No abstract

We study the photoelectron energy spectrum in direct two-photon double ionization of He, and show that in certain ranges of photon frequency it competes favourably with sequential double ionization. When the photon frequency allows two-photon double ejection in both the sequential and direct channels, the latter appears as a background dominated by the peaks of the sequential channel. However, in a different frequency regime, where the sequential channel requires three photons, the spectrum of the direct channel is separated in energy and dominates in magnitude. Our results have been obtained for a range of radiation intensity and pulse duration reasonably expected in high-order harmonic generation sources.

The interest in terahertz photometric and imaging measurements has motivated the development of bandpass resonant filters to be coupled to multiple-pixel devices such as bolometer arrays. Resonant grids are relatively simple to fabricate, exhibiting high transmission at the central frequency, a narrow bandpass, and good rejection of the side frequencies of the spectrum. We have fabricated filters centered at different frequencies between 0.4 and 10 THz, using photolithography and electroforming techniques. Transmission measurements have shown center frequencies and bandwidths close to the design predictions. The performance of the filters was found not to be critically dependent on small physical deformations in the mesh, becoming more noticeable at higher frequencies (i.e., for smaller physical sizes). Wider bandwidths, needed to attain higher sensitivities in the continuum, were obtained by changing the design parameters for filters at 2 and 3 THz.

No abstract