Lasers have four standard components: (1) a medium, (2) an energy pump, (3) mirrors, and (4) a delivery system. The medium can be a gas (e.g., carbon dioxide), liquid (e.g., rhodamine dye), or solid (e.g., alexandrite crystal, ruby crystal). An energy pump is used to excite electrons within the medium to emit photons that interact with mirrors at the ends of the laser cavity. These mirrors function to amplify the light energy, which then exits the laser through a specialized handpiece.

The light emitted from a laser has three inherent properties: (1) monochromatic, (2) coherent, and (3) collimated. Monochromaticity means that the laser light that is emitted is of a single wavelength. Coherence means that the light waves travel in phase, both temporally and spatially. Collimation refers to the light beams traveling parallel to one another. Devices that violate any one of these properties are not considered lasers. Intense pulsed light (IPL) devices are polychromatic, noncoherent, and noncollimated and are therefore not considered lasers.

Lasers emit light either continuously such as with a carbon dioxide (CO ₂ ) laser or through rapid pulses of light such as with a pulsed-dye laser (PDL). Pulses of laser light within the millisecond (ms), nanosecond (ns), or picosecond (ps) domains allow for the laser energy to be selectively absorbed by specific targets within the skin, called chromophores, with little to no heat dissipating and potentially damaging adjacent structures. This forms the basis of the concept of selective photothermolysis, first written by Anderson and Parrish in 1983. ^,

The three primary chromophores of the skin that can be targeted with lasers are water, hemoglobin, and melanin. Tattoo ink particles and lipids were later added to the list of chromophores. ^, Chromophores have characteristic light absorption bands that can be targeted with lasers of certain wavelengths. For example, water molecules have strong absorption in the infrared region, which makes 10,600-nm carbon dioxide lasers useful in targeting water molecules within the skin. Oxyhemoglobin has strong absorption bands in the ultraviolet, green, and yellow regions of the electromagnetic spectrum, which explains why green 532-nm potassium titanyl phosphate (KTP) lasers and yellow 585-nm PDLs can effectively target blood vessels in the skin.

When the laser beam hits the skin, about 4% to 7% of the light is reflected. The remaining light enters the skin where it scatters off collagen fibers in different directions or is absorbed by both target and competing chromophores. This scattering effect attenuates the depth of penetration of photons within the dermis and can reduce the number of photons reaching target chromophores. Scattering is directly proportional to the frequency (energy) of the light radiation. According to Planck’s law, frequency is inversely proportional to wavelength, and therefore, the longer the wavelength of the light radiation, the lesser is the degree of scatter. Scattering can be seen with wavelengths within the ultraviolet and visible light spectra (∼280 nm to 1,300 nm). For example, less scatter is observed with the 1,064-nm wavelength as opposed to a 755-nm wavelength. The laser’s spot size also contributes to the degree of scatter—the smaller the spot size, the more the photons are lost to the effects of scatter. Larger spot sizes allow for more photons to reach their intended target chromophores. For example, a 10-mm spot size for the handpiece of a PDL allows more photons to traverse the dermis to reach their target vascular chromophores compared to a 7-mm spot size.

While scattering affects the depth of penetration of photons to the depth of the intended target chromophore, the absorption of photons by competing chromophores also reduces absorption by the target chromophore. For example, a green 532-nm laser beam can be used to target oxyhemoglobin within facial telangiectasias; however, the melanin within the overlying epidermis may act as a competing chromophore that reduces the number of photons reaching the depth of the facial telangiectasias within the dermis. ^,

The principle of selective photothermolysis is the foundation of pulsed lasers today. ^{, , ,} Selective photothermolysis refers to the selective destruction of targets in the skin using lasers with minimal collateral damage to the surrounding tissue. This principle is used in the treatment of vascular lesions, pigmented lesions, and tattoo ink. The laser wavelength is chosen based on the light absorption peak of the target chromophore. When the target chromophore absorbs the laser energy at a given wavelength, it becomes heated and selectively destroyed. The principle of selective photothermolysis applies when the pulse duration of the laser (the time during which the laser energy is delivered) is equal to or shorter than the target’s thermal relaxation time (TRT), which is defined as the time required for the heated chromophore to cool down to half its original temperature. To avoid heat diffusion from the target chromophore into the surrounding nontarget tissues, the pulse duration of the laser must be equal to or less than the target chromophore’s TRT. The larger the target’s size, the longer the TRT. While the pulse durations to treat small tattoo ink particles are within the nanosecond and picosecond domains, the pulse durations for treating telangiectasias are within the millisecond domains. A millisecond pulsed laser should never be used to treat a tattoo ink particle since the pulse duration exceeds the tattoo ink particle’s TRT. This would lead to significant heat diffusion into the surrounding tissues, causing a thermal burn and scarring. ^,

