  We report on experimental results in a new regime of a relativistic
light-matter interaction employing mid-infrared (3.9-micrometer wavelength)
high-intensity femtosecond laser pulses. In the laser generated plasma, the
electrons reach relativistic energies already at rather low intensities due to
the fortunate lambda^2-scaling of the kinetic energy with the laser wavelength.
The lower intensity suppresses optical field ionization and creation of the
pre-plasma at the rising edge of the laser pulse efficiently, enabling an
enhanced efficient vacuum heating of the plasma. The lower critical plasma
density for long-wavelength radiation can be surmounted by using nanowires
instead of flat targets. In our experiments about 80% of the incident laser
energy has been absorbed resulting in a long living, keV-temperature,
high-charge state plasma with a density of more than three orders of magnitude
above the critical value. Our results pave the way to laser-driven experiments
on laboratory astrophysics and nuclear physics at a high repetition rate.
