360° Polarization Control of Terahertz Spintronic Emitters Using Uniaxial FeCo/TbCo₂/FeCo Trilayers

Abstract

Polarization control of THz light is of paramount interest for the numerous applications offered in this frequency range. Recent developments in THz spintronic emitters allow for a very efficient broadband emission, and especially unique is their ability of THz polarization switching through magnetization control of the ferromagnetic layer. Here we present an improved scheme to achieve full 360 • nearly coherent polarization rotation that does not require multipolar or rotating external magnetic bias nor complex cascaded emitters. By replacing the FM layer of the spintronic emitter with a carefully designed FeCo/TbCo 2 /FeCo anisotropic heterostructure, we experimentally demonstrate Stoner-Wohlfarth-like coherent rotation of the THz polarization over a full 2 azimuth only by a bipolar variation of the strength of the hard axis field, and with only a negligible decrease in the emission efficiency as compared to standard Pt/CoFeB/W inverse spin Hall emitters. THz measurements are in agreement with our model of the non-perfect Stoner-Wohlfarth behaviour. These emitters are well adapted for the implementation of polarimetric characterization not requiring any mechanically rotating polarizing elements. An example is given with the characterization of the birefringence in a quartz plate.

On-Chip Waveguided Spintronic Sources of Terahertz Radiation

“…With rising interest in photonic chips utilizing techniques for optical polarization control, ,, such a waveguided THz radiation source could seamlessly be integrated with optical circuitry that allows for THz power modulation. By replacing the Fe film of this same device with materials exhibiting magnetic anisotropy, it would be possible to create an on-chip THz radiation source capable of both THz polarization control and THz power modulation via innate device properties without reliance on external components. Such a THz radiation source would be a promising candidate for on-chip communications applications utilizing both OMA and NOMA techniques.…”

“…While advances in THz sources have resulted in real-world applications in communications, − spectroscopy, , medical and near-field imaging, − nondestructive monitoring and testing, − security, , and microscopy, , the lack of compact, efficient THz sources has stalled mass commercial adoption of this promising technology. Current methods for the generation of THz radiation on compact platforms include the utilization of nonlinear crystals, − solid-state sources, , photoconductive switches, − and spintronic THz radiation emitters. ,− Due to their bulk, growth requirements, and the need to achieve a high degree of crystallinity and phase-matching, nonlinear crystals are ill-suited to mass-fabrication techniques and multiplatform integration, while solid-state sources and photoconductive switches are mostly limited to lower THz frequencies while requiring high bias voltages and electronic circuitry. In recent years, spintronic THz radiation emitters, ultrathin film stacks consisting of nanometer-thick heterostructures of ferromagnetic (FM) and nonmagnetic (NM) films, have become the subject of increasing interest and popularity due to their thin-film nature and compatibility with a wide range of materials and substrates. − Flexibility with substrates and choice of material enable spintronic THz radiation emitters to be compatible with various fabrication platforms and provide a unique opportunity for on-chip integration.…”

“…While several modifications have been made to spintronic THz radiation emitters to allow for polarization control, ,− modulation of the output THz power is limited to modulation of the optical pump power itself, which requires additional elements such as filters that possess considerable bulk and must be manipulated through mechanical or electrical means, making such schemes unsuited for on-chip platforms where device footprint is a constraint. For such purposes, modulation must be achieved through means that are intrinsic to the device structure rather than relying on external mechanisms.…”

“…Waveguides fabricated on SiO 2 substrates allow one to control the output THz radiation power depending on the coupled optical modes by varying the polarization of the optical pump pulse. The spintronic waveguided device structure is compatible with methods to achieve high output THz power on the order of 40 μW via substrate cooling as well as with recently developed methods to control the polarization of the THz radiation electric field ,, as it is independent of the composition of the FM/NM film stack. Integration of these two schemes for polarization control and power modulation would result in on-chip spintronic THz radiation emitters that allow for both OMA and NOMA techniques via means intrinsic to the device achievable via chip-based modulators without the need for external components.…”

Magnetization rotation-based polarization control of spintronic terahertz emitter

“…[11][12][13] Recently, We have proved to control the emitted polarization using induced magnetic anisotropy. 14,15 It makes polarization control possible through the Stoner-Wohlfarth behaviour of magnetization.…”