physics

Optical microcavity

Optical microcavity 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 microcavity rather than just read about it. In short: An optical microcavity or microresonator is a structure formed by reflecting faces on the two sides of a spacer layer or optical medium, or by wrapping a waveguide in a circular fashion to form a ring. The former type is a standing wave cavity, and the latter is a traveling wave cavity.

Key takeaways

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

Reference excerpt

An optical microcavity or microresonator is a structure formed by reflecting faces on the two sides of a spacer layer or optical medium, or by wrapping a waveguide in a circular fashion to form a ring. The former type is a standing wave cavity, and the latter is a traveling wave cavity. The name microcavity stems from the fact that it is often only a few micrometers thick, the spacer layer sometimes even in the nanometer range. As with common lasers, this forms an optical cavity or optical resonator, allowing a standing wave to form inside the spacer layer or a traveling wave that goes around in the ring.

Applications and effects The fundamental difference between a conventional optical cavity and microcavities is the effects that arise from the small dimensions of the system, but their operational principle can often be understood in the same way as for larger optical resonators. Quantum effects of the light's electromagnetic field can be observed. For example, the spontaneous emission rate and behaviour of atoms is altered by such a microcavity, a phenomenon that is referred to as inhibited spontaneous emission. One can imagine this as the situation that no photon is emitted, if the environment is a box that is too small to hold it. This leads to an altered emission spectrum, which is significantly narrowed. Moreover, nonlinear effects are enhanced by orders of magnitude due to the strong light confinement, leading to the generation of microresonator frequency combs, low-power parametric processes such as down-conversion, second-harmonic generation, four-wave mixing and optical parametric oscillation. Several of these nonlinear processes themselves lead to the generation of quantum states of light. Another field that harnesses the strong confinement of light is cavity optomechanics, where the back-and-forth interaction of the light beam with the mechanical motion of the resonator becomes strongly coupled. Even in this field, quantum effects can start playing a role. Microcavities have many applications, frequently at present in optoelectronics, where vertical cavity surface emitting lasers VCSEL are probably the best known. Recently, a single photon emitting device was demonstrated by placing a quantum dot in a microcavity. These light sources are interesting for quantum cryptography and quantum computers. An overview is given in the review article published in the journal Nature.

Types

Standing-wave For a microcavity supporting a single-mode or a few standing-wave modes, the thickness of the spacer layer determines the so-called "cavity-mode", which is the one wavelength that can be transmitted and will be formed as a standing wave inside the resonator. Depending on the type and quality of the mirrors, a so-called stop-band will form in the transmission spectrum of the microcavity, a long range of wavelengths, that is reflected and a single one being transmitted (usually in the centre). There are different means of fabricating standing-wave microcavities, either by evaporating alternating layers of dielectric media to form the mirrors (DBR) and the medium inside the spacer layer or by modification of semiconductor material or by metal mirrors.

Traveling-wave Often just called "microresonators", traveling wave microcavities have a wave going around in a loop-like fashion in a preferred direction, depending on the input light direction. They can be in the form of whispering-gallery resonators, or as integrated ring resonators. Typical materials from which they are made could be semiconductors like Silicon, Silicon dioxide, silicon nitride, crystalline fluorides (CaF2, MgF2, SrF2) or lithium niobate. The material is chosen such that it is low-loss and transparent in the wavelength of application desired. Typically, such structures are fabricated by either diamond turning or micromachining a cylindrical rod of a material (especially for fluorides and lithium niobate), or by photolithography and electron-beam lithography to produce a patterned resonator on chip (for silicon-based materials). When an integer number of wavelengths in the material fits in the circumference of the resonator, a resonant wave is excited by constructive interference. At resonance, the light field can be enhanced by several hundred to several million times, quantified by the Finesse Coefficient of the resonator. This also leads to an ultrahigh quality factor, meaning that light travels around the circumference many million times before decaying into the surroundings.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Optical microcavity

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

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

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

Frequently asked questions

What is Optical microcavity in simple terms?

Why does Optical microcavity 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 microcavity?

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 microcavity.