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Insufficient light absorption in hard dental tissues makes laser ablation in near UV, visible or near IR spectral ranges very inefficient to be employed for tooth cavity preparations. We used deposition of a liquid absorber, indocyanine green (ICG) dye, to overcome this problem. Experiments employed Alexandrite laser anticipating that future near IR diode laser technology will replace existing medical lasers. Ablation kinetics and mechanisms for both free-running and Q-switched modes of Alexandrite laser were studied with the aim to determine optimal parameters of laser irradiation and optimal volume of the dye. Four experimental parameters were monitored during each ablation event: (1) incident laser fluence, (2) temporal profile of the laser pulse, (3) temporal profile and magnitude of laser-induced stress transients, (4) temporal profile and spectrum of plasma emission. We also examined kinetics of plume by probing ablation products with CW He-Ne laser beam. Results depicted ablation process as a complex multistage phenomenon. Two distinct stages associated with the tooth ablation are revealed in the free-running mode: (1) ablation of a dye droplet from a tooth surface by the first laser micropulse of a 250-microsecond(s) macropulse, (2) plasma mediated ablation of a melted layer of enamel produced by thermal explosion of the dye. Plasma jet formation was delayed 10-100 microsecond(s) against the beginning of free-running pulse. Ablation stages and their efficiency are defined by laser irradiation parameters, dye concentration and its total volume. In contrast, Q-switched (nanosecond) laser ablation occurs as a one stage process, and, therefore, less efficient. In addition, Q-switched mode irradiation induces shock waves amplitudes that are about an order of magnitude higher compared with that induced by the free-running irradiation. Experimental comparison of Q-switched and free-running modes of irradiation is evident in favor of free-running mode that produces a nice smooth crater without noticeable thermomechanical damage to surrounding tissues.
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