Active control of metasurface via integrated spintronic terahertz emitter

In the past two decades, increasing attention has been devoted to the terahertz (THz) science and technologies (typically within the frequency range of 0.1–10 THz) due to their potential for a wide range of applications, such as wireless communication [1–3], security imaging [4, 5], and spectroscopy for material and biomedicine [6, 7]. In these applications, it is highly required to manipulate THz waves efficiently. In order to break intrinsic constrains of natural materials that commonly have weak responses to THz waves, metasurfaces (referring to arrays of delicately designed structures with ultrathin thickness) have been developed to achieve highly efficient and arbitrary THz responses. The realized THz functional devices include but are not limited to polarization conversion [8, 9], sensing [10, 11], spectral lineshape manipulation [12], beam steering [13, 14], vortex beam [15], and hologram [16]. Despite the great flexibility in the THz response design, most of the early THz metasurfaces along with their functions were fixed after fabrication, which can only act as passive devices. Nevertheless, for practical and extensive applications, active devices are highly expected to provide greater adaptabilities.

Nowadays, various concepts, materials, designs, and processes have been proposed and realized to support active THz metasurfaces. Here we give a brief overview of the advances from the perspective of manipulation mechanisms. (a) Tailoring loss by integrating active materials to manipulate amplitudes or phase delays. The active materials can be a whole layer attached to the metasurface or a constitutional part filling the gap, as schematically shown in figure 1(a), including but not limited to semiconductors [17–20], 2D materials [21–24], superconductors [25–28], and phase change materials [29–31] under electrical gating or light pumping. (b) Tailoring refractive index of surroundings to alter the resonance frequency of metasurface (figure 1(b)). To this end, liquid crystals are widely used with electrical gating [32–36]. (c) Tailoring resonant modes by switching on/off states of phase change materials or semiconductors (figure 1(c))—for example, from LC resonance to dipole mode [37] or collective mode [38], and switch of polarization states [39]. (d) Tailoring coupling between adjacent resonators—for example, from capacitive coupling to conductive coupling, which adjusts the number and strength of coupled modes [40, 41] (figure 1(d)). (e) Structural reconfiguration using microelectromechanical systems (MEMS) where in-plane and/or out-of-plane reshaping are widely applied [42–45] (figure 1(e)). In addition to the above metasurfaces with various active components, coherent control of passive metasurfaces using two opposite THz beams provides another pathway to manipulate the amplitude, absorption, and polarization of THz waves [46–48], as schematically shown in figure 1(f). It indicates that the mode change of incident THz wave can also play the role of active counterpart. Thus a question is naturally raised: is it benefit to integrate metasurfaces with tunable THz sources?

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In this work, we introduce a novel concept of active control of metasurfaces via integrating a tunable and programmable THz emitter—spintronic THz emitter (STE). Since the orientation of the linearly polarized THz emission from STE can be rotated continuously by an external magnetic field, the resultant THz response, in principle, can be tuned smoothly when integrating STE with an anisotropic metasurface. Here we show an example of STE integrated broadband metasurface quarter-wave plate, which enables full polarization control on the entire Poincaré sphere. We also make a future outlook that the STE integrated metasurface can be readily programmed by using a commercial spatial light modulator (SLM), which enables more advanced wavefront shaping and multifunction switching. This work may pave a new pathway to design active metasurfaces for THz spectroscopy and time domain applications.

STE is a recently emerged THz source which is distinct from all other THz emitters as it utilizes both spin and charge degrees of freedom. The core region of STE is a heterostructure of ferromagnetic/nonmagnetic (FM/NM) nanofilms [49], whose THz emission mechanism is as follows: (a) a femtosecond (fs) laser pulse generates ultrafast spin currents in the FM layer which subsequently injects into the NM layer; (b) a spin-charge conversion process occurs via the inverse spin Hall effect, which induces transient charge currents at a time scale of picosecond; (c) the transient charge currents generate electromagnetic radiation at THz frequencies into the free space. Due to the ultrafast process without phonon absorption, the bandwidth of the field spectrum can span to 30 THz without any gaps [50], which is superior to the conventional optically pumped THz sources, such as the widely used photoconductive antenna and ZnTe crystal. Since then, great efforts have been made to improve its efficiency [50–56]. Up to now, the highest efficiency of STE can reach the same level of 1-mm-thick ZnTe crystal, though its thickness is only a few nanometers. Such excellent performance enables STE to replace ZnTe crystal in the terahertz time-domain spectroscopy (THz-TDS), which has taken place in several research labs [57].

