How lasers work

Lasers’ striking advantages in medicine rely on two special characteristics: they emit light in a slender high-intensity beam and their light is confined to one color of incredible purity. To better understand how lasers work, we first will consider the structure of matter at the atomic level.

All matter, including all body tissues and fluids, is composed of atoms. Atoms consist of central nuclei surrounded by electrons, tiny particles with a negative electrical charge. The nuclei themselves consist of protons, which have a positive electrical charge, and neutrons, which are electrically neutral. Each atom is electrically neutral overall: its negative and positive electrical charges sum to zero because it has the same number of electrons as protons. Its number of neutrons can vary. By specifying the number of protons in an atom, we also specify which chemical element it corresponds to.

Atoms are sometimes drawn with the electrons shown orbiting around the nucleus, just as the Moon orbits around the Earth. This picture is an oversimplification, however. A better image would be to think of the positionsof the electrons as being determined by a diffuse cloud called an orbital around the nucleus. Electrons can inhabit many different types of orbitals, and each type has a characteristic energy and shape. We are concerned with the organization of electrons in an atom or molecule because each electron has energies characteristic of the orbital it inhabits. These orbitals and their energies depend upon the chemical elements with which the electrons are associated and their organization into molecules.

The fact that electrons can have only restricted energies means that an electron can only gain or lose energy by steps. These steps, called quanta (singular, quantum), are determined by the difference, ΔE, between electron energies. The fact that the electron’s energy can only change in steps has several consequences. First, the electrons can only exchange energy with their environment by the values of ΔE. Electrons can absorb energy and be promoted to a higher level, a process called excitation. This can happen by the absorption of a photon of light or through collisions with other atoms. The photon must have exactly the right energy, ΔE, and corresponding wavelength to promote the electron to a new energy level, no more and no less. Once excited, the electron eventually will fall back down spontaneously to a lower energy orbital, usually by emitting a second photon also having energy ΔE. The above system can either absorb or emit photons with energies exactly equal to one of these energy differences. Thus, the electrons belonging to an atom will absorb and emit light at only particular wavelengths. These wavelengths are a fingerprint of the atom’s chemical element. If a gas of an element is excited so that it gives off photons by this process, a characteristic color will result. For example, the sodium vapor lamps used in street lights are yellowish because the ΔE values of emitted photons lie in the yellow. For similar reasons, if one shines light of all wavelengths through a gas of sodium atoms, only yellow light of the same wavelengths will be absorbed. Neon lamps emit very pure red light, and neon vapors absorb the same wavelength in the red.

Electrons can be excited to higher energy states by absorbing energy through a number of processes, such as collisions in a gas, the flow of an electrical current, or the absorption of photons. Once an electron reaches an excited state, it eventually gives off a photon and decays to its ground state. This process is called spontaneous emission. Ordinary incandescent lightbulbs give off light by spontaneous emission. They emit light of many wavelengths because no particular energy spacing is selected for, so electrons are excited into many different energy levels and decay into a variety of lower energy states. As they do so, they emit photons with many different energies and wavelengths. The photons are emitted from the lightbulb uniformly in all directions since no physical limitation confines their directions. However, spontaneous emission is only one way by which an excited electron can emit light. Another possibility exists: the excited electron can interact with a pre-existing photon that just happens to be passing by. If the photon’s energy coincides with the spacing between energy levels in the atom, it induces the electron to decay. Upon decaying, the electron emits a new photon. This process is called stimulated emission because the first photon initiated the emission of the second. Stimulated emission occurs only when the first photon has the same energy as the difference between the ground and excited states. Hence, the second photon has the same energy, wavelength, and color as the first photon.

In addition, a more complete physical picture of this process shows that the emitted photon also travels in the same direction and exactly in step with the first photon. Physicists describe this by saying the second photon is in phase with the first. This synchronization in energy and time gives rise to temporal coherence; among other things, it means that laser light is emitted essentially at a single, extremely pure wavelength.

