Zhi Luo

Adjustable annular rings of periodic surface structures induced by spatially shaped femtosecond laser

In this study, we propose a favorable method to modulate femtosecond laser Gaussian beams into annular beams by using an axicon for the fabrication of periodic surface structures. Adjustable annular rings of uniform oriented periodic surface structures with a period of approximately 600~750 nm are fabricated in one illumination step without complicated movements of the translation stage. The orientations of the fabricated periodic surface structures are controllable by changing the electric vector of the laser beam. In addition, by adjusting the distance between the sample and microscope objective, the diameters of the fabricated annular rings can be flexibly controlled in a wide range without changing the optical elements and realignment of the optical path.

One-step fabrication of annular microstructures based on improved femtosecond laser Bessel–Gaussian beam shaping

In this work, a favorable approach is proposed for the high-efficiency fabrication of annular microstructures by shaping femtosecond Gaussian laser into Bessel–Gaussian beam. An adjustable annular beam is generated by a combination of an axicon, a lens, and an objective. Uniform annular microstructures are fabricated on the surface of fused silica in a single illumination step with only one pulse and without stage translation, the parameters of which are coincident with theoretical design. By adjusting the relative distance between the lens and objective, the diameters of the annular microstructures can be flexibly controlled in a wide range. In addition, it has been discussed that the effects of laser pulse energy and off-focus on the depth and line-width of the fabricated annuli.

Highly Sensitive Strain Sensor Based on a Novel Mach-Zehnder Interferometer with TCF-PCF Structure

A highly sensitive strain sensor based on a novel fiber in line Mach-Zehnder interferometer (MZI) was demonstrated experimentally. The MZI was realized by splicing a section of photonic crystal fiber (PCF) with the same length of thin core fiber (TCF) between two single mode fibers (SMFs). The fringe visibility of MZI can reach as high as 20 dB in air. In particular, the strain sensitivity of −1.95 pm/με was achieved within a range from 0 to 4000 με. Furthermore, the strain properties of different length of MZI was investigated. It was found that the sensitivity was weekly dependent on the length of MZI. The strain sensitivities corresponding to the MZI with 35 mm PCF, 40 mm PCF and 45 mm PCF at 1550 nm band were −1.78 pm/με, −1.73 pm/με and −1.63 pm/με, respectively. Additionally, the sensor has advantages of simple fabrication, compact size and high sensitivity as well as good fringe visibility.

A Novel Strain Sensor with Large Measurement Range Based on All Fiber Mach-Zehnder Interferometer

We have proposed a high sensitive photonic crystal fiber (PCF) strain sensor based on the Mach-Zehnder interferometer (MZI). The sensing head is formed by all-fiber in-line single mode-multimode-photonic-crystal-single mode fiber (SMPS) structure, using only the splicing method. Such a strain sensor exhibited a high sensitivity of −2.21 pm/με within a large measurement range of up to 5000 με and a large fringe visibility of up to 24 dB. Moreover, it was found that the strain sensitivity was weekly dependent of the length of PCF or MMF. In addition, the sensor exhibited the advantages of simplicity of fabrication, high sensitivity and larger fringe visibility.

Temperature sensitivity enhancement of platinum-nanoparticle-coated long period fiber gratings fabricated by femtosecond laser

The temperature sensing performance of long period fiber gratings (LPFGs) written by femtosecond laser pulses coated with platinum nanoparticles (PtNPs) is proposed and demonstrated. It is found that the PtNPs increase the wavelength sensitivity of the LPFG significantly compared with the bare LPFG. The temperature sensitivities of the bare LPFG for three dips, corresponding to 1438, 1485, and 1585 nm, are 74.04, 77.23, and 86.26  pm/°C, respectively, when the temperature changes from 10°C to 500°C, whereas the corresponding sensitivities of PtNP coated LPFGs are up to 90.58, 93.51, and 103.43  pm/°C, respectively. Moreover, the PtNP coated LPFG has shown better wavelength repeatability. A small wavelength hysteresis of ∼0.5  nm is observed when the temperature is less than 300°C.

