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We report a continuously operated Cu Kα x-ray source produced by a commercial kilohertz submillijoule femtosecond laser system. The source has an x-ray conversion of ∼4×10−5 into Kα line emission at 8.05 keV. The microplasma x-ray source has a size of 8 μm (full width at half maximum) produced by focusing 260 μJ laser pulses on a moving Cu-wire target. An average photon flux of ∼1.1×109 photons/sr/s is obtained using the above laser pulses. The source has been used to record phase contrast images of test samples. This compact x-ray source can serve as a low cost operating system for phase contrast imaging in clinical applications.

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An optical damage threshold for crystalline silicon from single femtosecond laser pulses was determined by detecting a permanent change in the refractive index of the material. This index change could be detected with unprecedented sensitivity by measuring the resonant wavelength shift of silicon integrated optics microring resonators irradiated with femtosecond laser pulses at 400 nm and 800 nm wavelengths. The threshold for permanent index change at 400 nm wavelength was determined to be 0.053 ± 0.007 J/cm2, which agrees with previously reported threshold values for femtosecond laser modification of crystalline silicon. However, the threshold for index change at 800 nm wavelength was found to be 0.044 ± 0.005 J/cm2, which is five times lower than the previously reported threshold values for visual change on the silicon surface. The discrepancy is attributed to possible modification of the crystallinity of silicon below the melting temperature that has not been detected before.