Evidence of coherence in strong-field electron photoemission from a semiconductor

Strong-field quantum electronics is emerging as a potential candidate in information processing but still coherence vs decoherence is a primary concern of the concept. Strong-field coherent processes in band gap materials have led during the last decade to the emergence of high harmonic generation in semiconductors, petahertz electronics, or strong-field quantum states. However, the coherent behavior of the sub-optical cycle-driven electrons has never been directly observed. We report here on the experimental evidence of coherent ultrashort emission of hot electrons from a nanostructured semiconductor. Our method uses sub-wavelength electric field enhancement to localize the electron emission within a nanometer-scale spot. We found similarities with the electron emission from metallic nanotips in the strong-field regime, a topic that has opened a vast domain of applications during the last decade. The electron spectra display both odd and even harmonic orders of the driving femtosecond laser frequency, a signature of the coherent nature of the electron emission and their attosecond timing. Our findings complete our knowledge of phenomena governing coherent strong-field processes in semiconductors and open perspectives for the generation of future quantum devices operating in the strong-field regime.