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Cambridge University Press eBooks

Optical shielding and suppression

J. Weiner

Published Dec 4, 2003• 1 citation
Publisher

Abstract

Introduction Photoassociation uses optical fields to produce bound molecules from free atoms. Optical fields can also prevent atoms from closely approaching, thereby shielding them from shortrange inelastic or reactive interactions and suppressing the rates of these processes. Recently several groups have demonstrated shielding and suppression by shining an optical field on a cold atom sample. Figure 6.1(a) shows how a simple semiclassical picture can be used to interpret the shielding effect as the rerouting of a ground-state entrance channel scattering flux to an excited repulsive curve at an internuclear distance localized around a Condon point. An optical field, blue detuned with respect to the asymptotic atomic transition, resonantly couples the ground and excited states. In the cold and ultracold regime particles approach on the ground state with very little kinetic energy. Excitation to the repulsive state effectively halts their approach in the immediate vicinity of the Condon point, and the scattering flux then exits either on the repulsive excited state or on the ground state. Figure 6.1(b) shows how this picture can be represented as a Landau–Zener (LZ) avoided crossing of field-dressed potentials. As the blue-detuned suppressor laser intensity increases, the avoided crossing gap around the Condon point widens, and the semiclassical particle moves through the optical coupling region adiabatically. The flux effectively enters and exits on the ground state, and the collision becomes elastic.

