Harmonic generation in the limit of ultra-steep density gradients is studied experimentally. Observations demonstrate that while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale-lengths (Lp/λ < 1) the absolute efficiency of the harmonics declines for the steepest plasma density scale-length Lp → 0, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the Relativistic Oscillating Mirror (ROM) was estimated to be in the range of 10 −4 − 10 −6 of the laser pulse energy for photon energies ranging from 20 − 40 eV, with the best results being obtained for an intermediate density scale-length.PACS numbers: 52.59.Ye, 52.38.-r Keywords: surface high-harmonic generation, relativistic laser plasma interaction, attosecond pulse generation Ultrashort XUV pulses are a promising tool for a wide range of applications including attosecond laser physics and seeding of free-electron X-ray lasers. Typically, they are created by the nonlinear frequency up-conversion of an intense femtosecond driving laser field in a gaseous medium. Remarkable progress has been made to the present date with efficiencies reaching the level of 10 −4 at 20 nm wavelengths [1,2]. Such efficiencies are not yet available at shorter wavelengths or for attosecond pulse generation and the low intensities at which harmonic conversion takes place in gaseous media, makes harnessing the high peak power in the 0.1−1PW regime challenging. High-harmonic generation at a sharp plasma-vacuum interface via the Relativistically Oscillating Mirror (ROM) mechanism [3] is predicted to overcome these limitations and result in attosecond pulses of extreme peak power [4,5].While other mechanisms such as Coherent Wake Emission (CWE) can also emit XUV harmonics [6], the ROM mechanism is generally reported to dominate in the limit of highly relativistic intensities, where the normalized vector potential a 2 0 = Iλ 2 /(1.37 · 10 18 µm 2 W/cm 2 ) 1. The efficiency of ROM harmonics is predicted to converge to a power law for ultra-relativistic intensities [7], such that the conversion efficiency is given by η ≈ (ω/ω 0 ) −8/3 up to a threshold frequency ω t ∼ γ 3 , beyond which the spectrum decays exponentially. Here, γ is the maximum value of the Lorentz-factor associated with the reflection point of the ROM process. While these predictions correspond well with the observations made in experiments using pulse durations of the order of picoseconds in terms of highest photon energy up to keV [8,9] and the slope of the harmonic efficiency [10], no absolute efficiency measurements have been reported to date.The plasma density scale-length plays a critical role in determining the response of the plasma to the incident laser radiation. In the picosecond regime, the balance between the laser pre...

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Optical coherence tomography (OCT) is a non-invasive technique for cross-sectional imaging. It is particularly advantageous for applications where conventional microscopy is not able to image deeper layers of samples in a reasonable time, e.g. in fast moving, deeper lying structures. However, at infrared and optical wavelengths, which are commonly used, the axial resolution of OCT is limited to about 1 μm, even if the bandwidth of the light covers a wide spectral range. Here, we present extreme ultraviolet coherence tomography (XCT) and thus introduce a new technique for non-invasive cross-sectional imaging of nanometer structures. XCT exploits the nanometerscale coherence lengths corresponding to the spectral transmission windows of, e.g., silicon samples. The axial resolution of coherence tomography is thus improved from micrometers to a few nanometers. Tomographic imaging with an axial resolution better than 18 nm is demonstrated for layer-type nanostructures buried in a silicon substrate. Using wavelengths in the water transmission window, nanometer-scale layers of platinum are retrieved with a resolution better than 8 nm. XCT as a nondestructive method for sub-surface tomographic imaging holds promise for several applications in semiconductor metrology and imaging in the water window.

Experimental results on relativistic surface HHG at a repetition rate of 10 Hz are presented. Average powers in the 10 µW range are generated in the spectral range of 51 to 26 nm (24-48 eV). The surface harmonic radiation is produced by focusing the second-harmonic of a high-power laser onto a rotating glass surface to moderately relativistic intensities of 3 × 10 19 W cm −2 . The harmonic emission exhibits a divergence of 26 mrad. Together with absolute photon numbers recorded by a calibrated spectrometer, this allows for the determination of the extreme ultraviolet (XUV) yield. The pulse energies of individual harmonics are reaching up to the µJ level, equivalent to an efficiency of 10 −5 . The capability of producing stable and intense high-harmonic radiation from relativistic surface plasmas may facilitate experiments on nonlinear ionization or the seeding of free-electron lasers.