Fig. 1. Experimental setup and terahertz signals. (a) Schematic of the experimental setup. The x−y−z coordinates are adapted to the laboratory frame and the lattice coordinates are labeled as a−b−c. The crystal azimuthal angle φ, polar angle θ, and the laser-polarization angle α are defined. Femtosecond-laser-induced electrons are injected from the NM layer, resulting in transient electron currents jze along z. In the experiment, the excitation pulse is primarily incident from the EAC layer side. However, for ease of illustration, it is depicted as incident from the NM layer side. Owing to the electrical anisotropy in the EAC layer, transverse electron currents (jte) are generated flowing at an angle of φ relative to the x axis. Inset: illustration of superdiffusive electron currents jze induced by laser-pulse excitation. (b) Schematic of the ellipsoid of conductivity tensor of RuO2 and IrO2 in the laboratory frame, displaying anisotropy in electrical conductivities (σ∥<σ⊥). The schematic uses the same coordinate and angle definitions as described in panel (a). (c) Terahertz waveforms generated from (c1) Pt/IrO2(101), (c2) Pt/RuO2(101), (c3) RuO2(101), and (c4) Pt/Fe devices. The signal from the RuO2(101) thin film is scaled 10 times for comparison. (d) Terahertz spectra obtained via fast Fourier transform (FFT) of the waveforms in panel (c).

Fig. 2. Effect of NM materials and laser polarization states. (a) Terahertz waveforms generated by NM/RuO2(101) devices with NM materials of Ir, Cu, W, and Pt. The thickness of the NM layer is 2 nm. (b) Terahertz signal amplitude as a function of the NM materials used for the NM/RuO2(101) devices (red bars). For comparison, OACs at the laser wavelength of 1.03μm (blue bars) and spin-Hall angles γ (green bars) of the respective NM materials are also shown. (c) Terahertz signal amplitudes of the Ex and Ey components from the Pt/RuO2(101) device as a function of the polarization angle α of the linearly polarized excitation laser. Inset: Ex−Ey projection of the terahertz waves for different values of α. (d) Terahertz waveforms from the Pt/RuO2(101) device excited by excitation pulses with linear, right- and left-circular polarizations.

Fig. 3. Effect of crystal orientations. (a) Ex and Ey components of terahertz waveforms generated by NM/RuO2 devices with different crystal orientations and polar angles θ. (b) Ex and Ey components of terahertz waveforms generated by Pt/RuO2(101) at different azimuthal angles φ while keeping θ fixed at 34.7 deg. (c) Terahertz signal amplitude of Ex and Ey components from the Pt/RuO2(101) device at different azimuthal angles φ. The solid lines represent the sine and cosine fitting to the results.

Fig. 4. Optimizing the conversion efficiency. (a) Terahertz signal amplitude as a function of incident laser fluence from the Pt/RuO2(101), Pt/IrO2(101), and Pt/Fe samples. The red and blue dashed lines represent the linear fits to the low-fluence experimental results of Pt/IrO2(101) and Pt/RuO2(101), respectively. The slope of the red dashed line is ∼4.8 times of that of the blue dashed line. (b) Terahertz signal amplitude as a function of thickness of the RuO2 layer (dEAC) of the Pt/RuO2(101) device. (c) Terahertz signal amplitude as a function of thickness of the Pt layer (dNM) of the Pt/RuO2(101) device. The solid lines in (b) and (c) represent a global fit using the thickness-dependent model (see Appendix C).

Table 1. Longitudinal (σ∥) and transverse (σ⊥) conductivities, crystal polar angles θ, and the coefficients of electrical anisotropy β0 of RuO2(101) and IrO2(101).