Solid-state harmonics beyond the atomic limit

A direct comparison of high harmonic generation in the solid and gas phases of Ar and Kr reveals higher harmonics in these rare-gas solids caused by strong interband couplings; evidence of recollisions implies that gas-phase techniques for attosecond pulse generation and orbital tomography could be adapted for solids. Harmonics in solid- and gas-phase Ar and Kr In recent years the technique of high harmonic generation of light, known for decades for gases, has been be translated to solids, opening up a range of possible applications in table-top short-wavelength light sources and in exploring matter at ultrafast timescales. However, the underlying physical mechanisms are still under debate. Here Georges Ndabashimiye et al . report on high harmonic generation experiments in single-element systems, where modelling is not complicated by covalent bonding, as it has been for solids such as ZnSe and GaSe in previous studies. They study argon and krypton in both solid and gas form and observe high harmonic spectra for the solid phases that extend much further than for the gas form. This effect could be explained by strong interband couplings. Strong-field laser excitation of solids can produce extremely nonlinear electronic and optical behaviour. As recently demonstrated, this includes the generation of high harmonics extending into the vacuum-ultraviolet and extreme-ultraviolet regions of the electromagnetic spectrum 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 . High harmonic generation is shown to occur fundamentally differently in solids and in dilute atomic gases 1 , 2 , 3 , 4 , 5 , 6 , 9 , 10 , 11 , 12 , 13 . How the microscopic mechanisms in the solid and the gas differ remains a topic of intense debate 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 14 , 15 , 16 , 17 , 18 . Here we report a direct comparison of high harmonic generation in the solid and gas phases of argon and krypton. Owing to the weak van der Waals interaction, rare (noble)-gas solids are a near-ideal medium in which to study the role of high density and periodicity in the generation process. We find that the high harmonic generation spectra from the rare-gas solids exhibit multiple plateaus extending well beyond the atomic limit of the corresponding gas-phase harmonics measured under similar conditions. The appearance of multiple plateaus indicates strong interband couplings involving multiple single-particle bands. We also compare the dependence of the solid and gas harmonic yield on laser ellipticity