Abstract
The interaction of intense femtosecond pulses with metals allows for generating ultrashort hard x-rays. In contrast to plasma theories, tunneling from the target into vacuum is introduced as electron generation step, followed by vacuum acceleration in the laser field and re-entrance into the target to generate characteristic x-rays and Bremsstrahlung. For negligible space charge in vacuum, the Kα flux is proportional to the incident intensity and the wavelength squared, suggesting a strong enhancement of the x-ray flux by mid-infrared driving pulses. This prediction is in quantitative agreement with experiments on femtosecond Cu Kα generation.
Highlights
Hard x-ray generation has exploited electrons which are accelerated outside a metal target and produce characteristic x-ray emission and Bremsstrahlung after re-entering it
024102-8 Weisshaupt et al FIG. 4. (a) Electron extraction probability due to quantum mechanical tunneling as a function of the electric field amplitude perpendicular to the metal surface E? 1⁄4 Einj1 þ rpðk; hÞj sinðhÞ. (b) Generated x-ray flux per surface area, i.e., Ka photons into solid angle 4p/lm2 as a function of E? for a 50 fs driver pulse at k 1⁄4 0.8 lm and an 80 fs pulse at k 1⁄4 3.9 lm
After re-entering the metal target, the accelerated electrons do not experience the electric field of the laser anymore and are decelerated by energy loss mechanisms within the metal
Summary
Non-perturbative light-matter interactions are induced by optical fields which are comparable to or stronger than intraatomic or interionic electric fields, resulting in light-driven coherent electron dynamics on the time scale of the optical cycle, ionization processes, and plasma generation from dense materials. Apart from their fundamental relevance for probing nonequilibrium properties of matter, such phenomena have been exploited extensively for generating short-wavelength radiation from the extended ultraviolet (EUV) to the hard x-ray range. High-harmonic generation from gaseous targets, which covers the EUV and soft x-ray range, is based on light-induced tunneling of electrons into vacuum, field-driven coherent motion of the free electron, and recombination with the ionized atom. Hard x-ray generation has exploited electrons which are accelerated outside a metal target and produce characteristic x-ray emission and Bremsstrahlung after re-entering it. Non-perturbative light-matter interactions are induced by optical fields which are comparable to or stronger than intraatomic or interionic electric fields, resulting in light-driven coherent electron dynamics on the time scale of the optical cycle, ionization processes, and plasma generation from dense materials.. Non-perturbative light-matter interactions are induced by optical fields which are comparable to or stronger than intraatomic or interionic electric fields, resulting in light-driven coherent electron dynamics on the time scale of the optical cycle, ionization processes, and plasma generation from dense materials.1–7 Apart from their fundamental relevance for probing nonequilibrium properties of matter, such phenomena have been exploited extensively for generating short-wavelength radiation from the extended ultraviolet (EUV) to the hard x-ray range.. The space charge enhances the x-ray flux for k < 3.5 lm slightly and causes a gradual saturation for longer wavelengths
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