Ⅲ-1. Atomic & Solid State Physics (Ion)

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Improvement of the signal-to-noise ratio of Rb D1 ﬂuorescence insuperﬂuid helium using picosecond time-resolved detection
H. Endo,∗1,∗2 K. Ishii,∗3 K. Imamura,∗2 A. Takamine,∗2 Y. Matsuo,∗1,∗2 T. Tahara,∗3 and H. Ueno∗2
Our research group is developing a laser spectroscopy technique (OROCHI) for atoms in superﬂuid
helium (He II). When atoms are injected into He II,
the surrounding helium atoms are pushed out by thePauli repulsion force.1) The resulting vacuum region
is referred to as an atomic-bubble. As the shape ofthe electron orbit of the impurity atom is deformed
according to its energy state, the atomic bubble is also
deformed following the shape change of the atomic orbit. The wavelengths of the atomic transitions for boththe absorption and emission in He II are shifted fromthose in vacuum due to this deformation cycle.2) It
is estimated that the bubble deformation requires anorder of a few picoseconds;1) however, till date, therelaxation time has not been measured in the time domain in bulk He II. Because the emission wavelengthis expected to change with the degree of the bubble
deformation, we aim to determine the relaxation timeof Rb atomic bubbles in He II through time-resolved
emission measurements at diﬀerent wavelengths.3)
We have successfully observed laser-induced ﬂuorescence (LIF) from Rb atoms of the D1 line (absorption center wavelength in He II: 778.0 nm) picosecond
laser excitation and detection with time-correlated single photon counting (TCSPC).3) To measure the ﬂuorescence that is weaker at shorter wavelengths thanthe center wavelength, the signal-to-noise ratio must
be increased. Here, we report the introduction of ametal cell with anti-reﬂection (AR) coated windows
instead of a conventional quartz-cell to reduce the effect of scattered light from the excitation laser.
Figure 1 shows the experimental setup. We used apicosecond mode-locked Ti:sapphire laser (laser power:
100 mW, repetition rate: 80 MHz, pulse width:
1.6 ps, center wavelength: 778.2 nm) as the excitation
laser. LIF was detected using an avalanche photodiode (APD) through a monochromator. The transmission of AR-coated windows is greater than 99.4% in
the wavelength range of 780 nm to 830 nm. Because alarge proportion of the background photons are considered to originate from the scattered light on the surfaceof the quartz cell, the introduction of the AR-coatedwindows is expected to reduce the scattered light, inthe LIF detection eﬃciency.
Figure 2 shows the observed photon intensities as afunction of delay time. The peaks in Fig. 2 are at thetime of the excitation laser incidence, from which theRb atoms decay along their spontaneous emission life∗1∗2∗3

Department of Advanced Sciences, Hosei University
RIKEN Nishina Center
RIKEN Center for Advanced Photonics- 126 -Fig. 1. Experimental setup of LIF detection.

Fig. 2. Intensity detected by TCSPC using (a) a quartz
cell, (b) AR coated windows.

time of 27 ns. Figure 2(a) shows the result of the LIF
observation using the conventional quartz cell without
AR coating. Both the LIF signal (blue) and a relatively large amount of laser-scattered light (orange)
were observed. Note that the background also contains
ambient light other than the laser in the environment
and areas with a value of 0 correspond to the timing when the detection device can not count photons.
Figure 2(b) shows that LIF increased when AR-coatedwindows were used, whereas laser scattered light wasalmost negligible. We plan to observe weaker emission
on the short wavelength side through further improvement of the signal-to-noise ratio by increasing the dataacquisition time.
References1) B. Tabbert et al., J. Low Temp. Phys. 109, 653 (1997).
2) Y. Takahashi et al., Phys. Rev. Lett. 71, 1035 (1993).
3) Y. Takeuchi et al., RIKEN Accel. Prog. Rep. 54, 105(2020).