We observe enhanced terahertz (THz) radiation generated from a Si3N4 film-coated GaAs photoconductive dipole antenna. Compared to an uncoated antenna with identical electrode geometry and optical excitation power, the...We observe enhanced terahertz (THz) radiation generated from a Si3N4 film-coated GaAs photoconductive dipole antenna. Compared to an uncoated antenna with identical electrode geometry and optical excitation power, the Si3N4 film-coated antenna has a higher effective DC resistance and larger breakdown voltage. As a result, the peak amplitude of generated THz radiation is significantly enhanced due to the Si3N4 film-coated layer.展开更多
Optoelectronic terahertz generation and detection play a key role in the applications of non-destructive testing,which involves different areas such as physics,biological,material science,imaging,explosions detection,...Optoelectronic terahertz generation and detection play a key role in the applications of non-destructive testing,which involves different areas such as physics,biological,material science,imaging,explosions detection,astronomy applications,semiconductor technology and superconductiong electronics. In this article,we present a reviewof the principle and performance of typical terahertz sources,detectors and non-destructive testing applications. On this basis,the newdevelopment and trends of terahertz radiation detectors are also discussed.展开更多
Terahertz(THz)wave generation from laser-induced air plasma generally requires a short temporal laser pulse.In contrast,it was observed that THz radiation from ionized liquid water prefers a longer pulse,wherein the m...Terahertz(THz)wave generation from laser-induced air plasma generally requires a short temporal laser pulse.In contrast,it was observed that THz radiation from ionized liquid water prefers a longer pulse,wherein the mechanism remains unclear.We attribute the preference for longer pulse duration to the process of ionization and plasma formation in water,which is supported by a numerical simulation result showing that the highest electron density is achieved with a subpicosecond pulse.The explanation is further verified by the coincidence of our experimental result and simulation when the thickness of the water is varied.Other liquids are also tested to assure the preference for such a pulse is not exclusive to water.展开更多
Enhanced terahertz wave generation via a Stokes cascade process has been demonstrated using picosecond pulse pumped terahertz parametric generation at 1 kHz repetition rate.Clear cascade saturation of terahertz output...Enhanced terahertz wave generation via a Stokes cascade process has been demonstrated using picosecond pulse pumped terahertz parametric generation at 1 kHz repetition rate.Clear cascade saturation of terahertz output was observed,and the corresponding cascade-Stokes spectra were analyzed.The maximum terahertz wave average power was 22μW under a pump power of 30 W,whereas the maximum power conversion efficiency was 8×10^(-7)under a pump power of 21 W.The THz power fluctuation was measured to be about 1%in 20 min.This THz parametric source with a relatively stable output is suitable for a variety of practical applications.展开更多
To make further understanding of terahertz(THz)wave generation from liquid water,we study THz wave emission from water lines of different diameters.The water line with a smaller diameter generates a stronger THz elect...To make further understanding of terahertz(THz)wave generation from liquid water,we study THz wave emission from water lines of different diameters.The water line with a smaller diameter generates a stronger THz electric field for the diameters from 0.2 mm to 0.5 mm.The THz electric field strength and polarity change with the relative position between the incident laser and water line.Moreover,the THz energy has an optimal radiation angle of about 60°.A two-dimensional dipole array model is introduced to illustrate the phenomenon.Our observations contribute to optimizing the scheme of the liquid THz source.展开更多
The ability to generate and manipulate broadband chiral terahertz waves is essential for applications in material imaging,terahertz sensing,and diagnosis.It can also open up new possibilities for nonlinear terahertz s...The ability to generate and manipulate broadband chiral terahertz waves is essential for applications in material imaging,terahertz sensing,and diagnosis.It can also open up new possibilities for nonlinear terahertz spectroscopy and coherent control of chiral molecules and magnetic materials.The existing methods,however,often suffer from low efficiency,narrow bandwidth,or poor flexibility.Here,we propose a novel type of laser-driven terahertz emitters,consisting of metasurface-patterned magnetic multilayer heterostructures,that can overcome the shortcomings of the conventional approaches.Such hybrid terahertz emitters combine the advantages of spintronic emitters for being ultrabroadband,efficient,and highly flexible,as well as those of metasurfaces for the powerful control capabilities over the polarization state of emitted terahertz waves on an