Abstract

In [1] and [2] high harmonic generation (HHG) of ultrashort laser radiation in a gas jet is reported. To increase conversion efficiency the power of the laser pulses is enhanced in an external cavity. The high harmonics are coupled out geometrically through a small hole with ~100 µm diameter [1] or a slit with ~100 µm width [2] in one of the cavity mirrors. The mirror surface of such an output coupler should besides the opening remain unaffected by the manufacturing process in terms of contamination and deformation to minimize losses which decrease enhancement and thus lower harmonic yield. In addition to that the holes and slits need to be undercut by ~8° because the mirror is used under a finite angle of incidence. The openings have been structured into the mirror substrates by Inverse Laser Drilling (ILD) [3]. In this patented process [4], the uncoated mirror substrate is placed on a vertically movable translation table with the mirror surface facing upwards. The laser beam is focused through the top surface onto the bottom surface of the bulk. The beam is deflected by scanning mirrors in the horizontal plane; this way the opening geometry’s bottom layer is ablated. By moving the translation table and ablating layers step by step the entire geometry can be ablated or “drilled”. The laser source used in this case is a Diode Seeded Regenerative Amplifier [5], with ~100 µJ pulse energy and ~800 ps pulse duration. The described process showed to be well suited for manufacturing output couplers for cavity-enhanced HHG. However, the edges of all the openings are chipped, thus increasing the area causing losses to the cavity in addition to the free aperture (Fig. 1a). Minimization of the chipping should either improve enhancement or allow for a larger free aperture.

© 2015 IEEE