Rotational, Vibrational, and Electronic Modulations in N_{2}^{+} Lasing at 391 nm: Evidence of Coherent B^{2}Σ_{u}^{+}-X^{2}Σ_{g}^{+}-A^{2}Π_{u} Coupling

Bidirectional Cascaded Superfluorescent Lasing in Air Enabled by Resonant Third Harmonic Photon Exchange from Nitrogen to Argon

Cavity-free lasing in atmospheric air has stimulated intense research toward a fundamental understanding of underlying physical mechanisms. In this Letter, we identify a new mechanism-a third-harmonic photon mediated resonant energy transfer pathway leading to population inversion in argon via an initial three-photon excitation of nitrogen molecules irradiated by intense 261 nm pulses-that enables bidirectional two-color cascaded lasing in atmospheric air. By making pump-probe measurements, we conclusively show that such cascaded lasing results from superfluorescence rather than amplified spontaneous emission. Such cascaded lasing with the capability of producing bidirectional multicolor coherent pulses opens additional possibilities for remote sensing applications.

Amplification of light pulses with orbital angular momentum (OAM) in nitrogen ions lasing

Nitrogen ions pumped by intense femtosecond laser pulses give rise to optical amplification in the ultraviolet range. Here, we demonstrated that a seed light pulse carrying orbital angular momentum (OAM) can be significantly amplified in nitrogen plasma excited by a Gaussian femtosecond laser pulse. With the topological charge of ℓ = ±1, we observed an energy amplification of the seed light pulse by two orders of magnitude, while the amplified pulse carries the same OAM as the incident seed pulse. Moreover, we show that a spatial misalignment of the plasma amplifier with the OAM seed beam leads to an amplified emission of Gaussian mode without OAM, due to the special spatial profile of the OAM seed pulse that presents a donut-shaped intensity distribution. Utilizing this misalignment, we can implement an optical switch that toggles the output signal between Gaussian mode and OAM mode. This work not only certifies the phase transfer from the seed light to the amplified signal, but also highlights the important role of spatial overlap of the donut-shaped seed beam with the gain region of the nitrogen plasma for the achievement of OAM beam amplification.

Spectral splitting of the lasing emission of nitrogen ions pumped by 800-nm femtosecond laser pulses

We report on a spectral splitting effect of the cavity-less lasing emission of nitrogen ions at 391.4 nm pumped by 800-nm femtosecond laser pulses. It was found that with the increase of the nitrogen gas pressure and pump pulse energy, both R and P branches experience spectral splitting. With an external injected seeding pulse, a similar split spectral line is observed for the amplified emission. In contrast, for the fluorescence radiation, no such spectral splitting phenomenon is observed with much more abundant R branch structures. Our theoretical model considers gas ionization by the pump pulse, the competition of excitation of all relevant electronic and vibrational states, and an amplification of the seeding pulse in the plasma with a population inversion. Our simulation reproduces this spectral splitting effect, which is attributed to the gain saturation resulting in the oscillation of the amplitude of the amplified signal.

Transition from triggered super-radiance to seed amplification in N2 + lasing

Air lasing induced by laser filamentation opens a new route for research on atmospheric molecular physics and remote sensing. The generation of air lasing is composed of two processes, i.e., building up optical gain of air molecules in femtosecond time scale and emitting coherent radiation in picosecond time scale. Here, we focus on the emission mechanisms of N₂ ⁺ air lasing and reveal, by examining the intensities and temporal profiles of N₂ ⁺ lasing at 391 nm generated respectively in a time-varying polarization-modulated and a linearly polarized pump laser field under different nitrogen gas pressures, that the N₂ ⁺ lasing can emit through either triggered super-radiance or seed amplification. We find that the two pressure-sensitive factors, i.e., the dipole dephasing time T₂ and the population inversion density n, determine which of these two mechanisms dominates the N₂ ⁺ lasing emission process, enabling manipulation of the transition from triggered super-radiance to seed amplification or vice versa. Our findings clarify the emission mechanism of N₂ ⁺ lasing under different pressures and provide a deeper understanding of N₂ ⁺ air lasing not only in the establishment of optical gain but also in the lasing emission process.

