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Weyl semimetals are a group of materials with a single touching point of valence and conduction bands and a linear electron dispersion in the adjacent electronic bandstructure, giving rise quasi-massless electrons and topologically protected surface states. However, previous works on Weyl semimetals were largely limited to bulk material and therefore limited in means of flexibility and on-chip device applications.
Our work demonstrates first steps towards nanophotonics applications with Weyl semimetals by thoroughly characterizing the linear and nonlinear optical properties of recently realized epitaxial grown thin films of the prototypical Weyl semimetal niobium phosphide (NbP). Despite the dominant absorption over the entire spectrum, pronounced third-harmonic generation could be detected, reaching a maximum efficiency of 10-4 %, being orders of magnitude higher than for common dielectric materials. Furthermore, non-degenerate pump-probe measurements with 10 fs pulses were performed to investigate the ultrafast electronic and optical response of bulk NbP crystal and NbP thin film. For the first, absorption effects and subsequent scattering processes dominate the ultrafast response and show the characteristics of the linear dispersion regime in the electronic bandstructure. On the other hand, the NbP thin films reveal a response that is generated by the nonlinear optical Kerr effect and gives an ultrafast modulation depth of close to 1 % and with a complete relaxation within 100 fs. These findings would allow all-optical switching based on NbP with full-width half maximum bandwidth of 10-20 MHz.
Altogether, our results show that the epitaxial grown NbP thin films inherit the optical properties of the bulk crystal and further boost the nonlinear optical interactions. Combined with the possibility of common top-down fabrication techniques enabled by thin films, this paves the way for future applications of nanophotonic devices based on the promising properties of Weyl semimetals.
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