  Nanoscale amplification of non-linear processes in solid-state devices opens
novel applications in nano-electronics, nano-medicine or high energy conversion
for example. Coupling few nano-joules laser energy at a nanometer scale for
strong field physics is demonstrated. We report enhancement of high harmonic
generation in nano-structured semiconductors using nanoscale amplification of a
mid-infrared laser in the sample rather than using large laser amplifier
systems. Field amplification is achieved through light confinement in
nano-structured semiconductor 3D waveguides. The high harmonic nano-converter
consists of an array of zinc-oxide nanocones. They exhibit a large
amplification volume, 6 orders of magnitude larger than previously reported and
avoid melting observed in metallic plasmonic structures. The amplification of
high harmonics is observed by coupling only 5-10 nano-joules of a 3.2 {\mu}m
high repetition-rate OPCPA laser at the entrance of each nanocone. Harmonic
amplification (factor 30) depends on the laser energy input, wavelength and
nanocone geometry.
