physics

Optical decay

Optical decay 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 decay rather than just read about it. In short: Optical decay is process of relaxation of excitation of an excited quantum system, usually due to the spontaneous emission of a photon or a phonon. Optical decay is dominant mechanism of quenching of excitation of active optical media.

Key takeaways

Optical decay 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 decay to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Optical decay from memory before moving on to harder problems.

Reference excerpt

Optical decay is process of relaxation of excitation of an excited quantum system, usually due to the spontaneous emission of a photon or a phonon. Optical decay is dominant mechanism of quenching of excitation of active optical media. In solid-state lasers the optical decay limits the storage of energy in active medium. Also, the term optical decay is used to the effect of quick reduction of luminosity of astrophysical objects

Phenomenology of optical decay In the first approximation, the optical decay can be treated as just spontaneous emission, and its rate is determined with the Einstein Coefficients. For the most of laser systems, the effects of decoherence determine the spectral width of the emitted photons, and there is no reason to consider in detail the evolution of isolated quantum-mechanical systems which show the optical decay.

Shape of spectral line at the optical decay of an idealized atom The "isolated" quantum system (atom, ion, molecule or even a quantum dot) may have metastable states, weakly coupled to the outer world. (Some couliknd should be taken into account; otherwise, there is no optical decay.) In the idealized case, the evolution of the system is disturbed only by the interaction with the continuum of the modes of the electromagnetic field. Then, the spectral width of the emitted photons is determined by the relaxation rate. For the narrow spectral lines, the decay is almost exponential; then, the profile of the spectral line is determined by the Fourier transform of the exponential decay of the quantum amplitude of probability that the system is still excited; this profile is Lorentian.

Deformed vacuum The decay rate can be affected by the distortion of uniform density of states of photons, due, for example, an external cavity

or a nanofiber located in vicinity of the atom or by simply placing the system near a dielectric or metallic boundary.

References

Worked examples

Example 1 — a first encounter with Optical decay

Start with the simplest possible case. Write down what Optical decay 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 decay 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 decay 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 decay

In research: Optical decay 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 decay 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 decay is common in secondary-school and first-year university syllabi. It links to neighbouring topics Laser science, so understanding it makes those chapters shorter.
In everyday life: Look for Optical decay 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 decay in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Optical decay 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 decay out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Optical decay in simple terms?

Why does Optical decay 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 decay?

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 decay.