Abstract
Spintronic terahertz emitters promise terahertz sources with an unmatched broad frequency bandwidth that are easy to fabricate and operate, and therefore easy to scale at low cost. However, current experiments and proofs of concept rely on free-space ultrafast pump lasers and rather complex benchtop setups. This contrasts with the requirements of widespread industrial applications, where robust, compact, and safe designs are needed. To meet these requirements, we present a novel fiber-tip spintronic terahertz emitter solution that allows spintronic terahertz systems to be fully fiber-coupled. Using single-mode fiber waveguiding, the newly developed solution naturally leads to a simple and straightforward terahertz near-field imaging system with a 90%-10% knife-edge-response spatial resolution of 30 µm.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Spintronic terahertz emitters (STEs) convert an ultrashort laser pulse into an ultrabroadband terahertz pulse through a sequence of spin-current generation, spin-to-charge-current conversion, and current-to-field conversion within a thin-film stack of ferromagnetic metal (FM) and normal metal (NM) layers [1,2]. In 2010, M. Battiato, K. Carva, and P. M. Oppeneer theoretically predicted a spin-dependent superdiffusive transport of laser-excited electrons resulting in a strong spin injection from an FM into an NM layer [3]. In the following years, A. Melnikov et al. were the first to confirm this effect experimentally [4] and T. Kampfrath et al. could show that this effect combined with the inverse spin Hall effect (ISHE) in an FM-NM heterostructure can lead to the emission of terahertz pulses [5]. Latest since T. Seifert et al. demonstrated an efficient tri-layer spintronic terahertz emitter in 2016, the field has attracted considerable interest [6]. It raised the prospect of terahertz sources with unmatched broad frequency bandwidth that could be scaled conveniently and cost effectively. Therefore, a considerable amount of research has been done to further improve the optical-to-terahertz conversion efficiency and the total terahertz output power. Detailed overviews can be found in the references [1,7–9]. To highlight some examples, it has been shown that a (W/Fe/Pt/SiO$_{2}$)$_{\text {2}}$ tri-layer stack can yield a 1.7 fold increase in terahertz conversion efficiency compared to a single (W/Fe/Pt/SiO$_{2}$)$_{\text {1}}$ tri-layer structure [10]. Similarly, a photonic cavity structure was shown to increase the terahertz conversion efficiency by a factor of 1.7 [11]. In addition to improving the laser pumping process, it was shown that attaching a hyper-hemispherical silicon lens [12] or coupling an H-dipole antenna [13] to the STE significantly improves the terahertz outcoupling process. In addition, groundbreaking proofs of concept for a variety of possible applications have been demonstrated: Since the emitted terahertz electric field is to a very good approximation perpendicular to the FM magnetization [1], controlling and modulating the direction of the FM magnetization promises applications such as spintronic-based terahertz ellipsometry or terahertz wireless communication. The FM magnetization can be controlled not only by rotating the external magnetic field [14,15], but also by a simple bipolar variation of the magnetic field strength [16], or by magneto-electrically inducing a mechanical strain [17,18]. In addition to these achievements in terahertz polarization control, a terahertz magneto-optical imager consisting of an electro-optic crystal sensor stacked on an STE is noteworthy [19].
However, in these demonstrations, free-space ultrashort-pulsed pump lasers are used alongside rather complex benchtop setups. This does not meet industrial requirements for robustness, compactness, and safety. A transition from free-space laboratory setups to fully fiber-based systems is therefore desirable.
Here we present a novel fiber-tip spintronic terahertz emitter solution that allows spintronic terahertz systems to be fully fiber-coupled. This is an important step towards industrial-grade systems that exploit the many advantages of STEs.
As will be shown in the following sections, our novel fiber-tip spintronic terahertz emitter solution not only makes full fiber-coupling possible, but also inherently provides terahertz sources with sub-wavelength dimensions that are suitable for broadband terahertz near-field imaging [20,21]. In comparison to the fiber-based terahertz near-field imaging sources developed by M. Yi et al. who created a terahertz source with sub-wavelength dimensions by bonding an epitaxially grown InAs crystal to a 45-degree wedge end facet of an optical fiber [22], our terahertz near-field imaging sources require a less complex fabrication process, can be operated wavelength-independently, and exhibit higher bandwidth as well as higher near-field resolution. Integrating one of our fiber-tip STEs into a standard terahertz time-domain spectroscopy setup seamlessly leads to a terahertz near-field imaging system, that is much simpler and more robust than state-of-the-art terahertz scattering-type scanning near-field optical microscopy (THz s-SNOM) systems [23–28]. As a final remark, two other noteworthy spintronic-based concepts for terahertz near-field imaging were published recently: Z. Bai et al. demonstrated a spintronic terahertz near-field biosensing approach [29] and Chen et al. presented a spintronic microscopy concept based on the ghost near-field imaging technique with sub-10 $\mathrm{\mu}$m spatial resolution [30]. While the first is very limited in bandwidth and tailored toward the demonstrated use case, the latter requires again a complex laboratory setup including free-space lasers and two digital micromirror devices as programmable masks making these approaches cumbersome.
