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

TL;DR

This paper presents novel plasma-based methods for generating ultra-short, tunable IR to THz pulses through frequency downshifting in plasma wakes and upshifting via relativistic ionization fronts, enabling broad spectral coverage.

Contribution

It introduces two innovative plasma techniques for generating and tuning ultra-short IR to THz pulses, extending capabilities beyond existing methods.

Findings

Generation of multi-millijoule IR pulses (3-20 μm) using plasma wake downshifting.

Production of sub-joule, terawatt, single-cycle THz pulses (2-12 THz) with a CO₂ laser.

Frequency upshifting of CO₂ laser pulses via relativistic Doppler effect at ionization fronts.

Abstract

This paper discusses numerical and experimental results on frequency downshifting and upshifting of a 10 $\mu$m infrared laser to cover the entire wavelength (frequency) range from $\lambda$=1-150 $\mu$m ($\nu$=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 $\mu$m) 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 $\mu$m) pulses can be generated by replacing the drive laser with a picosecond 10 $\mu$m CO$_2$ laser and a different shaped plasma structure. The second plasma technique employs frequency upshifting by colliding a CO$_2$ laser with a rather sharp relativistic ionization front created by ionization of a gas in less than half cycle (17 fs) of the CO$_2$ laser. Even though the electrons in the ionization front carry no energy, the frequency of the CO$_2$ laser can be upshifted due to the relativistic Doppler effect as the CO$_2$ laser pulse enters the front. The wavelength can be tuned from 1-10 $\mu$m by simply changing the electron density of the front. While the upshifted light with $5 <\lambda(\mu$m$)< 10$ propagates in the forward direction, that with $1 <\lambda(\mu$m$)< 5$ is back-reflected. These two plasma techniques seem extremely promising for covering the entire molecular fingerprint region.