When the pulse duration of the laser is longer than the thermal relaxation time of the chromophore, there is heat diffusion from the chromophore into the surrounding structures. However, when the true target is not necessarily the chromophore, but instead the surrounding structures, then this concept of intentional heat diffusion to adjacent structures is known as the extended theory of selective photothermolysis . ^, This concept applies to situations when the laser operator wants to target and damage nonpigmented structures in the skin that normally would not be able to absorb the laser energy. The primary example is seen in laser hair removal, where the laser targets melanin within the hair shaft with the intention that heat would diffuse from the hair shaft to the nearby nonpigmented stem cells within the hair bulge, causing permanent damage to these stem cells.

Fractional photothermolysis refers to creating numerous microscopic channels of thermal injury in the skin, called microthermal treatment zones (MTZs). These MTZs promote collagen remodeling and desquamation of the overlying epidermis. Only a fraction of the skin is treated, and the normal intact skin in between the columns of thermal injury helps promote healing. Water is the chromophore targeted in fractional photothermolysis. Ablative fractional photothermolysis uses 10,600-nm CO ₂ or 2,940-nm Er:YAG (erbium-doped yttrium-aluminum-garnet) lasers to vaporize small microscopic channels within the skin surrounded by zones of coagulation. On the other hand, nonablative fractional photothermolysis uses mid-infrared wavelengths (e.g., 1,550 nm) to create only microscopic columns of tissue coagulation. These technologies are used to treat dermatoheliosis or stimulate neocollagenesis in the treatment of acne scars, surgical scars, traumatic scars, and static rhytides. ^,

Epidermal cooling is essential to avoid epidermal thermal injury and postinflammatory pigmentary changes, especially in darker skin types. The three primary cooling mechanisms are: (1) dynamic cryogen spray cooling, (2) contact cooling, and (3) cold air cooling. ^{, ,} The dynamic cryogen spray uses a liquid fluorocarbon molecule (tetrafluoroethane) that is sprayed onto the skin milliseconds before the laser pulse. This provides a form of precooling to protect the epidermis prior to the laser pulse. This is helpful with rapid pulse durations shorter than 5 ms. This technology has been especially used with PDLs for vascular lesions and long-pulsed alexandrite lasers for laser hair removal. Next, the contact cooling method uses a cold sapphire window that is in direct contact with the skin. This method is particularly helpful for devices with pulse durations exceeding 5 ms. This technology is also used in diode lasers for laser hair removal, as well as in KTP (potassium titanyl phosphate) lasers and intense pulsed light (IPL) devices for the treatment of vascular lesions. Finally, cold air cooling, although not as effective as cryogen spray cooling or cold sapphire contact cooling, can be used before, during, and after laser treatments to help reduce discomfort, redness, and swelling. This technology is commonly used with nonablative fractional laser devices.

Oxyhemoglobin has absorption peaks at 418 nm, 542 nm, and 577 nm. Therefore, 532-nm green light KTP (potassium titanyl phosphate) lasers and 585-nm to 600-nm yellow light PDLs (pulsed dye lasers) are most commonly used to treat vascular lesions, such as facial telangiectasias, port-wine stains, erythematous scars, and angiomas. The 1,064-nm Nd:YAG (neodymium-doped yttrium aluminum garnet) laser can also be used to treat vascular lesions but has less hemoglobin absorption and thus requires higher energies to effectively treat these lesions. ^, However, because of their longer wavelength, the 1,064-nm Nd:YAG lasers are safer in darker skin types due to less epidermal melanin absorption. Additionally, filtered xenon flashlamps (IPL devices) emit polychromatic light (∼500 nm to 1,500 nm) that can be used to treat vascular lesions, but these devices are not as selective in targeting oxyhemoglobin and not as powerful as lasers in treating vascular lesions. ^{, ,}

