Computed tomography has been proven to be useful for non-destructive inspection of structures and materials. We build a three-dimensional imaging system with the photonically generated incoherent noise source and the ...Computed tomography has been proven to be useful for non-destructive inspection of structures and materials. We build a three-dimensional imaging system with the photonically generated incoherent noise source and the Schottky barrier diode detector in the terahertz frequency band （90–140GHz）. Based on the computed tomography technique, the three-dimensional image of a ceramic sample is reconstructed successfully by stacking the slices at different heights. The imaging results not only indicate the ability of terahertz wave in the non-invasive sensing and non-destructive inspection applications, but also prove the effectiveness and superiority of the uni-traveling-carrier photodiode as a terahertz source in the imaging applications.展开更多
To satisfy the demands for compact,inexpensive terahertz(THz)sources with power of hundreds of watts,a radial sheet beam THz source which does not require an external magnetic field and is driven by a radial pseudospa...To satisfy the demands for compact,inexpensive terahertz(THz)sources with power of hundreds of watts,a radial sheet beam THz source which does not require an external magnetic field and is driven by a radial pseudospark discharge plasma electron gun(PSDP-EGUN)is proposed.Radial design has been used in pseudospark switches,but in this paper the design of a PSDP-EGUN to drive a radial THz source is presented for the first time.Being different from the latest reported axial quasi-rectangular sheet beam THz sources driven by an axial PSDP-EGUN,a new desig n consisti ng of a circular plate-shaped sheet beam that is directly gen erated by the radial PSDP-EGUN is reported.As compared to an axial system,the radial configuration may result in a larger beam current and a larger beam-wave interaction area together with a higher potential of THz output power.Theoretical analysis and pailicle-in-cell simulation have been employed in the design of the radial sheet beam THz source.Output powers in the kilowatt range have been observed in the simulation of this 0.22 THz source.Preliminary experimental results of the radial PSDP-EGUN are also presented.展开更多
The terahertz wave is considered to have great values and plentiful applications, such as in material science, analysis of molecular spectra, information and communication technology, biology and medical science, nond...The terahertz wave is considered to have great values and plentiful applications, such as in material science, analysis of molecular spectra, information and communication technology, biology and medical science, nondestructive evaluation, and national security. High-power widely tunable terahertz sources are required in the above practical applications of terahertz technologies. A promising approach for monochromatic terahertz generation is based on second-order nonlinear optical effect, e.g.展开更多
电子束注入器是决定自由电子激光太赫兹源(Free Electron Laser for Terahertz,FEL-THz)性能的核心部件,基于射频加速器的注入器能够产生驱动高功率THz辐射所需的高品质电子束。同时,出于商业化和市场化的考虑,注入器还需具备结构紧凑...电子束注入器是决定自由电子激光太赫兹源(Free Electron Laser for Terahertz,FEL-THz)性能的核心部件,基于射频加速器的注入器能够产生驱动高功率THz辐射所需的高品质电子束。同时,出于商业化和市场化的考虑,注入器还需具备结构紧凑的特点。采用独立调谐式结构(Independently Tunable Cavity,ITC)的束流注入器具有平衡高品质、低成本以及紧凑性的优势,在大功率FEL-THz装置小型化方面有广阔的应用前景。然而,随着束线长度的缩短和装置整体规模的压缩,注入器在运行过程中面临着在线调节手段和诊断能力不足等挑战。本文依托华中科技大学与中国科学技术大学共建的一台紧凑型FEL-THz注入器,首先介绍其基于ITC结构的优化思路和设计结果,然后在系统稳定的基础上,重点阐述调试和运行过程中所涉及的束流间接诊断和系统优化措施,并结合束流动力学仿真和在线实验开展了相关方案和研究的验证。仿真和实验结果均表明,通过优化设计并结合间接诊断技术,ITC注入器能够兼顾FEL-THz高功率和小型化的需求,而间接诊断技术的应用也能够弥补调试过程中诊断手段不足的问题,为ITC注入器的进一步推广奠定了基础。展开更多
We present the generation of high-repetition-rate strong-field terahertz(THz)pulses from a thin 4-N,N-dimethylamino-4’-N’-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate(DSTMS)organic crystal pumped by an ytterb...We present the generation of high-repetition-rate strong-field terahertz(THz)pulses from a thin 4-N,N-dimethylamino-4’-N’-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate(DSTMS)organic crystal pumped by an ytterbium-doped yttrium aluminum garnet laser.The generated THz pulse energy reaches 932.8 nJ at 1 kHz repetition rate,with a conversion efficiency of 0.19%and a peak electric field of 819 kV/cm.At a repetition rate of 10 kHz,it is able to maintain a peak electric field of 236 kV/cm and an average THz power of 0.77 mW.The high-repetition-rate,strong-field THz source provides a convenient tool for the study of THz matter manipulation and THz spectroscopy.展开更多
非线性差频产生(difference frequency generation,DFG)是实现太赫兹(terahertz,THz)源的重要方式之一.利用微纳结构的DFG产生THz源可以不考虑相位匹配,同时是器件小型化、可集成化的重要研究方向.借助微纳结构的共振模式增强的局域电...非线性差频产生(difference frequency generation,DFG)是实现太赫兹(terahertz,THz)源的重要方式之一.利用微纳结构的DFG产生THz源可以不考虑相位匹配,同时是器件小型化、可集成化的重要研究方向.借助微纳结构的共振模式增强的局域电场在宽波段范围内实现高效的、可调谐的THz源是该领域的研究重点.本文研究了宽波段范围内具有高Q因子的光栅-波导结构中的DFG产生高效可调谐的THz辐射.理论上,通过调控相邻光栅中其中一个的位置扰动,从而实现光栅周期的加倍,进而使得布里渊区发生折叠,光线下方波导层中导模色散曲线折叠到光锥上方,形成超高Q因子的导模共振,可以实现在宽光谱范围内增强的THz产生.以硫化镉(cadmium sulfide,CdS)光栅-波导为例,数值研究表明,在两束基频光光强均为100 kW/cm2时,THz的转换效率可达到10-8W-1的量级,为相同厚度CdS薄膜转换效率的10~9倍.通过改变两束基频光入射角,可实现不同共振基频组合,实现任意频率THz波产生,从而实现了在宽光谱范围内高效可调谐的THz源.展开更多