In principle, STE has several advantages for being integrated with metasurfaces. (a) Due to the nanofilm property, it can be readily deposited on dielectric [54, 55], semiconductor [58], and flexible substrate [52] with large area and low cost. (b) Since the emitted electric field is always perpendicular to the external magnetic field, the orientation of its polarization can be rotated continuously by controlling the external magnetic field [51, 59]. (c) When excited by spatially encoded femtosecond laser beam, the emitted THz beam presents associated patterns with the pixel size as small as a few micrometers [60], which indicates that the STE can be conveniently programmed via commercial SLM. Several STE integrated devices have been reported for polarization conversion [61], magneto-optic sensing [62], and biosensing [63].

In the following, we present a prototype design of STE integrated metasurface, which enables full polarization control over the entire Poincaré sphere via independently rotating the external magnetic field and the sample. To realize this functional device, a metasurface acting as a quarter-wave plate is required. Fortunately, almost all metasurface designs, from mono-layer to multilayer structures, are compatible with STE. Here we employ a bilayer design consisting of metallic wire gratings [9], which is readily fabricated and offers broadband performance. It is attached to a MgO substrate, while a STE is deposited on the opposite side of the substrate, as schematically shown in figure 2(a). We note that although the STE can be deposited on the metasurface directly, there is a primary benefit brought by introducing a high-permittivity MgO interlayer between the STE and the metasurface—more energy of THz pulses emitting into the high-permittivity side.

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Firstly, we investigate the effect of rotating the external magnetic field while fixing the metasurface. Suppose that the 'fast' axis of the metasurface is oriented at 45° with respect to the x-axis. When the magnetic field H is aligned with the x-axis, the emitted THz electric field from the STE is along the y-axis. In such case, the transmitted THz wave from the metasurface is left-handed circularly polarized (LCP). As H rotates, the circular polarization gradually turns to elliptical polarization with the major axis oriented at 135°, till it changes to a completely linear polarization at θ= 45° (where θ denotes the angle between H and the x-axis), as depicted in red in figure 2(b). With the further rotation of H, the polarization turns to be right-handed elliptical, and reaches the right-handed circularly polarization (RCP) at θ= 90°. When 90°< θ< 135°, the polarization is also right-handed elliptical but with the major axis oriented at 225°. As θ further increases, the polarization changes from linear (θ= 135°) to left-handed elliptical (135°< θ< 180°), and finally reaches LCP again at θ= 180°. Seeing it from the Poincaré sphere, the 180° rotation of H leads to the evolution of THz polarization states over a unit circle along a longitude line (red line in figure 2(c)).

Additional degree of freedom can be realized by alternatively rotating the integrated metasurface. Note that the orientation of the THz field from the STE is only dependent on H but robust to the laser polarization. Hence, the rotation of the metasurface is physically equivalent to the rotation of the 'fast' axis. More importantly, when the metasurface is rotated to a certain angle, a 180° rotation of H contributes to the evolution of THz polarization states along another longitude line of Poincaré sphere. For example, the case of rotating the sample by 45° clockwise is depicted in green in figures 2(b) and (c). Analogously, the THz polarization state can arbitrarily span the entire Poincaré sphere via the superposition of two rotation operations—rotating H and the metasurface itself from 0 to 180°. This is a straightforward example that utilizes the advantage of STE to achieve active control of a passive metasurface.

A sample was designed and fabricated to realize the concept in figure 2(a). Firstly, the bilayer metasurface was fabricated on a 0.5 mm thick MgO substrate by using conventional photolithography and metallization process. As schematically shown in figure 3(a), the bottom layer of the metasurface is a gold wire grating with the width (w₁) of 135 μm and the interval (g₁) of 135 μm, which is oriented at 45° with respect to the x-axis. The top layer is another gold wire grating with the width (w₂) of 32 μm and the interval (g₂) of 350 μm, which is aligned with the y-axis. The interlayer is made of polyimide (PI) with thickness (h₁) of 30 μm, and two PI capping layers with thickness (h₂) of 50 μm are employed to sandwich the bilayer wire grating. Figure 3(b) is a top view of a (square) unit cell with period (p) of 382 μm. A STE composed of SiO₂(110 nm)/Pt(1.8 nm)/Fe(1.8 nm)/W(1.8 nm) films was deposited on the opposite side of the MgO substrate as in our previous work [54]. The optical image of the fabricated sample is shown in figure 3(c).

