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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 report fabrication of three-dimensional (3D) high aspect ratio microchannels from the rear-surface of thick silica glass with femtosecond laser pulses. To avoid nonlinear self-focusing in the conventional focusing scheme which leads to intensity clamping in the interaction of the ultrafast laser pulses with the silica glass, simultaneous spatiotemporal focusing is employed which gives rise to high aspect ratio (∼30) straight and curved microchannels in 10 mm thick silica glass substrates.

Femtosecond laser induced selective etching (FLISE) of dielectric materials is a promising technique for fabricating various microfluidic devices. Here we experimentally studied the dependence of the selective etching speed in fused silica glass on laser pulse energy, repetition rate, and inscription speed using a 1030 nm femtosecond laser. The evolution of micromorphology of the laser inscribed lines was revealed with optical microscopy, scanning electron microscopy, as well as anisotropic diffraction of the optical gratings formed by these inscribed lines. A single pulse energy threshold is required to initiate the FLISE. Further, a laser repetition rate window between an upper threshold and a lower threshold was observed, which were limited by the thermal-induced disruption of the nanogratings and by the disconnection of successive pulses modified spots respectively. The synergetic influences of the above factors were evaluated by the exposure laser energy density, which shows a common threshold for different inscription conditions and demonstrates itself to be an excellent criterion for choosing appropriate parameters in FLISE. The formation of continuous nanogratings is confirmed to be the major mechanism of FLISE in fused silica. Our observations not only help one to understand the micro mechanism in FLISE of fused silica, but also are of great use for fabricating large-scale microfluidic circuits.

We report on fabrication of three dimensional (3D) microstructures in glass with isotropic spatial resolutions. To achieve high throughput fabrication, we expand the focal spot size with a low-numerical-aperture lens, which naturally results in a degraded axial resolution. We solve the problem with simultaneous spatial temporal focusing which leads to an isotropic laser-affected volume with a spatial resolution of ~100 micron., 7 pages, 5 figures

Intrinsic absorption and subsequent heat generation have long been issues for metal-based plasmonics. Recently, thermo-plasmonics, which takes the advantage of such a thermal effect, is emerging as an important branch of plasmonics. However, although significant temperature increase is involved, characterization of metal permittivity at different temperatures and corresponding thermo-derivative are lacking. Here we measure gold permittivity from 300K to 570K, which the latter is enough for gold annealing. More than one order difference in thermo-derivative is revealed between annealed and unannealed films, resulting in a large variation of plasmonic properties. In addition, an unusual increase of imaginary permittivity after annealing is found. Both these effects can be attributed to the increased surface roughness incurred by annealing. Our results are valuable for characterizing extensively used unannealed nanoparticles, or annealed nanostructures, as building blocks in future thermo-nano-plasmonic systems.

We report on fabrication of depressed cladding optical waveguides buried in lithium niobate crystal with shaped femtosecond laser pulses. Depressed cladding waveguides of variable mode-field sizes are fabricated by forming the four sides of the cladding using a slit-beam shaping technique. We show that the waveguides fabricated by our technique allows single-mode propagation of the light polarized in both vertical and horizontal directions., Comment: 6 pages, 3 figures (19 panels). arXiv admin note: text overlap with arXiv:1602.01570

We employ a self-assembly method to fabricate dielectric microsphere arrays that can be transferred to any desired positions. The arrays not only enable far-field, broad-band, high-speed, large-area, and wide-angle field of views but also achieve superresolution reaching λ/6.4. We also find that many proposed theories are insufficient to explain the imaging properties; including the achieved superresolution, effects of immersion, and unusual size-dependent magnification. The half-immersed microspheres certainly do not behave like any ordinary solid immersion lenses and new mechanisms must be incorporated to explain their unusual imaging properties.

We report on measurements of both the critical power for self-focusing of a Ti: Sapphire 800 nm femtosecond laser and the peak intensity clamped inside a single filament in an ethanol-air flame on an alcohol burner array. By observing the shift of focal position of femtosecond laser pulses, we determine the critical power in the flame to be 2.2 ± 0.3 GW, which is 4-5 times smaller than the usually quoted one in air. The clamped laser intensity inside the filament is measured to be roughly half of that in air. Our results provide insights into the understanding of femtosecond laser filamentation in flames for practical application of combustion diagnostics.

We provide evidence that the chirality of collagen can give rise to strong second-harmonic generation circular dichroism (SHG-CD) responses in nonlinear microscopy. Although chirality is an intrinsic structural property of collagen, most of the previous studies ignore that property. We demonstrate chiral imaging of individual collagen fibers by using a laser scanning microscope and type-I collagen from pig ligaments. 100% contrast level of SHG-CD is achieved with sub-micrometer spatial resolution. As a new contrast mechanism for imaging chiral structures in bio-tissues, this technique provides information about collagen morphology and threedimensional orientation of collagen molecules.