Self-induced transparency with focused light pulses

Abstract

In the field of optical image transmission, a focused light pulse is used to excite a target sample. Most of the problems in this area are due to the effects of an intervening medium placed between sample and source. The medium prevents most of the pulse energy from reaching the sample either by absorbing the light or scattering it. This is common in areas such as medical imaging of living tissues, undersea visibility and manufacturing inspections.
In this paper, optical transmission of focused light pulses through a highly absorbing medium is investigated numerically. Under normal circumstances, the light pulse will be absorbed by the medium after passing through a few Beer's absorption lengths. As the intensity of the pulse is increased, the absorbing medium suddenly becomes transparent to it. The pulse, as it propagates, has low energy losses and remains in the medium longer than its low intensity speed. This phenomenon is known as self-induced transparency (SIT).
Existing theories in SIT do not take into account the effects of diffraction and focusing. These factors were neglected due to the difficulty of incorporating them in the optical Maxwell-Bloch (MB) equations that govern the propagation of a light pulse in the absorber. Using a method known as ASPW, the paper addresses this problem. It has been used in solving wave propagation and diffraction problems. An arbitrary wave field is separated into its plane wave components. The resulting wave field at some other point is then solved by linear superposition of all its plane wave components taking into account the amplitude and phase change for each component.
In imaging applications, diffraction ultimately limits the resolving performance of the system and focusing is necessary to deliver sufficient excitation energy to the sample. This paper reports to the author's knowledge the first theoretical and numerical study of SIT incorporating both diffraction and focusing effects simultaneously. Only Gaussian light pulses are considered.