Benign pigmented lesions consist of lentigines, ephelides (freckles), seborrheic keratoses, cafe-au-lait macules, nevus of Ota, and melasma. The treatment of benign melanocytic nevi is controversial as it is unclear if lasers can change the biologic behavior of nevi or mask visible signs of malignant transformation. Benign pigmented lesions contain melanin, which has a broad absorption spectrum that can be targeted by visible light and near-infrared lasers. Pulsed dye lasers (585 nm to 600 nm), KTP lasers (532 nm), ruby lasers (694 nm), alexandrite lasers (755 nm), diode lasers (810 nm), Nd:YAG lasers (1,064 nm) and IPL devices (polychromatic ∼500 nm to 1,500 nm) can be used to treat benign pigmented lesions. Nonablative fractional lasers (1,927 nm, 1,440 nm, 1,550 nm) as well as both ablative fractional and ablative full-field lasers (2,940 nm, 10,600 nm) target water molecules and can treat benign pigmented lesions by mechanisms of coagulation or vaporization through heating of water molecules. ^,

A tattoo refers to the deposition of pigmented material into the skin. Different materials are used to give tattoo pigments their specific colors, such as black, blue, green, yellow, and white. ^, Black tattoos can be created using carbon or iron oxide. Blue tattoos can be created using cobalt aluminate. Green tattoos can be created using chromium oxide, malachite green, and phthalocyanine dyes. Red tattoos can be created using mercury sulfide, cadmium selenide, and azo dyes. Yellow tattoos can be created using cadmium sulfide. White tattoos can be created using titanium dioxide. Skin-colored tattoos can be created using iron oxides. Once tattoo ink has been deposited into the skin, it is engulfed by macrophages, fibroblasts, and mast cells, where it then resides within lysosomes. The size of ink particles is roughly about 40 nm. ^, Tattoo inks can be targeted using nanosecond domain lasers (Q-switched lasers) or picosecond domain lasers. These lasers help break down tattoo ink particles through not only thermal effects via selective photothermolysis, but also through photoacoustic sound waves created by sudden ink particle expansion that then mechanically breaks apart the tattoo ink particles. ^, Additionally, these photoacoustic effects cause cavitation of the lysosomes containing the tattoo ink particles, allowing them to be released into the extracellular space, where they can be removed by the body’s lymphatic system.

Laser hair removal uses laser energy to damage hair follicular stem cells that reside within the hair bulge. The laser energy is absorbed by melanin within the hair shaft with diffusion of heat to the nearby nonpigmented follicular stem cells, causing their destruction but with minimal damage to additional surrounding tissues. Laser wavelengths used in hair removal include 694 nm (ruby laser), 755 nm (alexandrite laser), 810 nm (diode laser), and 1,064 nm (Nd:YAG laser). IPL devices with appropriate cut-off filters may also be used. Hair removal works more effectively for darkly pigmented hairs due to the availability of the melanin chromophore to absorb the laser energy. The ideal patients are lighter skin types with brown or black hair.

Pulsed dye lasers (PDLs) emitting wavelengths between 585 and 600 nm can be used. Typical settings range from fluences of 4 to 15 J/cm ² , pulse duration of 0.45 ms to 20 ms, and spot sizes of 3 mm to 15 mm. The resultant desired effect is vessel cavitation or vessel rupture. The primary side effect is purpura, which may be lessened by delivering the energy over a longer pulse duration so that there is less sudden trauma to the vessel wall. Purpura is caused by microvascular rupture and hemorrhage, and a delayed vasculitis can occur a few days after treatment. With pulse durations greater than 20 ms, there is less chance of microvascular rupture and hemorrhage, but a small vessel vasculitis may occur a few days after treatment. ^{, ,}

The 532-nm KTP (potassium titanyl phosphate) lasers do not penetrate the skin as deeply as PDLs, but can also target oxyhemoglobin and have the advantage of having a lower risk of purpura. ^, The 1,064-nm Nd:YAG lasers have a longer wavelength that allows them to penetrate the skin more deeply and target deeper dermal structures compared to KTP or PDLs. ^, Additionally, 1,064-nm Nd:YAG lasers may be safer in darker-skinned individuals due to less overlying epidermal melanin absorption, which could result in postinflammatory dyspigmentation. However, they should be used with caution on the face as they can unintentionally target deeper arterioles and cause tissue necrosis. Compared to PDLs and KTP lasers, the 1,064-nm Nd:YAG lasers require higher fluences to achieve their therapeutic clinical endpoint due to lower hemoglobin absorption. It has been shown that hemoglobin absorption at 1,064 nm is about 100 to 250 times less than at 532 nm.

