Laser ablation effect can be used to cut and grind the hard tooth tissues, thereby offering several advantages over traditional instruments. As a result, laser ablation technology is applicable to various dental procedures. The ablation effect of lasers on hard biological tissues is highly dependent on wavelength. As is well known, the erbium-doped yttrium aluminum garnet (Er∶YAG) laser is the most frequently used dental laser because its wavelength is close to the strong absorption peaks of water and hydroxyapatite. However, this wavelength laser has a low ablation rate and must be used in conjunction with water spray. According to previous research, the 9.3 μm CO₂ laser has a higher absorption coefficient for apatite than the Er∶YAG laser and offers a number of unique advantages when it comes to biological hard tissue ablation. For example, irradiating teeth with a CO₂ laser at a wavelength of 9.3 μm can improve their acid resistance, increase the anticaries effect of fluoride on teeth, and strengthen the bond between composite resin materials and teeth. However, there is currently no systematic evaluation report on the ablation characteristics of this wavelength laser on biological hard tissues, and data on the ablation of human dental tissue is also sparse.

Experimental samples were randomly assigned to the A and B groups. In group A (the quantity of samples n=20), without spraying water, a single laser pulse irradiated the dentin and enamel on each sample with energy densities of 4.23, 4.61, 5.05, 5.56, 5.84, 6.14, 6.83, and 7.69 J/cm² to determine the dentin and enamel laser ablation thresholds. In group B (n=20), to investigate the ablation characteristics of various energy densities, a single laser pulse irradiated the dentin and enamel with energy densities of 9.73, 37.14, 65.43, 88.42, and 106.10 J/cm² under water spray (water flow rate of 0.25 mL/s) and non-water spray conditions. Following laser irradiation, the experimental samples were observed and photographed using a digital microscope. We defined the ablation threshold as the energy density that corresponds to 80% of the ablation probability, the ablation efficiency as the depth of ablation per unit of energy, and the ablation rate as the volume of ablation per unit time. Due to the relatively large time measurement error associated with a single pulse, the ablation rate in this study was expressed as the ratio of the measured volume to the elapsed time represented by the line scan (line length is 0.9 mm, scan speed is 60 mm²/s, and frequency is 500 Hz).

Due to the composition and structure of dentin and enamel, the ablation threshold of the 9.3 μm CO₂ laser is greater for enamel (6.07 J/cm²) than that of dentin (5.76 J/cm²) (Fig. 3). Additionally, we discovered that the energy density of the pulse has a significant effect on the ablation characteristics of hard tooth tissues. On the one hand, as energy density increases, both the geometric size of the ablation crater and the rate of ablation increase (Fig. 5, Fig. 7). Moreover, the dentin and enamel laser ablation rates reach their maximum values when the energy density reaches 106.10 J/cm² and the frequency is 500 Hz, and the maximum values are (0.308±0.026 ) mm³/s and (0.510±0.032) mm³/s, respectively. The crater created by the 106.10 J/cm² laser energy density is inverted cone-shaped and noncarbonized, but the surface morphologies of dentin and enamel are different (Fig. 4). On the other hand, the mean value of laser ablation efficiency of dentin decreases as the energy density increases, whereas the mean value of laser ablation efficiency of enamel initially decreases and then increases as the energy density increases (Fig. 6).

To the best of our knowledge, this is the first study to investigate the ablation characteristics of hard tooth tissues (enamel and dentin) using a 9.3 μm CO₂ laser. In this work, we determined the ablation threshold of enamel and dentin exposed to the wavelength of the laser and subsequently investigated the relationship between ablation crater geometry, ablation efficiency, and ablation rate as a function of energy density. The experimental results indicate that the ablation threshold of 9.3 μm wavelength laser-irradiated tooth enamel is greater than the ablation threshold of laser-irradiated dentin. Additionally, the ablation depth, ablation diameter, and ablation rate of irradiated dentin and enamel increase in direct proportion to the energy density. Moreover, when the energy density is 106.10 J/cm² and the laser frequency is 500 Hz, the ablation rate reachs its maximum, with no obvious sign of carbonization. The research findings will contribute significantly to a better understanding of the biohard tissue ablation mechanism, and the ablation threshold and photodosimetry characteristic parameters obtained from our study will thereby serve as a foundation for promoting the clinical application of this technology.