NSF Award
2320937
When electromagnetic radiation is sent through a gas, specific frequencies can be absorbed, and then re-emitted. The re-emitted radiation can be re-absorbed somewhere else in the gas, re-emitted there, and so on. This absorption/re-emission is known as “radiation trapping”. Because each absorption/re-emission induces a delay and can change the direction as well as induce a slight frequency change, radiation trapping changes the fundamental characteristics of the radiation – in other words, the radiation arriving at a detector has characteristics that are different from that of the radiation that was originally incident on the gas. As mentioned above, radiation trapping happens mainly at specific (very high) frequencies. As modern wireless systems are moving to higher and higher frequencies, there are thus more situations where radiation trapping is important. Wireless systems in those frequency ranges might be used for communication, sensing, or both. In any of these cases, it is important to analyze the impact of radiation trapping – either to simply assess its impact, to find ways to mitigate its detrimental effects, or to actively exploit it. This project pursues an in-depth investigation of both the fundamentals of radiation trapping and its effects on next-generation wireless communications and sensing. The results of these investigations will form a foundation for future wireless system design in these ultra-high-frequency regions, and will be broadly disseminated. Furthermore, significant outreach activities, aimed at broadening the participation in this research area, are planned.<br/> <br/>The radiation trapping process has important consequences for the properties of the resonance radiation emerging from the gas. Firstly, the line shape is distorted: since photons at the center frequency of the absorption line “see” a high absorption coefficient, the probability of reaching the detector is low, while photons in the “wings” of the line shape can escape more easily. Secondly, the emerging radiation suffers from both delay dispersion, frequency dispersion (the re-emitted frequencies are shifted from the absorbed frequencies), and spatial dispersion (photons can be re-emitted in any direction, though there can be a nontrivial relationship between directional dispersion and frequency dispersion). The project will start by investigating the interaction of resonance radiation with molecular transition and the interaction between Orbital Angular Momenta (OAM) radiation with atoms and molecules. Solution methods for the fundamental equation of radiation trapping, the Holstein equation, will be investigated for conditions that are practically relevant for transmission in (Earth) atmosphere. The project then will investigate how trapping can impact sensing systems. Finally, the implications for communications systems, including multi-carrier and single-carrier systems, will be investigated. The project takes an interdisciplinary approach, combining insights from atomic/molecular physics, chemical physics, communication theory, wireless system design, and experimental design.<br/><br/>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.
