0.1-30 THz) have experienced an unprecedentedly rapid progress with potential in fundamental sciences and real applications, especially under the fast development of ultrafast laser technology. [1] Recently, terahertz technologies have also been successfully employed in laboratories as powerful tools for triggering other novel related researches and applications, such as electron accelerations, [2] strong-field nonlinear phononics, [3] nonlinear terahertz photonics, [4] nonionization imaging, [5] and so on. Significantly large potential of terahertz technology could be envisaged in its real applications such as nondestructive spectroscopy [6] and ionization-free imaging, highly sensitive sensing, [7] short-range wireless communication at terahertz bit rates, [8,9] and so on. However, hindering the development of this fascinating technology from real applications lies in the lack of highly efficient terahertz sources, versatile functional devices, and sensitive detectors, as well as compact and robust systems. Among them, terahertz sources are significantly important, especially for those integrated with flexible manipulation of terahertz polarization states. As well demonstrated, when carrying with optical spin and angular momentum, [10] it enables another freedom of utilizing terahertz waves and boosts Flexible manipulation of terahertz wave polarizations during the generation process is very important for terahertz applications, especially for the next-generation on-chip functional terahertz sources. However, current terahertz emitters cannot satisfy such demand, hence calling for new mechanisms and conceptually new terahertz sources. Here an efficient and broadband terahertz source with magnetic-field-controlled flexible switching for the polarizations between linear and elliptical states in ferromagnetic heterostructures driven by femtosecond laser pulses is demonstrated. More importantly, the chirality, azimuthal angle, and ellipticity of the generated elliptical terahertz wave can be precisely manipulated by harnessing external magnetic fields via effectively tailoring the photoinduced spin currents. Such an ultrafast photomagnetic interaction-based, magnetic-field-controlled, and broadband tunable solid-state terahertz source integrated with polarization tunability functions not only provides 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 communications, and accelerating the development of ultrafast terahertz optospintronics. Terahertz RadiationThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

Free‐space super‐strong terahertz (THz) electromagnetic fields offer multifaceted capabilities for reaching extreme nonlinear THz optics. However, the lack of powerful solid‐state THz sources with single pulse energy >1 mJ is impeding the proliferation of extreme THz applications. The fundamental challenge lies in hard to achieve high efficiency due to high intensity pumping caused crystal damage, linear absorption, and nonlinear distortion induced short effective interaction length, and so on. Here, through cryogenically cooling the crystals, tailoring the pump laser spectra, chirping the pump pulses, and magnifying the laser energies, 1.4‐mJ THz pulses are successfully realized in lithium niobates under the excitation of 214‐mJ femtosecond laser pulses via tilted pulse front technique. The 800 nm‐to‐THz energy conversion efficiency reaches 0.7%, and a free‐space THz peak electric and magnetic field reaches 6.3 MV cm−1 and 2.1 Tesla. Numerical simulations reproduce the experimental optimization processes. To show the capability of this super‐strong THz source, nonlinear absorption in high conductive silicon induced by strong THz electric field is demonstrated. Such a high‐energy THz source with a relatively low peak frequency is very appropriate not only for electron acceleration toward table‐top X‐ray sources but also for extreme THz science and nonlinear applications.

We demonstrate generation of 0.2 mJ terahertz (THz) pulses in lithium niobate driven by Ti:sapphire laser pulses at room temperature. Employing tilted pulse front technique, the 800 nm-to-THz energy conversion efficiency has been optimized to 0.3% through chirping the sub-50 fs pump laser pulses to overcome multi-photon absorption and to extend effective interaction length for phase matching. Our approach paves the way for mJ-level THz generation via optical rectification using existing Ti:sapphire laser systems which can deliver Joule-level pulse energy with sub-50 fs pulse duration.

Polarization shaped terahertz sources play a key factor in terahertz wireless communications, biological sensing, imaging, coherent control in fundamental sciences, and so on. Recently developed spintronic terahertz emitters have been considered as one of the next-generation promising high performance broadband terahertz sources. However, until now, polarization control, especially for twisting the circularly polarized terahertz waves at the spintronic terahertz source, has not yet been systematically explored and experimentally achieved. In this work, we not only demonstrate the generation of circularly polarized terahertz waves in cascade spintronic terahertz emitters via delicately engineering the amplitudes, applied magnetic field directions, and phase differences in two-stage terahertz beams but also implement the manipulation of the chirality, azimuthal angle, and ellipticity of the radiated broadband terahertz waves. We believe our work can help with further understanding of the ultrafast optical magnetic physics and may have valuable contributions for developing advance terahertz sources and optospintronic devices.

Arbitrary manipulation of broadband terahertz waves with flexible polarization shaping at the source has great potential in expanding numerous applications, such as imaging, information encryption, and all-optical coherent control of terahertz nonlinear phenomena. Topological insulators featuring unique spinmomentum-locked surface state have already exhibited very promising prospects in terahertz emission, detection, and modulation, which may lay a foundation for future on-chip topological insulator-based terahertz systems. However, polarization-shaped terahertz emitters based on topological insulators with an arbitrarily manipulated temporal evolution of the amplitude and the electric-field vector direction have not yet been explored. We systematically investigated the terahertz radiation from topological insulator Bi 2 Te 3 nanofilms driven by femtosecond laser pulses and successfully realized the generation of efficient chiral terahertz waves with controllable chirality, ellipticity, and principal axis. The convenient engineering of the chiral terahertz waves was interpreted by a photogalvanic effect (PGE)-induced photocurrent, while the linearly polarized terahertz waves originated from linear PGE-induced shift currents. Our work not only provides further understanding of femtosecond coherent control of ultrafast spin currents but also describes an effective way to generate spin-polarized terahertz waves at the source.