Editorial Feature

How Do Solid-State Lasers Emit Light?

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Laser stands for light amplification by stimulated emission radiation. In reality lasers do not amplify light, they generate it! A laser beam is a specific type of light that is generated with three critical properties: it is monochromatic, meaning it has only one wavelength, unlike white light which is made up of a spectrum of wavelengths; it is highly directional, unlike light emitted from a light bulb that has many directions; and it is coherent, meaning all the photons share the same frequency and the waves are in phase with one another, unlike light from a torch which is incoherent.

Today, lasers are used in scientific research, military defence, medical imaging, surgical procedures, and in industrial and commercial production; they are used so commonly that it is almost impossible to find a product or medical treatment that does not involve one.

Einstein’s theory of stimulated emission, proposed in 1917, is the basic principle behind laser operation. He theorized that in addition to spontaneously absorbing and emitting light (spontaneous emission), it should be possible to stimulate electrons to emit photons with a specific wavelength. A photon is a single quantum of light; it is an elementary particle of electromagnetic radiation with wave-particle duality. The energy or wavelength that a photon possesses decides the colour of light emitted. As explained before, lasers emit only one wavelength of light and therefore only one colour; thus, unbeknownst to him, Einstein began the quest for the laser.

Einstein’s theory finally saw fruition 37 years later in 1954, in the form of an ammonia maser (microwave amplification by stimulated emission of radiation). The device, constructed by a group at Colombia University led by Charles H. Townes and Herbert J. Zeiger, generated coherent, monochromatic microwaves from the process of stimulated emission. From this point on the idea of building a maser-like device that operated in the optical region began to take off. Townes continued working in this area, as did Nikolai G. Basov and Alexander M. Prokhorov at The Lebedev Physical Institute in Moscow, as well as Gould Gordon, a graduate of Colombia University. Gordon first conceptualised the idea of a laser in 1957, proven by his lab book that was notarized at the time. A famous 30 year patent war ensued, between Gordon and the United States Patent office. Eventually, Gordon was granted with 48 patents to his inventions.

Despite Townes, Basov and Prokhorov winning the 1964 Nobel Prize in Physics for fundamental work in quantum electronics, leading to the construction of oscillators and amplifiers based on the maser-laser principle, Gordon is still renowned by many for the invention of the laser and holds the important patent for an optical pumping set-up.

The first solid-state laser:

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The first working laser was a solid-state laser (SSL), credited to Theodore H. Maiman of Hughes Research Laboratories in 1960. It used a cylinder of synthetic ruby measuring 1 cm in diameter by 2 cm in length and photographic flash lamps to optically pump the ruby, inducing the process of stimulated emission. A deep red colour light, at a wavelength of 694.3 nm, was emitted. In this case the active lasing medium was the ruby, chosen for its broad absorption profile and long florescence lifetime (approximately 3 milliseconds), allowing high energy pumping of the crystal.

There are five laser classes, categorized by their active lasing medium: liquid, gas, plasma, diode and solid-state. Diode lasers also use a solid lasing medium but, crucially, SSLs use a transparent, crystalline solid such as a glass or ceramic material, whereas diode lasers use a semiconductor medium. The optical pumping source surrounds the lasing medium to effectively irradiate it, and excite the electrons. The lasing medium of an SSL is in the optical cavity (or resonator), based on a Fabry–Pérot cavity, consisting of two parallel reflective plates (mirrors).

The light bounces between the two mirrors, amplifying the energy. A portion of the light can leave the optical cavity through the partially reflective mirror, known as the output coupler, emerging as the laser beam. The other portion of light is reflected back into the optical cavity, adding to the chain reaction of energy amplification. This set up ensures the beam is narrow and uni-directional.

Solid-state lasing medium:

The lasing medium is the source of optical gain. In SSLs it is often a crystal doped with rare earth or transition metal ions. The medium is responsible for converting electrical or optical energy into light as a direct result of excitation of the lasing medium. This excitation is usually induced by optical pumping, as is the case of the ruby laser, in which the medium is electronically excited via irradiation of light and population inversion is achieved. Population inversion simply refers to a greater proportion of higher energy levels being occupied than lower energy levels, which is an inversion of the thermal equilibrium state for which the opposite is always true.

Optical pumping is often chosen as the excitation stimulus when the lasing medium is electrically insulating or when certain advantages are desired, such as high power combined with high beam quality (resulting in high brightness). Since most SSL media are electrically insulating crystals, optical pumping is used because it is the only way to supply the ions in the lasing medium with the necessary energy for excitation. For diode lasers however, electrically pumping the lasing medium is possible but not always desirable due to the advantages and simplicity of the optical pumping set-up.

Stimulated emission:

When choosing a suitable crystal for an SSL, the absorption spectrum and fluorescence lifetime is important to consider because the material needs to be able to absorb enough light energy for population inversion to occur. Once the material has been excited the electrons in the material begin to decay to lower energy states. The material emits photons with equal energy to the change in energy level (the energy lost). This is stimulated emission and results in optical gain from the laser transition that has occurred.

In terms of energy states, the material chosen has a metastable state that is higher than the ground state and slightly lower than the energy absorbed via optical pumping. Once the electrons are excited to a higher energy level, through collisions with one another, they drop down by fast decay to the metastable state. This state is more favourable and thus the electrons can remain in this state for a prolonged time period. Population inversion occurs between this and the ground state in a three-level lasing medium, such as ruby: a photon is emitted due to this laser transition.

Since all the electrons start off at the ground state, it takes a substantial input of external energy to induce a state where more electrons are in the metastable state than the ground state. Once this has been achieved, population inversion cannot be maintained for a substantial period of time, since any electrons that decay to ground state act against the inversion. This means that three-level lasers cannot emit continuous light, instead they have to be pulsed, to ensure enough energy is being pumped to re-establish population inversion.

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