A gas or solid dielectric illuminated with a very high intensity infrared laser can emit high-order laser harmonics by means of a non-linear optical mechanism. The intense illumination (terawatt/cm²) needed to generate harmonics is most often obtained by playing with the shortness of the laser exciter pulses (femtosecond: 10^-15 s).

The researchers illuminated a ZnO crystal with a 21-watt femtosecond infrared laser (3.2 µm wavelength) and obtained the blue emission of the seventh-order harmonic at 0.457 µm. They repeated the experiment with a crystal whose surface was "micro-structured" by engraving a network of twenty-five cones, resulting in the observation of a much more intense blue emission from the cone tips. The harmonic generation was thus reinforced (gain of 10 to 30) for the "weakest" laser illuminations (<0.35 terawatts/cm²).

What is happening here? Because of their size, resonant electronic oscillations (plasmons) are excited within the micro-cone tips, which increases the electric field and enhances the non-linear effect.

Thanks to this result, the physics of strong fields is becoming accessible to the laser capacities available in laboratories.

This work was conducted with researchers from the Institute of Photonic Sciences (ICFO) in Barcelona and the University of Hanover (Germany).