ABSTRACT
BACKGROUND: During in-office bleaching, appropriate light sources are applied in order to enhance the activity of the bleaching gels applied onto teeth. For this method to be effective, a high absorption of light within the gel is necessary. Variation in the light attenuation capability of the gel, the duration of application and light activation can contribute towards safety hazards associated with this procedure. METHODS: In this study, seven different gels and hydrogen peroxide have been evaluated for their optical properties by means of spectrophotometry (440-1000 nm). The transmitted light spectrum was used to estimate the intensity loss for each gel. The mean intensity decreases observed were statistically analysed using an analysis of variance (ANOVA). RESULTS: The five more-pigmented gels tested indicated a very similar intensity loss of around 80%, whereas the remaining two gels showed significantly less attenuation (predominantly, p < 10-6). CONCLUSIONS: Throughout the spectrum of wavelengths examined, and according to the underlying studies evaluated, five of the gels assessed demonstrated an attenuation high enough to possibly avoid overheating of the underlying enamel dentine and pulp. An evaluation of appropriate irradiation parameters is proposed.
ABSTRACT
Plasma filaments in air induced by femtosecond laser pulses lead to the generation of strong shock waves. This letter presents a systematic study, both experimental and theoretical, of the acoustic radiation by femtosecond laser-generated filaments. A theoretical model is developed based on the experimental results and is used to evaluate the directivity of the filament's acoustic radiation within and beyond the audible frequency range. It is shown that the acoustic directivity of plasma filaments can be derived from the model of a weighted acoustic line source, consisting of elementary point sources with N-shaped excitation.
ABSTRACT
The role of thin-film metal transducers in ultrafast laser-generated longitudinal acoustic phonons in Si (100) monocrystal substrates is investigated. For this purpose degenerate femtosecond pump-probe transient reflectivity measurements are performed probing the Brillouin scattering of laser photons from phonons. The influence of the metallic electron-phonon coupling factor, acoustical impedance and film thickness is examined. An optical transfer matrix method for thin films is applied to extract the net acoustic strain relative strength for the various transducer cases, taking into account the experimental probing efficiency. In addition, a theoretical thermo-mechanical approach based on the combination of a revised two-temperature model and elasticity theory is applied and supports the experimental findings. The results show highly efficient generation of acoustic phonons in Si when Ti transducers are used. This demonstrates the crucial role of the transducer's high electron-phonon coupling constant and high compressive yield strength, as well as strong acoustical impedance matching with the semiconductor substrate.
