Huailiang Xu

• PhD
• Professor (Full) at Jilin University

Hydrogen migration in methanol induced by an intense laser field (0.2 PW/cm(2)) is investigated in real time by a pump-probe coincidence momentum imaging method. The observed temporal evolution of the kinetic energy spectra reveals that there are two distinctively different stages in the hydrogen migration processes in the singly charged methanol:...

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-inve...

We show that the intensity of self-seeded N2+ lasing at 391 nm, assigned to the B2Σu+(v′=0)→X2Σg+(v′′=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 N2 is irradiated. We find that this dramatic enhancement of the 391 nm lasing is sensit...

Soot, which is produced through the rapid growth of hydrocarbon molecules in fuel-rich flames, has been proved to be responsible for global warming and harmful to the respiratory system of human. However, the complex formation process of soot in flames has not been understood yet because of the lack of efficient time-resolved methods for monitoring...

Laser ignition (LI) allows for precise manipulation of ignition timing 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...

Structured air laser generated through establishing high-gain air media in a cavity-free scheme by intense ultrashort pulses is promising for optical manipulation and quantum communication at standoff distances. However, the mechanism how the orbital angular momentum (OAM) information can be entangled into strong-field-induced gain media is still c...

The orbital angular momentum (OAM) of light, which can be created through the helical phase front, has attracted significant attention and found numerous applications in both classical and quantum optics. The manipulation of light’s OAM has evolved from the generation of a single OAM mode to the simultaneous generation of multiple OAM modes. As a t...

Rapid and real-time monitoring of the concentrations of metal elements in water is essential for water quality evaluation and freshwater production through water desalination. Here we show the ability of the deep reinforcement learning (DRL) in assisting the filament-induced breakdown spectroscopy (FIBS) technique for high-sensitivity and standoff...

To date, supercontinuum light in the visible and near‐infrared ranges is readily realizable by the optical Kerr effect through self‐phase modulation of ultrashort laser pulses in transparent media. However, it is still a challenge to extend the supercontinuum spectrum down to the deep‐ultraviolet (DUV) range, which is particularly needed for explor...

Exciton polaritons—quasi-particle excitations consisting of strongly coupled photons and excitons—present fascinating possibilities for photonic circuits, owing to their strong nonlinearity, ultrafast reaction times and their ability to form macroscopic quantum states at room temperature via non-equilibrium condensation. Past implementations of tra...

In this study, we have explored the ultrafast formation and decay dynamics of exciton-polariton fluids at non-zero momenta, non-resonantly excited by a small-spot femtosecond pump pulse in a ZnO microcavity. Using the femtosecond angle-resolved spectroscopic imaging technique, multidimensional dynamics in both the energy and momentum degrees of fre...

Laser ignition (LI) is promising for green combustion of lean-fuel mixtures with controllable ignition timing and location. It was recently discovered that despite the inferior energy deposition and low thermal temperature in femtosecond (fs) laser-induced plasma, fs laser pulses can achieve a robust ignition of lean-fuel mixture through forming a...

We prepare N2+ mostly in the vibrational ground X2Σg+ state by an ultraviolet 267-nm pulse and create the free induction decay (FID) of the B2Σu+−X2Σg+(0, 0) transition at 391 nm by a resonant excitation pulse. By controlling the phase shift of the FID using a near-IR control pulse through the dynamical Stark effect and rotational coherence, we rev...

The efficiency of energy coupled to plasma during femtosecond (fs) laser filamentation plays a decisive role in a variety of filament applications such as remote fabrication and spectroscopy. However, the energy deposition characterization in the fs laser filament formed by a telescope, which provides an efficient way to extend the filament distanc...

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 d...

Coherent Raman spectroscopy (CRS) with air-laser-based hybrid femtosecond/picosecond (fs/ps) pulses has shown promising potential for remote detection and surveillance of atmospheric species with high temporal and frequency resolution. Here, to enhance the sensitivity and extend the detection distance, we generate the CRS spectra of air molecules i...

Zinc metal is emerging as the promising anode for aqueous Zn-ion batteries. However, corrosion and undesirable Zn dendrite growth limit their practical application in the large-scale energy storage area. Herein, a mountain-valley micro/nanostructure is successfully fabricated on the surface of the Zn anode via a femtosecond-laser filament texturing...

Parametric scattering dynamics are general and of crucial importance for cavity exciton polaritons. Here, parametric scattering process driven by exciton polariton condensates has been revealed in a 1D ZnO microcavity between the whispering-gallery mode and quasiwhispering-gallery mode. When the occupation of the produced polariton condensate is de...