ultracompact platform.Taking a stripe-patterned metasurface as an example,we demonstrate the efficient generation and manipulation of broadband chiral terahertz waves.The ellipticity can reach>0.75 over a broad terahertz bandwidth(1 to 5 THz),representing a high-quality and efficient source for few-cycle circularly polarized terahertz pulses with stable carrier waveforms.Flexible control of ellipticity and helicity is also demonstrated with our systematic experiments and numerical simulations.We show that the terahertz polarization state is dictated by the interplay between laser-induced spintronic-origin currents and the screening charges/currents in the metasurfaces,which exhibits tailored anisotropic properties due to the predesigned geometric confinement effects.Our work opens a new pathway to metasurface-tailored spintronic emitters for efficient vector-control of electromagnetic waves in the terahertz regime.展开更多
Ultra-broadband,intense,coherent terahertz(THz)radiation can be generated,detected,and manipulated using laser-induced gas or liquid plasma as both the THz wave transmitter and detector,with a frequency coverage spann...Ultra-broadband,intense,coherent terahertz(THz)radiation can be generated,detected,and manipulated using laser-induced gas or liquid plasma as both the THz wave transmitter and detector,with a frequency coverage spanning across and beyond the whole THz gap."Such a research topic is termed plasma-based THz wave photonics in gas and liquid phases."In this paper,we review the most important experimental and theoretical works of the topic in the non-relativistic region with pump laser intensity below 1018 W/cm^(2).展开更多
To accommodate the ever-increasing wireless capacity,the terahertz(THz)orbital angular momentum(OAM)beam that combines THz radiation and OAM technologies has attracted much attention recently,with contributing efforts...To accommodate the ever-increasing wireless capacity,the terahertz(THz)orbital angular momentum(OAM)beam that combines THz radiation and OAM technologies has attracted much attention recently,with contributing efforts to explore new dimensions in the THz region.In this paper,we provide an overview of the generation and detection techniques of THz-OAM beams,as well as their applications in communications.The principle and research status of typical generation and detection methods are surveyed,and the advantages and disadvantages of each method are summarized from a viewpoint of wireless communication.It is shown that developing novel THz components in generating,detecting and(de)multiplexing THz-OAM beams has become the key engine to drive this direction forward.Anyway,beneficial from the combination of infinite orthogonal modes and large bandwidth,THz-OAM beams will have great potential in delivering very large capacity in next generation wireless communications.展开更多
The applications of metasurfaces are currently a highly active research field due to their extraordinary ability to manipulate electromagnetic waves. The ultra-thin characteristics of metasurfaces allow the miniaturiz...The applications of metasurfaces are currently a highly active research field due to their extraordinary ability to manipulate electromagnetic waves. The ultra-thin characteristics of metasurfaces allow the miniaturization and integration of metasurface devices. However, these devices work typically under a low efficiency and narrow bandwidth condition. In this work, we design eight multilayered unit cells with similar amplitudes and a phase interval of π/4, which convert the polarization states of the terahertz(THz) waves between two orthogonal directions. The average cross-polarized transmission amplitudes of these cells are all around 0.9 in an ultra-broad frequency range from 0.5 THz to 1.4 THz. Furthermore,unit cells are used to construct both an ultra-thin anomalous refraction metalens and a vortex phase plate. Our simulation results show that the anomalous refraction for the transmitted linear polarization component is comparable to the theoretical prediction, and the maximum error is determined to be below 4.8%. The vortex phase plate can also generate an ideal terahertz vortex beam with a mode purity of 90% and more. The distributions of longitudinal electric field, intensity, and phase illustrate that the generated vortex beam has excellent propagation characteristics and a weak divergence. Simulations of the two types of metasurface devices, based on the eight unit cells, exhibit very high efficiencies in a wide bandwidth. Our research will assist in the improvement in the practical applications of metasurfaces. It also provides a reference for the design of high efficiency and broadband devices that are applied to other frequency ranges.展开更多
