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.
Femtosecond control of electron spin not only promises the capability of satisfying the ever-increasing demand of storage information and ultrafast manipulation of magnetization in mediums, but also delivering controllable, highlyefficient, cost-effective and compact terahertz sources. Femtosecond spin dynamics have been extensively investigated these years with the methods of ultrafast magnetic-optical Kerr effect, inverse Faraday effect, inverse spin Hall effect and so on. Recently emerged coherent terahertz emission spectroscopy can also be employed to study this ultrafast spin dynamics with its unique advantages. For example, terahertz emission spectroscopy is a coherent, time-resolved, contactless Ampere-meter, which can be used to deduce the spin-charge conversion. However, femtosecond laser interaction with magnetic mediums is a rather complex process, there are still lots of physical mechanisms waiting to be unveiled. Here, we systematically investigate the femtosecond spin dynamics in ferromagnetic materials via polarization-resolved terahertz emission spectroscopy. We obtain detectable electromagnetic field radiation with its polarization parallel to the external magnetic field direction, which was not observed in the same materials in previous work. Inverse spin-orbit torque tilting is responsible for the observed phenomenon. Based on this mechanism, the efficiency and polarization of the generated terahertz waves can be coherently controlled and manipulated not only by the external magnetic fields, but also by the sample structures and the pumping femtosecond laser pulses. Our work not only helps further deepen understanding of the physical mechanism of all-optical magnetization reversal, boosting future spin recording technology, but also offers a very promising way for developing novel and efficient terahertz functional sources and devices.
The behaviors of hot electrons in femtosecond laser-plasma interaction have been studied systematically under laser irradiance of 5 X 1015 Wcm-2micrometers 2. A very directional jet emission of hot electrons with energies above 170 keV has been observed in the normal direction to the target surface. The angular distribution of the jet emission of hot electrons has been found to be dependent on the energy of hot electrons. By measuring the Faraday rotation angle of the backscattered emission, a magnetic field in the axial direction has been detected for the first time. The maximum value of the magnetic field was estimated to be as high as 1.76 +/- 0.7 Mgauss at such a modest irradiance. It is believed that this axial magnetic field is generated by the dynamo effect in the laser-plasma interaction.
Intense lasing at 18.9, 20.3 and 28.5 nm from nickel-like molybdenum, niobium and neon-like chromium ions has been observed by using two 200 ps laser pulses with a total energy of 50 J at 1.053 micrometers from XingGuang II laser facility. This shows the possibility of extending nickel- like and neon-like x-ray lasing in low-Z elements and paves the way towards small scale x-ray lasers for applications at university laboratories. A comparison has been made of performance of the neon-like chromium soft x-ray lasing at 28.5 nm driven by a double 900 ps pulse at 6 TW(DOT)cm-2, with that driven by a double 200 ps pulse at similar irradiance. The double 200 ps pulse has been found to be more efficient to drive the neon-like x-ray lasing.
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