Nonlinear Optical Effects in Weyl Semimetals and Other Strongly Correlated Materials

Abstract

The study of Weyl semimetals has been of interest since their first prediction in pyrochlore iridates in 2011 and their experimental discovery in TaAs in 2015. Since then, several classifications of Weyl semimetals have been identified, including electronic and magnetic, type-I and type-II, chiral, and multifold. In addition, the nonlinear optical properties of these materials have stood out as being particularly remarkable. This is due in part to the topological nature of Weyl semimetals, which affects the strength of the nonlinear properties and yields the potential for quantization, and in part due to the symmetry properties of Weyl semimetals, which dictates the anisotropy of these effects.

Second harmonic generation and the photogalvanic effect have both been shown to be of particular interest in Weyl semimetals. A strong second harmonic response has already been identified in type-I Weyl semimetals even at optical wavelengths. However, there is still some discussion as to whether this is directly attributable to the topological nature of the type-I semimetal band structure. Photocurrent measurements, and particularly the circular photogalvanic effect, have also stood out as a potential probe of the topology of the Weyl semimetal band structure. Yet experiments to date have attributed photocurrent generation to multiple physical origins, including the linear and circular photogalvanic effects, photon drag, and the photothermal, photoelectric, and photovoltaic effects.

In this thesis, we present a survey of a variety of nonlinear optical studies on several Weyl semimetals. In particular, we study the second harmonic responses of the type-II Weyl semimetals Td-WTe2 and Td-MoTe2 and of the chiral Weyl semimetal CoSi. We look at the rotational anisotropy of those responses and use point-symmetry analyses to attribute that anisotropy to the symmetries of the crystalline structures of these materials. We also characterize the sizes of the responses in these materials. Although we cannot identify with certainty a topological contribution to the strength of the second harmonic, our analysis suggests that the topology of these materials may play some role in the outsized responses observed.

We further present information on impulsive stimulated Raman scattering in the type-II Weyl semimetal Td-WTe2, which is observed through excitations of the 0.25 THz shear mode in a time-resolved optical reflectivity experiment. By analyzing the phase of the excitation of this mode, we also see indications of the shear displacement through the Pockels effect. Additionally, the asymmetric Fano line shape of this mode suggests possible coupling to the Weyl fermion quasiparticle excitations in this material.

We also study photocurrent generation in the chiral Weyl semimetal CoSi. We survey several facets of this crystal and use a point-symmetry analysis to pin down the origins of both a linear and circular photogalvanic effect in this material. Spatially resolved photocurrent measurements suggest that the experimental geometry used can potentially cause extraneous polarization-dependent photocurrent responses in the case of laser illumination of the electronics on the sample surface.

Finally, one of the defining characteristics which allows for the existence of the Weyl semimetal state is strong spin-orbit coupling. Thus, I end by presenting a study of the second harmonic response in another strongly correlated material, the complex oxide RbFe(MoO4)2. Investigating the structural phase transition of this material, we identify for the first time a ferrorotational ordering, and offer a point-symmetry analysis to identify potential coupling fields for this new electronic state.