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.

Measurements of the quality factor Q ഠ 8 3 10 9 are reported for the whispering-gallery modes (WGM's) of quartz microspheres for the wavelengths 670, 780, and 850 nm; these results correspond to finesse F ഠ 2.2 3 10 6 . The observed independence of Q from wavelength indicates that losses for the WGM's are dominated by a mechanism other than bulk absorption in fused silica in the near infrared. Data obtained by atomic force microscopy combined with a simple model for surface scattering suggest that Q can be limited by residual surface inhomogeneities. Absorption by absorbed water can also explain why the material limit is not reached at longer wavelengths in the near infrared. © 1998 Optical Society of America OCIS codes: 270.0270, 230.5750, 160.6030. Small optical resonators with large quality factors Q have diverse technological and scientif ic applications, ranging from frequency stabilization of semiconductor diode lasers 1 to environmental sensing by trace absorption detection in compact, integrated structures. 2Our own interest in such resonators arises within the realm of cavity QED, in which it is necessary to achieve both small-cavity mode volumes (for large electric f ields per photon) and ultralow resonator losses (for long photon-storage times). 3In the optical domain these conditions have been met only in small Fabry-Perot cavities, most recently for single atoms coupled one by one to a cavity of mode volume V ഠ 2 3 10 5 mm 3 and finesse F ഠ 2 3 10 5 . 4An attractive alternative for achieving these conditions is the whispering-gallery modes (WGM's) of quartz microspheres, as was pointed out in the pioneering research of Ref. 5. These modes can have both large coupling coefficients for the atomcavity interaction (mode volume V ഠ 5 3 10 3 mm 3 ; Refs. 6 -8) and extremely low losses. 6,9,10 Moreover, measurements in Ref. 11 demonstrate that experimental Q factors can reach the intrinsic absorption limit of fused silica [at 633 nm, Q ͑8 6 1͒ 3 10 9 was reported]. For use in cavity QED and elsewhere, the microsphere WGM's are thus worthy of immediate further investigation, especially with the possibility of even higher Q factors in the near infrared (NIR), as the bulksilica absorption coefficient decreases more than fivefold as the wavelength moves from 600 nm to 1 mm.In this Letter we report what we believe to be the highest Q's for WGM's that have been achieved to date for wavelengths in the NIR. In addition, a measurement of Q 4.9 3 10 9 at 670 nm in a 345-mm-diameter sphere corresponds to what is to our knowledge the highest f inesse ͑F 2.3 3 106 ͒ yet recorded for an optical resonator. Furthermore, our data reveal that Q is largely independent of wavelength, so that the bulk absorption limit demonstrated in Ref. 11 is not tracked into the NIR, and the correspondingly higher Q values are not obtained. A systematic investigation of the surface of the microsphere by atomic force microscopy (AFM) allows the identif ication of residual surface inhomogeneities of size s Ӎ 1.7 nm, and a simple model of ...

A systematic shift of the photon recoil momentum due to the index of refraction of a dilute gas of atoms has been observed. The recoil frequency was determined with a two-pulse light grating interferometer using near-resonant laser light. The results show that the recoil momentum of atoms caused by the absorption of a photon is n variant Planck's k, where n is the index of refraction of the gas and k is the vacuum wave vector of the photon. This systematic effect must be accounted for in high-precision atom interferometry with light gratings.