Mid-infrared ultrafast laser pulses induced third harmonic generation in nitrogen molecules on an excited state

Abstract

We report on generation of third harmonic from nitrogen molecules on the excited state with a weak driver laser pulse at a mid-infrared wavelength. The excited nitrogen molecules are generated using a circularly polarized intense femtosecond pulse which produces energetic electrons by photoionization to realize collisional excitation of nitrogen molecules. Furthermore, since the third harmonic is generated using a pump-probe scheme, it enables investigation of the excited-state dynamics of nitrogen molecules produced under different conditions. We also perform a comparative investigation in excited argon atoms, revealing different decay dynamics of the molecules and atoms from the excited states in femtosecond laser induced filaments.

Initiation of air ionization by ultrashort laser pulses: evidence for a role of metastable-state air molecules

“…This provides a convincing explanation for our observations. The ionization probability of this non-radiative excited state approaches 100% under our laser fluences [39] and thus the air ionization is enhanced in multi-pulse irradiation regimes if the pulse repetition rate is high enough (a few 100 Hz or more)…”

“…The mechanism of populating the C 3  u state of N 2 , which radiates in the dominating band, is not yet unambiguously determined. This can be either multiphoton excitation [39] or depopulation of some highly excited electronic states through collisions [29].…”

“…The A 3 Σ u + state of N 2 , the lowest excited state with an excitation energy of 6.2 eV, is metastable with a radiative lifetime of about 2 s [20,40]. Its quenching occurs mainly through collisions with a rate of ~ 3×10 -19 cm 3 s -1 that corresponds to a decay time of about 100 ms under normal conditions [39,41]. Since the de-activation probability increases with temperature [40] and due to the presence of oxygen [41] we expect the quenching time for the N 2 A 3 Σ u + state in the focal region to be on the order of 10 ms. For instance, at 1000 K, a temperature easily reachable in air under the considered conditions [8], the average number of molecular collisions to de-activate the N 2 A 3 Σ u + state was found to be ~ 5×10 7 [40] that gives us a decay time of ~ 20 ms at 1 bar (assuming nitrogen collisions with a gas kinetic cross section of 4×10 -15 cm 2 ).…”

Time-resolved ultrafast transient polarization spectroscopy to investigate nonlinear processes and dynamics in electronically excited molecules on the femtosecond time scale

“…Some of these techniques include time-resolved photoelectron spectroscopy 1 , transient absorption spectroscopy, 2 fluorescence up-conversion, 3 pump degenerate four-wave mixing, 4 and third harmonic generation. 5 For many molecular systems, the 3 rd -order optical susceptibility 𝜒 (3) is the lowest order nonlinear optical response 6 and many techniques have been developed that use 𝜒 (3) as a sensitive probe of electronic structure and excited state dynamics. [7][8][9][10] With additional parameters such as independent frequencies of two or more probe pulses, multiple time delays, and phase-matching conditions, nonlinear spectroscopy can be highly differential and exquisitely sensitive to dynamics such as electron-hole coherence, or populations of specific electronic states.…”

“…Molecules in excited electronic states may exhibit a large second hyperpolarizability, which is the molecular property contributing to 𝜒 (3) , if strong dipole coupling exists between the excited state and virtual intermediate states, or if a high density of nearby real and virtual electronic states is present. 5,7,11,12 Techniques that utilize this enhancement of 𝜒 (3) to study dynamics of electronically excited systems include third harmonic generation, which has been applied as a probe to study the dynamics of fishnet metamaterials (multilayer nanoscale patterned solid state materials) as well as atmospheric air samples 8,13 . In these experiments the excitation pulse is followed by an intense near infrared (NIR) pulse that generates the third harmonic signal.…”