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Quantum coherence in quantum optics is an essential part of optical information processing and light manipulation. Alkali metal vapors, despite the numerous shortcomings, are traditionally used in quantum optics as a working medium due to convenient near-infrared excitation, strong dipole transitions and long-lived coherence. Here, we proposed and experimentally demonstrated photon retention and subsequent re-emittance with the quantum coherence in a system of coherently excited molecular nitrogen ions (N2+) which are produced using a strong 800 nm femtosecond laser pulse. Such photon retention, facilitated by quantum coherence, keeps releasing directly-unmeasurable coherent photons for tens of picoseconds, but is able to be read-out by a time-delayed femtosecond pulse centered at 1580 nm via two-photon resonant absorption, resulting in a strong radiation at 329.3 nm. We reveal a pivotal role of the excited-state population to transmit such extremely weak re-emitted photons in this system. This new finding unveils the nature of the coherent quantum control in N2+ for the potential platform for optical information storage in the remote atmosphere, and facilitates further exploration of fundamental interactions in the quantum optical platform with strong-field ionized molecules.

With the parallel and perpendicular components of high harmonics generated by using aligned N

We experimently and theoretically investigate temporal structures of ${{\mathrm{N}}_{2}}^{+}$ lasing at the wavelengths of 391 and 428 nm. Our results show that the resonant interaction of a femtosecond laser with ${{\mathrm{N}}_{2}}^{+}$ ions with a picosecond dipole relaxation time will induce a long-lasting polarization, which exists in cases of both absorption and amplification of the external seed. The induced polarization will be amplified in population-inverted ${{\mathrm{N}}_{2}}^{+}$ ions, giving rise to the retarded radiation (i.e., ${{\mathrm{N}}_{2}}^{+}$ lasing). The temporal profile of the retarded radiation is closely related to the dipole relaxation time, population inversion density, and propagation length. The combined experimental and theoretical study reveals the physical origin of the retarded seed amplification in ${{\mathrm{N}}_{2}}^{+}$ ions.

We report a comparative experimental investigation of femtosecond laser-induced third harmonic generation (THG) in an ethanol flame and in air. It was found that the third harmonic (TH) signal produced in the presence of a combustion field can be greatly enhanced, in comparison to that generated in air, and that the enhancement factor depends strongly on the experimental parameters, such as the focal length, relative position of the flame and filament, and laser repetition frequency. Moreover, by replacing the flame with a point temperature controller, a similar signal enhancement of THG was observed, and the TH signal exhibited a nonlinear growth with the heating temperature. Further analysis indicated that the observed enhancement of THG originates from the suppression of Gouy-phase-induced destructive interference due to the disturbed gas density under high-temperature heating. The high sensitivity of the TH signal to the combustion temperature helps formulate an effective and straightforward approach to achieve nonintrusive temperature measurements in the combustion field.

A systematic study of nonsequential double ionization (NSDI) of alkaline-earth metal atoms with mid-infrared femtosecond pulses is reported. We find that the measured NSDI yield shows a strong target dependence and it is more suppressed for alkaline-earth metal with higher ionization potential. The observation is attributed to the differences in the recollision induced excitation and ionization cross sections of alkaline-earth metals. This work indicates that NSDI of alkaline-earth metals can be generally understood within recollision picture and sheds light on ultrafast control of electron correlation and dynamics of ionic excited states during NSDI of atoms with complex structures.

We report on a comparative study of strong-field ionization of alkaline-earth-metal atoms by intense femtosecond laser pulses from near-infrared to midinfrared wavelengths. By collecting the ionization signals only produced within the central portion of the laser focus, the focus volume effect is largely reduced and the saturation intensities for different alkaline-earth-metal atoms are reliably determined, which permits us to directly test the strong-field-ionization theories. We demonstrate that the Perelomov-Popov-Terent'ev model accurately predicts the experimental ionization yields and saturation intensities in general for arbitrary values of the Keldysh parameter, while the Ammosov-Delone-Krainov simulations agree with the experiments for the tunneling-ionization regime and also for the regime when the Keldysh parameter is around 1. Our work presents benchmark data for strong-field ionization of alkaline-earth metals over a broad range of laser parameters and confirms the validity of Keldysh's picture for such atoms.