Graphene-based surface plasmon waveguides(SPWs) show high confinement well beyond the diffraction limit at terahertz frequencies. By combining a graphene SPW and nonlinear material, we propose a novel graphene/AlGaAs ...Graphene-based surface plasmon waveguides(SPWs) show high confinement well beyond the diffraction limit at terahertz frequencies. By combining a graphene SPW and nonlinear material, we propose a novel graphene/AlGaAs SPW structure for terahertz wave difference frequency generation(DFG) under near-infrared pumps.The composite waveguide, which supports single-mode operation at terahertz frequencies and guides two pumps by a high-index-contrast AlGaAs∕Al Oxstructure, can confine terahertz waves tightly and realize good mode field overlap of three waves. The phase-matching condition is satisfied via artificial birefringence in an AlGaAs∕Al Ox waveguide together with the tunability of graphene, and the phase-matching terahertz wave frequency varies from 4 to 7 THz when the Fermi energy level of graphene changes from 0.848 to 2.456 eV. Based on the coupled-mode theory, we investigate the power-normalized conversion efficiency for the tunable terahertz wave DFG process by using the finite difference method under continuous wave pumps, where the tunable bandwidth can reach 2 THz with considerable conversion efficiency. To exploit the high peak powers of pulses, we also discuss optical pulse evolutions for pulse-pumped terahertz wave DFG processes.展开更多
Interest of the research in terahertz(THz)wave has been strongly motivated by its wide applications in the fields of physics,chemistry,biology,and engineering.Developing efficient and reliable THz source is of uttermo...Interest of the research in terahertz(THz)wave has been strongly motivated by its wide applications in the fields of physics,chemistry,biology,and engineering.Developing efficient and reliable THz source is of uttermost priority in these researches.Numerous attempts have been made in fulfilling the THz generation.Greatly benefited from the progress of the ultrafast pulses,the laser-induced-plasma is one of the auspicious tools to provide desirable THz waves,owing to its superiorities in high power threshold,intense THz signal,and ultrawide THz spectrum.This paper reviews the physics and progress of the THz generation from the laser-induced plasmas,which are produced by gas,liquid,and solid.The characteristics of the emitted THz waves are also included.There are many complicated physical processes involved in the interactions of laser-plasma,making various laser-plasma scenarios in the THz generations.In view of this,we will only focus on the THz generation classified by physical mechanisms.Finally,we discuss a perspective on the future of THz generation from the laser-induced plasma,as well as its involved challenges.展开更多
The progress achieved on power scaling and compact and portable THz sources is reviewed.By reversely stacking the GaP plates,the photon conversion efficiency is improved from 25% to 40% which corresponds to the maximu...The progress achieved on power scaling and compact and portable THz sources is reviewed.By reversely stacking the GaP plates,the photon conversion efficiency is improved from 25% to 40% which corresponds to the maximum value.When the number of the plates is increased from four to five,the output power decreases because of back conversion.The THz generation is also investigated by mixing the two frequencies generated by a single Nd：YLF solid-state laser.The average output power reaches 1 μW.The introduction of two Nd：YLF crystals significantly improves the output power to 4.5 μW.This configuration facilitates the generation of different output frequencies.展开更多
Terahertz generation driven by dual-color filaments in air is demonstrated to be remarkably enhanced by applying an external electric field to the filaments. As terahertz generation is sensitive to the dual-color phas...Terahertz generation driven by dual-color filaments in air is demonstrated to be remarkably enhanced by applying an external electric field to the filaments. As terahertz generation is sensitive to the dual-color phase difference, a preformed plasma is verified efficiently in modulating terahertz radiation from linear to elliptical polarization. In the presence of preformed plasma, a dual-color filament generates terahertz pulses of elliptical polarization and the corresponding ellipse rotates regularly with the change of the preformed plasma density. The observed terahertz modulation with the external electric field and the preformed plasma provides a simple way to estimate the plasma density and evaluate the photocurrent dynamics of the dual-color filaments. It provides further experimental evidence of the photo-current model in governing the dual-color filament driven terahertz generation processes.展开更多
基金This work is supported by the National Natural Science Foundation of China (No. 50077017) and the U.S.National Science Foundation. X.-C. Zhang is the author to whom the correspondence should be addressed,
文摘We observe enhanced terahertz (THz) radiation generated from a Si3N4 film-coated GaAs photoconductive dipole antenna. Compared to an uncoated antenna with identical electrode geometry and optical excitation power, the Si3N4 film-coated antenna has a higher effective DC resistance and larger breakdown voltage. As a result, the peak amplitude of generated THz radiation is significantly enhanced due to the Si3N4 film-coated layer.
