Jielei Ni

Fabrication of microfluidic channels with a circular cross section using spatiotemporally focused femtosecond laser pulses

We report on the fabrication of hollow microfluidic channels with a circular cross-sectional shape embedded in fused silica by spatiotemporally focusing the femtosecond laser beam. We demonstrate both theoretically and experimentally that the spatiotemporal focusing of femtosecond laser beam allows for the creation of a three-dimensionally symmetric spherical intensity distribution at the focal spot.

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.

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,

Independent control of aspect ratios in the axial and lateral cross sections of a focal spot for three-dimensional femtosecond laser micromachining

We theoretically and experimentally show that independent control of aspect ratios of cross-sectional shapes of a focal spot in both axial and lateral directions can be achieved for three-dimensional (3D) femtosecond laser micromachining by the use of a combination of a slit beam shaping technique and a temporal focusing technique. The simultaneous employment of the spatial and temporal beam shaping techniques allows us to achieve isotropic resolution in 3D space even for an objective lens of low numerical aperture. We also present analytical expressions of the peak-intensity distributions near the focus for the spatiotemporally focused femtosecond laser beams with and without utilizing the slit beam shaping technique.

High-fidelity visualization of formation of volume nanogratings in porous glass by femtosecond laser irradiation

Formation of self-organized nanogratings in bulk glasses with femtosecond laser pulses is one of the most intriguing phenomena in the interactions of light with transparent materials. With the feature sizes far beyond the optical diffraction limit, these nanostructures have found widespread applications in nanophotonics and nanofluidics. The physics of the phenomenon is still far from being fully understood, largely because of the lack of a technique for noninvasively visualizing the formation of the nanogratings embedded within bulk glasses. Here, we access the snapshots of morphologies in the laser-affected regions in a porous glass that reveal the evolution of the formation of nanogratings with an increasing number of laser pulses. Combined with further theoretical analyses, our observation provides important clues that suggest that excitation of standing plasma waves at the interfaces between areas modified and unmodified by the femtosecond laser irradiation plays a crucial role in promoting the growth of periodic nanogratings. The proposed universal nanostructure growth mechanism involving laser-induced plasma wave formation at the interfaces of the seed structures may tie together many previous observations, meanwhile linking the in-volume nanograting formation to previously discovered mechanisms for surface nanoripple formation.

Characterization and control of peak intensity distribution at the focus of a spatiotemporally focused femtosecond laser beam.

We report on experimental examination of two-photon fluorescence excitation (TPFE) at the focus of a spatially chirped femtosecond laser beam, which reveals an unexpected tilted peak intensity distribution in the focal spot. Our theoretical calculation shows that the tilting of the peak intensity distribution originates from the fact that along the optical axis of objective lens, the spatiotemporally focused pulse reaches its shortest duration exactly at the focal plane. However, when moving away from the optical axis along the direction of spatial chirp of the incident pulse, the pulse reaches its shortest duration either before or after the focal plane, depending on whether the pulse duration is measured above or below the optical axis as well as the sign of the spatial chirp. The tilting of the peak intensity distribution in the focal spot of the spatiotemporally focused femtosecond laser beam can play important roles in applications such as femtosecond laser micromachining and bio-imaging.

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.