physics

Optical rectification

Optical rectification is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Optical rectification rather than just read about it. In short: 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 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.

Key takeaways

Optical rectification belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Optical rectification to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Optical rectification from memory before moving on to harder problems.

Reference excerpt

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 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. It was reported for the first time in 1962, when radiation from a ruby laser was transmitted through potassium dihydrogen phosphate (KDP) and potassium dideuterium phosphate (KDdP) 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-pulse-width laser, 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 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 surfaces by similar effect as surface second harmonic generation. 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. This is different from other processes of terahertz generation such as polaritonics where a polar lattice vibration is thought to generate the terahertz radiation.

See also Terahertz time-domain spectroscopy

References

Illustrations

Optical rectification: An electron (purple) is being pushed side-to-side by a sinusoidally-oscillating force, i.e. the light's electric field. But because the electron is in an anharmonic potential (black curve), the electron motion is not sinusoidal. The three arrows show the Fourier series of the motion: The blue arrow corresponds to ordinary (linear) susceptibility, the green arrow corresponds to second-harmonic generation, and the red arrow corresponds to optical rectification. (When there is no oscillating force, the electron sits at the potential minimum, but when there is an oscillating force, it is, on average, further to the right, by an amount shown by the red arrow.) — An electron (purple) is being pushed side-to-side by a sinusoidally-oscillating force, i.e. the light's electric field. But because the electron is in an anharmonic potential (black curve), the electron motion is not sinusoidal. The three arrows show the Fourier series of the motion: The blue arrow corresponds to ordinary (linear) susceptibility, the green arrow corresponds to second-harmonic generation, and the red arrow corresponds to optical rectification. (When there is no oscillating force, the electron sits at the potential minimum, but when there is an oscillating force, it is, on average, further to the right, by an amount shown by the red arrow.)

Worked examples

Example 1 — a first encounter with Optical rectification

Start with the simplest possible case. Write down what Optical rectification claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Optical rectification before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Optical rectification ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Optical rectification

In research: Optical rectification appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Optical rectification in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Optical rectification is common in secondary-school and first-year university syllabi. It links to neighbouring topics Nonlinear optics, Terahertz technology, so understanding it makes those chapters shorter.
In everyday life: Look for Optical rectification outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Optical rectification in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Optical rectification means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Optical rectification out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Optical rectification in simple terms?

Why does Optical rectification matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Optical rectification?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Optical rectification.