Spintronic terahertz emitters (STEs), with high stability, a low cost, an ultrabroad bandwidth, and controllable polarization, have come a long way since the early days of this topic and show promising applications in communications, medicine, and imaging [1–6]. In terms of integration and miniaturization, one of the limitations is the emission intensity in a practical application. Especially for terahertz (THz) sources in information technology and quantum devices [7–11], there is a necessary requirement for highly efficient STEs: the low dissipation of energy. Some improvements for spintronic THz radiation enhancement have been surveyed; for example, searching for the interface state, the spin Hall angle, the index of refraction, and the absorption of layers. Luo
Optical structures can be easily accomplished with a vast choice of materials and manufacturing technology, as well as greater expandability [20–22]. Recently, excellent optical gain properties have been explored, such as bound state in the continuum (BIC) resonances , epsilon-near-zero (ENZ) films , and 1D photonic crystals [21,25–27]. Hybridizing a metasurface with ENZ films has emerged as a promising alternative to promote THz radiation and shape the THz polarization . Due to the weak compatibility and complicated manufacture process, however, it is difficult to widely use in other kinds of THz emitters. In terms of BIC resonances, it can cause strong electromagnetic resonances in low ohmic loss materials like lithium niobate but is difficult to use on metal materials with high ohmic loss. Similarly, ENZ films are very limited on materials that have weak THz radiation. Alternating periodic multilayers via different refractive indexes can create a photonic band gap along the propagation direction when it satisfies the special optical confining condition . Note that the photonic band gap has an excellent filter property; i.e., an increased sensitivity against an unwanted wavelength. Based on Maxwell’s equations, the band of a photonic crystal can be calculated and adjusted. As a result, photonic crystals have emerged as excellent reflectors when designing an optical structure against a specific wavelength of incident light. Related approaches that rely on planar metal and dielectric heterostructures have resulted in a few potentially viable strategies for light absorptivity, such as Tamm plasmon coupling (TPC) [29–31]. In this state, most of the light radiation is trapped on the metal/dielectric interface and is absorbed by the metal. Unlike the SPP that has been found in spin-thin films and a dielectric layer with a complicated structure (e.g., prisms ), TPC can be excited directly between the metal/dielectric interface . For TPC, most of the existing methods for strong absorptivity of the incident light require a thick metal . However, in terms of spintronics THz emission, it has an optimal thickness of the STE within several nanometers, resulting in a low absorptivity of the pump laser [35,36]. A method that offers strong absorptivity and optimal thickness of the STE, however, remains elusive, despite several potential applications.
In this paper, we created a photonic nanostructure design for TPC between the spin THz thin films and a 1D photonic crystal using a simple manufacturing technology. The spin thin films can act as both the THz emitter and planar metal for the TPC structure. The simulation and experimental results show that a 94.3% narrow absorptance spectrum at 780 nm and a broad transmissivity () in the THz range are obtained. Compared to pure spin thin films, spin thin films with TPC directly excited THz radiation with a 264% improvement without polarization mode confinement. Our approach enables modulation of the center of the incident laser wavelength for diverse scenarios by adjusting the dielectric properties, such as the refractive index of the dielectric and the insert layer’s thickness.
