# Enhanced higher harmonic generation from nodal topology

## TL;DR

This paper demonstrates that the unique topological features of nodal loop semimetals enhance higher harmonic generation in the THz regime, offering a new physical property linked to their topology with potential technological applications.

## Contribution

It is the first work to show that nodal linkages in semimetals protect and enhance higher harmonic generation, connecting topology with nonlinear optical responses.

## Key findings

- Nodal linkages enforce non-monotonic intra-band responses.
- Enhanced HHG observed in known nodal materials.
- Potential applications in THz technology and material characterization.

## Abstract

Among topological materials, nodal loop semimetals (NLSMs) are arguably the most topologically sophisticated, with their valence and conduction bands intersecting along arbitrarily intertwined nodes. But unlike the well-known topological band insulators with quantized edge conductivities, nodal loop materials possess topologically nontrivial Fermi surfaces, not bands. Hence an important question arises: Are there also directly measurable or even technologically useful physical properties characterizing nontrivial nodal loop topology? In this work, we provide an affirmative answer by showing, for the first time, that nodal linkages protect the higher harmonic generation (HHG) of electromagnetic signals. Specifically, nodal linkages enforce non-monotonicity in the intra-band semi-classical response of nodal materials, which will be robust against perturbations preserving the nodal topology. These nonlinearities distort incident radiation and produce higher frequency peaks in the teraHertz (THz) regime, as we quantitatively demonstrate for a few known nodal materials. Since THz sources are not yet ubiquitous, our new mechanism for HHG will greatly aid applications like material characterization and non-ionizing imaging of object interiors.