As in the books of JRR Tolkien, the Research Group of Laser Applications and Photonics (ALF-USAL) continues their adventure in the search of the Ring.
After finding the Phase-Matched Ring in Argon based High-Harmonic Generation , now they’ve gone a step beyond to find the Phase-Matched Ring in graphene. It turns out that when a graphene layer is illuminated by an intense laser beam, ultrafast electronic dynamics result in the emission of higher frequency radiation.
This emission can be understood as a result of an excitation of electrons from the valence to the conduction band, a subsequent acceleration in such conduction band, and a final recombination with the holes that were left in the valence band. Such process, known as high-order harmonic generation, is very sensitive to the intensity of the incoming laser field, and thus to the spatial profile of the beam.
As a consequence, the radiation emitted from different parts of the graphene layer can interfere, both destructively or constructively. It turns out that an adequate matching between the emissions from different parts of the graphene layer is required to obtain bright high-frequency radiation.
In their latest work, the ALF-USAL group has demonstrated that when a graphene layer is illuminated by an intense Gaussian beam, the high-order harmonic emission is dominated by an annular region: the Phase-Matched Ring. This finding gives light into the macroscopic physics of high harmonic generation in graphene, offering a tool to engineer the process, and to increase its efficiency.
After finding the Phase-Matched Ring in Argon and in Graphene, the researchers of ALF-USAL group continue their adventures to rule intense light-matter interactions.
 “Carrier-envelope-phase insensitivity in high-order harmonic generation driven by few-cycle laser pulses”, C. Hernández-García, W. Holgado, L. Plaja, B. Alonso, F. Silva, M. Miranda, H. Crespo, and I. J. Sola, Optics Express 23, 21497 (2015)