Interdigitated finger capacitance of a continuous-wave terahertz photomixer is calculated using the finite element method.For the frequently used electrode width（0.2 μm） and gap width（1.8 μm）,the finger capacita...Interdigitated finger capacitance of a continuous-wave terahertz photomixer is calculated using the finite element method.For the frequently used electrode width（0.2 μm） and gap width（1.8 μm）,the finger capacitance increases quasi-quadratically with the number of electrodes increasing.The quasi-quadratic dependence can be explained by a sequence of lumped capacitors connected in parallel.For a photomixer composed of 10 electrodes and 9 photoconductive gaps,the finger capacitance increases as the gap width increases at a small electrode width,and follows the reverse trend at a large electrode width.For a constant electrode width,the finger capacitance first decreases and then slightly increases as the gap broadens until the smallest finger capacitance is formed.We also investigate the finger capacitances at different electrode and gap configurations with the 8 μm × 8 μm photomixer commonly used in previous studies.These calculations lead to a better understanding of the finger capacitance affected by the finger parameters,and should lead to terahertz photomixer optimization.展开更多
We demonstrate a physical mechanism for terahertz(THz) generation from surface plasmon polaritons(SPPs). In a structure with a bulk Dirac semimetals(BDSs) film deposited on a dielectric substrate, the energy of the as...We demonstrate a physical mechanism for terahertz(THz) generation from surface plasmon polaritons(SPPs). In a structure with a bulk Dirac semimetals(BDSs) film deposited on a dielectric substrate, the energy of the asymmetric SPP mode can be significantly enhanced to cross the light line of the substrate due to the SPP-coupling between the interfaces of the film. Therefore, the SPPs can be immediately transformed into Cherenkov radiation without removing the wavevector mismatch. Additionally, the symmetric SPP mode can also be dramatically lifted to cross the substrate light line when a buffer layer with low permittivity relative to the substrate is introduced. In this case, dual-frequency THz radiation from the two SPP modes can be generated simultaneously. The radiation intensity is significantly enhanced by over two orders due to the field enhancement of the SPPs. The radiation frequency can be tuned in the THz frequency regime by adjusting the beam energy and the chemical potential of the BDSs. Our results could find potential applications in developing room temperature, tunable, coherent, and intense THz radiation sources to cover the entire THz band.展开更多
Coherent diffraction radiation (CDR), generated by relativistic femtosecond electron bunches passing through an aperture in a metallic foil, is becoming widely used as a high brightness, coherent, polarized, and broad...Coherent diffraction radiation (CDR), generated by relativistic femtosecond electron bunches passing through an aperture in a metallic foil, is becoming widely used as a high brightness, coherent, polarized, and broad- band terahertz radiation source. It is one of the most promising technique for non-destructive beam diagnostics in ac- celerators. In this paper, how to produce CDR is studied by utilizing femtosecond electron bunches at wavelengths equal to or longer than the bunch length with the aperture size at the condition of a≤γλ/(2π), which is provided by Shanghai Deep Ultraviolet-Free Electron Laser (SDUV-FEL) facility. General characteristics of CDR at SDUV-FEL are analyzed and numerically calculated. The results show that the radiance of this radiation within the frequency range from 0.2 up to 5 terahertz greatly exceeds that available from conventional black body radiation or synchrotron radiation.展开更多
基金Supported by the Hundred Talents Program of Chinese Academy of Sciencesthe National Basic Research Program of China under Grant No 2014CB339803+2 种基金the Major National Development Project of Scientific Instrument and Equipment under Grant No2011YQ150021the National Natural Science Foundation of China under Grant Nos 61575214,61574155,61404149 and 61404150the Shanghai Municipal Commission of Science and Technology under Grant Nos 14530711300,15560722000 and 15ZR1447500
文摘Computed tomography has been proven to be useful for non-destructive inspection of structures and materials. We build a three-dimensional imaging system with the photonically generated incoherent noise source and the Schottky barrier diode detector in the terahertz frequency band （90–140GHz）. Based on the computed tomography technique, the three-dimensional image of a ceramic sample is reconstructed successfully by stacking the slices at different heights. The imaging results not only indicate the ability of terahertz wave in the non-invasive sensing and non-destructive inspection applications, but also prove the effectiveness and superiority of the uni-traveling-carrier photodiode as a terahertz source in the imaging applications.
