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 10 8 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.[S0031-9007 (97)03456-X] PACS numbers: 32.80.Pj, 39.90. + d

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We have carried out dual-comb spectroscopy and observed in a simultaneous acquisition a 140-THz-wide spectrum from 1.0 to 1.9 µm using two fiber-based frequency combs phase-locked to each other. This ultra-broad wavelength bandwidth is realized by setting the difference between the repetition rates of the two combs to 7.6 Hz using the sub-Hz-linewidth fiber combs. The recorded spectrum contains five vibration-rotation bands of C 2 H 2 , CH 4 , and H 2 O at different wavelengths across the whole spectrum. The determined transition frequencies of C 2 H 2 agree with those from the previous sub-Doppler resolution measurement of individual lines using CW lasers within 2 MHz.

We observe TM and TE Laguerre-Gaussian (LG) light beams reflected and transmitted at a dielectric interface near critical incidence. The intensity distribution of the reflected beam is transversely deformed near the beam waist, and that of the transmitted beam is similar to that of a diagonal Hermite-Gaussian beam. The former rotates around the optical axis by approximately pi/2 with propagation, and the latter returns to that of the incident LG beam. These observations agree well with numerical calculations based on an angular spectral analysis and are attributable to the helical wavefront of the LG beams, the sharp incidence-angle dependence of the Fresnel reflection and transmission coefficients, and the Gouy phase.

We derive a general expression for the field distribution of Hermite- and Laguerre-Gaussian (LG) beams reflected at a dielectric interface. The intensity distributions of the reflected LG light beam at the beam waist are also observed experimentally in the vicinity of the critical incidence. They are greatly deformed because of the nonspecular transverse shift induced by the orbital angular momentum of the LG beam. The observed and calculated intensity distributions agree well and indicate that a large fraction of the electromagnetic energy flows as much as the transverse beam size.