Search

Metal surfaces with low reflectance have received considerable attention for their great optical, electrical, and thermal properties. However, the difficulty in achieving low reflectance on curved metal surfaces has hindered their practical applications. We propose a rapid and flexible method for processing a three-dimensional surface with antireflective properties. A Bessel beam created using an axicon is employed to generate ripple structures on the curved surface, thereby assisting subsequent thermal oxidation. Ripple structures coated with oxide semiconductor nanowires are then processed on a Cu substrate, thus further reducing reflectance. Antireflective properties with a minimum reflectance of less than 0.015 at a wavelength of 500–1200 nm could be achieved by using this method. This presented approach reduces dimensionality in laser processing, subsequently improving processing efficiency, and provides a foundation for the practical application of metal antireflective surfaces.

We achieved the controllable formation of laser-induced periodic surface structures (LIPSSs) on ZnO films deposited on fused silica induced by modulated temporally shaped femtosecond (fs) laser pulses (800 nm, 50 fs, 1 kHz) through the laser scanning technique. Two-dimensional (2D) high spatial frequency LIPSSs (HSFLs) with a period from 100 to 200 nm could be flexibly modulated based on the preprocessed nanostructures with appropriate fs laser irradiation conditions (fluence, scanning speed, and pulse delay). The finite-difference time-domain (FDTD) method combined with the Drude model was employed to calculate the redistributions of electric fields, which suggested the origin of HSFL formation.

We proposed combining temporally shaped (double-pulse train) laser pulses with spatially shaped (Bessel beam) laser pulses. By using a temporally shaped femtosecond laser Bessel-beam-assisted chemical etching method, the energy deposition efficiency was improved by adjusting the pulse delay to yield a stronger material modification and, thus, a higher etching depth. The etching depth was enhanced by a factor of 13 using the temporally shaped Bessel beam. The mechanism of etching depth enhancement was elucidated by localized transient-free electrons dynamics-induced structural and morphological changes. Micro-Raman spectroscopy was conducted to verify the structural changes inside the material. This method enables high-throughput, high-aspect-ratio microchannel fabrication in fused silica for potential applications in microfluidics.

This work presents a novel method for fabricating repeatable, uniform, large-area, highly sensitive, surface-enhanced resonance Raman scattering (SERRS) substrates combined with silicon nanopillar arrays and silver nanoparticles. The proposed method consists of two steps: (1) induce periodic ripples in deionized water using a linearly polarized femtosecond laser; and (2) generate dense 80-nm-diameter nanopillar arrays with silver nanoparticles in silver nitrate solution with a 90° rotated polarization, femtosecond double-pulse train. As the pulse delay increases from 0 to 1000 fs, the mean size of the silver nanoparticles reduces, and the average number of nanoparticles increases, which, in turn, increases the enhancement factor of SERRS signals up to 1.1×10(9). Furthermore, melamine (down to 125 ppb) was detected by the fabricated SERRS substrates.

An all-in-fiber prototype optofluidic device was fabricated by femtosecond laser irradiation and subsequent selective chemical wet etching. Horizontal and vertical microchannels can be flexibly created into an optical fiber to form a fluidic cavity with inlets/outlets. The fluidic cavity also functions as an optical Fabry–Perot cavity in which the filled liquid can be probed. The assembly-free microdevice exhibited a fringe visibility of 20 dB and was demonstrated for measurement of the refractive index of the filling liquids. The proposed all-in-fiber optofluidic micro device is attractive for chemical and biomedical sensing because it is flexible in design, simple to fabricate, mechanically robust, and miniaturized in size.

It is widely believed that laser-induced periodic surface structures (LIPSS) are independent of material crystal structures. This Letter reports an abnormal phenomenon of strong dependence of the anisotropic formation of periodic ripples on crystal orientation, when Si (100) is processed by a linearly polarized femtosecond laser (800 nm, 50 fs, 1 kHz). LIPSS formation sensitivity with a π/2 modulation is found along different crystal orientations with a quasi-cosinusoid function when the angle between the crystal orientation and polarization direction is changed from 0° to 180°. Our experiments indicate that it is much easier (or more difficult) to form ripple structures when the polarization direction is aligned with the lattice axis [011]/[011¯] (or [001]). The modulated nonlinear ionization rate along different crystal orientations, which arises from the direction dependence of the effective mass of the electron is proposed to interpret the unexpected anisotropic LIPSS formation phenomenon. Also, we demonstrate that the abnormal phenomenon can be applied to control the continuity of scanned ripple lines along different crystal orientations.

