Terahertz Photoconductivity in Optimally and Underdoped YBa2Cu3O7-δ

We have studied photoconductivity under the action of strong laser terahertz pulses in optimally doped and underdoped films of the high-T
_c
superconductor YBa
₂
Cu
₃
O
_7-δ
in the vicinity of the critical temperature. In the optimally doped film, the negative photoconductivity with delayed signal increment and decay has been observed. In the underdoped film, a fast positive photoresponse has been detected on top of the negative photoconductivity background at temperatures above the transition temperature. Possible mechanisms responsible for the effects observed are discussed.