Haiyi Sun

Electro-optic integration of embedded electrodes and waveguides in LiNbO 3 using a femtosecond laser

We describe the fabrication of a Mach-Zehnder optical modulator in LiNbO(3) by femtosecond laser micromachining, which is composed of optical waveguides inscripted by a femtosecond laser and embedded microelectrodes subsequently fabricated using femtosecond laser ablation and selective electroless plating. A half-wave voltage close to 19 V is achieved at a wavelength of 632.8 nm with an interaction length of 2.6 mm. This simple and cost-effective technique opens up new opportunities for fabricating integrated electro-optic devices.

Direct fabrication of homogeneous microfluidic channels embedded in fused silica using a femtosecond laser

We demonstrate direct fabrication of homogeneous microfluidic channels embedded in fused silica by femtosecond laser direct writing, followed by wet chemical etching and glass drawing. In addition, the glass drawing process significantly reduces the inner surface roughness of the fabricated channels, and centimeter-level microfluidic channels with an aspect ratio above 1000 can be realized.

Selective metallization on insulator surfaces with femtosecond laser pulses

We report selective metallization on surfaces of insulators (glass slides and lithium niobate crystal) based on femtosecond laser modification combined with electroless plating. The process is mainly composed of four steps: (1) formation of silver nitrate thin films on the surfaces of glass or crystal substrates; (2) generation of silver particles in the irradiated area by femtosecond laser direct writing; (3) removal of unirradiated silver nitrate films; and (4) selective electroless plating in the modified area. We discuss the mechanism of selective metallization on the insulators. Moreover, we investigate the electrical and adhesive properties of the copper microstructures patterned on the insulator surfaces, showing great potential of integrating electrical functions into lab-on-a-chip devices.

Fabrication of microfluidic optical waveguides on glass chips with femtosecond laser pulses

We describe the fabrication of microfluidic channel structures on the surface of a borosilicate glass slide by femtosecond laser direct writing for optical waveguide application. Liquid with a variable refractive index is fed into the microchannel, serving as the core of the waveguide. We demonstrate that either a multimode or a single-mode waveguide can be achieved by controlling the refractive index of the liquid.

Laser filamentation-induced condensation and snow formation in a cloud chamber

Using 1 kHz, 9 mJ femtosecond laser pulses, we demonstrate laser-filamentation-induced spectacular snow formation in a cloud chamber. An intense updraft of warm moist air is generated owing to the continuous heating by the high-repetition filamentation. As it encounters the cold air above, water condensation and large-sized particles spread unevenly across the whole cloud chamber via convection and cyclone like action on a macroscopic scale. This indicates that high-repetition filamentation plays a significant role in macroscopic laser-induced water condensation and snow formation.

Redistribution of elements in glass induced by a high-repetition-rate femtosecond laser.

The redistribution of elements in a multicomponent oxyfluoride glass is induced by a 250 kHz femtosecond laser. Elemental distribution in the cross section of the modified region along the laser propagation axis is analyzed by an electron microprobe analyzer. The results indicate that the relative concentrations of network formers of the glass are higher in the central area of the modified region and lower in the periphery of the modified region compared with the unirradiated areas. However, the relative concentrations of network modifiers are as opposed to that of network formers. Fluorescence spectra confirm that the distribution of fluorescence intensity of Yb(3+) in the modified region is consistent with that of its concentration. The effects of spherical aberration of the incident beam on the elemental redistribution are also discussed.

Direct observation of laser guided corona discharges

Laser based lightning control holds a promising way to solve the problem of the long standing disaster of lightning strikes. But it is a challenging project due to insufficient understanding of the interaction between laser plasma channel and high voltage electric filed. In this work, a direct observation of laser guided corona discharge is reported. Laser filament guided streamer and leader types of corona discharges were observed. An enhanced ionization took place in the leader (filament) through the interaction with the high voltage discharging field. The fluorescence lifetime of laser filament guided corona discharge was measured to be several microseconds, which is 3 orders of magnitude longer than the fluorescence lifetime of laser filaments. This work could be advantageous towards a better understanding of laser assisted leader development in the atmosphere.

Multiphoton-excited upconversion luminescence of Nd: YVO 4

Fluorescence spectra of Nd:YVO(4) under excitation of a continuous wave (CW) diode laser and a femtosecond laser at 800nm were investigated. It was found that Nd:YVO(4) shows different upconversion and downconversion luminescencent behaviors when excited by the diode laser and the femtosecond laser. The dependence of the upconversion luminescence intensity on the pump power of the femtosecond laser was discussed. The populations of the upper energy levels for upconversion and downconversion luminescence were calculated based on the Bloch equations. The calculations agree well with the experimental results.

Laser filamentation induced air-flow motion in a diffusion cloud chamber.

We numerically simulated the air-flow motion in a diffusion cloud chamber induced by femtosecond laser filaments for different chopping rates. A two dimensional model was employed, where the laser filaments were treated as a heat flux source. The simulated patterns of flow fields and maximum velocity of updraft compare well with the experimental results for the chopping rates of 1, 5, 15 and 150 Hz. A quantitative inconsistency appears between simulated and experimental maximum velocity of updraft for 1 kHz repetition rate although a similar pattern of flow field is obtained, and the possible reasons were analyzed. Based on the present simulated results, the experimental observation of more water condensation/snow at higher chopping rate can be explained. These results indicate that the specific way of laser filament heating plays a significant role in the laser-induced motion of air flow, and at the same time, our previous conclusion of air flow having an important effect on water condensation/snow is confirmed.

Microstructures induced in the bulk of SrTiO3 crystal by a femtosecond laser

Microstructures with the total length of hundreds of mu m were induced by fixing the focal point of the femtosecond laser at a certain depth in the bulk of SrTiO(3) crystal. By different combination of the focusing conditions with the laser parameters, different morphologies have been observed, such as void array, necklace-shaped structures, continuous/ segmental filaments and etc. The possible mechanism of the formation of those diversiform structures is discussed.