The first laser was invented in 1960. The first laser treatments of tattoo was performed by Goldman in 1965. Q-switched laser evolved to a standard technique for removing pigments from skin in the past several decades. However, scientific knowledge about the mechanisms of laser tattoo removal is still incomplete. The very basic theory of laser tattoo removal is proposed or interpreted by Rox Anderson in his 1983 science paper on selective photothermolysis. The theory of selective photothermolysis dictates that both the laser wavelength and laser pulse width are important parameters for optimizing treatment outcome. Indeed, they are the most critical factors in addition to laser influence and laser beam size for laser tattoo removal. For a certain color of tattoos, the right laser wavelength should be used in the first place. The larger laser influence and beam diameter, the deeper pigments can be treated. The selection of laser pulse duration is based on the concept of minimal collateral damage when the laser pulse duration is less than the thermal relaxation time of the pigmented particle or ink. However, the theory of selective photothermolysis does not directly relate lase pulse duration with the clearance of pigments. The purpose of this post is to provide the most truthful and accurate review on the picosecond laser tattoo removal and help people to make most informed decision regarding to their tattoo treatment options.
Picosecond laser tattoo removal concept was first proposed and examined by Ross  who compare a multi-mode, passive mode-locking, 35-ps picoseond laser with a traditional nanosecond laser in treating tattoos. When the same low laser fluence setting is used for both lasers, Ross showed that picosecond laser is more effective than a traditional nanosecond laser for tattoo clearance. However, the winner would be the nanosecond laser if both lasers were operated at their full strength. So the real contest would have been undoubtedly won by the nanosecond laser before the first high-power picosecond laser became available in 2012. Since 2012, people are wondering whether the picosecond laser tattoo removal is worthy because it costs 2-3 times than the gold standard nanosecond laser tattoo removal. Despite 7-years of marking efforts, the acceptance of picosecond lasers is still not ideal due to both the poorly understood mechanisms and the lack of conclusive clinical evidences in scientific literature.
Laser tattoo removal is frequently explained as a particle fragmentation process by laser. However, Ross's results  showed that there are no microscopic difference between picosecond-laser-treated black inks and nanosecond-laser-treated black inks regarding to particle sizes. Thus, the idea that picosecond laser treatment might result in smaller particles was not experimentally supported by Ross's experiments. Ross hypothesized that higher peak temperature rise induced by a picosecond laser pulse may cause more chemical reactions and reduces more absorption of carbon inks than a nanosecond laser in his experiment. However, Ross  might neglect one mechanism behind it, i.e. photoacoustic waves excited by laser irradiations. A short picosecond laser pulse can always confine its pulse energy deposition into pigmented inks better than a nanosecond laser no matter the size of the ink aggregations and may result in higher interface temperature rise at the ink surface. According to the scientific theory of photoacoustics discovered by Alexander Graham Bell in 1880, the excited photoacoustic wave pressure is proportional to the Grüneisen coefficient of the tissue, which is temperature dependent. When the fluence and optical absorption coefficient of ink are the same for both lasers, the picosecond laser can excite larger photoacoustic wave since its frontal part of its laser pulse could induce higher peak temperature rise at interface due to better heat confinement and the remainder of the pulse could generate larger photoacoustic pressure due to larger Grüneisen coefficient enhanced by higher interface temperature. The third mechanism is related to the high temperature rise that could evaporate the surrounding water in the skin tissue and created acoustic shock wave, which can disperse the ink aggregations much more efficient than the photoacoustic waves. There are many advocators of picosecond lasers who claimed a fourth mechanism, i.e. the "mechanical" mechanism. In laser medicine, the laser-tissue-interaction mechanism could migrate from thermal (photoacoustic) mechanism to mechanical mechanism when the pulse duration migrates from nanosecond to picosecond regime when the laser intensity of ultra-short laser pulse becomes so high that the laser pulse essentially evaporate tissue from surface mechanically and non-selectively. Obviously, such a "mechanical" interaction should be avoided in laser tattoo removal. It may set an upper limit on the maximum laser intensity or lower limit on the laser pulse duration. So the "mechanical" mechanism of picosecond laser tattoo removal is wrong.
Although it is not new in laser physics literature that short laser pulsed can be significantly compressed with an absorption-saturation medium, it was not utilized by a tattoo removal laser until 2012. It was marketed as a "picosecond" laser while it was, in fact, a compressed version of Q-switch laser in sub-nanosecond domain. Since the new "picosecond" laser is compressed from a traditional Q-switch nanosecond laser, its full laser fluence is always less than its original nanosecond laser considering the loss introduced in the process of non-linear pulse compression. Nanosecond laser has been proved to be safe in tattoo removal practice even it is run at its full strength. Significant adverse events such as scarring are very rare. According to the theory of photoacoustics, the picosecond laser has to overcome the disadvantage of lower laser fluence in order to compete with a nanosecond laser. In the meantime, the picosecond laser should also has a safe laser intensity (laser fluence divided by pulse duration) that will not mechanically evaporate surface tissue.
As Ross suggested in his comments  at British Journal of Dermatology, most clinical studies [3-8] attesting to the efficacy of ps lasers lack appropriate experimental settings for the comparison of two types of lasers. Most studies that claimed picosecond laser as winner did not use the full strength of a nanosecond laser to compare with a picosecond laser or simply claim of efficacy of a picosecond laser in various case studies without comparing to a nanosecond laser.
