Formation of Optical Bullets in Laser-Driven Plasma Bubble Accelerators

Electron density bubbles--wake structures generated in plasma of density n{sub e{approx}}10{sup 19} cm{sup -3} by the light pressure of intense ultrashort laser pulses--are shown to reshape weak copropagating probe pulses into optical 'bullets'. The bullets are reconstructed using frequency-domain interferometric techniques in order to visualize bubble formation. Bullets are confined in three dimensions to plasma-wavelength size, and exhibit higher intensity, broader spectrum and flatter temporal phase than surrounding probe light, evidence of their compression by the bubble. Bullets observed at 0.8 < or approx. n{sub e} < or approx. 1.2x10{sup 19} cm{sup -3} provide the first observation of bubble formation below the electron capture threshold. At higher n{sub e}, bullets appear with high shot-to-shot stability together with relativistic electrons that vary widely in spectrum, and help relate bubble formation to fast electron generation.