Interfacial refractive index sensing using visible-excited intrinsic zinc oxide photoluminescence coupled to whispering gallery modes

Abstract

Whispering gallery modes (WGMs) excited by the intrinsic photoluminescence (PL) in zinc oxide microspherical resonators are investigated in this work. The microspheres were synthesized via a one-pot hydrothermal technique. A polymer was applied after the synthesis to fill remaining pores in the oxide particle. Defect-related ZnO PL was excited in the visible, coupling to WGMs. The observed WGMs red-shift with increasing refractive index of the surrounding medium with a sensitivity of 90–100 nm/refractive index unit. The spherical microresonators may be used to investigate binding to and structure at the particle/solution interface.

Whispering gallery modes in intrinsic TiO2 microspheres coupling to the defect‐related photoluminescence after visible excitation

“…The sharp resonances shown in Fig. 2 on top of the broad luminescence background are caused by the coupling of the luminescence to WGMs, as observed in a number of other systems [27,28,39].…”

“…Similar to the situation in ZnO , visible light can be used in photocatalytic processes involving TiO 2 . This work shall show that a recently established coupling method for ZnO microspheres – similar to free space coupling – enables the excitation of WGMs in intrinsic TiO 2 microspheres without doping elements. Here, we shall discuss the coupling of defect induced photoluminescence (PL), which can be excited in the visible, to WGMs in a TiO 2 microsphere resonator.…”

Whispering Gallery Mode Devices for Sensing and Biosensing

“…Finally the intrinsic microsphere fluorescence can be exploited to achieve similar results. For example, zinc oxide [137] and titanium oxide [138] microspheres have been used for such purpose with a photoluminescence emission ranging from 550 to 750 nm.…”

“…However using smaller microspheres, especially in aqueous environments, for biosensing applications implies that the resonator's refractive index has to be large enough to achieve sufficient optical confinement. This typically requires the use of polymers such as polystyrene (PS) [132,141] or melamine formaldehyde [140], high refractive index glasses such as Er 2+ or Tm 3+ doped tellurite [134,142], or other oxides [137,138]. Fluorescent microspheres can also be used to decrease the lifetime of the fluorescent medium at the resonance wavelengths, via the Purcell effect [143].…”

Strain-induced spectral tuning of the whispering gallery modes in a cylindrical micro-resonator formed by a polymer optical fiber

“…Numerous reports demonstrate the tuning of WGM resonant wavelengths of spherical MRs by changing the surrounding refractive index [15], applying mechanical strain to the resonator [16], changing internal pressure [17], applying external electric field [18], etc. Self-tuning of WGM resonances has been demonstrated for a stand-alone silica microsphere by changing its temperature under the influence of the gas flow surrounding the resonator [19].…”