24F12A Tunable Radiation Source from Laser-plasma Generated Electrons
Z. Jin,1 Z. L. Chen,3 H.B. Zhuo,3,5 A. Kon,3 M. Nakatsutsumi,3 H. B. Wang,2 B. H. Zhang,2 Y. Q.
Gu,2 Y. C. Wu,2 B. Zhu,2 L. Wang,2 M. Y. Yu,6,7 Z. M. Sheng,8,9 and R. Kodama1,3,4
1Photon Pioneers Center, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871 Japan
2National Key Laboratory of Laser Fusion, CAEP, Mianyang 621900, China
3Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871 Japan
4
Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka, 565-0871 Japan
5School of Science, National University of Defense Technology, Changsha 410073, China
6Institute for Fusion Theory and Simulation, Zhejiang University, Hangzhou 310027, China
7
Institute for Theoretical Physics I, Ruhr University, Bochum D-44780, Germany
8Key Laboratory for Laser Plasmas (Ministry of Education) and Department of Physics, Shanghai Jiao Tong University,Shanghai 200240, China
9Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, CAS, Beijing 100080, China
Near-infrared radiation with wavelength around 1000 nm is experimentally observed by coupling aperiodical metal grating to the MeV electron beams with high current density generated in the intense ultra
short pulse laser and solid target interaction. Simulations prove a prospect of tuning such kinds of radiation
into sub-mm waves, which holds a promise for a tunable compact “table-top” powerful Tera-Hertz source.

1. Introduction
Specialradiation
calledSmith-Purcell
Radiation (SPR) could be generated whenelectrons pass by a metal surface with periodical
grating structure. Such radiation was firstly
observed by S. J. Smith and E. M. Purcell, andproved to be tightly related to the electron
velocity, the grating period, and the observation
angles in both the experiments and the theories.
The radiation wavelength  is found to have arelationship as [1]l 1( - cos  )n (1),

where l is grating period,  is observation
angle, n is harmonic order of the radiation,  v / c is electron velocity, and c is lightspeed.
In this paper, we present a different
configuration to generate such kinds of radiationby use of the energetic electron beams produced
from the intense ultra short pulse laser andplasma interaction. Near-infrared radiation (NIR)
with wavelength around 1000 nm is clearlyobserved in our experiments and shows an exact
satisfaction to the Eq. (1). PIC simulations provea prospect of tuning such kinds of radiation into
Tera-Hertz (THz) wave, strongly indicating ahope for the fruitful applications by having atunable “table-top” THz source.
2. Experimental results and discussions

Fig.1. (a) Experimental configuration of the electron
beam and the grating structure. Images at the emission,or upside, surface of the solid target for targets withsmooth (b) and grated (c) surfaces.

Experiments were performed in both theSILEX-I laser facility in National Key Laboratoryof Laser Fusion, CAEP in China, and the P3 laser
facility in Graduate School of Engineering, Osaka
UniversityinJapan.
Theexperimental

configuration is shown in Fig. 1(a). A 1/830 mm
grating is chosen to realize output radiation in theNIR range, to which our spectrometer and CCD
camera are sensitive. For comparison, another target
with the same specifications but having a smoothflat upside surface is also used. Both targets are
made of BK7 glass, with the upside surface coatedwith gold.
The spatial distributions of the NIR radiationfrom the targets with grated and smooth upsidesurfaces are shown in Fig. 1(b)-(c). The effect of
the grating on the radiation can be clearly seen bycomparing the radiation patterns from the grated
and smooth surfaces in Fig. 1(b) and (c),respectively. A bright radiating spot, of width
500  m and nearly flat intensity distribution, on thegrated surface is recorded by the CCD camera. In
contrast, with the smooth surface the emission
intensity reaches a peak close to the spot where thelaser hits the target and decreases exponentiallyaway from it. The energetic electron beam
originating from the laser spot first propagatesinside target plasma, and SPR is emitted when it
reaches and propagates along the grated surface.

crescent-shaped wave fronts of the low frequency
radiation shown in Figs. 3(a) correspond to that of
the classical Smith-Purcell result. The Fourier
spectra of the emission fields are shown in Figs.
3(b). One can clearly see that the results from thefirst order Smith–Purcell relation agree well withthat from the simulations. And the result indicates acoherent radiation with tens GV/m field strength.

Fig. 3. Snapshots from the 2D PIC simulations. (a)The emission field eE z / mc0 and (b) the Fourier

spectrum E z k  for l =10  m at 200 laser periods.
The black dots and the red dashes represent thecalculated first order SPR results.

3. Acknowledgments
This work is supported by the Core Research for
Evolutional Science and Technology, Japan Scienceand Technology Agency. H. B. Zhuo and M. Y. Yu
would like to acknowledge the support of theNational Natural Science Foundation of China(grants 10976031, 10975185, and 10835003).
Z.M.S. acknowledges the support of the National
Basic Research Program of China (Grants
2007CB310406,2007CB815100and2009GB105002).
Fig. 2. Dependence of the radiation wavelength on theobservation angle θ. The black (I), the red (II), and thegreen (III) solid lines give the radiation spectra emitted
from the side mirror surface and the side grating surface
at the observation angle of θ=80.2º, and from θ=79.6º,respectively.

Radiation spectra in Fig. 2 measured by thespectrometer clearly show a characteristic radiation
peak around 1000 nm emitted from the grating
surface whereas no emission peak appears from themirror surface. The spectrum peak shifts from 985.6
nm to 997.6 nm as the observation angle changes
from 79.6° to 80.2°, which is exactly coincident
with the theoretical calculation results based on theEq. (1) where the shift is from 983 nm to 996 nm atthese two observation angles.
2D PIC simulations are carried out using thePDLPICC2D code [2]. Figure 3 illustrates thesimulation results for grating with l  10  m . The

References[1] S. J. Smith and E. M. Purcell: Phys. Rev. 92 (1953)
1069.
[2] H. B. Zhuo et al., Phys. Rev. Lett. 105 (2010)
065003.