Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 51 (2018)

Development of an intense mid-infrared coherent light source formuonic hydrogen spectroscopy
S. Kanda,∗1 S. Aikawa,∗2 K. Ishida,∗1 M. Iwasaki,∗1 Y. Ma,∗1 Y. Matsuda,∗3 K. Midorikawa,∗4 Y. Oishi,∗5S. Okada,∗6 N. Saito,∗4 M. Sato,∗5 A. Takamine,∗1 K. S. Tanaka,∗7 H. Ueno,∗1 S. Wada,∗4 and M. Yumoto∗4The proton is a fundamental constituent of the matter. However, its internal structure is complicatedand perplexing. The internal structure of a proton
is described by the electric and magnetic form factors. These form factors appear in the charge radius which has been measured by using electron-proton
scattering and spectroscopy of hydrogen-like atoms.
Since the proton charge radius was determined by
the spectroscopy of Lamb shift in muonic hydrogen atPSI,1) there has been a signiﬁcant discrepancy between
the electronic and muonic measurement results of thecharge radius.2) Both the experiments have been validated; however, the discrepancy was reproduced.3,4)
Alternatively, the proton structure is expressed by
the Zemach radius, which is deﬁned by a convolutionof the charge distribution with the magnetic moment
distribution. The Zemach radius can be extracted fromthe hyperﬁne splitting (HFS) of muonic hydrogen as acontribution arising from the ﬁnite volume eﬀect ofthe proton. We aim to determine the proton Zemachradius via laser spectroscopy of the ground-state hyperﬁne splitting in muonic hydrogen. The HFS energyof muonic hydrogen is 183 meV, and it corresponds toa light having the wavelength of 6.8 µm. The hyperﬁnetransition is E1-forbidden, and an intense mid-infraredcoherent light source is essential to the experiment.
In order to perform precision spectroscopy of muonichydrogen HFS, a pulse energy of 20 mJ and a spectral
width of 100 MHz are required for the transition laser.
The coherent light with a wavelength of 6.8 µm is generated by an optical parametric oscillator (OPO) usinga ZnGeP2 (ZGP) nonlinear optical crystal. The OPO
is pumped with a Tm3+ , Ho3+ co-doped YAG ceramiclaser. A quantum cascade laser (QCL) is adopted asa narrowband seeder. The output beam of the OPO
is ampliﬁed by the ZGP optical parametric ampliﬁers(OPAs). Figure 1 illustrates a diagram of the proposedlaser system.
As a ﬁrst step to develop the laser system, theTm,Ho:YAG ceramic laser was developed. A YAG ceramic rod was pumped by laser diodes and a quasicontinuous light output was pulsated by an acoustooptic Q-switch. A pulse energy of 20 mJ or higher and
∗1
∗2
∗3
∗4
∗5
∗6
∗7RIKEN Nishina Center
Department of Physics, Tokyo Institute of Technology
Graduate School of Arts and Sciences, The University ofTokyo
Photonics Control Technology Team, RIKENHigh Energy Accelerator Research Organization (KEK)
Atomic, Molecular, and Optical Physics Laboratory, RIKEN
Cyclotron and Radioisotope Center, Tohoku University- 214 -a pulse width of 150 ns or less are required for theoutput beam of Tm,Ho:YAG ceramic laser. Figure 2
shows the measured pulse energy and pulse width ofthe laser beam as a function of the current applied tothe laser diodes. A TEM00 mode beam proﬁle wasobtained, and the beam radius was 1 mm. Suﬃcientperformance of the light source was achieved by alignment optimization of each optical component.
Since the development of pumping light source was
successful, the OPO pumped with the Tm,Ho:YAG ceramic laser will be demonstrated as the next step. TheQCL as a seeder is under development and needs to betested.

Fig. 1. Diagram of the laser system. The system comprises of three stages: the Tm,Ho:YAG ceramic laser;
the QCL-seeded ZGP-OPO; and the ZGP-OPAs. A
quarter-waveplate is placed after the OPA to obtain acircularly polarized light. In the spectroscopy experiment, two sets of the laser system will be employed fora total energy of 20 mJ.

Fig. 2. Output characteristics of the Tm,Ho:YAG ceramic
laser. The black circles correspond to the pulse energy,
which refers to the left ordinate. The red squares correspond to the pulse width, which refers to the rightordinate. The inset represents the beam profile.

References
1) R. Pohl et al., Nature (London) 466, 213 (2010).
2) P. J. Mohr, B. N. Taylor, D. B. Newell, Rev. Mod. Phys.
84, 1527 (2012).
3) A. Antognini et al., Science 339, 417 (2013).
4) J. C. Bernauer et al., Phys. Rev. C 90, 015206 (2014).