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In this chapter, we present femtosecond laser micromachining to fiber optics, focusing on surface qualities. Some techniques applied in the field are introduced to date and a review of some of the current applications for this type of technology. Section 2 describes laser-induced periodic surface structures (LIPSSs), which are induced in low-and highfluence regime. Section 3 describes the influences of laser-induced structures for the fabrication of fiber-optic sensors, with experimental techniques and results in our research group. These sections explore ultrashort laser pulses applications, roughly going from lower to higher energy (power, intensity) ones.

In this paper, an improvement in the signal-to-noise (S/N) ratio of a nanocuboid array near-infrared (NIR) photodetector is demonstrated by using a combination of a small Schottky junction area and a converging lens. The combination of the small Schottky junction area and the lens showed a higher S/N ratio than a Schottky area of 1 cm2 for wavelengths above 1.2 μm, indicating 3.5 and 9.5 dB improvements in the S/N ratio at wavelengths of 1.2 and 1.5 μm, respectively. The structural parameters of the nanocuboids were determined to be 600 nm in length, 535 nm in height, and 900 nm in periodicity with high absorption efficiency in the broadband NIR region. The photocurrent signal was obtained over a broadband in the NIR region from 1.1 to 1.5 μm in wavelength by converging light with a convex lens above the nanocuboid array with a Schottky junction area of 1 mm2.

In this study, a low Schottky-barrier photodetector with a plasmonic assist using a two-dimensional (2D) nanohole array was demonstrated, which receives mid-infrared (MIR) light at room temperature. In the structural design, it was confirmed that the 2D nanohole-array photodetector has high absorbance in the MIR region using rigorous coupled-wave analysis. The result showed that the nanoholes formed in p-type silicon (p-Si), platinum silicide (PtSi), to form Schottky barriers, and gold (Au), for photocurrent extraction, had high absorbance in the MIR region along with the Fabry–Perot resonance mode toward the depth of the nanohole. The 2D nanohole array, with Au/PtSi/p-Si layers, has high absorbance for illuminating MIR light near 3.46 µm from the backside. The current–voltage characteristics indicated a low Schottky barrier of 0.32 eV, confirming the photoresponsive potential in the MIR photodetection. The photocurrent response to the modulation signal was obtained at room temperature. In addition, signal processing through transimpedance and lock-in amplifiers enabled us to obtain characteristics with high linearity for light intensities in milliwatts. Light acquisition for 2.5–3.8-µm-long MIR wavelength became possible, and applications in gas sensing, including vibrational absorption bands of alkane groups, are expected.