Nonlinear Absorption
In physics, nonlinear absorption is the process in which a material absorbs energy from an incident wave but does not re-emit that energy as a new wave. Instead, the material stores some of the energy and re-emits it at a lower frequency. The effect is similar to what happens when light shines on a piece of glass: some of the light is transmitted through the glass, while some is reflected off the surface. However, unlike reflection, nonlinear absorption doesn’t require a surface; it can happen deep within a material.
Nonlinear absorption was first observed in 1853 by British physicist David Brewster (1781-1868). He was studying double refraction, in which light passing through certain crystals is split into two beams that vibrate in different directions (a phenomenon known as birefringence). While investigating calcite under high magnification, Brewster noticed that when he placed his eye close to the crystal, one of the beams disappeared. This led him to conclude that something inside the crystal was absorbing energy from the beam and re-emitting it at another wavelength.
It wasn’t until nearly 100 years later that scientists realized what was happening at a molecular level. In 1937, German physicist Fritz Zernike (1888-1966) developed a theory that explained how molecules could absorb and re-emit light without changing their overall shape. He showed that when light hits a molecule, it can cause electrons within the molecule to move around or change their energy levels. These changes create new states within the molecule that are lower in energy than the original state. As these excited molecules return to their ground state, they emit photons with lower energies than those of the incident photons. In other words, they absorb high-energy photons and emit low-energy photons – leading to nonlinear absorption.
Today, we know that nonlinear absorption can occur in many different materials – including semiconductors, metals, liquids, gases, and even biological tissues – under various conditions such as intense laser irradiation or strong electrical fields. The degree of nonlinearity varies widely depending on factors such as wavelength (shorter wavelengths are more likely to be absorbed), intensity (higher intensities lead to greater absorption), duration (the longer exposure time ,the greater chance for molecules to become excited and return to their ground state), and temperature (lower temperatures increasenonlinearity). Additionally, some materials may exhibit negative nonlinearity under certain conditions; this means they actually amplify incident light instead of absorbing it . Nonlinear effects are used extensivelyin optical fibers for telecommunications applications such as fiber lasers and amplifiers