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Laser cooling and trapping using Laguerre-Gaussian laser beams

Sharon A. Kennedy

Published Jan 1, 2003• 0 citations
OpenAlex

Abstract

Laser cooling and trapping has led to a revolution in Atomic and Molecular physics. Optical traps created with blue-detuned laser beams are desired for numerous studies. One method of creating such a trap is with Laguerre-Gaussian laser beams. Laguerre-Gaussian (LG) laser beams are interesting because of their orbital angular momentum and radial intensity modes. Previous methods of creating higher-order LG beams did not yield pure LG modes. In collaboration with Digital Optics Corporation diffraction optics have been manufactured that yield higher-order LG laser beams with purities consistent with 100% of the intensity in the desire mode. We then create a donut-mode LG beam with this method to confine atoms from a MOT in both a blue-detuned and a red-detuned optical dipole force trap.

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Creation of Laguerre-Gaussian laser modes using diffractive optics

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Diffractive optics are used to create low- and high-order Laguerre-Gaussian (LG) beams from the output of a diode laser. We examine the mode purities, conversion efficiencies, extinction ratios, and propagation characteristics. We present detailed analyses of the beam profiles for one low-order $({\mathrm{LG}}_{0}^{1})$ and one high-order $({\mathrm{LG}}_{1}^{2})$ Laguerre-Gaussian mode. Modeling the ${\mathrm{LG}}_{1}^{2}$ beam as a superposition of LG modes, we find $(99.3\ifmmode\pm\else\textpm\fi{}0.9%)$ of the laser intensity in the ${\mathrm{LG}}_{1}^{2}$ mode, a mode purity greater than for LG beams created by other methods external to the laser cavity. The high mode purity may be useful in making atom traps for precision measurements and for Bose-Einstein condensation.

Physical Review A•Oct 4, 2002•104 citations

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Tightly confined atoms in optical dipole traps

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We are exploring the possibilities offered by Spatial Light Modulators to create dipole potentials with arbitrary shape in order to apply to cold atoms. As a first experiment we have realized a guide for atoms by shaping the laser beam into a Laguerre-Gaussian mode, creating a sort of tube, empty of light, where the atoms rest confined during their flight in the gravity field. We are, now, creating new potentials by modifying the Laguerre-Gaussian hologram in order to create new original traps for atoms.