基金supported by the Cooperative Innovation Center of Terahertz Science , the National Basic Research Program of China (Grant No. 2014CB339800)the National Natural Science Foundation of China (Grant Nos. 61138001, 61420106006)+1 种基金the Program for Changjiang Scholars and Innovative Research Team in University (grant No. IRT13033)the Major National Development Project of Scientific Instruments and Equipment of China (Grant No. 2011YQ150021)
文摘Optoelectronic terahertz generation and detection play a key role in the applications of non-destructive testing,which involves different areas such as physics,biological,material science,imaging,explosions detection,astronomy applications,semiconductor technology and superconductiong electronics. In this article,we present a reviewof the principle and performance of typical terahertz sources,detectors and non-destructive testing applications. On this basis,the newdevelopment and trends of terahertz radiation detectors are also discussed.
基金This work was supported by the U.S.National Science Foundation(No.ECCS-1916068)the Army Research Office(No.W911NF-17-1-0428)the Air Force Office of Scientific Research(No.FA9550-18-1-0357).
文摘Terahertz(THz)wave generation from laser-induced air plasma generally requires a short temporal laser pulse.In contrast,it was observed that THz radiation from ionized liquid water prefers a longer pulse,wherein the mechanism remains unclear.We attribute the preference for longer pulse duration to the process of ionization and plasma formation in water,which is supported by a numerical simulation result showing that the highest electron density is achieved with a subpicosecond pulse.The explanation is further verified by the coincidence of our experimental result and simulation when the thickness of the water is varied.Other liquids are also tested to assure the preference for such a pulse is not exclusive to water.
基金funded by the National Natural Science Foundation of China (Grant Nos.U22A20353,U22A20123,62175182,and 62275193)Daheng Atlas (Beijing)Laser Technology Co.Ltd.for their support。
文摘Enhanced terahertz wave generation via a Stokes cascade process has been demonstrated using picosecond pulse pumped terahertz parametric generation at 1 kHz repetition rate.Clear cascade saturation of terahertz output was observed,and the corresponding cascade-Stokes spectra were analyzed.The maximum terahertz wave average power was 22μW under a pump power of 30 W,whereas the maximum power conversion efficiency was 8×10^(-7)under a pump power of 21 W.The THz power fluctuation was measured to be about 1%in 20 min.This THz parametric source with a relatively stable output is suitable for a variety of practical applications.
基金supported by the Natural Science Foundation of Beijing(No.JQ18015)the National Natural Science Foundation of China(Nos.61905271 and 61935001)the China Postdoctoral Science Foundation funded project(No.2019M660217).
文摘To make further understanding of terahertz(THz)wave generation from liquid water,we study THz wave emission from water lines of different diameters.The water line with a smaller diameter generates a stronger THz electric field for the diameters from 0.2 mm to 0.5 mm.The THz electric field strength and polarity change with the relative position between the incident laser and water line.Moreover,the THz energy has an optimal radiation angle of about 60°.A two-dimensional dipole array model is introduced to illustrate the phenomenon.Our observations contribute to optimizing the scheme of the liquid THz source.
基金support from the National Key Research and Development Program of China (Grant No.2017YFA0303504)the National Natural Science Foundation of China(Grant No.11734007)+8 种基金the Natural Science Foundation of Shanghai(Grant No.20JC1414601)support from the National Natural Science Foundation of China(Grant Nos.11734006 and 11974079)the National Key Research and Development Program of China(Grant No.2016YFA0300703)the Shanghai Municipal Science and Technology Major Project(Grant No.2019SHZDZX01)financial support from the National Natural Science Foundation of China(Grant No.11874121)the Shanghai Municipal Science and Technology Basic Research Project(Grant No.19JC1410900)support from the National Natural Science Foundation of China(Grant No.11874123)the Shanghai Science and Technology Committee(Grant No.20ZR1406000)the Alexander von Humboldt Foundation for support.
文摘The ability to generate and manipulate broadband chiral terahertz waves is essential for applications in material imaging,terahertz sensing,and diagnosis.It can also open up new possibilities for nonlinear terahertz spectroscopy and coherent control of chiral molecules and magnetic materials.The existing methods,however,often suffer from low efficiency,narrow bandwidth,or poor flexibility.Here,we propose a novel type of laser-driven terahertz emitters,consisting of metasurface-patterned magnetic multilayer heterostructures,that can overcome the shortcomings of the conventional approaches.Such hybrid terahertz emitters combine the advantages of spintronic emitters for being ultrabroadband,efficient,and highly flexible,as well as those of metasurfaces for the powerful control capabilities over the polarization state of emitted terahertz waves on an ultracompact platform.Taking a stripe-patterned metasurface as an example,we demonstrate the efficient generation and manipulation of broadband chiral terahertz waves.The ellipticity can reach>0.75 over a broad terahertz bandwidth(1 to 5 THz),representing a high-quality and efficient source for few-cycle circularly polarized terahertz pulses with stable carrier waveforms.Flexible control of ellipticity and helicity is also demonstrated with our systematic experiments and numerical simulations.We show that the terahertz polarization state is dictated by the interplay between laser-induced spintronic-origin currents and the screening charges/currents in the metasurfaces,which exhibits tailored anisotropic properties due to the predesigned geometric confinement effects.Our work opens a new pathway to metasurface-tailored spintronic emitters for efficient vector-control of electromagnetic waves in the terahertz regime.
