We manipulate the transient terahertz (THz) waves emitted from metal-graphite interfaces, where potential barriers were formed because of work function differences. To adaptively control the phase of the THz waves, two distinct groups of metals were evaporated on n-type doped highly oriented pyrolytic graphite (HOPG): group A, which consisted of Pt, Au, and Ag with work functions larger than that of HOPG and group B, which consisted of Al and Ti with work functions smaller than that of HOPG. The phase of the transient THz lineshapes from group A was opposite to that of group B under infrared laser excitation, which is indicative of opposite band bending and concomitant interfacial doping for ambipolar transport at the metal-graphite junctions. The amplitude of the THz waves could be further substantiated by the work function differences and modified minority carrier mobilities at the depletion regions.
Semi-metals might offer a stronger interaction and a better confinement for terahertz wave than semiconductors, while preserve tunability. Particularly, graphene-based materials are envisioned as terahertz modulators, filters and ultra-broadband sources. However, the understanding of terahertz generation from those materials is still not clear, thus limits us recognizing the potential and improving device performances. Graphite, the mother material of graphene and a typical bulk semi-metal, is a good system to study semi-metals and graphene-based materials. Here we experimentally modulate and …
Terahertz spin waves could be generated on-demand via all-optical manipulation of magnetization by femtosecond laser pulse. Here, we present an energy balance model, which explains the energy transfer rates from laser pulse to electron bath coupled with phonon, spin, and magnetization of five different magnetic metallic thin films: Iron, Cobalt, Nickel, Gadolinium and Ni$_{2}$MnSn Heusler alloy. Two types of transient magnetization dynamics emerge in metallic magnetic thin films based on their Curie temperatures (T$_{C}$): type I (Fe, Co, and Ni with T$_{C}$ > room temperature, RT) and type II…
Local phase control of electromagnetic wave, the basis of a diverse set of applications such as hologram imaging, polarization and wave-front manipulation, is of fundamental importance in photonic research. However, the bulky, passive phase modulators currently available remain a hurdle for photonic integration. Here we demonstrate full-range active phase modulations in the Tera-Hertz (THz) regime, realized by gate-tuned ultra-thin reflective metasurfaces based on graphene. A one-port resonator model, backed by our full-wave simulations, reveals the underlying mechanism of our extreme phase mo…
As a fascinating topological phase of matter, Weyl semimetals host chiral fermions with distinct chiralities and spin textures. Optical excitations involving those chiral fermions can induce exotic carrier responses, and in turn lead to novel optical phenomena. Here, we discover strong coherent chiral terahertz emission from the Weyl semimetal TaAs and demonstrate unprecedented manipulation over its polarization on a femtosecond timescale. Such polarization control is achieved via the colossal ultrafast photocurrents in TaAs arising from the circular or linear photogalvanic effect. We unravel …
External electric fields are highly attractive for dynamical manipulation of excitons in two-dimensional materials. Here, we theoretically study the ionization of excitons in monolayer transition metal dichalcogenides (TMDs) by intense pulsed electric fields in the terahertz (THz) regime. We find that THz pulses with realistic field strengths are capable of ionizing a significant fraction of photogenerated excitons in TMDs into free charge carriers. Short THz pulses are therefore an efficient, non-invasive method of dynamically controlling the free carrier concentration in monolayer TMDs, whic…
Electromagnetic field confinement is crucial for nanophotonic technologies, since it allows for enhancing light-matter interactions, thus enabling light manipulation in deep sub-wavelength scales. In the terahertz (THz) spectral range, radiation confinement is conventionally achieved with specially designed metallic structures - such as antennas or nanoslits - with large footprints due to the rather long wavelengths of THz radiation. In this context, phonon polaritons - light coupled to lattice vibrations - in van der Waals (vdW) crystals have emerged as a promising solution for controlling li…
Efficient manipulation of magnetization at ultrashort time scales is of particular interest for future technology. Here, we numerically investigate the influence of the so-called field-derivative torque, which was derived earlier based on relativistic Dirac theory [Mondal et al., Phys. Rev. B 94, 144419 (2016)], on the spin dynamics triggered by ultrashort laser pulses. We find that only considering the THz Zeeman field can underestimate the spin excitation in antiferromagnetic oxide systems as, e.g., NiO and CoO. However, accounting for both, the THz Zeeman torque and the field-derivative tor…
Coherent terahertz control of metastable magnetization in FePS3
Massachusetts Institute of Technology（麻省理工学院）；Max Planck Institute for the Structure and Dynamics of Matter（马普物质结构与动力学研究所）；Universidad del Pais Vasco（巴斯克大学）；Seoul National University（首尔国立大学）；The Flatiron Institute（平顿研究所）
Nonlinear manipulation of nuclear and electron spins is the basis for all advanced methods in magnetic resonance including multidimensional nuclear magnetic and electron spin resonance spectroscopies, magnetic resonance imaging, and in recent years, quantum control over individual spins. The methodology is facilitated by the ease with which the regime of strong coupling can be reached between radiofrequency or microwave magnetic fields and nuclear or electron spins respectively, typified by sequences of magnetic pulses that control the magnetic moment directions. The capabilities meet a bottle…
The melting transition in solid monolayers of a series of short-chained n-alkanes, n-octane (n-C8H18), n-decane (n-C10H22), and n-dodecane (n-C12H26) physisorbed onto the graphite basal plane are studied through use of molecular dynamics simulations. Utilizing previous experimental observations of the solid phase behavior of these monolayers, this study investigates the temperature dependence of the phases and phase transitions in these three monolayers during the solid-fluid phase transition, and compares the observed melting behavior to previous studies of hexane and butane monolayers. In pa…
Artificial semiconductor heterostructures played a pivotal role in modern electronic and photonic technologies, providing a highly effective mean for the manipulation and control of carriers, from the visible to the Terahertz (THz) frequency range. Despite the exceptional versatility, they commonly require challenging epitaxial growth procedures due to the need of clean and abrupt interfaces, which proved to be a major obstacle for the realization of room-temperature (RT), high-efficiency devices, like source, detectors or modulators, especially in the far-infrared. Two-dimensional (2D) layere…
