Formation of optical bullets in laser-driven plasma bubble accelerators

We show that luminal velocity electron density bubbles produced by relativistically intense ultrashort drive laser pulses propagating through near-atmospheric-density plasma re-shape co-propagating probe pulses into optical ``bullets" that we reconstruct using frequency-domain interferometric techniques and use to visualize the spatio-temporal profile of the plasma bubble. A study of bubble-compressed bullets together with bubble-accelerated electrons reveals three regimes: (I) bullets of sub-plasma-wavelength (sub-$\lambda _{p})$ size, trapped and compressed inside plasma bubbles, appear at 1 $<$ n$_{e} \quad <$ 1.5 $\times $ 10$^{19}$ cm$^{-3}$ without production of relativistic electrons; (II) enlongated bullets spanning $\sim $2 $\lambda _{p}$, signifying temporary merging of sequential bubbles, are observed frequently at n$_{e} \quad >$ 2 $\times $ 10$^{19}$ cm$^{-3}$ together with poly-energetic relativistic electrons; (III) mono-energetic electrons are observed only in conjunction with intense sub-$\lambda _{p}$ bullets signifying stable or contracted bubbles, generally at n$_{e} \quad >$ 2.5 $\times $ 10$^{19}$ cm$^{-3}$. The results help to relate bubble structure to the properties of laser-wakefield-accelerated electrons in the blowout regime.