2. Fiber-tip spintronic terahertz emitters
In our new concept, a spintronic tri-layer structure [W(2.0 nm) / FeCoB(1.8 nm) / Pt(2.0 nm)] is sputtered directly onto the tip of a fiber. Therefore, we call it a fiber-tip spintronic terahertz emitter. Fig. 1 shows an example of such a fiber-tip STE. The fiber is enclosed in a standard 2.5 mm fiber ferrule to achieve two advantages: first, it makes handling and integration very convenient, and second, the pump fiber can be connected via a standard fiber-optic connector with physical contact (FC/PC). The scanning electron microscopy zoom shown in Fig. 1 c) reveals the surface and dimensions of the fiber-tip STE from Fig. 1 a). Using single-mode fibers optimized for our pump wavelength of 1550 nm, the pump mode field diameter at the surface can be reduced to only about 10 $\mathrm{\mu}$m. We have fabricated fiber-tip STEs with two different fiber materials (glass and sapphire) and with different diameters ranging from 100 $\mathrm{\mu}$m to 425 $\mathrm{\mu}$m.
3. Methods
3.1 Setup
To characterize the fiber-tip spintronic terahertz emitters, we used a standard fiber-based terahertz time-domain spectroscopy setup consisting of a single mode-locked Erbium-doped fiber laser (ML EDFL) with two equal output ports, each emitting a pulse train with 100 MHz repetition rate, 1560 nm center wavelength, and 70 fs FWHM temporal pulse width at the emitter and detector positions, two identical Erbium-doped fiber amplifiers (EDFA), and a delay stage (Fig. 2). The pump fiber was butt-coupled to the fiber-tip STE via a simple FC/PC connection with reflection losses of less than 5 %. The magnetic field was provided by two small NdFeB magnets integrated near the fiber tip in a 3D printed one inch ferrule holder. The emitted terahertz signal was modulated by a rotating chopper blade and focused by two parabolic mirrors onto a photoconductive antenna (PCA) detector. The induced current was converted to voltage by a trans-impedance amplifier and finally detected by a lock-in amplifier. For terahertz near-field imaging, a high-resolution metal mask was mounted on a motorized XYZ stage and moved in front of the fiber-tip STE.
3.2 Fabrication
The spintronic W(2.0 nm) / FeCoB(1.8 nm) / Pt(2.0 nm) tri-layer structure was deposited on the fiber tips using a thin-film sputtering process. Prior to deposition, a cleaning procedure involved 5 min ultrasonication steps in successive baths of acetone, isopropyl alcohol, and de-ionized water. The W/FeCoB/Pt spintronic emitters were deposited by RF-diode sputtering in LEYBOLD Z550 equipment where the individual layers are sputtered from circular 4 inches targets. In particular, the Fe$_{60}$Co$_{20}$B$_{20}$ alloy is obtained from a target with this stoichiometry. To ensure high precision in the thicknesses, the deposition was carried on a rotatory turn-table substrate holder in an oscillation mode: each time the substrate passes under the target, which corresponds to one oscillation, a layer of the material is deposited with a thickness that depends on the angular speed of the turn-table. The deposition is carefully calibrated beforehand by typically depositing a film with 100 oscillations.
For the W(2.0 nm) / FeCo(0.5 nm)/TbCo$_{2}$(0.8 nm)/FeCo(0.5 nm) / Pt(2.0 nm) emitters, the TbCo$_2$ and FeCo alloys composing the ferromagnetic layer are obtained from composite targets. Given their thickness, the CoFe/TbCo$_2$/CoFe tri-layer acts as an exchange-coupled multilayer. During deposition, the tips are placed in an in-plane field with an approximate strength of 80 kA/m to imprint an in-plane magnetic anisotropy along a chosen direction in the ferromagnetic tri-layer.
4. Results
4.1 Terahertz signals
Figure 3 exemplary shows a 30 ps section of a recorded terahertz waveform with a time-domain window of 100 ps and the associated frequency spectrum. We achieved a dynamic range of 28 dB and a bandwidth of 2.5 THz at an average pump power of 25 mW. This proves the full functionality of our newly developed concept. It is worth noting that the simple FC/PC butt coupling of the pump fiber allows us to exchange and operate a fiber-tip spintronic emitter within a few minutes.
