By harnessing the power of light through lasers, you can use lasers for a variety of purposes and understand them better by studying the underlying physics and chemistry that makes them work.
Generally, a laser is produced by a laser material, be it solid, liquid or gas, that gives off radiation in the form of light. As an acronym for "light amplification by stimulated emission of radiation," the method of stimulated emissions shows how lasers differ from other sources of electromagnetic radiation. Knowing how these frequencies of light emerge can let you harness their potential for various uses.
Lasers can be defined as a device that activates electrons to emit electromagnetic radiation. This laser definition means radiation can take the form of any kind on the electromagnetic spectrum, from radio waves to gamma rays.
Generally the light of lasers travels along a narrow path, but lasers with a broad range of emitted waves are possible, too. Through these notions of lasers, you can think of them as waves just like ocean waves on the seashore.
Scientists have described lasers in terms of their coherence, a feature that describes whether the phase difference between two signals is in step and they have the same frequency and waveform. If you imagine lasers as waves with peaks, valleys and troughs, the phase difference would be how much one wave isn't quite in sync with another or how far apart the two waves would be from overlapping.
The frequency of light is how many wave peaks pass through a given point in a second, and the wavelength is the entire length of a single wave from trough to trough or from peak to peak.
Photons, individuals quantum particles of energy, make up the electromagnetic radiation of a laser. These quantized packets mean that the light of a laser always has the energy as a multiple of the energy of a single photon and that it comes in these quantum "packets." This is what makes electromagnetic waves particle-like.
How Laser Beams are Made
Many types of devices emit lasers, such as optical cavities. These are chambers that reflect the light from a material that emits electromagnetic radiation back to itself. They're generally made of two mirrors, one at each end of the material such that, when they reflect light, the beams of light become stronger. These amplified signals exit through a transparent lens on the end of the laser cavity.
When in the presence of an energy source, such as an external battery that supplies current, the material that emits electromagnetic radiation emits the light of the laser at various energy states. These energy levels, or quantum levels, depend on the source material itself. Higher energy states of electrons in the material are more likely to be unstable, or in excited states, and the laser will emit these through its light.
Unlike other lights, such as the light from a flashlight, lasers give off light in periodic steps with itself. That means the crest and trough of each wave of a laser line up with those of the waves that come before and after, making their light coherent.
Lasers are designed this way such that they give off light of specific frequencies of the electromagnetic spectrum. In many cases, this light takes the form of narrow, discrete beams that the lasers emit at precise frequencies, but some lasers do give off broad, continuous ranges of light.
One feature of a laser powered by an external energy source that may occur is a population inversion. This is a form of stimulated emission, and it occurs when the number of the number of particles in an excited state outnumber the ones in a lower level energy state.
When the laser achieves population inversion, the amount of this stimulated emission that light can create will be greater than the amount of absorption from the mirrors. This creates an optical amplifier, and, if you place one inside a resonant optical cavity, you've created a laser oscillator.
These methods of exciting and emitting electrons form the basis for lasers being a source of energy, a laser principle found in many uses. The quantized levels that electrons can occupy range from low energy ones that don't require much energy to be released and high energy particles that stay close and tight to the nucleus. When the electron releases due to the atoms colliding with each other in the right orientation and energy level, this is spontaneous emission.
When spontaneous emission occurs, the photon emitted by the atom has a random phase and direction. This is because the Uncertainty Principle prevents scientists from knowing both the position and momentum of a particle with perfect precision. The more you know of a particle's position, the less you know of its momentum, and vice versa.
You can calculate the energy of these emissions using the Planck equation
for an energy E in joules, frequency ν of the electron in s-1 and Planck's constant h = 6.63 × 10-34 m2 kg / s. The energy that a photon has when being emitted from an atom can also be calculated as a change in energy. To find the associated frequency with this change in energy, calculate ν using the energy values of this emission.
Categorizing Types of Lasers
Given the broad range of uses for lasers, lasers can be categorized based on purpose, type of light or even the materials of the lasers themselves. Coming up with a way to categorize them needs to account for all of these dimensions of lasers. One way of grouping them is by the wavelength of light they use.
The wavelength of a laser's electromagnetic radiation determines the frequency and strength of energy they use. A greater wavelength correlates with a smaller amount of energy and a smaller frequency. In contrast, a greater frequency of a beam of light means it has more energy.
