Femtosecond (fs) laser is proved a powerful tool in the field of 3-dimensional internal micromachining inside the transparent dielectrics. There are already many types of femtosecond laser systems widely applied nowadays, among which Ti: sapphire femtosecond laser and femtosecond fiber laser are the two most frequently used typical femtosecond laser systems. However, according to our study, the differences in the laser parameters between these two femtosecond laser systems may induce significant discrepancy in manufacturing internal hollow structures inside the polymethyl methacrylate (PMMA) substrate. The experimental results show that when a 65fs laser beam is focused inside the PMMA substrate and scans a route of a straight line at an average power of 1.5W and 1 kHz repetition rate, a hollow microchannel is successfully fabricated and no melted region is found around the microchannel. However, if the PMMA substrate is constantly irradiated with the 1.5W laser pulses that generated by a 400fs femtosecond fiber laser system at a repetition rate of 100 kHz, a growing hollow cavity is observed and a melted-resolidified region is formed around the cavity. According to the numerical simulation of heat accumulation effect, we explain the ‘heat accumulation effect’ caused in the femtosecond fiber laser manufacturing and the ‘cold processing property’ of the Ti: sapphire femtosecond laser processing, respectively. These experimental and numerical results may broaden the understanding of thermal effect during the process of the femtosecond laser micromachining and provides more opportunities in the manufacturing of polymeric integrated microfluidic chip with laser direct writing technology in the future.
We investigated the interaction between femotsecond laser and polyimide with a high repetition femtosecond fiber laser and a precisely motorized 3D stage. We have found that high repetition femtosecond laser pulse train can effectively fabricate double-layer electrical conductive tracks inside a polyimide (PI) sheets by a single-time irradiation. This interaction comprised multi-photon absorption, dissociation of polymer molecules and the thermal accumulation. The experiment unveiled that dual-layer carbonization was a consequence of an inside micro-lens formed instantly as laser was just focused into the inside of polyimide. This micro-lens further focused the subsequent laser pulse to carbonize the polymer through multi-photon excitation, bond breaking and graphite layer reformation and eventually form the second electronic conductive layer. The second conductive layer was generated below the focal point. With the laser irradiating is kept at the same height, the top layer at the focused plane continued to absorb laser energy then carbonized into the conductive layer. We called the process as a kind of self-focusing phenomenon. We study the focus effect of inside microlenses under different laser powers and irradiation times. The gap of double electronic tracks embedded in the polyimide matrix can be adjusted with the laser processing parameters. When the gap is more than 30 micrometer, two conductive layers are electrically insulating. While the gap is smaller than 10 micrometer, two conductive layers are electrically connected. Various applications, such as, supercapacitors, capacitive sensors and the field effect transistors were investigated in the flexible PI sheets using this 3D double-layer electrical conductive architecture.
With the developing of wearable electronics and information society, integrated energy storage devices are urgently demanded to be integrated on flexible substrates. We successfully demonstrated using direct laser-reduction of the hydrated GO and chloroauric acid (HAuCl4) nanocomposite to fabricate in-plane micro-supercapacitors (MSCs) with fast ion diffusion on paper. The electrode conductivity of these flexible nanocomposites reaches up to 1.1 x 10<sup>6</sup> S m<sup>-1</sup>, which enhances superior rate capability of micro-supercapacitors, and large specific capacitances of 0.77 mF cm<sup>-2</sup> (17.2 F cm<sup>-3</sup> for volumetric capacitance) at 1 V s<sup>-1</sup>, and 0.46 mF cm<sup>-2</sup> (10.2 F cm<sup>-3</sup>) at 100 V s<sup>-1</sup>. We also have demonstrated that pulsed laser irradiation rapidly converts the polyimide (PI) sheets into an electrically conductive porous carbon structure in ambient conditions. The specific capacitance of single layer surface supercapacitors can reach 20.4 mF/cm<sup>2</sup> at 0.1 mA/cm<sup>2</sup> discharge current density. Furthermore, we successfully fabricate the multi-layer supercapacitor with the PI substrate using 3D femtosecond laser direct writing, and the specific capacitances of three layers supercapacitors is 37.5 mF/cm<sup>2</sup>.
Recent progresses in femtosecond laser (fs) manufacturing have already proved that fs laser is a powerful tool in three dimensional internal structure fabrications. However, most studies are mainly focused on realize such structures in inorganic transparent dielectric, such as photosensitive glass and fused silica, etc. In this study, we present two methods to fabricate embedded internal 3D structures in a polymer dielectric material polymethyl methacrylate (PMMA). Both continuous hollow structure such as microfluidic channels and discrete hollow structures such as single microcavities are successfully fabricated with the help of femtosecond lasers. Among them, complicated 3D microchannel with a total length longer than 10mm and diameters around 80μm to 200μm are fabricated with a low repetition rate Ti: sapphire femtosecond laser by direct laser writing at a speed ranging from 25μm/s to 2000μm/s; microcavities which function as concave microball lenses (CMBLs) and can be applied in super-wide-angle imaging are fabricated with a high repetition rate femtosecond fiber laser due to the distinct heat accumulation effect after 5s irradiation with the tightly focused fs laser beam. These new approaches proved that femtosecond laser direct writing technology has great application potential in 3D integrated devices manufacturing in the future.
Femtosecond laser induced nonthermal processing is an emerging nanofabrication technique for delicate plasmonic
devices. In this work we present a detailed investigation on the interaction between ultra-short pulses and silver
nanomaterials, both experimentally and theoretically. We systematically study the laser-silver interaction at a laser fluent
from 1 J/m<sup>2</sup> to 1 MJ/m<sup>2</sup>. The optimal processing window for welding of silver nanowires occurs at fluences of 200-450
J/m<sup>2</sup>. The femtosecond laser-induced surface melting allows precise welding of silver nanowires for "T” and “X” shape
circuits. These welded plasmonic circuits are successfully applied for routining light propagation.
We report a naturally grown stripe structure with a nanometer scale wavelength in REBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> (RE = Sm and Eu) superconductors investigated with scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). Such a periodic array was unveiled owning to the 3 dimensionally spatial oscillation of RE and Ba around the stoichiometric ratio. The study displayed that novel nanostripes function as robust pinning sites and effectively enhance the peak effect and the irreversibility line at 77K. This illustrates an approach to fabricate high performance REBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> superconductors for application in liquid nitrogen temperature.