The advent of polarization spectroscopy has facilitated scientific and technological advancements in communication,1^–3 memory,4^–6 computing,7^–9 and biomedicine.10^–12^,13 Terahertz (THz) waves14^,15 have become a focal point of research due to the same vibration frequencies as many biological macromolecules and low photon energies.16^,17 These characteristics enable noninvasive identification and imaging of multimaterials.18 The THz circularly polarized wave is outstanding for contributions to understanding biomolecules10^,19 and exploring chiral solid-state physics.20^,21 However, the current generation of chiral THz waves mainly relies on scattering elements,22 photo-elastic modulators,23 segmented wave plates,24 and metamaterials,25^,26 which are often constrained to narrow bandwidths, leading to cumbersome and inefficient optical systems.

Metasurfaces,27^–30 based on the generalized laws of refraction, can be reasonably designed to achieve negative refractive index and controllable chirality through tailoring effective permittivity.31^–34 By cooperating with changing external magnetic fields,35 integrating with planar optical elements such as wave plates,24^,36^,37 liquid crystals,38^–41 THz polarizers,42^–44 and reforming optical systems,19^,45^,46 anisotropic metasurfaces can flexibly modulate polarization states precisely at subwavelength resolutions.29^,47^–49 The liquid-integrated metadevices are quite mature in wavefront control but lack of process compatibility limits their promotion.50^,51 Recently, fabricating spintronic THz emitters13^,52^–64 into metasurfaces shed a bright light on the novel road to realize efficient flexible broadband chirality control.65^–67 Once excited in spintronic-metasurface emitters, the laser-induced transient current will be controlled by subwavelength patterns directly to manipulate the radiated THz waveforms in a compact and high transmission system.68^–70 Because THz manipulation intersperses with the emission process, polarization states are determined by the coordination of metasurface, external fields, and pump laser, endowing the device with versatile manipulation options. Furthermore, simplicity and low manufacturing cost can promote this integrated emitter for mass production and application. It has been reported that a spintronic-metasurface THz emitter can reach an ellipticity of >0.75 by altering the orientation of the external magnetic field.71 The acquisition of circularly polarized waves, characterized by precise chirality control, is anticipated to be feasible through the rectification of existing deficiencies in the systematic study of polarization control mechanisms and bolstering experimental validation within spintronic-metasurface devices. These strides are poised to constitute a pivotal breakthrough in the realm of THz chirality control.

In this work, we propose seven groups of emitters with different aspect-ratio periodically striped arrays to understand the effect of pattern on THz emission and modulation. The effective irradiated area and the number of stripes are both fixed in these devices. We find that the capacity to modulate various polarization states of chiral THz is proportional to the aspect ratio of stripes, which fits our proposed model well. Importantly, the conversion of linear, elliptical, and left and right-handed circular polarization can be realized with ease in the large aspect ratio. Over broadband bandwidth from 0.74 to 1.66 THz, the absolute value of the ellipticity rate we achieve can be kept greater than 0.85. The novel and creative integrated THz multifunctional element provides new insights into developing next-generation on-chip tunable optoelectronic devices, which lays the groundwork for future THz applications in biomedical detection and high-speed communication.

The charge current induced in spintronic THz emitters flows through the conductor surface under a femtosecond (fs) laser. Charge displacement in the finite area will result in positive and negative charge accumulation on opposite boundaries and form built-in electric field Ei to modulate THz polarization states.72 Here, the fs laser (800-nm central wavelength, 35-fs pulse duration, and 1-kHz repetition rate) generated by an amplified Ti:sapphire laser source is used to excite the spintronic-metasurface device. The excitation pulse power is ∼150mW, and the beam radius on the emitter is ∼1cm.