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2. RESULTS AND DISCUSSION
As shown in Fig. 1(a), the STE with TPC is composed of a 1D photonic crystal ( and ) and spin thin films (Pt/Co/MgO). When the pump beam is incident from the left of the spin thin films, the reflection coefficient for the incident light propagating to the spin thin films from the side of the 1D photonic crystal to the spin thin films is , the reflection coefficient of a wave incident on the dielectric layers from just after the spin thin films is , and these two reflection coefficients can be calculated by the transfer matrix method. More details can be found in Appendix A. In the simulations, the refractive indexes of Co, Pt, and MgO are adopted from Refs. [37–39]. The refractive indexes of and are 1.446 and 1.99, respectively. The dielectric layers are composed of 20 groups of alternately stacked and with thicknesses of 92 nm and 113 nm, respectively. Then the intrinsic mode field in the insert cavity layer can be represented by the transfer matrix [40,41]
Figure 1.(a) Schematic illustration of the spin thin films without TPC and with TPC structure for THz radiation. (b) Phase of
Thus, the phase match condition for the excitation of the TPC is
From the analysis above, it is easy to find that the thickness of the insert layer can affect the resonance wavelength of the TPC. By varying the thickness of the insert layer from 0 nm to 600 nm, the optical absorptance from 750 nm to 950 nm can be adjusted, as shown in Fig. 2(a). With the continuous increase of , the optical absorptance caused by TPC has a periodic change that is related to the resonant mode of the insert layer, as expressed by 
Figure 2.(a) Simulated absorptance varying with
After adding the proper insert layer, a narrow and high resonance absorption can be generated at 780 nm corresponding to the wavelength of the pump beam. Figure 2(b) shows the absorptance spectra with different . As Fig. 2(b) shows, a weak resonance absorption at the wavelength of 750 nm can be found when the thickness of the insert is 0. Moreover, the absorptance can be as high as 94.38% at 780 nm, when the thickness of the insert is increased to . Figures 2(c)–2(e) illustrate the normalized profile electric field distributions of points , , and . Note that the normalized electric field distributions are up to four times greater at point than at point , indicating the energy is well trapped at point . This is because point lies at the edge of the band gap of the 1D photonic crystal. Figures 2(d) and 2(e), show that point is comparable to point in its ability to generate TPC, and they correspond to the odd mode and even mode of the TPC, respectively. Considering the economical fabrication accessibility, a insert layer with a thickness of 57 nm is finally chosen. To investigate the optimal groups of alternating layers, we simulated the electric field distribution under different groups, as shown in Fig. 7 in Appendix C. With the increment of alternating layers, the electric field intensity increases before 20 groups, but approaches saturation after that number of groups. Thus, 20 groups of alternating layers are selected as a balanced consideration between the optical performance and simple preparation are discussed below.
After making a trade-off between spin to charge conversion efficiency and optical cavity performance, the well-defined alternating layers were prepared. To prepare the Tamm plasmon-enhanced STE, the 1D photonic crystal dielectric multilayer consisting of an insert layer () and 20 groups of alternating layers ( and ) was first fabricated by plasma-enhanced chemical vapor deposition (PECVD). Then, the spin thin films (Pt/Co/MgO) for THz emission were deposited on the top of the dielectric layers using magnetron sputtering. At the same time, for comparison, pure spin thin films Pt/Co/MgO without TPC were prepared by magnetron sputtering. Figure 3(a) shows the cross-sectional SEM image of the spin thin films with a TPC structure. It is clear that a good homogeneity in the layer thicknesses and no layer interpenetration exist, which assures that the TPC structure will have an excellent optical gain property. The average thicknesses of and are, respectively, about 120 nm and 93 nm. For the sake of THz emission, a 4 nm MgO film was deposited between the Pt/Co and dielectric layers. It can buffer the roughness of dielectric layers from PECVD. Figures 3(b)–3(d) show the average roughness of different samples using an AFM. The average roughnesses of the spin thin films with a TPC structure, the pure spin thin films, and the pure dielectric layers surface are, respectively, 4.29 nm, 0.16 nm, and 4.85 nm. It is easy to see that the roughness of pure dielectric layers is obviously smoothened by the buffer layer.
Figure 3.(a) Cross-sectional SEM image of the spin thin films with a TPC structure. The average roughness of (b) spin thin films with TPC, (c) pure spin thin films, and (d) pure dielectric layers surface.
