Atoms, Molecules and Optical Physics 2 pp 71-134 | Cite as
Coherence and Photons
Abstract
After the previous extensive exploration into the wave character of light, the present chapter focuses on its particle properties and on the statistical properties of photons. In Sect. 2.1 concepts such as “quasi-monochromatic” and “partially coherent” light will be defined and exemplified by simple models for a laser and a classical light source. We shall familiarize ourselves with the fundamental experiments, beginning with the famous “Hanbury Brown-Twiss experiment”. In Sect. 2.2 we shall try to find a pragmatic approach to the quantum mechanical description of photon states – giving an introduction for “pedestrians” so to say. Finally, we shall in Sect. 2.3 apply the new tools to the theory of absorption and emission of light – this time with explicit consideration of the quantum nature of photons. This will allow us for the first time to derive the basic formulas for spontaneous emission – as opposed to the previous, hand waving introduction of this inherently quantum mechanical phenomenon.
Keywords
Spontaneous Emission Coherence Time Photon State Temporal Coherence Angular DiameterNotes
Acronyms and Terminology
‘In statistical thermodynamics defined as the amount of energy or work that is necessary to change the number of particles of a system (by 1) without disturbing the equilibrium of the system’, see μ in Sect. 1.3.4, Vol. 1.
‘Continuous wave’, (as opposed to pulsed) light beam, laser radiation etc.
‘Electric dipole’, transitions induced by the interaction of an electric dipole with the electric field component of electromagnetic radiation.
‘Electric quadrupole’, transitions induced by the interaction of a quadrupolar charge distribution with the electromagnetic radiation field.
‘Electron paramagnetic resonance’, spectroscopy, also called electron spin resonance ESR (see Sect. 9.5.2 in Vol. 1).
‘European southern observatory’, in Chile, hosting four of today’s largest telescopes of the world, with 8.5 m diameter each.
‘electrostatic units’, old system of unities, equivalent to the Gauss system for electric quantities (see Appendix A.3 in Vol. 1).
‘Fabry-Pérot interferometer’, for high precision spectroscopy and laser resonators (see Sect. 6.1.2 in Vol. 1).
‘Full width at half maximum’.
‘Hanbury Brown and Twiss’, experiment, to determine the lateral correlation of light by a second-order interferometric measurement (see Sect. 2.1.6).
‘Infrared’, spectral range of electromagnetic radiation. Wavelengths between \(760\operatorname{nm}\) and \(1\operatorname{mm}\) according to ISO 21348 (2007).
‘Magnetic dipole’, transitions induced by the interaction of a magnetic dipole with the magnetic field component of electromagnetic radiation.
‘Near infrared’, spectral range of electromagnetic radiation. Wavelengths between \(760\operatorname{nm}\) and Open image in new window
according to ISO 21348 (2007).
‘Nuclear magnetic resonance’, spectroscopy, a rather universal spectroscopic method for identifying molecules (see Sect. 9.5.3 in Vol. 1).
‘Quantum electrodynamics’, combines quantum theory with classical electrodynamics and special relativity. It gives a complete description of light-matter interaction.
‘Radio frequency’, range of the electromagnetic spectrum. Technically, one includes frequencies from \(3\operatorname{kHz}\) up to \(300\operatorname{GHz}\) or wavelengths from \(100\operatorname{km}\) to \(1\operatorname{mm}\); ISO 21348 (2007) defines the RF wavelengths from \(100\operatorname{m}\) to \(0.1\operatorname{mm}\); in spectroscopy RF usually refers to \(100\operatorname{kHz}\) up to some \(\operatorname{GHz}\).
‘Rotating wave approximation’, allows to solve the coupled equations for a two level system in a strong electromagnetic field in closed analytical form (see Sect. 10.2.3).
‘Second harmonic generation’, doubling of a fundamental frequency, for infrared or visible light typically by methods of nonlinear optics.
‘Ultraviolet’, spectral range of electromagnetic radiation. Wavelengths between \(100\operatorname{nm}\) and \(400\operatorname{nm}\) according to ISO 21348 (2007).
‘Visible’, spectral range of electromagnetic radiation. Wavelengths between \(380\operatorname{nm}\) and \(760\operatorname{nm}\) according to ISO 21348 (2007).
‘Very long baseline interferometry’, worldwide network of radio telescopes for interferometry.
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