Formation of Optical Bullets in Laser-Driven Plasma Bubble Accelerators

Electron density bubbles-wake structures generated in plasma of density (n) over bar (e) similar to 10(19) cm(-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 less than or similar to (n) over bar (e) less than or similar to 1.2 x 10(19) cm(-3) provide the first observation of bubble formation below the electron capture threshold. At higher (n) over bar (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.