Jinping Yao

High-brightness switchable multiwavelength remote laser in air

We demonstrate a harmonic-seeded switchable multiwavelength laser in air driven by intense midinfrared femtosecond laser pulses, in which population inversion occurs at an ultrafast time scale (i.e., less than \ensuremath{\sim}200 fs) owing to direct formation of excited molecular nitrogen ions by strong-field ionization of inner-valence electrons. The bright multiwavelength laser in air opens the perspective for remote detection of multiple pollutants based on nonlinear optical spectroscopy.

Generation of a coherent x ray in the water window region at 1 kHz repetition rate using a mid-infrared pump source

We demonstrate the generation of a coherent x ray in the water window region in a gas cell filled with neon gas using a wavelength-tunable mid-IR femtosecond laser operating at 1 kHz repetition rate. The cutoff energy and conversion efficiency of the water window x ray can be optimized by tuning gas pressure as well as the focal position.

Remote creation of coherent emissions in air with two-color ultrafast laser pulses

We experimentally demonstrate the generation of narrow-bandwidth emissions with excellent coherent properties at 391 and 428nm from N + (B 2 6 + (v 0 = 0) ! X 2 6 + g (v = 0,1)) inside a femtosecond filament in air by an orthogonally polarized two-color driver field (i.e. 800nm laser pulse and its second harmonic). The durations of the coherent emissions at 391 and 428nm are measured to be 2.4 and 7.8ps, respectively, both of which are much longer than the duration of the pump and its second harmonic pulses. Furthermore,

A pure organic heterostructure of μ-oxo dimeric iron(III) porphyrin and graphitic-C3N4 for solar H2 roduction from water

Due to the two-dimensional flexible structure and abundant pendant amine, graphitic-C3N4 (g-C3N4) may be easily modified by organic molecules as a promising photocatalyst for solar H-2 production from water. Here, through a simple liquid chemical reaction between g-C3N4 and the precursor of m-oxo dimeric iron(III) porphyrin [(FeTPP)(2)O], we provide a novel route to construct a pure organic heterostructure of g-C3N4/(FeTPP)(2)O on the basis of the pi-pi and the Fe-amine interactions. The experimental results demonstrated that (FeTPP)(2)O acted not just as a photosensitizer, but also played the role of a charge promotor to prohibit the recombination of the excited electrons and holes of g-C3N4. As compared to pure or mixed g-C3N4 and/or (FeTPP)(2)O, the obtained pure organic g-C3N4/(FeTPP)(2)O heterostructure exhibited dramatic photocatalytic H-2 production under solar light without any cocatalysts.

Self-assembled two-dimensional superlattice of Au–Ag alloy nanocrystals

The self-assembled two-dimensional superlattice of octanthiol-passivated Au–Ag alloy nanoparticles is reported in this letter. These nanoparticles show good composition-tunable properties. In the UV-visible spectra, the absorption peaks shifted linearly from the high-energy to the lower-energy side with increasing molar faction of Au.

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

We experimentally demonstrate ultrafast dynamic of generation of the 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 C(3)Π(u) and B(3)Π(g) states of N(2) 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.

Identification of the physical mechanism of generation of coherent N 2 + emissions in air by femtosecond laser excitation

Recently, amplification of harmonic-seeded radiation generated through femtosecond laser filamentation in air has been observed, giving rise to coherent emissions at wavelengths corresponding to transitions between different vibrational levels of the electronic B(2)Σ(u)(+) and X(2)Σ(g)(+) states of molecular nitrogen ions [Phys. Rev. A. 84, 051802(R) (2011)]. Here, we carry out systematic investigations on its physical mechanism. Our experimental results do not support the speculation that such excellent coherent emissions could originate from nonlinear optical processes such as four-wave mixing or stimulated Raman scattering, leaving stimulated amplification of harmonic seed due to the population inversion generated in molecular nitrogen ions the most likely mechanism.

A self-induced white light seeding laser in a femtosecond laser filament

We report on the generation of remote self-seeding laser amplification by using only one 800 nm Ti:sapphire femtosecond laser pulse. The laser pulse (~40 fs) is first used to generate a filament either in pure nitrogen or in ambient air in which population inversion between the and states of is realized. Self-induced white light inside the filament covering the transition is then serving as the seed to be amplified. The self-induced narrow-band laser at 428 nm has a pulse duration of ~2.6 ps with perfect linear polarization properties. This finding opens new possibilities for remote detection in the atmosphere.

Multiwavelength amplified harmonic emissions from carbon dioxide pumped by mid-infrared femtosecond laser pulses

We demonstrate multiwavelength amplified harmonic emissions in carbon dioxide gas driven by intense mid-infrared femtosecond laser pulses. The simultaneous amplified emissions at 315, 326, 337 and 351 nm from the A2Πu-X2Πg transition of CO2+ are found to have a high gain coefficient of ~4.43 cm−1 with the polarizations parallel to that of the pump laser. This is ascribed to the seeding effect by the third harmonic of pump laser. The coherence property of the amplified harmonic emissions is further evidenced by its diffraction pattern.

Generation of extended filaments of femtosecond pulses in air by use of a single-step phase plate

We experimentally demonstrate that by use of a phase plate that is placed between an adjustable aperture and a focusing lens, the length of the filament can be dramatically extended in air with a femtosecond laser pulse. In addition, the far-field beam profile captured after the filament indicates that the supercontinuum is strongly confined on axis, and a single filament appears to be attainable at relatively high input pulse power when the phase plate is used.