Cascaded Amplification and Manipulation of Terahertz Emission by Flexible Spintronic Heterostructures

Abstract

Spintronic heterostructures consisting of ferromagnetic (FM) and nonmagnetic (NM) have become increasingly important devices for terahertz (THz) pulse generation, named as spintronic THz emitter (STE). The recycling of the laser pump energy is known to have a key impact on further improvement of the THz emission. Here, an efficient and practical approach to enhance and manipulate THz generation based on a cascaded Pt/CoFe/Ta trilayer fabricated on a flexible polyethylene terephthalate (PET) substrate is shown. Considering the refractive index n of PET is ≈1.61 at 1.55 eV and ≈1.75 at THz frequencies, it is anticipated that the femtosecond (fs) laser pulse propagates with a similar group velocity as the generated THz pulse. Therefore, the cascaded design enables all THz emission from each PET/STE propagating almost in‐phase and yields a 1.55 times amplification compared to a single PET/STE. Two proof‐of‐concept demonstrations are experimentally presented. First one, a metasurface can be assembled with the cascaded PET/STE to manipulate the THz signal. Second one, the cascaded PET/STE is used to perform a spectroscopic measurement of riboflavin. These demonstrations highlight the potential of flexible PET/STE for building blocks of advanced functionalities, such as compact THz emitters, THz spectroscopic, and imaging systems.

Anomalous Nernst Effect Induced Terahertz Emission in a Single Ferromagnetic Film

“…The terahertz (THz) wave refers to the electromagnetic radiation within the spectral window between the electronics and optics. , This region is where many fundamental collective excitations or ultrafast charge/phonon/spin dynamics occur. THz spectroscopy thus can serve as a sensitive probe to the physical insights of various materials. − Besides the importance in fundamentals, the THz wave also exhibits great potential in many applications, such as communications, spectroscopy, imaging, biomedical, security, etc. − In parallel, the rich excitations in many different materials also yield a broad platform to explore novel THz devices. − One of the recent and fascinating examples is the magnetic heterostructure-based spintronic THz emitter, ,,− which provides the advantages such as ultrabroad bandwidth, high efficiency, high flexibility, and controllable polarization. As a result, the spintronic THz emitter has been successfully employed in various applications, − and THz spintronics has emerged as a new and vivid research field. − …”

Terahertz Spectral Signatures of Ultrafast Spin Transports in Ferromagnetic Heusler Alloy

“…Thus, the light‐induced spin currents in Co 2 FeSi differ either from semiconductors [ 78–80 ] or metals. [ 52–55 ]…”

“…Thus, the light-induced spin currents in Co 2 FeSi differ either from semiconductors [78][79][80] or metals. [52][53][54][55] The photonenergy dependent THz emission are further performed to identify the photo-induced thermalization and the optical transitions from majority/minority spins to the out-ofequilibrium spin current injection of Co 2 FeSi/Pt heterostructure. Figure 4a,c shows the representative THz waveforms from Co 2 FeSi/Pt and Fe/Pt measured from 0.8 to 2.48 eV, under the same experimental condition (excitation fluence is 0.5 mJ cm −2 ).…”

“…[44][45][46][47][48] Therefore, another challenge is a clear experimental distinguishing between contributions to the ultrafast spin dynamics from majority and minority spins in Heusler alloy. [49] In this study, we address these challenges using ultrafast THz emission TDS, which gives direct access to the ultrafast demagnetization, [50,51] and spin-to-charge current conversion in FM based heterostructures, such as FM/NM, [52][53][54][55] FM/2D hybrid metal halides, [56] antiferromagnets/NM, [57] FM/topological insulator, [58][59][60] FM/MoS 2 [61] and semimetals/NM, [62,63] on the (sub-) picosecond timescales in a contact-free and rapid fashion. Following optical excitation of Co 2 FeSi/Pt, we observe sizable emission of broadband THz wave generation.…”

Terahertz Emission Spectroscopy of Ultrafast Coupled Spin and Charge Dynamics in Nanometer Ferromagnetic Heterostructures

“…The rise and decay of the j res ( t ) mainly depends on the conductivity of the heterostructure. Therefore, a more proper consideration of the complex interplay between spin voltage profile, spin-to-charge conversion, and material properties is required to maximize and shape the THz emissions [ 60 ], which will be our future research focus.…”