Impact of pump wavelength on terahertz emission of a cavity-enhanced spintronic trilayer

Abstract

We systematically study the pump-wavelength dependence of terahertz pulse generation in thin-film spintronic THz emitters composed of a ferromagnetic Fe layer between adjacent nonmagnetic W and Pt layers. We find that the efficiency of THz generation is essentially flat for excitation by 150 fs pulses with center wavelengths ranging from 900 to 1500 nm, demonstrating that the spin current does not depend strongly on the pump photon energy. We show that the inclusion of dielectric overlayers of TiO 2 and SiO 2 , designed for a particular excitation wavelength, can enhance the terahertz emission by a factor of of up to two in field. PACS numbers: Valid PACS appear hereTerahertz (THz) radiation is non-ionizing and, therefore, safe for many applications, ranging from microand macroscopic imaging and spectroscopy to wireless communication 1-4 . However, the spectroscopically interesting THz frequency band near 1 THz is not easily accessible. Electronic sources such as oscillators can only provide high (milliwatt) output levels up to a few 100 GHz 5 , while optical sources such as quantum cascade lasers are typically limited to frequencies >2 THz at room temperature 4,6 . To fill this "gap", considerable effort has been garnered towards sources capable of frequency mixing and optical rectification 7 , typically driven by femtosecond lasers.To date, most THz emitting materials have been found to be insulators or semiconductors 8 . Recently, THz emitters based on magnetic, metallic thin films have been demonstrated which emit THz radiation under illumination by femtosecond pulses 9-15 . We here focus on trilayer thin-film emitters formed from a ferromagnetic (FM) layer between two non-ferromagnetic (NM) layers. A two-step process is thought to generate THz radiation: 16 Upon excitation by the femtosecond pump pulse, an ultrashort out-of-plane spin current polarized along the FM magnetization is injected from the FM into the NM layers. Thereafter, the inverse spin Hall effect converts the laser-induced spin current into a transverse in-plane charge current within the NM layer which leads to the emission of a terahertz pulse into the optical farfield 16-18 .One of the most efficient films of this type 9 comprises W, CoFeB and Pt layers. Importantly, Pt and W feature a spin Hall angle of opposite sign, resulting in a constructive superposition of the two charge currents in these layers. The result is an ultrabroadband THz emitter, capable of delivering pulses spanning 0.1 to 30 THz. 9 a) rh522@exeter.ac.uk T i O 2 W C o F e B P t S a p p h i r e B P u m p T H z P u l s e y z x S i O 2 FIG. 1: Schematic of a spintronic trilayer with added dielectric cavity, grown on 0.5 mm of sapphire (Al 2 O 3 ).The near-infrared pump pulse, incident through the substrate, is partially absorbed in the metallic layers, launching a spin current from the ferromagnetic (FM) layer into the nonmagnetic (NM) layers. The inverse spin Hall effect converts this ultrashort out-of-plane spin current into an in-plane charge current resulting in the emis...

Magnetically and optically tunable terahertz radiation from Ta/NiFe/Pt spintronic nanolayers generated by femtosecond laser pulses

“…Although our data in Fig. 1(b) seem to contradict recent results by Herapath et al, 24 where no wavelength dependence on THz generation was reported, we stress that the measurements in Ref. 24 were performed not only on a different material system, but, first of all, exclusively at infrared wavelengths (900-1500 nm).…”

“…1(b) seem to contradict recent results by Herapath et al, 24 where no wavelength dependence on THz generation was reported, we stress that the measurements in Ref. 24 were performed not only on a different material system, but, first of all, exclusively at infrared wavelengths (900-1500 nm). High-energy, blue photons used in our studies are certainly more efficiently absorbed by metallic nanolayers and generate larger (up to a factor of three, according to our measurements) concentration of hot electrons that give rise to an enhanced spin current.…”

Laser‐Driven Strong‐Field Terahertz Sources

“…[122] By shortening the pump-pulse width from 40 to 10 fs, a spectrum covering the full interval from 1 to 30 THz can be expected. [112] • STE operation is independent of the pump wavelength, [123,124] most likely because the primary function of the pump pulse is heating of the electron subsystem. Therefore, large-scale STEs can be driven by any high-power laser, independent of its output wavelength.…”

“…Numerous improvements can be anticipated, e.g., enhanced pump absorption, [124] optimization of F [125][126][127] and N materials, better heat dissipation by the substrate, shorter pump pulses, [112] and antenna structures. Numerous improvements can be anticipated, e.g., enhanced pump absorption, [124] optimization of F [125][126][127] and N materials, better heat dissipation by the substrate, shorter pump pulses, [112] and antenna structures.…”

Modification of spintronic terahertz emitter performance through defect engineering

“…When triggered by ultrafast femtosecond (fs) laser pulses, they generate pulsed terahertz (THz) electromagnetic radiation due to the inverse spin Hall effect (ISHE), a mechanism that converts the spin currents originating in the magnetized FM layer into transient transverse charge currents in the NM layer resulting in THz emission 2 . Different strategies have been followed in order to explore the THz amplitude and bandwidth of the signal: different material compositions of FM/NM systems with a variety of thicknesses 3–8 , ferri- and antiferromagnetic metal/Pt structures 9,10 , spintronic emitters assisted by metal-dielectric photonic crystal 11 , metallic trilayer structures with different interface materials 12–14 , THz emission from Rashba type interfaces 15,16 and THz emission using different excitation wavelengths 7,17 . The spin-to-charge-conversion mechanism was additionally probed in metallic and insulating magnetic/NM interfaces 18–20 showing, however, much lower efficiency of the THz emission compared to the metallic magnetic layers.…”