Air lasing induced by laser filamentation opens a new route for research on atmospheric molecular physics and remote sensing. The generation of air lasing is composed of two processes, i.e., building up optical gain of air molecules in femtosecond time scale and emitting coherent radiation in picosecond time scale. Here, we focus on the emission me...

Quantum dot−sensitized solar cells (QDSSC) have been regarded as one of the most promising candidates for effective utilization of solar energy, but its power conversion efficiency (PCE) is still far from meeting expectations. One of the most important bottlenecks is the limited collection efficiency of photogenerated electrons in the photoanodes....

Femtosecond filament-induced breakdown spectroscopy (FIBS) is an efficient approach in remote and in situ detection of a variety of trace elements, but it was recently discovered that the FIBS of water is strongly dependent on the large-bandgap semiconductor property of water, making the FIBS signals sensitive to laser ionization mechanisms. Here,...

The research deals with fabrication of CdS/CdSe-sensitized quantum dot sensitized solar cells (QDSSCs) with laser-engineered black rutile TiO 2 nanoparticles (TiO 2-NPs) as the photoanode material. Three QDSSC devices with pristine and two types of black rutile TiO 2-NPs engineered respectively in water and ethanol by intense femtosecond laser prop...

We experimentally demonstrate an externally seeded N2+ lasing action on the transition between the B2Σu+(v=0) and X2Σg+(v′=0) states at 391.4 nm by irradiating a N2 gas with an intense 400-nm pump laser pulse and reveal that the populations in the rotational levels of J′=7–17 in the B2Σu+(v=0) state are responsible for the lasing based on the rotat...

We have proved that the chemical reaction induced by femtosecond laser filament in the atmosphere produces CO, HCN and NO, and the production CO and HCN were observed for the first time. The concentrations of the products were measured by mid-infrared tunable laser absorption spectroscopy. In reduced pressure air, the decomposition of CO 2 was enha...

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 hi...

Exciton polaritons have shown great potential for applications such as low-threshold lasing, quantum simulation, and dissipation-free circuits. In this paper, we realize a room temperature ultrafast polaritonic switch where the Bose-Einstein condensate population can be depleted at the hundred femtosecond timescale with high extinction ratios. This...

By using the femtosecond angle-resolved spectroscopic imaging technique, the ultrafast buildup dynamics of room-temperature polariton condensation is explicitly visualized in a ZnO whispering gallery mode microcavity. The buildup time of polariton condensation with respect to the arrival of the femtosecond pump pulse decreases with the increasing p...

Soot nanoparticles result from incomplete combustion of fossil fuels, and have been exhibited, when released into the atmosphere, to be detrimental to air quality and human health. However, because of the inert and non-luminescent properties, probing the dynamics of soot in situ is still a challenge. Here we report a strong near-infrared laser pump...

We investigate femtosecond laser-induced plasma spectroscopy (fs-LIPS) of water, and find that at different input laser energies the measured characteristic fs-LIPS emissions from trace metal elements in water exhibit different dependences on the chirped pulse duration. As the pulse is temporally stretched from 50 fs to longer durations up to 500 f...

Laser ignition of lean fuels offers a promising route for green combustion with high combustion efficiency and low exhaust emissions. The fundamental limitations which apply to femtosecond laser ignition (fs-LI) of lean fuels are the inferior energy deposition and low thermodynamic temperature. However, it was discovered recently that the fs laser...

Based on ZnO microcavities with high quality factors, where the gain medium exhibits confinement of wave packets due to the intrinsically formed whispering gallery microcavity, strong coupling between excitons and cavity photons can be obtained at room temperature resulting in hybrid quasiparticles, e.g. exciton polaritons. In this work, polariton...

We report a principle-of-proof approach for non-destructive and high-sensitivity plasma diagnosis based on third-harmonic generation (THG) of an ultrashort-pulsed laser. We show that the on- and off-axial spectral intensities of the generated TH depend strongly on the electron density of a discharged plasma, even when the laser is non-contact to th...

While the efficiency of quantum dot sensitized solar cells (QDSSCs) has been long limited by photogenerated electron extraction of photoanode, we propose here an alternative approach for boosting the light harvesting efficiency of QDSSCs, which is based on the modifications of the TiO2 photoanode absorbance and impedance by irradiating crystalline...

Laser ablation ignition is an efficient approach in igniting lean-fuels with significantly reduced minimum pulse energy (MPE), but it requires a precise position control between the laser focal spot and the target surface, and is hardly for long-term use due to laser-induced surface damage. Here we adopt femtosecond filamentating laser to ablate a...

