Amal Jose

Similar to microspheres, thin-walled microbubble resonators support whispering gallery modes (WGMs) that combine ultrahigh Q -factors and small effective mode volumes. In contrast, their hollow nature enables enhanced interactions with encapsulated materials and lower spectral mode density due to the tight radial confinement of the optical modes. However, the existence of a high axial-mode density still leads to significant mode mixing and modal interference that can complicate spectral shift measurements, thereby limiting sensing applications. To address this limitation, we have fabricated geometric filters directly on the surface of microbubbles using focussed ion beam milling. Based on numerical calculations, we first designed and then fabricated large tapered patterns, such as circular surface dips or holes, that could effectively filter modes while minimising optical scattering losses. Local lateral mode confinement and partial recovery of the loaded Q -factors of up to 58% were experimentally achieved by adding shallow slit patterns. Using few-mode engineered microbubble resonators, we subsequently demonstrated pressure sensing and spectral tuning of WGMs free from mode-mixing artefacts. This precision engineering approach may provide improved mode isolation, tunable directional emission, and high sensitivity measurements in microbubble resonator devices.

Long-persistent luminescent (LPL) materials store photon energy as charge and emit light over extended periods due to charge recombination. The LPL decay profiles follow a power-law rather than an exponential decay, enabling the confirmation of charge accumulation from the emission decay characteristics. While charge generation in organic materials has primarily been discussed in the context of photo-induced charge separation at electron donor-acceptor (D/A) interfaces, charge generation in single-component luminescent materials has not been extensively addressed. Here, we investigated the mechanism of charge generation in organic solids by dispersing the same luminescent material in various host media and performing slow transient emission analyses. This approach allowed us to evaluate ionization through accumulated triplet excited states and detect extremely weak charge accumulation, which is challenging with conventional transient analyses.