We developed a metastability-exchange optical pumping technique for metastable triplet helium (3He) atoms utilizing the 23S1-33PJ transition at 389 nm with radiation generated by a frequency-doubled cw Ti:Sapphire laser and compared the results with our results obtained utilizing the 23S1-23PJ transition at 1083 nm with a diode laser. We optically measured the nuclear polarization of 3He in the hollow cathode discharge cell by monitoring the absorption of a weak longitudinal probe laser at 1083 nm. This provided, for the first time, nuclear polarization utilizing the 23S1-33PJ transition at 389 nm.
NMR based on laser-polarized 3He gases has been attracted as a powerful tool for characterizing physical parameters of
porous media and then imaging human lungs. In this paper, the feasibility study of nuclear polarization of 3He atoms
utilizing the 23S-33P transition at 389 nm is reported in comparison with the conventional 23S-23P transition at 1083 nm.
The 389-nm light has been available readily with the development of various indium gallium nitride light-emitting
diodes (InGaN LEDs). In this work, the frequency-doubled light of a 778-nm CW Ti:sapphire laser with the nonlinear
crystal (BiB3O6) was used as the optical pumping light at 389 nm. The other light from a Littrow external cavity diode
laser was also used for optical pumping at the 1083-nm wavelength and then measurement of the nuclear polarization.
The nuclear polarization of 1.8% with optical pumping at the 23S-33P transition was demonstrated and then it was found
that the (23S1, F=1/2)-(33P0, F=1/2) transition was the most efficient transition of 23S-33P lines for the magnetic field of
1.6 mT and the gas pressure of 0.5 Torr.
Optical pumping is the efficient way to polarize a nuclear spin of helium-3 (3He). The nuclear spin polarization of 3He
has been often demonstrated in a discharge cell under the low B-field through the metastability exchange optical
pumping (MEOP) technique for a short time. Since the appearance of a high power near-infrared light source, optical
pumping with a circularly polarized light tuned to the resonant frequency of 23S→23P transition at 1083nm has been
investigated extensively. We, however, are focusing on another optical transition 23S1→23P0 at 389nm that has not been
investigated yet for the polarization. Therefore, we developed a single-frequency 389-nm coherent light source which
enables to optical-pump metastable 3He atoms via the transition. This light source is based on the second harmonic
generation of a single-frequency 778-nm CW Ti:Sapphire laser light with a BiB6O3 (BIBO) nonlinear crystal in an
external cavity for the enhancement. We have demonstrated very efficient frequency doubling with high conversion
efficiency of 56%, which is obtained at the second harmonic generation of 380mW in the cavity and is corresponding to
an efficiency of 81%/W, and we examined the optical pumping for the nuclear spin polarization and polarization
analysis.
A hyperpolarized helium-3 (3He) has been researched extensively for application in fields such as polarization analysis, 3He spin filtration for n (p, d) γ, and medical imaging. In the medical field, nuclear magnetic resonant imaging (MRI) using hyperpolarized gas is attracted recently because air space in human lungs can be monitored in detail by aspirating polarized gas. There are two methods of producing hyperpolarized 3He gas: spin-exchange optical pumping (SEOP) and metastability exchange optical pumping (MEOP). It is well known that the way of making polarization by the MEOP method is more shortly and more directly. In this study, however, the metastable 3He atoms provided by hollow cathode discharge are polarized by circularly polarized light. When 3He atoms pumped optically, the 23S-23P transition at the wavelength of 1083nm is leveraged typically. The 3He atom actually has the optical transition 23S→ 23P at 389nm, but the transition has never been used. Therefore, we developed the 389-nm coherent light source to polarize 3He gas by MEOP method. This light source utilized the second harmonic generation (SHG) of continuous-wave Ti:sapphire laser operating at a wavelength of 778nm based on BiB3O6 (BiBO) in an external cavity.
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