Giant Electrical Modulation of Terahertz Emission in $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{)}_{0.7}{\mathrm{Ti}}_{0.3}{\mathrm{O}}_3/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{Pt}$ Structure

Spintronic terahertz (THz) emitters based on ferromagnetic (FM)-nonmagnetic (NM) heterostructures are the focus of many experimental endeavors for THz sources. However, existing spintronic THz sources control emission by changing the external magnetic field, which is inconvenient for compact integration. Therefore, various modulated mechanisms are in high demand. Here, we present an electric-field-controlled spintronic THz emitter based on the ferroelectric (FE)-FM/NM structure that can emit THz radiation with voltage-modulated amplitude and polarization angle. The modulation depth of the THz amplitude is up to 69%, and the maximum rotation of the THz polarization angle is 28°. In addition, after removing the electric field, two different THz-emission states can remain unchanged, demonstrating nonvolatile behavior. Modulation is attributed to magnetoelectric coupling between the FM and FE layers, which modifies magnetization of the FM layer by the electric-field-induced strain in the FE layer, and thus, modulates THz emission. Furthermore, the possibility of a programmable array-type THz source with varied bias voltage is theoretically demonstrated, allowing a spatially modulated THz light to be generated directly without the use of additional spatial light modulators. We believe that this electric-field-controlled THz emitter has the potential to be used in next-generation on-chip THz sources, THz memory devices, and THz ghost imaging.

Figure 1

(a) Schematic illustration of NM/FM/FE-based spintronic THz emitter. Emitter consists of 4-nm $\mathrm{Pt}$, 8-nm $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$, 0.5-mm (011)-cut $\mathrm{PMN}\text{−}\mathrm{PT},$ and 100-nm ITO layer, respectively. External magnetic field is applied to magnetize the sample along the y axis and then removed before the test. External electric field is added to control the THz emission in the −z direction. (b) Mechanism of electrical modulation on THz emission from the sample. (c) Emitted THz waveforms under bias voltage of 200 V (pink line) and −200 V (blue line). Inset shows a fast Fourier transform of time-domain THz pulse.

Figure 2

Dependence of THz-emission-peak signals and modulation depth on bias voltage (S-E curve) for magnetization along the in-plane [01−1] direction (a) and [100] direction (b). (c) THz-emission-peak signals changes with the angle of polarizer under 200 V (pink square) and −200 V (blue circles) for magnetization along the [01−1] direction. Inset illustrates the polarization characterization setup. (d) Dependence of THz polarization angles on bias voltage for magnetization along the [01−1] direction.

Figure 3

(a) Magnetic hysteresis loops measured along the [01−1] direction at V = 200 V (red circles) and −200 V (blue circles). Dependence of remanent magnetization on bias voltage (b) for the in-plane [01−1] direction and (c) for the in-plane [100] direction after magnetizing the sample along the in-plane [01−1] direction. (d) Dependence of in-plane remanent magnetization (purple line) and angle $\mathrm{\Phi }$ (dark cyan line) between the remanent magnetization and [01−1] direction on the bias voltage.

Figure 4

Polarization of the $\mathrm{PMN}\text{−}\mathrm{PT}(011)$ substrate under an electric field for rhombohedral (R) phase (a) and orthorhombic (O) phase (b). Red and green arrows represent polarization directions for the R phase and O phase. Gray arrows represent polarization directions for the R phase that have vanished under the electric field. Schematics of magnetization in the films under bias voltages of −200 V (c) and 200 V (d). Ginger, violet, and cerulean arrows denote tensile strain along the [0−11] dieection, in-plane magnetization direction, and THz electric field, respectively. Film undergoes tensile strain in the [0−11] direction and compressive strain in the [100] direction at −200 V. Owing to tensile strain along the [0−11] direction and compressive strain along the [100] direction increasing, in-plane magnetization of the sample becomes larger and rotates a few degrees when the bias voltage changes to 200 V.

Figure 5

(a) Nonvolatile and repeatable high and low states of normalized THz-peak signal (green circles) switched by electric field (dark purple squares). (b) THz-emission signals under bias voltages of ±0 V.

Figure 6

(a) Schematic of encoding THz-emitter array (size of every pixel is 300 × 300 µm²). (b) Mapping of THz electric field when THz wave propagates to locations of 0.2λ, (c) 0.5λ, and (d) 0.8λ calculated by comsol software.