Cooling is very important to reduce the risk of unwanted epidermal thermal injuries resulting in blistering, scarring, and dyspigmentation. PDLs typically use cryogen spray cooling to precool the skin and protect the epidermis prior to the laser pulse, whereas KTP lasers typically use parallel skin cooling using cold sapphire contact windows. Nd:YAG lasers may use either cryogen spray cooling or cold sapphire contact window cooling. Filtered xenon flashlamps (IPL devices) use cold window contact cooling. ^{, ,} It is crucial that the cryogen spray aligns properly with the laser pulse to avoid burn injuries. Similarly, devices that use cold contact windows require good contact with the skin during treatment to avoid epidermal injury.

When operating lasers or filtered xenon flashlamps to treat a vascular lesion, the therapeutic clinical endpoint is immediate purpura, transient vessel graying, or immediate vessel clearance. These therapeutic clinical endpoints are visible skin responses to the laser pulses that signify effective treatment parameters and laser settings. Purpura and transient vessel graying are classically seen with PDLs, while immediate vessel clearance (i.e., the vessel disappearing instantly after the laser pulse) is classically seen with KTP and Nd:YAG lasers. When using PDLs, short pulse durations (between 450 µsec and 3 ms) cause purpura from vascular hemorrhage that results from the quick burst of energy. In order to reduce the risk of purpura, the pulse duration can be increased so that the energy is delivered over a longer time duration and thus is not as traumatic to the blood vessels being targeted. ^{, ,} While purpura signifies an effective treatment, it may not be aesthetically acceptable to patients who prefer little to no downtime. Therefore, it may be more convenient for the patient to use nonpurpuric PDL settings (i.e., pulse duration ≥ 6 ms) or use a KTP laser.

Filtered xenon flashlamps, or IPL devices, may target either melanin or hemoglobin with polychromatic light. To filter out unwanted wavelengths for the purposes of treating vascular lesions, special filters are used to remove wavelengths below 560 nm while allowing wavelengths between 560 nm and 1,200 nm to exit from the handpiece. Many devices have cold window contact cooling, which allows for higher fluences to be used without epidermal injury. Cold ultrasound gel or a proprietary gel is used to improve the penetration of the polychromatic light by reducing the refractive index between air and skin. The ultrasound gel also acts as a heat sink to reduce bulk heating and minimize the risk of epidermal injury. ^,

Port wine stains are capillary malformations made of very small vessels requiring short pulse durations to treat (typically purpuric settings between 450 µs and 3 ms). For port wine stains, the lowest fluence should be used that provides the clinical endpoint of purpura. The handpiece should be held perpendicularly to the skin with 10% to 15% overlap between pulses. Port wine stains are classically treated with PDLs but as patients become older, the port wine stains may become thicker due to soft tissue hypertrophy. This requires lasers with longer wavelengths (e.g., long-pulsed 755-nm alexandrite laser) and longer pulse durations (e.g., 1.5 ms to 3 ms) to target the deeper and thicker components. ^{, ,}

