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.

We propose, fabricate, and evaluate strain-induced InGaAs/InAlAs superlattice (SL), which can efficiently radiate broadband terahertz (THz) waves. By means of optical pump-probe measurements, we demonstrate ultrashort photocarriers relaxation times of τ∼1.7ps without Be-doping of InGaAs photoconductive layers. We assume two dominant mechanisms to be responsible for a sharp reduction of τ in strained SL, which are photocarriers scattering at InGaAs/InAlAs heterointerface roughness and the decrease in the energy bandgap of InGaAs photoconductive layers due to the residual strain. The THz time-domain spectroscopic measurements reveal the rise in both emitted THz waveform and spectrum amplitudes with an increase of the residual strain in SL, in particular, at the low-frequency region. We refer this to the band structure engineering due to the residual strain in SL—since InGaAs photoconductive layers become compressively strained, this reduces the semiconductor’s energy bandgap, thus more photocarriers can contribute to the THz emission. The results might be of specific interest for the development of portable THz pulsed spectroscopic and imaging systems and other fundamental and applied aspects of the THz science and technology.

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Electrical control of conjugate degrees of freedom in multiferroics provides the advantage of reducing energy consumption to femto-and even attojoules per switch in spintronics and memory devices. This is achieved through the development of technologies that make it possible to fabricate artificial materials with constantly improving properties. Here, we present the design, physics, and characteristics of a composite multiferroic spintronic emitter, which provides electrical control of the emitted terahertz (THz) wave polarization. The effect is due to electrical control of the magnetization in a high-quality magnetostrictive superlattice, TbCo2/FeCo, deposited on an anisotropic piezoelectric substrate. In our approach, several mechanisms are realized in the system simultaneously: the strain-mediated coupling of the magnetic and piezoelectric subsystems, which operate in the range of the spin-reorientation transition of the magnetic superlattice, and THz-wave generation in the superlattice by an optical femtosecond pulse. This provides flexibility and control of the set of parameters. We determine the magnetoelectric parameter, which is responsible for THz polarization control. Our results offer a significant fundamental insight into the physics of composite multiferroic systems that can be used for applications of multiferroicity, primarily for THz spintronic emitters. We believe that our findings represent a decisive step towards technologies for other types of spintronic and memory devices.

We report an increase in terahertz (THz) radiation efficiency due to FeCo/WSe2 structures in the reflection geometry. This can be attributed to an absorption increase in the alloy FeCo layer at the input FeCo/WSe2 interface due to constructive interference, as well as to the backward transport of hot carriers from FeCo to WSe2. In contrast to the transmission geometry, the THz generation efficiency in the reflection is much less dependent on the magnetic layer thickness. Our results suggest a cheap and efficient way to improve the characteristics of THz spintronic emitters with the conservation of a full set of their important properties.