The dependence of the etching rate on the ultrafast pulse shaping is observed when microchannels are fabricated in fused silica glass using the method of femtosecond laser irradiation followed by chemical etching. In comparison with the conventional femtosecond pulses, the temporally shaped pulse trains can greatly enhance the etching rate under the same processing conditions. The enhancement is mainly attributed to the localized transient electron dynamics control by shaping the ultrafast pulse, resulting in higher photon absorption efficiency and uniform photomodification zone. Furthermore, processing parameters, including pulse delay and pulse energy distribution ratio, have also been investigated to optimize microchannels fabrication.

This Letter presents a simple, efficient approach for high surface-enhanced Raman scattering by one-step controllable fabrication of hierarchical structures (nanoparticles+subwavelength ripples) on silicon substrates in silver nitrate solutions using femtosecond double pulses based on nanoscale electron dynamics control. As the delays of the double pulses increase from 0 fs to 1 ps, the hierarchical structures can be controlled with (1) nanoparticles--the number of nanoparticles in the range of 40-100 nm reaches the maximum at 800 fs and (2) ripples--the subwavelength ripples become intermittent with decreased ablation depths. The redistributed nanoparticles and the modified ripple structures contribute to the maximum enhancement factor of 2.2×10(8) (measured by 10(-6) M rhodamine 6G solution) at the pulse delay of 800 fs.

A high-temperature sensor based on a Mach–Zehnder interferometer (MZI) in a conventional single-mode optical fiber is proposed and fabricated by concatenating two microcavities separated by a middle section. A femtosecond laser is used to fabricate a microhole on the center of a fiber end. Then a micro-air-cavity is formed by splicing the microholed fiber end with a normal fiber end. The interferometer is applied for high-temperature sensing, in the range of 500–1200 °C, with a sensitivity of 109 pm/°C that, to the best of our knowledge, is highest in silica fiber temperature sensors. Also, the interferometer is insensitive to external refractive index (RI), which is desirable for temperature sensors.

We have conducted experimental investigations for the micromachining of dielectrics (fused silica) using an integrated femtosecond (fs) and nanosecond (ns) dual-beam laser system at different time delays between the fs and ns pulses. We found that the maximum ablation enhancement occurs when the fs pulse is shot near the peak of the ns pulse envelope. Enhancements up to 13.4 times in ablation depth and 50.7 times in the amount of material removal were obtained, as compared to fs laser ablation alone. The fs pulse increases the free electron density and changes the optical properties of fused silica to have metallic characteristics, which increases the absorption of the ns laser energy. This study provides an opportunity for efficient micromachining of dielectrics.

We report a simple Fabry-Perot interferometer (FPI) embedded in a glass chip, which is capable of precisely measuring the refractive indices of liquid samples. The microdevice is the integration of a single-mode optical fiber and a microchannel in the photosensitive glass fabricated by femtosecond laser followed by thermal treatment, wet etching, and annealing. The function of the FPI is demonstrated by measuring the refractive indices of water and methanol. The interference visibility is more than 4.0 dB, which is sufficient for most sensing applications. This refractive index sensor with rigid structure could be further integrated to become a more complex 3D lab-on-a-chip for reliable biomedical applications.

We propose an approach to realize polarization-independent etching of fused silica by using temporally shaped femtosecond pulse trains to control the localized transient electrons dynamics. Instead of nanograting formation using traditional unshaped pulses, for the pulse delay of pulse trains larger than 1 ps, coherent field-vector-related coupling is not possible and field orientation is lost. The exponential growth of the periodic structures is interrupted. In this case, disordered and interconnected nanostructures are formed, which is probably the main reason of etching independence on the laser polarization. As an application example, square-wave-shaped and arc-shaped microchannels are fabricated by using pulse trains to demonstrate the advantage of the proposed method in fabricating high-aspect-ratio and three-dimensional microchannels.

