Optimization of Whispering Gallery Mode Microresonators for Graphene Nanoribbons Emission

Ultra‐narrow graphene nanoribbons (GNRs) demonstrate bright photoluminescence (PL) with parameters determined by GNR geometry. Combination of wide variable spectrum and fast electronic relaxation permits to consider GNRs as a potential active laser material. To avoid reabsorption of light and to realize laser generation, it is necessary to use nonstandard resonators. This study theoretically investigates microspherical resonators supporting whispering gallery modes for lasing in the visible/near‐infrared range, focusing on optimizing the design for efficient light amplification. The selected spectral range is well suited for GNRs, which offer a compelling alternative to traditional gain materials as a result of their tunable electronic properties and high PL intensity. The maximization of the Q‐ factor, essential for lasing, is limited by parasitic modes with higher radial numbers. Specifically, for a commonly used 6 µm diameter polystyrene microsphere (refractive index N = 1.57) in air, modes with radial numbers n = 1, n = 2, and n = 3 coexist within the spectral range of 550–850 nm, with a mode spacing of less than 1 nm. This results in groups of closely spaced modes ( n = 1, n = 2, and n = 3) appearing throughout the resonant spectrum, with the lowest Q mode ( n = 3) dictating the overall performance of each group. We explored strategies to suppress parasitic modes, including reducing the diameter of the microsphere and modifying the refractive index of the surrounding medium. Single‐mode operation ( n = 1) within the selected spectral range is achievable, but at the cost of reducing the Q ‐factor of the fundamental mode (the highest Q mode, absent parasitic modes) to approximately 10 which can be achieved by selecting a polystyrene microsphere with a diameter of approximately 2.8 µm. Interestingly, operating with two modes ( n = 1 and n = 2) can offer a compromise, as the n = 2 mode can exhibit a Q ‐factor comparable to the fundamental mode in the absence of higher‐order modes, potentially reaching Q near 10. Operation with two modes ( n = 1 and n = 2) is achievable by choosing a larger diameter microsphere, approximately 4.8 µm. Parasite modes limit achievable Q. Suppressing them via microsphere diameter reduction presents a practical route toward creating functional microresonator devices.