Enhancing the radiation efficiency of dye doped whispering gallery mode microresonators

Whispering-gallery mode lasers for biosensing: a rationale for reducing the lasing threshold

“…Polystyrene microspheres with a nominal diameter (Ø) of 10.52 μm with a standard deviation (ΔØ) of 0.25 μm and a refractive index of 1.591 (Polysciences Inc., USA) were doped with a fluorescent laser dye (Nile Red, λ abs ~ 532 nm, λ em ~ 590 nm, Sigma Aldrich) using a liquid two phase system [10] . The fluorescent dye was first dissolved into xylene until the solubility limit was reached.…”

“…At the resonance wavelengths of the microcavity, the Purcell effect increases the emission rate of the gain medium [9] . As a consequence, the resulting WGM spectrum observable remotely in the far field shows up as a fluorescent signal modulated by sharp peaks corresponding to the microcavity resonances [10] . While this approach offers advantages in terms of practicality such as robustness of the coupling scheme, easy excitation and collection of the WGM modulated fluorescence, the Q factor observed for the radiation modes are often 3 or 4 orders of magnitude lower than those observed using the evanescent field coupling approach, eventually limiting the use of such a WGM excitation strategy in terms of resolution for sensing applications [11] .…”

“…While this approach offers advantages in terms of practicality such as robustness of the coupling scheme, easy excitation and collection of the WGM modulated fluorescence, the Q factor observed for the radiation modes are often 3 or 4 orders of magnitude lower than those observed using the evanescent field coupling approach, eventually limiting the use of such a WGM excitation strategy in terms of resolution for sensing applications [11] . A way forward to address this issue is to operate the microresonator beyond its fluorescence regime, inducing stimulated emission, resulting in a higher Q factor [10] . Examples of such lasing WGMs have been published by Kuwata, Gonokami and Takeda, who used dye doped polymer microspheres [12] .…”

Optimization of whispering gallery mode sensor design for applications in biosensing

“…Despite the consideration of the three loss mechanisms, there remains a noticeable difference in the measured and predicted Q values (Q ~ 1000 [30] measured and Q ~ 2x10 5 predicted for a 15μm polystyrene sphere in water), as their individual contributions are not noticeable. This difference is though to be a result of one of the assumptions made in deriving the Chew model, namely that the resonator being considered is a perfect dielectric sphere.…”

Dynamic Self-Referencing Approach to Whispering Gallery Mode Biosensing and Its Application to Measurement within Undiluted Serum

“…Fluorescent or active resonators are particularly interesting in this context as they allow remote excitation of the resonator, thereby alleviating some of the practical limitations of passive resonator configurations 9,[23][24][25] . Active resonators do display lower Q-factors 26 in comparison with passive resonators 27,28 , however, techniques exist that can facilitate in improving the Q-factor such as operating the resonator within the stimulated emission regime 29 or breaking the symmetry of the resonator by placing it onto the tip of a microstructured optical fiber (MOF) 30 . This second technique has the additional advantage of creating a robust and easy to use dip-sensing architecture 1 .…”