Experimental observation of short-pulse upshifted frequency microwaves from a laser-created overdense plasma Noboru Yugami, 1, * Toshihiko Niiyama, 2 Takeshi Higashiguchi, 3,† Hong Gao, 1 Shigeo Sasaki, 1 Hiroaki Ito, 1,‡ and Yasushi Nishida 1,§ 1 Department of Energy and Environmental Science, Graduate School of Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan 2 Ishikawajima-Harima Heavy Industries Company, Limited, Tokyo 100-0005, Japan 3 Department of Electrical and Electronic Engineering, Miyazaki University Gakuen Kibanadai Nishi 1-1, Miyazaki 889-2192, Japan Received 30 July 2001; published 5 March 2002 A short and frequency upshifted from a source microwave pulse is experimentally generated by the over- dense plasma that is rapidly created by a laser. The source wave, whose frequency is 9 GHz, is propagating in the waveguide filled with tetrakis-dimethyl-amino-ethylene gas, which is to be converted to the overdense plasma by the laser. The detected frequency of the pulse is over 31.4 GHz and its duration is 10 ns. This technique has the potential for the generation of a tunable frequency source. DOI: 10.1103/PhysRevE.65.036505 PACS numbers: 52.59.Ye, 52.25.Os, 52.50.Jm In the last decade the interaction between a plasma and the electromagnetic em wave has been studied for plasma applications of short and tunable radiation source. Lampe et al. have studied the phenomena of the frequency upshift of the em wave using a relativistic, overdense ionization front with the speed of light 1. The em wave reflects at the ion- ization front and the frequency is upshifted by the double Doppler effect. The creation of the ionization front is now easy to be realized by the intense short laser pulse through photoionization 2. A frequency upshift and a radiation of the em wave have been studied by using photon acceleration 3 and relativistic underdense ionization front 4 in the plasma application in the 1990s. The frequency upshift of the em wave with the laser produced underdense ionization front having a speed of light was studied by Mori in 1991, when the em wave was incident on the underdense ionization front 4. This phenomenon of the frequency upshift is the phase modulation of the incident wave by the ionization front. Sav- age et al. studied experimental frequency upshift of the mi- crowave frequency from 35 GHz over 170 GHz by using the laser produced underdense ionization front 5. The frequency upshift can also be expected from a peri- odic electrostatic field instead of the initial em wave, because the periodic electrostatic field is observed as the incident em wave with the frequency of  ' 0 in the frame moving with the ionization front. In the laboratory frame, the transmitted wave in the plasma behind the ionization front is directly observed as the em wave radiation pulse. The observed fre- quency, however, is upshifted from zero frequency, that is, this scheme can directly convert the dc field energy to the em wave radiation. This phenomenon is called dc to ac radiation converter 6–13. Wilks and co-workers have investigated the effect of quickly creating a plasma around an em source wave, on the time scales in the order of a cycle of the wave 13,14. They have found that this can generate upshifted em wave by changing the plasma density. Experimental works have been successfully performed 15,16; however, this is the first time, to the best of our knowledge, that the phenomenon has been explored experimentally using laser-created overdense plasma. In this paper we describe experimental results in which 9 GHz source is upshifted in frequency up to 31.4 GHz by the laser-created plasma. The mechanism that is called ‘‘flash ionization frequency upshift phenomena’’ can be explained as follows. The source em wave with an angular frequency  0 ( k 0 ) is propagating in the z direction. Suddenly, in a time interval much shorter than 2  /  0 , a plasma is created around the em wave along the propagation direction keeping the wave number of this source wave, k 0 , fixed at the initial value. But the em wave should obey the dispersion relation in the plasma given by  f 2 =k 0 2 c 2 + p 2 = 0 2 + p 2 . 1 Therefore, the final frequency  f of the wave is represented to be   0 2 + p 2 , where  p is the plasma angular frequency and we can expect the frequency upshift that is represented by   =  0 2 + p 2 - 0 Fig. 1. When the length of the plasma, d, in the propagation direction of em wave is much longer than the wavelength of the em wave,  0 , the pulse duration of the upshifted em wave is decided by the plasma length d. Therefore, this mechanism has a potential for the high frequency and the ultrashort pulse generation by chang- ing the plasma density and length of plasma. The upshifted wave has two components with the em wave propagation, i.e., copropagation and counterpropaga- tion components. The maximum electric field of the wave is represented by E  = E 0 2  1   0   0 2 + p 2  , 2 *Email address: yugami@cc.utsunomiya-u.ac.jp † Email address: higashi@opt.miyazaki-u.ac.jp ‡ Email address: hito@cc.utsunomiya-u.ac.jp § Email address: nishiday@cc.utsunomiya-u.ac.jp PHYSICAL REVIEW E, VOLUME 65, 036505 1063-651X/2002/653/0365055/$20.00 ©2002 The American Physical Society 65 036505-1