Electro-optic rectification (EOR), also referred to as optical rectification, is a non-linear optical process that consists of the generation of a quasi-DC

polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...

in a non-linear medium at the passage of an intense optical beam. For typical intensities, optical rectification is a second-order phenomenon which is based on the inverse process of the

electro-optic effect Electro–optics is a branch of electrical engineering, electronic engineering, materials science, and material physics involving components, electronic devices such as lasers, laser diodes, LEDs, waveguides, etc. which operate by the prop ...

. It was reported for the first time in 1962, when radiation from a ruby laser was transmitted through

potassium dihydrogen phosphate Monopotassium phosphate (MKP) (also, potassium dihydrogenphosphate, KDP, or monobasic potassium phosphate) is the inorganic compound with the formula KH2PO4. Together with dipotassium phosphate (K2HPO4.(H2O)x) it is often used as a fertilizer, ...

(KDP) and

potassium dideuterium phosphate Deuterated potassium dihydrogen phosphate (KD2PO4) or DKDP single crystals are widely used in non-linear optics as the second, third and fourth harmonic generators for Nd:YAG and Nd:YLF lasers. They are also found in electro-optical applicatio ...

(KD_dP) crystals.

Explanation

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. Optical rectification is analogous to the electric rectification effect produced by diodes, wherein an AC signal can be converted ("rectified") to DC. However, it is ''not'' 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 ''changing'' 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. When the applied electric field is delivered by a

femtosecond A femtosecond is a unit of time in the International System of Units (SI) equal to 10 or of a second; that is, one quadrillionth, or one millionth of one billionth, of a second. For context, a femtosecond is to a second as a second is to about 3 ...

- pulse-width

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...

, 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

terahertz Terahertz or THz may refer to: * Terahertz (unit), a unit of frequency, defined as one trillion (1012) cycles per second or 1012 hertz * Terahertz radiation, electromagnetic waves within the ITU-designated band of frequencies from 0.3 to 3 terahe ...

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 zinc telluride. Optical rectification also occurs on

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typi ...

surfaces by similar effect as

surface second harmonic generation Surface second harmonic generation is a method for probing interfaces in atomic and molecular systems. In second harmonic generation (SHG), the light frequency is doubled, essentially converting two photons of the original beam of energy ''E'' into ...

. The effect is however influenced e. g. by nonequilibrium electron excitation and generally it manifests in a more complicated way. Similar to other nonlinear optical processes, optical rectification is also reported to become enhanced when surface plasmons are excited on a metal surface.

Applications

Together with carrier acceleration in semiconductors and polymers, optical rectification is one of the main mechanisms for the generation of terahertz radiation using lasers.Tonouchi, M, "Cutting-edge terahertz technology," ''Nature Photonics'' 1, 97 (2007), {{doi, 10.1038/nphoton.2007.3 This is different from other processes of terahertz generation such as polaritonics where a polar

lattice vibration In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, specifically in solids and some liquids. A type of quasiparticle, a phonon is an excited state in the quantum mechanic ...

is thought to generate the

terahertz radiation Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of fre ...

References

Terahertz technology Nonlinear optics

Explanation

Applications

See also

References