**4. Influence of pulse width on tissue removal**

Although the pulse duration of most commercial lasers (range of µs) is shorter than the thermal relaxation time of dental hard tissues, laser ablation promotes irregular cavities (depending on composition of target tissue), desiccation of the surface (due to the removal of underlying water) and the presence of few microcracks (related to the energy density), the amount of water coolant and the repetition rate must be adjusted during the clinical procedure.

The adjustment of repetition rate is important to assure that the inter-pulse period is longer than the thermal relaxation time of tissues; in this way, it is possible that the temperature of the irradiated tissues decrease between laser pulses (McDonald et al., 2001). Another strategy for cooling the tissue during laser irradiation is reducing the pulse duration (Seka et al., 1995). Depending on the pulse duration (<1 ps), the process of ablation is changed and the non-linear processes (or non-thermal ones) take place (Ana et al., 2006; Freitas et al., 2010; Kruger et al., 2008; McDonald et al., 2001; Niemz, 2004; Strassl et al., 2008).

According to Niemz (1995), lasers with pulse durations in the range of ms (10-3 s), µs (10-6 s) or ns (10-9 s) generate considerable heat during ablation of dental hard tissues, in a mechanism mediated by *thermal interaction*. On the other hand, lasers with pulse durations of ps (10-12 s) and fs (10-15 s) ablate the tissues by forming an ionizing plasma. These lasers, commonly called as USPL (ultra short pulse lasers), operates at very high repetition rate (larger than 15 kHz) and energy per pulse typically of hundreds of µJ (Wieger et al., 2006).

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concerning the use of USPLs for medical applications started more than 15 years ago. In fact, one of the first studies that report the use of lasers with pulse widths of ps and fs was performed by Stern *et al*. (1989), relating applications for corneal ablation. Since then, efforts were made to understand the effects of these lasers on biological tissues and to develop of practically applicable systems. Although the majority of the studies report the use of laboratorial equipments for biological purposes, nowadays it is possible to find commercially available equipments for ophthalmology and for laboratorial use; this fact indicates that, in a near future, commercial equipments can be available for dentistry

The USPLs are lasers with pulse duration ranging from 100 fs to 500 ps, with power densities above 1011 W/cm2 in solids (Niemz, 2004). The main characteristics of these complex systems (Freitas et al., 2010) are the very low pulse duration and the high precision that can be acquired due to the extremely small focalization area, in which a peak power up to 1.5 TW (Freitas et al., 2010) can be obtained. Also, these lasers can operate at repetition rates higher than 15 kHz and energy per pulse of hundreds of µJ (Wieger et al., 2006). In this way, these lasers offer the advantage of promoting precise smooth ablation without a heataffected zone, effects that cannot be controlled when using lasers with pulse duration of µs or ns. Some researchers report that the main advantage of using the USPLs in dentistry is to achieve a controlled material removal and, as a consequence, reducing the pain caused by the vibration and friction heat (Kruger et al., 1999). According to Neev *et al*. (1996), the main advantages of USPLs are: the decreased energy density to ablate the material; minimal mechanical and thermal damages due to the extremely short laser pulses; minimal dependence of the tissue composition for ablation; precision in the ablation depth; low noise level in comparison with high-speed bur; ability to texture surface and precise spatial

The USPLs are solid-state lasers, such as Nd:YLF, Ti:Al2O3 , Cr:LiSAF (Alexandrite), Cr:BeAl204, Cr:LiSGaF, Cn:LiCAF, Cr:YAG, Ti:Al2O3/Nd:glass, Er:glass. These lasers interact with the tissues by a mechanism called *plasma-induced ablation* or *plasma mediated ablation*, in which the phenomenon of *optical breakdown* occurs. In a few words, the ablation is caused by plasma ionization, in which laser irradiation produces an extremely high electric field that forces the ionization of the molecules and atoms, promoting a breakdown and, then, the ablation or ejection of target tissue (Niemz, 2004). During the cutting, it is possible to observe the formation of a bright plasma spark, and a typical low noise, characteristic of

Considering the strictly short pulse durations and the low energy per pulse in USPLs systems, it is possible to infer that the ablation process is practically not dependent on the wavelength or the composition and absorption characteristics of the tissue (Perry et al., 1999). Also, the removal of ablated material is faster than the heat propagation on the tissue, i.e., the pulse length is lower than the heat conduction time of target tissue (Perry et al., 1999); in this way, there is no transmission of heat to pulp or surrounding tissues, for example, as well, no thermal damages to the irradiated tissues. Other advantage of using USPLs in dentistry is that these systems can remove any kind of restorative material, including amalgam (Freitas et al., 2010), which is not possible using other systems due to the

applications too.

control.

plasma formation.

reflection of light or overheating of the material.

Although USPLs have extremely higher repetition rate (> KHz) and peak power (up to TW), previous studies relate that a single ultra short laser pulse removes significantly less volume of dental tissue when compared to conventional Er:YAG laser removal (Strassl et al., 2008). This fact occurs due to the differences in focal size and penetration depth of USPLs (which are severely lower when compared to Er:YAG lasers that operate at pulse width of µs); in this way, the pulse repetition rate had to be increased in USPLs to obtain a similar ablation volume than those obtained by Er:YAG (Wieger et al., 2006).

Some literature studies compared the morphological aspects, as well the depth of craters during ablation of dental hard tissues with lasers operating with distinct pulse widths. Niemz (1995) relates that the Nd:YLF laser (λ = 1053 nm) operating with pulse duration of 30 ps provide cavity preparation on sound and decayed enamel without severe thermal or mechanical damages, with negligible shock-wave effects. Also, in the same paper, they showed that the ablation of carious enamel was 10 times more efficient than the ablation of sound enamel. A study performed by McDonald *et al*. (2001) showed that the total deposited energy on tissue as well the laser pulse duration change the crater depth generated on dentin, and the Nd:YAG with pulse width of 35 ps is unable to promote carbonization of dentin in comparison with a Nd:YAG laser with pulse width of ms.

The heating of dental hard tissues can induce composition and crystallographic changes on these tissues which are dependent on temperature rises. In this way, both morphological aspects and chemical analysis are indicative of thermal effects of lasers on enamel and dentin. A study performed by Kamata *et al*. (2004) showed that the chemical properties of hydroxyapatite (HAp) are unchanged after ablation with lasers operating with pulse widths of 50 fs, 500 fs and 2 ps. These results suggest that USPLs do not significantly increase the temperature of HAp. On the other hand, the use of Nd:YAG operating with pulse duration of 6 ns and 200 ns on enamel promote melting and recrystallization of this tissue (Antunes et al., 2005), indicating temperature rises up to 1200o C. Also, with the pulse duration of 6 ns, Nd:YAG promoted changes on organic content of enamel and dentin (Antunes et al., 2006).

Thermal measurements were performed using a laser with pulse width of fs on enamel using thermocouples, and it was detected temperature rises about 2o C on enamel surfaces after a 8 ms train of 70 fs pulses (Pike et al., 2007). This fact indicates that the USPLs do not induce significant thermal rises on surfaces and on surrounding tissues and can be used with safety even without refrigeration.
