Xue-Qing Liu

A SERS-active microfluidic device with tunable surface plasmon resonances†

A surface‐enhanced Raman scattering (SERS)‐active microfluidic device with tunable surface plasmon resonances is presented here. It is constructed by silver grating substrates prepared by two‐beam laser interference of photoresists and subsequent metal evaporation coating, as well as PDMS microchannel derived from soft lithography. By varying the period of gratings from 200 to 550 nm, surface plasmon resonances (SPRs) from the metal gratings could be tuned in a certain range. When the SPRs match with the Raman excitation line, the highest enhancement factor of 2×107 is achieved in the SERS detection. The SERS‐active microchannel with tunable SPRs exhibits both high enhancement factor and reproducibility of SERS signals, and thus holds great promise for applications of on‐chip SERS detection.

Sapphire-Based Fresnel Zone Plate Fabricated by Femtosecond Laser Direct Writing and Wet Etching

Here, we report a sapphire-based Fresnel zone plate (FZP), which is fabricated by femtosecond laser direct writing assisted with subsequent wet etching. With this method, we solved the problem of high surface roughness caused by ultrafast femtosecond laser processing. We have obtained ~12-nm average surface roughness smaller than 1/25 of the optical working wavelength. As-formed sapphire FZP also exhibited a well-defined geometry. More importantly, ultraviolet (UV) light focusing and imaging can be easily achieved. Due to the high material hardness, thermal and chemical stabilities of sapphire, such sapphire FZP, may have great potential in UV imaging and focusing under some harsh environments.

Measurement of Two-Photon Absorption Cross Section of Metal Ions by a Mass Sedimentation Approach

The photo-reduction of metal ions in solution induced by femtosecond laser is an important and novel method for fabricating three-dimensional metal microstructures. However, the nonlinear absorption cross section of metal ions remains unknown because its measurement is difficult. In the present study, a method based on Two-Photon Excited Sedimentation (TPES) is proposed to measure the two-photon absorption cross section (TPACS) of metal ions in solution. The power-squared dependence of the amount of sediment on the excitation intensity was confirmed, revealing that 800 nm femtosecond laser induced reduction of metal ions was a two photon absorption process. We believe that the proposed method may be applied to measure the TPACS of several metal ions, thereby opening a new avenue towards future analysis of two-photon absorption materials.

Mask-free construction of three-dimensional silicon structures by dry etching assisted gray-scale femtosecond laser direct writing

A mask-free micro/nano fabrication method is proposed for constructing arbitrary gradient height structures on silicon, combining gray-scale femtosecond laser direct writing (GS-FsLDW) with subsequent dry etching. Arbitrary two-dimensional patterns with a gradient concentration of oxygen atoms can be fabricated on the surface of undoped silicon wafer by FsLDW in air. After dry etching, various three-dimensional (3D) gradient height silicon structures are fabricated by controlling the laser power, scanning step, etching time, and etching power. As an example, a well-defined 3D Fresnel zone plate was fabricated on silicon wafer, which shows excellent focusing and imaging properties. The combination of high precision from dry etching and 3D fabrication ability on non-planar substrates of FsLDW, may broaden its applications in microelectronics, micro-optics, and microelectromechanical systems.

Pressure dependence of excited-state charge-carrier dynamics in organolead tribromide perovskites

Excited-state charge-carrier dynamics governs the performance of organometal trihalide perovskites (OTPs) and is strongly influenced by the crystal structure. Characterizing the excited-state charge-carrier dynamics in OTPs under high pressure is imperative for providing crucial insights into structure-property relations. Here, we conduct in situ high-pressure femtosecond transient absorption spectroscopy experiments to study the excited-state carrier dynamics of CH3NH3PbBr3 (MAPbBr3) under hydrostatic pressure. The results indicate that compression is an effective approach to modulate the carrier dynamics of MAPbBr3. Across each pressure-induced phase, carrier relaxation, phonon scattering, and Auger recombination present different pressure-dependent properties under compression. Responsiveness is attributed to the pressure-induced variation in the lattice structure, which also changes the electronic band structure. Specifically, simultaneous prolongation of carrier relaxation and Auger recombination is achieved in the ambient phase, which is very valuable for excess energy harvesting. Our discussion provides clues for optimizing the photovoltaic performance of OTPs.Excited-state charge-carrier dynamics governs the performance of organometal trihalide perovskites (OTPs) and is strongly influenced by the crystal structure. Characterizing the excited-state charge-carrier dynamics in OTPs under high pressure is imperative for providing crucial insights into structure-property relations. Here, we conduct in situ high-pressure femtosecond transient absorption spectroscopy experiments to study the excited-state carrier dynamics of CH3NH3PbBr3 (MAPbBr3) under hydrostatic pressure. The results indicate that compression is an effective approach to modulate the carrier dynamics of MAPbBr3. Across each pressure-induced phase, carrier relaxation, phonon scattering, and Auger recombination present different pressure-dependent properties under compression. Responsiveness is attributed to the pressure-induced variation in the lattice structure, which also changes the electronic band structure. Specifically, simultaneous prolongation of carrier relaxation and Auger recombination is ...

Silicon three-dimensional structures fabricated by femtosecond laser modification with dry etching

In this paper, a maskless, high efficiency, and flexible technology is developed to fabricate three-dimensional (3D) microstructures on a silicon wafer, which is based on the combination of femtosecond laser modification and subsequent dry etching. The silicon atoms in 2D patterned areas were insufficiently oxidized after femtosecond laser irradiation. Complex 3D structures can be fabricated on the silicon wafer after etching, such as micro gears, comb drive actuators, and micro cantilevers applied in microelectromechanical systems (MEMS) and micro Fresnel zone plates applied in micro optics. What is more, surface roughness of the laser structured wafer can be improved with increased etching time in the dry etching process. This technology shows its unique capacity to fabricate various 3D microstructures for applications in MEMS and micro optics.