China
Africa
Explore chapters and articles related to this topic
Filamentation Phenomena and Generation of Supercontinuum in Propagation of Laser Pulses in a Nonlinear Medium
Published in Yu. N. Kulchin, Modern Optics and Photonics of Nano and Microsystems, 2018
It should also be noted that, in addition to high intensity, the laser pulse should also have a short duration (femtosecond). Otherwise, instead of multiphoton ionization of the medium, a multistage ionization process may begin in which the electron concentration becomes so large that they begin to ionize the molecules even far from the region occupied by the propagating laser beam. This leads to an imbalance between self‐focusing and defocusing, and the laser beam ceases to be focused and quickly diverges. Filamentation is observed in gaseous, liquid and solid transparent dielectrics and is accompanied by the formation of plasma channels, the super‐broadening of the frequency and angular momentum spectra, and nonlinear optical effects. Self‐focusing (see section 1.3) of radiation is the main physical cause of the formation of extended light filaments. The phenomenon of self‐focusing of electromagnetic waves in general form was predicted in 1962 by G.A. Askar’yan.
Nanoparticles Induced by Femtosecond Lasers
Published in Shalom Eliezer, Kunioki Mima, Applications of Laser–Plasma Interactions, 2008
Furthermore, a very important advantage of the femtosecond lasers is the accuracy in material removal due to the ultrashort pulse duration (Du et al., 1994; Nolte et al., 1997; Gamaly et al., 1999). Using molecular dynamics simulation (Allen and Tildesley, 1987; Vidal et al., 2001; Lorazo et al., 2003; Cheng and Xu, 2005) or thermodynamic calculations (Eliezer et al., 1986, 2004; Gamaly et al., 2004), the phase transition occurs near the critical point. The femtosecond laser pulses are much shorter than the heat conduction timescale; therefore, the irradiated metal can reach a state of superheated liquid (Henis and Eliezer, 1993) with a temperature higher than its critical temperature Tc. During adiabatic cooling the liquid decomposes into tiny droplets and vapor. The size of the droplet is determined by the correlation length λc, given by () λc=r0(T−TcTc)−v
Why Femtosecond?
Published in Marcos Dantus, Femtosecond Laser Shaping, 2017
From a medical standpoint, an accidental exposure to an amplified femtosecond laser pulse led to the realization that these lasers can cut cleanly through the transparent cornea of the eye without causing collateral damage. This accident occurred at the University of Michigan, and the eye doctor that examined the scientist with the eye injury was sufficiently intrigued to initiate a conversation on how to use femtosecond lasers for performing eye surgeries. Material processing and corrective laser eye surgeries are presently the most important commercial uses of femtosecond lasers. These applications have already created multibillion dollar industries.
Terahertz generation and detection of 1550-nm-excited LT-GaAs photoconductive antennas
Published in Journal of Modern Optics, 2021
Zhi-Chen Bai, Xin Liu, Jing Ding, Hai-Lin Cui, Bo Su, Cun-Lin Zhang
The test system for the THz photoconductive antennas is shown in Figure 6. The femtosecond laser has a wavelength of 1550 nm, pulse width of 75 fs, and repetition rate of 100 MHz. After passing through a half-wave plate and a polarization beam splitter, the laser is divided into a pump beam and a probe beam. After the probe beam passes through the translation stage, lens L1 (focal length: 5 cm) is used to focus it into the slit between the two gold electrodes of the LT-GaAs epitaxial wafer antenna. After passing through mirrors M4 and M5 and lens L2 (focal length: 5 cm), the probe beam is focused on the film in the middle of the LT-GaAs thin-film antenna. A 100-V pulse voltage is applied to both ends of the LT-GaAs epitaxial wafer antenna. The electrode of the LT-GaAs thin-film antenna is connected to the locked-in amplifier, and the frequency is set to 10 kHz. The laser power of the pump beam is 8 mW, the laser power of the probe beam is 10 mW, and the distance between the two antennas is 2 mm.
800-nm femtosecond-laser-written core-and-cladding integrated fibre Bragg grating inscribed by line-by-line method through fibre coating
Published in Journal of Modern Optics, 2020
He Wei, Dong Yunhui, Dong Mingli, Meng Fanyong, Zhu Lianqing
The proposed femtosecond FBG manufacturing system is shown in Figure 2(a). The IR femtosecond laser frequency is 1 kHz, pulse width is 35 fs, and laser wavelength is 800 nm (Astrella, Coherent Inc.). Initially, a 2-m-long polyimide-coated fibre was fixed on the 3D platform, in order to guarantee the tensile strength around grating area, the FBG was inscribed through polyimide coating, and the femtosecond light spot was focused on the single-mode fibre core. In the experiment, for a 1.5-µm-waveband first-order FBG, the 3D platform movement speed was 541 µm/s and the inscription lines were fabricated through the plane-by-plane method with the interval of 541 nm. The FBG was inscribed into the core with the centre wavelength of 1565.3 nm; the reflection spectrum is shown in Figure 2(b). To demonstrate the reasonability of the core-and-cladding-integrated FBG, one FBG was fabricated through the plane-by-plane method only into the fibre cladding part. The image of the inscription area is shown in Figure 2(c); the length of each line is 10 µm and the interval between two lines is 541 nm. As shown in Figure 2(d), because the cladding part has a lower refractive index than the fibre core, the cladding FBG reflection wavelength is 1559.7 nm. From the above experiment, it is possible to realize two reflection peaks during one inscription time.
Multiscale Investigation of Femtosecond Laser Pulses Processing Aluminum in Burst Mode
Published in Nanoscale and Microscale Thermophysical Engineering, 2018
Yiming Rong, Pengfei Ji, Mengzhe He, Yuwen Zhang, Yong Tang
Femtosecond laser owes the advantage of small collateral damage in fabricating precise microstructures over other long pulse lasers and continuum wave lasers. In the past decades, tremendous approaches were attempted to enhance the micro-/nano-fabrication quality and efficiency, which can be categorized from the laser side (such as tuning the laser parameters) [1–4] and the material side (such as pre-/post-processing the material with hybrid manufacture method) [5–8]. With the rapid development of femtosecond laser pulse source technology and approaches manipulating the pulse transportation, the high pulse repetition rate up to megahertz (MHz) and high power up to tens of watts can be achieved, enabling the efficient productivity of femtosecond laser fabrication from academic research to industrial applications [9].