Criminal masterminds Auric Goldfinger and Dr. Evil were fascinated by amplified light, “a sophisticated heat beam which we called a ‘laser,’” but fortunately in the 21st century, most lasers are used instead for therapeutic purposes. Although the vast array of available lasers can be confusing, laser physics are straightforward, and only a basic understanding of light energy is necessary to understand how lasers work. Light energy can be described as either a series of particles (photons) or as a wave phenomenon. The color of light is determined by the distance between two successive waves (the wavelength, usually measured in nanometers). The human eye can see only a narrow range of the electromagnetic spectrum (visible light), and many lasers produce invisible light in the infrared range.

A molecule or atom in its resting state is composed of a nucleus and circulating electrons. If energy is added to the system, the electrons become excited and circulate at a higher orbit. Eventually, an excited electron will fall back to its resting orbit, releasing a specific packet of energy–a photon. That photon has a wavelength specific to that molecule. Some molecules have more than one excited orbit, and therefore, the light emitted may have more than one wavelength. If a photon collides with an excited electron, that electron falls back to its resting orbit, thereby releasing another photon. The two photons are in phase, meaning their wave patterns are synchronized and therefore reinforce each other. As these photons hit other excited electrons, more photons are released and the light energy increases (Figure 18.1).

A laser tube has a mirror at each end and contains a solid, liquid, or gas medium within it whose electrons are in a resting state. As energy is added to the system, the majority of the electrons become excited (population inversion) and begin to release photons. Only those photons that hit the mirrors directly are reflected back into the lasing medium, creating more and more photons that travel back and forth between the mirrors, parallel to the tube. Since the photons are in phase, the intensity of the light increases in the tube. This phenomenon has been described as light amplification by the stimulated emission of radiation, or the more familiar term LASER. (In contrast, TASER is an acronym for Thomas A. Swift’s Electric Rifle and has nothing to do with light energy.) To allow light to escape from the tube, one of the mirrors is only partially reflecting. The emitted light is coherent; it is in phase, parallel, and in most cases monochromatic. In contrast, incandescent light is noncoherent, meaning it has many wavelengths and is not parallel.

Light energy can be visible or invisible depending upon its wavelength. The spectrum of electromagnetic radiation ranges from long radio waves (wavelength > 10 cm) to extremely short gamma rays (<10^-11 m). The entire spectrum includes radio, microwaves, infrared, visible (400 to 700 nm), ultraviolet, X-ray, and gamma rays.

The laser tube may contain either a gas, liquid, or solid lasing medium (Table 18.1), and new lasers are constantly being invented and promoted to the medical community. The first laser, invented in 1960, used a synthetic ruby rod, and other solid crystal lasers include yttrium aluminum garnet (YAG). The YAG crystal contains neodymium, erbium, or holmium ions, each with its own specific wavelength and tissue interactions. In a dye laser, the medium is a solution of a fluorescent dye in a solvent such as methanol. Organic rhodamine dye is used in the yellow dye laser, and although the earlier dye lasers had adjustable (tunable) wavelengths ranging from yellow to red, currently available dye lasers offer single wavelength light energy. In a helium-neon laser, it is a mixture of the gases helium and neon. In a diode laser, it is a thin layer of semiconductor material sandwiched between other semiconductor layers. Excimer lasers (the name is derived from the terms excited and dimers) use reactive gases, such as chlorine and fluorine, mixed with inert gases such as argon, krypton, and xenon. When electrically stimulated, a pseudomolecule (dimer) produces light in the ultraviolet range.

When the laser strikes an object, a variety of desirable and undesirable effects may result as the light is reflected, scattered, transmitted, or absorbed. A series of reflecting mirrors directs CO₂ laser light to the handpiece, but reflected CO₂ light off of a shiny surgical instrument is hazardous. The risk of inadvertent light reflection can be reduced by using ebonized instruments. The dull finish scatters laser light, diffusing the concentrated energy beam. Glass and clear liquids will transmit some types of laser light, allowing photocoagulation through glass slides, the vitreous of the eye, and water. Some lasers will also pass through the epidermis, allowing the energy to reach dermal vessels and pigment without disrupting the epidermal layer. It is the absorbed light that causes desirable or undesirable biologic effects. Except for the excimer lasers that break chemical bonds, most laser energy is converted into thermal (heat) energy. Depending upon the rate of tissue heating, surgical effects include welding, coagulation, protein denaturation, dessication, and vaporization. Some lasers will indiscriminantly target living tissue, while other lasers will semiselectively target a specific chromophore such as oxyhemoglobin, melanin, and tattoo pigmentation. Selective photothermolysis describes the ability of lasers to target blood vessels or pigment without harming the surrounding epidermis or dermis. It is generally safer to deliver cutaneous laser light in pulses rather than as a continuous beam, as the interval between pulses allows the tissue to cool before the heat is transferred to the surrounding dermis. Pulsed lasers respect the thermal relaxation time of dermal vessels (the time to dissipate the heat absorbed during a laser pulse).