Trapping Tiny Laser beams

Bertram M. Schwarzschild

The first color photographs ever to grace the pages of Physical Review Letters show us the fluorescent glow from some 500 sodium atoms caught in the first successfully demonstrated optical trap for neutral atoms. The 21 July Letter by Steven Chu, John Bjorkholm, Arthur Ashkin and Alex Cable at Bell Labs, reports that this tiny “optical dipole” trap, formed at the focus of a single laser beam, was able to hold onto an ultracold, high-density accumulation of atoms for several seconds and manipulate them in space. In recent days the group has been able to hold a much larger collection of sodium atoms for a fraction of a minute with a newer optical-trap design.

Barbara Goss Levi

Researchers have been pushing atoms around with laser beams for many years, cooling them to record low temperatures and capturing them in confined spaces. Most such efforts require many photons. The field of a single photon is not generally strong enough to corral an atom—unless, that is, the light field is enhanced by confinement within a tiny cavity and the atom enters at a crawl. Then the coupling energy of the atom to the field can exceed the atom's kinetic energy, and the atom can become ensnared. A group from Caltech and the University of Auckland, and also a group at the Max Planck Institute for Quantum Optics in Garching, Germany, recently created the required conditions by combining cavity quantum electrodynamics with laser trapping and cooling; both groups succeeded in confining single atoms within optical cavities with an average of one photon in the cavity.

Deshui Yu, Frank Vollmer

Microlasers, relying on the strong coupling between active particles and optical microcavity, exhibit fundamental differences from conventional lasers, such as multi-threshold/thresholdless behavior and nonclassical photon emission. As light sources, microlasers possess extensive applications in precision measurement, quantum information processing, and biochemical sensing. Here we propose a whispering-gallery-mode microlaser scheme, where ultracold alkaline-earth metal atoms, i.e., gain medium, are tightly confined in a two-color evanescent lattice that is in the ring shape and formed around a microsphere. To suppress the influence of the lattice-induced ac Stark shift on the moderately-narrow-linewidth laser transition, the red-detuned trapping beams operate at a magic wavelength while the wavelength of the blue-detuned trapping beam is set close to the other magic wavelength. The tiny mode volume and high quality factor of the microsphere ensure the strong atom-microcavity coupling in the bad-cavity regime. As a result, both saturation photon and critical atom numbers, which characterize the laser performance, are substantially reduced below unity. We explore the lasing action of the coupled system by using the Monte Carlo approach. Our scheme may be potentially generalized to the microlasers based on the forbidden clock transitions, holding the prospect for microscale active optical clocks in precision measurement and frequency metrology.

Harold J. Metcalf, P. van der Straten