Both stimulated and spontaneous emission take place all the time in ordinary light sources, but spontaneous emission is more common. Why? An electron in an atom stays excited for very short times, ordinarily. It generally emits a photon before encountering a photon with the right energy for stimulated emission. The atoms in a system are much more likely to absorb a photon (or just ignore it altogether) than to undergo stimulated emission. For stimulated emission to happen frequently, the atoms and their electrons must be carefully prepared. Many more electrons must be held in an excited state than in the ground state. This ensures that there are few electrons able to absorb the incoming photons, and many ready to be stimulated into releasing a new photon. Only then is there a fighting chance for the less frequent process of stimulated emission to take place. This situation is called a population inversion, and it requires a means for keeping electrons stuck in an excited state for a long time.

We call the process by which electrons are pushed into the relatively long-lasting excited state pumping. To understand this phenomenon further, let us study the specific case of the red helium neon laser. This laser consists of a glass tube filled with a mixture of gases of the chemical elements helium and neon. The gas is called the active medium. The helium and neon gases’ atoms are heated by an electrical current. Collisions in the hot gases excite the helium atoms’ electrons into many different energy states, most of them short-lived. One state is very long-lived and electrons get stuck there for quite a while. We say this state is metastable and cannot easily decay by spontaneous emission of light. These helium atoms’ electrons can escape to the ground state by transferring their energy to a neon atom’s electrons through a collision. This excites the neon electron from its ground state. This is because, purely by chance, neon has an energy level almost equal to the helium energy level with slow decay. This coincidental agreement is taken advantage of in order to promote the neon atoms’ electrons to an excited state. Now, many electrons bound to neon atoms are pumped into a particular excited energy state, many more than are in the state to which it decays. This represents the desired condition for a population inversion. Eventually an electron belonging to a neon atom spontaneously decays and emits a photon. This event sets off a chain reaction whereby the spontaneously emitted photon can go on to cause the stimulated emission of a second photon. Then both photons can go on to create even more photons through stimulated emission. Soon many identical photons are coursing through the gas, all with the same energy and wavelength. In order to make a laser out of this situation, the tube containing the gas of helium and neon atoms is placed into a configuration called an optical resonating cavity. This can be thought of as two parallel mirrors placed a fixed distance apart. Now, each time a photon of light passes through the active medium, it induces more stimulated emissions. These photons travel in the same direction as the first. However, if a photon travels at an angle to the mirrors, it will reflect outside the cavity and fail to return. Only photons traveling straight down the optical resonating cavity can make multiple passes. This restriction maintains the directionality of the lasers and is responsible for their spatial coherence and narrow beam. Each successful photon enlists many other photons, each with the same energy and phase as the first, each traveling in the same direction. This mechanism for multiplying the number of photons is called amplification or gain. A typical value for the gain is 5% more photons for each trip. In 100 passes down the tube, the photons increase their numbers by 130 times and in 200 passes by 17,000 times. If one of the mirrors is only partially silvered, transmitting only 1 % or 2 % of the light hitting it, some of this light escapes out the end. This small loss is not enough to stop the process. This escaping light is what you see emerging as the slender, highly parallel laser beam. The laser’s light is a single wavelength due to stimulated emission. The light travels in a single direction because of the optically resonating cavity and the tendency of stimulated emission to create photons in phase with one another. The power lost as the laser’s beam escapes from the optical resonating cavity must be replaced by electrical power. In the helium neon laser, an lectrical current is used to constantly heat the atoms in the gases, creating a constant supply of newly excited electrons and a steady intensity of laser light. In other systems, a white light, called a flashlamp, or even another laser is used to pump the excited electrons. Electrical energy sources power the flashlamp and hence the laser.

`The preceding description was given for a red helium neon laser. Other lasers work using similar processes that take place in other gases. For example, the carbon dioxide (infrared), argon ion (green), krypton (red), and helium cadmium (blue) lasers all utilize similar mechanisms taking place in the gases that give them their names. Because each atom or molecule has a different set of energy spacings, each laser also has a characteristic wavelength and color. The active medium need not be a gas. Examples of lasers with solid active media are diode lasers, used in various medical applications, compact disc players, and supermarket scanner devices. Dye lasers instead use a liquid active medium, a solution of large organic dye molecules in water, which allow them to emit light at a variety of of the resonating cavity can be adjusted to select only one wavelength for operation at any time. The yellow light from dye lasers is extremely useful in dermatology, for example. Now that we have a brief background for appreciating the special properties of lasers, we will return to an investigation of just how those properties are turned into medical applications.