“…Within the plethora of non-linear effects already discovered [19], one of the most promising is High Harmonic Generation (HHG). First observed in gases, this phenomenon has also been induced by strong fields in atoms and molecules [20][21][22][23][24][25][26], the surfaces of both metal and dielectric solids [27][28][29][30][31], and in bulk crystals [32][33][34][35][36][37][38][39][40][41]. The ability to generate light at frequencies many multiples greater than the initial excitation provides a tool for observation and manipulation at the attosecond timescale [42][43][44][45].…”