Cylindrical shockwaves inside polymethyl methacrylate (PMMA) generated simultaneously with two hemispherical shockwaves induced by a femtosecond Gaussian beam laser were investigated using an ultrafast pump–probe imaging technique. The evolutions of these three shockwaves with probe delay and incident pulse number have been systematically analyzed. The plasma intensity and filament length in the center of cylindrical shockwave both decayed with pulse number. Moreover, the self-focused filament moved downstream towards the output surface with an increased pulse number. The experimental results and mechanism illustrated that energy deposition was suppressed by a degraded nonlinear effect due to a pre-ablated structure in multi-pulse irradiation.

We propose a temperature-insensitive refractive index (RI) fiber sensor based on a Mach–Zehnder interferometer. The sensor with high sensitivity and a robust structure is fabricated by splicing a short photonic crystal fiber (PCF) between two single-mode fibers, where two microcavities are formed at both junctions because of the collapse of the PCF air holes. The microcavity with a larger equatorial dimension can excite higher-order cladding modes, so the sensor presents a high RI sensitivity, which can reach 244.16 nm/RIU in the RI range of 1.333–1.3778. Meanwhile it has a low temperature sensitivity of 0.005 nm/°C in the range of 33°C–360°C.

An optical fiber extrinsic Fabry–Perot interferometer (EFPI) is designed and fabricated for refractive index (RI) sensing. To test the RI of liquid, the following two different methods are adopted: the wavelength tracking method and the Fourier-transform white-light interferometry (FTWLI). The sensitivities of sensors with cavity lengths of 288.1 and 358.5 μm are 702.312 nm/RIU and 396.362 μm/RIU, respectively, by the two methods. Our work provides a new kind of RI sensor with the advantages of high sensitivity, mechanical robustness, and low cross sensitivity to temperature. Also, we provide a new method to deal with gold film with a femtosecond laser.

A simple and repeatable method to fabricate high-aspect-ratio (HAR) and high-quality microgrooves in silica is reported. The method consists of two steps: (1) formation of laser-modified regions by femtosecond Bessel beam irradiation, and (2) removing laser-modified regions through HF etching. Uniform, straight microgrooves can be fabricated and the highest aspect ratio that can be reached is ～52. The phenomenon is attributed to the uniform energy distribution in the long propagation distance, which leads to the long and uniform laser-modified regions and subsequent HF acid etching of laser-modified regions with high selectivity. This method will have potential applications in fabrication of HAR microgrooves in transparent materials.

A surprising phenomenon can be discovered by using femtosecond double-pulse ablation of silicon and germanium in ethanol. The ablation areas present an oscillation increase phenomenon when the pulse delay increases from 200 fs to 1 ps in the fluence range of 0.5–0.6 J/cm2. In contrast, the ablation areas exhibit an oscillation decrease phenomenon as the pulse delay increases when the laser fluence F<0.5 J/cm2, which is consistent with the results of the experiment in air. It is considered that the adjustment of the photon–electron coupling efficiency by pulse train technology plays an important role in the ablation process.

We report a simple, cost-effective and repeatable method for fabricating a large area and uniform substrate for surface-enhanced Raman scattering (SERS). The silicon, micromachined by a femtosecond laser, is coated with gold film and then treated through the dewetting process. The morphology shows a higher electric field enhancement due to light trapping. The enhancement factor of the SERS substrate is 9.2×107 with a 5 nm-thick film coated. Moreover, it also exhibits a uniform signal through Raman mapping and chemical stability with the greatest intensity deviation of 6% after a month. The proposed technique provides an opportunity to equip microchips with the SERS capabilities of high sensitivity, chemical stability, and homogeneous signals.