Laser (Light Amplification by the Stimulated Emission of Radiation) is based on Einstein’s quantum theory and the concept of stimulated emission of light. In brief, when an atom returns to ground state following excitation, it emits a photon of light at a specific wavelength. If the photon collides with a similarly excited atom, the atom will return to ground state and emit a photon that is synchronized in time and space with the incoming photon. Energy, in the form of intense flashes of light or electrical discharge, is used to excite a population of atoms. The “inversion” of the excited population of atoms creates monochromatic, coherent light which is reflected by mirrors, further stimulating the emission of radiation. One of the mirrors is half-silvered allowing some light to escape, and this is the laser light. There are several excellent reviews of lasers written for physicians/dermatologists [8-10].
There are many different lasers which can be categorized based on the type of lasing material. For medical applications, these are solid, for example, ruby, gas, (e. g., carbon dioxide) liquid (e. g., rhodamine G6), and semiconductors (e. g., diode). The lasing medium determines the wavelength of the light emitted. Further, the light can be continuous or quality switched (Q-switched). Continuous light is relatively low power with long bursts, for example, milliseconds, compared to pulsed or Q-switched, the latter of which emits a
Visible
1064 nm (Nd:YAG)
Figure 23.1 Spectrum of electromagnetic radiation.
high power burst of very short duration, for example, nanoseconds. In general, the longer the wavelength, for example, 1064 nm Nd:YAG versus 694 nm ruby, the deeper the light will penetrate into the skin [11]. Examples of commercially available lasers are presented in Table 23.1. A brief description of such lasers is summarized here.