DOI
10.1017/cbo9780511535215.007

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Introduction Photoassociation uses optical fields to produce bound molecules from free atoms. Optical fields can also prevent atoms from closely approaching, thereby shielding them from shortrange inelastic or reactive interactions and suppressing the rates of these processes. Recently several groups have demonstrated shielding and suppression by shining an optical field on a cold atom sample. Figure 6.1(a) shows how a simple semiclassical picture can be used to interpret the shielding effect as the rerouting of a ground-state entrance channel scattering flux to an excited repulsive curve at an internuclear distance localized around a Condon point. An optical field, blue detuned with respect to the asymptotic atomic transition, resonantly couples the ground and excited states. In the cold and ultracold regime particles approach on the ground state with very little kinetic energy. Excitation to the repulsive state effectively halts their approach in the immediate vicinity of the Condon point, and the scattering flux then exits either on the repulsive excited state or on the ground state. Figure 6.1(b) shows how this picture can be represented as a Landau–Zener (LZ) avoided crossing of field-dressed potentials. As the blue-detuned suppressor laser intensity increases, the avoided crossing gap around the Condon point widens, and the semiclassical particle moves through the optical coupling region adiabatically. The flux effectively enters and exits on the ground state, and the collision becomes elastic.
Research focusing on the formation of ultracold atomic and molecular quantum gases is a continuously expanding field due to its envisioned applications such as quantum-controlled chemistry or quantum simulation. The aim of our theoretical work is to find ways to suppress inelastic or reactive processes between colliding particles in ultracold quantum gases. Using a laser field detuned to the blue of a relevant transition, we propose to couple the initial colliding particle state to a repulsive excited one, thus preventing the particles to come close to each other. This "blueshielding" technique is applied for ultracold quantum gases, close to quantum degeneracy. We investigate the optical shielding in ultracold39K-Cs collision by using close-coupling channel method. As the (3)3Σ+and (2)3Π+molecular states are repulsive in long range[1], one verified effective optical shielding can be realized via driving the transition from the ground α3Σ+state to these excited states in the presence of linear-polarized light. We demonstrate that a strong field with a small blue detuning to the atomic transition is to the benefit of shielding since the Condon point is farther away from the inner region of ground triplet state. The spontaneous emission effect is taken into account by considering excited optical potentials with the modified formula of shielding rate [2]. We expect the shielding efficiency could be improved by using circular-polarized light [3].
Inelastic collision rates between two very cold ground-state atoms can be strongly decreased by using an optical shielding laser to repel the colliding atoms at very large internuclear separation so that they never get close to each other. This can be achieved by using a sufficiently intense laser to excite a repulsive state of the quasimolecule formed by the two colliding atoms. We study a simple two-state model and compare the predictions of fully quantum-mechanical close coupling and time-dependent wave packet methods with semiclassical approximations based on the concept of a Landau-Zener curve crossing of field-dressed states. The effect of spontaneous emission in the excited state is treated by the Monte Carlo wave-function method. Calculations are given for a model sodium system at 250 \ensuremath{\mu}K. Strong optical shielding, without significant heating due to the quasimolecular excitation, can be achieved at relatively modest laser power. We find that Landau-Zener models give near quantitative agreement with the fully quantum calculations in spite of the extremely low temperature.
Introduction If, while approaching on an unbound ground-state potential, two atoms absorb a photon and couple to an excited bound molecular state, they are said to undergo photoassociation. Figure 5.1 illustrates the process. At long range electrostatic dispersion forces give rise to the ground-state molecular potential varying as C 6 / R 6 . If the two atoms are homonuclear, then a resonant dipole–dipole interaction sets up ± C 3 / R 3 excited-state repulsive and attractive potentials. Figure 5.2 shows the actual long-range excited potential curves for the sodium dimer, originating from the 2 S ½ + 2 P 3/2 and 2 S ½ + 2 P ½ separated atom states. For cold and ultracold photoassociation processes the long-range attractive potentials play the key role; the repulsive potentials figure importantly in optical shielding and suppression, the subject of Chapter 6. In the presence of a photon with frequency ω p the colliding pair with kinetic energy k B T couples from the ground-state to the attractive molecular state in a free–bound transition near the Condon point R C , the point at which the difference potential just matches ћω p . Scanning the probe laser ω p excites population of vibration–rotation states in the excited bound potential and generates a free–bound spectrum. This general class of measurements is called photoassociative spectroscopy (PAS) and can be observed in several different ways. The observation may consist of bound-state decay by spontaneous emission, most probably as the nuclei move slowly around the outer turning point, to some distribution of continuum states on the ground potential controlled by bound–free nuclear Franck–Condon overlap factors.
Two recent developments in ultracold atomic collision physics are discussed. The first is the “optical shielding” of ultracold atomic collisions by optical radiation that is tuned to the blue of an atomic resonance. This radiation couples a pair of colliding atoms to repulsive excited states and prevents their close approach. This effect may substantially suppress inelastic collision rates, and is an interesting example of the control of a cold collision process by a weak perturbing field. The second topic is ultracold atom photoassociation spectroscopy. In this work, high resolution free‐bound absorption spectra are obtained which provide detailed information on the long‐range states of the two interacting atoms. The physics of these states has an appealing simplicity, and in addition is of importance for a complete understanding of the effects of atomic interactions in cold atom experiments. The photoassociation spectra also provide a unique probe of cold collisions.
We propose a method to suppress the chemical reactions between ultracold bosonic ground-state ^{23}Na^{87}Rb molecules based on optical shielding. By applying a laser with a frequency blue-detuned from the transition between the lowest rovibrational level of the electronic ground state X^{1}Σ^{+}(v_{X}=0,j_{X}=0), and the long-lived excited level b^{3}Π_{0}(v_{b}=0,j_{b}=1), the long-range dipole-dipole interaction between the colliding molecules can be engineered, leading to a dramatic suppression of reactive and photoinduced inelastic collisions, for both linear and circular laser polarizations. We demonstrate that the spontaneous emission from b^{3}Π_{0}(v_{b}=0,j_{b}=1) does not deteriorate the shielding process. This opens the possibility for a strong increase of the lifetime of cold molecule traps and for an efficient evaporative cooling. We also anticipate that the proposed mechanism is valid for alkali-metal diatomics with sufficiently large dipole-dipole interactions.
We have developed a full three-dimensional quantum scattering approach to optical suppression of ultracold-collision rates. These calculations are carried out assuming colliding atoms without fine or hyperfine structure, which have a $^{1}\ensuremath{\rightarrow}^{1}$P transition. The three-dimensional model predicts that the optical suppression of ultracold-collision rates saturates with light intensity much more slowly than predicted by two-level curve-crossing models. Circularly polarized light is significantly more effective for optical suppression, and causes less increase in atomic kinetic energy due to excited-state production than linearly polarized light. The suppressor optical field can also cause orders of magnitude increases in ground-state elastic-scattering rates.