Fig. 1. Characterization of the Mg₃Bi₂ film.

Fig. 2. Schematic view of helicity-dependent terahertz radiation process.

Fig. 3. (a) Schematic of the THz emission configuration. Polar plots of terahertz waveforms (0~4.65 ps) as a function of α at (b) θ =-45º and (c) +45º, respectively, for φ = 0º. The colors represent the amplitude of the terahertz emissions. Terahertz waveforms for excitation with left-hand circularly polarized, linearly polarized, and right-hand circularly polarized optical pulses at (d) θ =-45º and (e) +45º, respectively.

Fig. 4. (a) and (b) represent the α-dependent terahertz amplitude at t = 2.46 ps. The red solid lines are the best fit with Eq. (2). The symbol ↔, the counterclockwise arrow ↺, and the clockwise arrow ↻ denote linearly polarized (black: α = 0°), left-hand circularly polarized (pink: α = 45°), and right-hand circularly polarized (green: α = 135°) incident photons, respectively. (c) and (d) display the α-dependent coefficients C (red line), L₁ (blue line), L₂ (orange line), and D (blue line) extracted [using Eq. (2)] individually from (a) and (b).

Fig. 5. (a) THz waveforms at different photoexcitation power with the wavelength center in 800 nm. (b) Fast Fourier transformations (FFT) spectra for the directly measured terahertz waveforms that are shown in (a). (c) THz amplitude versus the power of the incident laser beam. (d) The azimuthal dependent (φ-dependent) absolute amplitude of THz emission waveforms with linear polarized optical pulses.

Fig. 6. (a) THz amplitude as a function of the linear polarization angle α. The red solid line shows the fitting results using a sinusoidal function. (b) Terahertz waveforms for excitation with a linearly polarized angel at α = 0º, 45 º and 90º, respectively.