Air lasing: Phenomena and mechanisms

Air lasing is a concept that refers to remote no-cavity(mirrorless) optical amplification in ambient air with the air constituents as the gain media. Due to the high potential of air lasing in view of applications in atmospheric sensing, a variety of pumping schemes have been proposed so far for building up population-inverted gain media in air and producing forward and/or backward directional lasing emissions. This review paper presents an overview of recent advances in the experimental observations and physical understanding of air lasing in various pumping schemes of air molecules by intense laser fields. Special emphasis is given to the strong-field-induced N^+2 air lasing, the mechanism of which is currently still in a hot debate.

Observation of the optical X2Σg+–A2Πu coupling in N2+ lasing induced by intense laser field

We propose a simple pump-coupling-seed scheme to examine the optical X^2Σg^+–A^2Πu coupling in N2^+ lasing. We produce the N2^+ lasing at 391 nm, corresponding to the B^2Σu^+(v = 0)–X^2Σg+(v = 0) transition, by externally seeding the N^2+ gain medium prepared by irradiation of N2 with an intense pump pulse. We then adopt a weak coupling pulse in between the pump and seed pulses, and show that the intensity of the 391-nm lasing can be efficiently modulated by varying the polarization direction of the coupling pulse with respect to that of the pump pulse. It is found that when the polarization directions of the pump and coupling pulses are perpendicular, the 391-nm lasing intensity is more sensitive to the coupling laser energy, which reflects the inherent nature of the perpendicular X^2+Σg^–A^2Πu transition.

Review:Coincidence Momentum Imaging:Principle and Application in Molecular Reaction Dynamics

Coincidence Momentum Imaging(CMI) is a powerful imaging technique that can determine the full momentum vectors of all particles released from a single parent molecule in coincidence and thus provide detailed information on transient molecular structures.So far,the CMI technique has been extensively employed for investigating a variety of molecular reaction dynamics induced,e.g.,by particle collisions,intense laser fields and synchrotron radiation.In this article,we first introduce the principle of the CMI technique,which is followed by several typical experimental designs of the CMI systems realizing the coincidence momentum detections.We then present representative examples of studying molecular reaction dynamics using the CMI technique.

Strong-Field-Induced N2+ Air Lasing in Nitrogen Glow Discharge Plasma

We investigate N_(2)^(+) air lasing at 391 nm,induced by strong laser fields in a nitrogen glow discharge plasma.We generate forward N_(2)^(+) air lasing on the B^(2)Σ_(u)^(+)(v’=0)-X^(2)Σ_(g)^(+)(v"=0) transition at 391 nm by irradiating an intense 35-fs,800-nm laser in a pure nitrogen gas,finding that the 391-nm lasing quenches when the nitrogen gas is electrically discharged.In contrast,the 391-nm fluorescence measured from the side of the laser beam is strongly enhanced,demonstrating that this discharge promotes the population in the B^(2)Σ_(u)^(+)(v’=0) state.By comparing the lasing and fluorescence spectra of the nitrogen gas obtained in the discharged and laser-induced plasma,we show that the quenching of N_(2)^(+) lasing is caused by the efficient suppression of population inversion between the B^(2)Σ_(u)^(+) and X^(2)Σ_(g)^(+) states of N_(2)^(+),in which a much higher population occurs in the X^(2)Σ_(g)^(+) state in the discharge plasma.Our results clarify the important role of population inversion in generating N_(2)^(+) air lasing,and also indicate the potential for the enhancement of N_(2)^(+) lasing via further manipulation of the population in the X^(2)Σ_(g)^(+) state in the discharged medium.

Diet and feeding behavior of a group of high-altitude rhesus macaques: high adaptation to food shortages and seasonal fluctuations