The first randomized controlled single-blinded clinical study that used the full strength of a nanosecond laser is the paper  published by Pinto on British Journal of Dermatology in 2017. Pinto concluded that the use of picosecond pulse does not provide better clearance than nanosecond pulses except the reduced pain. However, the design of Pinto's study did not answer whether a picosecond laser will outperform a nanosecond laser when the same laser fluence setting is used. Secondly, he included both pretreated tattoos and non-treated tattoos in his studies in order to address the claims that picosecond laser is more efficient on pre-treated tattoos. Ross  criticized Pinto's design because he believes that picosecond laser might fragment ink particles into smaller particles, an attractive concept that might not be true. If both Ross  and Pinto  have drawn correct conclusions from their studies, we will need more powerful picosecond lasers to really demonstrate its advantage over a traditional nanosecond laser. For example, the Picoway picosecond laser  could only generate 1064-nm laser fluence at 1.4 J/cm2 for the 6-mm diameter setting while the MedliteC6 laser can generate 3.5J/cm2 laser fluence for the 6-mm diameter setting. Its laser influence need to be improved to at least 3.5J/cm2 at the 6-mm diameter setting to show its clear advantage over a MedliteC6 laser if the conclusion of Ross  is right.
Another randomized clinical study was published by Lorgeou  in 2018. The authors claimed a statistically significant superiority of the picosecond lasers compared to the nanosecond laser for tattoo clearance. Additionally, the authors claimed that picosecond lasers were superior to the nanosecond laser for professional tattoos; However, no difference was observed for amateur and cosmetic tattoos; Picosecond lasers were superior to the nanosecond laser for black and/or blue tattoos but they provided similar efficacy for polychromatic tattoos; The difference in terms of side-effects was also minimal with a tendency of picosecond lasers to be less painful but without reaching the level of statistical and clinical significance. Lorgeou's study is as valuable as Pinto because very limited data is available. Lorgeou's finding that the picosecond laser clears deeper tattoos better than the nanosecond laser is very interesting. It could only be explained by the possible absorption saturation due to the high laser intensity of the picosecond laser. The first weakness of this study is that one of the picosecond lasers (Enlighten, Cutera) is actually a combo laser, including a nanosecond laser and a subnanosecond laser. This may reduce the credibility of Lorgeou's conclusion. The second weakness of the study is that no detailed treatment parameters were provided except fluence range of all lasers. It is difficult to determine whether the nanosecond laser was ran at full strength in the study. The third weakness of the study is that the authors claimed financial ties with one picosecond laser manufacture.
In summary, the claim that current picosecond laser is significantly better than a full-strength, traditional nanosecond laser such as a Medlite C6 is not conclusively supported by current scientific literatures. More independent studies with an experimental design similar to studies [1,9,10] are encouraged. It is possible that when the picosecond laser pulse energy become comparable to the nanosecond laser, the picosecond laser may start to show clear advantages. If a picosecond-laser-tattoo-removal service provider claims that their picosecond laser can remove your tattoos two time faster, you should ask for reference of clinical studies that compared their picosecond laser to a full-strength, gold standard nanosecond laser. Some people might want to know what are the best option to remove unwanted tattoos most efficiently at this moment. The only option that has been confirmed by several studies in scientific literature is the R20/R0 protocol or the protocol with multiple laser exposures in a single visit. Check out our post on the topic of R20/R0 protocol for more information.
Dr. Bin Rao
 V. Ross, et. al, "Comparison of responses of tattoos to picosecond and nanosecond Q-switched neodymium:YAG laser", Arch Dermatol 134:167-171,1998.
 V. Ross, "The picosecond revolution and laser tattoo treatments: are shorter pulse really better?", British Journal of Dermatology 176:288-306, 2017.
 JA. Brauer,et. al,"Successful and rapid treatment of blue and green tattoo pigment with a novel picosecond laser", Arch Dermatol 148:820-823, 2012.
 N. Saedi, et. al, "Treatment of tattoos with a picosecond alexandrite laser: a prospective trial " Arch Dematol 148: 1360-1363, 2012.
 EF. Bernstein, et. al, "A novel dual-wavelength, Nd:YAG, picosecond-domain laser safely and effectively removes multicolor tattoos" Lasers Surg Med 47(7): 542–548, 2015.
 SG. Ho, et. al, " Laser tattoo removal: a clinical update", J Cutan Aesthet Surg 8:9-15, 2015.
 DJ. Friedman, et. al, "Successful treatment of a red and black professional tattoo in skin type VI with a picosecond dual-wavelength, neodymium-doped yttrium aluminium garnet laser", Dermatol Surg 42: 1121-1123, 2016.
 Mi Soo Choi, et. al, "Effects of picosecond laser on the multicolored tattoo removal using Hartley guinea pig: A preliminary study", PLoS ONE 13(9):e02003370,2018.
 F. Pinto, et. al, "Neodymium-doped yttrium aluminium garnet (Nd:YAG) 1064-nm picosecond laser vs Nd:YAG 1064 -nm nanosecond laser in tattoo removal: a randomized controlled single-blind clinical trial", British Journal of Dermatology 176:457-464, 2017.
 A. Lorgeou, et. al,"Comparison of two picosecond lasers to a nanosecond laser for treating tattoos: a prospective randomized study on 49 patients", European Academy of Dermatology and Venereology 32: 265-270,2018.