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 spin-momentum–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 Bi2Te3 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.
Tilted pulse front technique in lithium niobate has been widely used for strong-field terahertz generation in laboratories and with this method, lots of strong terahertz field induced phenomena have been observed. However, for mJ-level pulse energy, focused electric field >10 MV/cm solid state terahertz sources, there are still many scientific and technical challenges waiting to be explored. For real applications, the properties of intense terahertz source is very important, such as spatial chirp effect. In this work, we systematically investigate the spatial dispersion of intense terahertz generation process in lithium niobate. We also observe obvious non-uniform spatial terahertz frequency distribution with respect to the emission plane using a knife-edge measurement. Higher frequency generation is obtained when the emission spot is far away from the cutting edge of the crystal, while lower frequency emission is detected when the emission spot is close to the crystal edge. This phenomenon is contrary to the original predicts, of which higher frequencies will experience longer propagation distance resulting in weak contribution. The possible mechanism is the nonlinear distortion effect caused by high energy laser pumping. Our study is very important and useful for building intense terahertz systems with the applications in extreme terahertz science, and time-resolved nonlinear spectroscopy.
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