  The brightness of a light source defines its applicability to nonlinear
phenomena in science. Bright low frequency terahertz (< 5THz) radiation
confined to a diffraction-limited spot size is a present hurdle due to the
broad bandwidth and long wavelengths associated with single-cycle terahertz
pulses as well as due to the lack of terahertz wavefront correctors. Here,
using a present-technology system, we employ a new concept of terahertz
wavefront manipulation and focusing optimization. We demonstrate a
spatio-temporal confinement of terahertz energy at its physical limits to the
least possible 3-dimensional light bullet volume of lambda cubic. This leads to
a new regime of extremely bright terahertz radiation reaching 40 PW/m2
intensity. The presented work is focused on the sub-5 THz range using small
aperture organic crystals DSTMS and OH1. The obtained peak field of up to 5
GV/m and 17 Tesla is order of magnitude higher than any reported single-cycle
field oscillation in the entire THz range from a laser-based system and
surpassing large scale accelerator systems. The presented results are foreseen
to have a great impact on future nonlinear terahertz applications in different
science disciplines.
