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<p><b>Second-harmonic generation</b> (<b>SHG</b>), also known as <b>frequency doubling</b>, is the lowest-order wave-wave nonlinear interaction that occurs in various systems, including optical, radio, atmospheric, and magnetohydrodynamic systems. As a prototype behavior of waves, SHG is widely used, for example, in doubling laser frequencies. SHG was initially discovered as a <a href="page.php?w=nonlinear_optics">nonlinear optical</a> process in which two <a href="page.php?w=photon">photon</a>s with the same frequency interact with a nonlinear</p><p>
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