Diet and feeding behavior data are crucial to a deep understanding of the behavioral response and adaptation of primates to a high-altitude environment.From August 2019 to June 2021,we collected data on the feeding behavior of a high-altitude rhesus macaque Macaca mulatta group from Yajiang County,Western Sichuan Plateau,which has an altitude of over 3,500 m.The results showed that feeding(33.0±1.8%)and moving(28.3±2.6%)were the dominant behavior of rhesus macaques.Macaques ate 193 food items,comprising 11 food categories from 90 species.Our study found that plant roots(30.9±30.1%)and young leaves(28.0±33.1%)were the main foods eaten by macaques.The preferred foods of rhesus macaques were young leaves,fruits,and seeds,and the consumption of these items was positively correlated with its food availability.When the availability of preferred foods was low,macaques took plant roots,barks,and fallen leaves as fallback foods.In particular,roots were a dominant food item in winter,and this way of feeding became a key survival strategy.Our results suggest that,facing the relative scarcity and strong seasonal fluctuations of food resources in high-altitude habitat,macaques adopt active foraging strategies,relying on a variety of food species and adjusting flexibly their food choices based on food availability,which may help to maximize the energy efficiency of high-altitude macaques.

Significant enhancement of multiple resonant sidebands in a soliton fiber laser

Resonant sidebands in soliton fiber lasers have garnered substantial interest in recent years due to their crucial role in understanding soliton propagation and interaction dynamics.However,most previous studies and applications were restricted to focusing on only the first few low-order resonant sidebands because higher-order sidebands usually decay exponentially as their wavelengths shift far away from the soliton center and are negligibly weak.Here we report numerically and experimentally significant enhancement of multiple resonant sidebands in a soliton fiber laser mode-locked by a nonlinear polarization evolution mechanism.The birefringence and the gain profile of the laser cavity were shown to be critical for this phenomenon.Multiple intense resonant sidebands were generated whose maximum intensity was more than 30 d B higher than that of the soliton,which is the highest yet reported,to our knowledge.To accurately predict the wavelengths of all high-order resonant sidebands,an explicit formula was derived by taking the third-order dispersion effect into account.The temporal features of multiple orders of resonant sidebands were characterized,which all exhibit exponentially decaying leading edges.This study provides insight into understanding the properties of high-order resonant sidebands in a soliton laser and opens possibilities for constructing multi-wavelength laser sources.

Robust and ultralow-energy-threshold ignition of a lean mixture by an ultrashort-pulsed laser in the filamentation regime

Laser ignition(LI)allows for precise manipulation of ignition tinning and location and is promising for green combustion of automobile and rocket engines and aero-turbines under lean-fuel conditions with improved emission efficiency;however,achieving completely effective and reliable ignition is still a challenge.Here,we report the realization of igniting a lean methane/air mixture with a 100%success rate by an ultrashort femtosecond laser,which has long been regarded as an unsuitable fuel ignition source.We demonstrate that the minimum ignition energy can decrease to the sub-mJ level depending on the laser filamentation formation,and reveal that the resultant early OH radical yield significantly increases as the laser energy reaches the ignition threshold,showing a clear boundary for misfire and fire cases.Potential mechanisms for robust ultrashort LI are the filamentation-induced heating effect followed by exothermal chemical reactions,in combination with the line ignition effect along the filament.Our results pave the way toward robust and efficient ignition of lean-fuel engines by ultrashort-pulsed lasers.

Femtosecond laser processing of microcavity lasers

In this paper, we reviewed the fabrications of functional microcavity lasers in soft materials such as polymer and protein by femtosecond laser processing. High-quality （Q） microdisks with a laser dye （Rhodamine B, RhB） acting as gain medium were fabricated that produced whispering-gallery-mode （WGM） lasing output. We also obtained unidirectional lasing output with a low lasing threshold in a deformed spiral microcavity at room temperature. Photonic-molecule （PM） microlasers were prepared to investigate the interaction and coupling effects of different cavities, and it was found that the distance between the two disks plays an important role in the lasing behaviors. Single-mode lasing was realized from a stacked PM microlaser through Vernier effect. Furthermore we adopted the biocompatible materials, RhB-doped proteins as a host material and fabricated a three-dimensional （3D） WGM microlaser, which operated well both in air and aqueous environment. The sensing of the protein micro- lasers to Na2SO4 concentration was investigated. Our results of fabricating high-Q microlasers with different materials reveal the potential applications of femtosecond laser processing in the areas of integrated optoelectronic and ultrahigh sensitive bio-sensing devices.

Air-Laser-Based Standoff Coherent Raman Spectrometer

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 800nm and cavity-free picosecond N,air-laser pulses at 391nm 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 and the coupling processes of the rovibrational wavepacketof N,molecules canbeprobed.