We experimentally investigated the rotationally resolved polarization characteristics of N$_2^+$ lasing at 391 and 428 nm using a pump-seed scheme. By varying the relative angle between the linear polarizations of the pump and seed, it is found that the polarizations of the P and R branches of 391-nm lasing are counter-rotated. By contrast, both branches of 428-nm lasing remain polarized along the pump. The origin of the puzzled abnormal polarization characteristics is found based on a complete physical model that simultaneously includes the transient photoionization and the subsequent coupling among the electronic, vibrational and rotational quantum states of ions.It suggests that the cascaded resonant Raman processes following ionization create negative coherence between the rotational states of $J$ and $J$+2 in the ionic ground state X$^2\Sigma_g^+(\nu=0)$, which leads to mirror-symmetrical polarization for the P and R branches of 391-nm lasing. Both the experiment and theory indicate that the demonstrated rotational coherence plays an extremely pivotal role in clarifying the gain mechanism of N$_2^+$ lasing and opens up the route toward quantum optics under ultrafast strong fields., Comment: submitted to referenced Journal in Jan 12, 2020

We develop a background-free single-beam coherent Raman scattering technique enabling the high-sensitivity detection of greenhouse gases. In this scheme, Raman coherence prepared by a femtosecond laser is interrogated by self-generated narrowband air lasing, thus allowing single-beam measurements without complex pulse shaping. The unique temporal and spectral characteristics of air lasing are beneficial for improving the signal-to-noise ratio and spectral resolution of Raman signals. With this method, SF6 gas present at a concentration of 0.38% was detected in an SF6–air mixture. This technique provides a simple and promising route for remote detection due to the low divergence of Raman signals and the availability of high-energy pump lasers, which may broaden the potential applications of air lasing.

It has been observed that in a pre-excited medium produced by femtosecond laser pulses, nonlinear interaction can be dramatically enhanced. The underlying mechanism is still under debate because of the lack of detailed information on the species, energy states, and temporal dynamics of the excited medium. Here, we report a dramatic enhancement of seventh-harmonic generation in pre-excited argon atoms. By measuring the evolution of the enhanced signal with the propagation distance, we are able to separate the contributions from the improved phase matching and the enhanced nonlinear susceptibility. The time- and spectrum-resolved measurements clearly show that generation of excited states is responsible for the enhanced nonlinear susceptibility. The maximum population of excited states on the picosecond and nanosecond timescales can be attributed to two possible pathways.

Molecular ions, produced via ultrafast ionization, can be quantum emitters with the aid of resonant electronic couplings, which makes them the ideal candidates to study strong-field quantum optics. In this work, we experimentally and numerically investigate the necessary condition for observing a collective emission arising from macroscopic quantum polarization in a population-inverted N 2 + gain system, uncovering how the individual ionic emitters proceed into a coherent collection within hundreds of femtoseconds. Our results show that for a relatively high-gain case, the collective emission behaviors can be readily initiated for all the employed triggering pulse area. However, for a low-gain case, the superradiant amplification is quenched since the building time of macroscopic interionic quantum coherence exceeds the dipole dephasing time, in which situation the seed amplification and free induction decay play an essential role. These findings not only clarify the contentious key issue regarding to the amplification mechanism of N 2 + lasing but also show the unique characteristics of ultrashort laser-induced amplification in a molecular ion system where both the microscopic and macroscopic quantum coherence might be present.

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 A 2 Π i state of C O + , which is the result of multi-channel interference during the resonant excitation process. High resolution of Fourier transform spectra up to 0.067 c m − 1 allows us to obtain the fine energy levels of the A 2 Π 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.

