Coherent Control of Light Propagation and Absorption in Complex Media and Resonators

Yale University, New Haven CT

Investigators

Abstract

NONTECHNICAL SUMMARY This award supports theoretical research and related education and training towards advancing our ability to control light interacting with matter. The first of two main thrusts aims to manipulate and control the ability of a material to absorb light by varying either illumination conditions or other external control parameters. Normally, the absorption of light is considered as a fixed property of a specific material, which cannot be easily and reversibly changed. However, if the material is appropriately structured at a microscopic level, it can be made to preferentially absorb, or even perfectly absorb input light with specific characteristics. Such a structure is called a Coherent Perfect Absorber or Anti-laser, since it behaves in a certain sense like a laser in reverse, absorbing instead of emitting light of specific frequency. The current project will study how to predict and control the absorption of very strong light signals. It will also study how to design structures that will perfectly absorb light. These and similar microstructures that allow the manipulation of light routing are of interest for communications and sensing, as well as for on-chip optical processing of information. The second thrust will study how to control light when it is scattered by small nonabsorbing particles, such as those that make up white paint. Typically, such scattering media make it impossible for a beam of light to penetrate through without spreading out and losing its focus. Modern optical instruments provide techniques for synthesizing particular beams of light that can be focused through such media, given enough input information about the medium. This project will extend the theory of such focusing in scattering/opaque media so as to focus to a specific point in space at a specific time after the beam has entered the medium. This new type of optical control modality shows promise for improved optical imaging of regions deep within biological media, such as living tissue. In addition to the research, PhD students will be trained in critical areas of modern optical technology, as identified in the national photonics initiative. The PI will communicate the importance of condensed matter and optical science to general audiences and precollege students through his popular science writings and lecturing at diverse venues. TECHNICAL SUMMARY This award supports theoretical research and related education and training towards advancing our ability to control light interacting with matter. The project comprises two research thrusts on the physics of light-matter interactions in disordered media and resonant structures. Potential applications of the research have great relevance to important priorities in applied science, relating to communications, information processing, materials research and imaging. The first thrust addresses the concept of coherent perfect absorption or time-reversed lasing, introduced recently by the PI. As previously developed, this phenomenon only applies to linear absorption, which limits the power that can be transmitted and absorbed via this mechanism. The proposed work will extend the theory to describe saturable absorbers and to predict the properties required to maintain perfect absorption at high input powers. An ab initio theory will be developed that takes into account nonlinear effects of saturation, along with algorithms and codes to implement the theory that will apply to absorbing systems and resonators of arbitrary geometry and complexity. The coexistence of perfect absorption and optical bistability and the properties of such bistable absorbers as an on-off optical switch will be studied. Finally, the conditions for coherent perfect absorption at an exceptional point (non-hermitian degeneracy) will be determined and shown to enable intrinsically chiral resonant absorption. The second research thrust is to develop a theory of coherent control of light in opaque strong-scattering media, specifically focusing light strongly to a region in space within or at the opposite side of such a medium at a particular time. This is achieved by finding special pulsed input states that exploit multipath interference to bias the diffusion of light and achieve coherent focusing. The correlations between the light intensity at different points in space will play a critical role in enabling such coherent control. This work builds heavily on insights from quantum transport in mesoscopic electronic systems. The work will provide a general theoretical framework based on microscopic calculations and random-matrix theory for controlling the propagation and absorption of light in strong scattering media for a variety of optimization goals. In addition to the research, PhD students will be trained in critical areas of modern optical technology, as identified in the national photonics initiative. The PI will communicate the importance of condensed matter and optical science to general audiences and precollege students through his popular science writings and lecturing at diverse venues. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.