Laser filamentation generated when intense laser pulses propagate in air has been an attractive phenomenon having a variety of potential applications such as detection and spectroscopy of gases at far distant places. It was discovered recently that the filamentation in air induces ‘lasing’, showing that electronically excited N₂⁺ is population-inverted, exhibiting marked contrast to the common understanding that molecular ions generated by intense laser fields are prepared mostly in their electronic ground states. Here, to clarify the mechanism of the population inversion, we adopt few-cycle laser pulses, and experimentally demonstrate that the lasing at 391 nm occurs instantaneously after N₂⁺ is produced. Numerical simulations clarify that the population inversion is realized by the post-ionization couplings among the lowest three electronic states of N₂⁺. Our results shed light on the controversy over the mechanism of the air lasing, and show that this post-ionization coupling can be a general mechanism of the atmospheric lasing.

High-intensity lasers are opening up a new realm of light-matter interactions. In his Perspective, Yamanouchi reviews recent progress in this field, focusing on two intensity regimes: the Coulombic regime, which mostly deforms molecular structures and causes tunneling ionization, and the relativistic regime, where high-intensity lasers produce x-rays and high-energy particles and may cause nuclear fusion reactions. Efforts are under way to increase laser intensity further for accessing the next frontier.

Laser-assisted electron scattering (LAES) in a femtosecond near-infrared intense laser field was observed through the scattering of 1~keV electrons by xenon atoms. The intensities, angular distributions, and laser polarization dependence of the observed LAES signals for the energy gain (+$\hbar${}$\omega${}) and energy loss (-$\hbar${}$\omega${}) processes were interpreted well by numerical simulations. As an application of this femtosecond-LAES process, a new method of time-resolved gas electron diffraction for probing geometrical change of molecules with high precision is proposed.

We show that the intensity of self-seeded ${\mathrm{N}}_2^{+}$ lasing at 391 nm, assigned to the $B^2{\mathrm{\Sigma }}_u^{+}(v^{\prime }=0)\to X^2{\mathrm{\Sigma }}_g^{+}(v^{\prime \prime }=0)$ emission, is enhanced by 2 orders of magnitude by modulating in time the polarization of an intense ultrashort near-IR (40 fs, 800 nm) laser pulse with which ${\mathrm{N}}_2$ is irradiated. We find that this dramatic enhancement of the 391 nm lasing is sensitive to the temporal variation of the polarization state within the laser pulse while the intensity of the spontaneous fluorescence emission at 391 nm is kept constant when the polarization state varies. We conclude that a postionization multiple-state coupling, through which the population can be transferred from the $X^2{\mathrm{\Sigma }}_g^{+}$ state of ${\mathrm{N}}_2^{+}$ to the first electronically excited $A^2{\mathrm{\Pi }}_u$ state, leads to the depletion of the population in the $X^2{\mathrm{\Sigma }}_g^{+}$ state, and consequently, to the population inversion between the $X^2{\mathrm{\Sigma }}_g^{+}$ state and the $B^2{\mathrm{\Sigma }}_u^{+}$ state.

Among currently available optical spectroscopic methods, Raman spectroscopy has versatile application to investigation of dynamical processes of molecules leading to chemical changes in the gas and liquid phases. However, it is still a challenge to realize an ideal standoff coherent Raman spectrometer with which both high temporal resolution and high-frequency resolution can be achieved, so that one can remotely probe chemical species in real time with high temporal resolution while monitoring the populations in their respective rovibronic levels in the frequency domain with sufficiently high spectral resolution. In the present study, we construct an air-laser-based Raman spectrometer, in which near-infrared femtosecond (fs) laser pulses at 800 nm and cavity-free picosecond N ₂ ⁺ air-laser pulses at 391 nm generated by the filamentation induced by the fs laser pulses are simultaneously used, enabling us to generate a hybrid ps/fs laser source at a desired standoff position for standoff surveillance of chemical and biochemical species. With this prototype Raman spectrometer, we demonstrate that the temporal evolution of the electronic, vibrational, and rotational states of N ₂ ⁺ and the coupling processes of the rovibrational wave packet of N ₂ molecules can be probed.

We investigate lasing of a ${\mathrm{N}}_2$ gas induced by intense few-cycle near-IR laser pulses. By the pump-probe measurements, we reveal that the intensity of the $B{{}^2\mathrm{\Sigma }}_u^{+}-X{{}^2\mathrm{\Sigma }}_g^{+}$ lasing emission of ${\mathrm{N}}_2^{+}$ oscillates at high (0.3–0.5 PHz), medium (50–75 THz), and low ($\sim 3\mathrm{THz}$) frequencies, corresponding to the energy separations between the rovibrational levels of the $A{{}^2\mathrm{\Pi }}_u$ and $X{{}^2\mathrm{\Sigma }}_g^{+}$ states. By solving the time-dependent Schrödinger equation, we reproduce the oscillations in the three different frequency ranges and show that the coherent population transfer among the three electronic states of ${\mathrm{N}}_2^{+}$ creates the population inversion between the $B{{}^2\mathrm{\Sigma }}_u^{+}$ and $X{{}^2\mathrm{\Sigma }}_g^{+}$ states, resulting in the lasing at 391 nm.