We investigate the interwoven dynamic evolutions of neutral nitrogen molecules together with nitrogen ions created through transient tunnel ionization in an intense laser field. By treating the molecules as open quantum systems, it is found that considering real-time injection of ions and strong couplings among their electronic states, nitrogen molecular ions are primarily populated in the electronically excited states, rather than staying in the ground state as predicted by the well-known tunneling theory. The unexpected result is attributed to sub-cycle switch-on of time-dependent polarization by transient ionization and dynamic Stark shift mediated near-resonant multiphoton transitions. Their combined contribution also causes that the vibrational distribution of N$_2^+$ does not comply with Franck-Condon principle. These findings corroborate the mechanism of nitrogen molecular ion lasing and are likely to be universal. The present work opens a new route to explore the important role of transient ionization injection in strong-field induced non-equilibrium dynamics., 4 figurs

We report a combined theoretical and experimental investigation on the unambiguous role of electronic coherence underlying the generation of laserlike coherent emission from ${{\mathrm{N}}_{2}}^{+}$ ions launched in a strong laser field. The coherence manifests itself by giving rise to a dramatic enhancement of ${{\mathrm{N}}_{2}}^{+}$ laserlike radiation with two temporally separated pump laser pulses centered at 800- and 1580-nm wavelengths, respectively. We further show that coherent control of ${{\mathrm{N}}_{2}}^{+}$ lasing in a subfemtosecond timescale can be achieved by replacing the single 800-nm laser pulse with a pair of 800-nm laser pulses of adjustable time delay. The experimental results are well reproduced in theory using a three-level quantum model of ${{\mathrm{N}}_{2}}^{+}$ ions under the density matrix representation.

We report on a systematic investigation of wavelength scaling strong-field double ionization of Mg in intense laser fields. A significant decrease of nonsequential double ionization (NSDI) yield with increasing wavelength from 800--2000 nm is observed. Our data is well reproduced by a three-dimensional Monte Carlo simulation considering recollision impact excitation cross section. We demonstrate that the NSDI of Mg mainly occurs via the first ionic excited state ${\mathrm{Mg}}^{+*}(3p\phantom{\rule{0.16em}{0ex}}^{2}P_{3/2,1/2})$ pumped by returning electron impact process. The recollision impact direct ionization pathway plays a minor role here. The wavelength dependence of the NSDI ratio is due to the recollision energy-dependent excitation cross section as well as the electron wave packet diffusion effects, both sensitively depending on the wavelength. Our work represents a step towards strong-field double ionization experiments on Mg in the long wavelength limit and sheds light on the NSDI mechanism of alkaline-earth metal atoms.

We experimentally investigate air lasing emission from singly ionized nitrogen molecules exposed to bicircular two-color (BTC) femtosecond laser pulses. The light driven electron recollision is suppressed in the corotating BTC field as compared to the counter-rotating one. It allows us to straightforwardly examine the role of electron recollision on the air lasing generation by simply changing the rotating sense of the BTC fields. Lasing emission is observed for both corotating and counter-rotating BTC fields without noticeable difference when the rotating sense is switched from one to the other case. It indicates a minor contribution of electron recollision on the air lasing emission. In addition to the well-observed linear polarization, we demonstrate that the polarization of the newborn lasing emission can be remotely manipulated from linear to circular by controlling the polarization of the driving second-harmonic field which at the same time acts as the seed.

We report on the generation of electronic quantum coherence in the ionized nitrogen molecules. The coherence is revealed by observing N2+ air lasing with a near-infrared laser and a delayed mid-infrared laser.

