## 2 The Mechanism of Action in Picosecond Laser Treatment  
A picosecond laser interacts with various tissues through two primary mechanisms: photo-thermo-mechanical disruption (PTMD) and laser-induced optical breakdown (LIOB) [7]. The former relates to non-fractionated laser beams and is based on the principle that ultrafast temperature changes, which are induced by the picosecond pulses, generate intense acoustic shock waves within the targeted chromophores, creating tensile stress which exceeds the rupture threshold of a chromophore [2]. A significant photomechanical interaction occurs when the duration of the pulse is shorter than stress relaxation time [7]. This photomechanical (photoacoustic) effect is the main method of fragmenting sub-dermal ink particles (tattoo) and cellular melanosomes using picosecond laser irradiation, facilitating their clearance by macrophages and other phagocytic cells [2]. In tattoo removal with Q-switched nanosecond lasers, the duration of the laser pulse is shorter than the thermal relaxation time of the tattoo pigments, allowing for the achievement of a thermal confinement. At the same time, another important phenomenon, known as stress confinement, must be taken into account. Stress relaxation can be explained as follows--when a particle is heated, it undergoes thermal expansion. The expansion propagates to the surrounding tissue as a vibrational wave, a process which is called stress diffusion. If a particle is heated very briefly, the stress generated in the particle does not have enough time to dissipate, resulting in stress confinement. If the stress level exceeds a critical threshold, the particle will fracture. This mechanism is analogous to thermal confinement and rapid temperature rise described in the thermal relaxation theory, which posits that if a structure is heated to a certain temperature, thermal energy will spread to the surrounding tissues by thermal conduction. However, if a structure is heated over an extremely brief time period, the temperature rises rapidly because heat diffusion does not occur--this is known as thermal confinement. Once thermal confinement is achieved, selective photothermolysis of the target structure becomes possible. Achieving thermal confinement depends on the time threshold of the structure, defined as thermal relaxation time (TRT). TRT is determined by the absorption coefficient and the heat diffusion coefficient of the structure [8]. The energy of a picosecond laser may be fractionated using different optical lenses, which allows for the concentration of higher peak energies within the microbeams and the preservation of the adjacent tissues [2]. The commercially available fractional optical delivery technologies include diffractive lens arrays (DLA), micro-lens arrays (MLA), and holographic optical matrices. Distinct optical arrays, with varying spot sizes, have been used in different PS laser devices. DLA technology is used in the 755 nm alexandrite Picosure laser (Picosure TM, Cynosure, Westford, MA, USA). It consists of closely packed individual hexagonal lenses, with a 500 \(\upmu\)m pitch. In turn, a neodymium-doped yttrium aluminum garnet (Nd:YAG) (532 nm and 1064 nm) picosecond laser PICOPLUS (PICOPLUS; Lutronic Corp., Goyang, Republic of Korea) employs the MLA technology with a 4-mm spot size to generate zones of high-intensity tissue damage, while the adjacent structures remain unaffected [9]. Closely packed microlenses divide the picosecond pulses into columns of high fluence, which irradiate only 10% of the target area, while the background receives laser pulses of lower fluence. The sites which receive high-fluence laser irradiation absorb approximately 20-fold more energy than the background regions exposed to low-fluence laser pulses. High-fluence fractionated laser emission promotes the synthesis of collagen and elastin through photothermal and photomechanical effects, without surface ablation [10].