Laser pulses with a power of 10' W and a duration of 10 ' s were focused onto both gas and solid targets. Strong emission of pulsed radiation at terahertz frequencies was observed from the resulting plasmas. The most intense radiation was detected from solid density targets and was correlated with the emission of MeV x rays and electrons. Results indicate that radiative processes in such plasmas are driven by ponderomotively induced space charge fields in excess of 10 V/cm. This work constitutes the first direct observation of a laser-induced wake field.PACS numbers: 52.40.Nk, 42.65.Re, 52.25.Rv, 52.35.Mw Plasmas created by high-intensity laser pulses with subpicosecond duration have received considerable attention as novel sources of radiation. The observed emission includes coherent radiation at high harmonics of the laser frequency [1], incoherent soft x-ray bursts with subpicosecond duration [2], and the generation of hard x rays with photon energies extending beyond 1 MeV [3]. At the low frequency end of the electromagnetic spectrum, strong emission of coherent far-infrared radiation (FIR) at terahertz frequencies was recently predicted [4]. This radiation results from the space charge fields developed in such plasmas. In this Letter we report the first observation of this eff'ect.The generation of strong electric and magnetic fields in laser produced plasmas has been considered before. Electric fields on the order of 10 V/cm have been inferred in experiments involving plasma-wave accelerators [5]. In the context of high-intensity short-pulse laser interaction with plasmas, electric fields of 10 V/cm [6] and magnetic fields of up to 10 G [7,8] were predicted by several groups. Our experiments allow a comparison with this previous work by direct measurement of such fields. We note that the generation of terahertz radiation through the use of femtosecond laser pulses has been considered in a variety of schemes [9]. For example, intense pulses with energies up to 0.8 pJ were produced by illuminating a biased GaAs wafer with short laser pulses [10].In our experiment, the mechanism of FIR generation involves ponderomotive forces present at the focus of an intense laser pulse. These forces generate a large density difference between ionic and electronic charges since the laser pulse length is short enough to inertially confine the ions [6,11]. This charge separation results in a powerful electromagnetic transient [4].To estimate the magnitude of the terahertz emission we employed a hydrodynamic model for the plasma dynamics. We calculated the spatial and temporal dependence of the charge density and acceleration within the focal region and thereby determined the far-field radiation pattern. The electron fiuid can be assumed to be cold, i.e. , the thermal energy is small compared to the ponderomotive energy, U~". U~" is defined in Ref. [12]. The cold Auid approximation is justified since plasmas produced by short pulse lasers tend to have temperatures~10 eV [13], while the ponderomotive energies for our experime...

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We demonstrate a new technique for enhancing the absorption of high-intensity, ultrashort-duration laser pulses by solids. Targets consisting of gold gratings and gold clusters were found to absorb greater than 90% of the incident high-intensity laser light. This is in contrast to less than 10% absorption by flat surfaces. As a result of this strong coupling of the laser to a high-density plasma, conversion efficiency of laser energy to x rays of greater than 1% was observed for x rays above 1 keV. Efficiency of nearly 25% was observed for emissions greater than 30 eV. These conversion efficiencies are more than an order of magnitude greater than those measured from flat targets.

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