Quantum and quasi-classical effects in the strong field ionization and subsequent excitation of nitrogen molecules

The processes leading to the N₂ ⁺ lasing are rather complex and even the population distribution after the pump laser excitation is unknown. In this paper, we study the population distribution at electronic and vibrational levels in N₂ ⁺ driven by ultra-short laser pulse at the wavelengths of 800 nm and 400 nm by using the quantum-mechanical time-domain incoherent superposition model based on the time-dependent Schrödinger equation and the quasi-classical model assuming instantaneous ionization injection described by density matrix. It is shown that while both models provide qualitatively similar results, the quasi-classical instantaneous ionization injection model underestimates the population inversions corresponding to the optical transitions at 391 nm, 423 nm and 428 nm due to the assumption of quantum mixed states at the ionization time. A fast and accurate correction to this error is proposed. This work solidifies the theoretical models for population at vibrational states in N₂ ⁺ and paves the way to uncover the mechanism of the N₂ ⁺ lasing.

Few-Femtosecond Isotope Effect in Polyatomic Molecules Ionized by Extreme Ultraviolet Attosecond Pulse Trains

Following ionization by an extreme ultraviolet (XUV) attosecond pulse train, a polyatomic molecule can be promoted to more-than-one excited states of the residual ion. The ensuing relaxation dynamics is often facilitated by several reaction coordinates, making them difficult to disentangle by the usual spectroscopic means. Here, we show that in atto-chemistry isotope labeling can be an efficient tool for unraveling the relaxation pathways in highly excited photoionized molecules. Employing an XUV pump pulse and a near-infrared probe pulse, we found the nuclear as well as coupled electron-nuclear dynamics in ethylene to be almost 40% faster compared to that of its deuterated counterpart. The findings, which are supported by advanced nonadiabatic dynamics calculations, led to the identification of the relevant nuclear coordinates controlling the relaxation. Our experiment highlights the relevance of ultrashort XUV pulses to capture the isotopic effect in few-femtosecond molecular photodynamics.

Observation of rotational coherence in an excited state of CO. OPTICS LETTERS 2021;46:3893-3896. [PMID: 34388768 DOI: 10.1364/ol.432315] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The vacuum ultraviolet (VUV) radiation is generated in the strong-field-ionized CO molecules through 2+1 resonance excitation with two-color femtosecond laser pulses. When scanning the relative delay between two pump pulses, the rotational-resolved VUV radiations show periodic oscillations lasting as long as 500 ps. Fourier analysis reveals that these oscillations correspond to rotational beat frequencies of the A²Π_i state of CO⁺, which is the result of multi-channel interference during the resonant excitation process. High resolution of Fourier transform spectra up to 0.067cm^-1 allows us to obtain the fine energy levels of the A²Π_i state. The theoretical calculation is in good agreement with the experimental observation. This work reveals the rotational coherence of the ionic excited state and shows the prospect of rotational coherence spectroscopy in measuring fine structures of molecular ions.

Optical amplification from high vibrational states of ionized nitrogen molecules generated by 800-nm femtosecond laser pulses

We experimentally investigated the interaction between nitrogen molecules and intense femtosecond laser pulses. When irradiated by an 800-nm pump laser and a delayed 355-nm seed laser, the spectral lines around 353.3 nm and 353.8 nm are observed to be greatly amplified, no matter whether the pump laser is circularly or linearly polarized. The two spectral lines correspond to the transition of N₂⁺ (B, ν' = 5 → X, ν = 4) and N₂⁺ (B, ν' = 4 → X, ν = 3), respectively. In comparison with the spectral lines related with ground vibrational states of nitrogen molecular ion, the observed amplification exhibits different polarization dependence of the pump laser. This distinctive change can be explained by the population variation of high vibrational states caused by the pump laser with different polarizations.

Ramsey interferometry through coherent A2Πu-X2Σg+-B2Σu+ coupling and population transfer in N2+ air laser

Motivated by the hot debate on the mechanism of laser-like emission at 391 nm from N₂ gas irradiated by a strong 800 nm pump laser and a weak 400 nm seed laser, we theoretically study the temporal profile, optical gain, and modulation of the 391 nm signal from N2+. Our calculation sheds light on the long standing controversy on whether population inversion is indispensable for optical gain and show the Ramsey fringes of the emission intensity at 391 nm formed by additionally injecting another 800 nm pump or 400 nm seed, which provides strong evidence for the coherence driven modulation of transition dipole moment and population transfer between the A²Π_u(ν=2)-X²Σg+ states and the B²Σu+(ν=0)-X²Σg+ states. Our results show that the 391 nm optical gain is susceptible to the population inversion within N2+ states manipulated by the Ramsey technique and thus clearly reveal their symbiosis. This study reveals not only the physical picture of producing N2+ population inversion but also versatile control of the N2+ air laser.

