Figure 4 shows the gain as a function of excitation intensity,as compared to the theory. The solid curve is the best fitting with P 0 = 3.91 × 10 − 8 Wcm − 2 and n = 1.05. Our data agrees well with the theory at relative high excitation intensity, showing a high gain up to 10 4 . And the responsivity of the device is calculated to be in the order of 10 4 AW − 1 , with the excitation intensity as low as 10 − 5 Wcm − 2 . At a lower excitation intensity ( P → 0), we predict the gain can be as high as 10 7 . To compare with the performance of ZnO nanowire