基金supported by the National Natural Science Foundation of China(Nos.62075157,61875151,and 62235013)Tianjin Municipal Fund for Distinguished Young Scholars(No.20JCJQJC00190)+3 种基金Key Fund of Shenzhen Natural Science Foundation(No.JCYJ20200109150212515)National Key Research and Development Program of China(No.2017YFA0701000)Prof.Xi-Cheng Zhang in The Institute of Optics at University of Rochester has support from the Air Force Office of Scientific Research(Nos.FA9550-21-1-0389 and FA9550-21-1-0300)the National Science Foundation(No.ECCS-2152081).
文摘Ultra-broadband,intense,coherent terahertz(THz)radiation can be generated,detected,and manipulated using laser-induced gas or liquid plasma as both the THz wave transmitter and detector,with a frequency coverage spanning across and beyond the whole THz gap."Such a research topic is termed plasma-based THz wave photonics in gas and liquid phases."In this paper,we review the most important experimental and theoretical works of the topic in the non-relativistic region with pump laser intensity below 1018 W/cm^(2).
基金the National Key Research and Development Program of China(2020YFB18057002018YFB1801500&2018YFB2201700)the Natural National Science Foundation of China under Grant 61771424the Natural Science Foundation of Zhejiang Province under Grant LZ18F010001 and Zhejiang Lab(no.2020LC0AD01).
文摘To accommodate the ever-increasing wireless capacity,the terahertz(THz)orbital angular momentum(OAM)beam that combines THz radiation and OAM technologies has attracted much attention recently,with contributing efforts to explore new dimensions in the THz region.In this paper,we provide an overview of the generation and detection techniques of THz-OAM beams,as well as their applications in communications.The principle and research status of typical generation and detection methods are surveyed,and the advantages and disadvantages of each method are summarized from a viewpoint of wireless communication.It is shown that developing novel THz components in generating,detecting and(de)multiplexing THz-OAM beams has become the key engine to drive this direction forward.Anyway,beneficial from the combination of infinite orthogonal modes and large bandwidth,THz-OAM beams will have great potential in delivering very large capacity in next generation wireless communications.
基金Project supported by the National Natural Science Foundation of China (Grant No. 62071312)the National Key Research and Development Program of China (Grant No. 2021YFB3200100)+1 种基金the Important Research and Development Projects of Shanxi Province, China (Grant No. 201803D121083)the Fund from the Shanxi Scholarship Council, China (Grant No. 2020-135)。
文摘The applications of metasurfaces are currently a highly active research field due to their extraordinary ability to manipulate electromagnetic waves. The ultra-thin characteristics of metasurfaces allow the miniaturization and integration of metasurface devices. However, these devices work typically under a low efficiency and narrow bandwidth condition. In this work, we design eight multilayered unit cells with similar amplitudes and a phase interval of π/4, which convert the polarization states of the terahertz(THz) waves between two orthogonal directions. The average cross-polarized transmission amplitudes of these cells are all around 0.9 in an ultra-broad frequency range from 0.5 THz to 1.4 THz. Furthermore,unit cells are used to construct both an ultra-thin anomalous refraction metalens and a vortex phase plate. Our simulation results show that the anomalous refraction for the transmitted linear polarization component is comparable to the theoretical prediction, and the maximum error is determined to be below 4.8%. The vortex phase plate can also generate an ideal terahertz vortex beam with a mode purity of 90% and more. The distributions of longitudinal electric field, intensity, and phase illustrate that the generated vortex beam has excellent propagation characteristics and a weak divergence. Simulations of the two types of metasurface devices, based on the eight unit cells, exhibit very high efficiencies in a wide bandwidth. Our research will assist in the improvement in the practical applications of metasurfaces. It also provides a reference for the design of high efficiency and broadband devices that are applied to other frequency ranges.
