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<title>Optical rectification</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Optical rectification</span></span>

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<p><b>Electro-optic rectification</b> (EOR), also referred to as <b>optical rectification</b>, is a <a href="Non-linear_optics" class="mw-redirect" title="Non-linear optics">non-linear optical process</a> that consists of the generation of a quasi-DC <a href="Polarization_(electrostatics)" class="mw-redirect" title="Polarization (electrostatics)">polarization</a> in a non-linear medium at the passage of an intense optical beam. For typical intensities, optical rectification is a second-order phenomenon<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> which is based on the inverse process of the <a href="Electro-optic_effect" class="mw-redirect" title="Electro-optic effect">electro-optic effect</a>. It was reported for the first time in 1962,<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> when radiation from a <a href="Ruby_laser" title="Ruby laser">ruby laser</a> was transmitted through <a href="Potassium_dihydrogen_phosphate" class="mw-redirect" title="Potassium dihydrogen phosphate">potassium dihydrogen phosphate</a> (KDP) and <a href="Potassium_dideuterium_phosphate" title="Potassium dideuterium phosphate">potassium dideuterium phosphate</a> (KD<sub>d</sub>P) crystals.

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<div class="mw-heading mw-heading2"><h2 id="Explanation">Explanation</h2></div>

<p>Optical rectification can be intuitively explained in terms of the symmetry properties of the non-linear medium: in the presence of a preferred internal direction, the polarization will not reverse its sign at the same time as the driving field. If the latter is represented by a sinusoidal wave, then an average DC polarization will be generated.

</p><p>Optical rectification is analogous to the <a href="Rectifier" title="Rectifier">electric rectification effect</a> produced by <a href="Diode" title="Diode">diodes</a>, wherein an AC signal can be converted ("rectified") to DC. However, it is <i>not</i> the same thing. A diode can turn a sinusoidal electric field into a DC current, while optical rectification can turn a sinusoidal electric field into a DC polarization, but not a DC current. On the other hand, a <i>changing</i> polarization is a kind of current. Therefore, if the incident light is getting more and more intense, optical rectification causes a DC current, while if the light is getting less and less intense, optical rectification causes a DC current in the opposite direction. But again, if the light intensity is constant, optical rectification cannot cause a DC current.

</p><p>When the applied electric field is delivered by a <a href="Femtosecond" title="Femtosecond">femtosecond</a>-<a href="Pulsed_laser#Pulsed_operation" title="Pulsed laser">pulse-width</a> <a href="Laser" title="Laser">laser</a>, the spectral bandwidth associated with such short pulses is very large. The mixing of different frequency components produces a beating polarization, which results in the emission of electromagnetic waves in the <a href="Terahertz_radiation" title="Terahertz radiation">terahertz</a> region. The EOR effect is somewhat similar to a classical electrodynamic emission of radiation by an accelerating/decelerating charge, except that here the charges are in a bound dipole form and the THz generation depends on the second order susceptibility of the nonlinear optical medium. A popular material for generating radiation in the 0.5–3 THz range (0.1 mm wavelength) is <a href="Zinc_telluride" title="Zinc telluride">zinc telluride</a>.

</p><p>Optical rectification also occurs on <a href="Metal" title="Metal">metal</a> surfaces by similar effect as <a href="Surface_second_harmonic_generation" title="Surface second harmonic generation">surface second harmonic generation</a>. The effect is however influenced e. g. by nonequilibrium electron excitation and generally it manifests in a more complicated way.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>

</p><p>Similar to other nonlinear optical processes, optical rectification is also reported to become enhanced when <a href="Surface_plasmons" class="mw-redirect" title="Surface plasmons">surface plasmons</a> are excited on a metal surface.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Applications">Applications</h2></div>

<p>Together with carrier acceleration in semiconductors and polymers, optical rectification is one of the main mechanisms for the generation of terahertz radiation using lasers.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> This is different from other processes of terahertz generation such as <a href="Polaritonics" title="Polaritonics">polaritonics</a> where a polar <a href="Lattice_vibration" class="mw-redirect" title="Lattice vibration">lattice vibration</a> is thought to generate the <a href="Terahertz_radiation" title="Terahertz radiation">terahertz radiation</a>.

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<ul><li><a href="Terahertz_time-domain_spectroscopy" title="Terahertz time-domain spectroscopy">Terahertz time-domain spectroscopy</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

<li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text">Rice <i>et al.</i>, "Terahertz optical rectification from <110> zinc-blende crystals," <i>Appl. Phys. Lett.</i> <b>64</b>, 1324 (1994), <style data-mw-deduplicate="TemplateStyles:r1238218222">

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</style><a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1063%2F1.111922">10.1063/1.111922</a></span>

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<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text">Bass <i>et al.</i>, "Optical rectification," <i>Phys. Rev. Lett.</i> <b>9</b>, 446 (1962), <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1103%2FPhysRevLett.9.446">10.1103/PhysRevLett.9.446</a></span>

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<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text">Kadlec, F., Kuzel, P., Coutaz, J. L., "Study of terahertz radiation generated by optical rectification on thin gold films," <i>Optics Letters</i>, <b>30</b>, 1402 (2005), <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1364%2FOL.30.001402">10.1364/OL.30.001402</a></span>

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<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text">G. Ramakrishnan, N. Kumar, P. C. M. Planken, D. Tanaka, and K. Kajikawa, "Surface plasmon-enhanced terahertz emission from a hemicyanine self-assembled monolayer," <i>Opt. Express</i>, <b>20</b>, 4067-4073 (2012), <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1364%2FOE.20.004067">10.1364/OE.20.004067</a></span>

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<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text">Tonouchi, M, "Cutting-edge terahertz technology," <i>Nature Photonics</i> <b>1</b>, 97 (2007), <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1038%2Fnphoton.2007.3">10.1038/nphoton.2007.3</a></span>

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