Terahertz Emission from $\mathrm{Bismuth}$ Thin Films Induced by Excitation with Circularly Polarized Light

We demonstrate that $\mathrm{Bi}$ thin films emit terahertz radiation when irradiated by circularly polarized femtosecond laser pulses. Terahertz emission that is polarized in a direction perpendicular to the plane of incidence is significantly enhanced when the pump pulse is circularly polarized compared with when it is linearly polarized. The enhanced terahertz emission reverses its polarity with the reversal of pump helicity. This helicity dependent terahertz emission is absent in the horizontal direction along the plane of incidence, and is odd with respect to the incident angle. Combined with the layer thickness dependence measurement, the dominant origin of terahertz emission is attributed to the photoinduced inverse spin Hall effect (ISHE) in $\mathrm{Bi}$. The temporal dynamics of the ultrafast photocurrents are extracted and the spin and charge dynamics are discussed. The observation of terahertz emission from the photoinduced ISHE in a thin film consisting of a single material indicates the unique character of $\mathrm{Bi}$, i.e., the long spin diffusion length and the large spin Hall angle, demonstrating the potential ability of the material to be applied to terahertz emitters and terahertz spintronics.

Figure 1

(a) Schematic of the sample structure. (b) Schematic of the experimental setup. The incident angle is set to be $\theta \sim {45}^{\circ }$. The polarization of the excitation pulse can be changed from right circular polarization ${\sigma }_{+}$ to left circular polarization ${\sigma }_{-}$, or an elliptical polarization in between. WGP is wire grid polarizer, QWP is quarter wave plate. (c) Waveform of the emitted terahertz pulse from 30-nm-thick sample. Red and blue curves are obtained when the excitation pulse is circularly polarized. The green curve is obtained when the excitation pulse is linearly (p-) polarized. (d) The corresponding power spectra of the emitted terahertz wave.

Figure 2

(a) The peak EO sampling signal $S_{\mathrm{THz},\mathrm{peak}}$ of the Y polarized terahertz wave from 30-nm-thick sample plotted against the rotation angle α of the QWP. The red circles are the experimental data and the blue solid line is the fitted result using Eq. (1). The dashed lines are the fitting results decomposed into each term of Eq. (1) except for the constant D. The green, magenta and orange dashed lines correspond to the contribution of the first, second, and third term in Eq. (1). (b) The absolute values of the fitting parameters obtained by the fitting shown in (a). (c), (d) Identical plots to (a) and (b) respectively, for X polarized terahertz waves.

Figure 3

(a) Helicity dependent component of the X polarized terahertz wave (green) and the Y polarized terahertz wave (red) emitted from 30-nm-thick sample. See main text for the definition of $S_{\mathrm{THz},\mathrm{circ}}$. A schematic of the measurement configuration is shown above. (b) Incident angle dependence of the helicity dependent component of the Y polarized terahertz wave emitted from 30 nm thick sample. The green waveform is obtained for normal incidence $(\theta =0^{\circ })$, while the red and blue waveforms are obtained for oblique incidence. Schematics of the measurement configuration are shown above. (c), (d) Schematic explaining the origin of the incident angle dependence (b) under the proposed photoinduced ISHE picture. Reversing the incident angle while holding the excitation helicity effectively reverses the in-plane component of the excited spin, causing the photoinduced spin current and the CPC to be odd with respect to the incident angle.

Figure 4

(a) $\mathrm{Bi}$ layer thickness dependence of the terahertz emission. The green, magenta, orange and blue data points correspond to the absolute values of the fitting results $C,L_1,L_2$, and D using Eq. (1) at the peak position. The green dashed line is the fitting result using Eq. (2) with ${\lambda }_{\mathrm{rel}}=30\mathrm{nm}$. (b) Schematic of terahertz emission from photoinduced ISHE. (c) Extracted terahertz electric field (blue solid line) from 30-nm-thick sample along with the original EO sampling signal (red dashed line) plotted in the same scale for comparison. (d) Temporal dynamics of CPCs. Traces are normalized by their maximum value.

Figure 5

(a), (b), (c) Waveforms of X polarized terahertz waves when the 30-nm-thick sample is excited by right circular polarization $({\sigma }_{+})$, left circular polarization $({\sigma }_{-})$, and linear polarization (lin. pol.), for different incident angles as schematically illustrated in (d).