Facial telangiectasias, with or without underlying rosacea, are typically treated with PDLs, KTP lasers, or IPL devices. ^{, , , ,} PDL or KTP laser settings for treating facial telangiectasias range from fluences 7 to 12 J/cm ² and pulse durations of 10 ms to 20 ms with spot sizes of 7 mm to 12 mm. Multiple passes with adequate cooling in between passes may help with efficacy. KTP lasers and IPL devices have less risk of purpura compared to PDLs. A split face study comparing KTP lasers and PDL devices in the treatment of facial telangiectasias showed that, on average, 62% clearance was achieved after one treatment with the KTP laser compared to 49% clearance with PDL. Three weeks after the third treatment, 85% clearance was achieved with KTP compared to 75% with PDL. However, more patients with KTP treatments experienced swelling greater than one day compared to PDL. To target deeper vessels or treat vessels in darker skin type patients, the 1,064-nm Nd:YAG may be used. However, with less absorption of 1,064-nm light by oxyhemoglobin, higher fluences are necessary to reach a therapeutic clinical endpoint. The use of 1,064-nm lasers for facial telangiectasias must be done very carefully to avoid the risk of treating deeper vessels inadvertently with subsequent tissue necrosis. These higher fluences require significant epidermal cooling to protect the skin as well as sufficient time and distance between pulses to prevent bulk heating. Long-pulsed 1,064-nm Nd:YAG lasers operate within the millisecond pulse duration range to target vascular lesions. More recently, millisecond domain 1,064-nm Nd:YAG lasers were developed and have been used to successfully treat telangiectasias and other vascular lesions. These millisecond 1,064-nm lasers may be considered a first-line option in patients with darker skin types and have a better safety profile compared to the long-pulsed Nd:YAG lasers with less risk of scarring and dyspigmentation.

Cherry angiomas can also be treated with PDL, KTP lasers, IPL devices, and 1,064-nm Nd:YAG lasers; however, the most popular devices for these lesions are PDL and KTP lasers. Smaller spot sizes and shorter pulse durations are used given the small size of these lesions. The endpoint is immediate purpura.

Poikiloderma of Civatte refers to the telangiectasias, epidermal atrophy, and mottled brown discoloration of the neck and upper chest due to chronic sun damage. Treatments include PDL, KTP lasers, IPL devices, and also nonablative and ablative fractional lasers. Lower fluences and more conservative settings are used when treating the neck and chest as compared to the face to avoid complications, such as burns and scarring. The neck and chest have less adnexal structures compared to the face, and these adnexal structures normally help with wound healing. IPL devices can target both the telangiectasias and sun-induced dyspigmentation, and studies have shown 50% to 75% improvement in both the redness and brown discoloration with IPL devices. Combination treatments using both vascular and fractional lasers may also lead to significant improvement. When using vascular lasers or IPL devices, it is important to have appropriate overlap or spacing between pulses to avoid a “honeycomb” pattern from the circular spot of vascular lasers or a “stamping” pattern from the rectangular spots of IPL devices.

Lentigines or ephelides (freckles) can be targeted with long-pulsed lasers, flashlamps (IPL devices), Q-switched lasers, and picosecond lasers. When treating benign pigmented lesions, the therapeutic clinical endpoint is subtle epidermal darkening or a slate-gray appearance with long-pulsed (millisecond) devices, versus immediate epidermal whitening with Q-switched (nanosecond) or picosecond lasers. For example, subtle epidermal darkening reflects an effective therapeutic clinical endpoint after treatment with the long-pulsed 532-nm KTP laser, 585- to 600-nm PDL, 755-nm alexandrite laser, and IPL device. However, immediate whitening reflects an effective clinical endpoint after treatment with a Q-switched or picosecond laser. In order to reduce the effects of competing chromophores, notably hemoglobin, compression may be used with long-pulsed devices to blanch the area and help focus the laser energy toward the melanin-containing targets ^, . Special compression glass windows may come with PDLs to compress and blanch the treatment area during the laser pulse. Q-switched and picosecond lasers are generally more effective than long-pulsed lasers and flashlamps in the treatment of benign pigmented lesions because of the added benefit of a photomechanical effect with the sudden burst of energy. ^, While 532-nm lasers are helpful in superficial pigmented lesions, the more deeply penetrating 694-nm ruby lasers, 755-nm alexandrite lasers, and 1,064-nm Nd:YAG lasers are safer in darker skin types and more effective in treating deeper pigmented lesions such as a nevus of Ota. ^{, ,}

Nonablative fractional lasers, ablative fractional lasers and fully ablative lasers can treat benign pigmented lesions through mechanisms of thermal injury or tissue vaporization. For these lasers, the energy setting dictates the depth of laser penetration into the skin. For fractional lasers, the density setting dictates how much of the skin surface is treated (e.g., 10% coverage, 50% coverage). The higher the density reflects more of the skin being treated, which results in a longer healing time.

