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Journal ArticleDOI

Population inversion in N 2 + by vibrationally mediated Rabi oscillation at 400 nm

TL;DR: In this paper, the authors investigated the lasing of a nitrogen gas induced by femtosecond laser pulses at around 400 nm and found that the population inversion occurs at the $B\phantom{\rule{0.28em}{0ex}}+}$ (0, 1) and (1, 2) emissions.
Abstract: We investigate lasing of a nitrogen gas induced by intense femtosecond laser pulses at around 400 nm. By examining both the self-induced and externally seeded forward emission spectra, we unambiguously identify the ${{\mathrm{N}}_{2}}^{+}$ lasing actions at 427.8 and 423.6 nm assigned respectively to the $B\phantom{\rule{0.16em}{0ex}}{}^{2}\mathrm{\ensuremath{\Sigma}}{{}_{u}}^{+}\ensuremath{-}X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ (0, 1) and (1, 2) emissions and show that the lasing mechanism is totally different from the lasing induced by near-infrared (800 nm) laser pulses, in which the population transfer from $X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ to $A^{2}\mathrm{\ensuremath{\Pi}}_{u}$ proceeds through the resonant $X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}\ensuremath{-}A^{2}\mathrm{\ensuremath{\Pi}}_{u}$ transition, facilitating the population inversion between the $B\phantom{\rule{0.16em}{0ex}}{}^{2}\mathrm{\ensuremath{\Sigma}}{{}_{u}}^{+}\ensuremath{-}X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ states [Phys. Rev. Lett. 123, 203201 (2019)]. We simulate the population distributions among the vibrational levels in the three lowest electronic states of ${{\mathrm{N}}_{2}}^{+}$ by the 400-nm laser pumping and find that the population is efficiently transferred between the $X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ state and the $B\phantom{\rule{0.16em}{0ex}}{}^{2}\mathrm{\ensuremath{\Sigma}}{{}_{u}}^{+}$ state by a Rabi oscillation combined with a Raman-type transition, leading to the population inversion so that the lasing occurs at the $B\phantom{\rule{0.16em}{0ex}}{}^{2}\mathrm{\ensuremath{\Sigma}}{{}_{u}}^{+}\ensuremath{-}X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ (0, 1) and (1, 2) emissions.
Citations
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Journal ArticleDOI
TL;DR: In this paper , an externally seeded 400-nm pump laser pulse was used to demonstrate the lasing action on the transition between the rotational levels of a gas with an intense 400nm pump pulse and a control pulse.
Abstract: We experimentally demonstrate an externally seeded ${\mathrm{N}}_{2}{}^{+}$ lasing action on the transition between the $B{\phantom{\rule{4pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}{}^{+}(v=0)$ and $X{\phantom{\rule{4pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}{}^{+}({v}^{\ensuremath{'}}=0)$ states at 391.4 nm by irradiating a ${\mathrm{N}}_{2}$ gas with an intense 400-nm pump laser pulse and reveal that the populations in the rotational levels of ${J}^{\ensuremath{'}}=7--17$ in the $B{\phantom{\rule{4pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}{}^{+}(v=0)$ state are responsible for the lasing based on the rotational revival structure in the lasing intensity recorded by the pump-probe measurements. By introducing additionally an 800-nm control pulse, we find that the lasing intensity is suppressed when the timing of the control pulse is set between the 400-nm pump and 400-nm seed laser pulses while it can be enhanced when the control pulse overlaps temporally the seed pulse. By solving the time-dependent Schr\"odinger equation including the continuous ionization of ${\mathrm{N}}_{2}$ and multistate coupling among the ${B}^{\phantom{\rule{4pt}{0ex}}2}{\mathrm{\ensuremath{\Sigma}}}_{u}{}^{+}$, ${A}^{\phantom{\rule{4pt}{0ex}}2}{\mathrm{\ensuremath{\Pi}}}_{u}$, and $X{\phantom{\rule{4pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}{}^{+}$ states in ${\mathrm{N}}_{2}{}^{+}$ induced by the control pulse, we show that the population inversion in ${\mathrm{N}}_{2}{}^{+}$ can be achieved by the 400-nm pump laser pulse and can be further modulated by the control pulse. We show also that the ${\mathrm{N}}_{2}{}^{+}$ lasing intensity can be suppressed or enhanced depending on the timing and intensity of the control pulse originating from the competition between the two dynamical processes, that is, the ionization of ${\mathrm{N}}_{2}$ and the population transfer among the three electronic states of ${\mathrm{N}}_{2}{}^{+}$.

2 citations

Journal ArticleDOI
TL;DR: In this article , 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 was studied.
Abstract: 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.

