Doppler-free spectroscopy of metastable Sr atoms using a hollow cathode lamp.

We report on the demonstration of Doppler-free spectroscopy of metastable Sr atoms using a hollow cathode lamp (HCL). We employed a custom Sr HCL, which is filled with a mixture of 0.5 Torr Ne and 0.5 Torr Xe as a buffer gas to suppress velocity changing collisions and increase the populations in all of the (5s5p)3PJ(J=0,1,2) metastable states. We performed frequency modulation spectroscopy for the (5s5p)3P0-(5s6s)3S1, (5s5p)3P1-(5s6s)3S1, (5s5p)3P2-(5s5d)3D2, and (5s5p)3P2-(5s5d)3D3 transitions with sufficient signal-to-noise ratios for laser frequency stabilization. We also observed the hyperfine transitions of (5s5p)3P2-(5s5d)3D3 of Sr87. This method would greatly facilitate laser cooling of Sr.

A simplified 461-nm laser system using blue laser diodes and a hollow cathode lamp for laser cooling of Sr.

We develop a simplified light source at 461 nm for laser cooling of Sr without frequency-doubling crystals but with blue laser diodes. An anti-reflection coated blue laser diode in an external cavity (Littrow) configuration provides an output power of 40 mW at 461 nm. Another blue laser diode is used to amplify the laser power up to 110 mW by injection locking. For frequency stabilization, we demonstrate modulation-free polarization spectroscopy of Sr in a hollow cathode lamp. The simplification of the laser system achieved in this work is of great importance for the construction of transportable optical lattice clocks.

Buffer-gas-assisted polarization spectroscopy of 6Li.

We report on the demonstration of Doppler-free polarization spectroscopy of the D2 line of (6)Li atoms. Counterintuitively, the presence of an Ar buffer gas, in a certain pressure range, causes a drastic enhancement of the polarization rotation signal. The observed dependence of the signal amplitude on the Ar buffer pressure and the pump laser power is reproduced by calculations based on simple rate equations. We performed stable laser frequency locking using a dispersion signal obtained by polarization spectroscopy for laser cooling of (6)Li atoms.

Precise intensity correlation measurement for atomic resonance fluorescence from optical molasses.

We measured the intensity correlation of true thermal light scattered from cold atoms in an optical molasses. Using a single-mode fiber as a transverse mode filter, measurement with maximally high spatial coherence was realized, allowing us to observe ideal photon bunching with unprecedented precision. The measured intensity correlation functions showed a definite bimodal structure with fast damped oscillation from the maximum value of 2.02(3) and slow monotonic decay toward unity. The oscillation can be understood as an interference between elastic and inelastic scattering fields in resonance fluorescence.

Design of a high-Q air-slot cavity based on a width-modulated line-defect in a photonic crystal slab.

We have presented a novel design of a photonic crystal slab (PCS) nanocavity, in which the electric field of the cavity mode is strongly localized in free space. The feature of the cavity is a linear air slot introduced to the center of the mode-gap confined PCS cavity. Owing to the discontinuity of the dielectric constant, the electric field of the cavity mode is strongly enhanced inside the slot, allowing strong matter-field coupling and large interaction volume in free space. Using finite-difference time-domain method, we calculate the properties of the cavity mode as a function of the slot width. The calculated quality factor is still as high as 2 x 10(5) and the mode volume is as small as 0.14 of a cubic wavelength in a vacuum, even if 200-nm-wide slot is introduced to the PCS.

Holographic storage of multiple coherence gratings in a Bose-Einstein condensate.

We demonstrate superradiant conversion between a two-mode collective atomic state and a single-mode light field in an elongated cloud of Bose-condensed atoms. Two off-resonant write beams induce superradiant Raman scattering, producing two independent coherence gratings with different wave vectors in the cloud. By applying phase-matched read beams after a controllable delay, the gratings can be selectively converted into the light field also in a superradiant way. Because of the large optical density and the small velocity width of the condensate, a high conversion efficiency of >70% and a long storage time of >120 micros were achieved.

Superradiant light scattering from thermal atomic vapors.

Superradiant light scattering from noncondensed, thermal atomic vapors was experimentally studied. We found that superradiant gain is independent of quantum degeneracy and determined only by the shape of the atomic cloud and a contained number of atoms. Superradiant pump-probe spectroscopy was also developed to measure the atomic correlation function, revealing the Doppler-width-limited coherence time of the thermal gas and sudden buildup of long-lived coherence below the transition temperature.

Frequency stabilization of a laser diode with use of light-induced birefringence in an atomic vapor.

We present a simple modulation-free technique to stabilize a laser frequency to the Doppler-free spectra of an atomic vapor. Polarization spectroscopy with use of a balanced polarimeter allows us to obtain a background-free dispersion signal suitable for high-speed and robust frequency stabilization. We employed the method to the 5S(1/2) F = 2 --> 5P(3/2) F' = 3 transition of 87Rb atoms. The achieved feedback bandwidth was approximately 100 kHz, and an efficient suppression of the frequency noise in a laboratory environment was attained.

The onset of matter-wave amplification in a superradiant Bose-Einstein condensate.

The interaction of short and strong laser pulses with an atomic Bose-Einstein condensate is found to generate patterns of recoiling atoms that are different from those seen in previous light-scattering experiments. This phenomenon can only be explained by optical stimulation, showing that the previous description of superradiance as atomic stimulation is incomplete and that matter-wave amplification in Bose-Einstein condensates is suppressed at short times. Our experiments clarify the nature of bosonic stimulation in the four-wave mixing of light and atoms.