基金National Natural Science Foundation of China(NSFC)(11547187,11405073,61405073)Shandong Provincial Key R&D Program(2017CXGC0416)
文摘Graphene-based surface plasmon waveguides(SPWs) show high confinement well beyond the diffraction limit at terahertz frequencies. By combining a graphene SPW and nonlinear material, we propose a novel graphene/AlGaAs SPW structure for terahertz wave difference frequency generation(DFG) under near-infrared pumps.The composite waveguide, which supports single-mode operation at terahertz frequencies and guides two pumps by a high-index-contrast AlGaAs∕Al Oxstructure, can confine terahertz waves tightly and realize good mode field overlap of three waves. The phase-matching condition is satisfied via artificial birefringence in an AlGaAs∕Al Ox waveguide together with the tunability of graphene, and the phase-matching terahertz wave frequency varies from 4 to 7 THz when the Fermi energy level of graphene changes from 0.848 to 2.456 eV. Based on the coupled-mode theory, we investigate the power-normalized conversion efficiency for the tunable terahertz wave DFG process by using the finite difference method under continuous wave pumps, where the tunable bandwidth can reach 2 THz with considerable conversion efficiency. To exploit the high peak powers of pulses, we also discuss optical pulse evolutions for pulse-pumped terahertz wave DFG processes.
基金This work was supported by the National Natural Science Foundation of China(Grant Nos.11774246 and 121774271)the National Key R&D Program of China(Grant No.2019YFC1711905)+2 种基金the Beijing Talents Project(Grant No.2018A19)the Sino-German Mobility Program of the Sino-German Center for Science Funding(Grant No.M-0225)the Capacity Building for Science&Technology Innovation-Fundamental Scientific Research Funds(Grant No.00820531120017).
文摘Interest of the research in terahertz(THz)wave has been strongly motivated by its wide applications in the fields of physics,chemistry,biology,and engineering.Developing efficient and reliable THz source is of uttermost priority in these researches.Numerous attempts have been made in fulfilling the THz generation.Greatly benefited from the progress of the ultrafast pulses,the laser-induced-plasma is one of the auspicious tools to provide desirable THz waves,owing to its superiorities in high power threshold,intense THz signal,and ultrawide THz spectrum.This paper reviews the physics and progress of the THz generation from the laser-induced plasmas,which are produced by gas,liquid,and solid.The characteristics of the emitted THz waves are also included.There are many complicated physical processes involved in the interactions of laser-plasma,making various laser-plasma scenarios in the THz generations.In view of this,we will only focus on the THz generation classified by physical mechanisms.Finally,we discuss a perspective on the future of THz generation from the laser-induced plasma,as well as its involved challenges.
基金supported by the U.S. National Science Foundation
文摘The progress achieved on power scaling and compact and portable THz sources is reviewed.By reversely stacking the GaP plates,the photon conversion efficiency is improved from 25% to 40% which corresponds to the maximum value.When the number of the plates is increased from four to five,the output power decreases because of back conversion.The THz generation is also investigated by mixing the two frequencies generated by a single Nd：YLF solid-state laser.The average output power reaches 1 μW.The introduction of two Nd：YLF crystals significantly improves the output power to 4.5 μW.This configuration facilitates the generation of different output frequencies.
基金supported by the National Key Scientific Instrument ProjectChina(Grant No.2012YQ150092)+3 种基金the National Basic Research Program of China(Grant No.2011CB808105)the National Natural Science Foundation of China(Grant No.11434005)the China Postdoctoral Science Foundation(Grant No.2014M560348)the Fund from the Shanghai Municipal Science and Technology Commission,China(Grant No.14JC1401600)
文摘Terahertz generation driven by dual-color filaments in air is demonstrated to be remarkably enhanced by applying an external electric field to the filaments. As terahertz generation is sensitive to the dual-color phase difference, a preformed plasma is verified efficiently in modulating terahertz radiation from linear to elliptical polarization. In the presence of preformed plasma, a dual-color filament generates terahertz pulses of elliptical polarization and the corresponding ellipse rotates regularly with the change of the preformed plasma density. The observed terahertz modulation with the external electric field and the preformed plasma provides a simple way to estimate the plasma density and evaluate the photocurrent dynamics of the dual-color filaments. It provides further experimental evidence of the photo-current model in governing the dual-color filament driven terahertz generation processes.
