Terahertz (THz) frequency occupies a unique position in the electromagnetic spectrum, serving as a transitional band between macro-scale electronics and micro-scale photonics. As an important electromagnetic wave, terahertz (THz) wave has strong practicability in non-destructive testing, wireless communication and imaging sensor technology. Ultra-high data rates in communication technology push toward exploring broadband and commercially available terahertz (THz) signal sources. The application and development of terahertz (THz) technology are largely limited by the level of THz sources. Therefore, obtaining stable and high-performance THz sources has been a major focus of current research.
Due to the feature of high stability and economic efficiency, ultrabroad bandwidth, spintronic THz emitters (STEs) have been a hot topic in the field of THz sources. This approach offers advantages such as independence from the driving wavelength, ultra-wide bandwidth, and high damage threshold. The principle involves using femtosecond laser pumping on spintronic THz heterostructures composed of ferromagnetic and non-magnetic thin films. The ferromagnetic layer undergoes ultrafast demagnetization, thereby inducing the generation of ultrafast spin currents. The ultrafast spin currents diffuse from the ferromagnetic layer into the non-magnetic layer, where they undergo spin-orbit coupling, transforming into ultrafast charge currents in non-magnetic thin films. This leads to THz radiation.
The mechanism of spintronics THz emission and its modulation has been extensively studied over the years and made a significant progress. However, spintronics THz emitters and THz time domain system still face some challenges: (1) limited utilization of pump light resulting in weak THz generation, and (2) the inability to effectively employ high-energy pump light in the THz spectroscopic system, leading to potential damage to subsequent optical components. Moreover, the current attenuators contribute to undesirable THz wave attenuation.
To solve the aforementioned challenges, the group of Prof. Weisheng Zhao and Xiaoqiang Zhang from Beihang University proposed a high performance spintronics THz emitter. Here, through the combination of optical physics and ultrafast photonics, the Tamm plasmon coupling (TPC) facilitating THz radiation is realized between spin THz thin films and photonic crystal structures. The relevant research results were published in Photonics Research, Volume 11, No. 6, 2023 (Yunqing Jiang, Hongqing Li, Xiaoqiang Zhang, Fan Zhang, Yong Xu, Yongguang Xiao, Fengguang Liu, Anting Wang, Qiwen Zhan, Weisheng Zhao. Promoting spintronic terahertz radiation via Tamm plasmon coupling[J]. Photonics Research, 2023, 11(6): 1057).
In this work, most of the light radiation is trapped on the metal/dielectric interface and absorbed by the metal. Unlike the surface plasmonic polariton that has been found in spin-thin films and a dielectric layer with complicated structure, e.g. prisms, the TPC can be excited directly between the metal/dielectric interface. To design this innovative spintronics THz emitter, the research group initially optimized the parameters of the one-dimensional photonic crystal and spintronics THz thin films using the transfer matrix theory and Tamm plasmon theory. According to the Tamm plasmon theory, the excitation requires satisfying the phase matching condition, given by , where r1, r2, dinsert represent the reflection coefficients of the spintronics THz thin films, the reflection coefficients of the one-dimensional photonic crystal, and the thickness of the intermediate layer between the spintronics THz thin films and the one-dimensional photonic crystal, respectively. The research group conducted simulations and investigations of the reflection phases at different wavelengths for the spintronics THz thin films, the one-dimensional photonic crystal, and the value of at a thickness of 57 nm for the inserted layer. They found that TPC state can be generated at a wavelength of 780 nm. Furthermore, the research results demonstrated efficient transmission of THz waves through the designed one-dimensional photonic crystal, providing a theoretical basis for subsequent device fabrication.
Fig. 1. (a) Schematic illustration of the spin thin films without TPC and with TPC structure for THz radiation. (b) the phase of r1, r2 and r1r2exp[i(4πnSiO2dinsert)/λ] as a function of wavelength, when the thickness of the optical cavity is 57 nm; (c) Simulated reflectance spectra of the dielectric layers as a function of incidence angle and wavelength for TM polarization.
To prepare the Tamm plasmon-enhanced STE, the one-dimensional photonic crystal dielectric multilayer consisting of an insert layer (SiO2) and twenty groups of alternating layers (SiO2 and Si3N4) is fabricated by the plasma-enhanced chemical vapor deposition (PECVD) firstly. Then, the spin thin films (Pt(4 nm)/Co(4 nm)/MgO(4 nm)) for THz emission are deposited on the top of the dielectric layers using magnetron sputtering. The research group conducted tests on the spectral absorption of the spintronics THz emitter, providing theoretical and experimental verification, that it can achieve Tamm plasmon coupling state.
Subsequently, using a self-built terahertz time-domain spectroscopy system, the research group evaluated the terahertz emission performance of this novel spintronics THz emitter. Compared to traditional Pt/Co spintronics THz emitters, the new spintronics THz emitter exhibited a significant improvement in pump light utilization, increasing from 36.8% to 94.3%. It generated terahertz waves with an amplitude 2.64 times higher than that of traditional spintronics THz emitters. The efficient absorption of the pump light and the presence of the one-dimensional photonic crystal structure enabled the emitter to effectively block the transmission of the pump light while facilitating efficient transmission of the generated terahertz waves.
Fig. 2. (a) The illustration of four comparison experiments that pure spin thin films (spin thin films w/o TPC), spin thin films with TPC (spin thin films w/ TPC), the spin thin films with SiO2 substrate (w/o TPC + SiO2), the spin thin films with pure dielectric layers (w/o TPC + dielectric layers). The THz waveforms (b) and the frequency-domain THz signal (c) from the spin thin films w/o TPC (blue line, MAX is 406), the spin thin films w/ TPC (red line, MAX is 1072), w/o TPC + SiO2 (black line, 354), and w/o TPC + dielectric layers (green line, MAX is 356). at the pump fluence of 12.7 μJ/cm2. (d) The simulation absorptance spectra of the spin thin films with TPC and without TPC. (e) The maximum amplitude of generated THz electric field as a function of the pump laser power when its spot diameter is 1 cm.
The research group stated, "This work fully demonstrates the advantages of Tamm plasmon coupling in enhancing spintronics THz emission and promoting integration in THz systems. Moreover, this approach offers possibilities for addressing the compatibility issues between optical structure design and low energy consumption in ultrafast THz opto-spintronics and similar devices. In the future, the team will further engage in the research and development of spintronics THz emitters to enhance device stability and improve THz generation efficiency."
