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3D all optical local cooling in a bichromatic standing wave

W. Alge, K. M. Gheri, K. Ellinger, H. Stecher, H. Ritsch

Published Apr 15, 1999• 0 citations
OpenAlex

Abstract

\n \nWe theoretically investigate an all optical scheme for three-dimensional \ntrapping and cooling of atoms in a single bichromatic standing \nwave with a Gaussian transverse intensity profile \nas, e.g. formed by two different longitudinal modes of a linear \noptical cavity. The atoms are cooled through an efficient Sisyphus \nmechanism and trapped at those anti-nodes (maxima) of the stronger \nfield where the second weaker field has a node. \nThis generates a large effective atom-field coupling as it is desired \nin nonlinear optical experiments with clouds of atoms. The scheme effects \nhigh local densities modulated at the beat frequency of the \ntwo involved modes. In the appropriate parameter regime the results \nfrom a three-dimensional semi-classical approach are confirmed by \na 1D full QMCWF-simulation. Extending our model to a \nmore realistic case, we include loss channels out of the system and repumping. \nFurthermore, we generalise our approach to Λ-type level schemes,\nwhich exhibit promising optical nonlinearities. Trapping and cooling \nof such atoms is predicted to be compatible with maintaining high \ncooperativities needed for large nonlinear effects.\n

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\n \nWe theoretically investigate an all optical scheme for three-dimensional \ntrapping and cooling of atoms in a single bichromatic standing \nwave with a Gaussian transverse intensity profile \nas, e.g. formed by two different longitudinal modes of a linear \noptical cavity. The atoms are cooled through an efficient Sisyphus \nmechanism and trapped at those anti-nodes (maxima) of the stronger \nfield where the second weaker field has a node. \nThis generates a large effective atom-field coupling as it is desired \nin nonlinear optical experiments with clouds of atoms. The scheme effects \nhigh local densities modulated at the beat frequency of the \ntwo involved modes. In the appropriate parameter regime the results \nfrom a three-dimensional semi-classical approach are confirmed by \na 1D full QMCWF-simulation. Extending our model to a \nmore realistic case, we include loss channels out of the system and repumping. \nFurthermore, we generalise our approach to Λ-type level schemes,\nwhich exhibit promising optical nonlinearities. Trapping and cooling \nof such atoms is predicted to be compatible with maintaining high \ncooperativities needed for large nonlinear effects.\n
We propose a scheme for three-dimensional trapping and cooling of atoms in a bichromatic standing wave formed by two different longitudinal modes of an optical cavity. The atoms are cooled through an efficient Sisyphus mechanism and trapped at those anti-nodes (maxima) of the stronger field where the second weaker field has a node, this leading to a large effective atom-field coupling. Contrary to the case of a single strong standing wave, the final width of the momentum distribution is considerably less than the depth of the optical potential implying long storage times. The scheme offers the possibility of capturing atoms out of the background and achieving high local densities modulated with the beat frequency of the two involved modes. The results from a three-dimensional semi-classical calculation are confirmed by a full QMCWF simulation as well as by a quantum rate equation model in the one-dimensional case in the proper limits.
We present a calculation of cooling of Rydberg atoms in bichromatic standing light waves. The atoms in our model are assumed to have a cascade three-level transition scheme, with the second excited level being the Rydberg state. The cooling fields, which also excite the atoms to the Rydberg state, consist of two standing waves whose frequencies are tuned near the two transition frequencies of the cascade atomic system. It is shown that the mechanisms of Sisyphus cooling and velocity-selective coherent population trapping can co-exist in such a configuration, and under certain conditions the majority of the laser-cooled atoms are in the Rydberg state.
We present a general theoretical description of laser cooling of three-level atoms in a Λ configuration in two standing waves. Detunings, intensity, and spatial phase difference of the two waves are arbitrary. We demonstrate that, for a zero spatial phase shift, sub-Doppler cooling requires equal detunings. The basic mechanism is then coherent population trapping. For different detunings sub-Doppler cooling occurs only when the spatial phases of the two standing waves are different. This effect can be traced back to a mechanism of the Sisyphus type. These schemes of sub-Doppler cooling require neither polarization gradients nor magnetic fields.
We propose and analyze theoretically a scheme for atomic guiding and cooling in a dark hollow laser beam generated from the collimated output beam of a LP01{\mathrm{LP}}_{01}LP01​ mode in a micrometer-sized hollow optical fiber. In the scheme, cold atoms from a magneto-optical trap are loaded into the blue-detuned dark hollow beam, move down in the dark hollow beam, and experience transverse Sisyphus cooling induced by the dark hollow beam and a weak repumping beam. In the longitudinal direction, the guided atoms experience heating from the gravity field and cooling from the upward-propagating dark hollow beam and repumping beam. We estimate the transverse two-dimensional equilibrium temperature, the final longitudinal mean velocity, and the total loss of the guided atoms. Our calculations show that a transverse equilibrium temperature of $\ensuremath{\sim}1.2\ensuremath{\mu}\mathrm{K},$ a final longitudinal mean velocity of 0--4.41 m/s, and a guiding efficiency of 65--95 % may be obtained.
We show that a three-level atom in the cascade configuration can be stably trapped and cooled in one dimension by an intense bichromatic standing wave. At the two-photon resonance, rectified dipole forces result in a deep potential well which can be used to localize the atoms in space. In the vicinity of the rectified potential minimum, the spatial dependence of the dressed state energies can lead to a velocity dependence of the force which produces damping of the atomic motion. Consideration of the heating effects of momentum diffusion indicates that cooling and stable trapping at low temperatures is possible in such a bichromatic field.
We propose a trap scheme for cold alkali-metal atoms with an efficient intensity-gradient induced Sisyphus cooling in a weak standing-wave hollow-beam gravito-optical trap, which is composed of the interference of two well collimated, counterpropagating doughnut hollow beams with an intensity difference and a plug beam. We calculate the intensity distribution of the weak standing-wave hollow-beam field and its intensity gradient one, and find that such an optical dipole trap with an extremely high intensity gradient is desirable to realize an efficient intensity-gradient cooling for alkali-metal atoms in the trap. We also calculate the optical potentials, instantaneous dipole forces and spontaneous emission rates for a three-level dressed atom and study the dynamic process of intensity-gradient cooling of 87Rb^{87}\mathrm{Rb}87Rb atoms in the weak standing-wave hollow-beam trap by Monte Carlo simulations. Our study shows that the minimum optical potential at each node in our dipole trap is high enough to trap almost all cold atoms with a temperature of $120\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ released from a standard magneto-optical trap, and an ultracold 87Rb^{87}\mathrm{Rb}87Rb atomic sample with a temperature of $\ensuremath{\sim}0.73\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ can be obtained in the trap. Starting from this stage, an all-optical Bose-Einstein condensation (BEC) could be realized by using the optical-potential evaporative cooling.