In Fig. 1(a), the schematic of THz chirality modulation from stripe-patterned spintronic THz devices is shown. At the initial moment, stripes are along the y axis, whereas the external magnetic field with a field magnitude of 2000 Oe is fixed along the y axis to saturate the magnetization. The azimuth angle φ is defined as the angle between the emitters and the x axis. The emitters in this study were made from trilayer heterostructures of W (5 nm)/CoFeB (2 nm)/Pt (2 nm) nanofilms grown on a 500-μm-thick SiO2 substrate in a direct current (DC) and radio frequency (RF) sputtering system. We fabricated seven groups of stripe-array emitters with varying aspect ratios (l:w) denoted as D1 to D7 in the order of 1:1, 1.5:1, 2:1, 10:1, 20:1, 32:1, and 50:1. The ratio of l and d remains the same as 5.66:1. Each group contains 22 stripes, and every stripe keeps the same area of 20,000μm2. The fabrication process of patterns is shown in Section S1 in the Supplementary Material. As detailed in the inset, when the devices rotate by angle φ, the charge current jc can be considered the vector sum of the currents along both sides of the stripe, represented as jp=jcsinφ and jv=jccosφ.

The mechanism of THz emission here is elaborated in Fig. 1(b). A fs laser is employed to irradiate the heterostructures and yields an ultrafast spin voltage.73^,74 The laser-induced spin voltage triggers a spin-polarized current js into the nonferro-magnetic (NM) layer (W and Pt).74^,75 According to the inverse spin Hall effect (ISHE), js is transformed into an ultrafast transverse in-plane charge current jc and radiates THz waves.

The diagrammatic sketch of the interaction between built-in electric field Ei and gap electric field Es when φ is 0 deg is illustrated in Fig. 1(c). Upon charges generated, electrons accumulate at the edge of the stripe in the W and Pt layers, whereas an equal number of positive charges gather at the other side accordingly due to the law of charge conservation. Here, the electric field inside the stripe is denoted as the built-in electric field Ei, and the electric field in the gap between stripes is called the gap electric field Es. Charges on both edges of a stripe attract each other by Ei and Es helps them keep at boundaries. According to Coulomb’s law, Ei=∫θ2arcsinll2+w2+θ114πε0Qiwsinθdθ,(1)Es=∫α2arcsinll2+d2+α114πε0Qidsinαdα,(2)where l is the stripe length, w is the stripe width, d is the gap length between stripes, ε0 is the dielectric constant, and Qi represents linear charge density. In addition, θ is the angle between dEi formed by a pair of heterologous charges and the long edge, and θ1 (θ2) is the angle between dEi and upper (lower) edge, respectively. Similarly, α, α1, and α2 are applicable to the situation where the gap electric field dEs is formed by a pair of heterologous charges in the gap of adjacent stripes. According to Eqs. (1) and (2), l/w and l/d are key elements for Ei and Es, and the value of Qi is jointly determined by Ei and Es. If l/w is larger than l/d, Ei is larger than Es, and the force Fi exerted by Ei on charges is greater than Fs from Es according to Coulomb’s law. In this case, charges cannot keep at edges and flow opposite to jc, which is defined as the backflow ji. Based on this phenomenon, as presented in Figs. 1(d)–1(f), the linearly polarized, left-handed and right-handed elliptically polarized THz waves can be obtained when we rotate emitters with large Ei. The phase difference and polarization rotation can be observed in the Lissajous projection curves of the x−y plane.

THz amplitude, frequency domain, and pump dependence of these seven groups was detected by rotating emitters. Combined with j=σE, σ is the conductance of the metal layer and E is the electric field intensity. Given that the direction of the electric field Ei is opposite that of the current density jc, the induced current density ji also opposes jc. Consequently, ji exerts a reduction effect on jc, leading to a diminished amplitude of the THz wave. Nevertheless, if l/w is less than l/d, no significant changes occur in THz radiation because electrons remain stable at edges. When φ is 0 deg, as Fig. 2(a) displays, the trend of amplitude from D1 to D7 is consistent with the above expectation. As samples rotate by 90 deg, the amplitude from D1 to D7 in Fig. 2(d) increases as l/w increases and as Fi and ji decrease without the gap electric field existing.

