Isotopic difference in trap loss collisions of laser cooled rubidium atoms

Experimental Investigation of the Influence of the Laser Beam Waist on Cold Atom Guiding Efficiency

The primary purpose of this study is to investigate the influence of the vertical guiding laser beam waist on cold atom guiding efficiency. In this study, a double magneto-optical trap (MOT) apparatus is used. With an unbalanced force in the horizontal direction, a cold atomic beam is generated by the first MOT. The cold atoms enter the second chamber and are then re-trapped and cooled by the second MOT. By releasing a second atom cloud, the process of transferring the cold atoms from MOT to the dipole trap, which is formed by a red-detuned converged 1064-nm laser, is experimentally demonstrated. And after releasing for 20 ms, the atom cloud is guided to a distance of approximately 3 mm. As indicated by the results, the guiding efficiency depends strongly on the laser beam waist; the efficiency reaches a maximum when the waist radius (w₀) of the laser is in the range of 15 to 25 μm, while the initial atom cloud has a radius of 133 μm. Additionally, the properties of the atoms inside the dipole potential trap, such as the distribution profile and lifetime, are deduced from the fluorescence images.

Giant and tunable optical torque for micro-motors by increased force arm and resonantly enhanced force

Micro-motors driven by light field have attracted much attentions for their potential applications. In order to drive the rotation of a micro-motor, structured optical beams with orbital angular momentum, spin angular momentum, anisotropic medium, and/or inhomogeneous intensity distribution should be used. Even though, it is still challenge to increase the optical torques (OT) in a flexible and controllable way in case of moderate incident power. In this paper, a new scheme achieving giant optical torque is proposed by increasing both the force arm and the force amplitude with the assistance of a ring resonator. In this case, the optical torque doesn’t act on the target directly by the incident beam, but is transmitted to it by rotating the ring resonator connected with it. Using the finite-difference in time-domain method, we calculate the optical torque and find that both the direction and the amplitude of the torque can be tuned flexibly by modifying the frequency, or the relative phases of the sources. More importantly, the optical torque obtained here by linearly polarized beams can be 3 orders larger than those obtained using the structured beams. This opt-mechanical-resonator based optical torque engineering system may find potential applications in optical driven micro-machines.

Phase-space properties of magneto-optical traps utilising micro-fabricated gratings

We have used diffraction gratings to simplify the fabrication, and dramatically increase the atomic collection efficiency, of magneto-optical traps using micro-fabricated optics. The atom number enhancement was mainly due to the increased beam capture volume, afforded by the large area (4cm(2)) shallow etch (~ 200nm) binary grating chips. Here we provide a detailed theoretical and experimental investigation of the on-chip magneto-optical trap temperature and density in four different chip geometries using (87)Rb, whilst studying effects due to MOT radiation pressure imbalance. With optimal initial MOTs on two of the chips we obtain both large atom number (2×10(7)) and sub-Doppler temperatures (50 μK) after optical molasses.

High efficient loading of two atoms into a microscopic optical trap by dynamically reshaping the trap with a spatial light modulator

We demonstrated trapping two neutral (87)Rb atoms in a two site optical ring lattice generated by reflecting a single laser beam from a computer controlled spatial light modulator directly. The ring lattice was transformed into a Gaussian trap by dynamically displaying the holograms animation movie on the modulator. The trapped atoms follow the evolution of traps and move into the same microscopic dipole trap at the end. The detected success rate of this manipulation is larger than 90%. Under imposing the near resonance light, we observed strong light-induce collision between two atoms.

Alternative to the hyperfine-change-collision interpretation for the behavior of magneto-optical-trap losses at low light intensity

Usually, the large trap loss rates observed in MOTs at the low light intensity regime have been associated with hyperfine change collisions (HCC). We propose an alternative mechanism to explain the sudden raise up of trap loss rates at low intensity without relying on HCC. Using the Gallagher-Pritchard model [Phys. Rev. Lett. 63, 957 (1989)] together with an intensity dependent escape velocity, we were able to reproduce qualitatively well some existing experimental results, including recent observations by Nesnidal et al. [Phys. Rev. A 62, 030701(R) (2000)]. This result reopens the discussion in order to better understand the physical mechanisms and their actual contribution to the trap losses.

Cold rubidium molecules formed in a magneto-optical trap

We report the first observation of translationally cold ( approximately 90 &mgr;K) Rb2 molecules. They are produced in a magneto-optical trap in their triplet ground state. The detection is performed by selective mass spectroscopy after two-photon ionization into Rb+2, resonantly enhanced through the intermediate a (3)Sigma(+)(u)-->2 (3)Pi(g) molecular band. The two rubidium isotopes present very different types of behavior that are interpreted in terms of their respective collisional properties.

Magneto-optic trap of Rb atoms with an injection-seeded laser that operates at two frequencies

We captured Rb atoms in a magneto-optic trap by using two frequencies in an injection-seeding process of a diode laser without the use of a separate repumping laser. In the incomplete injection-seeding condition, the power of the free-running slave laser at the repumping frequency can be partially converted into power at the master laser frequency, which is stabilized at the cooling frequency. We measured conversion efficiencies and frequency shifts based on the injecting power and the preset frequency difference between the master laser and the slave laser.

Simultaneous cooling and trapping of (85)Rb and (87)Rb in a magneto-optical trap

Two different isotopes-(85)Rb and (87)Rb-have been captured simultaneously in one magneto-optical trap. The trap parameters could be chosen nearly independently for the two isotopes. The trap loss rates that are due to collisions between atoms of different isotopes have been measured in the low-intensity limit.

Use of trapped atoms to measure absolute photoionization cross sections

We describe a new technique for accurate measurement of absolute photoionization cross sections. By measuring the loss rateof atoms from a laser trap in the presence of ionizing light, we directly measure the photoionization rate. The only quantities requiring absolute calibration are the ionizing laser intensity and the fractional population in the relevant state. Our technique is capable of detecting extremely small ionization rates, which means that low-power cw sources can be used. We have applied this method to photoionization from the 5P(3/2) state of rubidium at wavelengths of 413 and 407 nm.The cross sections are 1.36(12) x 10(-17) and 1.25(11) x 10(-17) cm(2) , respectively.