The Taming of the Light Screw

When an intense laser field interacts with a crystalline solid, higher-order harmonic fields are emitted whose polarization states are determined by crystal symmetry and can be controlled by the strong-field dynamics. The colormap surface shows the ellipticity of the ninth harmonic from silicon. (Source: J. Harms, MPSD)

The nonlinear process of high-order harmonic generation (HHG) in gases is one of the corner­stones of attosecond science and is widely used in many different areas of science nowadays, ranging from physics to chemistry to biology. This strong-field phen­omenon converts many low-energy photons from an intense laser pulse into a photon of much higher energy. Whereas the HHG process is well understood in atomic and molecular gases, the mechanism underlying frequency conversion in solid materials is currently still the subject of scientific controversy.

By combining HHG experiments and state-of-the-art theoretical simu­lations, scientists from the Deutsches Elektronen-Synchro­tron (DESY) and the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science (CFEL) in Hamburg now introduce polari­zation-state-resolved high-harmonic spectro­scopy of solids, that permits deeper insights into both electronic and structural dynamics occurring on time scales shorter than one oscil­lation of the light field.

The emitted harmonic fields can oscillate in a linear fashion, or they can rotate ellip­tically or circularly with clockwise or anti­clockwise helicity – just like a screw of light. The scientists now reveal how the harmonics’ polari­zation states and their handedness encode valuable information on the crystal structure and ultrafast strong-field dynamics, and how the harmonics’ polari­zation states can be controlled. Moreover, since the harmonics are created within a single period of the incident driving field, the method inherently comes with a sub-optical-cycle temporal resolution.

The present work inves­tigates the prototype materials silicon and quartz to establish the new spectro­scopic technique. Yet the method is versatile and expected to find important appli­cations in future studies of novel quantum materials such as strongly correlated materials, topo­logical insulators, and magnetic materials. (Source: MPSD)

Reference: N. Klemke et al.: Polarization-state-resolved high-harmonic spectroscopy of solids, Nat. Commun. 10, 1319 (2019); DOI: 10.1038/s41467-019-09328-1

Link: Ultrafast Optics and X-Rays Division, CFEL, Hamburg, Germany

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