Speaker
Prof.
Koichi Sasaki
(Division of Quantum Science and Engineering, Hokkaido University, Japan)
Description
The measurement of electric field by laser Stark spectroscopy aimed the detection of high Rydberg states for a long time to realize a high sensitivity. We changed our approach to the detection of the Stark effects of lower-lying states. The detection of a low-energy state is easier than that of a Rydberg state since it has a large transition probability. However, since the magnitude of the energy shift of the low-energy state by the Stark effect is much smaller than that of a Rydberg state, the energy shift is buried under the resolutions of conventional spectroscopic methods. In this work, we compensated this problem by employing saturation spectroscopy with a Doppler-free spectral resolution [1,2].
A planar electrode was inserted in an inductively coupled hydrogen plasma. The electrode was connected to a dc power supply. We injected pump and probe laser beams, which were obtained from a tunable diode laser system, into the sheath region in front of the electrode from the counter directions. The wavelength of the laser was scanned over the whole range of the Doppler-broadened Balmer-alpha line of atomic hydrogen. The saturation spectrum was obtained from the difference between the absorption spectra of the probe beam in the presence and the absence of the pump beam.
We observed the change in the fine-structure spectrum of the Balmer-alpha line as a function of the distance from the electrode surface. The experimental spectra were compared with spectra obtained by the theoretical calculation, and we deduced the magnitude of the electric field. It was possible to detect the 10 MHz shift of the fine-structure line with the help of the Doppler-free resolution of saturation spectroscopy. The shift of 10 MHz corresponds to the detection limit of 10 V/cm in the evaluation of the electric field. The spatial resolution was essentially limited by the diameter of the laser beams, and was less than 0.2 mm. The measurement ambiguity of the electric field was evaluated to be 30 and 10 V/cm in strong and weak electric fields, respectively. Since the developed method utilizes an economical, maintenance-free diode laser system, it may be useful in various experiments which need the measurements of electric fields in plasmas.
[1] S. Nishiyama, K. Katayama, H. Nakano, M. Goto, and K. Sasaki, Appl. Phys. Express 10, 036101 (2017).
[2] S. Nishiyama, H. Nakano, M. Goto, and K. Sasaki, J. Phys. D: Appl. Phys. 50, 234003 (2017).
Primary author
Prof.
Koichi Sasaki
(Division of Quantum Science and Engineering, Hokkaido University, Japan)
Co-author
Dr
Shusuke Nishiyama
(Division of Quantum Science and Engineering, Hokkaido University, Japan)
