Youwang Hu

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.

Sensitivity Improvement of SAW NO 2 Sensors by p-n Heterojunction Nanocomposite Based on MWNTs Skeleton

As one of the major air pollutants, nitrogen dioxide (NO2) is seriously harmful to humans and the environment. The development of surface acoustic wave (SAW) sensors provides great potential for manufacturing miniaturized NO2 sensor with high sensitivity, and high stability. In this paper, a SAW NO2 sensor with a sensing layer formed by nanocomposite materials mixed with multi-walled carbon nanotubes (MWNTs) is presented. The SAW device used in this paper is a commercialized two-port Rayleigh wave resonator and the main sensitive material to NO2 is phthalocyanine copper (CuPc). In order to increase the sensitivity of CuPc to NO2 at relative low temperature, MWNTs and metal-oxide nanoparticles are doped into the sensitive films. The p-n heterojunctions formed between the metal-oxide-semiconductor and the CuPc/MWNTs sensitive materials improve the performance of the sensor at low temperature down to 50 °C. The sensor shows good linearity and repeatability to NO2 with bulk concentration of 1-80 ppm, and the sensitivity is 50.5 Hz/ppm at 50 °C.

Ultrafast electron dynamics of a Na4 cluster under resonant femtosecond laser pulse train irradiation

In this study, the photoexcitation and ionization of a Na4 cluster under intense femtosecond laser irradiation is investigated using first-principles calculations. 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, including the dipole response, electron emission, energy absorption and transient electron density distribution. The calculated photoabsorption spectra show good correspondence with the experimental results and the simulations describe well the behavior of the ionization process. The effects of pulse train on electron dynamics are discussed for both the off resonance and on resonance cases. In addition, the time evolutions of experiment-linked probabilities for producing different charged ionic states at various laser conditions are presented.

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.

Effects of key pulse train parameters on electron dynamics during femtosecond laser nonlinear ionization of silica

The controlled, well-characterized evolution of the amplitude envelope and carrier-frequency sweep of ultrafast laser pulses permits the measurement and control of quantum transitions on a femtosecond time scale. This opens new perspectives to manipulate/adjust/interfere with the transient localized electron dynamics, corresponding material properties and phase change mechanisms, which are critical in laser micro/nano fabrication. This study presents the first-principles calculations of nonlinear electron–photon interactions during femtosecond pulse train ablation of silica. A real-time and real-space time-dependent density functional theory (TDDFT) is used to describe the transient localized electron dynamics such as photon absorption, electron excitation and free electron distribution. The effects of key pulse train parameters (including the pulse separation, the number of pulses per train and temporal pulse energy distribution) on electron dynamics are investigated.

Refractive index and temperature-sensing characteristics of a cladding-etched thin core fiber interferometer

A high refractive index (RI) sensor based on an in-line Mach–Zehnder mode interferometer (MZI) is proposed. The sensor was realized by splicing a 2-cm length of cladding-etched thin core fiber (TCF) between two single mode fibers (SMFs). The TCF-structured MZI exhibited good fringe visibility as high as 15 dB in air and the high RI sensitivity attained a value of 1143.89 nm/RIU at a RI of 1.447. The experimental data revealed that the MZI has high RI sensitivity after HF etching realizing 2599.66 nm/RIU. Studies were performed on the temperature characteristics of the device. It is anticipated that this high RI sensor will be deployed in new and diverse applications in the chemical and biological fields.

Investigation on Eigenfrequency of a Cylindrical Shell Resonator under Resonator-Top Trimming Methods

The eigenfrequency of a resonator plays a significant role in the operation of a cylindrical shell vibrating gyroscope, and trimming is aimed at eliminating the frequency split that is the difference of eigenfrequency between two work modes. In this paper, the effects on eigenfrequency under resonator-top trimming methods that trim the top of the resonator wall are investigated by simulation and experiments. Simulation results show that the eigenfrequency of the trimmed mode increases in the holes-trimming method, whereas it decreases in the grooves-trimming method. At the same time, the untrimmed modes decrease in both holes-trimming and grooves-trimming methods. Moreover, grooves-trimming is more efficient than holes-trimming, which indicates that grooves-trimming can be a primary trimming method, and holes-trimming can be a precision trimming method. The rigidity condition after grooves-trimming is also studied to explain the variation of eigenfrequency. A femtosecond laser is employed in the resonator trimming experiment by the precise ablation of the material. Experimental results are in agreement with the simulation results.

Characteristics of core refractive index axial asymmetry LPFGs written by femtosecond laser

Direct writing of long period fibre gratings (LPFGs) in standard single mode fibres (SMF) with infrared femtosecond laser pulses is experimentally investigated. It is found that the transmission spectra of LPFGs can be controlled by infrared femtosecond laser focus condition. The transmission spectra of LPFGs present single resonance peak when the microscopic objective is 25´, which is distinct from the LPFGs manufactured by UV laser irradiation with mask. However, when the microscopic objective is 10´, the LPFGs transmission spectra with four rejection bands can be obtained. When the distribution of femtosecond laser energy is changed by inserting a diaphragm, the LPFGs transmission spectra present multi resonance peaks.