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Register a new userUltrabroadband single-cycle terahertz pulses with peak fields of 300 kV cm$^{-1}$ from a metallic spintronic emitter
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To explore the capabilities of metallic spintronic thin-film stacks as a source of intense and broadband terahertz electromagnetic fields, we excite a W/CoFeB/Pt trilayer on a large-area glass substrate (diameter of 7.5 cm) by a femtosecond laser pulse (energy 5.5 mJ, duration 40 fs, wavelength 800 nm). After focusing, the emitted terahertz pulse is measured to have a duration of 230 fs, a peak field of 300 kV cm$^{-1}$ and an energy of 5 nJ. In particular, the waveform exhibits a gapless spectrum extending from 1 to 10 THz at 10% of amplitude maximum, thereby facilitating nonlinear control over matter in this difficult-to-reach frequency range and on the sub-picosecond time scale.
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Terahertz electromagnetic radiation is extremely useful for numerous applications such as imaging and spectroscopy. Therefore, it is highly desirable to have an efficient table-top emitter covering the 1-to-30-THz window whilst being driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source which relies on tailored fundamental spintronic and photonic phenomena in magnetic metal multilayers: ultrafast photo-induced spin currents, the inverse spin-Hall effect and a broadband Fabry-Perot resonance. Guided by an analytical model, such spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort pulses fully covering the 1-to-30-THz range. Our novel source outperforms laser-oscillator-driven emitters such as ZnTe(110) crystals in terms of bandwidth, terahertz-field amplitude, flexibility, scalability and cost.
Charge-density wave (CDW) is one of the most fundamental quantum phenomena in solids. Different from ordinary metals in which only single particle excitations exist, CDW also has collective excitations and can carry electric current in a collective fashion. Manipulating this collective condensation for applications has long been a goal in the condensed matter and materials community. Here we show that the CDW system of 1T-TaS2 is highly sensitive to light directly from visible down to terahertz, with current responsivities around the order of ~1 AW-1 at room temperature. Our findings open a new avenue for realizing uncooled, ultrabroadband and sensitive photoelectronics continuously down to terahertz spectral range.
Flexible manipulation of terahertz-wave polarization during the generation process is very important for terahertz applications, especially for the next-generation on-chip functional terahertz sources. However, current terahertz emitters could not satisfy such demand, hence calling for new mechanism and conceptually new terahertz source. Here we demonstrate a magnetic-field-controlled, highly-efficient, cost-effective, and broadband terahertz source with flexible switch of terahertz polarization states in ferromagnetic heterostructures driven by femtosecond laser pulses. We verify that the chirality, azimuthal angle, and ellipticity of the generated elliptical terahertz waves can be independently manipulated by delicately engineering of the external applied magnetic fields via effectively manipulating the photo-induced spin currents. Such an ultrafast photomagnetic interaction-based, magnetic-field-controlled, and broadband tunable solid-state terahertz source integrated with terahertz polarization tunability function not only has the capability to reveal physical mechanisms of femtosecond spin dynamics, but also demonstrates the feasibility to realize novel on-chip terahertz functional devices, boosting the potential applications for controlling elementary molecular rotations, phonon vibrations, spin precessions, high-speed terahertz communication, and accelerating the development of ultrafast terahertz opto-spintronics.
Phase-locked ultrashort pulses in the rich terahertz (THz) spectral range have provided key insights into phenomena as diverse as quantum confinement, first-order phase transitions, high-temperature superconductivity, and carrier transport in nanomaterials. Ultrabroadband electro-optic sampling of few-cycle field transients can even reveal novel dynamics that occur faster than a single oscillation cycle of light. However, conventional THz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielectric functions averaged over the size, structure, orientation and density of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved THz spectroscopy (20 - 50 THz) to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM). We trace the time-dependent dielectric function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast ($<$50 fs) formation of a local carrier depletion layer.
We investigate the response of palladium-cobalt bi-layer thin films to hydrogen charging at atmospheric pressure for spintronic applications. We find that hydrogen absorption by the palladium layer results in the narrowing and shifting of the ferromagnetic resonance line for the material. We explain the observed phenomena as originating from reduction in spin pumping effect and from variation in the magnetic anisotropy of the cobalt film through an interface effect. The shift of the resonance frequency or field is the easiest to detect. We utilize it to demonstrate functionality of the bi-layer films as a hydrogen sensor.
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