Basics of Lasers in Dermatology

Lasers have become a critical part of the dermatologist’s armamentarium for modulating cutaneous biology, both in treating skin disorders and providing tangible cosmetic alterations to the skin. Although advances in technology and convenient user interfaces have made modern lasers relatively straightforward to use, they are in fact quite complex and powerful instruments that are capable of considerable damage if not used correctly. Thus it is necessary to establish a framework for the safe and responsible use of lasers in dermatology; fundamental to this tenet is an understanding of the development and physics of lasers. In this article, the fundamental concepts of lasers as well as their interactions with the skin will be discussed to impart a working knowledge of lasers to allow for better, safer use of these important tools.

Development of Lasers

The term laser is an acronym for “light amplification by the stimulated emission of radiation.” Albert Einstein established the framework for the functioning of lasers in his seminal work, “On the Quantum Theory of Radiation,”¹ in which he described how an electron in an atom in an excited state can return to a lower state by emitting energy in the form of a photon of light. Light comprises a portion of the electromagnetic spectrum, rangingfrom UV (200–400 nm) to visible (400 to about 700 nm) to infrared light (about 700 to >3000 nm). The unique properties of light that affect the function of lasers include reflection (eg, seeing a mirror image of a mountain on the surface of a still lake) and refraction (eg, your hand looking larger under the surface of a pool of water).

Despite early theories on lasers, it was not until the late 1950s that the technology finally started to catch up to the science. Researchers experimenting with microwave fields were able to generate a beam of excited ammonia molecules through a resonant cavity, resulting in a uniform (albeit low power) emission of radiation.² Maiman³ expounded on this development by building the first working prototype of a device that radiated light without the use of a microwave. So how exactly do lasers work?

Basic Physics of Lasers

To understand how lasers work, one must have a rudimentary understanding of quantum mechanics. Bohr⁴ revealed that an atom is comprised of a nucleus that is orbited by electrons at discrete distances (ie, only at specific radii), which have corresponding energy levels that increase as the distance from the nucleus increases. With the application of energy, an electron may be excited to a higher energy level, thus increasing its distance from the nucleus, but will then spontaneously return to the lower energy level. By the law of conservation of energy, the excess energy is released as a photon. Although this small amount of energy would not be of much interest at the single particle level, Einstein and Bose discovered that photons were uniquely “gregarious” with the tendency to join together in a common state, leading to the ability to generate a coherent beam of light by simultaneously exciting multiple atoms and their electrons, whereby the return of one electron to a lower energy state generated a chain reaction among the other excited electrons, subsequently prompting the release of photons with the same characteristics as the initial incident photon and returning to a lower energy state.⁵ This process requires several steps to occur in order. First, absorption of energy has to occur among a population of atoms, thus exciting the electrons to higher energy states. When one of the electrons returns to a lower energy level, spontaneous emission will occur with the release of a photon of light. The photon has a certain probability of colliding with other atoms, thereby causing their electrons to return to a lower energy state and release additional photons of light with the same wavelength and in the same direction as the incident photon in a process that is referred to as stimulated emission.⁶ When this process occurs in a cavity with a large number of atoms, the result may lead to a high-energy beam of photons, which becomes the laser beam.

There are some caveats to consider regarding electron population dynamics as outlined by the Boltzmann principle whereby only a small proportion of molecules are in the first excited state and the vast majority are in the ground state (lowest energy) at any given time, but the details of higher-energy transitions in quantum mechanics are beyond the scope of this article.⁷ Primarily, it is important to understand that the ultimate power of a laser’s output depends largely on the population of electrons that are residing at a higher energy state at any given point in time, and the goal of many types of lasers is to achieve a large number of electrons in a high-energy state as opposed to their usual ground state, a process known as population inversion.⁸