Resistive transition and superconducting properties of optically illuminated tin microstrips

We report on experiments in which superconducting tin microstrips at temperatures >1/2T/sub c/ were illuminated, both cw and pulsed, by an argon laser. The illuminated films cannot in general be modeled in terms of a spatially uniform effective temperature. Even when care is taken to ensure that the illumination is spatially uniform to within approx.5% over the area of the strips, the resistive transition far from T/sub c/ extends over a range of laser powers which is approx.25% of the power necessary to drive the strips completely normal. The critical current of each strip decreases smoothly to zero and the current--voltage characteristics smoothly approach that of the normal state as the laser power is increased, indicating the probable absence of a first-order transition in any section of the strips. However, the behavior of the current--voltage characteristics of one strip is consistent with what might be expected if a spatially inhomogeneous state were to develop suddenly as the laser power is increased. Possible explanations are discussed