A systematic investigation on the proper utilization of defect levels present in ZnO is very much in demand to avail many applications of photonics in visible and near infrared (NIR) regions. In this paper, we have engineered intrinsic defects of zinc oxide (ZnO) to achieve high-quality intense whispering gallery modes (WGMs) in a single ZnO microsphere optical resonator by doping with alkali metal ions. Here, a single microsphere of undoped and doped ZnO was considered to investigate WGMs by recording luminescence spectra using a microphotoluminescence system under green laser excitation having a central wavelength of 532 nm and a fixed power of 55 mW/cm2. We have found that there is a significant enhancement in the intensity of WGMs in the case of doped ZnO in comparison to undoped ones. Among all the doped ZnO microspheres, 2 mol. % Li-doped ZnO yields the strongest and intense WGMs, which are accompanied by high-quality (Q)-factors. Furthermore, the pump power dependence measurement performed in 2 mol. % Li-doped ZnO reveals the lasing action in the visible optical window by explicitly exploiting the defect levels present in the material. Thus, our proposed defect engineered ZnO microsphere may represent a promising optical microresonator for developing highly sensitive WGMs based optical sensors.
Smooth surfaced and crystalline undoped and ytterbium doped zinc oxide (ZnO) microspheres having an approximate size of 3–5 μm were synthesized by hydrothermal process. Out of these microspheres, a single microparticle was chosen and engaged as a whispering gallery wave microresonator. The defect induced luminescence from an individual ZnO microsphere was investigated with micro-photoluminescence measurement in the spectral range of 565 to 740 nm under the excitation of a green laser having a centered wavelength at 532 nm. The defects-related emissions from a single ZnO microsphere show optical resonance peaks so-called “whispering gallery modes” (WGMs) which are confirmed with the theoretical calculation. Further, ZnO microspheres were chemically doped with the different molar percentages of Ytterbium (Yb), and enhancement in their emission properties was investigated. Our experimental results show that ZnO microspheres with 0.5 mol. % doping of Yb gives the strongest optical emission and has highest Q-factor which can be employed in the development of WGM based optical biosensor or laser.
Optical whispering gallery mode microresonators (WGM-µRs) are powerful sensitive components with many analytical applications. Here, spherical WGM-µRs have been synthesised in a single-step microwave (MW)-assisted heterophase polymerisation. The microresonators are based on poly(styrene) beads into which the organic lasing dye nile red was incorporated as gain medium in situ during the polymerisation. The particle diameter and diameter distribution of the synthesised particles were tuned in the range of around 200 nm up to 50 µm by adjusting the concentration between stabiliser poly-(N-vinyl pyrrolidone) (PVP) and monomer styrene, and the solvent composition in the dispersion process. Lower water content enabled the synthesis of spherical particles with large size polydispersity, from which WGM-µRs with a variety of diameters were selected. Microspheres with diameters ≳3.5 µm supported WGMs. The WGMs were excited through free space via the fluorescence of the laser dye. Pumping power levels <1 µW were sufficient to excite WGMs. WGM shifts of beads with diameter between ≈5 and 30 µm measured in air and water show a sensitivity up to 54 nm/RIU for the smallest particles. Dye doped WGM-µR in the low µm size range obtained by the
Photoluminescence (PL) spectra of high-Q ZnO microspheres of various degrees of inhomogeneity are studied. Multiple sharp resonance features with distinct patterns associated with whispering gallery modes (WGM) of these microspheres are observed. Relative WGM peak strengths in these PL spectra are found to be closely related to defects and the surface roughness of the microspheres. A theoretical model based on Mie theory and Green’s function method is used to simulate the optical emission spectra of these microspheres. The effects of defects and surface roughness can be characterized by a frequency-dependent external quality factor (Q ex ) with a prefactor. By adjusting the prefactor in Q ex and a parameter describing the contribution of stimulated emission in the PL spectra, we can fit the observed PL spectra well. Through this study, we gain a better understanding of the relation between the diffusive scattering and the lineshapes of spectral features in PL.
In this work, we investigated both spontaneous and stimulated whispering gallery mode (WGM) emissions of 2 mol. % Li+-doped ZnO (Li-ZnO) microspheres with different sizes under 325 and 488 nm wavelength laser excitations, respectively. It was found that all the microspheres exhibit stimulated emissions under a visible laser excitation source of 488 nm wavelength after the threshold pumping power. Thereafter, we studied the dependence of threshold pumping power on the size of microresonators to achieve stimulated emissions by individual microspheres. Furthermore, two microspheres (MS2 and MS3) are excited via a 325 nm UV laser, and surprisingly, the WGM peaks of higher intensity are observed in the visible rather than in the UV spectral region. We expected that most of the emissions are achieved via defect states transitions instead of inter-band transitions in the microresonators. It was found that WGMs in each microsphere exhibit a linear spectral shift of 3–5 nm with increasing pumping power of 488 nm excitation laser source. We believe that these proposed microspheres can be utilized effectively as WGM-based visible lasers and sensors.
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