Microcavity Mach–Zehnder Interferometer Sensors for Refractive Index Sensing

Two types of robust Mach-Zehnder interferometer (MZI) sensors based on microcavity in a single mode optical fibers (SMFs) are proposed, which are fabricated by femtosecond laser inscription and chemical etching. The SMFs are modified by an improved point-by-point inscription method (called a transversal-scanning method) or a line-by-line scanning inscription method, which lead to form a rectangular-shaped or a V-shaped MZI after etching. The MZIs show high refractive index (RI) sensitivity above 105 nm/RIU with good linearity. Especially, the rectangular-shaped MZI exhibits a ultra-high RI sensitivity of -17503.73 nm/RIU with a linearity of 0.999 in the range of 1.3371-1.3407. The results of which are coincident with theoretical calculation. In addition, the MZI structures have good mechanical strength and temperature sensitivities in water are also studied.

Femtosecond laser fabrication of robust underwater superoleophobic and anti-oil surface on sapphire

Due to the presence of unique micro/nanostructures on the surface, fish’s scale exhibits underwater superoleophobicity and keeps clean even in oil-polluted water. Inspired from this, we propose a facile method for the fabrication of underwater superoleophobic and anti-oil sapphire surface with line-patterned nanostructures by femtosecond laser. The as-prepared surface shows great superoleophobicity that the oil contact angles can reach up to 153° for 1, 2-dichloroethane droplets in water and low oil-adhesion. At the same time, the relationship between the microgrooves’ period and surface wettability is studied, and the results indicate that the underwater superoleophobicity and low oil-adhesion can be achieved using a wide range of processing parameters. Meanwhile, the obtained surface is demonstrated to exhibit excellent stability. Moreover, the self-cleaning anti-oil ability of the as-prepared surface is conducted, and the potential mechanism of which is discussed. This technique has potential applicati...

Pulse train dependence of electron dynamics during resonant femtosecond laser nonlinear ionization of a Na4 cluster

In this study, a real-time and real-space time-dependent density functional theory (TDDFT) is applied to describe nonlinear electron–photon interactions during a resonant femtosecond laser pulse train photoionization of a Na4 cluster. The effects of key pulse train parameters, such as the spatial/temporal pulse energy distribution, pulse number per train, pulse separation and pulse phase on resonant absorption, are discussed. The calculations show that the resonant effect and the nonlinear electron dynamics, including energy absorption, electron emission, dipole response and ionization probability, can be controlled by shaping the ultrafast laser pulse train.

Quantum multiscale modeling of electron dynamics and material properties during femtosecond laser-material interactions

The controlled, well-characterized evolution of the amplitude envelope and carrier-frequency sweep of ultrafast laser pulses permits measurement and control of quantum transitions on a femtosecond time scale. This opens new perspectives for controlling the transient localized electron dynamics, corresponding material properties and phase change mechanisms, which are critical in laser micro/nano fabrication. In this study, the first-principles calculations and plasma model are used to theoretically investigate the changes of localized transient electron dynamics and the corresponding material properties during femtosecond laser pulse trains ablation of fused silica. Theoretical results show that the electron dynamics including photon absorption, electron excitation and free electron distributions can be changed; the corresponding material properties such as thermal and optical properties can then be controlled; and hence, high quality structures can be obtained.

Transient electron dynamics of nitrogen molecule irradiated by femtosecond and XUV attosecond laser pulse train

This study investigates the photoexcitation and ionization of a nitrogen molecule under ultrafast (femtosecond/attosecond) laser pulse irradiation. The real-time and real-space time-dependent density functional (TDDFT) is applied to describe the electron dynamics during the linear and nonlinear electron-photon interactions. The calculations describe well the behavior of the ionization process, and the results of ionization rates show good correspondence with the experimental results. In addition, the effects of near-infrared femtosecond laser pulse trains and the selected extreme ultraviolet attosecond laser pulse trains on electron dynamics are discussed. Theoretical results show that pulse number, laser frequency, and pulse delay are the key parameters for the control of electron dynamics including the electron excitation, energy absorption, electron density, and electron density oscillation.