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Numerical simulations were performed by the time domain solver of CST studio. A pulsed THz plane wave source is set at the end of the MgO substrate to emulate the role of the STE. The boundary conditions are set as 'periodic' in the x–y plane, and the maximum frequency in the simulated region is fixed at 0.77 THz (below the Floquet modes cutoff at normal incidence [64]). For the above sample, the best broadband performance is expected to occur when the polarization of the THz source is aligned with the y-axis [9]. The electric field amplitudes of the co-polarized component (E_yy ) and the cross-polarized component (E_xy ) are plotted in figure 4(a), which are almost equal within the frequency range of 0.43–0.75 THz. The phase retardance is exhibited in figure 4(b), which is nearly −90° from 0.56 THz to 0.72 THz. The corresponding ellipticity is shown by the solid line in figure 5(a). One can see that the frequency region for the value larger than 0.9 spans from 0.42 THz to 0.76 THz, which denotes a quite good relative bandwidth, ∼53% (centered at 0.64 THz). In contrast, the broadband performance deteriorates when the polarization of the THz source is rotated by 90° (aligned with the x-axis). The ellipticity is plotted as the dashed line in figure 5(a). The frequencies for the value over 0.9 are only from 0.65 THz to 0.75 THz (0.2 THz smaller than the best performance). This is an inherent constraint of the bilayer-grating design, where the wire grating in the top layer is aligned with the y-axis. The introduced anisotropy in the x–y plane leads to the variation of the electric field amplitudes for x- and y-polarized incidence. When the electric field amplitudes of the co-polarized component (E_yy ) and the cross-polarized component (E_xy ) are almost equal in a broad band for y-polarized incidence, those for x-polarized incidence (E_xx and E_yx ) can only be matched in a small bandwidth. It is worth mentioning that such anisotropy can be removed in reflective metasurface designs—for instance, the configuration in [8]. As we found that the electric field transmitted through a 5.4 nm-thick STE could be as high as 74% [54], the STE is also applicable for being integrated with reflective metasurface, which will be worth investigating in further study.

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In the THz experiments, THz-TDS was used to characterize the performance. The sample was pumped by a femtosecond Ti: sapphire oscillator delivering linearly polarized laser pulses with a duration of 100 fs, a center wavelength of 800 nm, a repetition rate of 80 MHz, and an average power of 2 W. The THz waves were detected by a photoconductive antenna, and two polarizers were mounted before the photoconductive antenna in order to measure the full polarization states.

The measured time-domain THz signals with continuously rotated magnetic fields (θ= 0°–180°) and fixed samples are shown in figure 6. The amplitude-x and amplitude-y depict the THz electric field amplitude projected on the x-axis and y-axis, respectively, and the z-axis depicts the time flow. Figure 6(a) corresponds to the case of θ = 0°, where the THz signal was designed to be LCP. It is observed that the electric field orientation rotates clockwise in the direction of the z-axis (the opposite direction of the THz wavevector ${\vec k_{{\text{THz}}}}$), which is in accordance with the LCP. In general, the bandwidth of the metasurface quarter-wave plate is limited in comparison to the ultra-broadband STE. Thus the time-domain THz signals seem not to be perfect LCP, as they also contain frequency components out of the LCP bandwidth. The RCP case with θ = 90° is shown in figure 6(d), where the electric field orientation rotates counterclockwise in the direction of the z-axis. To evaluate the bandwidth, we converted the time domain signal into the frequency domain by employing fast Fourier transformation, and then depicted the ellipticity in figure 5(b). The features are generally in agreement with the simulated results in figure 5(a). The deviations are mainly reflected in the two aspects: slight shifting of frequencies to the lower band and slight lowering of ellipticity at frequencies from 0.4 to 0.6 THz. Such behavior probably originates from the fabrication errors, such as the fluctuation of the PI thickness and the deviation of the relative angle between two gratings.

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The case of an arbitrary θ (17°) is depicted in figure 6(b), which appears a left-handed elliptical polarization. Figures 6(c) and (e) exhibit linear polarizations oriented at 135° and 225° with respect to the x-axis, respectively, which correspond to the magnetic field rotations of θ= 45° and θ= 135°, respectively. When θ increases to 180°, the polarization changes to LCP again, as shown in figure 6(f). The only difference from the case of 0° (figure 6(a)) is the opposite sign of the THz signals in the time domain. To sum up, the experimental results confirms that when the external magnetic field is rotated from 0° to 180°, the polarization of the THz signals evolve over a unit circle along a longitude line of the Poincaré sphere, in accordance with the red line in figure 2(c).

