Two-photon vibrational excitation of air by long-wave infrared laser pulses

Abstract

Ultrashort long-wave infrared (LWIR) laser pulses can resonantly excite vibrations in N 2 and O 2 through a two-photon transition. The absorptive, vibrational component of the ultrafast optical nonlinearity grows in time, starting smaller than, but quickly surpassing, the electronic, rotational, and vibrational refractive components. The growth of the vibrational component results in a novel mechanism of 3 rd harmonic generation, providing an additional two-photon excitation channel, fundamental + 3 rd harmonic. The original and emergent two-photon excitations drive the resonance exactly out of phase, causing spatial decay of the absorptive, vibrational nonlinearity. This nearly eliminates two-photon vibrational absorption. Here we present simulations and analytical calculations demonstrating how these processes modify the ultrafast optical nonlinearity in air. The results reveal nonlinear optical phenomena unique to the LWIR regime of ultrashort pulse atmospheric propagation.

Ultrashort infrared 25–11 μm pulses: spatiotemporal profiles and absolute nonlinear response of air constituents

“…As in our prior results at λ ≤ 2.4µm in the mid-IR [11], there is little dispersion in 𝑛𝑛 2 . In the region near λ=8.0µm for N2, which was scanned continuously through 7.5-8.5 µm, we observed no signature of 2-photon resonant absorption [10].…”

“…In this Article we present measurements of the detailed spatiotemporal profiles of femtosecond laser pulses in the infrared wavelength range 𝜆𝜆 = 2.5 − 11𝜇𝜇𝜇𝜇, and the absolute nonlinear response of major air constituents (N2, O2, and Ar) over this range. We also investigate possible resonant 2-photon vibrational excitation of N2 near λ=8µm, which is of interest for high power LWIR laser pulse propagation [10]. The spatiotemporal measurements reveal the wavelength-dependent pulse front tilt and temporal stretching induced by the difference frequency generation scheme used to generate the infrared pulses.…”

Wavelength-scaled laser filamentation in solids and plasma-assisted subcycle light-bullet generation in the long-wavelength infrared

“…Recent theoretical work on mid-IR pulse propagation in gases has predicted that at long wavelengths, selfcompression and self-steepening give rise to single-cycle pulses and a supercontinuum that spans multiple octaves [17]. Mid-IR/LWIR ultrashort pulses have been shown to exhibit drastically different propagation dynamics than pulses in the near-IR, including optical shock formation [17][18][19][20], accelerated third-harmonic generation via vibrational excitation of N 2 and O 2 [21], extended and smooth filamentation [22,23], and a significant defocusing contribution due to many-body effects, leading to long-range self-channeling [24]. Furthermore, Panagiotopoulos et al [18] have theoretically demonstrated long-range, highpower 3-D spatio-temporal solitary waves (i.e., light bullets [25,26]) in air via mid-IR laser filamentation.…”

Vibrational excitation of hydrogen molecules by two-photon absorption and third-harmonic generation

“…because the tan −1 converges to a constant. The modified formula, equation (8), indicates that the qualitative behavior of THG is changed by the relationship between Δ Laser and γ. Palastro et al [2] discussed the interference between two-photon absorption and THG. We do not consider the interference effect because the target length used in our study is much shorter than that in [2] and the third harmonic is considerably weak compared with the pumping pulses.…”

“…Of the many theoretical and experimental studies on general THG, She and Billman [1] investigated the theory of using the vibrational states of diatomic molecules for THG and found that a high conversion efficiency of up to 30% could be expected in hydrogen gas using an optimized column density and an input laser intensity of 34 MW cm −2 . Palastro et al [2] discussed vibrational two-photon absorption and THG by nitrogen and oxygen and their effects on the propagation of ultrafast long-wave infrared pulses. However, as far as we know, there have been no experimental studies on THG using the vibrational states of diatomic molecules, perhaps because it may be difficult to generate high-intensity mid-infrared pulses with a narrow linewidth, which are desired for the preparation of high coherence.…”