As shown in Figs. 2(b) and 2(e), the built-in electric field also plays an important role in the frequency domain. In the process of THz generation, the high-speed backflow of heterologous charges can be regarded as violent oscillation within the stripe, which is very consistent with the radiation process of dipole antennas.76 When φ is 0 deg, the center frequency fc of D1 to D3 climbs as the ratio of l/w increases, which matches the working frequency f0 of the corresponding antennas. As for D4 to D7, they share similar fc and encounter information loss at 0 to 0.2 THz, due to the capacitive reactance. However, benefiting from the maintenance of information from 1.2 to 1.5 THz, the full width at half-maximum ΔF expands in D4 to D7 compared with D1 to D3. When φ reaches 90 deg, all emitters exhibit inductive impedance, and their fc are also close to each other. As l increases, information loss alleviates, and ΔF gradually rises from D1 to D7. The working principle of antennas77^,78 and detailed analysis are provided in Section S5 in the Supplementary Material.

With regard to pump power dependence of THz emission, a saturation behavior is gradually displayed from D1 to D7 when φ is 0 deg, as shown in Fig. 2(c). Stimulated electrons are boomed by the increase of pumping power and cause the enhancement of jc and Ei. As for the same growth rate of jc, a more intense suppressor effect on THz amplitude from Ei will lead to slower climbing of THz amplitude and saturating at lower pump power. Similarly, D1 to D3 show stronger pump dependence due to the negligible ji. When samples rotate to 90 deg, the intensity of the built-in electric field is too weak to affect jc. Therefore, all emitters show similar regularity of pump dependence, as shown in Fig. 2(f).

To achieve polarization controllability, we investigated the chiral performance of D1 to D7 at various azimuth angles from 0 deg to 90 deg. When emitters rotate, forces on boundaries and current flows are revealed [see Fig. 3(a)]. The detailed analysis of the current components in different directions is shown in Section S6 in the Supplementary Material. When the device is at 0 deg, ji can be divided into ja and jb based on the boundaries they flow along. In D1 to D3, Fi<Fs induces the absence of ja no matter how these devices rotate, and only jc and jb remain here. According to Eqs. (S22)–(S25) in the Supplementary Material, the vanishing of ja makes it impossible to change and modulate the polarization state by φ. As for D4 to D7, built-in electric fields are excessively large enough to induce both charges backflow ja and jb to modulate THz radiation. To confirm the existence of the backflow, we use the THz waveforms of D1, D4, and D7 measured at 20 deg to extract the components of ja and jb. As Fig. 3(b) summarizes, the backflow in each sample is similar to our expectations, jb is too weak, and the value of ja is directly proportional to the strength of the built-in electric field. To verify the accuracy of the model, we also extracted ja and jb from the THz signal of D7 at 60 deg. They nearly coincide with the components of D7 at 20 deg, indicating that the backflow is a key factor in modulating the THz emission within the pattern.

Our meticulous extraction and analysis of the amplitude and phase of D7 from 0 to 1.8 THz at 20 and 60 deg were undertaken to uncover the extraordinary potential of large Ei in chirality modulation. Figures 3(c) and 3(d) demonstrate that backflow induces substantial alterations in both the amplitude and phase of jv and jp across the entire frequency spectrum. This effect leads to discernible disparities in phase and amplitude within the broadband signal in the x and y directional components. These findings are pivotal for elucidating the fundamental mechanisms underpinning the chiral modulation capability in broadband applications. The phase difference distribution in the full-frequency domain is shown in Fig. 3(e). In the presence of a strong built-in electric field, the phase difference Δβ=βx−βy at 20, 45, and 60 deg tends to remain constant within a specific frequency range. The coexistence of jb and ja ensures significant changes in phase difference between varying azimuth angles, meeting the key requirement of different chirality controlled by rotating samples. Especially at 20 deg, D7 is able to maintain a phase difference of about π/2 within the 1.3 to 1.7 THz band, which highlights the built-in electric field’s ability to provide stable control across a wide range of frequencies.

