CIF: Small: Impact of radiation trapping on sensing and communication systems in the THz, infrared, and optical regime - foundations, challenges, and opportunities

当电磁辐射穿过气体时，特定的频率可以被吸收，然后重新发射。重新释放的辐射可以在气体的其他地方被重新吸收，在那里重新释放，以此类推。这种吸收/再发射称为“辐射捕获”。由于每次吸收/再发射都会引起延迟，并且可以改变方向以及引起轻微的频率变化，因此辐射捕获会改变辐射的基本特性-换句话说，到达探测器的辐射具有与最初入射到气体上的辐射不同的特性。如上所述，辐射捕获主要发生在特定的（非常高的）频率。随着现代无线系统向越来越高的频率移动，因此有更多的情况下，辐射捕获是重要的。这些频率范围内的无线系统可用于通信、传感或两者兼而有之。在任何一种情况下，分析辐射捕获的影响都是很重要的——要么简单地评估其影响，要么找到减轻其有害影响的方法，要么积极利用它。该项目将深入研究辐射捕获的基本原理及其对下一代无线通信和传感的影响。这些研究结果将为这些超高频区域的未来无线系统设计奠定基础，并将广泛传播。此外，还计划进行重要的外联活动，以扩大对这一研究领域的参与。辐射捕获过程对从气体中产生的共振辐射的性质有重要的影响。首先，线形扭曲：由于光子在吸收线中心频率处“看到”一个高的吸收系数，到达探测器的概率很低，而在线形“翅膀”处的光子更容易逃逸。其次，新出现的辐射受到延迟色散、频率色散（重新发射的频率从吸收的频率偏移）和空间色散（光子可以在任何方向重新发射，尽管方向色散和频率色散之间可能存在重要关系）的影响。该项目将首先研究共振辐射与分子跃迁的相互作用以及轨道角动量（OAM）辐射与原子和分子的相互作用。对于与（地球）大气中传输实际相关的条件，将研究辐射捕获的基本方程，即霍尔斯坦方程的解方法。然后，该项目将调查陷阱如何影响传感系统。最后，对通信系统的影响，包括多载波和单载波系统，将进行调查。该项目采用跨学科的方法，结合原子/分子物理，化学物理，通信理论，无线系统设计和实验设计的见解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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. 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.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.