High‐Performance THz Emitters Based on Ferromagnetic/Nonmagnetic Heterostructures

Wu, Yang; Elyasi, Mehrdad; Qiu, Xuepeng; Chen, Mengji; Yang, Liu; Ke, Lin; Yang, Hyunsoo

doi:10.1002/adma.201603031

Cited by 216 publications

(245 citation statements)

References 41 publications

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“…It is apparent that the THz polarity is reversed when either the applied magnetic field or pumping direction is reversed; the former indicates that the THz emission is of magnetic origin [25][26][27] whereas the latter implies that there is a reversal of the sign of the driving force that is responsible for the generation of the THz wave. Similar sign dependence on pumping direction has been observed in FM/NM bilayer emitters [2,6,8]. But, in those cases, it is understood that the sign reversal of THz wave polarization is caused by the reversal of the direction of spin polarized current due to sample flipping [3,10].…”

Section: A Ahe Origin Of Thz Emissionsupporting

confidence: 68%

Terahertz Emission from Anomalous Hall Effect in a Single-Layer Ferromagnet

Zhang

Luo

et al. 2019

Phys. Rev. Applied

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We report on terahertz emission from a single layer ferromagnet which involves the generation of backflow nonthermal charge current from the ferromagnet/dielectric interface by femtosecond laser excitation and subsequent conversion of the charge current to a transverse transient charge current via the anomalous Hall effect, thereby generating the THz radiation. The THz emission can be either enhanced or suppressed, or even the polarity can be reversed, by introducing a magnetization gradient in the thickness direction of the ferromagnet. Unlike spintronic THz emitters reported previously, it does not require additional non-magnetic layer or Rashba interface.

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Section: A Ahe Origin Of Thz Emissionsupporting

confidence: 68%

Terahertz Emission from Anomalous Hall Effect in a Single-Layer Ferromagnet

Zhang

Luo

et al. 2019

Phys. Rev. Applied

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“…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][10][11][12][13][14][15] . We here focus on trilayer thin-film emitters formed from a ferromagnetic (FM) layer between two non-ferromagnetic (NM) layers.…”

Section: Pacs Numbers: Valid Pacs Appear Herementioning

confidence: 99%

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

Herapath

Hornett

Seifert

et al. 2019

Applied Physics Letters

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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...

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“…It is worth noting that THz emission by spintronic multilayers consisting of nanometer-thick ferromagnetic (FM) and nonmagnetic (NM) metals has led to the development of efficient THz emitters 19,25,26,27,28,29 which are capable of providing a broadband and continuous spectrum from 1 to 30 THz without gaps (for details see ref. 19).…”

Section: Introductionmentioning

confidence: 99%

Terahertz spectroscopy for all-optical spintronic characterization of the spin-Hall-effect metals Pt, W and Cu₈₀Ir₂₀

Seifert

Tran

Gueckstock

et al. 2018

J. Phys. D: Appl. Phys.

101

107

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Identifying materials with an efficient spin-to-charge conversion is crucial for future spintronic applications. In this respect, the spin Hall effect is a central mechanism as it allows for the interconversion of spin and charge currents. Spintronic material research aims at maximizing its efficiency, quantified by the spin Hall angle and the spin-current relaxation length . We develop an all-optical contact-free method with large sample throughput that allows us to extract and . Employing terahertz spectroscopy and an analytical model, magnetic metallic heterostructures involving Pt, W and Cu80Ir20 are characterized in terms of their optical and spintronic properties. The validity of our analytical model is confirmed by the good agreement with literature DC values. For the samples considered here, we find indications that the interface plays a minor role for the spin-current transmission. Our findings establish terahertz emission spectroscopy as a reliable tool complementing the spintronics workbench. Figure 1. Schematic of the experiment. (a) Terahertz emission experiment. A femtosecond nearinfrared pump pulse excites electrons in both the ferromagnetic (FM, in-plane magnetization ) and non-magnetic (NM) metal layer. Due to the asymmetry of the heterostructure, a spin current is injected from the FM into the NM material where it is converted into an in-plane charge current by the inverse spin Hall effect (ISHE). The sub-picosecond charge-current burst leads to the emission of a terahertz (THz) pulse into the optical far-field. (b) Terahertz transmission experiment. A THz transient is incident onto either the bare substrate or onto the substrate coated by a thin metal film. By comparing the two transmitted waveforms and , the metal conductivity at THz NM THz pulse FM Femtosecond pump M j s j c ISHE (a) (b) Metal Substrate frequencies is determined.Figure 2. Typical THz emission raw data and sample characterization. (a) THz emission signal measured from a C40F40B20(3 nm)|Pt(3 nm) bilayer for two opposite orientations of the sample magnetization (± ). (b) Normalized pump-power dependence of the THz signal amplitude (RMS) for one orientation of the sample magnetization. (c) Pump-light absorptance, transmittance and reflectance as function of the Pt-layer thickness. (d) Frequency-dependent THz conductivities measured by THz transmission experiments (black and red dots) along with fits obtained by the Drude model (black and red solid lines).

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High‐Performance THz Emitters Based on Ferromagnetic/Nonmagnetic Heterostructures

Cited by 216 publications

References 41 publications

Terahertz Emission from Anomalous Hall Effect in a Single-Layer Ferromagnet

Terahertz Emission from Anomalous Hall Effect in a Single-Layer Ferromagnet

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

Terahertz spectroscopy for all-optical spintronic characterization of the spin-Hall-effect metals Pt, W and Cu₈₀Ir₂₀

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High‐Performance THz Emitters Based on Ferromagnetic/Nonmagnetic Heterostructures

Cited by 216 publications

References 41 publications

Terahertz Emission from Anomalous Hall Effect in a Single-Layer Ferromagnet

Terahertz Emission from Anomalous Hall Effect in a Single-Layer Ferromagnet

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

Terahertz spectroscopy for all-optical spintronic characterization of the spin-Hall-effect metals Pt, W and Cu80Ir20

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Terahertz spectroscopy for all-optical spintronic characterization of the spin-Hall-effect metals Pt, W and Cu₈₀Ir₂₀