Polarization control of THz emission using spin-reorientation transition in spintronic heterostructure

Polarization of electromagnetic waves plays an extremely important role in interaction of radiation with matter. In particular, interaction of polarized waves with ordered matter strongly depends on orientation and symmetry of vibrations of chemical bonds in crystals. In quantum technologies, the polarization of photons is considered as a “degree of freedom”, which is one of the main parameters that ensure efficient quantum computing. However, even for visible light, polarization control is in most cases separated from light emission. In this paper, we report on a new type of polarization control, implemented directly in a spintronic terahertz emitter. The principle of control, realized by a weak magnetic field at room temperature, is based on a spin-reorientation transition (SRT) in an intermetallic heterostructure TbCo2/FeCo with uniaxial in-plane magnetic anisotropy. SRT is implemented under magnetic field of variable strength but of a fixed direction, orthogonal to the easy magnetization axis. Variation of the magnetic field strength in the angular (canted) phase of the SRT causes magnetization rotation without changing its magnitude. The charge current excited by the spin-to-charge conversion is orthogonal to the magnetization. As a result, THz polarization rotates synchronously with magnetization when magnetic field strength changes. Importantly, the radiation intensity does not change in this case. Control of polarization by SRT is applicable regardless of the spintronic mechanism of the THz emission, provided that the polarization direction is determined by the magnetic moment orientation. The results obtained open the prospect for the development of the SRT approach for THz emission control.


Supplementary Note 1: THz generation efficiency as function of pump pulse fluence
To determine the efficiency of THz radiation generation in the TbCo2/FeCo structure, the ΔS peak signal (Supplementary S5) was measured at different values of the incident radiation power (Supplementary Figure 1). Detected signal, which is proportional to 2 [1], nonlinearly depends on the pump fluence due to nonlinearity of inverse spine Hall effect [2].  Figure  2). It was found that the peak-to-peak signal remains constant for any direction of the pump polarization.

Supplementary Note 3: Experimental Schematic
To determine the magnitude and direction of the polarization of THz radiation generated by the investigated sample and its dependence on the magnetic field, the method of electro-optical sampling was used [3]. The experimental setup is shown in Supplementary -orientation of linearly polarized THz electric field vector (after the WGP polarizer); 1 , 2 are the refractive indices of an anisotropic crystal at ≠ 0, is the orientation of the main axis of the ellipsoid.
In the absence of the THz field, the probe polarization coincides with the crystal axis [001] and the axis of the refractive index ellipsoid. As a result of this interaction, the polarization of the probe beam remains linear and does not change its orientation. This beam is blocked by the GTP. Electric field of THz pulse rotates the ellipsoid of the refractive indices and results in the ellipticity of the probe beam. The intensity of the signal detected by the diode is described by the following expression [1]: = 0 2 (2 ( 2 )) 2 ( 2 ) (S1) where 0 is the probe beam intensity, ( 2 ) an angle of ellipsoid axis with the [-110] ZnTe axis on orientation angle φ 2 , Γ is the relative phase shift between the two orthogonal components of the laser field.
where λprobe is the wavelength of the probe wave, d is the ZnTe thickness, nZnTe is the ZnTe refractive index for λprobe, r41 is the electro-optical coefficient, is the field amplitude at the output of the THz polarizer. The amplitude of the THz field depends on the mutual polarization orientation of the THz field and the WGP axis: So, the resulting intensity of the probe beam measured by the diode is given by: where the smallness of the argument under sine is taken into account.
In order to increase the signal-to-noise ratio, the final signal recorded by a synchronous detector (Lock-in amplifier SR830) is given by: where is the phase difference between Δ ( ) and the reference signal, t is the delay time [3].

SUPPLEMENTARY NOTE 5: Configuration of and WGP in polarization detection experiment
If the THz polarization generated by the heterostructure is linearly polarized, then the following typical cases are probable (Supplementary Figure 5). Figure 5: Dependence of the signal recorded by the photodiode during WGP rotation for two characteristic orientations of the linearly polarized THz field: a) = 0, Ω = Ω ; b) = 2.4 kOe, Ω = Ωx