Melasma is a complex condition characterized by a mottled blue-gray or brown pigmentation on the face, mostly in women. The bluish or brown color is the result of the depth of pigmentation in the skin, either epidermal, dermal, or both. It results from an unclear and complex process involving hormonal triggers (e.g., pregnancy, oral contraceptive pills) and ultraviolet light. More recent studies show that visible light may also exacerbate symptoms of melasma. Medical treatments include strict sun protection, mineral-based sunscreens, and topical lightening agents. Topical lightening agents include hydroquinone, retinoids, and tyrosinase inhibitors, such as topical tranexamic acid, kojic acid, and azelaic acid. Oral tranexamic acid has also been shown to be highly effective in the treatment of melasma but with a high recurrence rate of symptoms once the medication is stopped. The biggest difficulty in treating melasma is the tendency for it to return or flare after treatment. This is especially seen if strict sun protection is not followed. Low-energy, low-density nonablative fractional lasers may be used to treat melasma but recurrence is common and may be mitigated with strict sun protection and concurrent use of topical lightening agents. ^, Q-switched and picosecond lasers that target melanin have also been shown beneficial in melasma when used with low energy settings, a concept commonly referred to as laser toning (or picotoning when using picosecond lasers). The low fluence settings help reduce the risk of postinflammatory hyperpigmentation, and studies have shown picotoning as having a less risk of melasma flares and dyspigmentation compared to conventional laser toning with Q-switched devices. ^, Picosecond lasers improve melasma primarily through a photomechanical process with less of a photothermal effect. The photomechanical process involves shock wave generation and plasma formation. The plasma formation is known as laser-induced optical breakdown, which forms vacuoles within the epidermis and helps eliminate pigment. Melasma treatment parameters are typically fluences of 0.5 to 0.8 J/cm ² with spot sizes ranging from 7 to 10 mm.

Lasers or IPL devices that operate with wavelengths between 600 nm and 1,100 nm may be used for laser hair removal or photoepilation, respectively. The pulse duration should be equal to or longer than the thermal relaxation time of the hair follicle, which is 10 to 50 ms. This would promote thermal damage to the surrounding nonpigmented follicular stem cells within the hair bulge. Thicker course hairs require longer pulse durations than thinner hairs. Conservative fluence (energy) settings should be used in areas with a high hair density, such as the chin, lip, and genitals, due to significant absorption of laser energy with resultant bulk heating and possible complications, such as burns and dyspigmentation. Epidermal cooling is very important to prevent thermal injury to the epidermis. The most effective methods of cooling are dynamic cryogen spray cooling (precooling) and contact cooling (parallel cooling). In darker skin types, the epidermal melanin acts as a competing chromophore, and so it is important to choose longer laser wavelengths (e.g., 810-nm diode laser, 1,064-nm Nd:YAG laser) that can bypass the epidermal melanin. For example, in skin types 1 to 3 it is most effective to use a 755-nm alexandrite laser, but in a skin type 6 patient it is important to choose a 1,064-nm Nd:YAG laser. The ideal patients for laser hair removal have light skin color and dark brown or black hairs. Multiple treatments spaced 4–8 weeks apart are often recommended.

Ablative 10,600-nm CO ₂ lasers target water molecules within the skin to create tissue ablation and vaporization. The minimum energy to create tissue ablation is calculated as 5 J/cm ² , and below this fluence the tissue is simply coagulated instead of truly ablated. ^, Current CO ₂ lasers allow parameters such as depth of ablation and the degree of coagulation to be adjusted. While the depth of ablation is typically around 20 micrometers, the combination of ablation and coagulation can reach as far deep as 150 micrometers. The coagulation helps with hemostasis, collagen remodeling, and skin tightening; however, too much coagulation may also lead to thermal injury, dyspigmentation, and scarring. With each laser pulse, there is an area of tissue vaporization, surrounded by an area of coagulation called the residual thermal damage. The longer the laser’s pulse duration, then the larger the residual thermal damage. ^{, ,} The ideal pulse duration for an ablative CO ₂ laser is ≤1 ms. Pulse durations longer than 1 ms are associated with a larger residual thermal damage, more thermal injury, and higher risk of scarring. With subsequent passes on top of recently treated charred skin, there is less vaporization and instead more nonspecific thermal damage. Therefore, it is important to remove the charred material after the first pass before proceeding with another treatment pass. Visual tissue contraction is seen with CO ₂ laser treatments. The final endpoint with fully ablative CO ₂ laser treatments is a yellowish color, referred to as chamois, which reflects penetration through the papillary dermis with exposure to the reticular dermis. ^, This can be difficult to visualize and is the point at which further ablation should not be performed to avoid complications such as scarring.