Journal of Physics Conference Series•Nov 1, 2009•0 citations
Laser cooling and trapping has led to a revolution in Atomic and Molecular physics. Optical traps created with blue-detuned laser beams are desired for numerous studies. One method of creating such a trap is with Laguerre-Gaussian laser beams. Laguerre-Gaussian (LG) laser beams are interesting because of their orbital angular momentum and radial intensity modes. Previous methods of creating higher-order LG beams did not yield pure LG modes. In collaboration with Digital Optics Corporation diffraction optics have been manufactured that yield higher-order LG laser beams with purities consistent with 100% of the intensity in the desire mode. We then create a donut-mode LG beam with this method to confine atoms from a MOT in both a blue-detuned and a red-detuned optical dipole force trap.
Diffractive optics are used to create low- and high-order Laguerre-Gaussian (LG) beams from the output of a diode laser. We examine the mode purities, conversion efficiencies, extinction ratios, and propagation characteristics. We present detailed analyses of the beam profiles for one low-order (LG01)({\mathrm{LG}}_{0}^{1})(LG01​) and one high-order (LG12)({\mathrm{LG}}_{1}^{2})(LG12​) Laguerre-Gaussian mode. Modeling the LG12{\mathrm{LG}}_{1}^{2}LG12​ beam as a superposition of LG modes, we find $(99.3\ifmmode\pm\else\textpm\fi{}0.9%)$ of the laser intensity in the LG12{\mathrm{LG}}_{1}^{2}LG12​ mode, a mode purity greater than for LG beams created by other methods external to the laser cavity. The high mode purity may be useful in making atom traps for precision measurements and for Bose-Einstein condensation.
Summary form only given. Laguerre-Gaussian (LG) beams of non-zero order (also called doughnut or vortex beams), with their unusual phase and intensity spatial variation, have lately attracted interest in atom trapping and cooling research. These modes have a cylindrical geometry with respect to the direction of propagation and an intensity variation described by I(r)/spl prop/r/sup 21/e/sup -2r2/, where r is the radial distance from the axis of propagation measured in units of the beam waist and l is the order of the mode.
This thesis reports on the design, setup, and operation of a new atom trap apparatus, which is developed to confine few rubidium atoms in ultrahigh vacuum by means of taylored and dynamically variable light potentials (dipole traps), thus making the atoms available for spectroscopy and controlled manipulations. To maintain low background pressure, atoms of a vapour cell are transferred into a cold atomic beam by laser cooling techniques, and accumulated by a magneto-optic trap (MOT) in a separate part of the vacuum system. The laser cooled atoms are then transferred into dipole traps made of focused far-off-resonant laser fields in single or crossed-beam geometry, which are superimposed with the center of the MOT. Gaussian as well as hollow Laguerre-Gaussian (LG01) beam profiles are used with red-detuned and blue-detuned light. Microfabricated dielectric phase objects allow the efficient and robust mode conversion of Gaussian into Laguerre-Gaussian laser beams. Trap geometries can easily be changed due to the highly flexible experimental setup. The dipole trap laser beams have powers up to several hundred milliwatts and are focused to around 10 m. At a detuning of several tens of nanometers from the atomic resonance, the resulting traps are a few millikelvin deep, and the trapped atoms exhibit motional frequencies near 30 kHz, trap light scattering rates of a few hundred photons per second per atom, and storage times of several seconds. The number of dipole-trapped atoms ranges from more than ten thousand to below ten. The dipole-trapped atoms are detected either by a photon counting system with efficient straylight discrimination, or by recapture into the MOT, which is imaged onto a sensitive photodiode and a CCD-camera. Due to the strong AC-Stark shift imposed by the high-intensity trapping light, energy-selective resonant excitation and detection of the stored atoms is possible. The measured energy distribution is consistent with a harmonic potential shape and allows for the determination of temperatures and heating rates. From initial measurements, the thermal energy is found to be about 10 % of the trap depth. Using a crossed-beam geometry with red-detuned laser light, efficient transfer of atoms between the beams is observed. Optimum transfer occurs when the two beams cross at a radial offset, which can qualitatively be understood when the particle energy and geometrical properties of the two-beam trapping potential are considered. Numerical simulations reproduce the general features of the measured transfer efficiency vs. radial beam offset. This experiment is a first step towards dynamically variable potentials. Atoms have been radially confined in a blue-detuned hollow beam, and their decay time has been determined. This configuration is currently extended to a three-dimensionally confining blue-detuned dipole trap. For advanced laser cooling, state manipulation and spectroscopy of trapped atoms, a system of two diode lasers has been set up which are phase-coherently locked with a difference frequency near 6.834 GHz, to drive Raman transitions between the hyperfine-split ground states of 87Rb atoms. Resonances of such a transition have been recorded in a buffer gas loaded rubidium vapour cell with linewidths below 100 Hz.
We have constructed a novel optical trap for neutral atoms by using a Laguerre-Gaussian (doughnut) beam whose frequency is blue detuned to the atomic transition. Laser-cooled rubidium atoms are trapped in the dark core of the doughnut beam with the help of two additional laser beams which limit the atomic motion along the optical axis. About 108{10}^{8}108 atoms are initially loaded into the trap, and the lifetime is 150 ms. Because the atoms are confined at a point in a weak radiation field in the absence of any external field, ideal circumstances are provided for precision measurements. The trap opens the way to a simple technique for atom manipulation, including Bose-Einstein condensation of gaseous atoms.
We are exploring the possibilities offered by Spatial Light Modulators to create dipole potentials with arbitrary shape in order to apply to cold atoms. As a first experiment we have realized a guide for atoms by shaping the laser beam into a Laguerre-Gaussian mode, creating a sort of tube, empty of light, where the atoms rest confined during their flight in the gravity field. We are, now, creating new potentials by modifying the Laguerre-Gaussian hologram in order to create new original traps for atoms.