Mark E. Rogers

Laser induced bubble formation in the retina

The immediate thermodynamic effects of absorption of a laser pulse in the retina are theoretically studied to understand underlying physical damage mechanisms at threshold fluences. Damage is most likely to occur at threshold levels in the retinal pigment epithelium due to the strong absorption by the melanosomes.

Melanin granule model for laser-induced thermal damage in the retina

An analytical model for thermal damage of retinal tissue due to absorption of laser energy by finite-sized melanin granules is developed. Since melanin is the primary absorber of visible and near-IR light in the skin and in the retina, bulk heating of tissue can be determined by superposition of individual melanin granule effects. Granules are modeled as absorbing spheres surrounded by an infinite medium of water. Analytical solutions to the heat equation result in computations that are quick and accurate. Moreover, the model does not rely on symmetric beam profiles, and so arbitrary images can be studied. The important contribution of this model is to provide a more accurate biological description of sub-millisecond pulse exposures than previous retinal models, while achieving agreement for longer pulses. This model can also be naturally extended into the sub-microsecond domain by including vaporization as a damage mechanism. It therefore represents the beginning of a model which can be applied across the entire pulse duration domain.

Nonlinear refraction in vitreous humor

We extend the application of the z-scan technique to determine the nonlinear refractive index (n(2)) for human and rabbit vitreous humor, water, and physiological saline. In these measurements there were nonlinear contributions to the measured signal from the aqueous samples and the quartz cell that held the sample. Measurements were made with 60-ps pulses at 532 nm. To our knowledge, this is the first measurement of the nonlinear refractive properties of biological material.

Effect of nonlinear optical phenomena on retinal damage

Recent studies of retinal damage due to ultrashort laser pulses have shown interesting behavior. Laser thresholds for retinal damage from ultrashort (i.e. <EQ 1 ns) laser pulses are produced at lower energies than in the nanosecond (ns) to microsecond (microsecond(s) ) laser pulse regime. We examine how nonlinear optical phenomena affect the characteristics of light impinging the retina and hence, changes the minimum energy required to produce damage. Nonlinear optical phenomena which occur in homogeneous materials like the ocular media include self-focusing, stimulated Brillouin scattering, supercontinuum generation, laser induced breakdown, and nonlinear absorption. We will discuss all relevant thresholds and determine which nonlinear optical phenomena play a role in mediating the reduction in energy required to produce minimum visible lesion damage to the retina.

Laser-induced bubble formation in the retina

Bubble formation in the retinal pigment epithelium by submicrosecond laser pulses may be a source of laser induced retinal damage. Heat conduction away from absorbing melanin granules requires timescales on the order of microseconds. For pulses of shorter duration, all energy absorbed is effectively absorbed as a (delta) -function in time, and energy concentration may be high enough to cause vaporization of the surrounding medium. This occurs at lower fluences than required for thermal denaturation of a significant volume of cellular material. The adiabatic nature of the absorption and subsequent expansion is used to develop expressions for the calculation of maximum bubble size as a function of laser intensity and melanosome properties such as radius and absorption coefficients. We describe the analysis that went into the development of the bubble size expression and also present the results for representative calculations of maximum bubble radius. We find that our expression leads to a threshold for the formation of bubbles in the retinal pigment epithelium that is close to the ED50 experimentally measured for laser induced retinal damage.