Above we demonstrated a fundamental design of STE integrated metasurface utilizing the magnetic control and nanofilm properties of STE. More interestingly, STE has another significant advantage: it can be conveniently programmed via being excited by spatially encoded fs laser beam, which has been successfully applied to the THz single-pixel near-field imaging with the spatial resolution of as high as 6.5 μm [60]. Such characteristic and capability are quite promising for the future evolution of metasurface—to be programmable and multifunctional. In contrast to the existing programmable manners, such as semiconductors, phase change materials, and MEMS, the scheme of using STE does not need any complicated fabrication process other than conventional passive metasurfaces. In principle, it can empower arbitrary passive metasurface to be programmable by simply integrating with a uniform STE and using a commercial SLM for encoded excitation. Once the interval between the STE and the metasurface is far less than the THz wavelength, the pattern of the spatially modulated pumping laser can be mapped into the resultant THz patterns generated by the STE [60].

The configuration is schematically shown in figure 7. If one uses an amplitude-only SLM whose elements are individually programmable in binary states of either 'on' or 'off', the uniform STE will exhibit the same pattern with 'on' and 'off' distributions. There are several scenarios can be expected:

• (a)  
  STE integrated with single functional metasurface. In addition to the magnetic degree of freedom, beam steering and focusing can be realized when the pumping laser presents grating or Fresnel zone patterns. Taking the STE integrated metasurface quarter-wave plate for example, the polarization states and beam steering/focusing can be independently manipulated without interruption.
• (b)  
  STE integrated with hybrid metasurface. For the hybrid metasurface with multiple elements in a super cell, THz patterns can be tailored to selectively illuminate the specific elements. Consequently, different functions as well as their combinations can be switched.
• (c)  
  STE integrated with active metasurface. Since the STE is compatible with semiconductors, 2D materials, phase change materials, and liquid crystal, the arbitrary manipulation of amplitude, phase, and frequency band can also be employed in addition to the polarization control and encoded excitation. Taking the STE integrated metasurface quarter-wave plate for example, although there have been several routes for broadband performance, the bandwidth of metasurface is still limited in compared with the spectrum width of the STE. If the active components are designed to shift the frequency band of metasurface, the whole structure will offer more flexibility in real-world applications.

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Furthermore, anisotropy can also be introduced into the STE itself to provide more degree of freedom. Stripe-patterned STEs have been reported to own the capability of manipulating amplitude, polarization, and spectrum of the emitted THz wave [51, 65]. One can expect versatile multifunctional designs based on the obvious advantages of integrating STE with both passive and active metasurfaces. Nevertheless, it is worth mentioning that the constraints are also apparent. It lies in the fact that the STE can only be efficiently excited by fs laser up to date, and thus it can only act as a THz pulse emitter. It suggests that the STE integrated metasurface will meet the application scenarios most for THz spectroscopy and time domain imaging, but may be not applicable for THz communication and radar. Hence, it is important to design the metasurfaces according to specific applications.

In summary, we have explored a new concept for active control of metasurfaces via integrating a tunable and programmable STE. By utilizing the unique property of the STE that the orientation of the linearly polarized THz emission can be rotated continuously by external magnetic field but is irrelevant to the pumping laser, we demonstrated a design of STE integrated metasurface quarter-wave plate, which enables full polarization control over the entire Poincaré sphere. An example was given based on a typical broadband bilayer-grating metasurface, whose largest bandwidth can be as high as 53% for specific polarized incidence according to the simulation results. Experiments were performed to realize this idea, where only small deviation occurred (probably originating from the fabrication errors). And the possibility of STE integrated reflective metasurface was enlightened as well.

Thanks to the unique advantage of STE—enabling convenient programming via spatially encoded laser pumping, we also made a future outlook that the integrated STE can empower conventional metasurface to be programmable and multifunctional by readily using a commercial SLM but without the requirement of complicated design and fabrication. Up to date, the STE has demonstrated its great potentials for THz spectroscopy and time domain applications. We are highly expecting that the combination with metasurface may stimulate new and fertile ideas for promoting more innovative applications.

This work was supported by the National Natural Science Foundation of China (NSFC) (62027807, 62005256, 61905225, 62222106) and the National Key R&D program of China (2021YFA1401400).

All data that support the findings of this study are included within the article (and any supplementary files).