Ultra-short pulse generation from mid-IR to THz range using plasma wakes and relativistic ionization fronts

Abstract

This paper discusses numerical and experimental results on frequency downshifting and upshifting of a 10 μm infrared (IR) laser to cover the entire wavelength (frequency) range from λ = 1 to 150 μm (ν = 300–2 THz) using two different plasma techniques. The first plasma technique utilizes frequency downshifting of the drive laser pulse in a nonlinear plasma wake. Based on this technique, we have proposed and demonstrated that in a tailored plasma structure, multi-millijoule energy, single-cycle, long-wavelength IR (3–20 μm) pulses can be generated by using an 810 nm Ti:sapphire drive laser. Here, we extend this idea to the THz frequency regime. We show that sub-joule, terawatts, single-cycle terahertz (2–12 THz or 150–25 μm) pulses can be generated by replacing the drive laser with a picosecond 10 μm CO2 laser and a different shaped plasma structure. The second plasma technique employs frequency upshifting by colliding a CO2 laser with a rather sharp relativistic ionization front created by ionization of a gas in less than half cycle (17 fs) of the CO2 laser. Even though the electrons in the ionization front carry no energy, the frequency of the CO2 laser can be upshifted due to the relativistic Doppler effect as the CO2 laser pulse enters the front. The wavelength can be tuned from 1 to 10 μm by simply changing the electron density of the front. While the upshifted light with 5<λ(μm)<10 propagates in the forward direction, that with 1<λ(μm)<5 is back-reflected. These two plasma techniques seem extremely promising for covering the entire molecular fingerprint region.