Coherent anti-Stokes Raman scattering (CARS) microscopy is an attractive technique for label-free biochemical-specific characterization of biological specimens. However, it has very low sensitivity in monitoring and imaging molecules present in extremely low concentrations or at fast speeds. To improve this sensitivity, especially for multiplex CARS, the intensity of the pump beam and broadband Stokes beam should be enhanced simultaneously. Therefore, the gold shell particle and gold surface are demonstrated to enhance the forward and backward CARS, respectively. Results show that a signal enhancement factor of ∼25,000 can be achieved for the gold surface and an even higher enhancement factor can be achieved for the gold shell particles. Thus, we can obtain an enhanced CARS signal in a broad spectral range, which will substantially improve the detection sensitivity of hyperspectral CARS spectroscopy and imaging.
Metal nanowire fabrication has drawn tremendous attention in recent years due to its wide application in electronics, optoelectronics, and plasmonics. However, conventional laser fabrication technologies are limited by diffraction limit thus the fabrication resolution cannot meet the increasingly high demand of modern devices. Herein we report on a novel method for high-resolution high-quality metal nanowire fabrication by using Hermite-Gaussian beam to ablate metal thin film. The nanowire is formed due to the intensity valley in the center of the laser beam while the surrounding film is ablated. Arbitrary nanowire can be generated on the substrate by dynamically adjusting the orientation of the intensity valley. This method shows obvious advantages compared to conventional methods. First, the minimum nanowire has a width of ~60 nm (≈1/13 of the laser wavelength), which is much smaller than the diffraction limit. The high resolution is achieved by combining the ultrashort nature of the femtosecond laser and the low thermal conductivity of the thin film. In addition, the fabricated nanowires have good inside qualities. No inner nanopores and particle intervals are generated inside the nanowire, thus endowing the nanowire with good electronic characteristics: the conductivity of the nanowires is as high as 1.2×107 S/m (≈1/4 of buck material), and the maximum current density is up to 1.66×108 A/m2. Last, the nanowire has a good adhesion to the substrates, which can withstand ultrasonic bath for a long time. These advantages make our method a good approach for high-resolution high-quality nanowire fabrication as a complementary method to conventional lithography methods.
Precise assembly of carbon nanotubes (CNTs) in arbitrary 3D space with proper alignment is critically important and desirable for CNT applications but still remains as a long-standing challenge. Using the two-photon polymerization (TPP) technique, it is possible to fabricate 3D micro/nanoscale CNT/polymer architectures with proper CNT alignments in desired directions, which is expected to enable a broad range of applications of CNTs in functional devices. To unleash the full potential of CNTs, it is strategically important to develop TPP-compatible resins with high CNT concentrations for precise assembly of CNTs into 3D micro/nanostructures for functional device applications. We investigated a thiol grafting method in functionalizing multiwalled carbon nanotubes (MWNTs) to develop TPP-compatible MWNT-thiol-acrylate (MTA) composite resins. The composite resins developed had high MWNT concentrations up to 0.2 wt%, over one order of magnitude higher than previously published work. Significantly enhanced electrical and mechanical properties of the 3D micro/nanostructures were achieved. Precisely controlled MWNT assembly and strong anisotropic effects were confirmed. Microelectronic devices made of the MTA composite polymer were demonstrated. The nanofabrication method can achieve controlled assembly of MWNTs in 3D micro/nanostructures, enabling a broad range of CNT applications, including 3D electronics, integrated photonics, and micro/nanoelectromechanical systems (MEMS/NEMS).
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.
Microholes drilling has attracted extensive research efforts for its broad applications in photonics, microfluidics, optical fibers and many other fields. A femtosecond (fs) laser is a promising tool for high-precision materials processing with reduced recast/microcracks and minimized heat affected zones. But there remain many challenges in hole drilling using conventional fs laser with Gaussian beams, such as low aspect ratio and taper effects. We report small-diameter and high-aspect-ratio microholes with taper free drilling in PMMA (polymethyl methacrylate) using single-pulse fs laser Bessel beams. Axicon is used to transform Gaussian beams into Bessel beams, which then irradiate in the sample by a telescope consisting of plano-convex lens and microscope objective. Using this technique, we enhance the aspect ratio of microholes by 55 times as compared with Gaussian beams. We attribute this high aspect ratio and high quality microholes formation to the unique spatial intensity distribution and propagation stability of Bessel beams, which can effectively adjust the transient localized electron density distribution leading to a long and uniform localized-interacted zone. By using the optimized pulse energy and focal depth position, the microholes diameter ranges between 1.4-2.1 μm and the aspect ratio can exceed 460. This efficient technique is of great potentials for fabrication of microphotonics devices and microfluidics.
With ultrashort pulse durations and ultrahigh power densities, femtosecond laser presents unique advantages of high precision and high quality fabrication of microchannels in transparent materials. In our study, by shaping femtosecond laser pulse energy distribution in temporal or spatial domains, localized transient electrons dynamics and the subsequent processes, such as phase changes, can be controlled, leading to the dramatic increases in the capability of femtosecond laser microchannels fabrication. The temporally shaped femtosecond laser pulse trains can significantly enhance the material removal rate in both water-assisted femtosecond laser drilling and femtosecond laser irradiation followed by chemical etching. Besides, high-aspect-ratio and small-diameter microchannels are drilled by spatially shaped femtosecond laser pulses.