We report generation of cascaded rotational Raman scattering up to 58th orders in coherently excited CO_2 molecules. The high-order Raman scattering, which produces a quasiperiodic frequency comb with more than 600 sidebands, is obtained using an intense femtosecond laser to impulsively excite rotational coherence and the femtosecond-laser-induced N_2^+ lasing to generate cascaded Raman signals. The novel configuration allows this experiment to be performed with a single femtosecond laser beam at free-space standoff locations. It is revealed that the efficient spectral extension of Raman signals is attributed to the specific spectra-temporal structures of N_2^+ lasing, the ideal spatial overlap of femtosecond laser and N2+ lasing, and the guiding effect of molecular alignment. The Raman spectrum extending above 2000 cm^-1 naturally corresponds to a femtosecond pulse train due to the periodic revivals of molecular rotational wavepackets., Comment: 17 pages, 4 figures

We report generation of spectrally bright vacuum ultraviolet (VUV) and deep UV (DUV) coherent radiations at the wavelengths of 192 nm, 198 nm and 204 nm. These DUV/VUV radiations originate from resonant four-wave mixing assisted by quantum coherence in tunnel-ionized CO molecules. The electronic coherence is created when the pump pulses resonantly excite electronic transitions of CO+. The technique allows for the selective switch of DUV/VUV wavelengths provided by the abundant energy levels of molecular ions. The developed source can have arbitrary polarization states by manipulating the polarization of the pump pulse. It also exhibits a spectral bandwidth of 5∼7 cm−1, a divergence angle of 3∼5 mrad, a pulse duration of ∼10 ps, and a photon flux of ∼1010 photons/s. The superior temporal and spectral properties give rise to a broadband Raman comb in the DUV/VUV region.

With the parallel and perpendicular components of high harmonics generated by using aligned N2 molecules, we propose a method to retrieve the alignment distribution induced by the aligning laser based on the quantitative rescattering theory. And the intensity of pump laser and gas temperature can be precisely determined as well. We find that the intensity ratio between two harmonic components is very sensitive to the inclusion of multiple-orbital contribution in the theory. We thus suggest that it could be used to identify the interference from inner orbitals by tuning input laser power or extending the spectral region of high harmonics.

Ultrafast optics and ultra-intense laser technology, which are important frontiers in the fields of optics and lasers, are expected to promote fundamental science discoveries and high-technology developments. In this paper, the current status and future trends of ultrafast optics and ultra-intense laser technology are introduced briefly, including the applications of ultrafast and ultra-intense lasers in particle acceleration, attosecond science, ultrafast nonlinear optics, micro/nano-fabrication, broadband optical frequency combs, etc. The future prospects and key techniques for generating ultra-intense and ultra-short laser pulses with higher performance are also described.

We investigate free-space lasing actions from molecular nitrogen ions (N2+) at the wavelengths of ~391 nm and ~428 nm. Our results show that pronounced gain can be measured at either 391 nm or 428 nm laser wavelength with a pump laser centered at 800 nm wavelength, whereas the gain at 391 nm laser wavelength completely disappears when the wavelength of the pump laser is tuned to 1500 nm. Our theoretical analysis reveals that the different gain behaviors can be attributed to the vibrational distribution of populations in X2Σg+(v=0) and X2Σg+(v=1) states as the N2+ ions are generated by photoionization in the laser fields, giving rise to more robust (i.e., less sensitive to the pump laser wavelength) population inversion for generating the 428 nm laser.

It is known that in an intense laser field of a sufficiently high strength combined with a sufficiently long wavelength, i.e., in the regime of the Keldysh parameter γ

We report on lasing actions and extraordinary supercontinuum generation in strong-field ionized nitrogen molecules. It was revealed that these unexpected emissions result from a series resonant nonlinear optical processes in molecular ions.

We present a method based on simultaneous spatiotemporal focusing (SSTF) of the femtosecond laser pulses that enables to fabricate 3D structures on the centimeter scale with TPP. Several Centimeter-scale structures were fabricated using this method.

