How does the alexandrite laser work in the skin?

Melanin absorbs 755 nm strongly

The laser takes its name from the crystal that produces the light: alexandrite. That crystal emits light at 755 nm, just beyond visible red. Melanin absorbs that light more strongly than the other natural chromophores in the skin, about three times as strongly as light at 1064 nm, the wavelength of the Nd:YAG laser ^[1].

Blood absorbs 755 nm much less than green or yellow light. Deoxygenated blood does have a small peak around 757 nm, where it absorbs markedly more than oxygenated blood ^[2]. That is one of the reasons 755 nm also has vascular uses. Water barely absorbs in this range ^[1].

Hair removal: the heat has to reach the stem cells

With millisecond pulses, the melanin in the hair shaft and bulb absorbs the light and converts it into heat. In the classic form of selective photothermolysis, the pulse is shorter than the time the target needs to cool down, so the heat stays there ^[3].

With hair, however, the actual target lies next to the pigment: the stem cells in the follicle wall and the bulge, which themselves contain little melanin. According to the extended theory of selective photothermolysis, the pulse may therefore last longer than the cooling time (thermal relaxation time) of the hair shaft, so that the heat from the pigmented shaft can reach those cells ^[4].

Fine hair contains less pigment and cools faster than thick, dark hair. Because melanin absorbs 755 nm more strongly than 1064 nm, alexandrite generates more heat in such hair. Which pulse duration and energy are appropriate depends on the device, the hair and the skin.

Diode lasers at around 800–810 nm are also widely used for hair removal; they are not part of our range. Their wavelength lies between those of alexandrite and Nd:YAG, and so does their melanin absorption. Which devices we offer for hair removal, and when a second wavelength helps, is covered on the hair removal page.

Where 755 nm reaches its limit

The melanin that heats the hair is also present in the epidermis. The darker the skin, the more light the epidermis already absorbs and the greater the risk of blistering, burns and pigmentary changes. The same applies to recently tanned skin, which contains more melanin.

A review article describes alexandrite as safe in skin types I to IV ^[6]; in darker skin, however, pigmentary changes have been reported, even with cooling ^[7]. That makes skin type IV a borderline area. In a randomised study in skin types III and IV, alexandrite and Nd:YAG gave comparable hair reduction after 18 months ^[8].

For dark skin, review articles recommend a longer wavelength, with Nd:YAG (1064 nm) as the safest choice, together with a longer pulse and good cooling of the skin surface ^[6][7][9].

Pigmented spots and vessels

Besides hair, 755 nm also targets pigmented spots and, through deoxygenated blood, vessels.

Solar lentigines and freckles

Solar lentigines (age spots) and freckles lie in the epidermis and contain a lot of melanin. The classic treatment is a Q-switched laser; picosecond lasers are also used ^[10]. A split-face study of twenty Chinese patients compared Q-switched alexandrite (50 ns) with alexandrite using 100-microsecond pulses. After one treatment and twelve weeks of follow-up, there was no significant difference in improvement. The longer pulse did, however, cause less pain and less post-inflammatory hyperpigmentation (6% versus 22%) ^[11]. The millisecond pulses used in hair removal were not studied in that trial.

Pigment can also increase temporarily after treatment with longer pulses, especially in darker skin. Any spot not confirmed as benign is assessed first, before a laser is used. For more on treating pigmented spots, see pigmentation.

Breaking up pigment and tattoo ink

With picosecond pulses, the same wavelength does something different. Pigment granules and ink particles are so small that they lose their heat within nanoseconds to a microsecond ^[3][15]. The nanosecond pulses of a Q-switched laser are already shorter than that. According to Kasai, they work largely photothermally, with a small mechanical component ^[16].

A picosecond pulse is also shorter than the time a pressure wave needs to cross the particle; for tattoo pigment, that is just under a nanosecond. The pressure then cannot dissipate, builds up in the particle and breaks it. This is called stress confinement. According to Kasai, the photoacoustic effect then dominates and the thermal effect is small ^[16].

After treatment, macrophages take up the ink fragments and carry them away through the lymphatic system; some are shed with the crusts ^[17]. Dermal pigment is cleared in the same way after fragmentation, through phagocytosis and the lymphatic system ^[7].

Which inks 755 nm targets depends on their colour. Black ink absorbs across the whole red and near-infrared range. Blue and green ink absorb between 600 and 800 nm and therefore respond well to 755 nm. Red, orange and yellow ink need 532 nm ^[6]. PicoSure Pro works at 755 nm; optional handpieces convert that light to 532 or 1064 nm. For the devices we offer for tattoos, see tattoo removal.

A fractional lens spreads the beam over a grid of small points. At each point, the fluence is much higher than in the surrounding beam. On PicoSure Pro, this lens is called the Focus™ Lens Array. In a histological study, it created small cavities in the epidermis. The author attributes these to laser-induced optical breakdown (LIOB): a microscopic plasma triggered by melanin absorbing the light. These zones flaked off within about three weeks, and the author describes new collagen and elastic tissue in the dermis ^[18].

What the research says

Hair removal

Alexandrite and the diode laser are the best-studied lasers for hair removal ^[20], and the literature supports long-term hair reduction with alexandrite, diode, Nd:YAG and IPL ^[9]. The result is hair reduction, not permanent hair removal; that is something to discuss with the patient beforehand.

In a retrospective comparison, alexandrite gave more hair reduction than Nd:YAG ^[21]. In the randomised study in skin types III and IV, the difference after 18 months was not significant. Combining both wavelengths did not give a better result there, but did cause more hyperpigmentation and pain ^[8].

Pigment and ink

For naevus of Ota, picosecond alexandrite gave better clearance in a randomised split-lesion study, with fewer sessions, less pain and fewer pigmentary changes than Q-switched alexandrite ^[19]. A systematic review finds the strongest evidence for picosecond lasers in tattoo removal ^[10].

Alexandrite alongside other technologies

Nd:YAG, long pulse (1064 nm)

Less melanin absorption and therefore safer in dark skin; in skin type IV, both wavelengths are used ^[8]. The light reaches deeper, which matters for large, deep vessels ^[6][9]. Clarity II and Elite iQ have 1064 nm alongside 755 nm. LongPulse is a 1064 nm handpiece for the Etherea MX and the ZYE platforms, and the ZYE YAG has its own 1064 nm laser.

IPL

A flashlamp with a broad spectrum. Depending on the filter, it reaches hair, pigment or vessels, less selectively than a laser. A review supports long-term hair reduction with IPL ^[9]. Our IPL devices are IPL SQ and ReClear IPL.

Nd:YAG, picosecond and Q-switched

For pigment and ink, 1064 nm reaches deeper, and melanin in the epidermis absorbs it less, which matters in dark skin; 532 nm reaches red, orange and yellow ink ^[1][6]. These wavelengths are available on PicoPlus and the handPICO handpiece (picosecond), on the Hollywood Spectra (Q-switched) and as an option on PicoSure Pro.

Vascular lasers (532 and 577 nm)

Blood absorbs green and yellow light much more strongly than 755 nm, but that light does not reach as deep ^[2][6]. That is why these lasers are used mainly for fine, red, superficial vessels. Our vascular lasers are DermaV and FlexSys Laser.