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High Harmonic Generation
Published in Hitendra K. Malik, Laser-Matter Interaction for Radiation and Energy, 2021
In SHG, when two similar frequency photons interact with a nonlinear material, after their effective combination there is a generation of a new photon whose energy is twice that of the incident photons. In other words, new photon is generated with half the wavelength and twice the frequency of the incident photons (Figure 8.2). Therefore, SHG is also designated as the frequency doubler process. The tendency of occurrence of SHG in a medium is characterized by its second-order nonlinear susceptibility (χ(2)). There are some cases in which the approximate complete convergence of light energy into second-harmonic frequency is observed. In such cases, intense pulsed laser beams are passed through large crystals and phase matching is attained by attentive alignment. Whereas in some cases such as second-harmonic imaging microscopy, a small amount of light energy is transformed to second-harmonic frequency. Good SHG-generating materials are peripheral nerves and collagen fibers.
Gold Nanoclusters with Atomic Precision: Optical Properties
Published in Yan Zhu, Rongchao Jin, Atomically Precise Nanoclusters, 2021
Second harmonic generation (SHG) is a nonlinear optical phenomenon and is the most important and widely used technique in nonlinear optical mixing. SHG occurs when the fundamental light of a certain intensity passes through the nonlinear optical crystal to generate an electromagnetic wave with twice the original frequency, and it is therefore also called the double-frequency effect. In 1961, Franken et al. first discovered the second harmonic phenomenon [150]. They used a ruby laser (0.6943 mm) to perform a second harmonic experiment on quartz crystal. It was found that the output of the 694 nm laser passed through the quartz crystal and obtained 347.1 nm of ultraviolet light. This provides a convenient method for obtaining ultraviolet light using a near-infrared laser. Since then, research on SHG has been rapidly developed and applied to achieve laser modulation and to obtain various lasers as short as UV and as long as infrared. The SHG signal is related to both the intensity of the optical electric field and the polarizability of the medium. Because the dielectric polarizability reflects the microscopic characteristics of the medium, such as its symmetry and alignment, the second harmonic signal can be used to characterize the microstructure of the medium. In recent years, second harmonic imaging has often been used in the field of biological imaging [159, 160].
Imaging Cell Adhesion and Migration
Published in Margarida M. Barroso, Xavier Intes, In Vivo, 2020
Chandrani Mondal, Julie Di Martino, Jose Javier Bravo-Cordero
Third harmonic generation (THG) has been used to image infiltrating leukocytes (Rehberg et al., 2011) and the microenvironment surrounding melanoma cell invasion (Weigelin et al., 2012). In contrast to SHG, THG requires the simultaneous arrival of three photons and the emission of a single photon at one-third the wavelength and triple the energy (Weigelin et al., 2016). THG occurs at cell and tissue interfaces (e.g. between lipid-rich structures and aqueous fluids, or protein-rich structures and aqueous fluids), including cell and nuclear membranes, intracellular and extracellular vesicles, adipocytes, blood vessels, erythrocytes, nerve fibers, and muscle fibers (Weigelin et al., 2016). Both SHG and THG allow for label-free imaging of cell-matrix interactions during cell motility in vivo.
Second-harmonic generation of hollow Gaussian laser beams in inhomogeneous plasmas in the presence of wiggler magnetic field
Published in Waves in Random and Complex Media, 2021
M. Hashemzadeh, M. Abbasi-Firouzjah
The interaction of high-power laser pulses with plasmas has been investigated by many authors in the last few decades [1–3]. In this interaction, a number of nonlinear effects can be observed. Some of these nonlinear effects are self-focusing [4,5], magnetic field generation [6], generation of terahertz waves [7], shock generation [8], inertial confinement fusion [9], harmonic generation [10] etc. Second-harmonic generation (SHG) is an attractive subject of nonlinear effects and can be used in various application areas. Some of these areas are the microscopic resonance imaging [11,12], medical science [13], optoelectronics [14], superconductors with inversion symmetry [15], photonic topological metasurfaces [16], soft X-ray as an interfacial probe [17] etc. In SHG, two photons with the same energy are combined to generate a single photon with twice the energy. SHG is a useful technique that can convert infrared lasers to the shorter wavelength in the visible or near-visible range. In the interaction between a high-power laser with plasma, an electron plasma wave can be excited by linear mode conversion near the critical layer or by decay instability [18,19]. The presence of the external magnetic field and the electric and magnetic fields of the laser pulse can induce linear and nonlinear electron density, electron velocity and ponderomotive force. These electron densities and velocities induce linear and nonlinear transverse plasma currents, which lead to the generation of coherent harmonic radiation in the forward direction. The inhomogeneity of the initial electron density and the inhomogeneity of the external magnetic field can affect the SHG.