hhg can happen both in gases as well as in laser ablated plasma plumes. in gas high harmonics, a gas jet usually acts as the nonlinear media and the femtosecond laser pulse interacts with the gas to emit high harmonics. hence, only one laser pulse is required in gas harmonics. however to generate high harmonics from plasma plumes, we require another laser pulse focused onto the surface of a solid target to create a plume of laser ablated plasma. this plasma plume acts as a nonlinear medium for the nonlinear interaction. typically, a long picosecond laser pulse is used for the purpose of plasma creation.