Matter interaction with light

All branches of spectroscopy follow the way that photons of light interact with matter, e.g. by absorption. The main difference between the distinct... 

Polarized light interaction with matter, OPTICAL ROTATION Polyadenylate polymerase,... 

Figure 2.1 Photophysical processes of a beam of light interacting with matter absorption (A), scattering (S), luminescence (L) the rest of the beam is transmitted...

When light interacts with matter, several processes take place, sometimes simultaneously. It can be absorbed, which results in a decrease of the primary intensity, or it can be transmitted without attenuation. The propagation velocity v of light depends on the optical density of the medium. It is related to the absolute (vacuum) velocity of light c by... 

Several other optical techniques that rely on various mechanisms by which light interacts with matter, including absorption, reflection, elastic scattering and autofluorescence, are also being developed for cancer diagnostics. This section will discuss these optical biopsy techniques. 

In this chapter we introduce some of the fundamental concepts needed to understand how light interacts with matter. We start by examining a system of classical charged particles that interacts with a pulse of electromagnetic radiation. We then quantize the particle variables and develop the semiclassical theory of light interacting with quantized particles. The details of the derivations are not required for subsequent chapters. However, the resultant equations [Eqs. (1.50) to (1.52)] form the basis for the theoretical development presented in Chapter 2, which deals with both the interaction of weak lasers with molecules and with photodissociation processes. 

When light interacts with matter, and the photons are not absorbed, it does so by inducing a polarization in the medium. Since the interaction energy between the electric field of the incident radiation and the molecules making up the medium is small compared to the total energy of the molecules, the incident radiation can be treated as a perturbation to the total energy of the medium. (This is true for pulsed laser beams as well as ambient light [13].) Therefore, the polarization of the medium, P, can be expanded as a power series in the electric field [13,14]. 

In addition to behaving as a wave, visible light (and other types of electromagnetic radiation) exhibits the properties of particles such as mass and acceleration (Einstein s observation that energy has mass, or E = me2, applies to the photon.) When light interacts with matter, it does so in discrete packets of energy called photons. The energy e of a photon is proportional to the frequency of the radiation. 

Figure 4.4 Jablonski-type energy diagrams for possible excited energy states when light interacts with matter, (a) Three possible transition pathways for return to ground state without radiation, (b) Two possible transition pathways with fluorescent light emission as final step on return to ground state, (c) Two possible transition pathways with phosphorescent light emission as final step on return to ground state.

Menzel, Ralf. Photonics Linear and Nonlinear Interactions of Imsct Light and Matter. 2d ed. Berlin Springer-Verlag, 2007. For the advanced student, covers how light interacts with matter, both for linear and nonlinear cases. The bibliography contains an enormous set of references. 

Polarized light interacts with matter in the direction of the electric vector. This is the polarization direction. Using this property it is possible to use polarized light to probe different materials and identify them by how they interact with light. 

We will also see how light interacts with matter, or, rather, how it interacts with the atoms and molecules that compose matter. It is this interaction that produces color. 

Fig. 15.8 Typical ways light interacts with matter in a cuvette. The eye in the emission represents the detector location...

Besides intensity and energy, another characteristic of light that can be affected when light interacts with matter and can, thus, be used on the characterization of materials is its polarization. The best known technique that uses the polarization of light to characterize materials is the ellipsometry. In ellipsometry we measure the change on the state of polarization of light upon reflection by or transmission through a sample, which depends on its complex dielectric function. 

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