Next, the performance of all samples was tested on a home-built THz time-domain spectroscopy system. A femtosecond pump laser (Spectra-Physics) with a repetition rate of 1 kHz, a central wavelength of 780 nm, and a pulse duration of 50 fs was used to pump the spintronic THz emission. A couple of permanent magnets used for in-plane magnetization along the axis acted on the spintronic samples, as illustrated in Fig. 1(a), and provided 20 mT of the centric magnetic field. Using a 1 mm thick ZnTe electrooptic crystal, the spintronics THz radiation can be detected by a balanced detector. All the experiments were performed at room temperature and in a environment. In our THz experiments, four comparison experiments were considered. Their illustrations are exhibited in Fig. 4(a), where the pump laser is located on the side of spin thin films and the THz radiation is detected after all samples and substrates. To ensure the accuracy of the results, all THz TDS measurements were repeatedly tested. In Fig. 4(a), two of THz waveforms were generated by the pure spin thin films without TPC, and the spin thin films with TPC, respectively. The other two THz waveforms were generated by the pure spin thin films and then passed through one substrate (without ) or dielectric layers deposited on a substrate (without TPC dielectric layers), respectively. There are two reasons why one substrate was used for comparison after the pure spin thin films: i) the dielectric layers are deposited at the substrate and ii) the substrate has an absorptance of 12.7% for the THz signal, as shown in Fig. 4(b). Excluding these irrelevant variables, the substrate was used to verify that the pure dielectric layers had no negative effects on the THz radiation. In Fig. 4(b), we found that the dielectric layers have little influence on the transmission of the generated THz wave, and it conformed to the simulation, as shown in Fig. 6 in Appendix B. As illustrated in Fig. 4(b), the pure dielectric layers also cannot enhance the THz radiation. Hence, it confirms that spin thin films with TPC structures play a crucial role in THz radiation enhancement.
Figure 4.(a) Illustration of four comparison experiments of pure spin thin films without TPC, spin thin films with TPC, spin thin films with
Figure 5.Absorptance spectra of the spin thin film with TPC under different incident angles and wavelengths for (a) a TM pump beam and (b) a TE pump beam. (c) THz waveforms from the spin thin films with TPC under different polarization states at the pump fluence of
Figure 6.Simulated transmittance of THz wave on the dielectric layers.
Figures 4(b) and 4(c) display the THz time-domain waveforms and the respective magnitude in frequency domain from the four samples. As shown in Fig. 4(c), our samples can obtain a 3 THz bandwidth. By contrast, the THz waveforms and bandwidth remain unchanged between the spin thin films with TPC or not, except for a stronger THz amplitude from the spin thin films with TPC. It means the dielectric layers have little influence on the frequency of the THz waves, which is supported by the simulation results in Fig. 6 in Appendix B. To survey the properties of the samples quantificationally, the value of the peak-to-peak THz electric field is defined as the maximum amplitude (MAX). Here, the maximum amplitudes of the spin thin films with TPC and without TPC are and , respectively. is 406 at the pump fluence of , as shown in Fig. 4(b), while can achieve 1072 with TPC enhancement at the same pump fluence. Strikingly, we observed a predictable 264% enhancement in the spin thin films samples by comparing their maximum amplitude and frequency domain spectra. The normalized THz amplitudes of 0.5 mm ZnTe, spin thin films without TPC and with TPC are compared in Fig. 8 in Appendix D. Note that spin thin films with TPC have an approximate value compared to the 0.5 mm thick ZnTe THz emitter. The enhancement of the THz electric field is caused by the absorption dependence of incident pump laser according to 
Figure 7.Normalized electric field profile distributions of (a) five groups, (b) 10 groups, (c) 20 groups, and (d) 25 groups of alternating layers.
Figure 8.THz waveforms from 0.5 mm thick
Figure 9.(a) Reflectance, (b) transmittance, and (c) absorptance spectra of pure spin thin films (spin thin films without TPC, blue line), spin thin films with TPC (red line), and pure dielectric layers (black line).
Figure 10.THz waveforms from the spin thin films: (a) without TPC and (b) with TPC as a function of laser power when the spot diameter is 1 cm.
