High Order Harmonic Generation (HHG) Facility

An HHG facility is available on the 1 kHz laser system.

The facility, developed by the LUXOR (INFM-CNR) Laboratory in Padova, consists of a generation chamber and a monochromator, and is schematically shown in fig. 1.
 

Before entering into a continuously variable attenuator (CVA, from Altechna Ltd.), the beam waist size is halved by means of a telescope (1a, 1b). The CVA consists of a half wave plate (2a) and two Brewster mirrors (2b). The near-IR vertically polarized beam is then directed onto a 0.2 mm thick Beta Barium Borate (BBO) type I non-linear crystal (4) in order to generate the second harmonic (SH) of the fundamental Ti:Sapphire laser emission. Two high reflectivity band-pass mirrors, set at 400 nm, are used to reflect the generated horizontally polarized SH beam filtering the residual near-IR field (5).
Depending on the chosen experimental conditions it is possible to reflect, into the high-vacuum “generation” chamber (GC), the near-IR or the SH beams keeping blocked the previous one by means of a third band-pass (removable) mirror (6c).
The generation chamber is kept at a background pressure of approximately 10^-7 mbar.
Two plane-convex lenses, with 250 mm (6a) and 170 mm (6b) focal lengths respectively, can focus the near-IR or the SH beams onto a static (cylindrical shaped) cell, filled with inert gas (A).
The gas cell is mounted onto a three dimensional manipulator stage in order to optimize the alignment between the inert gas target and the incoming laser beam. In addition, the possibility to control the position of the gas cell along the axial coordinate allows to optimize the phase-matching conditions in order to maximize the EUV photons flux.
The generated high-order harmonics propagate collinearly with the laser beam: a monochromator (MC) and a detector (Channeltron ) are set to study the spectral properties of the generated photons.
The generation chamber (GC) and monochromator one (MC) are separated by a 200 nm thick aluminium filter.
The monochromator consists of a gold coated spherical grating (1000 mm radius) and an exit slit. The grating has 576 grooves/mm and blazed profile. The distances from the grating to the exit slit and from the grating to the source are 350 and 450 mm, respectively.
In fig. 2 is shown a typical high-order harmonics spectrum generated by near-IR (red spectrum) and its SH (blue spectrum) in Ne, at the operating gas pressure, measured in the generation chamber, of 1 x 10^-3 mbar.
 

For further information:

Alberto Simoncig
Giulio Vampa