Self-assembled conjugated polymer spheres as fluorescent microresonators

Abstract

Confinement of light inside an active medium cavity can amplify emission. Whispering gallery mode (WGM) is one of mechanisms that amplifies light effectively by confining it inside high-refractive-index microstructures, where light propagates along the circumference of a sphere via total internal reflection. Here we show that isolated single microspheres of 2–10 μm diameter, formed from self-assembly of π-conjugated alternating copolymers, display WGM photoemission induced by laser pumping. The wavelengths of the emission peaks depend sensitively on the sphere size, position of the excitation spot and refractive index of each polymer. The Q-factor increases with increasing sphere diameter and displays a linear correlation with the reciprocal radius, indicating that the small curvature increases the efficacy of the total internal reflection. WGM photoemission from π-conjugated polymer microspheres is unprecedented and may be of high technological impact since the microspheres fulfill the role of fluorophores, high-refractive-index media and resonators simultaneously, in addition to their simple fabrication process.

Color-Tunable Resonant Photoluminescence and Cavity-Mediated Multistep Energy Transfer Cascade

“…The average PL intensity ratios, r G→O and r G→R , are 1.02 and 0.79, respectively, whereas those of the reverse direction, r O→G and r R→G , are only 0. 16 Figure S11c and d), yet in the latter case, the PL intensity after propagation became very small because of the low energy transfer efficiency. Note that, according to the results on an excitation of bispheres of coupled nondoped and doped PS microspheres, the influence of the direct excitation of the adjacent sphere by the propagation of the laser beam is negligible.…”

Whispering Gallery Resonance from Self-Assembled Microspheres of Highly Fluorescent Isolated Conjugated Polymers

“…According to the ellipsometry study ( Figure S12), the k value at the photoemission wavelength range of P6 was less than 0.08, which is smaller than those of the previously reported polymers (k ∼ 0.2). 19 The plots of Q max versus reciprocal radius, i.e., the curvature of the spheres, showed an approximately linear correlation (Figure 3e) because the bending radiation loss of circular resonators declines as the curvature decreases. 19,43 In addition, we checked the excitation power dependency of the PL spectra and the intensity of WGM emission peaks ( Figure S14).…”

“…This value (Q max ) was 1.5-fold higher than that previously reported for the π-conjugated polymer microsphere with much larger d (Q max ∼ 600 for d = 10 μm sphere). 19 One possible reason for such high Q-factors is lower attenuation of the present microspheres. According to the ellipsometry study ( Figure S12), the k value at the photoemission wavelength range of P6 was less than 0.08, which is smaller than those of the previously reported polymers (k ∼ 0.2).…”

“…25−31 However, one drawback of the fluorescent microresonators we reported is attributed to the low intrinsic photoemission properties of the copolymers, which exhibit an absolute fluorescent quantum yield (ϕ FL ) of less than 0.02 in their solid state. 19,32,33 In this article, we focus our attention on isolated conjugated polymers (ICPs). 34−40 Most ICPs show high ϕ FL even in the solid state because their π-conjugated main chains are highly isolated from one another by insulating cyclic side chains.…”

“…Interestingly, upon laser excitation of a single sphere, clear whispering gallery modes (WGM) were observed in photoemission. 19 Because the refractive indices (η) of these polymers are in the range of 1.6− 1.8, which is high enough in comparison with the refractive index of air (η air ∼ 1.0), fluorescence generated inside the microspheres is efficiently confined via total internal reflection at the polymer/air interface. As a result, sharp and periodic emission lines appear in their photoluminescence spectra with Q-factors up to 600.…”