Optical Limiting In Media With Absorbing Microparticles

Abstract

We have characterized the nonlinearities observed in suspensions of carbon black particles in liquids (CBS). We have developed a preliminary explanation of the optical limiting characteristic of the CBS that qualitatively explains the low thresholds, broad-band response and other limiting characteristics. In this model, the microscopic carbon particles are heated by linear absorption to a temperature at which a plasma can be created by the optical field. These microplasmas rapidly expand, thus scattering the incident light and limiting the transmittance. This model is consistent with our observations that nonlinear scattering dominates transmission losses. We find that limiting depends on the input fluence (J/cm2) rather than irradiance (W/cm2). Therefore, limiting works well (i.e., low limiting energy) for long pulses (≥ 10 nsec) but is less effective for short pulses (≈ psec). In addition, the CBS rapidly degrades with repetitive laser firings, thus, flowing or moving the liquid between firings is necessary.