Enhancement in optically induced ultrafast THz response of MoSe2MoS2 heterobilayer

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

Ultrafast THz radiation generation from ferromagnetic/nonmagnetic (FM/NM) spintronic heterostructures generally exploits the spin-charge conversion within the nonmagnetic layer and its interface with the ferromagnetic layer. Various possible sub-contributions to the underlying mechanism need to be exploited not only for investigating the intricacies at the fundamental level in the material properties themselves but also for improving their performance for broadband and high-power THz emission. Here, we report ultrafast THz emission from (CoFeB,Fe)/(Ta,Pt) bilayers at varying sample temperatures to unravel the role of intrinsic and extrinsic spin-charge conversion processes through the extracted values of spin-Hall conductivities. An enhanced THz emission along with temperature-dependent THz signal polarity reversal is observed in the case of annealed CoFeB/Ta. These results demonstrate a large interfacial contribution to the overall spin-Hall angle arising from the modified interface in the annealed CoFeB/Ta.

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Temperature-dependent THz emission from spintronic heterostructures

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THz polarity reversal at a certain temperature in the annealed CoFeB/Ta

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Determination of spin-Hall conductivity from the efficiency of THz emission

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Distinguishing the intrinsic/extrinsic contributions to spin-charge conversion

Ultrafast, broadband and tunable terahertz reflector and neutral density filter based on high resistivity silicon

We report THz transmission and reflection properties of an ultrafast optically excited highly resistive silicon wafer. Amplified Ti:Sapphire femtosecond laser pulses at 800 nm were used to create fluence-dependent carrier density on the front surface of the wafer which modifies the dielectric properties at the THz frequencies. Time-resolved experiments in the optical pump-THz probe configuration were conducted in which THz pulses reflected off from the surface at 0° and 45° angles of incidence make it possible to measure the pump-fluence dependent ultrafast evolution of the reflection and transmission coefficients in 0.5-6 THz range. An analytical model, where both the Drude contributions from the photo-excited electrons and holes account for the change of the dielectric constant of the photo-excited silicon, has been used to evaluate the THz reflection and transmission coefficients at steady state. Thus obtained results match well with the experimental results and demonstrate an all-optical means to convert a silicon wafer into an ultrafast, tunable and broadband neutral density filter or reflector in the THz frequency range.

Optical damage limit of efficient spintronic THz emitters

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/cm².

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THz generation efficiency of (CoFeB,Fe)/(Pt,Ta) spintronic film heterostructures

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Determination of optical damage threshold at NIR excitation

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Mean value of the optical damage threshold is ∼60 GW/cm²