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

RIKEN Accel. Prog. Rep. 49 (2016)

Developmentfor thethemeasurement
measurementofof
Developmentofofmid-infrared
mid-infrared laser formuonichydrogenhydrogenatom
atom hyperfine
hyperfine splitting
splittingenergy
energymuonicS. Aikawa,*1,*2 K. Ishida,*2 M. Iwasaki,*1,*2 S. Kanda,*4 Y. Ma,*2 Y. Matsuda,*5 T. Matsuzaki,*2 K. Midorikawa,*3 Y. Oishi,*2,*6S. Okada,*2 M. Sato,*2 N. Saito,*3 A. Takamine,*2 K. Tanaka,*2,*5 H. Ueno,*2 S. Wada,*3 and M. Yumoto*3
Recently, the proton charge radius was measured using la- MHz, we will introduce a single mode laser diode (LD) ofser spectroscopy of the 2s-2p Lamb shift in muonic hydrogen 100 kHz as a seed laser. In this paper, we report the output(μp). Laser spectroscopy of μp resulted in 10 times higher characteristics of a Cr:ZnSe master oscillator (laser) and a
precision of the proton charge radius determination than that single pass Cr:ZnSe power amplifier.
Figure 2 shows the outof the ordinary measurements such as e-p scattering and hydrogen spectroscopy.1) As verification of the charge radius put energy of the Cr:ZnSemeasurement, e-p scattering experiments were conducted at laser. A maximum pulseMainz2) and the Jefferson Lab3), but the discrepancy in the energy of 5.8 mJ was obtresults was not resolved. This raised a question as to whether ained at 2.4 μm with athe magnetic radius of the proton measured with muons will pump energy of 18 mJ.
be consistent with that measured by electrons. Thus, we plan The conversion efficiency
an independent measurement of the proton structure, i.e., the reached 32% at maximum.
Zemach radius, to answer this question by using the hyper- This value is excellent
Fig. 2. Output energy of the forfine transition from singlet to triplet in the 1s state of muonic compared with those ofother lasers for example,
Cr:ZnSe laserhydrogen atoms using the laser-spin-pumping method. 4)
Two important requirements of the laser in this experime- the Ti:Al2O3 laser and Nd:YAG laser.
nt are a high energy and narrow spectrum width. We need toThe output enegy extracted from the Cr:ZnSe power amplgenerate pulse energy exceeding 40 mJ at 6.7 μm. In addition, ifier is shown in Fig. 3. The incident energy from the Cr:Zn
Cr:ZnSe master oscillator
a narrow spectrum width Se laser was 5.5 mJ atof 50 MHz is required. A 2.4 μm, and it was amplmid-infrared laser system ified to 26.3 mJ with incthat produces pulse ener- reasing pump energy. ACr:ZnSe amplifier
gy of 10 mJ at 6.1 μm has maximum conversion efalready been developed at ficiency of 41.5 % wasRIKEN.5)
obtained with a pump eBased on this, we plan to nergy of 50.2 mJ. This is,
Optical parametric oscillators and amplifiers
OFig. 3. Output energy extracted
construct a new laser syste- to the best of our knowlefrom the Cr:ZnSe powerm. The schematic of the la- dge, the highest conversiser is shown in Fig. 1. The on efficiency ever attainedamplifier.
laser system consists of a for a single-pass Cr:ZnSe
Cr:ZnSe master oscillator, power amplifier.
Cr:ZnSe power amplifiers,
Based on the above results, to obtain an energy of 100 mJ
and optical parameteric os- at 2.4 μm, we need a pump energy of 240 mJ. However, ifcillators (OPOs) / amplifie- we introduce such an energy to the Cr:ZnSe, it will be damaFig. 1. Schematic of the laser ||ers (OPAs). The Cr:ZnSe
ged because the damage threshold on the surface of Cr:ZnSe
master oscillator and the p- is about 90 mJ. Therefore, we will construct three-stage
ower amplifier generate 100 mJ pulse energy per line at 2.4 Cr:ZnSe power amplifiers to solve this problem.
In the next stage, we will optimize the conversion efficienμm. The pulse is introduced to the ZnGeP2 nonlinear crystal(ZGP) OPO to achieve radiation at 6.7 μm. Two independent cy of the Cr:ZnSe laser and the power amplifiers by controlloptical parametric oscillators and amplifiers will be prepared ing the crystal length and Cr2+ concentration. In addition, weto achieve 40 mJ laser power in total and avoid damages to will construct a three-stage Cr:ZnSe power amplifier and an
the ZGP crystal, which is difficult to synthesize with a large OPO to generate radiation at 6.7 μm.
diameter. To realize a narrow band spectrum width of 50          References
AOTF Cr:ZnSe

10 mJ@ 2.4 μm

Multi-stage Cr:ZnSe amplifier

ZnGeP2

ZnGeP2

Department of Physics, TokyoTech
RIKEN Nishina Center
3
RIKEN Center for Advanced Photonics
4
Department of Physics, The University of Tokyo
5Graduate School of Arts and Science, The University of Tokyo
6_IMSS, KEK121) R. Pohl et al., Nature 466, 213 (2010).
2) J. C. Bernauer et al., Phys. Rev. Lett.105 242001 (2010).
3) X. Zhan et al., Phys. Lett. B705 59 (2011).
4) M. Sato et al., JPS Conf. Proc., 025005 (2015)5) M. Yumoto et al., OSA/Nonlinear optics, NW4A.19 (2013)- 231 -