Extreme ultraviolet (XUV) single-order high harmonic pulses with 10 6 photons/pulse were separated from multiple harmonic orders by a time-compensated toroidal-grating monochromator consisting of a pair of toroidal gratings. The first grating separates the harmonic order and the second one compensates for the pulse-front tilt. The center photon energies were tunable between 42 and 23 eV. The separated single harmonic pulses were spatially and temporally characterized as having a spot size of 58 ± 3 µm at the focus, with a shortest pulse duration of 47 ± 2 fs. The developed XUV light source is versatile for application to time-and space-resolved spectroscopy.

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Scanning near-field optical microscopy using a slit-type probe is discussed. The slit-type probe has a width of much less than a wavelength, λ, and a length on the order of λ, and thus has high transmission efficiency. Two dimensional near-field images of objects have been constructed using an image reconstruction algorithm based on computerized tomographic imaging. Experiments performed at 60 GHz (λ=5 mm) show that this type of near-field microscopy can achieve a spatial resolution of better than λ/45 for two dimensional imaging. A method for fabricating a submicron width slit probe at the end of an optical fiber is presented for extending this microscopy to optical waves.

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We experimentally demonstrate a two-color two-photon resonant excitation of the doubly excited 2p 2 1 S state in helium atoms by the combination of 19th and 21st harmonic photons of a Ti:sapphire laser. Production of the 2p 2 1 S state is confirmed by the experimental observation that the electron emission from this state does not depend on the direction of harmonic polarization. Our ab-initio theoretical results through the solution of time-dependent Schrödinger equation are consistent with the experimental results and confirm the first successful production of a doubly excited state by high-harmonic pulses.

We experimentally demonstrate a two-color two-photon resonant excitation of the doubly excited 2p 2 1 S state in helium atoms by the combination of 19th and 21st harmonic photons of a Ti:sapphire laser. Production of the 2p 2 1 S state is confirmed by the experimental observation that the electron emission from this state does not depend on the direction of harmonic polarization. Our ab-initio theoretical results through the solution of time-dependent Schrödinger equation are consistent with the experimental results and confirm the first successful production of a doubly excited state by high-harmonic pulses.

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