Topological high-order harmonic spectroscopy in Communications Physics

It is reported that researchers from the University of Salamanca in Spain have demonstrated a high-order harmonic spectroscopy scheme generated by the interaction between a structured driving beam and a crystal solid target. This work promotes the topological analysis of high-order harmonic fields as a spectroscopic tool to reveal nonlinearity in the coupling of light and target symmetry. The relevant paper was published in Communications Physics under the title of "Topological high molecular spectroscopy copy".

High order harmonic generation (HHG) is an extreme nonlinear effect that occurs when a strong field laser is focused on a gas medium, resulting in hundreds of orders of high-energy harmonic photons.

In the paper, researchers demonstrated the high-order harmonic spectroscopy scheme generated by the interaction between structured driving beams and crystal solid targets. Unlike isotropic gas targets, researchers have demonstrated the coupling of crystal symmetry with the driving beam topology during high-order harmonic generation (HHG) processes. This coupling feature is encoded into a complex spatial structure that emits harmonics. In particular, researchers have revealed this interwoven photon conversion by studying the HHG of monolayer graphene driven by LPVB.

Figure 1: Overview of topological high-order harmonic spectra in graphene and argon gas.

Figure 2: Far field harmonic emission curves of circularly polarized components on the left (LCP) and right (RCP) sides.

Figure 3: Comparison of orbital angular momentum (OAM) carried by high-order harmonics emitted from anisotropic and isotropic targets.

Researchers have found that, unlike isotropic cases, the harmonics generated by crystal targets can break the conservation of the driving topology based on their compositional symmetry. Researchers have provided an analytical derivation that can (1) predict the topology of high-order harmonic beams from the anisotropic symmetry of the target, and (2) retrieve the anisotropic response of the target from the topology of high-order harmonic beams. Therefore, high-order harmonic spectroscopy based on topological structure can extract spatial resolution information of target nonlinear response, which cannot be obtained by standard spectroscopy techniques.

Figure 4: Near field harmonic emission profiles obtained in anisotropic and isotropic targets.

Figure 5: Retrieve nonlinear response from topological harmonic characteristics.

Although researchers have demonstrated the interaction between the topological structure of vector beam drivers and target symmetry in two-dimensional materials such as graphene, they believe that their research results open up a universal scenario for topological optics, where the non-linear response of the target is coupled with the topological structure of light. Researchers believe that this technology can be further used to characterize more complex targets, such as polycrystalline or heterostructures.

Source: Sohu