Terahertz Functional Device Investigation Based On Strong Field And Spintronics

Due to the rapid development and progress of terahertz radiation generation and detection technology,more and more novel physical effects in this frequency band have been discovered and successfully applied.Among them,the sources of novel physical effects may be divided into two branches,one of which is derived from the increasing field strength of terahertz radiation,which pushes the energy scale of the interaction between terahertz radiation and matter to the non-perturbative and nonlinear region.The other stems from the discovery and research of novel functional materials.Due to their special electronic energy bands or magnetic structures,these novel materials can not only utilize the charge of electrons,but also utilize their spin degrees of freedom to greatly enrich function of the device in this band.In this doctoral dissertation,from the perspective of the first branch,we systematically study the photovoltaic response due to nonlinear impact ionization in light-emitting diodes（LEDs）under strong terahertz radiation illumination.From the perspective of the second branch,we systematically study the generation and modulation of chiral terahertz radiation in topological insulators,and theoretically study the generation and modulation of the spin currents when terahertz radiation pumps antiferromagnetic insulator/nonmagnetic heavy metal heterojunctions.The details are as follows:（1）We homemade a inserted LED detection circuit on a PCB,and then connected it to an oscilloscope with a bandwidth of 1 GHz.In the strong-field terahertz radiation generation and diagnosis setup,the generated terahertz radiation is focused onto the LED at the center of the PCB by collimating the focusing optical path,and then the strong-terahertz-induced photovoltaic signal in the LED can be observed in the oscilloscope.In the experiment,before focusing the terahertz radiation optical path,we used two metal wire grid terahertz polarizers to adjust the terahertz radiation energy,and then simultaneously observed the photovoltaic signal changes under different terahertz radiation energies.Theoretically,we use Monte Carlo simulation to reproduce the response change processes in LEDs which depend on the terahertz pump field strength,and prove that the impact ionization process induced by the terahertz radiation electric field is generated in the LEDs.In addition,we observed such photovoltaic signals in LED lamps of various colors,proving the universality of this effect and proposing a unified model to explain this phenomenon.Finally,we independently developed two types of terahertz LED prototypes,scanning type and single-shot array type,to image the terahertz radiation spot,which proved that fast-response high-field terahertz radiation detection and imaging can be performed with inexpensive LED lamps.It saves a lot of cost and lowers the barrier for the detection of strong field terahertz radiation.Due to its nanosecond-level response time,it also provides a new feasible solution for the time synchronization of strong-field terahertz radiation and probe laser.（2）We use Al₂O₃ as the substrate to grow the topological insulator Bi₂Te₃ single-crystal nanofilm sample by molecular beam epitaxy（MBE）.After verifying the single crystal quality with atomic force microscopy（AFM）and X-ray diffraction（XRD）,we employed a transmission-type polarization-resolvable terahertz time-domain spectrometer to characterize the broadband terahertz emission spectrum generated in the topological insulator Bi₂Te₃ under femtosecond laser pumping.Using a polarization-resolvable terahertz time-domain spectrometer,we experimentally demonstrate that when the incident angle of the femtosecond laser,the azimuth angle of the sample,and the helicity of the incident laser are controlled,terahertz emission of any polarization state can be achieved,And the radiation efficiency is comparable to commercial Zn Te.Theoretically,we used the phenomenological Photogalvanic effect to explain the terahertz radiation generated in topological insulators semi-quantitatively,and found that this phenomenological explanation is a good guide for the operation of the experiment and the optimization of the terahertz helicity.This research provides a new experimental scenario for realizing a polarization-tunable terahertz radiation at the source,and the device does not require an external magnetic field.The device is compact and lightweight,and has high radiation efficiency,making it a candidate for a new generation of terahertz emitters.（3）The spin pumping effect in an antiferromagnet（AFM）has been proved experimentally,but due to the cut-off frequency of the measurement circuit,the measurement of the charge flow in the terahertz band cannot be realized.Using linear response theory,we systematically study the amplitudes of ac spin currents generated in antiferromagnetic/heavy metals under different polarization terahertz pumping conditions.It is found that when the pumping terahertz radiation resonates with the antiferromagnet and satisfies the chirality-match condition,the generated spin current signal does not depend on the magnitude of the applied dc magnetic field.And it is clear that the spin current,whether it is dc or ac,has the same scaling relationship with the intrinsic magnetic properties of the material.Finally,we also found that in both ferromagnets and antiferromagnets,the dc spin current generated by the spin pumping effect is proportional to the pump power,and the ac spin current is proportional to the pump field strength,revealing a universal law of spin pumping effect among magnetic materials.The above three works have deepened the understanding of the interaction between terahertz radiation and condensed matter,and extended the functions of terahertz devices both from the perspective of strong-fields induced effect and novel functional materials.Specifically,we realized the mechanism study and characterization of strong terahertz radiation detection with inexpensive LEDs,which greatly reduced the cost of strong terahertz radiation detection and time synchronization.We realize chirally tunable terahertz emission from topological insulator Bi₂Te₃,which provides an alternative solution for future pulsed chiral terahertz radiation sources.Finally,we theoretically study the behavior of spin current generation in antiferromagnetic systems,revealing the universal law of spin pumping in magnetic systems,which can also be used as a novel terahertz detection scenario.