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Two-photon absorption

Two-photon absorption 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 Two-photon absorption rather than just read about it. In short: In atomic physics, two-photon absorption (TPA or 2PA), also called two-photon excitation or non-linear absorption, is the simultaneous absorption of two photons of identical or different frequencies in order to excite an atom or a molecule from one state (usually the ground state), via a virtual energy level, to a higher energy, most commonly an excited electronic state. Absorption of two photons with the same frequ…

Two-photon absorption — main illustration
Two-photon absorption — illustration

Key takeaways

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

Reference excerpt

In atomic physics, two-photon absorption (TPA or 2PA), also called two-photon excitation or non-linear absorption, is the simultaneous absorption of two photons of identical or different frequencies in order to excite an atom or a molecule from one state (usually the ground state), via a virtual energy level, to a higher energy, most commonly an excited electronic state. Absorption of two photons with the same frequency is called degenerate two-photon absorption, while absorption of two photons with different frequencies is called non-degenerate two-photon absorption. The energy difference between the involved lower and upper states is equal or smaller than the sum of the photon energies of the two photons absorbed. Since TPA depends on the simultaneous absorption of two photons, the probability of two-photon absorption is proportional to the photon dose (D), which is proportional to the square of the light intensity D ∝ I2 thus it is a nonlinear optical process. Two-photon absorption is a third-order process, with absorption cross section typically several orders of magnitude smaller than one-photon absorption cross section. Two-photon absorption was originally predicted by Maria Goeppert-Mayer in 1931 in her doctoral dissertation. Thirty years later, the invention of the laser permitted the first experimental verification of two-photon absorption when two-photon-excited fluorescence was detected in a europium-doped crystal. Soon afterwards, the effect was observed in cesium vapor and then in cadmium sulfide, a semiconductor.

Description

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Illustrations

Two-photon absorption: Schematic of energy levels involved in two photons absorption
Schematic of energy levels involved in two photons absorption
Two-photon absorption: Schematic of energy levels involved in two photons excited fluorescence. First there is a two-photons absorption, followed by one non-radiative deexcitation and a fluorescence emission. The electron returns at ground state by another non-radiative deexcitation. The created pulsation ω 2 {\displaystyle \omega _{2}} is thus smaller than twice the excited pulsation ω 1 {\displaystyle \omega _{1}} .
Schematic of energy levels involved in two photons excited fluorescence. First there is a two-photons absorption, followed by one non-radiative deexcitation and a fluorescence emission. The electron returns at ground state by another non-radiative deexcitation. The created pulsation ω 2 {\displaystyle \omega _{2}} is thus smaller than twice the excited pulsation ω 1 {\displaystyle \omega _{1}} .
Two-photon absorption illustration

Worked examples

Example 1 — a first encounter with Two-photon absorption

Start with the simplest possible case. Write down what Two-photon absorption 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 Two-photon absorption 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 Two-photon absorption 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 Two-photon absorption

In research
Two-photon absorption 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 Two-photon absorption 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
Two-photon absorption is common in secondary-school and first-year university syllabi. It links to neighbouring topics Nonlinear optics, so understanding it makes those chapters shorter.
In everyday life
Look for Two-photon absorption 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 Two-photon absorption in 20 minutes

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

Frequently asked questions

What is Two-photon absorption in simple terms?

In atomic physics, two-photon absorption (TPA or 2PA), also called two-photon excitation or non-linear absorption, is the simultaneous absorption of two photons of identical or different frequencies in order to excite an atom or a molecule from one state (usually the ground state), via a virtual en…

Why does Two-photon absorption 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 Two-photon absorption?

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 Two-photon absorption.

Tags

  • Nonlinear optics

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