| Project Title: | IRTSCS: Impact of radiation trapping on sensing and communication systems in the THz, infrared, and optical regime - foundations, challenges, and opportunities |
| Funding Agency: | National Science Foundation |
| Award Number: | 2320937 |
When the frequency of electromagnetic radiation matches the energy gap between different atomic or molecular energy states, its absorption can bring the atom/molecule into a higher (excited) state. Such radiation is thus called resonance radiation. Radiation trapping describes the interaction of this resonance interaction with an ensemble of atoms or molecules, e.g., in a gas (or vapor). Assume an externally created photon, with a wavelength matching a resonant atomic transition, is incident on the gas and gets absorbed, exciting an atom to a higher state. Due to natural decay, the photon is reemitted after some time, but – since its wavelength still matches the atomic resonance, the probability is high that is re-absorbed by a nearby atom, re-emitted after some time, absorbed by yet another atom, and so on – until it finally escapes from the gas. The radiation trapping process has important consequences for the properties of the resonance radiation emerging from the gas. Firstly, the lineshape 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 lineshape can escape more easily. Secondly, the emerging radiation is suffering from both delay dispersion, frequency dispersion (the reemitted frequencies are different from the absorbed frequencies), and spatial dispersion (photons can be re-emitted into any direction, though there can be a nontrivial relationship between directional dispersion and frequency dispersion).
As modern wireless systems are moving to higher and higher frequencies, there are more situations where the operating frequency matches atomic or molecular transitions. This is true for THz signals that mostly interact with water vapor, as well as free-space optical communications in the infrared or visible spectrum, which might interact with a variety of molecules or atoms, from CO2 to various pollutants. These systems might be used for communication, sensing, or both. The project aims to provide an in-depth investigation of both the fundamentals of radiation trapping and its effects on next-generation wireless communications and sensing.