Cheng-Hsiang Lin

One-step fabrication of nanostructures by femtosecond laser for surface-enhanced Raman scattering

This paper reports an efficient fabrication of nanostructures on silicon substrates for surface-enhanced Raman scattering (SERS). Silicon wafer substrates in the aqueous solution of silver nitrate were machined by the femtosecond laser direct writing to achieve simultaneously in one-step the generation of grating-like nanostructures on the surface of the substrate and the formation of silver nanoparticles on the surface of the nanostructures via the laser-induced photoreduction effect. Parametric studies were conducted for the different concentrations of aqueous silver nitrate solutions and scanning speeds. The enhancement factor of the SERS is found to be higher than 10(9). The patterning technique provides an opportunity to incorporate the SERS capability in a functional microchip.

Laser-treated substrate with nanoparticles for surface-enhanced Raman scattering

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.

Fabry-Perot interferometer embedded in a glass chip fabricated by femtosecond laser

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.

Investigations of femtosecond-nanosecond dual-beam laser ablation of dielectrics.

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.

Adaptive-optics system with liquid-crystal phase-shift interferometer

We develop an adaptive-optics system based on a Mach-Zehnder radial shearing interferometer with liquid-crystal-device (LCD) phase-shift interferometry (PSI). Using accurate phase calibration and transient nematic driving of the LCD, the developed three-step PSI procedure can be achieved in a time of 5 ms. The proposed Mach-Zehnder radial shearing PSI method reconstructs the phase information using a digital signal processor (DSP). The DSP then computes appropriate control signals to drive a deformable mirror in such a way as to eliminate the wavefront distortion. The current adaptive-optics system is capable of suppressing low-frequency thermal disturbances with a signal-to-noise ratio improvement of more than 20 dB and a steady-state phase error of less than 0.02pi root mean square when the control loop is operated at a frequency of 30 Hz.

Real-time depth measurement for micro-holes drilled by lasers

An optical system based on the confocal principle has been developed for real-time precision measurements of the depth of micro-holes during the laser drilling process. The capability of the measuring system is theoretically predicted by the Gaussian lens formula and experimentally validated to achieve a sensitivity of 0.5 µm. A nanosecond laser system was used to drill holes, and the hole depths were measured by the proposed measuring system and by the cut-and-polish method. The differences between these two measurements are found to be 5.0% for hole depths on the order of tens of microns and 11.2% for hundreds of microns. The discrepancies are caused mainly by the roughness of the bottom surface of the hole and by the existence of debris in the hole. This system can be easily implemented in a laser workstation for the fabrication of 3D microstructures.

Femtosecond and nanosecond laser fabricated substrate for surface-enhanced Raman scattering.

We report a simple and repeatable method for fabricating a large-area substrate for surface-enhanced Raman scattering. The substrate was processed by three steps: (i) femtosecond (fs) laser micromachining and roughening, (ii) thin-film coating, and (iii) nanosecond laser heating and melting. Numerous gold nanoparticles of various sizes were created on the surface of the silicon substrate. The 3D micro-/nanostructures generated by the fs laser provide greater surface areas with more nanoparticles leading to 2 orders of magnitude higher of the enhancement factor than in the case of a flat substrate. Using an He-Ne laser with a 632.8 nm excitation wavelength, the surface-enhanced Raman scattering enhancement factor for Rhodamine 6G was measured up to 2×10(7).

Surface-enhanced Raman scattering microchip fabricated by femtosecond laser

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.

Enhancement of ablation efficiency by a femto/nano-second dual-beam micromachining system

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 Fouriers 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.

Femtosecond and Nanosecond Dual-Laser Optical Emission Spectroscopy of Gas Mixtures:

A method employing an integrated femtosecond (fs) and nanosecond (ns) dual-laser system was developed to generate plasma with desired radical species from gas mixtures via a fs laser pulse and then to excite selected radical species to higher electronic states using a wavelength-tunable ns laser pulse. An optical spectrometer was used to measure the emission spectra and identify the transition from the excited electronic state to the ground state. The proposed technique has been demonstrated for an N2–CO2 mixture with various time delays between the two fs and ns pulses. The results have indicated that the population of selected radical species at the excited electronic state can be increased using the subsequent ns laser pulse, which also enhances the intensity of emission spectra allowing better identifications of the radical species. This technique holds a promise of detection and identification of signature plasma species, particularly for trace elements and long-distance standoff detections.