Optical damage limit of efficient spintronic THz emitters

Abstract

Summary THz pulses are generated from femtosecond pulse-excited ferromagnetic/nonmagnetic spintronic heterostructures via inverse spin Hall effect. The highest possible THz signal strength from spintronic THz emitters is limited by the optical damage threshold of the corresponding heterostructures at the excitation wavelength. For the thickness-optimized spintronic heterostructure, the THz generation efficiency does not saturate with the excitation fluence even up till the damage threshold. Bilayer (Fe, CoFeB)/(Pt, Ta)-based ferromagnetic/nonmagnetic (FM/NM) spintronic heterostructures have been studied for an optimized performance for THz generation when pumped by sub-50 fs amplified laser pulses at 800 nm. Among them, CoFeB/Pt is the best combination for an efficient THz source. The optimized FM/NM spintronic heterostructure having α-phase Ta as the nonmagnetic layer shows the highest damage threshold as compared to those with Pt, irrespective of their generation efficiency. The damage threshold of the Fe/Ta heterostructure on a quartz substrate is ∼85 GW/cm 2 .

Optimum excitation wavelength and photon energy threshold for spintronic terahertz emission from Fe/Pt bilayer

“…Other thickness combinations of Fe and Pt may be explored to directly corroborate the observations of pump wavelength dependence due to pump absorptance in this work. Pump wavelength dependence measurements of the spintronic THz emission can also be utilized to probe other properties, such as the optical damage limit ( Kumar et al., 2021 ) of various spintronic heterostructures.…”

“…The successful demonstration of THz generation due to spin-to-charge current conversion in Fe/Au and Fe/Ru magnetic heterostructures ( Kampfrath et al., 2013 ) was replicated and explored further by other research groups using various combinations of FM and NM materials in bilayer, trilayer, and multilayer heterostructures. The published works to-date include investigations on optimum layer thicknesses ( Seifert et al., 2016 ; Yang et al., 2016 ; Wu et al., 2017 ; Torosyan et al., 2018 ; Zhang et al., 2018 ; Qiu et al., 2018a ; Kumar et al., 2021 ), material choices for substrates ( Wu et al., 2017 ; Torosyan et al., 2018 ; Nenno et al., 2019 ), incorporation of liquid crystal ( Qiu et al., 2018b ), dielectric cavity ( Herapath et al., 2019 ), striped patterns ( Yang et al., 2016 ; Jin et al., 2019 ), and antenna structures ( Nandi et al., 2019 ; Talara et al., 2021 ); different growth parameters ( Torosyan et al., 2018 ; Nenno et al., 2019 ), defect engineering ( Nenno et al., 2019 ), and optical damage limit ( Kumar et al., 2021 ). Several of these investigations have Pt as the NM material ( Seifert et al., 2016 ; Yang et al., 2016 ; Wu et al., 2017 ; Torosyan et al., 2018 ; Qiu et al., 2018a ; Qiu et al., 2018b ; Herapath et al., 2019 ; Jin et al., 2019 ; Nandi et al., 2019 ; Nenno et al., 2019 ; Matthiesen et al., 2020 ; Talara et al., 2021 ; Kumar et al., 2021 ).…”

Large interfacial contribution to ultrafast THz emission by inverse spin Hall effect in CoFeB/Ta heterostructure

“…Under pre-treatment procedure, the substrates were chemically cleaned by double ultrasonication in acetone and isopropyl alcohol for removing various types of impurities from the silicon surface. For the CoFeB/Ta and Fe/Pt bilayers, the thicknesses of FM and NM layers are such that a reasonably high THz signal is obtained from those combinations ( Kumar et al., 2021b ). A separate set of samples having individual layers of Pt (3nm) and Ta (3nm) were also deposited.…”

THz emission from Fe/Pt spintronic emitters with L10-FePt alloyed interface

“…Further optimization of STE was achieved by alternating the stack geometry of the STE: cascading FM/NM layers as multilayers ( Wu et al., 2017 ), using stacking sequence as NM1/FM/NM2 where NM1 and NM2 have opposite spin Hall angles ( Seifert et al., 2016 ), inserting the STE layer as an interlayer into a metal-dielectric photonic crystal ( Feng et al., 2018 ), and using metallic trilayer structures with different patterned structures, interface materials, and substrates ( Jin et al., 2019 ; Li et al., 2018 ; Seifert et al., 2018 ; Li et al., 2019 ; Mag-usara et al., 2019 ; Hibberd et al., 2019 ; Kong et al., 2019 ; Scheuer et al., 2020 ). Research effort has also been focused on a better utilization of the laser pulse energy by using different excitation wavelengths ( Papaioannou et al., 2018 ; Herapath et al., 2019 ) and exploring the optical damage threshold ( Kumar et al., 2021 ).…”