文摘To satisfy the demands for compact,inexpensive terahertz(THz)sources with power of hundreds of watts,a radial sheet beam THz source which does not require an external magnetic field and is driven by a radial pseudospark discharge plasma electron gun(PSDP-EGUN)is proposed.Radial design has been used in pseudospark switches,but in this paper the design of a PSDP-EGUN to drive a radial THz source is presented for the first time.Being different from the latest reported axial quasi-rectangular sheet beam THz sources driven by an axial PSDP-EGUN,a new desig n consisti ng of a circular plate-shaped sheet beam that is directly gen erated by the radial PSDP-EGUN is reported.As compared to an axial system,the radial configuration may result in a larger beam current and a larger beam-wave interaction area together with a higher potential of THz output power.Theoretical analysis and pailicle-in-cell simulation have been employed in the design of the radial sheet beam THz source.Output powers in the kilowatt range have been observed in the simulation of this 0.22 THz source.Preliminary experimental results of the radial PSDP-EGUN are also presented.
文摘The terahertz wave is considered to have great values and plentiful applications, such as in material science, analysis of molecular spectra, information and communication technology, biology and medical science, nondestructive evaluation, and national security. High-power widely tunable terahertz sources are required in the above practical applications of terahertz technologies. A promising approach for monochromatic terahertz generation is based on second-order nonlinear optical effect, e.g.
文摘电子束注入器是决定自由电子激光太赫兹源(Free Electron Laser for Terahertz,FEL-THz)性能的核心部件,基于射频加速器的注入器能够产生驱动高功率THz辐射所需的高品质电子束。同时,出于商业化和市场化的考虑,注入器还需具备结构紧凑的特点。采用独立调谐式结构(Independently Tunable Cavity,ITC)的束流注入器具有平衡高品质、低成本以及紧凑性的优势,在大功率FEL-THz装置小型化方面有广阔的应用前景。然而,随着束线长度的缩短和装置整体规模的压缩,注入器在运行过程中面临着在线调节手段和诊断能力不足等挑战。本文依托华中科技大学与中国科学技术大学共建的一台紧凑型FEL-THz注入器,首先介绍其基于ITC结构的优化思路和设计结果,然后在系统稳定的基础上,重点阐述调试和运行过程中所涉及的束流间接诊断和系统优化措施,并结合束流动力学仿真和在线实验开展了相关方案和研究的验证。仿真和实验结果均表明,通过优化设计并结合间接诊断技术,ITC注入器能够兼顾FEL-THz高功率和小型化的需求,而间接诊断技术的应用也能够弥补调试过程中诊断手段不足的问题,为ITC注入器的进一步推广奠定了基础。
基金supported by the National Key Research and Development Program of China(No.2022YFA1604401)the National Natural Science Foundation of China(Nos.12325409,62105346 and 12388102)+2 种基金the CAS Project for Young Scientists in Basic Research(No.YSBR-059)the Basic Research Project of the Shanghai Science and Technology Innovation Action Plan(No.20JC1416000)the Shanghai Pilot Program for Basic Research–Chinese Academy of Sciences,Shanghai Branch。
文摘We present the generation of high-repetition-rate strong-field terahertz(THz)pulses from a thin 4-N,N-dimethylamino-4’-N’-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate(DSTMS)organic crystal pumped by an ytterbium-doped yttrium aluminum garnet laser.The generated THz pulse energy reaches 932.8 nJ at 1 kHz repetition rate,with a conversion efficiency of 0.19%and a peak electric field of 819 kV/cm.At a repetition rate of 10 kHz,it is able to maintain a peak electric field of 236 kV/cm and an average THz power of 0.77 mW.The high-repetition-rate,strong-field THz source provides a convenient tool for the study of THz matter manipulation and THz spectroscopy.