This above phenomenon is also similarly reflected in the time domain,79^–82 as shown in Figs. 3(f) and 3(g). With the enhancement of the built-in electric field, the phase difference and ellipticity at all angles tend to reach saturation, and the effects of chiral control at various azimuth angles exhibit significant differences. At 20 deg, a gradual increase of the built-in electric field leads to the phase difference nearing π/2 and a dramatic increase in chiral THz ellipticity from 0.3 to 0.96, contrasting with the uniform ellipticity at other angles. This underscores the critical role of specific azimuth angles in achieving diverse THz polarization state modulation. The phase difference in the time domain could be obtained by correlation functions of Ex and Ey (Section S7 in the Supplementary Material), and the ellipticity calculation using the shift current in the short excitation model83^–87 (Section S8 in the Supplementary Material), and the Stokes parameters (Section S9 in the Supplementary Material) are highly consistent with the experimental data. The varying amplitude in the x and y directions at 20 deg in Fig. 3(h) and the Lissajous curves of all devices at 20 deg in Fig. 3(i) reveal that the combination of aspect ratio and azimuth angle enables the patterned THz emitters to achieve diversification in chirality modulation effects. Devices with a large aspect ratio are more likely to obtain the circular polarization state at 20 deg.

In line with the analysis of current components in Section S6 in the Supplementary Material, the current in the y direction undergoes a phase reversal of 180 deg after φ exceeding 90 deg as Eqs. (S23) and (S25) in the Supplementary Material describe, accompanied by the same phase sign of current in the x direction, as depicted in Eqs. (S22) and (S24) in the Supplementary Material. On the basis of the principle of polarization formation of electromagnetic waves, the left- and right-handed polarization states can be obtained at φ and 180 deg-φ with sufficient amplitude and phase difference in the x and y directions, as represented in Fig. 4(a). To grasp the influence of aspect ratio on the control of polarization states more intuitively, the measured 3D temporal waveforms from D1, D4, and D7 are adopted here. Figures 4(b)–4(d) shows the chirality-control performance in patterned THz emitters at φ=20deg and 160 deg. As l/w becomes larger, the THz polarization states undergo transitions from linear polarization (D1) and elliptical polarization (D4) to circular polarization (D7). The change from left- to right-handed polarization occurs at φ=20deg and 160 deg as a result of a phase reversal of the y component of THz waves. Figures 4(e) and 4(f) focus on the frequency domain for both azimuth angles in D4 and D7. Here, we ignored D1 due to its linear polarization feature. The variation of Ex and Ey with l/w agrees with that in the temporal domain. As for phase in each direction, the βx of φ=20deg and 160 deg remains consistent, whereas their βy maintains 180 deg disparity in all frequency ranges. The 180-deg phase difference in Δβ=βx−βy between φ=20deg and 160 deg reveals that excellent continuity and stability of conversion in left- and right-handed THz waves can be achieved by rotating devices. All Ex, Ey, and their phases are obtained from the Fourier transform of time-domain signals measured in the x and y directions.