2 citations

Journal ArticleDOI
TL;DR: In this article , an analytical pulse design protocol for controlling the vibrational dynamics of polar diatomic molecules within a given electronic state is presented, where the optimal pulse shapes are obtained by reverse engineering, that is, solving the Schr\"odinger equation of the nuclei inversely in a relevant subspace.
Abstract: We present an analytical pulse design protocol for controlling the vibrational dynamics of polar diatomic molecules within a given electronic state. Altering the potential energy function via the position-dependent electric permanent dipole moment, the vibrational state population dynamics is directly controlled using appropriately shaped laser pulses in the midinfrared regime. The optimal pulse shapes---that are expected to drive the molecule along user-defined quantum pathways---are obtained by reverse engineering, that is, solving the Schr\"odinger equation of the nuclei inversely in a relevant subspace. The proposed control scheme is validated by accurately solving the full time-dependent Schr\"odinger equation of the ${\mathrm{HeH}}^{+}$ molecular ion with two completely different methods: (1) propagating the complex population amplitudes of many field-free eigenstates or (2) propagating directly the nuclear wave packet on a grid. We find that besides smooth transitions, arbitrary Rabi oscillations as well as vibrational ladder climbing can be efficiently controlled with the present scheme. As a result, the molecule is successively excited beyond the potential barrier, leading to enhanced dissociation in the ground electronic state. Rotational effects and possible extensions of the presented control are also briefly discussed.
References
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Journal ArticleDOI
TL;DR: The experiment shows that external focusing strongly influences the plasma density and the diameter of femtosecond Ti-sapphire laser filaments generated in air and is in good qualitative agreement with the results of numerical simulations.
Abstract: Our experiment shows that external focusing strongly influences the plasma density and the diameter of femtosecond Ti-sapphire laser filaments generated in air. The control of plasma filament parameters is suitable for many applications such as remote spectroscopy, laser induced electrical discharge, and femtosecond laser material interactions. The measurements of the filament showed the plasma density increases from 10(15)cm(-3) to 2 x 10(18)cm(-3) when the focal length decreases from 380 cm to 10 cm while the diameter of the plasma column varies from 30 microm to 90 microm. The experimental results are in good qualitative agreement with the results of numerical simulations.

303 citations

Journal ArticleDOI
28 Jan 2011-Science
TL;DR: It is demonstrated that high gain can be achieved in the near-infrared region by pumping with a focused ultraviolet laser and low-divergence backward air lasing provides possibilities for remote detection.
Abstract: The compelling need for standoff detection of hazardous gases and vapor indicators of explosives has motivated the development of a remotely pumped, high-gain air laser that produces lasing in the backward direction and can sample the air as the beam returns. We demonstrate that high gain can be achieved in the near-infrared region by pumping with a focused ultraviolet laser. The pumping mechanism is simultaneous resonant two-photon dissociation of molecular oxygen and resonant two-photon pumping of the atomic oxygen fragments. The high gain from the millimeter-length focal zone leads to equally strong lasing in the forward and backward directions. Further backward amplification is achieved with the use of earlier laser spark dissociation. Low-divergence backward air lasing provides possibilities for remote detection.

263 citations

Journal ArticleDOI
06 Jun 2003
TL;DR: In this article, the backscattered fluorescence of nitrogen from long filaments generated by intense ultrafast Ti-sapphire laser pulses propagating in air is studied and it shows an exponential increase with increasing filament length, indicating amplified spontaneous emission (ASE).
Abstract: The backscattered fluorescence of nitrogen from long filaments generated by intense ultrafast Ti-sapphire laser pulses propagating in air is studied It shows an exponential increase with increasing filament length, indicating amplified spontaneous emission (ASE)

230 citations

Journal ArticleDOI
TL;DR: In this paper, a switchable multi-wavelength laser in air driven by intense mid-infrared femtosecond laser pulses is demonstrated, in which population inversion occurs at an ultrafast time scale (i.e., less than \ensuremath{\sim}200 fs) owing to direct formation of excited molecular nitrogen ions by strong-field ionization of innervalence electrons.
Abstract: We demonstrate a harmonic-seeded switchable multiwavelength laser in air driven by intense midinfrared femtosecond laser pulses, in which population inversion occurs at an ultrafast time scale (i.e., less than \ensuremath{\sim}200 fs) owing to direct formation of excited molecular nitrogen ions by strong-field ionization of inner-valence electrons. The bright multiwavelength laser in air opens the perspective for remote detection of multiple pollutants based on nonlinear optical spectroscopy.

226 citations

Journal ArticleDOI
TL;DR: A simple experiment to directly determine the critical power for self-focusing in air by measuring the focal shift of the focused femtosecond Ti:sapphire laser pulses.
Abstract: We report a simple experiment to directly determine the critical power for self-focusing in air by measuring the focal shift of the focused femtosecond Ti:sapphire laser pulses. The measured critical power is 10 GW for the 42 fs laser pulse; it gradually decreases to 5 GW for (chirped) pulse duration longer than 200 fs.

209 citations

Trending Questions (1)
What is the impact of population inversion on the efficiency of a four-level laser system?

The provided paper does not discuss the impact of population inversion on the efficiency of a four-level laser system.