Nonlinear optics and electron generation in the interaction of a high intensity laser with underdense plasma.

Abstract

This thesis presents a systematic study of the nonlinear optics and electron generation of a high power (up to 10 TW), ultrashort, (400 fs) laser pulse in an underdense plasma, which is formed by the photo-ionization of a helium gas target generated by a supsonic gas jet. The scattered light from the interaction region was measured spectrally and spatially from various directions as a function of laser intensity and plasma density, and correlated with the accelerated electron beam. The data shows that forward stimulated Raman scattering (SRS) is sensitive to the laser focal position relative to the nozzle. Together with spectrally resolved top-view images of the channel, the measurements indicate that SRS is seeded by the ionization front. This observation provides a method to control the Raman instability in laser-plasma interaction by controlling the ionization process. With the increase of the laser power, a novel nonlinear optical phenomenon, relativistic cross-phase modulation, is observed. The bandwidth of a Raman satellite is found to be broadened from 3.8 nm to 100 nm when the pump laser power is increased from 0.45 TW to 2.4 TW, corresponding to an intensity increase from <italic>I</italic> = 2.7 x 10<super>17 </super> Wcm<super>-2</super> to 1.3 x 10<super>18</super> Wcm<super> -2</super>. A signature of relativistic cross-phase modulation, namely, asymmetric spectral broadening of the Raman signal, is observed at a pump power of 2.4 TW (<italic>I</italic> = 1.3 x 10<super>18</super> Wcm<super> -2</super>). The experimental cross-phase modulated spectra compared well with theoretical calculations. Relativistic cross phase modulation will be useful for the generation of high-power attosecond light pulses. A proof-of-principle experiment of laser injection laser acceleration (LILAC) concept also has been done by overlapping terawatt-power 400-femtosecond (tau<italic>_laser</italic> > tau<italic>_plasma</italic>) laser pulses (&le;2.4 x 10<super>18</super> W/cm<super>2</super>) in plasma. The interference pattern creates the first optical trap capable of confining relativistic electrons, with kinetic energy &le;350 keV. Experiment, numerical simulation and analysis all indicate that the electron density becomes deeply modulated, reaching peak values of up to ten times the background density (<italic>n_e/n </italic>₀ &sim; 10), creating a DC electrostatic field on the order of 10<super>11</super> eV/m. These values are to our knowledge the largest ever measured in the laboratory. Significant energy transfer (&sim;50%) between the two laser beams is also observed in both simulation and experiment. Applications to table-top size electron accelerators and x-ray sources are discussed.