In this work, by using femtosecond angle-resolved spectroscopic imaging technique, the ultrafast dynamics of confined exciton-polaritons in an optical induced potential well based on a ZnO whispering-gallery microcavity is explicitly visualized. The sub-picosecond transition between succeeding quantum harmonic oscillator states can be experimentall...

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 N2+ lasing actions at 427.8 and 423.6 nm assigned respectively to the B2Σu+−XΣg+2 (0, 1) and (1, 2) emissions and show that the lasing...

The lasing of N2+ at 391 nm, originating from the population inversion between X2Σg+(v=0) and B2Σu+(v=0) achieved by the irradiation of a pair of few-cycle intense near-IR laser pulses in a pump-probe scheme, is investigated both experimentally and theoretically. The characteristic periodic intensity modulations recorded experimentally in both P- a...

In this chapter, we introduce our recent studies on the mechanisms responsible for the optical amplification in N2⁺ induced by irradiating nitrogen molecules N2 with intense laser fields. We demonstrate that the lasing intensities of N2⁺ at 391.4 nm, corresponding to the B²Σu⁺ (ʋ = 0)-X²Σg⁺ (ʋ″ = 0) transition, can be strongly modulated by manipula...

Cavity-free air lasing offers a promising route towards the realization of atmospheric lasers for various applications such as remote sensing and standoff spectroscopy; however, achieving efficient generation and control of air lasing in ambient air is still a challenge. Here we show the experimental realization of a giant lasing enhancement by thr...

Flexible pressure sensors are promising for biomedical diagnosis and health monitoring, but most of such sensors meet with problems of one kind or another, such as counterfeit sensitivity, poor durability, expensive fabrication cost and trade-off between sensitivity and sensing range. Herein, a high-performance flexible pressure sensor microstructu...

Whispering gallery modes in a microwire are characterized by a nearly equidistant energy spectrum. In the strong exciton-photon coupling regime, this system represents a bosonic cascade: a ladder of discrete energy levels that sustains stimulated transitions between neighboring steps. In this work, by using femtosecond angle-resolved spectroscopic...

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...

Highly sensitive and cost-effective flexible piezoresistive sensors are pursued as essential components of electronic skin (e-skin) for a variety of applications such as soft robotics and body prosthesis. A common strategy for achieving high sensitivity of these sensors is to fabricate their electrode material with complex 3-D microstructures, but...

Wavelength-tunable ultrashort laser pulses in the visible spectral range are generated in a femtosecond laser filament in air through four-wave mixing of an intense 808-nm near-infrared (NIR) laser pulse and a weak tunable infrared (IR: 1.3–2.2 µm) laser pulse. We found that the external focusing geometries strongly influence the four-wave mixing e...

Optical ionization of $\text{N}_2$ N 2 and subsequent population redistribution among the ground and excited states of $\text{N}_{2}^{+}$ N 2 + in an intense laser field are commonly accepted to be fundamentally responsible for the generation of $\text{N}_{2}^{+}$ N 2 + lasing. By finely controlling this two-step process, the optimization of $\text...

A fine manipulation of population transfer among molecular quantum levels is a key technology for control of molecular processes. When a light field intensity is increased to the TW-PW cm−2 level, it becomes possible to transfer a population to specific excited levels through nonlinear light-molecule interaction, but it has been a challenge to cont...

We experimentally demonstrate an asymmetric enhancement of the N2⁺ lasing at 391 nm for the transition between the B²Σ u ⁺ (v = 0) and X²Σ g ⁺ (v” = 0) states in an intense laser field with the ellipticity, ε , modulated by a 7-order quarter-wave plate (7-QWP). It is found that when the 7-QWP is rotated from α = 0 to 90°, where α is the angle betwe...

Hydroxyl (OH) radicals were generated by dielectric barrier discharge (DBD) from water vapor in a multi-pass cell with a volume of 8000 cm³. The cell was filled with the following water vapor at reduced pressure. The absolute OH number density was accurately determined by direct absorption spectroscopy using a tunable laser operating at 2.8 μm. The...

We propose an approach, by utilizing the propagation of femtosecond laser pulses in the filamentation region in air, to achieve black titanium dioxide (TiO2), a promising material for efficient photocatalysis. It is found that the black TiO2 engineered by femtosecond laser filament in both solution and air environments shows significantly improved...

Tellurium-doped black silicon with the absorptance of >85% over the wide range of 240–2500 nm is rapidly fabricated at a standoff distance of 1 m by ablating a tellurium-film-coated crystalline silicon wafer with a femtosecond laser filament. After an annealing temperature of 873 K, the black silicon is further prepared as an n⁺−n photodiode. By me...