This study proposes a method for adjusting subwavelength ripple periods and the corresponding double-grating structures formed on fused silica by designing femtosecond laser pulse trains based on localized transient electron density control. Four near-constant period ranges of 190-490 nm of ripples perpendicular to the polarization are obtained by designing pulse trains to excite and modulate the surface plasmon waves. In the period range of 350-490 nm, the double-grating structure is fabricated in one step, which is probably attributable to the grating-assisted enhanced energy deposition and subsequent thermal effects.

An interesting anisotropy phenomenon in femtosecond laser processing of crystalline silicon is revealed by changing the angle between the writing direction and the laser polarization. The experimental results indicate the surface patterning is dependent on the laser polarization direction, showing that it is beneficial to forming continuous, ordered, and better-controlled ripples when the writing direction is parallel to the laser polarization. The anisotropy is attributed mainly to the elliptical shape of the induced ripples. The formation mechanisms of the elliptical ripples are also discussed. This observation promotes the fabrication of self-assembled subwavelength structures, which is important for electro-optic devices.

A fiber inline Michelson interferometer was fabricated by micromachining a step structure at the tip of a single-mode optical fiber using a femtosecond laser. The step structure splits the fiber core into two reflection paths and produces an interference signal. A fringe visibility of 18 dB was achieved. Temperature sensing up to 1000°C was demonstrated using the fabricated assembly-free device.

A simple and repeatable method using femtosecond laser pulse train to fabricate nanostructured substrates with silver nanoparticles over a large area for surface-enhanced Raman scattering is reported. The method involves two steps: (1) femtosecond laser pulse train micromachining and roughening and (2) femtosecond laser processing of the substrates in a silver nitrate solution. Surface modification is investigated experimentally by varying the time delay of the double femtosecond laser pulse train. With time delay ranging from 200 to 600 fs, the different enhancement factors were observed. This study demonstrates that a maximum enhancement factor of 6.8×10(6), measured by 10(-6) M Rhodamine 6G solution, can be achieved at the time delay of 400 fs.

We report a surface-enhanced Raman scattering (SERS) microchip that is capable of measuring SERS signals of liquid samples with high sensitivity. The microdevice is an integration of a silicon-based SERS substrate, a multimode optical fiber (MMF), and a microchannel embedded in the photosensitive glass fabricated by the femtosecond laser followed by thermal treatment, wet etching, and annealing. The performance of the SERS microchip is evaluated by measuring rhodamine 6G using a 632.8 nm He-Ne laser at 4.3 mW excitation power, which reveals that the detection limit is lower than 10(-10) M at a 1 s short accumulation time.

A rapid and simple approach to fabricate a large area of nanostructured substrate for surface-enhanced Raman scattering (SERS) is reported. Gold nanoparticles ranging from 10 to 40 nm in diameter uniformly distributed on a silicon substrate were obtained by annealing the gold film precoated on the silicon substrate with UV nanosecond (ns) laser pulses. The gold nanoparticles were formed by surface tension of the melted gold layer heated by ns laser pulses. The enhancement factor of the SERS substrate for Rhodamine 6G at 632.8 nm excitation was measured to be higher than 10(5). The proposed technique provides the opportunity to equip a functional microchip with SERS capability of high sensitivity and chemical stability.

We report what we believe to be a new method to fabricate surface enhanced Raman scattering (SERS) fiber probe by direct femtosecond laser micromachining. Direct femtosecond laser ablations resulted in nanostructures on the cleaved endface of a multimode optical fiber with a 105/125 microm core/cladding diameter. The laser-ablated fiber endface was SERS activated by silver chemical plating. High-quality SERS signal was detected using Rhodamine 6G molecules (10(-8)-10(-6) M solutions) via back excitation with the fiber length of up to 1 m. The fiber SERS probe was compared with a planar fused silica substrate at a front excitation. The long lead-in fiber length and the backexcitation/collection setup make the SERS probe promising for remote sensing applications.