Feasibility of Free-Space Transmission using L-Band Maser Signals in Organic Gain Media

Abstract

Atmospheric conditions such as fog, humidity, and scattering by foliage routinely degrade optical free-space (FS) links, motivating alternatives that are robust in adverse conditions. Coherent microwave sources offer a compelling alternative for quantum-secure communication, yet their propagation outside enclosed resonators has remained untested. Here, we demonstrate room-temperature FS transmission of maser signals generated using organic L-band (1-2 GHz) gain media, pentacene doped p-terphenyl (Pc: PTP), and Diazapentacene-doped p-terphenyl (DAP:PTP). Spectral and temporal coherence is preserved over distances up to 25 cm, approximately one wavelength at the masing frequency. Tests included polarisation misalignment, high-humidity conditions, and partial occlusion by foliage to emulate realistic FS reception scenarios. Strong spin-photon coupling was maintained, as confirmed by persistent rabi oscillations and normal-mode splitting. An instantaneous peak output of 4.29 mW (+6.32 dBm) from a 0.01% DAP:PTP gain medium. with a pulse duration of ~4 {\mu}s, marks a performance benchmark for directly coupled masers. These findings demonstrate a proof-of-concept for masers as viable platforms for short-range, interference-resilient coherent microwave links, with future relevance to quantum sensing and secure communication technologies.