It is shown that the interaction of ultrashort laser pulses with nonstation
ary semiconductor plasmas can, under appropriate conditions, lead to a vari
ety of interesting phenomena including controlled upshifting of the laser f
requency, leading to the possibility of tunable lasers in a wide range of f
requencies, and trapping (nonpropagation) of a substantial part of the inci
dent pulse. [S1050-2947(99)09201-X].