Optical Response and Fabrication of ${\rm MgB}_{2}$ Nanowire Detectors

We report the fabrication of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanowire based on a liftoff-like technique and its optical response. A Si/C bilayer mask is formed as the negative pattern of the nanowire by e-beam lithography and the standard liftoff process. A 10 nm-thick MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin film is deposited on the pattern, which produces MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanowire with a width of down to 200 nm. We do not need to liftoff the Si/C bilayer because the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is well separated at the edge of the Si/C bilayer due to its overhang structure. The optical response at 1.5 mum wavelength is measured at 4.2 K. For a 300 nm-wide nanowire, photoresponse signals with the repetition rate of 100 MHz are observed, and all signals disappear as the laser intensity decreases below 1.8 times 10 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> photon/pulse. On the other hand, the signals become intermittent as the laser intensity decreases for a 200 nm-wide nanowire. The signal disappears below 4times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> photon/pulse. This shows that the nanowire works in the multi-photon detection regime.