arXiv subjects

Jinping Yao

Publications and source records attributed to Jinping Yao.

At least 19 recordsLinked to original sources

arXiv2024

Single-Shot Single-Beam Coherent Raman Scattering Thermometry Based on Air Lasing

Thermometric techniques with high accuracy, fast response speed and ease of implementation are desirable for the study of dynamic combustion environments, transient reacting flows, and non-equilibrium plasmas. Herein, single-shot single-beam coherent Raman scattering (SS-CRS) thermometry is developed, for the first time to our knowledge, by using air lasing as a probe. It's proved that the air-lasing-assisted CRS signal has a high signal-to-noise ratio enabling single-shot measurements at a 1 kHz repetition rate. The SS-CRS thermometry consistently exhibits precision better than 2% at different temperatures, but the inaccuracy grows with the increase in temperature. The high detection precision, 1 kHz acquisition rate and easy-to-implement single-beam scheme are achieved thanks to the unique temporal, spectral and spatial characteristics of air lasing. This work opens a novel avenue for high-speed CRS thermometry, holding tremendous potential for fast diagnostics of transient reacting flows and plasmas.

physics.optics↗

arXiv2023

Enhanced Population on Ionic Excited States by Synchronized Ionization and Multiphoton Resonance

We study population distributions and lasing actions of N_2^+ driven by femtosecond lasers with various wavelengths, and uncover an efficient ionic excitation mechanism induced by synchronized ionization and multiphoton resonance. Our results show that the strongest N_2^+ lasing appears around 1000 nm pump wavelength. At the optimal wavelength, the pump-energy threshold for air lasing generation is reduced by five folds compared with that required by the previous 800 nm pump laser. Simulations based on the ionization-coupling model indicate that although the Stark-assisted three-photon resonance can be satisfied within a broad pump wavelength range, the optimal pump wavelength arises when the dynamic three-photon resonance temporally synchronizes with the ionization injection. In this case, the ionic dipoles created at each half optical cycle have the same phase. The dipole phase locking promotes the continuous population transfer from ionic ground state to the excited state, giving rise to a dramatic increase of excited-state population. This work provides new insight on the photoexcitation mechanism of ions in strong laser fields, and opens up a route for optimizing ionic radiations.

physics.atom-ph↗

arXiv2023

Spatiotemporal vortex strings of light

Light carrying orbital angular momentum (OAM) holds unique properties and boosts myriad applications in diverse fields from micro- to macro-world. Endeavors have been made to manipulate the OAM in order to generate on-demand structured light and to explore novel properties of light. However, the generation of an ultrafast wave packet carrying numerous vortices with various OAM modes, that is vortex string, has been rarely explored and remains a significant challenge. Moreover, methods that enable parallel detection of all vortices in a vortex string are lacking. Here, we demonstrate that a vortex string with 28 spatiotemporal optical vortices (STOVs) can be successfully generated in an ultrafast wave packet. All STOVs in the string can be randomly or orderly arranged. The diffraction rules of STOV strings are also revealed theoretically and experimentally. Following these rules, the topological charges and positions of all STOVs in a vortex string can be easily recognized. The strategy for parallel generation and detection of STOV strings will open up exciting perspectives in STOV-based optical communications and also promote promising applications of the structured light in light-matter interaction, quantum information processing, etc.

physics.optics↗

arXiv2021

Photon retention in coherently excited nitrogen ions

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.

quant-ph↗

arXiv2020

A spectrally bright wavelength-switchable vacuum ultraviolet source driven by quantum coherence in strong-field-ionized molecules

We report generation of spectrally bright vacuum ultraviolet (VUV) and deep UV (DUV) coherent radiations driven by quantum coherence in tunnel-ionized carbon monoxide (CO) molecules. Our technique allows us to switch between multiple wavelengths provided by the abundant energy levels of molecular ions. The DUV/VUV sources can have arbitrary polarization states by manipulating the pump laser polarization. The superior temporal and spectral properties of the developed source give rise to a broadband Raman comb in the DUV/VUV region.

physics.optics↗

arXiv2020

Role of rotational coherence in femtosecond-pulse-driven nitrogen ion lasing

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$^2Σ_g^+(ν=0)$, 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.

physics.atom-ph↗

arXiv2019

Sub-cycle coherent control of ionic dynamics via transient ionization injection

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.

physics.atom-ph↗

arXiv2019

Extreme nonlinear Raman interaction of an ultrashort nitrogen ion laser with an impulsively excited molecular wavepacket

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.

physics.optics↗

arXiv2018

Electronic quantum coherence induced by strong field molecular ionization

The existence of electronic coherence can fundamentally change the scenario of nonlinear interaction of light with quantum systems such as atoms and molecules, which, however, has escaped from observation in the investigations of strong field nonlinear optics in the past several decades. Here, we report on the generation of electronic quantum coherence by strong field ionization of nitrogen molecules in an intense 800 nm laser field. The coherence is experimentally revealed by observing a resonant four-wave mixing process in which the two pump pulses centered at 800 nm and 1580 nm wavelengths are temporally separated from each other. The experimental observation is further reproduced by calculating the nonlinear polarization response of N_2^+ ions using a three-level quantum model. Our result suggests that strong field ionization provides a unique approach to generating a fully coherent molecular wavepacket encapsulating the rotational, vibrational, and electronic states.

physics.optics↗

arXiv2017

Near-resonant Raman amplification in the rotational quantum wavepackets of nitrogen molecular ions generated by strong field ionization

