Photoinduced operation and diagnostic of superconductivity in the MgB2 films

The occurrence of the optical second harmonic generation (SHG) at fundamental wavelength of Er:glass nanosecond 1,540 nm stimulated by two coherent CO2 microsecond laser beams at wavelengths 10.6 and 5.3 μm was observed for the MgB2 films. The SHG signal has shown its sensitivity to the occurrence of the superconducting (SC) phase transition. The SHG achieves its maximum at temperatures higher with respect to the SC transition. The effect is reversible with respect to the photo induced treatment. After the switching off of the external photoinducing treatment the effect almost disappears. However, this still strongly affects the SC properties of the MgB2 thin layers, as the critical current is increased by 50 % after illumination.