Molecular alignment control of terahertz emission from a two-color filament in air

Abstract

We demonstrate a method to control the terahertz (THz) emission from a two-color filament in air based on molecular alignment due to rotational Raman excitation. By tuning the delay time between rotational Raman excitation and THz excitation around the air molecule revival time, a significant modulation of THz emission is observed. The phenomenon is attributed to molecular alignment induced refractive index change, resulting in the changes of the nonlinearity (χ(3)) in neutrals and laser intensities inside the filaments.

Spatio-temporal evolution scenarios of femtosecond laser pulse filamentation in fused silica

“…The analytical interference model of SC formation [16] reproduces the SC frequency angular spectrum transformation for the case of laser pulse splitting into several subpulses and for multiple refocusing of the light field during femtosecond laser pulse filamenta tion in dispersive media. The filamentation in air is accompanied with third harmonic generation [17], and two color filament in air is used for generation of terahertz radiation [18][19][20]. The unique properties of femtosecond laser radiation in filamentary regime reveal new opportunities for development of atmo spheric optic systems [21], pulse compression [22][23][24], micro optics elements [25], various devices for nonlinear optical conversion of laser radiation [13,26].…”

Intense broadband THz generation from femtosecond laser filamentation

“…The emission was characterized by a heterodyne THz detector, which was sensitive at 0.1 THz with a bandwidth of 4 GHz. Wu et al [43] and Wang et al [44] reported the molecular alignment effect on THz emission from two-color filaments in air. Air molecules were pre-aligned by the FW pulses [43] or a pair of two-color laser pulses [44] .…”

“…• [44] . The linear polarizations and temporal schemes of the two pairs of two-color pulses after the BBO crystal are depicted as the inset of Fig.…”

“…5(a)) and full revival of N 2 with three quarter revival of O 2 ( Fig. 5(b)) given by the expression of R(t) = n 2,rot ∞ J=0 F J sin(−ω J t), where n 2,rot = 32β 2 (ω)π 2 N/hcn 0 is the overall magnitude of the rotational Raman effect, ω J = 4πBc(2J + 3) is the angular frequency difference between the coupled rotational levels, B, N, h, c, n 0 , and β(ω) are the rotational constant of the molecule, the density of the considered gas, Planck's constant, the speed of light, linear refractive index at the laser wavelength, and the anisotropy of the molecular polarizability, respectively [44] . F J = (ρ J+2 − ρ J )Z J (J + 2)(J + 1)/(2J + 3) is a function of the transition moment between two coupled rotational levels (J+2 and J), of the population difference (ρ J+2 −ρ J ) between these levels and of a weighting factor Z J describing the multiplicity of each level.…”

Joint Measurements of Terahertz Wave Generation and High-Harmonic Generation from Aligned Nitrogen Molecules Reveal Angle-Resolved Molecular Structures

“…The alignment and generation pulses are parallel to each other in polarization. We use supersonic gas jet to deliver pure nitrogen molecules and to minimize the plasma effect, which is different from previous experiments conducted in air [28,34,35]. To estimate the degrees of alignment, we compute the rotational wave packet of nitrogen molecules [36,37].…”