High Harmonics in Bulk Materials

When exciting crystals such as silicon by an intense elliptically or circularly polarized light pulse (red), circularly polarized higher harmonics (green & blue) can be generated. (Source: N. Tancogne-Dejean, J. M. Harms, MPSD)

The gene­ration of high-order harmonics in gases is nowadays routinely used in many different areas of sciences, ranging from physics, to chemistry and biology. This strong-field pheno­menon consists in conver­ting many low-energy photons coming from a very strong laser, to fewer photons with a higher energy. Despite the growing interest in this pheno­menon in solids, the mechanism behind the conver­sion of light is still under debate for solid materials. Scientists from the MPSD – Max Planck Institute for the Structure and Dynamics of Matter – and CFEL (Center for Free-Electron Laser Science) in Hamburg used state-of-the-art theo­retical simulation tools to advance the funda­mental under­standing of this pheno­menon in solids.

When atoms and molecules interact with strong laser pulses, they emit high-order harmonics of the funda­mental driving laser field. The high-harmonic gene­ration (HHG) in gases is regularly used nowadays to produce isolated atto­second pulses and coherent radiation ranging from visible to soft x-rays. Because of a higher elec­tronic density, solids are one promising route towards compact, brighter HHG sources. However, their use is currently hampered by the lack of a micro­scopic under­standing of the mecha­nism leading to HHG from solids.

Researchers from the MPSD and CFEL have shown that, by using ellip­tically polarized driving light, it is possible to unravel the complex inter­play between the two mecha­nisms respon­sible for HHG in solids. By means of extensive first-prin­ciples simu­lations they have shown how these two mechanisms are strongly and diffe­rently affected by the ellip­ticity of the driving laser field. The complex inter­play between these effects can be used to tune and improve harmonic emission in solids. In particular, they have shown that the maximal obtained photon energy can be increased by as much as 30% using a finite ellip­ticity of the driving laser field.

They also demonstrated the possi­bility of generating circu­larly polarized har­monics with alter­nating helicity from a single circularly polarized driving field, thus opening a new avenue for a better under­standing and control of HHG in solids based on ellip­ticity, with intri­guing new oppor­tunities in the spectro­scopy of magnetic materials. Their work shows that ellip­ticity provides an additional knob to experi­mentally control high-order harmonic generation in solids. (Source: MPSD)

Reference: N. Tancogne-Dejean et al.: Ellipticity dependence of high-harmonic generation in solids: unraveling the interplay between intraband and interband dynamics, Nat. Commun. 8, 745 (2017); DOI: 10.1038/s41467-017-00764-5

Links: Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany • Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany

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