According to the above analysis, the sample of D7 is the optimal device to integrate strong THz emission and flexible polarization state modulation. By characterizing the accuracy and limit of D7’s ability to control THz chirality, it could facilitate the improvement of such multifunctional components and the achievement of a high-sensitivity on-chip THz application. In this work, the elliptical THz waves are generated and controlled by comprehensively rotating the device, as shown in Fig. 5(a). Figure 5(b) illustrates the THz temporal spectral waves of the x (the bottom) and y (the upper) directions. The phase reversal in the y direction enables the switching between the left-handed and right-handed waves. In Fig. 5(c), Δβ of D7 achieves a broadband phase difference stability, such as ±π/2 at φ=20deg or 160 deg, and nearly zero at all frequencies at φ=80deg or 100 deg. The experimental results clearly highlight that a strong built-in electric field contributes to the diversity and availability of polarization modulation effects and stabilizes over a wide frequency band, which provides a promising prospect for improving the utilization of chiral THz in the full-frequency range. By combining jy/jx in Fig. 5(d) and Δβ of the time domain in Fig. 5(e), it demonstrates that D7 can achieve a flexible switching between linear polarization and elliptical polarization during rotation. In Fig. 5(e), the relationship between the azimuth angle and the time-domain phase difference, as calculated, aligns well with the experimental data. This alignment also theoretically substantiates that D7 has the potential to achieve polarization state diversity. The Lissajous waves of D7 in Fig. 5(f) reveal that a variety of polarization states can be realized by adjusting the azimuth. To assess the performance of controlling polarization states in the broadband range, we characterize the ellipticity of fully polarized plane THz waves by calculating Stokes parameters.88 The process is described in Section S9 in the Supplementary Material. As shown in Fig. 5(g), the value of ellipticity can remain greater than 0.85 in the range of ∼0.74 to 1.66 THz from 20 to 40 deg, and the ellipticity is able to stay around 0 in the full frequency domain at 0, 90, and 180 deg. The results reveal that the flexible broadband conversion between various polarizations can be achieved with the THz emission at room temperature. Remarkably, the same ellipticity from 0.1 to 0.65 can be distributed across low (0 to 0.6 THz), medium (0.6 to 1.2 THz), and high-frequency (1.2 to 1.7 THz) bands through tuning different azimuth angles, which endows the ability to frequency fully adjust and control the THz polarization states. As is well known, the Poincaré sphere has been widely applied in the polarization states of monochromatic light waves for the capacity of covering all polarization states and signifying the modulation trajectories of chirality.89 The Stokes parameters S1, S2, and S3 serve as three axes of Cartesian coordinates of the Poincaré sphere. S1, S2, and S3 represent any point on the spherical surface, with each point on the Poincaré sphere corresponding to the fully polarized state of light. The transformation trajectory pictured on the surface of the Poincaré sphere in Fig. 5(h) depicts the change of polarization states from 0.74 to 1.66 THz with seven different rotation angles. When φ is 20 or 160 deg, the trajectories are near the poles of the sphere, which shows that the ellipticity is approximate to ±1. As the azimuth angle approaches 90 deg, the trajectories gradually draw close to the equator, meaning the linear polarization state has been achieved when φ is 90 deg. It can be awarded from the route distribution that the apparatus is able to realize the broadband modulation of chirality flexibly with multiazimuth control at room temperature.

As Fig. 6(a) illustrates, the patterned spintronic THz emitters are capable of generating tunable chiral THz waves across a broadband frequency range. Below 0.7 THz, the amplitude of Ex,y and the phase difference between them are unstable with frequency growing, so the THz chirality modulation performance is set aside. The generated polarized THz spectra at three selected typical frequencies of 0.7, 1.1, and 1.5 THz, with changing azimuth angles, are displayed in Figs. 6(b)–6(d). In the frequency scope of our consideration, the variation of ellipticities with different azimuth angles is confirmed: circular polarization states can be achieved in both the lower and relatively higher frequency ranges; the THz waves generated at 60 and 120 deg maintain elliptical polarization states across the entire frequency spectrum, with the ellipticity increasing as a function of frequency; linear polarization is consistently observed when the azimuth angle φ is 90 deg. This indicates that the switching among different polarization states depends not only on controlling by rotating azimuth angles but also on the requirements of frequency bands, which provides chirality modulation and application in integrated THz multifunctional devices with an effective strategy for future exploration.

We demonstrated that trilayer heterostructures combining μm-scale stripes afford real-time broadband tunability of THz chirality at room temperature. The geometry allows the emergence of backflow ji induced by large built-in electric fields and a more enhanced suppression effect on charge flow jc with the increase of aspect ratio in the stripes. These effects play a crucial role in affecting THz amplitude and phase difference between the x and y directions, which gives way for polarization state modulation by varying azimuth angles of devices. The flexible transition among linear, elliptical, and circular polarization states can be attained by rotating the device when the intensity of the built-in electric field is sufficiently strong. The ellipticity can remain over 0.85 in a broadband frequency range of ∼0.74 to 1.66 THz. The patterned emitters are available for ellipticity modulation in various bands below the cut-off frequency of 1.7 THz. The lightweight capabilities, low power consumption, and high modulation efficiency open up the possibility of exploiting portable and high-performance ultrafast THz optical-spintronic devices, which may ensure advances in high-speed wireless communication and noninvasive biomedical detection.