Optimization of N2+ lasing by waveform-controlled polarization-skewed pulses

Optical ionization of N₂ and subsequent population redistribution among the ground and excited states of N2+ in an intense laser field are commonly accepted to be fundamentally responsible for the generation of N2+ lasing. By finely controlling this two-step process, the optimization of N2+ lasing is possibly achieved. Here, we design a waveform-controlled polarization-skewed (PS) pumping pulse, in which the leading and falling edges are orthogonally polarized, and their relative field strength and phase can be well controlled. We demonstrate that precise manipulation of the N2+ lasing at 391 nm and 428 nm emissions can be achieved by modulating both the relative phase and amplitudes of the two orthogonally polarized components of the pumping PS pulse. We find that the optimization of N2+ lasing depends not only on the competitive balance between the ionization and post-ionization coupling that varies in different pumping energies but also on the phase with the maximum intensity appearing at the phase of nπ. Orders of magnitude enhancement in the N2+ lasing intensity is observed as the phase changes from (n+1/2)π to nπ. The PS pulse with a controllable spatiotemporal waveform provides us a robust and straightforward tool to efficiently enhance the N2+ lasing emission.

Quantum erasing of laser emission in N2

Cavity-free lasing of N2+ induced by a femtosecond laser pulse at 800 nm is nearly totally suppressed by a delayed twin control pulse. We explain this surprising effect within the V-scheme of lasing without population inversion. A fast transfer of population between nitrogen ionic states X²Σg+ and A²Π_u, induced by the second pulse, terminates the conditions for amplification in the system. The appearance of short lasing bursts at delays corresponding to revivals of rotational wave packets is explained along the same lines.

Asymmetric enhancement of N2+ lasing in intense, birefringence-modulating elliptical laser fields

We experimentally demonstrate an asymmetric enhancement of the N₂⁺ lasing at 391 nm for the transition between the B²Σu⁺ (v = 0) and X²Σg⁺ (v" = 0) states in an intense laser field with the ellipticity, ε, modulated by a 7-order quarter-wave plate (7-QWP). It is found that when the 7-QWP is rotated from α = 0 to 90°, where α is the angle between the polarization direction of the input laser and the slow axis of the 7-QWP, the intensity of the 391-nm lasing is optimized at ε ∼ 0.3 with α∼ 10°-20° and 70°-80° respectively, but the optimization intensity at α∼ 10°-20° is about 4 times smaller than that at α∼ 70°-80°. We interpret the asymmetric enhancement based on a post-ionization coupling model, in which the birefringence of the 7-QWP induces an opposite change in the relative amplitudes of the ordinary (Eo) and extraordinary (Ee) electric components under the two conditions, so that the same temporal separation of Eo and Ee leads to a remarkably different coupling strength for the population transfer from the X²Σg⁺ (v "=0) to A²Π_u (v '=2) states.

Giant Enhancement of Air Lasing by Complete Population Inversion in N_{2}^{+}

A fine manipulation of population transfer among molecular quantum levels is a key technology for control of molecular processes. When a light field intensity is increased to the TW-PW cm^{-2} level, it becomes possible to transfer a population to specific excited levels through nonlinear light-molecule interaction, but it has been a challenge to control the extent of the population transfer. We deplete the population in the X^{2}Σ_{g}^{+}(v=0) state of N_{2}^{+} almost completely by focusing a dual-color (800 nm and 1.6  μm) intense femtosecond laser pulse in a nitrogen gas, and make the intensity of N_{2}^{+} lasing at 391 nm enhanced by 5-6 orders of magnitude. By solving a time-dependent Schrödinger equation describing the population transfer among the three lowest electronic states of N_{2}^{+}, we reveal that the X^{2}Σ_{g}^{+}(v=0) population is depleted by the vibrational Raman excitation followed by the electronic excitation A^{2}Π_{u}(v=2,3,4)←X^{2}Σ_{g}^{+}(v=1)←X^{2}Σ_{g}^{+}(v=0), resulting in the excessive population inversion between the B^{2}Σ_{u}^{+}(v=0) and X^{2}Σ_{g}^{+}(v=0) states. Our results offer a promising route to efficient population transfer among vibrational and electronic levels of molecules by a precisely designed intense laser field.