High harmonic generation by resonant nano-antennas: Phase matching at the nanometer scale

Plasmon resonances amplify the electromagnetic fields near metallic nanostructures. Therefore, resonant nano-antenna structures have been recently considered in new schemes to generate extreme-ultraviolet (EUV)/soft x-ray radiation from high-order harmonic generation (HHG) driven by infrared laser pulses [1, 2], as shown in Fig. 1a. As a main advantage, the resonant enhancement of the infrared driving field at the nano-antenna's gap allows for harmonic generation using high-repetition low-power laser pulses, as those coming from an oscillator. On the other side, the small interaction volume reduces substantially the harmonic signal in comparison to a typical HHG experiment — a laser focused into a gas jet or cell.