Speaker
Dr
Shinji Yoshimura
(National Institute for Fusion Science)
Description
We have been developing a new laser spectroscopy technique using a Laguerre-Gaussian (LG) beam, which is known as an optical vortex. One of the characteristics of optical vortices is helical equiphase surfaces associated with orbital angular momentum of light. Existing techniques using a plan-wave laser beam are insensitive to perpendicularly crossing motion of atoms, since they rely on the axial Doppler effect.
However, this limitation is possible to overcome by using a LG beam, because its three-dimensional phase structure gives rise to three-dimensional Doppler effect [1]. This additional Doppler effect is proportional to the azimuthal velocity component of atoms and to the topological charge of the LG beam and inversely proportional to the distance from the beam axis in which the beam intensity banishes due to phase singularity. For example, the additional frequency shift is 10 MHz under the conditions that the topological charge is one, the distance from the singularity 100 $\mu$m and azimuthal velocity 100 m/s. Therefore, the additional frequency shift inhering in the absorption spectrum depends on positions in the beam cross-section.
In order to detect this subtle frequency shift, we have performed laser absorption experiments for metastable argon atoms in an ECR discharge plasma using a LG beam produced by a spatial light modulator or by a q-plate. With scanning the laser frequency, images of the transmitted beam intensity were recorded by a beam profiler. Obtained frequency shift in absorption spectra qualitatively agreed with theoretical azimuthal Doppler shift in LG beams but not quantitatively. Pros and cons as well as the present status and problems of the absorption spectroscopy using an optical vortex beam will be reported.
[1] L. Allen, M. Babiker, W. L. Power, Opt. Commun. 112, 141-144 (1994).
Primary author
Dr
Shinji Yoshimura
(National Institute for Fusion Science)
Co-authors
Dr
Kenichiro Terasaka
(Kyushu University)
Prof.
Masayoshi Tanaka
(Kyushu University)
Prof.
Mitsutoshi Aramaki
(Nihon University)