Qing Yang is a PhD student at Beihang University. She received her bachelor’s degree in electrical information engineering from Northeastern University in 2016. In September 2019, she joined the School of Integrated Circuit Science and Engineering at Beihang University, pursuing a PhD under the supervision of Professor Tianxiao Nie. Her current research topics involve THz emission and modulation. She is also interested in spintronic THz emission from new materials.

Yan Huang received his bachelor’s degree from School of Electronic and Information Engineering of Beihang University in 2018 and is now pursuing his doctor degree in microelectronics and solid-state electronics. His research interest includes micro-nanofabrication, spintronics functional device and in-memory computing.

Houyi Cheng received his doctoral degree from the School of Integrated Circuit Science and Engineering at Beihang University. He is currently a lecturer at the School of Integrated Circuit Science and Engineering , with his main research interests in integrated circuit equipment and process research, as well as magnetic memory deposition processes.

Reza Rouzegar received his MSc degree in telecommunication engineering from Politecnico di Milano, Italy, in 2017. He joined the THz physics group at the Fritz Haber Institute of the Max Planck Society in 2018. He completed his PhD at the Freie Universität Berlin in 2023 and currently works there as a postdoc, focusing on THz spintronics, ultrafast magnetism and photonics.

Renyou Xu is a PhD student at Beihang University. He started his doctoral studies at the School of Integrated Circuit Science and Engineering in 2021. He joined Professor Weisheng Zhao’s research group, where his research focuses on orbitronics.

Shijie Xu, a postdoctoral fellow at Beihang University, was awarded his PhD from the School of Physical Science and Engineering at Tongji University in 2021, when he joined Weisheng Zhao’s research group. His research focuses on the electron transport properties of antiferromagnetic and ferromagnetic materials.

Jie Zhang is an assistant professor at the School of Integrated Circuit Science and Engineering, Beihang University, since 2019. She is working on low power consumption spintronic materials and devices, especially, focusing on the epitaxial growth of two-dimensional magnetic materials, terahertz emission phenomena, magnetization dynamics of two-dimensional magnetic heterojunctions, and spin orbital torque driven magnetic tunnel junction memory and logic devices.

Fan Zhang, associate professor, is the assistant director of the National Key Laboratory of Spintronics, Beihang University. Her research focuses on spin terahertz emission.

Yong Xu, associate professor, is the assistant director of the National Key Laboratory of Spintronics, Beihang University. His research focuses on ultrafast spintronics and broadband terahertz emitters. In the past five years, he has published over 40 SCI papers in journals such as Nature Communications and Advanced Materials.

Lianggong Wen is an associate professor at the School of Integrated Circuit Science and Engineering, Beihang University, and vice dean of the Microelectronics Research Institute, Beihang University Qingdao Research Institute. He joined Beihang University in 2017, having previously worked at the Interuniversity Microelectronics Centre (IMEC). His research interests include biomedical micro/nano systems, micro/nano devices, terahertz devices and sensors, and MEMS sensors.

Weisheng Zhao is currently the vice principal of Beihang University, he is also the director of Fert Beijing Research Institute and Beihang-Goertek Joint Microelectronics Institute. He graduated from University of Paris-Sud in 2007 and was nominated as tenured research scientist at CNRS in France from 2009 to 2013. He is the recipient of the prestigious IEEE Guillemin-Cauer Award (2017). In 2020, he became the editor-in-chief of IEEE Transactions on Circuits and Systems I: Regular Papers (TCAS-I). He has focused on the study of spintronics memory and logic devices, and established a multi-disciplinary research framework, from device design and novel materials research to circuit design. He serves on the editorial board of four SCI-indexed journals, including TCAS-I, IEEE Transactions on Nanotechnology, and IET Electronic Letters.

Tianxiao Nie received his BS degree from Shandong University, China, in 2006, and his PhD from the Department of Physics at Fudan University. He is currently a professor in the School of Integrated Circuit Science and Engineering at Beihang Univeristy. Before joining Beihang University, he worked as a postdoctoral fellow in the Electrical Engineering Department of UCLA. His research interests include the fields of spintronic terahertz, topological insulators, and vdW 2D ferromagnets.