You can also group lasers by the nature of the laser material. Solid state lasers use a solid matrix of atoms such as neodymium used in the crystal Yttrium Aluminum Garnet that houses the neodymium ions for these types of laser. Gas lasers use a mixture of gases in a tube like helium and neon that create a red color. Dye lasers are created by organic dye materials in liquid solutions or suspensions
Dye lasers use a laser medium that is usually a complex organic dye in liquid solution or suspension. Semiconductor lasers use two layers of semiconductor material that can be built into larger arrays. Semiconductors are materials that conduct electricity using the strength between that of an insulator and a conductor that use small amounts of impurities, or chemical introduced, because of introduced chemicals or changes in temperature.
Components of Lasers
For all their different uses, all lasers use these two components of a source of light in the form of solid, liquid or gas that gives off electrons and something to stimulate this source. This can be another laser or the spontaneous emission of the laser material itself.
Some lasers use pumping systems, methods of increasing the energy of particles in the laser medium that let them reach their excited states to make population inversion. A gas flash lamp can be used in optical pumping that carries energy to the laser material. In cases where the laser material's energy relies on collisions of the atoms within the material, the system is referred to as collision pumping.
The components of a laser beam also vary in how long they take to deliver energy. Continuous wave lasers use a stable average beam power. With higher power systems, you can generally adjust power, but, with lower power gas lasers like the helium-neon lasers the power level is fixed based on the content of the gas.
The helium-neon laser was the first continuous wave system and is known to give off a red light. Historically, they used radio frequency signals to excite their material, but nowadays they use a small direct current discharge between electrodes in the tube of the laser.
When the electrons in helium are excited, they give off energy to neon atoms through collisions that create a population inversion among the neon atoms. The helium-neon laser can also function in a stable manner at high frequencies. It's used in aligning pipelines, surveying and in X-rays.
Argon, Krypton and Xenon Ion Lasers
Three noble gases, argon, krypton and xenon, have shown use in laser applications across dozens of laser frequencies that span ultraviolet to infrared. You can also mix these three gases with each other to produce specific frequencies and emissions. These gases in their ionic forms let their electrons become excited by colliding against one another until they achieve population inversion.
Many designs of these kinds of lasers will let you select a certain wavelength for the cavity to emit to achieve the desired frequencies. Manipulating the pair of mirrors within the cavity can also let you isolate singular frequencies of light. The three gases, argon, krypton and xenon, allow you to choose from many combinations of light frequencies.
These lasers produce outputs that are highly stable and don't generate much heat. These lasers show the same chemical and physical principles that are used in lighthouses as well as bright, electric lamps like stroboscopes.
Carbon Dioxide Lasers
Carbon dioxide lasers are the most efficient and effective of continuous wave lasers. They function using an electrical current in a plasma tube that has carbon dioxide gas. The electron collisions excite these gas molecules that then give off energy. You can also add nitrogen, helium, xenon, carbon dioxide and water to produce different laser frequencies.
When looking at the types of a laser that may be used in different ares, you can determine which ones can create large amounts of power because they have a high efficiency rate such that they use a significant proportion of the energy given to them without letting much go to waste. While helium-neon lasers have an efficiency rate of less than .1%, the rate for carbon dioxide lasers is about 30 percent, 300 times that of helium-neon lasers. Despite this, carbon dioxide lasers need special coating, unlike helium-neon lasers, to reflect or transmit their appropriate frequencies.
Excimer lasers use ultraviolet (UV) light that, when first invented in 1975, attempted to create a focused beam of lasers for precision in microsurgery and industrial microlithography. Their name comes from the term "excited dimer" in which a dimer is the product of gas combinations that are electrically excited with an energy level configuration that creates specific frequencies of light in the UV range of the electromagnetic spectrum.
These lasers use reactive gases like chlorine and fluorine alongside amounts of noble gases argon, krypton and xenon. Physicians and researchers are still exploring their uses in surgical applications given how powerful and effective they can be used for eye surgery laser applications. Excimer lasers don't generate heat in the cornea, but their energy can break intermolecular bonds in corneal tissue in a process called "photoablative decomposition" without causing unnecessary damage to the eye.
- Always use eye protection when working around lasers. A laser can easily burn your retinas and blind you, and the higher power lasers can burn the skin as well.
About the Author
S. Hussain Ather is a Master's student in Science Communications the University of California, Santa Cruz. After studying physics and philosophy as an undergraduate at Indiana University-Bloomington, he worked as a scientist at the National Institutes of Health for two years. He primarily performs research in and write about neuroscience and philosophy, however, his interests span ethics, policy, and other areas relevant to science.