中央研究院 原子與分子科學研究所

This paper discusses numerical and experimental results on frequency downshifting and upshifting of a 10-um infrared (IR) laser to cover the entire wavelength (frequency) range from wavelength=1-150 um (frequency=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 um) 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 um) pulses can be generated by replacing the drive laser with a picosecond 10 um 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 um by simply changing the electron density of the front. While the upshifted light with 5 < wavelength(um) < 10 propagates in the forward direction, that with 1 < wavelength(um) < 5 is back-reflected. These two plasma techniques seem extremely promising for covering the entire molecular fingerprint region.