Direct measurement of ultrafast electron-hole plasma expansion at high density at an asymmetric GaAs quantum well

The ultrafast spatial expansion of photoexcited electron-hole plasma created by a femtosecond laser pulse excitation in an asymmetric GaAs single quantum well at 4.3 K was directly measured using a 3ps time resolution streak camera system. The experimental results show that the diffusion D is four orders of magnitude larger than the conventional ambipolar diffusivity and the ballistic velocity of the plasma is about four times larger than its Fermi-velocity. The mechanism which causes the photoexcited carriers to be so diffusive is discussed.