Tu4A.4.pdf Latin America Optics and Photonics Conference © OSA
2018
Below Threshold Harmonics Beams Characterization Using
the Knife-Edge Technique
Armando V. F. Zuffi, Andreia A. Almeida, Nilson D. Vieira Jr., Ricardo E. Samad*
Centro de Lasers e Aplicações – IPEN-CNEN/SP – Av. Prof. Lineu Prestes 2242, Cidade Universitária, 05508-000, São Paulo, SP, Brazil
*resamad@gmail.com
Abstract: Using the knife-edge method we have measured gas generated BTH beams sizes in the
VUV region and calculated their divergences, geometrically characterizing this light source.
OCIS codes: (140.7240) UV, EUV and X-ray lasers; (140.7090) Ultrafast lasers.
Introduction
The generation of harmonics below the ionization threshold in gases by ultrashort laser pulses has been widely
studied due to its enormous potential for coherent light generation in the VUV region, being of fundamental interest
for time resolved spectroscopy [1] and to study the electronic dynamics close to the gas ionization energy[2].
In our laboratory, Below Threshold Harmonics (BTH) are being generated in order to provide a light source in the
UV and VUV regions for future applications. For this, it was decided to geometrically characterize the BTH beams,
besides other characterizations of the harmonics. In this work, we report the generation of BTH in argon flowing
through a nozzle inside a vacuum chamber. Were generated the 3
rd
(centered in 255.5 nm), 5
th
(152.5 nm) and 7
th
(108.0 nm) harmonics, and here we describe these harmonics beam characterization using the knife-edge method.
Experimental Setup
BTH (3
rd
to 7
th
harmonic) were generated using up to 650 µJ, of 25 fs pulses centered at 785 nm, at 4 kHz repetition
rate (Femtolasers Femtopower Compact Pro HR/HP). The laser pulses were focused, by a f=50 cm lens, on a glass
gas nozzle with a 2.7 mm gas interaction length, through which Argon flowed at a 100 mbar pressure, placed inside a
vacuum chamber with a background pressure under 10
-6
mbar. The harmonics were separated by a VUV
monochromator (McPherson 234/302) and detected by a scintillator and a photomultiplier tube, as shown in Figure 1.
To determine the harmonics beams sizes and divergences, it was assumed that they were TEM
00
Gaussian beams, and
two knife edges (KE1 and KE2) were mounted inside the vacuum chamber on displacement stages with vacuum
compatible computer controlled actuators (Newport NSA12V6), at 65 and 221 mm from the nozzle. The
monochromator entrance slit was 820 mm away from the second knife-edge, and its aperture was set at 2 mm, larger
than the beams at its position. The exit slit aperture was also set at 2 mm.
Figure 1. scheme of the experimental setup in which the harmonics beams were generated and measured.
Knife-edges measurements were performed reading the PMT signal in a lock-in amplifier triggered by the pulses.
The obtained data were then fitted by[3]:
bg x x
w
erf amp f
) (
2
1
2
1
0
(1)
where w is the beam spot size defined in the usual way (radial distance where the intensity drops to 1/e
2
of the peak),
x
0
is the peak position, amp is the function amplitude and bg is a background value. The measured values of w at the