The 2,940-nm Er:YAG laser also targets water molecules within the skin, but has a higher affinity to water than the 10,600-nm CO ₂ laser. ^{, ,} This higher affinity translates to more efficient vaporization of tissue with very little coagulation around the zone of ablation. Therefore, Er:YAG lasers are traditionally known to have less hemostasis and tissue contraction compared to CO ₂ lasers. To overcome these limitations of Er:YAG lasers, newer models are capable of increasing the pulse duration to allow for better coagulation and tissue contraction. With CO ₂ lasers, there is less tissue ablation per pulse as the skin is replaced with residual thermal damage. However, Er:YAG lasers can continue to ablate through the residual thermal damage with each pulse, and therefore inexperienced laser surgeons can unintentionally drill deep into the skin if not paying attention to the depth of penetration. CO ₂ lasers provide greater tissue contraction and collagen remodeling compared to Er:YAG lasers, and therefore the degree of wrinkle reduction and tightening with CO ₂ lasers is greater. However, healing may be quicker with Er:YAG lasers.

Ablative fractional CO ₂ or Er:YAG lasers can be used to achieve reduced healing times and less risk of complications compared to fully ablative laser treatments. With fractional treatments, the laser creates microscopic columns of ablation and coagulation called MTZs, which are separated by normal untreated skin. Since only a fraction of the skin is treated, there is quicker healing and less risk of infection, pigmentary alteration, and scarring. Additionally, fractional treatments allow the laser operator to treat deeper into the skin with a significantly less risk of complications compared to a fully ablative treatment. The two parameters to control with ablative fractional lasers are (1) energy and (2) density. ^, With ablative fractional lasers, the energy level dictates the depth of laser penetration into the skin and therefore the depth of thermal injury. The density reflects how much of the skin is being treated (or the percentage of skin treated). Lower densities may lead to less dramatic results but are associated with less downtime and less risk of pigmentary complications, which is important in darker skin types. In addition, low densities paired with high energies are preferable in the treatment of surgical and traumatic scars. As the skin heals after fractional lasers, there is a sandpaper-like “peppering” on the skin due to microscopic epidermal necrotic debris. These microscopic epidermal necrotic debris are gradually exfoliated from the skin over several days, but sometimes take up to several weeks.

As the name implies, nonablative fractional lasers do not vaporize the skin, but instead create thousands of MTZs. The first nonablative fractional laser was a 1,550-nm erbium-doped fractionated laser that was released for clinical use in 2005, and a few adjustments were made a few years later to allow the zone of coagulation to reach a depth of 1.5 mm. This device was revolutionary in not only its nonablative fractional technology but also in its scanning handpiece using a technology called the Intelligent Optical Tracking System. Unlike traditional stamping pattern lasers, the Intelligent Optical Tracking System technology allows the laser operator to “roll” the handpiece on the skin with a uniform formation of MTZs on the skin surface. Several years later, a newer model was released that contained dual wavelengths (1,550-nm erbium-doped fiber and 1,927-nm thulium fiber). Today there are several nonablative fractional laser models, which employ wavelengths within the infrared spectrum such as 1,440 nm, 1,550 nm, 1,927 nm, and 1,940 nm. The deeper penetrating 1,440-nm and 1,550-nm wavelengths can be used to treat scars and fine lines, but the more superficial 1,927-nm and 1,940-nm wavelengths can be used for photorejuvenation and improvement of dermatoheliosis. A more recent advancement is the hybrid fractional laser that creates both vaporization and coagulation with its dual 2,940-nm and 1,470-nm wavelengths, respectively. The hybrid technology allows for simultaneous ablative and nonablative resurfacing to improve dermatoheliosis, texture, tone, and pigmentation.