An electro–optic effect is a change in the optical properties of a material in response to an

electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...

that varies slowly compared with the frequency of light. The term encompasses a number of distinct phenomena, which can be subdivided into * a) change of the

absorption Absorption may refer to: Chemistry and biology * Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which ...

** Electroabsorption: general change of the absorption constants **

Franz–Keldysh effect The Franz–Keldysh effect is a change in optical absorption by a semiconductor when an electric field is applied. The effect is named after the German physicist Walter Franz and Russian physicist Leonid Keldysh. Karl W. Böer observed first t ...

: change in the absorption shown in some bulk semiconductors **

Quantum-confined Stark effect The quantum-confined Stark effect (QCSE) describes the effect of an external electric field upon the light absorption spectrum or emission spectrum of a quantum well (QW). In the absence of an external electric field, electrons and holes within th ...

: change in the absorption in some semiconductor

quantum well A quantum well is a potential well with only discrete energy values. The classic model used to demonstrate a quantum well is to confine particles, which were initially free to move in three dimensions, to two dimensions, by forcing them to occupy ...

s ** Electrochromic effect: creation of an absorption band at some wavelengths, which gives rise to a change in colour * b) change of the

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

and

permittivity In electromagnetism, the absolute permittivity, often simply called permittivity and denoted by the Greek letter ''ε'' ( epsilon), is a measure of the electric polarizability of a dielectric. A material with high permittivity polarizes more in ...

Pockels effect The Pockels effect or Pockels electro-optic effect, named after Friedrich Carl Alwin Pockels (who studied the effect in 1893), changes or produces birefringence in an optical medium induced by an electric field. In the Pockels effect, also known a ...

(or linear electro-optic effect): change in the refractive index linearly proportional to the electric field. Only certain crystalline solids show the Pockels effect, as it requires lack of inversion symmetry **

Kerr effect The Kerr effect, also called the quadratic electro-optic (QEO) effect, is a change in the refractive index of a material in response to an applied electric field. The Kerr effect is distinct from the Pockels effect in that the induced index chang ...

(or quadratic electro-optic effect, QEO effect): change in the refractive index proportional to the square of the electric field. All materials display the Kerr effect, with varying magnitudes, but it is generally much weaker than the Pockels effect ** electro-gyration: change in the

optical activity Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...

. ** Electron-refractive effect or EIPM In December 2015, two further electro-optic effects of type (b) were theoretically predicted to exist but have not, as yet, been experimentally observed. Changes in absorption can have a strong effect on refractive index for wavelengths near the absorption edge, due to the Kramers–Kronig relation. Using a less strict definition of the electro-optic effect allowing also electric fields oscillating at optical frequencies, one could also include nonlinear absorption (absorption depends on the light intensity) to category a) and the

optical Kerr effect The Kerr effect, also called the quadratic electro-optic (QEO) effect, is a change in the refractive index of a material in response to an applied electric field. The Kerr effect is distinct from the Pockels effect in that the induced index chan ...

(refractive index depends on the light intensity) to category b). Combined with the photoeffect and

photoconductivity Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation. Wh ...

, the electro-optic effect gives rise to the photorefractive effect. The term ''"electro-optic"'' is often erroneously used as a synonym for ''"

optoelectronic Optoelectronics (or optronics) is the study and application of electronic devices and systems that find, detect and control light, usually considered a sub-field of photonics. In this context, ''light'' often includes invisible forms of radiat ...

"''.

Applications

Electro-optic modulators

Electro-optic

modulator In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...

s are usually built with electro-optic crystals exhibiting the Pockels effect. The transmitted beam is

phase modulated Phase modulation (PM) is a modulation pattern for conditioning communication signals for transmission. It encodes a message signal as variations in the instantaneous phase of a carrier wave. Phase modulation is one of the two principal forms ...

with the electric signal applied to the crystal.

Amplitude modulator Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to t ...

s can be built by putting the electro-optic crystal between two linear

polarizer A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of well ...

s or in one path of a

Mach–Zehnder interferometer The Mach–Zehnder interferometer is a device used to determine the relative phase shift variations between two collimated beams derived by splitting light from a single source. The interferometer has been used, among other things, to measure pha ...

. Additionally,

s can be constructed by deflecting the beam into and out of a small aperture such as a fiber. This design can be low loss (<3 dB) and polarization independent depending on the crystal configuration.

Electro-optic deflectors

Electro-optic deflectors utilize prisms of electro-optic crystals. The index of refraction is changed by the Pockels effect, thus changing the direction of propagation of the beam inside the prism. Electro-optic deflectors have only a small number of resolvable spots, but possess a fast response time. There are few commercial models available at this time. This is because of competing

acousto-optic Acousto-optics is a branch of physics that studies the interactions between sound waves and light waves, especially the diffraction of laser light by ultrasound (or sound in general) through an ultrasonic grating. Introduction Optics has had a ...

deflectors, the small number of resolvable spots and the relatively high price of electro-optic crystals.

Electro-optic field sensors

The electro-optic Pockels effect in nonlinear crystals (e.g. KDP, BSO, K*DP) can be used for electric field sensing via polarisation state modulation techniques. In this scenario, an unknown electric field results in polarisation rotation of a laser beam propagating through the electro-optic crystal; through inclusion of polarisers to modulate the light intensity incident on a photodiode, a time-resolved electric field measurement can be reconstructed from the obtained voltage trace. As the signals obtained from vgcc the crystalline probes are optical, they are inherently resistant to electrical noise pickup, hence can be used for low-noise field measurement even in areas with high levels of electromagnetic noise in the vicinity of the probe. Furthermore, as the polarisation rotation due to the Pockels effect scales linearly with electric field, ''absolute'' field measurements are obtained, with no need for numerical integration to reconstruct electric fields, as is the case with conventional probes sensitive to the time-derivative of the electric field. Electro-optic measurements of strong electromagnetic pulses from intense laser-matter interactions have been demonstrated in both the nanosecond and picosecond (sub-petawatt) laser pulse driver regimes.

References

{{FS1037C MS188

External links

AdvR - Electro-optic Devices & Research
Nonlinear optics