We generate unidirectional coherent radiations at 391 and 428 nm corresponding to the B2Σu+−X2Σg+ (0,0) and (0,1) emission of N2+ by irradiating neutral N2 with an elliptically modulated near-IR ultrashort laser field generated using a quarter-wave plate (QWP) with the different orders, and reveal that the lasing intensity at 391 nm becomes maximum...

We propose a near-infrared filament pump and an ultraviolet light probe approach to measure the dynamics of soot in flames with femtosecond time resolution, and demonstrate unexpected ultrafast swelling and shrinking processes of soot nanoparticles.

We propose a simple pump-coupling-seed scheme to examine the optical X 2 Σ g + –A ² Π u coupling in N 2 + lasing. We produce the N 2 + lasing at 391 nm, corresponding to the B 2 Σ u + ( v ′ = 0)– X 2 Σ g + ( v = 0) transition, by externally seeding the N 2 + gain medium prepared by irradiation of N 2 with an intense pump pulse. We then adopt a weak...

Heat effects on femtosecond laser annealing to crystallize doped amorphous Si films are studied. The structural, optical and electronic properties of phosphorus-doped amorphous Si films before and after femtosecond laser treatment are characterized. As the temperature increases from room temperature to 200 ∘C controlled by a hot-stage, the grain si...

We investigate lasing of a N2 gas induced by intense few-cycle near-IR laser pulses. By the pump-probe measurements, we reveal that the intensity of the BΣ2u+−XΣ2g+ lasing emission of N2+ oscillates at high (0.3–0.5 PHz), medium (50–75 THz), and low (∼3 THz) frequencies, corresponding to the energy separations between the rovibrational levels of th...

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 an...

Multi-functional metal surfaces with simultaneous superhydrophobic and antireflective properties have a wide range of industrial, medical, and defense applications. Rapid fabrication of such multifunctional surfaces on irregularly shaped metals and on a large scale has been a challenge. Here we demonstrate an approach for rapid, standoff fabricatio...

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 o...

Aiming at exploring the non-thermal annealing, crystallization of amorphous silicon is carried out by femtosecond laser irradiation. Phase transformation from amorphous to polycrystalline, confirming by the comparison of surface morphologies and micro-Raman spectra, is demonstrated to be realized even at room temperature. With careful experiment de...

Tree barks and leaves are increasingly used these days as environmental bioindicators for pollution assessment of their growing environment, but remote discrimination of tree species and their growing environments are rarely reported. Here the analytical potential of femtosecond filament induced breakdown spectroscopy (FIBS) for standoff detection...

Storage capacity is a key requirement for multiplexed quantum memory in long-distance quantum communication and quantum calculation. The data exchange and processing of a quantum system would speed up, with the benefit from the increasing storage capacity. In this paper, we demonstrate a frequency selected optical storage based on the electromagnet...

We report on the experimental demonstration of a technique for rapid fabrication of black silicon with enhanced absorption in the short-wavelength infrared (SWIR) region, directly in the ambient air, and at a standoff distance of about 1 m. Our approach is based on the irradiation of a surface of the crystalline silicon, coated by a 100-nm-thick al...

Water is the major natural resource that enables life on our planet. Rapid detection of water pollution that occurs due to both human activity and natural cataclysms is imperative for environmental protection. Analytical chemistry–based techniques are generally not suitable for rapid monitoring because they involve collection of water samples and a...

Cardiovascular diseases, the leading cause of death worldwide in the last two decades, are mainly due to the pathological changes inside the heart or blood vessels. Current treatment prescription is to replace obstructed blood vessels by synthetic alternatives, but it can only cure patients effectually when the vessel diameter is larger than a cert...

We demonstrate the generation of filament array with orthogonal polarizations in air by using specifically designed wedge-type birefringent quartz plates. Experimental results show that the number of the generated filaments can be expressed as N = 2ⁿ where n is the number of quartz plates inserted in the laser propagation path. By manipulating the...

Coherent radiation in the ultraviolent (UV) range has high potential applicability to the diagnosis of the formation processes of soot in combustion because of the high scattering efficiency in the UV wavelength region, even though the UV light is lost largely by the absorption within the combustion flames. We show that the third harmonic (TH) of a...

The achievement of an ultrastrong coupling regime between surface plasmon modes generated by gold conical pits arrays and excitons associated to squaraine dye is demonstrated. Numerical and experimental steady-state reflection measurements demonstrate a remarkable Rabi splitting of 860 meV, to date the largest reported value involving surface plasm...

