Highly enhanced hard x-ray emission from oriented metal nanorod arrays excited by intense femtosecond laser pulses

We report a 43-fold enhancement in the hard x-ray emission (in the 150--300 keV range) from copper nanorod arrays (compared to a polished Cu surface) when excited by 30-fs, 800-nm laser pulses with an intensity of ${10}^{16}$ W/cm${}^{2}$. The temperature of the hot electrons that emit the x rays is 11 times higher. Significantly, the x-ray yield enhancement is found to depend on both the aspect ratio as well as the cluster size of the nanorods. We show that the higher yield arises from enhanced laser absorption owing to the extremely high local electric fields around the nanorod tips. Particle-in-cell plasma simulations reproduce these observations and provide pointers to further optimization of the x-ray emission.