Physics

Atoms at their resting energy state, or ground state (E⁰), can be excited to a higher energy state (E*) when they absorb electrical, optical, or thermal energy (see first image below). At the E* level, atoms are unstable and spontaneously return to their E⁰ ground state, which liberates the absorbed energy as light or photons. This process is referred to as spontaneous emission of radiation (see second image below).

Atoms at their resting energy state, or ground state (E⁰), can be excited to a higher energy state (E*) when they absorb electrical, optical, or thermal energy.

At the E* level, atoms are unstable and spontaneously return to their E⁰ ground state; this process liberates the absorbed energy as light or photons and is referred to as spontaneous emission of radiation.

If, on their brief descent from E* to E⁰, the excited atoms or molecules at E* are further bombarded with the same energy that caused the initial transition from E⁰ to E* or a proportional amount, the net result is the liberation of an amount of energy twice the original (see image below). Thus, if a photon strikes an atom at E⁰, causing it to go to its E* level, and if a second photon strikes the atom as it returns to E⁰, the atom emits 2 photons of the same frequency.

If, on their brief descent from E* to E⁰, the excited atoms or molecules at E* are further bombarded with the same energy that caused the initial transition from E⁰ to E* or a proportional amount, the net result is the liberation of an amount of energy twice the original.

This emission occurs in phase (coherence) with and in the same direction as the first bombarding photon. This process is called stimulated emission. The 2 emitted photons may then each strike other excited atoms, further stimulating emission of photons with the same phase and frequency. As more atoms are excited to the upper energy level E* to the extent that the number of atoms in the active medium at E* is greater than those at E⁰, a population inversion occurs in the system. This chain reaction rapidly produces a powerful eruption of a coherent beam of radiation—a laser.

Basic Components

The basic laser device consists of 3 components: (1) an active medium, or lasing medium; (2) an optical cavity, or resonator; and (3) an energizing source, or pump. The active medium in lasers may be a solid, liquid, or gas. Different active media emit different energies or wavelengths of light. However, they all operate with the same basic principles.

The resonator contains an active medium. At each end of the resonator, parallel reflectors or mirrors are placed facing each other. The front of the output mirror is designed to be partially reflective. It reflects only a portion of the light impinging on it, allowing some portion of the total energy or light to escape. The rear mirror is a total reflector that reflects 100% of the energy impinging on it. The pump source provides the energy (thermal, electric, or optical [eg, a flash lamp]) for absorption by the active medium.

When the active medium is pumped with sufficient energy, a population inversion occurs, causing the spontaneous emission of photons. Some of these photons are reflected back and forth between the 2 mirrors (others are dissipated as heat) and then collide with atoms in the excited state; these collisions subsequently stimulate the emission of radiation. As other photons collide with excited atoms, energy within the resonator builds and is amplified by reflections between the parallel mirrors. At the front output mirror, a portion of the energy is permitted to escape. This energy is in the form of an intense beam of monochromatic (same wavelength), collimated (parallel, nondiverging), and coherent (same direction) light.

Laser Properties

Power density

When the laser beam exits the resonator, its diameter is often too large and diffuse, and the beam itself may have inadequate power to be useful. Therefore, the laser beam is passed through a focusing lens to reduce its diameter, which increases its intensity and energy so that it is of more suitable size for manipulation and practicality. Its intensity, referred to as its power density (Pd) or irradiance (E), is defined as the energy delivered per unit area of incident tissue. It is measured in terms of wattage of laser per diameter of the beam. That is, Pd varies inversely with the square of the diameter of the laser beam, as follows:

Pd = (100 W)/d², where W is the laser power in watts, and d is the diameter of the laser beam in centimeters (ie, 100W/cm²).

For a given wattage, a wide or unfocused beam has less penetration ability and is more useful for procedures such as skin resurfacing, vaporization of tissue, and coagulation of blood vessels. A focused beam penetrates to a greater depth and is more useful in procedures involving delicate cutting and volume ablation.

Fluence

To accurately determine the total amount of energy delivered to the tissue by the laser, the duration of exposure is vital. Prolonged exposures result in tissue destruction, and too short an exposure results in an inadequate effect. The dose, or fluence, is a measure of the total energy. It is determined by multiplying Pd by the exposure time (t) and is expressed in terms of energy per unit area of incident tissue, as follows:

Fluence = Pd(t) = J/A, where J is the energy in joules and A is the cross-sectional area of beam in square centimeters.

Wavelength

The effect of light on skin depends on the wavelength of the light. Light in the UV region (100-400 nm), which is invisible to the human eye, is known to cause deleterious effects such as erythema, hyperpigmentation, and cutaneous carcinoma. Light energy in the visible spectrum (380-700 nm) is mostly innocuous, but it can be absorbed and cause thermal damage when it is delivered to the skin at a high intensity. Light in the near IR region of the spectrum (780-3000 nm), which is also invisible to the human eye, causes skin and retinal defects. In general, the effects of light in the mid-to-far IR region of the spectrum (3-1000 µm) are limited to the superficial layers.

The degree of absorption and its thermal effect on skin vary with the amount and type of chromophores that are present in the recipient. As stated earlier, hemoglobin and melanin are natural endogenous chromophores. An example of an exogenous chromophore is tattoo ink. Different chromophores have different absorption coefficients. The absorption coefficient is a measure of the degree of absorption by the chromophores at a particular wavelength. Because the laser is monochromatic and because it has a very narrow bandwidth, it permits selective targeting of chromophores in the tissue for treatment. This property is one of the underlying principles of selective photothermolysis (SP).