Lasers are increasingly used by plastic surgeons to address issues such as wrinkles and textural changes, skin laxity, hyperpigmentation, vascularity, and excess fat accumulation. A fundamental understanding of the underlying science and physics of laser technology is important for the safe and efficacious use of laser in medical settings. The purpose of this article was to give clinicians with limited exposure to lasers a basic understanding of the underlying science. In that manner, they can confidently make appropriate decisions as to the best device to use on a patient (or the best device to purchase for a practice).
Key points
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Lasers are increasingly used by plastic surgeons to address issues such as wrinkles and textural changes, skin laxity, hyperpigmentation, vascularity, and excess fat accumulation.
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A fundamental understanding of the underlying science and physics of laser technology is important for the safe and efficacious use of laser in medical settings.
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Lasers are differentiated from incandescent light because the light is monochromatic, coherent, and collimated.
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When lasers are applied to tissue, various forms of laser-tissue interaction occur, the most clinically relevant of which is photothermal.
Introduction and history
The classic modalities for the treatment of soft tissue, vascular, and pigmented lesions of the skin include surgical incision, cauterization, and cryotherapy. The state of the art has progressed such that less-destructive modalities are not only available, but also desirable for reasons of patient comfort, recovery, and aesthetics. Laser (which is the acronym L ight A mplification by S timulated E mission of R adiation) and other light energy devices such as IPL (intense pulse light) devices and light-emitting diodes (LEDs) represent a significant segment of these new therapeutic options and play a major role in the care of the aesthetic patient.
The theoretic underpinnings of laser were first described by Einstein in 1917, and later by Townes, Gennadiyevich, and Mikhailovich in the 1950s (who were collectively awarded the Nobel Prize in Physics in 1964 for their fundamental work in quantum physics). This theory became a reality in 1960 when Maimen developed the first laser at Bell Laboratories. It was recognized very early that lasers held tremendous promise as a highly targeted, therapeutic modality. Although there are notable scenarios in which laser has become the standard of care, its wider application has been somewhat limited by cost, availability, and training.
The purpose of this article was to give clinicians with limited exposure to lasers a basic understanding of the underlying science. In that manner, they can hopefully confidently make appropriate decisions as to the best device to use on a patient (or the best device to purchase for a practice). The specific clinical applications of lasers and other light energy devices are covered in more detail elsewhere in this issue.
Introduction and history
The classic modalities for the treatment of soft tissue, vascular, and pigmented lesions of the skin include surgical incision, cauterization, and cryotherapy. The state of the art has progressed such that less-destructive modalities are not only available, but also desirable for reasons of patient comfort, recovery, and aesthetics. Laser (which is the acronym L ight A mplification by S timulated E mission of R adiation) and other light energy devices such as IPL (intense pulse light) devices and light-emitting diodes (LEDs) represent a significant segment of these new therapeutic options and play a major role in the care of the aesthetic patient.
The theoretic underpinnings of laser were first described by Einstein in 1917, and later by Townes, Gennadiyevich, and Mikhailovich in the 1950s (who were collectively awarded the Nobel Prize in Physics in 1964 for their fundamental work in quantum physics). This theory became a reality in 1960 when Maimen developed the first laser at Bell Laboratories. It was recognized very early that lasers held tremendous promise as a highly targeted, therapeutic modality. Although there are notable scenarios in which laser has become the standard of care, its wider application has been somewhat limited by cost, availability, and training.
The purpose of this article was to give clinicians with limited exposure to lasers a basic understanding of the underlying science. In that manner, they can hopefully confidently make appropriate decisions as to the best device to use on a patient (or the best device to purchase for a practice). The specific clinical applications of lasers and other light energy devices are covered in more detail elsewhere in this issue.
Generalized laser physics
In its simplest form, a laser device creates energy in the form of a beam of light that interacts with the target tissue to have a desired effect (known as “laser-tissue interaction”) ( Fig. 1 ).
Basics of Optics
Light and other forms of electromagnetic energy are composed of elementary particles known as photons (quantum of light). Photons travel at the speed of light (2.998 × 10 8 m/s) in empty space and are perpetually in motion in a sinusoidal wave pattern. The wavelength is the distance between 2 successive crests of the electromagnetic sine wave, and ranges from very short (gamma rays) to very long (AM radio waves) ( Fig. 2 ).
Visible light is in the relatively small spectrum of electromagnetic radiation located between 390 nm (violet) and 700 nm (red) that can be seen by the human eye. Most of the clinically relevant lasers fall in the visible light portion of the electromagnetic spectrum as well as in the longer wavelength infrared part of the electromagnetic spectrum ( Fig. 3 ).
Laser Device
All laser devices are fundamentally composed of an energy source and an optical resonator ( Fig. 4 ).
The energy source is what is used to initially move the electrons into an excited state so as to ultimately generate the photons. This energy source can take the form of lamps, electrical current, or even other lasers. The optical resonator is composed of the medium that is enclosed within a tube that has 2 mirrors: one mirror is completely opaque, and the other mirror is partially opaque (and therefore partially transmissible). The medium can be in the form of a solid, liquid crystal, liquid, or gas (of note, a liquid crystal has atomic organization that is intermediate between a solid and a liquid). It is the medium that determines the wavelength and identification of the laser (eg, ruby, argon gas, CO 2 , Er:YAG, alexandrite, diode, KTP, argon, Nd:YAG).
The energy source drives the electrons in the medium to an excited state. As they return to the resting state, photons of a specific wavelength are emitted in all directions within the tube. If an electron that is already in an excited state encounters another photon of the proper energy, it will emit a photon of the same wavelength without absorbing the additional photon, thereby constituting a “chain reaction” that leads to the further propagation of photons. Some of the photons that are traveling in a perfectly parallel direction will exit the optical resonator through the partially transmitting mirror, taking the form of the “laser beam.”
Laser Beam
The light in the resulting beam has a number of properties that distinguish it from a standard incandescent flashlight or lamp, and, therefore, give it its unique properties. Incandescent light is polychromic, noncoherent, and noncollimated. Laser light on the other hand is monochromatic , coherent , and collimated ( Fig. 5 ).
Monochromatic
Laser beams are monochromatic in that they are composed of photons that all have the same wavelength. This is in contrast to a flashlight, which emits photons of various wavelengths.
Coherent
The photons within a laser beam are coherent, in that the waves are all in phase in terms of both space and time.
Collimated
The laser beam is collimated, in that all photons are parallel to each other. The consequence of this is that a laser beam can travel extremely long distances with minimal distortion.
As a result of this, the laser beam has a high energy density. The absolute amount of energy generated by a laser is actually quite low, as the process is highly inefficient (only approximately 0.1% of the input energy is actually transformed into laser output). However, as the beam is collimated, the energy is very densely packed into a small volume.
It is the ability to transform energy (eg, electrical, light) into photons of a single wavelength and a collimated, coherent, and high-energy beam that makes laser a powerful technology.
Laser-Tissue Interactions
When a laser contacts tissue, there are a number of different, and not mutually exclusive interactions: reflection, transmission, absorption, and scatter ( Fig. 6 ).
