Broadband strong optical dichroism in topological Dirac semimetals with Fermi velocity anisotropy
Abstract
Prototypical threedimensional (3D) topological Dirac semimetals (DSMs), such as Cd_{3}As_{2} and Na_{3}Bi, contain electrons that obey a linear momentumenergy dispersion with different Fermi velocities along the three orthogonal momentum dimensions. Despite being extensively studied in recent years, the inherent Fermi velocity anisotropy has often been neglected in the theoretical and numerical studies of 3D DSMs. Although this omission does not qualitatively alter the physics of lightdriven massless quasiparticles in 3D DSMs, it does quantitatively change the optical coefficients which can lead to nontrivial implications in terms of nanophotonics and plasmonics applications. Here we study the linear optical response of 3D DSMs for general Fermi velocity values along each direction. Although the signature conductivityfrequency scaling, σ(ω) ∝ ω, of 3D Dirac fermion is wellprotected from the Fermi velocity anisotropy, the linear optical response exhibits strong linear dichroism as captured by the universal extinction ratio scaling law, Λ_{ij} = (v_{i}/v_{j})^{2} (where i ≠ j denotes the three spatial coordinates x,y,z, and v_{i} is the idirection Fermi velocity), which is independent of frequency, temperature, doping, and carrier scattering lifetime. For Cd_{3}As_{2} and Na_{3}Bi_{3}, an exceptionally strong extinction ratio larger than 15 and covering a broad terahertz window is revealed. Our findings shed new light on the role of Fermi velocity anisotropy in the optical response of Dirac semimetals and open up novel polarizationsensitive functionalities, such as photodetection and light modulation. *Project supported by Singapore Ministry of Education (MOE) Tier 2 Grant No. (2018T21007) and USA ONRG Grant No. (N629091912047). JL is supported by MOE PhD RSS. KJAO acknowledges the funding support of Xiamen University Malaysia Research Fund, Grant Nos. XMUMRF/2019C3/IECE/0003 and XMUMRF/2020C5/IENG/0025, and the Ministry of Higher Education Malaysia under the Fundamental Research Grant No. Scheme, Grant No. FRGS/1/2019/TK08/XMU/02. CZ acknowledges the funding support by the Australian Research Council (Grant No. DP160101474).
 Publication:

Chinese Physics B
 Pub Date:
 July 2020
 DOI:
 10.1088/16741056/ab928e
 arXiv:
 arXiv:2005.03363
 Bibcode:
 2020ChPhB..29g7802L
 Keywords:

 topological Dirac semimetal;
 optical anisotropy;
 linear dichroismn;
 linear response theory;
 78.20.e;
 78.20.Bh;
 78.20.Ci;
 Condensed Matter  Materials Science;
 Condensed Matter  Mesoscale and Nanoscale Physics;
 Physics  Applied Physics;
 Physics  Optics
 EPrint:
 8 pages, 3 figures