We investigate lasing action in aligned nitrogen molecular ions ($\mathrm{N}_2{}^{+}$) produced in an intense laser field. We find that, besides the population inversion between the ${\mathrm{B}}^2\Sigma _u{}^{+}-{\mathrm{X}}^2\Sigma _g{}^{+}$ states, which is responsible for the observed simulated amplification of a seed pulse, a rotational wave packet in the ground vibrational state ($v=0$) of the excited electronic ${\mathrm{B}}^2\Sigma _u{}^{+}$ state has been created in $\mathrm{N}_2{}^{+}$. The rotational coherence can faithfully encode its characteristics into the amplified seed pulses, enabling reconstruction of rotational wave packets of molecules in a single-shot detection manner from the frequency-resolved laser spectrum. Our results suggest that the air laser can potentially provide a promising tool for remote characterization of coherent molecular rotational wave packets.

Erbium doped integrated waveguide amplifier and laser prevail in power consumption, footprint, stability and scalability over the counterparts in bulk materials, underpinning the lightwave communication and large‐scale sensing. Subject to the highly confined mode in the micro‐to‐nanoscale and moderate propagation loss, gain and power scaling in such integrated devices prove to be more challenging compared to their bulk counterparts. In this work, a thin cladding layer of tantalum pentoxide (Ta ₂ O ₅ ) is employed in the erbium doped lithium niobate (LN) waveguide amplifier fabricated on the thin film lithium niobate on insulator (LNOI) wafer by the photolithography assisted chemo‐mechanical etching (PLACE) technique. Above 20 dB small signal internal net gain is achieved at the signal wavelength around 1532 nm in the 10 cm long LNOI amplifier pumped by the diode laser at ∼980 nm. Experimental characterizations reveal the advantage of Ta ₂ O ₅ cladding in higher optical gain compared with the air‐clad amplifier, which is further explained by the theoretical modeling of the LNOI amplifier including the guided mode structures and the steady‐state response of erbium ions.

We demonstrate an on-chip single-mode Er³⁺-doped thin-film lithium niobate (Er:TFLN) laser which consists of a Fabry–Perot (FP) resonator based on Sagnac loop reflectors (SLRs). The fabricated Er:TFLN laser has a footprint of 6.5 mm × 1.5 mm with a loaded quality (Q) factor of 1.6 × 10⁵ and a free spectral range (FSR) of 63 pm. We generate the single-mode laser at 1544 nm wavelength with a maximum output power of 44.7 µW and a slope efficiency of 0.18%.

We show that recently discovered rotational echoes of molecules provide an efficient tool for studying collisional molecular dynamics in high-pressure gases. Our study demonstrates that rotational echoes enable the observation of extremely fast collisional dissipation, at timescales of the order of a few picoseconds, and possibly shorter. The decay of the rotational alignment echoes in ${\mathrm{CO}}_2$ gas and ${\mathrm{CO}}_2\text{−}\mathrm{He}$ mixture up to 50 bar was studied experimentally, delivering collision rates that are in good agreement with the theoretical expectations. The suggested measurement protocol may be used in other high-density media, and potentially in liquids.

Molecular rotational spectroscopy based on a strong-field-ionization-induced nitrogen laser is employed to investigate the time evolution of the rotational wave packet composed by a coherent superposition of quantum rotational states created in a field-free molecular alignment. We show that this technique uniquely allows real-time observation of the ultrafast dynamics of the molecular rotational wave packet. Our analysis also shows that there exist two channels of generation of the nitrogen laser, shedding light on the population inversion mechanism behind the air laser generated by intense femtosecond laser pulses.