We report the far-infrared properties of superconducting ab plane-oriented
YBa2Cu3O7-delta films in magnetic fields of up to 27 T. The applications of
magnetic field (with H perpendicular to the ab plane and with unpolarized
light) at low temperature produces no discernible field dependence. This ob
servation differs from other previous far-infrared measurements in this tem
perature range. Only at fields and temperatures where the de resistance is
not zero - on account of dissipative flux motion - is a field-induced effec
t observed. (C) 1998 Elsevier Science Ltd. All rights reserved.