Lasers

In an important phenomenon that can occur when light and matter interact is the phenomenon known as light amplification by stimulated emission of radiation or laser (informally, ``laser'' is used as a noun, so the verb would be ``lasing''). If a molecule (or atom) is excited to a state of energy $E_1$ it can decay back down to the ground state $E_0$ and emit a photon of frequency $\nu_{01} = (E_1-E_0)/h$. The phase of the emitted photon will generally be random so that if many molecules or atoms in the state $E_2$ emit photons, the phases will, on average, have a random distribution leading to ordinary light. A second emission process called stimulated emission can also operate between these two energy states leading to photons that have the same phase and travel in the same direction. This is laser light.

If a substance has energy states $E_0$ and $E_1$, and we subject it to radiation of frequency $\nu_{01}$, then each time a photon of this radiation is absorbed, a molecule will be excited to the state $E_1$. If it is possible to have more molecules in the excited state than in the ground state, we achieve a phenomenon known as ``population inversion''. Moreover, if we can trap the beam in the sample and simply pass it back and forth many times, then the intensity of the beam will be amplified, giving the properties of laser light. The population inversion is usually achieved by ``pumping'' in extral thermal energy or by a technique known as ``optical pumping''. The laser cavity is equipped with two mirrors at either end of the cavity. When the first few photons are emitted from the material, they stimulate the emission of more photons, and the resulting wave is reflected back and forth within the cavity, stimulating more photons as it passes through the material. The mirrors ensure that the photons are all traveling in the same direction (photons not traveling along the axis joining the mirrors is simply not amplified) and eventually, their phases come into alignment. If one of the mirrors is only partially reflecting, then some of the coherent light can escape and be used for applications. The basic principle of lasing is illustrated in the figure below:

Figure: Illustration of optical pumping and the generation of laser light.

Lasers have an enormous range of applications. They are used in various types of spectroscopic experiments in chemistry and physics, as well as laser cooling and trapping of atoms. They also have everyday uses, such as CD and DVD players, laser points, and barcode scanners. In industry, they can be used for welding and cutting. In medicine, laser surgery is becoming increasingly popular.