Nonlinear and Quantum Optics
Nonlinear optics treats the response of matter to intense light, while quantum optics treats light's discrete, quantum nature; together they govern modern photonics.
Definition
The combined study of the nonlinear optical response of matter to intense light and of the quantized, non-classical properties of the electromagnetic field and its interaction with matter.
Scope
This area combines two closely related advanced branches of optics. Nonlinear optics studies how the polarization of a medium responds nonlinearly to strong optical fields, producing effects such as harmonic generation, frequency mixing, and intensity-dependent refraction that are absent at low intensity. Quantum optics studies the quantized electromagnetic field, the photon, and non-classical states of light, together with the quantum nature of light-matter interaction. The area covers the second- and third-order nonlinear susceptibilities and the processes they enable, parametric and frequency-conversion devices, photon statistics and squeezed and entangled light, and the quantum treatment of emission and absorption. It supplies the physics behind laser frequency conversion, quantum information, and precision measurement.
Sub-topics
Core questions
- How does a medium's optical response become nonlinear at high intensity?
- How can light at one frequency generate light at new frequencies?
- What does it mean to treat light as quantized photons?
- How do non-classical states of light differ from ordinary light?
Key concepts
- nonlinear susceptibility
- second-harmonic generation
- parametric processes
- phase matching
- photon
- coherent and squeezed states
- photon statistics
- entangled photons
Key theories
- Nonlinear optical susceptibility
- At high intensity the induced polarization of a medium contains terms proportional to higher powers of the field, with second- and third-order susceptibilities giving rise to harmonic generation, frequency mixing, and intensity-dependent refraction.
- Quantization of the electromagnetic field
- Treating each mode of the field as a quantum harmonic oscillator yields photons and a hierarchy of states, including coherent, number, squeezed, and entangled states with no classical counterpart.
Clinical relevance
Nonlinear optical processes enable multiphoton and second-harmonic microscopy that image living tissue with intrinsic contrast and deep penetration, and they provide the green and other wavelengths of frequency-converted surgical and ophthalmic lasers; quantum-optical methods promise enhanced-sensitivity imaging and sensing.
History
Nonlinear optics began in 1961 when Franken and colleagues observed second-harmonic generation soon after the laser made intense coherent light available, and Bloembergen developed its theoretical framework. In parallel, the Hanbury Brown and Twiss experiments of the 1950s and Glauber's quantum theory of optical coherence in 1963 founded quantum optics.
Key figures
- Nicolaas Bloembergen
- Peter Franken
- Roy J. Glauber
- Robert Hanbury Brown
Related topics
Seminal works
- boyd2020
- loudon2000
Frequently asked questions
- Why do nonlinear effects require a laser?
- Nonlinear responses grow with field strength and are negligible at everyday intensities; only the intense, coherent fields of lasers drive the higher-order polarization terms strongly enough to observe effects such as harmonic generation.
- What is a photon in quantum optics?
- A photon is a single quantum of excitation of a mode of the electromagnetic field; quantum optics describes light in terms of these quanta and the special states they can form, which classical wave optics cannot capture.