Two-photon polymerization (TPP) is a promising micro/nanofabrication technique, which is capable of fabricating 3D micro/nanostructures beyond the diffraction limit of light. However, the study of TPP process with a focus on the dependence of degree of conversion on TPP parameters using a non-destructive and efficient method is still lacking. We studied the quantitative relationships between the TPP parameters and the cross-linking of an acrylic-based IP-L 780 photoresist via systematic Raman characterization. The differences in the Raman spectra between the non-polymerized and the polymerized IP-L 780 photoresists were observed by probing the excitation of carbon-carbon double bond (C=C) vibrations. We obtained the relationship between the degree of conversion in TPP and the Raman spectra of the IP-L 780 resin, in which the intensity of the characteristic Raman peak of IP-L 780 at 1635 cm-1 decreases with the increase of the TPP laser dose. A mathematic model of the degree of conversion with respective to the TPP parameters, including laser average power and writing speed, has been established. The method provides a simple and effective way to characterize and optimize the TPP micro/nanofabrication processes. The established model for the degree of conversion as the function of TPP parameters will contribute to the advanced 3D TPP micro/nanofabrication by providing a guidance to optimize the laser doses, voxel sizes, and the mechanical strength of the polymers.
A technique was developed to achieve enhanced coherent anti-Stokes Raman scattering (CARS) imaging using selfassembled silica microspheres. In this study, a layer of optically transparent silica microspheres was self-assembled onto polymer grating samples to enhance the CARS signals. The highest enhancement of 12.5 was achieved using 6.1-μmdiameter microspheres for C-H molecule vibration. Finite-difference time-domain (FDTD) algorithm under the perfectly matched layer boundary condition was used to simulate the enhancement using silica microspheres of different diameters.
Direct fabrication of graphene patterns on SiO2/Si substrates was demonstrated using a single-step laser-induced chemical vapor deposition (LCVD) process. A laser beam was used to irradiate a nickel-coated SiO2/Si substrate in a methane-hydrogen environment to induce a local temperature rise on the laser focused area. Followed by a rapid cooling process by moving the laser beam, graphene patterns were formed on the laser scanning pathway. Nickel (Ni) layer under graphene patterns was removed by the Ni etchant diffused into the area under the graphene. Laser direct writing graphene patterns on SiO2/Si substrates was achieved. Energy dispersive X-ray diffraction spectroscopy was used to confirm the removal of Ni layers. The discovery and development of the LCVD growth process provide a route for the rapid fabrication of graphene-based electronic devices.
A fiber Mach-Zehnder interferometer (MZI) constructed by concatenating a micro cavity in the fiber core and a
taper is proposed and applied for temperature sensing. Femtosecond laser micro-machining technique and fusion
splicing technique are utilized to fabricate the micro cavity in a single mode optical fiber core. The fiber taper is
fabricated by electric arc-discharge with a conventional fusion splicer. A sensitivity of 68 pm/°C is obtained at the
temperature range of 200-400°C. The interferometer is also characterized by different strain.It exhibits very low
strain sensitivity because only low order cladding modes are excited by the micro cavity, which is a desirable merit
for temperature sensor to eliminate cross sensitivity to strain.
This study proposes a thinned-cladding zeolite coated long period fiber grating (LPFG) chemical sensor. The cladding
outside the grating zone is etched by hydrofluoric (HF) acid, and then the zeolite film is coated on the thinned grating.
The sensitivity characteristics of the LPFG to different external refractive indices are investigated. By etching cladding,
the resonant wavelength shifts towards the longer wavelength (red shift) while the attenuation band depth is increased. In
addition, with very large surface-to-mass ratio, zeolitic pores can efficiently adsorb molecules from the ambient for
highly sensitive detections. The adsorbed analyte molecules are collected and aligned in nanoscale zeolite pores. This
leads to changes in the refractive index of zeolite coating, which in turn, causes resonant wavelength shift. After zeolite
coating, the resonant wavelength shifts towards the shorter wavelength (blue shift) and the sensitivity to external
refractive index is enhanced. The resonant wavelength of uncoated LPFG is 1563.1 nm in air and 1560.5 nm in water,
with the wavelength shift of 2.6 nm, while the resonant wavelength of zeolite coated LPFG is 1543.2 nm in air.
Decreases in cladding diameters and ziolite coating can effectively enhance the refractive index sensitivity of LPFG
chemical sensor.
In this paper, a dual-beam laser micromachining system consisting of a femtosecond (fs) laser and a nanosecond (ns)
laser has been developed to enhance the ablation efficiency. Experiments were conducted in different materials including
dielectric (fused silica), semiconductor (silicon wafer), and metal (aluminum alloys). The amount of material being
removed was determined for fs pulses alone, ns pulses alone, and pairs of fs and ns pulses with different time lags in
between. It was found that the material removal efficiency increases in the dual-beam process for all materials being
studied as compared to the fs alone or ns alone, particularly for dielectrics. The highest ablation efficiency for fused
silica occurs when the fs pulse is shot near the peak of the ns pulse envelope. A corresponding numerical model for dual
beam ablation of dielectrics was also developed by integrating the plasma model, the improved two-temperature model,
and Fourier's law to understand the laser-material interaction. It was found that the fs laser pulse can significantly
increase the free electron density and change the optical properties of the dielectric, leading to the increase of absorption
for the subsequent ns pulse energy. This study provides a fundamental understanding for the enhancement of material
ablation efficiency, particularly for wide-bandgap dielectrics.
A multiscale model is developed to study to the femtosecond laser single pulse and pulse train processing of the metal
films. In our model, molecular dynamics simulation combined with the improved two-temperature model is employed in
the ablation area and the improved two-temperature model is applied in heat-affected zone. This paper extends the
improved two-temperature model to describe higher laser fluences processing by introducing the phase change. The
phase change mechanisms of the non-equilibrium thermal melting and vaporization are both analyzed, which has a
strong impact on the lattice temperature evaluation. The model can simulate phase change process of gold with higher
accuracy. It is demonstrated that the pulse train could improve the fabrication accuracy, repeatability, and controllability.
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