文摘非线性差频产生(difference frequency generation,DFG)是实现太赫兹(terahertz,THz)源的重要方式之一.利用微纳结构的DFG产生THz源可以不考虑相位匹配,同时是器件小型化、可集成化的重要研究方向.借助微纳结构的共振模式增强的局域电场在宽波段范围内实现高效的、可调谐的THz源是该领域的研究重点.本文研究了宽波段范围内具有高Q因子的光栅-波导结构中的DFG产生高效可调谐的THz辐射.理论上,通过调控相邻光栅中其中一个的位置扰动,从而实现光栅周期的加倍,进而使得布里渊区发生折叠,光线下方波导层中导模色散曲线折叠到光锥上方,形成超高Q因子的导模共振,可以实现在宽光谱范围内增强的THz产生.以硫化镉(cadmium sulfide,CdS)光栅-波导为例,数值研究表明,在两束基频光光强均为100 kW/cm2时,THz的转换效率可达到10-8W-1的量级,为相同厚度CdS薄膜转换效率的10~9倍.通过改变两束基频光入射角,可实现不同共振基频组合,实现任意频率THz波产生,从而实现了在宽光谱范围内高效可调谐的THz源.
基金Project supported by the National High Technology Research and Development Program of China (Grant No. 2011AAxxx2008A)Hundred Talent Program of the Chinese Academy of Sciences (Grant No. J08-029)the Main Direction Program of Knowledge Innovation of the Chinese Academy of Sciences (Grant No. YYYJ-1123-4)
文摘Interdigitated finger capacitance of a continuous-wave terahertz photomixer is calculated using the finite element method.For the frequently used electrode width（0.2 μm） and gap width（1.8 μm）,the finger capacitance increases quasi-quadratically with the number of electrodes increasing.The quasi-quadratic dependence can be explained by a sequence of lumped capacitors connected in parallel.For a photomixer composed of 10 electrodes and 9 photoconductive gaps,the finger capacitance increases as the gap width increases at a small electrode width,and follows the reverse trend at a large electrode width.For a constant electrode width,the finger capacitance first decreases and then slightly increases as the gap broadens until the smallest finger capacitance is formed.We also investigate the finger capacitances at different electrode and gap configurations with the 8 μm × 8 μm photomixer commonly used in previous studies.These calculations lead to a better understanding of the finger capacitance affected by the finger parameters,and should lead to terahertz photomixer optimization.
基金Project supported by the National Key Research and Development Program of China(Grant No.2017YFA0701000)the National Key Scientific Instrument and Equipment Development of China(Grant No.2018YFF01013001)the National Natural Science Foundation of China(Grant Nos.61701084 and61505022)。
文摘We demonstrate a physical mechanism for terahertz(THz) generation from surface plasmon polaritons(SPPs). In a structure with a bulk Dirac semimetals(BDSs) film deposited on a dielectric substrate, the energy of the asymmetric SPP mode can be significantly enhanced to cross the light line of the substrate due to the SPP-coupling between the interfaces of the film. Therefore, the SPPs can be immediately transformed into Cherenkov radiation without removing the wavevector mismatch. Additionally, the symmetric SPP mode can also be dramatically lifted to cross the substrate light line when a buffer layer with low permittivity relative to the substrate is introduced. In this case, dual-frequency THz radiation from the two SPP modes can be generated simultaneously. The radiation intensity is significantly enhanced by over two orders due to the field enhancement of the SPPs. The radiation frequency can be tuned in the THz frequency regime by adjusting the beam energy and the chemical potential of the BDSs. Our results could find potential applications in developing room temperature, tunable, coherent, and intense THz radiation sources to cover the entire THz band.
文摘Coherent diffraction radiation (CDR), generated by relativistic femtosecond electron bunches passing through an aperture in a metallic foil, is becoming widely used as a high brightness, coherent, polarized, and broad- band terahertz radiation source. It is one of the most promising technique for non-destructive beam diagnostics in ac- celerators. In this paper, how to produce CDR is studied by utilizing femtosecond electron bunches at wavelengths equal to or longer than the bunch length with the aperture size at the condition of a≤γλ/(2π), which is provided by Shanghai Deep Ultraviolet-Free Electron Laser (SDUV-FEL) facility. General characteristics of CDR at SDUV-FEL are analyzed and numerically calculated. The results show that the radiance of this radiation within the frequency range from 0.2 up to 5 terahertz greatly exceeds that available from conventional black body radiation or synchrotron radiation.