Combustion diagnostics is of particular importance in combustion science for raising the combustion efficiency with low-pollution products as well as for monitoring the nanoparticle growth by combustion synthesis. In this chapter, we discuss the feasibility in the detection and diagnosis of combustion intermediates using femtosecond filament-induce...

We demonstrate that the filamentation process is strongly influenced by the polarization state of the driver laser. When the laser polarization changes from linear to circular, the critical power for the self-focusing of a Ti: Sapphire laser (800 nm, 40 fs) in air increases from about 9.6 ± 1.0 to 14.9 ±1.5GW, while the second nonlinear refractive...

We present an approach for generation of air lasing at extended distances by coaxial dual-color femtosecond laser pulses. A strong 800 nm pulse prepares the population inversion in N 2 + during the filamentation in air, and a weak 400 nm pulse produced coaxially with the 800 nm light by frequency doubling in a BBO crystal seeds the N 2 + gain mediu...

We align nonadiabatically an ensemble of N2 in air, and irradiate the ensemble with linearly polarized intense few-cycle laser pulses to generate population-inverted N2+ ions. By probing the self-seeded lasing signals of N2+ at 391 nm, we show that the ultrafast population inversion in N2+ between the electronic ground XΣg2 state and the electronic...

Optical microcavities have been widely employed for sensing and detection, because of their ultrasensitive responsiveness to surrounding environment. Here we report an active disk-shaped Whispering Gallery Mode (WGM) microcavity fabricated from a water-sensitive material of dye-doped PEG-DA hydrogel through two-photon polymerization. We demonstrate...

Here, a facile fabrication of graphene-based humidity sensors is reported for smart device design. Focused sunlight photoreduction of graphene oxide (GO) helps to remove most of the oxygen-containing groups on GO sheets, which not only recovers their conductivity but also leads to the formation of a highly porous nanostructure, enabling the manufac...

In this article, we have mainly demonstrated the fabrication of a dye-doped polymer whispering gallery mode (WGM) microsphere by femtosecond laser two-photon polymerization, which shows good surface smoothness with a fabrication spatial resolution beyond the diffraction limit. The microcavity shows excellent lasing performance with a quality factor...

A multimode coherent hybrid system formed by strongly coupled surface plasmons in gold nanohole arrays and excitons in sulforhodamine 101 dyes is investigated by using both a steady-state spectroscopic method and a femtosecond transient absorption spectra approach. A double Rabi splitting up to 255 meV and 188 meV is observed in steady-state transm...

Optical microcavities have been paid enormous attentions as a basic integrated element in photonic devices including micro-modulators, micro-filters, and micro-sensors. In particular, the microcavities that can produce unidirectional and single-mode lasing emissions are greatly demanded to meet the challenge of integrating high-efficiency and high-...

We report an approach for remote and rapid fabrication of a broadband low-reflectivity black silicon surface by ablating crystalline silicon with femtosecond laser filaments in air. Porous microstructures on the processed silicon surface are formed, resulting in a significantly enhanced light trapping efficiency in a broadband (UV-IR) spectral rang...

The advances in femtosecond Ti-sapphire laser technology have led to the discovery of a profusion of new physics. This review starts with a brief historical account of the experimental realization of tunnel ionization, followed by high harmonic generation and the prediction of attosecond pulses. Then, the unique phenomenon of dynamic population tra...

We have investigated the strong coupling interaction between excitons of CdSe quantum dots (QDs) and surface plasmon polaritons (SPPs) of gold nanoholes array by steady-state spectroscopic method and transient absorption measurements. Numerical and experimental steady-state measurements demonstrate that the SPPs-QDs system can indeed undergo strong...

Through numerical simulations we reveal the scenario of 3.9−μm filament spectrum enrichment in the atmosphere in the cases of linear and circular polarization of the incident pulse. The discrete spectrum of odd harmonics transforms into the two-octave plateau in the case of linear polarization. In contrast, in the case of circular polarization of t...

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 l...

On page 6198, H.-Y. Wang, H.-B. Sun, R. Proietti Zaccaria, and co-workers describe the dynamics of two hybrid bands formed by the interaction of strongly coupled plasmon polaritons and excitons. A remarkable long life time of the higher energy band is found and ascribed to a newly identified trap state emerging under resonant excitation. This chara...

We have investigated the influence of Rabi splitting tuning on the dynamics of strongly coupled J-aggregate/surface plasmon polariton systems. In particular, the Rabi splitting was tuned by modifying the J-aggregate molecule concentration while a polaritonic system was provided by a nanostructure formed by holes array in a golden layer. From the pe...