4.2 Near-field imaging
Figure 4 shows the knife-edge responses for four different fiber-tip STEs varying in fiber type and diameter: a standard single-mode glass fiber and three coreless sapphire fibers with outer diameters of 100 $\mathrm{\mu}$m, 250 $\mathrm{\mu}$m, and 425 $\mathrm{\mu}$m, respectively. The measurements confirm the expected mode field diameter dependent near field resolution. Using the single-mode fiber tip STE with a pump mode field diameter of approximately 10.5 $\mathrm{\mu}$m (FWHM), we achieved a 90%-10% knife-edge resolution of 30 $\mathrm{\mu}$m. The coreless sapphire fibers with diameters of 425 $\mathrm{\mu}$m, 250 $\mathrm{\mu}$m, and 100 $\mathrm{\mu}$m resulted in 90%-10% knife-edge resolutions of 233 $\mathrm{\mu}$m, 178 $\mathrm{\mu}$m, and 97 $\mathrm{\mu}$m, respectively. To test these capabilities, we performed 2D near-field imaging of two metal strips for the different emitter types with the maximum of the terahertz waveform as the working point. The metal strips have a width of about 77.5 $\mathrm{\mu}$m and a spacing of about 122.5 $\mathrm{\mu}$m (optical microscopy image readout). As can be seen in Fig. 5, the stripes are clearly resolved with the single-mode fiber-tip spintronic terahertz emitter, only hinted at with the 100 $\mathrm{\mu}$m emitter, and not resolved with the 250 $\mathrm{\mu}$m emitter.
To increase the resolution further, the use of sharpened fiber-tips might evolve into a promising option. However, due to the small dimensions of the source in comparison to the emitted wavelength, outcoupling the terahertz radiation into the ambient air efficiently may pose a major challenge. In addition, the optical damage threshold could limit the signal-to-noise ratio considerably.
5. Challenges and outlook
5.1 Magnetic-bias-free fiber-tip STE
The results presented above were achieved using the well-known W/FeCoB/Pt thin film design. However, in this design, the generation of terahertz pulses still relies on an external magnetic bias, which could become a challenge for small devices and lab-on-a-chip technologies. Replacing the ferromagnetic FeCoB layer with an anisotropic FeCo/TbCo$_{2}$/FeCo heterostructure solves this challenge and allows the fiber-tip STEs to be operated without an external magnetic bias [16]. Figure 6 shows the recorded terahertz signal under the same conditions as Fig. 3. Although the peak-to-peak amplitude is lower than that of the FeCoB fiber-tip STEs with external magnetic bias, we consider a dynamic range of 21.5 dB a decent performance and an interesting opportunity to explore further. A challenge to be addressed is the steady demagnetization due to heat accumulation at higher pump powers.
5.2 Material destruction
A second challenge is a material change that we observe for pump powers above a mode field diameter dependent threshold. Since the terahertz signal decreases steadily on a second to minute time scale once the pump threshold is reached, and since we observe a darkening of the material in the region of the pump field (Fig. 7), we are confident that this is an irreversible chemical process due to the accumulation of heat. The fact that the material destruction threshold scales with increasing diameter supports this hypothesis (Fig. 8). Approximating the fluences based on the knife-edge responses shown in Fig. 4 and assuming a uniform pump mode field distribution, the fluence threshold is below 2 kW/cm$^2$ (0.02 mW/$\mathrm{\mu}$m$^2$) for both sapphire and glass fiber tip STEs.
6. Conclusions
In conclusion, we introduced our newly developed fiber-tip spintronic terahertz emitter solution, which allows spintronic terahertz systems to be fully fiber-coupled. The pump fiber can be connected via a standard FC/PC connector, and the emitters can be easily changed within a few minutes. We have recorded terahertz signals to demonstrate full functionality, and our near-field imaging with a 90%-10% knife-edge resolution of 30 $\mathrm{\mu}$m demonstrates a possible application. In addition, we presented an advanced magnetic-bias-free fiber-tip spintronic terahertz emitter and addressed the challenge of an existing material destruction threshold limiting the pump power. We anticipate our solution to lay the groundwork for a far-reaching transition from free-space setups to fiber-coupled systems and to catalyze further research in the field of fiber-tip spintronic terahertz emitters.
Funding
Horizon 2020 European Union Research and Innovation (860060 (MagnEFi), 863155 (s-Nebula)); Deutsche Forschungsgemeinschaft (TRR 173 - 268565370 - A01, B02, and B11).
Acknowledgement
The authors gratefully acknowledge funding from the Horizon 2020 European Union Research and Innovation program under FET-OPEN Grant agreement No. 863155 (S-Nebula) and Marie Skłodowska-Curie Grant agreement No. 860060 "Magnetism and the effect of Electric Field" (MagnEFi), as well as from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - TRR 173 - 268565370 (SPIN+X, projects A01, B02, and B11). Moreover, the authors would like to thank the French RENATECH Network and the Nano Structuring Center at the RPTU Kaiserslautern-Landau for their technical support during the development of the fiber-tip spintronic terahertz emitters.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are openly available in Dataset 1 [31].
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