We observed terahertz (THz) radiation from superconducting YBa2Cu3O7-delta
(YBCO) thin films excited by femtosecond laser pulses with a center wavelen
gth around 1.55 mum. The THz radiation mechanism is revealed to be a one ph
oton excitation process corresponding to an intraband transition in the in-
gap state. The property of the THz radiation excited by a 1.55 mum laser is
similar to that excited by an 800 nm laser.