. For physical systems with strongly history-dependent (non-Markovian) dynamics, there is currently no satisfying systematic theoretical approach to establish the associated spectrum. We present a detailed theoretical method to obtain the resonance fluorescence spectrum for a general system undergoing non-Markovian dynamics. This procedure can be used to systematically account for features in resonance-fluorescence spectra due to genuine non-Markovian dynamics. Our approach is based on a Nakajima-Zwanzig generalised master equation for the dynamics of the reduced density matrix. We apply this theory to study the resonance fluorescence in the non-Markovian dynamics of a three level lambda system, relevant to recent experiments on heavy-hole spin dynamics in a quantum dot.   

Quantum Revivals of the Morse Oscillator in Position Space and Momentum Space

Analytical solutions for the Morse oscillator are applied to investigate the quantum revivals both in position and momentum spaces. The properties of this anharmonic oscillator came across interesting space-time phenomena. These findings include simple Farey arithmetic revival structures. Such dynamic systems may have applications for quantum information technology and quantum computing.   

Trapping of particles in the ray optics regime using DNG materials

Optical tweezers use to confine and manipulate microscopic objects including living cells and bacteria, with high accuracy. The objective is calibrating the force on targets using DPS-DNG layered structure. Using this layered structure which acts as a tunable optical band-pass filter would assist calibration of the force on the target(s). Here shown that the proposed DNG-DPS structure would help to have highly focused calibrated tweezers without worrying about the polarization of optical wave. Calculation can describe well the experimental results.