As a fascinating topological phase of matter, Weyl semimetals host chiral fermions with distinct chiralities and spin textures. Optical excitations involving those chiral fermions can induce exotic carrier responses, and in turn lead to novel optical phenomena. Here, we discover strong coherent chiral terahertz emission from the Weyl semimetal TaAs and demonstrate unprecedented manipulation over its polarization on a femtosecond timescale. Such polarization control is achieved via the colossal ultrafast photocurrents in TaAs arising from the circular or linear photogalvanic effect. We unravel that the chiral ultrafast photocurrents are attributed to the large band velocity changes when the Weyl fermions are excited from the Weyl bands to the high-lying bands. The photocurrent generation is maximized at near-IR frequency range close to 1.5 eV. Our findings provide an entirely new design concept for creating chiral photon sources using quantum materials and open up new opportunities for developing ultrafast opto-electronics using Weyl physics.

We revisit the gravitational collapse of spherically symmetric Lemaître -Tolman -Bondi (LTB) dust models. A sufficient condition for global visibility of singularity is given. This condition also allows us to extend the condition of local visibility to mass functions which are not Taylor expandable near the centre.

Circularly polarized light (CPL) can induce an asymmetry between the number of left-and righthanded chiral quasiparticles in Dirac and Weyl semimetals. We show that if the photoresponse of the material is dominated by chiral quasiparticles, the total chiral charge induced in the material by CPL can be evaluated in a model-independent way through the chiral anomaly. In the presence of an external magnetic field perpendicular to the incident CPL, this allows to predict the linear density of the induced photocurrent resulting from the chiral magnetic effect. The predicted effect should exist in any kind of Dirac or Weyl materials, with both symmetric and asymmetric band structure. An estimate of the resulting chiral magnetic photocurrent in a typical Dirac semimetal irradiated by an infrared laser of intensity 5 × 10 6 W/m 2 and a wavelength of λ 10 µm in an external magnetic field B 2 T yields a current J 50 nA in the laser spot of size 50 µm. This current scales linearly with the magnetic field and wavelength, opening up possibilities for applications in photonics, optoelectronics, and THz sensing.

In strong magnetic field the longitudinal magnetoconductivity in 3D chiral materials is shown to exhibit a new type of quantum oscillations arising from the chiral magnetic effect (CME). These quantum CME oscillations are predicted to dominate over the Shubnikov-de Haas (SdH) ones in chiral materials with an approximately conserved chirality of quasiparticles at strong magnetic fields. The phase of quantum CME oscillations differs from the phase of the conventional SdH oscillations by π/2.The chiral magnetic effect (CME) [1] (see [2,3] for reviews and additional references) is a macroscopic quantum transport phenomenon induced by the chiral anomaly. In 3D systems possessing chiral fermions, an imbalance between the densities of left-and righthanded fermions generates a non-dissipative electric current along the direction of an external magnetic field. The CME has been predicted to occur in Dirac and Weyl semimetals (DSMs/WSMs) [1,[4][5][6][7][8][9], and has been recently experimentally observed through the measurement of negative longitudinal magnetoresistance in DSMs [10][11][12][13] as well as in WSMs [14][15][16][17][18][19].The CME electric current flowing along the external magnetic field B in the presence of a chiral chemical potential µ 5 is given byThe chiral chemical potential The rate of chirality production in parallel electric and magnetic fields due to the chiral anomaly is given bẏwhere ρ 5 = ρ R −ρ L is the difference between the densities of the right-handed and left-handed fermions, and the second term is introduced to take account of the chirality changing transitions with a characteristic time τ V . If the chirality flipping time τ V is much greater than the scattering time τ , the left-handed fermions and the righthanded fermions can exist in a steady state with different chemical potentials µ L and µ R . * Electronic address: sahal.kaushik@stonybrook.edu † Electronic address: dmitri.kharzeev@stonybrook.eduIn a uniform and constant magnetic field, the energies of the lowest Landau levels are = −vp z for left-handed and = +vp z for right-handed fermions, see Fig. 1 (v is the Fermi velocity; we assume that magnetic field B is along the z-axis). The energies of excited Landau levels are E = ±v p 2 z + 2eBn for n ≥ 1, for both chiralities. The density of Landau levels in the xy plane is eB/2π, whereas the density of states in the z-direction is p z /2π. Because the lowest Landau level is not degenerate in spin, it has right-handed fermions of positive charge traveling along B and left-handed ones of negative charge traveling in the opposite direction. This induces the CME current (1).The density of the chiral charge ρ 5 is related to the chiral chemical potential µ 5 through the chiral susceptibility χ ≡ ∂ρ 5 /∂µ 5 -at small µ 5 , ρ 5 = χµ 5 + ... so that µ 5 χ −1 ρ 5 . Note that in the absence of chirality loss corresponding to τ V → ∞ the CME current would grow linearly in time -in other words, it would behave as a superconducting current, see [27] for a discussion.At finite τ V , the density o...

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