Au Nanoplasma as Efficient Hard X-ray Emission Source

Abstract

Hard X-ray generation up to 15 keV (∼1 A) from Au nanoparticle suspensions was systematically investigated for different particle diameters ranging from 10 to 100 nm with a narrow size distribution of ±2%. Scaling law of X-ray generation is close to a 6-photon process before the onset of saturation for excitation by 40 fs laser pulses with central wavelength of 800 nm. This is consistent with bulk plasmon excitation at λpbulk ≃ 138 nm. The longitudinal E-field component due to nanoparticle focusing is responsible for the excitation of the longitudinal bulk plasmon. The proposed analysis also explains X-ray emission from water breakdown via an electron solvation mechanism at ∼6.2 eV. The most efficient emission of X-rays was observed for 40 ± 1 nm diameter nanoparticles which also had the strongest UV extinction. X-ray emission was the most efficient when induced by prechirped 370 ± 20 fs laser pulses and exhibited the highest intensity at a negative chirp.