Free-space\( \kern0.28em {\mathrm{N}}_2^{+}\kern0.28em \)lasers induced by strong-field ionization have shown a great potential for the standoff diagnostics of quantum rotational wave packets in air molecules and for the fingerprint identification of chemical species, thanks to their capability of remotely generating intense, narrow-bandwidth, coherent emissions. It has been observed that the laser signals are strongly influenced by the dynamics of the rotational wave packets in the \( {\mathrm{N}}_2^{+} \) ions produced by photoionization in the intense laser fields. This chapter gives an overview of the experimental results of the effects of molecular rotations in the free-space \( {\mathrm{N}}_2^{+} \) lasers. We will begin with the description of the ultrafast dynamics of coherent rotational wave packets using the \( {\mathrm{N}}_2^{+} \) laser as an optical probe. We will follow with a discussion of the coherent coupling of the rotational quantum states in a strong laser field, manipulation of the polarization of \( {\mathrm{N}}_2^{+} \) laser emission, and the observation of impulsive rotational Raman scattering induced by the \( {\mathrm{N}}_2^{+} \) laser. Finally, a summary and perspective will be given.

Experimental and theoretical investigations of the wavelength dependence of the nonsequential double ionization (NSDI) process of the xenon atom in intense laser fields are reported. The observed wavelength dependence of the ratio Xe2+/Xe+ deviates significantly from the prediction of the three-step model of ionization (sometimes called the "simple-man" model), both in the slope of the overall decrease with increasing wavelength and in the existence of some pronounced humps. A semiclassical model calculation reproduces the changing slope of the overall decrease, which is due to the interplay of wave-packet diffusion, Coulomb focusing, and a closely related Coulomb defocusing effect of the liberated electron. The hump is beyond the scope of the semiclassical model.

We extend the detection range of laser-induced breakdown spectroscopy by combining high-intensity femtosecond laser pulses with high-energy nanosecond CO2 laser pulses. The femtosecond laser pulses ionize the molecules and generate filament in air. The free electrons generated in the self-confined plasma channel by the femtosecond laser serve as the seed electrons which cause efficient avalanche ionization in the nanosecond CO2 laser field. We show that the detection distance has been extended by three times with the assistance of femtosecond laser filamentation.

We experimentally investigate the generation of above-threshold harmonics completely from argon atoms on an excited state using mid-infrared femtosecond laser pulses. The highly nonlinear dependences of the observed signal on the pulse energy and polarization of the probe laser pulses indicate its nonperturbative characteristic.

We investigate nonlinear interaction of nitrogen molecules with a two-color laser field composed by an intense 800 nm laser pulse and a weak 400 nm laser pulse. It is demonstrated that the spectrum of 400 nm pulses is dramatically broadened when the two beams temporally overlap. In comparison, the spectral broadening in argon is less pronounced, although argon atoms and nitrogen molecules have comparable ionization potentials. We reveal that the dramatic spectral broadening originates from the greatly enhanced nonlinear optical effects in the near-resonant condition of interaction between the 400 nm pulses and the nitrogen molecular ions.

We experimentally investigate the dependence of N2+ lasing radiation on the ellipticity of an 800 nm pump laser. The results show that 391 and 428 nm lasing signals exhibit different behaviors with the change of ellipticity. The 391 nm lasing signals show abnormal ellipticity dependence regardless of the pump intensity, whereas for the 428 nm lasing signals, the abnormal characteristic only occurs at a high laser intensity. The qualitative analysis reveals that the physical origin of the phenomenon is the postionization coupling between the ground X2Σg+ state and the intermediate A2Π u state of N2+ ions.

We carry out a combined theoretical and experimental investigation on the population distributions in the ground and excited states of tunnel ionized N2 molecules at various driver wavelengths in the near- and mid-infrared range. Our results reveal that efficient couplings (i.e., population exchanges) between the ground state and the excited states occur in strong laser fields. The couplings result in the population inversion between the ground and the excited states at the wavelengths near 800 nm, which is verified by our experiment by observing the amplification of a seed at ~391 nm. The result provides insight into the mechanism of free-space nitrogen ion lasers generated in remote air with strong femtosecond laser pulses., Comment: 18 pages, 4 figures

We discuss the non-purturbative and purturbative nonlinear optical processes involved in generation of atmospheric lasing with intense ultrashort laser pulses.