Generation of laser-like narrow bandwidth emissions from nitrogen molecular ions generated in intense near- and mid-infrared femtosecond laser fields has aroused much interest because of the mysterious physics underlying such a phenomenon as well as the potential application of such an effect in atmospheric spectroscopic sensing. Here, we perform a pump-probe measurement on the nonlinear interaction of rotational quantum wavepackets of nitrogen molecular ions generated in mid-infrared (e.g., at a wavelength centered at 1580 nm) femtosecond laser fields with an ultrashort probe pulse whose broad spectrum overlaps both P- and R-branch rotational transition lines between the upper and lower electronic states. The results show that in the near-resonant conditions, stimulated Raman amplification can efficiently occur which converts the broad bandwidth ultrashort probe pulse into the narrow bandwidth laser-like beam. Our finding provides an insight into the physical mechanism of strong field induced lasing actions in atmospheric environment.

physics.optics↗

arXiv2017

Generation of bright laser-like coherent emissions from nitrogen molecular ions in intense infrared laser fields: A unique quantum system for extreme nonlinear optics

We report on an investigation of simultaneous generation of several narrow-bandwidth laser-like coherent emissions from nitrogen molecular ions ( ) produced in intense mid-infrared laser fields. With systematic examinations on the dependences of coherent emissions on gas pressure as well as several laser parameters including laser intensity, polarization and wavelength of the pump laser pulses, we reveal that the multiple coherent emission lines generated in originate from a series of nonlinear processes beginning with four-wave mixing, followed with stimulated Raman scattering. Our analyses further show that the observed nonlinear processes are greatly enhanced at the resonant wavelengths, giving rise to high conversion efficiencies from the infrared pump laser pulses to the coherent emission lines near the transition wavelengths between the different vibrational energy levels of ground X and that of the excited B states.

physics.optics↗

arXiv2016

Harmonic-seeded resonant Raman amplification in strong-field ionized nitrogen molecules

Generation of free-space laser-like emissions of high spatiotemporal coherence and narrow bandwidth in atmosphere opens promising opportunities for remote spectroscopic sensing. Here, we report on generation of such laser-like emissions, which results from the combined contributions of perturbative and non-perturbative nonlinear optical effects in nitrogen molecules exposed to intense mid-infrared laser fields. We systematically investigate the dependence of the generated free-space laser spectrum on wavelength and power of the driver laser. It is revealed that the free-space laser is produced by resonant Raman amplification of the fifth harmonic of the driver pulses in rotational wavepacket of the molecular nitrogen ions.

physics.optics↗

arXiv2015

Generation of elliptically polarized nitrogen-ion laser fields using two-color femtosecond laser pulses

We experimentally investigate generation of molecular nitrogen-ion lasers with two femtosecond laser pulses at different wavelengths. The first pulse serves as the pump which ionizes the nitrogen molecules and excites the molecular ions to excited electronic states. The second pulse serves as the probe which leads to stimulated emission from the excited molecular ions. We observe that changing the angle between the polarization directions of the two pulses gives rise to elliptically polarized molecular nitrogen-ion laser fields, which is interpreted as a result of strong birefringence of the gain medium near the wavelengths of the molecular nitrogen-ion laser.

physics.optics↗

arXiv2015

Fabrication of a microresonator-fiber assembly maintaining a high-quality factor by CO2 laser welding

We demonstrate fabrication of a microtoroid resonator of a high-quality (high-Q) factor using femtosecond laser three-dimensional (3D) micromachining. A fiber taper is reliably assembled to the microtoroid using CO2 laser welding. Specifically, we achieve a high Q-factor of 2.12*10^6 in the microresonator-fiber assembly by optimizing the contact position between the fiber taper and the microtoroid.

physics.optics↗

arXiv2015

Population Redistribution among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields

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.

physics.optics↗

arXiv2015

Spatiotemporal focusing does not always improve axial intensity localization

We report on an experimental comparison on critical intensities of nonlinear self-focusing in air with conventional focusing and spatiotemporal focusing schemes. Our results show that the conventional focusing with the focal lens completely filled with the incident beam allows for the strongest axial intensity confinement against the self-focusing effect. This is because that in the high-numerical-aperture condition, the focal spot will have a compact size which results in a high focal intensity. Meanwhile, the Rayleigh length of the focused beam will be substantially shortened which efficiently postpones the onset of self-focusing.

physics.optics↗

arXiv2015

Backward air lasing actions induced by femtosecond laser filamentation: influence of population inversion lifetime

We experimentally investigate generation of backward 357 nm N2 laser in a gas mixture of N2/Ar using 800-nm femtosecond laser pulses, and examine the involved gain dynamics based on pump-probe measurements. Our findings show that a minimum lifetime of population inversion in the excited N2 molecules is required for generating intense backward nitrogen lasers, which is ~0.8 ns under our experimental conditions. The results shed new light on the mechanism for generating intense backward lasers from ambient air, which are highly in demand for high sensitivity remote atmospheric sensing application.

physics.optics↗

arXiv2014

Gain dynamics of a free-space nitrogen laser pumped by circularly polarized femtosecond laser pulses

We experimentally demonstrate ultrafast dynamic of generation of a strong 337-nm nitrogen laser by injecting an external seed pulse into a femtosecond laser filament pumped by a circularly polarized laser pulse. In the pump-probe scheme, it is revealed that the population inversion between the excited and ground states of N2 for the free-space 337-nm laser is firstly built up on the timescale of several picoseconds, followed by a relatively slow decay on the timescale of tens of picoseconds, depending on the nitrogen gas pressure. By measuring the intensities of 337-nm signal from nitrogen gas mixed with different concentrations of oxygen gas, it is also found that oxygen molecules have a significant quenching effect on the nitrogen laser signal. Our experimental observations agree with the picture of electron-impact excitation.

physics.optics↗