  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.
