Planar dielectric multilayers are known to possess resonant properties, which play a critical role in guiding and sustaining different kinds of optical modes. This is well corroborated in the literature. Among the existing electromagnetic surface modes, Bloch surface waves (BSWs) have drawn considerable interest owing to the advantages of the energy and momentum narrow resonances. The narrow resonances together with the evanescent field distributions, facilitate the interaction between the BSWs and the external medium.
Although a considerable amount of research on BSW excitation has been reported, most of them have mainly focused on momentum, spectral and spatial features associated with the coupling and manipulation of BSWs. However, the possible temporal effects have been substantially neglected despite its important implications in practical applications, especially those involving the use of BSW excitation to trigger ultrafast light-matter interactions. The temporal and spectral properties of pulses can be significantly modified in the presence of photonic bandgaps and resonances, resulting in temporal widening, splitting and tailing effects. Moreover, different deposition techniques enable the selection of suitable materials for fabricating multilayers sustaining BSWs across a broad spectral range.
On this account, researchers at the University of Eastern Finland: PhD candidate Atsu Asilevi, Dr. Henri Pesonen, Dr. Ségoléne Pelisset, Professor Matthieu Roussey and Professor Jari Turunen together with Professor Emiliano Descrovi from Polytechnic University of Turin studied the effects of BSW excitation on temporal characteristics of short optical pulses. The structure considered in this study was a dielectric multilayer consisting of N identical low-/high-index bilayers with an extra terminating layer separating the substrate and the superstrate. The BSW resonance and the spectral width of the incident pulse were assumed to be of the same order of magnitude. Their work is currently published in the journal, Optics Letters.
The research team showed that a sharp spectral phase variation around the BSW resonance resulted in a profound temporal modulation of the reflected pulse. The occurrence of such resonant excitation within the spectrum of the incident pulse resulted in the temporal splitting of the reflected pulse into leading and trailing parts, with the trailing part exhibiting an exponentially decaying tail. The role and importance of the number of bilayers and the absorption level in the multilayer stack were discussed. The observed temporal splitting effect could be implemented in BSW-based sensing platforms employing time-gated detection to filter out undesired backgrounds and improve the collection of useful signals. Furthermore, it was worth noting that, unlike incident pulses, the spectral width of the BSW resonance had a significant effect on the characteristics of the temporal modulation.
In summary, the authors reported a theoretical analysis of pulse modulation by BSW excitation as well as the effects of BSW resonances on the temporal properties of evanescent-wave pulses. Although the discussion was restricted to a plane-wave pulsed excitation, the analysis could be extended to finite-size confined pulses. This provided more insights into the temporal and spectral behaviors of the pulses inside and on top of the multilayer structure. In a statement to Advances in Engineering, first author, Atsu Asilevi pointed out their study contributes to advancement in the application of BSW excitation in pulse modulation.
Asilevi, A. L., Pesonen, H., Pelisset, S., Descrovi, E., Roussey, M., & Turunen, J. (2022). Pulse modulation by Bloch Surface Wave Excitation. Optics Letters, 47(10), 2574-2577.