UC Berkeley

Nonlinear optical properties of materials offer a diverse and powerful experimental probe into symmetry, electronic properties, and band structure. When paired with variable-wavelength light sources, these techniques are enhanced with the capability of resolving energy-dependent features. These properties are restricted to space groups that break inversion symmetry, but this dependence on crystal structure provides an opportunity to gain information about the material, and can help distinguish various electronic effects within a crystal. This point is crucial to this work, where bulk and surface optical properties in the Weyl semimetal RhSi are completely disentangled through symmetry considerations alone, with no need to rely on conjectures or assumptions about spectral features.

Weyl semimetals are a class of materials of special interest in condensed matter physics community within the last 10 years because their electronic band structures contain features which are analogous to Weyl fermions, a class of chiral fundamental particles which have not been observed in nature. While studied extensively experimentally, certain properties of Weyl semimetals have eluded measurement and theories have gone unconfirmed. One issue is that most Weyl semimetals only resemble an ideal model over a small energy range, limiting the optical sources available to study them within a relevant photon energy. Weyl semimetals may also contain trivial bands away from the Weyl fermion structures whose properties can be difficult to disentangle from those of interest.

In this work, nonlinear optical properties of the Weyl semimetal RhSi are measured. In particular, the theory of the Quantized Circular Photogalvanic Effect [35] in Weyl semimetals, a bulk property, is tested via measurements of terahertz generation and radiation. Precise numerical agreement with the theory is ruled out for the material, although we find the qualitative features of the prediction to hold true. Similar measurements are made in order to examine surface states on RhSi which have proven difficult to experimentally study with traditional transport and optical techniques. The bulk and surface properties are distinguished by considering experimental geometry in concert with crystal symmetry and orientation.