Abstract
The synthesis of isolated attosecond pulses (IAPs) in the extreme ultraviolet (XUV) spectral region has opened up the shortest time scales for time-resolved studies. It relies on the generation of high-order harmonics (HHG) from high-power few-cycle infrared (IR) laser pulses. Here we explore experimentally a new and simple route to IAP generation directly from 35 fs IR pulses that undergo filamentation in argon. Spectral broadening, self-shortening of the IR pulse and HHG are realized in a single stage, reducing the cost and experimental effort for easier spreading of attosecond sources. We observe continuous XUV spectra supporting IAPs, emerging directly from the filament via a truncating pinhole to vacuum. The extremely short absorption length of the XUV radiation makes it a highly local probe for studying the elusive filamentation dynamics and in particular provides an experimental diagnostic of short-lived spikes in laser intensity. The excellent agreement with numerical simulations suggests the formation of a single-cycle pulse in the filament.
Highlights
Experimental setupIn our experiment, sketched in figure 1, a commercial titanium–sapphire amplifier system (Dragon, KMLabs Inc.) delivers 35 fs pulses with a central wavelength of 780 nm and 1 mJ pulse energy at 3 kHz repetition rate
We have shown that high-order harmonics can be generated and extracted directly from a filament
We attribute this to the formation of intensity spikes in the laser field at certain positions
Summary
In our experiment, sketched in figure 1, a commercial titanium–sapphire amplifier system (Dragon, KMLabs Inc.) delivers 35 fs pulses with a central wavelength of 780 nm and 1 mJ pulse energy at 3 kHz repetition rate. An aperture of 7.5 mm diameter transmits 80% of the pulse energy, which yields about 21 GW peak power and 4.1 times the critical power for self-focusing in 1 atm argon, estimated with the nonlinear index coefficient n2 = 1.74 × 10−19 cm W−1. A laser-drilled pinhole in a metal plate (diameter: 500–800 μm) truncates the filament abruptly by terminating the high-pressure cell. Behind a second laser-drilled pinhole (diameter: about 250 μm) placed 1 cm distance from the first one, the background pressure is below 5 × 10−4 mbar. The absorption length at 1 atm is 10 μm for 20 eV radiation, and 50 μm for 38 eV, which is the high end of the observed harmonic spectra. After filtering by a 200 nm thick aluminum foil at 1 m distance from the pinholes, the harmonic radiation is recorded by using two XUV spectrometers (LHT 30, Horiba-Jobin-Yvon with 500 lines mm−1; 248/310-G, McPherson, with 300 lines mm−1 and CCD DH420A-F0, ANDOR Technology)
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