Arnold Guerra

Investigation of laser-induced cell lysis using time-resolved imaging

Using time-resolved imaging, we investigated the lysis of confluent PtK2 cell cultures by pulsed laser microbeam irradiation. Images obtained at time delays of 0.5 ns to 50 μs demonstrate lysis to be mediated by laser-induced plasma formation resulting in pressure wave propagation and cavitation bubble formation. Image analysis enabled quantitative characterization of the pressure wave and cavitation bubble dynamics. The zone of cell damage exceeded the plasma size and serves to implicate cavitation bubble expansion as the primary agent of cell injury.

Optoacoustic imaging using interferometric measurement of surface displacement

We describe an optoacoustic imaging technique based on time-resolved measurements of laser-induced thermoelastic expansion. Tomographic images of tissue phantoms are formed using such measurements made at several locations following irradiation with a Q-switched Nd:YAG (λ=1064nm) laser pulse. Our system is based on a modified Mach–Zehnder interferometer that measures surface displacement with a temporal resolution of 4ns and a displacement sensitivity of 0.3nm. Images formed from data sets acquired from several highly scattering tissue phantoms provide better than 200μm resolution and show great promise for high-resolution noninvasive imaging of heterogeneous tissues at depths approaching 1cm.

Imaging through quasi-particle transport

Quasi-particles travel down a one-dimensional concentration gradient in a thin metallic film composed of regions of only gold and regions of gold-titanium-gold multilayers. We obtain an image depending on whether the quasi-particles encounter a multilayer.

Experimental investigation of optical breakdown using nanosecond 532-nm and 1064-nm laser pulses delivered at high numerical aperture

We have conducted time-resolved studies of optical breakdown produced by the irradiation of water using 6 ns Nd:YAG laser pulses of 1064 nm and 532 nm wavelength focused at a numerical aperture of NA=0.9. We determined pulse energy threshold values for plasma formation to be 1.89 (mu) J and 18.3 (mu) J for 532 and 1064 nm irradiation, respectively. These energy thresholds correspond to irradiance thresholds of 0.77 x 109 W/mm2 for 532 nm irradiation and 1.87 x 109 W/mm2 for 1064 nm irradiation. For pulse energies 1x, 2x, 5x, and 10x above threshold, we determined the length of the laser induced plasma, the propagation speed and peak pressures of the emitted shock wave, and the mechanical energy dissipated by subsequent cavitation bubble formation, growth and collapse. This analysis demonstrates that both the breakdown threshold as well as the conversion efficiency of the incident laser energy into mechanical energy is smaller for irradiation at 532 nm than for 1064 nm. These results are consistent with laser parameters employed for a variety of nanosecond pulsed micro irradiation procedures using 1064 nm and 532 nm radiation focused by microscope objectives with large numerical apertures (NA >0.8). These results suggest that laser- induced breakdown is the primary mechanism that drives a variety of cellular micro manipulation techniques which employ nanosecond visible and near-infrared laser pulses.

Optoacoustic imaging via the interferometic measurement of surface displacement

A modified Mach Zehnder interferometer system has been developed to measure surface motion with 4 ns temporal resolution and 0.2 nm displacement sensitivity. We discuss its use to measure the optical properties of homogeneous turbid media and image heterogeneous volumes to depths of 1 cm with ~200 micrometer resolution.

POISe: pulsed optoacoustic interferometric spectroscopy and imaging

POISe is a spectroscopic imaging technique based on the measurement of surface motion resulting from thermoelastic stress waves produced by short pulse laser irradiation of optically heterogeneous turbid samples. Here we show the capability of POISe to form tomographic images of tissue phantoms using surface displacement measurements taken at several locations following irradiation of a sample with a Q-switched Nd:YAG laser λ=1064 nm. The principal component of POISe is a modified Mach-Zehnder interferometer that provides surface displacement measurements with a temporal resolution of 4 ns and a displacement sensitivity of 0.2 nm. By performing simple image reconstructions on data sets acquired from several tissue-like phantoms, we demonstrate the ability of POISe to provide better than 250 μm spatial resolution at depths of 6 to 8 mm in a strongly scattering medium (μs=1/mm). This technique shows great promise for high-resolution non-invasive imaging of superficial (< 1 cm) tissue structures.

Examination of laser-induced cell lysis by time resolved imaging

Highly focused laser microbeams are being used with increasing regularity for targeted cell lysis, cellular microsurgery and molecular delivery via transient cell membrane permeabilization. To examine the mechanisms of laser induced cell lysis, we performed time-resolved imaging of confluent PtK2 cell cultures following the delivery of a single 6 ns, 532 nm Nd:YAG laser pulse. The laser pulse energies employed correspond to 1x and 3x threshold for plasma formation. The resulting plasma formation, pressure wave propagation and cavitation bubble dynamics were imaged over a temporal range spanning 5 orders of magnitude (0.5 ns - 50 µs). Time-resolved imaging enabled determination of process characteristics including pressure wave speed and amplitude and cavitation bubble energies. The time-resolved images also revealed the onset of cellular damage to occur on nano-second time scales and complete within 1 µs. Moreover, the size of the damage zone was larger than the plasma but smaller than the maximum cavitation bubble size. This indicated that mechanisms apart from plasma vaporization namely pressure wave propagation and cavitation bubble expansion are contributors to cellular damage. Dye exclusion assays showed that the majority of cells experiencing considerable deformation due to fluid flow generated by the cavitation bubble expansion remain viable over 24 hours.

Quasi-electron and phonon interactions in the femtosecond time domain

Abstract The properties of optically excited particles in ultra thin metallic films differ markedly between the femto, and the picosecond time domain. In the specific case of Au films, the difference arises from the lack of thermal equilibrium in the femtosecond domain. It is also observed that some quasi-particles travel essentially unhindered, i.e., ballistically at near the Fermi velocity. It is further concluded that although a large number of phonons are produced through the decay of electrons, only a small fraction of the phonon energy returns to the quasi-electrons. It is further demonstrated that the experimental results are not consistent with the so-called “two temperature model (TTM)”, and that an attempt to use classical thermal dynamics, when a system is not in thermal equilibrium, is difficult to justify. We demonstrate that excellent agreement exists between experiment and Fermi-liquid theory (FLT), and that many parameters and functional relations, which describe quasiparticle dynamics and transport, can be recovered by the application of FLT to the experimental results.