Another character of TPC is that it can be flexibly excited in both TM and TE polarizations. Figures 5(a) and 5(b) present the simulated absorptance spectra of the spin thin film with TPC under different incident angles and wavelengths for TM and TE pump beams. With the incident angle increase from 20° to 40°, the responding pump wavelength of TPC appears to have a slight blue shift. Note that the absorptance for the 780 nm wavelength from 0° to 40° is still higher than that of the pure spin thin films since the TPC state excites a stronger resonance coupling. The incident angle, especially from 0° to 20°, continues to have a high absorptance tolerance. Significantly, the simulation absorptances in both the TM polarization and TE polarization are almost invariant at the same incident angle. In Fig. 5(c), the THz radiation experiments under both TM and TE polarization states are presented at a 0° incident pump laser beam, where the maximum amplitudes under TM and TE polarization states are and , respectively. Obviously, and are with no noticeable differences in Fig. 5(c), which are 1072 and 1056, respectively. Figure 5 shows the normalized maximum amplitude of THz waveforms from the spin thin films with TPC for TM and TE pump beams at different incident angles. In Fig. 5(d), the variation in the normalized MAX value under TE polarization is related to the incident angle of the pump laser from 0° to 40°. For the TM polarization, there is an acceptable reduction by varying the incident angle. All THz waveforms by varying incident angles are exhibited in Fig. 11 in Appendix G. Therefore, it can be seen that the experiments of the oblique incidence for the spin thin films with TPC are almost consistent with the simulation trend.
Figure 11.THz waveforms at
By combining spin thin films with a photonic crystal, a TPC state is established between them by theoretical simulation and experimental fabrication, significantly reducing the pump energy consumption. By adjusting the optical parameters, the designed structure possesses a significant photonic resonance absorption peak at the pump laser wavelength of 780 nm. Compared to the 36.8% absorptance of the pure spin thin films, 94.3% of the energy of the pump laser is trapped within the microcavity and absorbed by the spin thin films with TPC. Experimental results show that the THz electric field is enhanced nearly three times for the spin thin films with TPC. The dielectric layers as a photonic band gap offer a 98% reflectivity for the pump laser and a 95% transmission for the generated THz wave. Moreover, our design can provide a TPC polarization independent state and remain at a high absorptance under a multidirectional incident pump laser. Using a TPC to reduce the THz radiation consumption can also be easily adapted to other STE designs, greatly expanding the new avenues for THz radiation enhancement.
Acknowledgment. Y. Xiao acknowledges support from the cultivation projects of National Major R&D Project (92164108) and the Foundation of Innovation Center of Radiation Application (KFZC2020020901).
APPENDIX A: TRANSFER MATRIX METHOD FOR THE 1D PHOTONIC CRYSTAL AND THE SPIN THIN FILMS
To demonstrate the TPC between the spin thin films and the 1D photonic crystal, we modeled the optical reflectance and absorptance of the system based on the transfer matrix method. Derived from Maxwell’s equations and boundary conditions, the amplitudes of the electrical and magnetic field of the wave in the front and back interfaces of the -th layer can be expressed as [
The field amplitudes of the input and output waves in the one-dimensional photonic crystal can be characterized by the transfer matrix . Then the transmission and reflection coefficients of the system can be calculated as
On the other hand, because the spin thin films are composed of a Pt(4 nm)/Co(4 nm)/MgO(4 nm) multilayer heterostructure, the transfer matrix of this system is , where , , and are the transfer matrix of the MgO layer, Co layer, and Pt layer, and they can be calculated by the Eq. (
APPENDIX B: SIMULATED TRANSMITTANCE OF THz WAVE ON THE DIELECTRIC LAYERS
The simulated transmittance for the THz wave on the dielectric layers obtains 95%, and 96.35% within 2 THz in Fig.
APPENDIX C: NORMALIZED ELECTRIC FIELD PROFILE DISTRIBUTIONS
With the increment of alternating layers, the electric field intensity increases before 20 groups, but approaches saturation after that number of groups. Thus, 20 groups of alternating layers are selected as a balanced consideration between the optical performance and simple preparation.
APPENDIX D: COMPARISON WITH ZnTe CRYSTAL
The normalized THz amplitudes of 0.5 mm ZnTe, spin thin films without TPC and with TPC are compared in Fig.
APPENDIX E: REFLECTANCE, TRANSMITTANCE, AND ABSORPTANCE
To further confirm the absorptance of the samples, the reflectance and transmittance spectra of the different samples are measured by using a spectrometer system, as shown in Fig.
APPENDIX F: THz WAVEFORMS UNDER DIFFERENT LASER POWERS
All of tested THz time domain waveforms under different pump power can be found in Fig.
APPENDIX G: THz WAVEFORMS AT DIFFERENT INCIDENT ANGLES
All THz waveforms by varying incident angles are exhibited in Fig.
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