Anti-Correlated Plasma and THz Pulse Generation during Two-Color Laser Filamentation in Air

Abstract

The THz generation efficiency and the plasma density generated by a filament in air have been found anti-correlated when pumped by 800 nm+1600 nm two-color laser field. The plasma density near zero delay of two laser pulses has a minimum value, which is opposite to the trend of THz generation efficiency and contradicts common sense. The lower plasma density cannot be explained by the static tunneling model according to the conventional photocurrent model, but it might be attributed to the electron trapping by the excited states of nitrogen molecule. The present work also clarifies the dominant role of the drifting velocity accelerated by the two-color laser field during the THz pulse generation process. The results promote our understanding on the optimization of the THz generation efficiency by the two-color laser filamentation.

Local measurement of terahertz field-induced second harmonic generation in plasma filaments

“…35 a. The electron density can be estimated by using the expressions [ 141 ]: where α loss is the absorption coefficient in m −2 , κ is the extinction coefficient, A 1 is the OBCD amplitude at time τ = 0, A 2 is the signal amplitude at τ = 4 ps, ε is the complex dielectric function, ω p is the electron plasma frequency, and τ s is the electron scatter time. Assuming a scatter time of 1 ps in a clamped fs filament [ 25 ] and using the times ratio between the two waveform peaks, the estimated electron number density is on the order of 10 16 cm −3 as previously found in Sect.…”

Laser-Chirp Controlled Terahertz Wave Generation from Air Plasma

“…Two-color-laser-induced air plasma is a promising method for generating ultra-broad and coherent terahertz (THz) radiation, [1] with numerous applications in remote sensing, ultrafast control, nonlinear modulation, and bio-detection. [2][3][4][5][6] In order to optimize the THz yield, several attempts have been made, including laser polarization control, [7,8] phase delay variation between the two-color laser fields, [9,10] alteration of the driven wavelength, [3,[11][12][13] gas media type modification, [14] and adding an additional DC field. [15,16] To engineer and optimize the THz radiation, it is crucial to understand the generation mechanism.…”

“…Other fields of research, such as the near-field THz microscopy [46] and remote sensing, [47] which utilize optically induced plasma, can benefit from our results. Furthermore, in recent years, filament-based THz sources have been extensively studied using the long-wave infrared [3,[11][12][13][14] and other frequency ratios. [48,49] Although the present LDA model only deals with the THz wave generation from near-infrared two-color fields (800 nm + 400 nm), it may be possible to extend to other wavelengths for a better understanding of wavelength-dependent THz emission by incorporating electronic dynamics into dynamics of local dipoles, e.g., avalanche ionization, [50] many-body effects, [51] and trapping of excited states.…”

“…[48,49] Although the present LDA model only deals with the THz wave generation from near-infrared two-color fields (800 nm + 400 nm), it may be possible to extend to other wavelengths for a better understanding of wavelength-dependent THz emission by incorporating electronic dynamics into dynamics of local dipoles, e.g., avalanche ionization, [50] many-body effects, [51] and trapping of excited states. [13]…”