Upon absorption of laser energy, microparticles can convert the absorbed energy into temperature rises, pressure waves, and vaporization. All of these will affect the surrounding material as well as damaging the absorbing particle. The pressure signals display especially complex behavior because of two competing time scales: the duration of the laser pulse and the characteristic mechanical oscillation time of the absorber. As the pulse duration is lengthened, the pressure signals become increasingly more complicated. Using power spectra and Lyapunov exponents, we show that for pulse durations greater than the characteristic oscillation time, the pressure signals are chaotic. The chaotic nature of the pressure signal presents potentially dangerous uncertainty when using longer laser pulses in biomedical and engineering applications.
Resonance effects can occur upon laser absorption by micro and nanoparticles when a train of pulses is used. The pressure generated by the train of pulses may be significantly different than the pressure generated by a single pulse with the same total energy. For pulsed lasers with a gap duration between pulses that is an integer multiple of the characteristic oscillation time of the absorber, constructive interference occurs and the pressure is increased. For pulsed lasers with a gap duration between pulses that is an half integer multiple of the characteristic oscillation time of the absorber, destructive interference occurs and the pressure is significantly decreased. We present numerical computations showing the manifestation of this effect in gold particles with a radius of 100 nm. The resonance effects have implications for damage thresholds and therapeutic applications
of laser radiation.
We present initial results showing chaotic behavior in the pressure signals generated by laser absorption by a microparticle. Specifically, the system of a melanosome immersed in water is investigated. We describe how the system manifests chaos, and the implications for causing damage to the surrounding material. We also find that a characteristic acoustic time of the absorber, the time it takes a sound wave to traverse the absorber, known as the stress confinement time, defines an important time scale for laser pulse duration. For pulse durations shorter than the stress confinement time, the pressure response is periodic, while for pulse durations greater than the stress confinement time, chaotic pressure transients are observed.
Damage by pulsed lasers to the retina or other tissues containing strongly absorbing particles may occur through biophysical mechanisms other than simple heating. Shockwaves and bubbles have been observed experimentally, and depending on pulse duration, may be the cause of retinal damage at threshold fluence levels. We perform detailed calculations on the shockwave and bubble generation expected from pulsed lasers. For a variety of different laser pulse durations and fluences, we tabulate the expected strength of the shockwave and size of the bubble that will be generated. We also explain how these results will change for absorbing particles with different physical properties such as absorption coefficient, bulk modulus, or thermal expansion coefficient. This enables the assessment of biological danger, and possible medical benefits, for lasers of a wide range of pulse durations and energies, incident on tissues with absorbing particles with a variety of thermomechanical characteristics.
The concept of confinement is that if energy deposition into a system occurs during durations shorter than a confinement time, the response of the system depends only on the total energy deposited and not on the deposition time. For stress confinement, the relevant response is the pressure that is produced. We have shown previously that for laser absorption by a spherical absorber, stress confinement is not valid at the core of the absorber and the tensile stresses continue to grow as the pulse duration shrinks well below any characteristic response time of the system. We have now calculated the pressure response in the cellular medium outside the absorber. We find that for a variety of energies, stress confinement is valid. We find that the characteristic confinement time agrees well with that expected for pressure transmission across the absorber. We show that even though the peak pressure that is produced varies slowly as a function of pulse duration, there is a sudden onset of shock wave production when the pulse duration is shortened below the confinement time. Since damage results from pressure gradients, the sudden onset of shock waves implies a sharp increase in the potential for damage.
We study the response of a spherical absorber immersed in aqueous media. We investigate temporal resonant absorption and present initial numerical calculations of the same topic. Initial results indicate that, because of the dynamical characteristic of the system, the response after a sequence of energy pulses depends nonlinearly on the time between pulses. Specifically, the response exhibits resonant type behavior around a critical time, the time it would take a sound wave to traverse the absorber.
Theoretical work has been previously reported in which the full thermo-mechanical response of an absorber to an incident laser pulse has been calculated. For a laser pulse of any energy or duration, the temperature rise, explosive bubble formation, and shock wave generation in the surrounding medium can be predicted. This work allows the assessment of danger to biological or opto-electronic systems from laser pulses. The work also allows the thermo-mechanical properties of micro and nano particle absorbers, such as the thermal expansion coefficient and bulk modulus, to be calculated from measurements of the pressure waves generated in the medium. Previous results assumed a temporal profile for the laser pulse that was a square wave with infinitely fast rise and fall times. We report on new calculations that use a more realistic gaussian-like temporal profile for the laser pulse. We compare how the resulting thermo-mechanical responses are altered compared to the idealized temporal square wave laser pulse.