Yanyan Zhi - Academia.edu (original) (raw)

Papers by Yanyan Zhi

Research paper thumbnail of Refractometric sensitivity and thermal stabilization of fluorescent core microcapillary sensors: theory and experiment

Applied Optics, 2015

ABSTRACT Fluorescent-core microcapillaries (FCMs) present a robust basis for the application of o... more ABSTRACT Fluorescent-core microcapillaries (FCMs) present a robust basis for the application of optical whispering gallery modes toward refractometric sensing. An important question concerns whether these devices can be rendered insensitive to local temperature fluctuations, which may otherwise limit their refractometric detection limits, mainly as a result of thermorefractive effects. Here, we first use a standard cylindrical cavity formalism to develop the refractometric and thermally limited detection limits for the FCM structure. We then measure the thermal response of a real device with different analytes in the channel and compare the result to the theory. Good stability against temperature fluctuations was obtained for an ethanol solvent, with a near-zero observed thermal shift for the transverse magnetic modes. Similarly good results could in principle be obtained for any other solvent (e.g., water), if the thickness of the fluorescent layer can be sufficiently well controlled.

Research paper thumbnail of Refractometric Sensing with Silicon Quantum Dots Coupled to a Microsphere

Plasmonics, 2012

ABSTRACT Quantum dots (QDs) coupled to an optical microsphere can be used as fluorescent refracto... more ABSTRACT Quantum dots (QDs) coupled to an optical microsphere can be used as fluorescent refractometric sensors. The QD emission couples to the whispering gallery resonances of the microsphere, leading to sharp, periodic maxima in the fluorescence spectrum. Silicon QDs (Si-QDs) are especially attractive fluorophores because of their low toxicity and ease of handling. In this work, a thin layer of Si-QDs was coated onto the surface of a microsphere made by melting the end of a tapered optical fiber. Refractometric sensing experiments were conducted using two methods. First, the sphere was immersed directly into a cuvette containing methanol–water mixtures. Second, the sphere was inserted into a silica capillary and the solutions were pumped through the capillary channel. The latter method enables microfluidic operation, which is otherwise difficult to achieve with a microsphere. In both geometries, high-visibility (V = 0.83) modes were observed with Q factors up to 1,700. Using standard signal processing methods applied to the whispering gallery mode (WGM) spectrum, sensorgram-type measurements were conducted using single Si-QD-coated microspheres. The WGM resonances shifted as a function of the refractive index of the analyte solution, giving sensitivities ranging from ~30 to 100 nm/refractive index unit (RIU) for different microspheres and a detection limit on the order of 10−4 RIU.

Research paper thumbnail of Microcavity effects in ensembles of silicon quantum dots coupled to high-Q resonators

physica status solidi (a), 2011

Page 1. Microcavity effects in ensembles of silicon quantum dots coupled to high-Q resonatorsPabl... more Page 1. Microcavity effects in ensembles of silicon quantum dots coupled to high-Q resonatorsPablo Bianucci, Yanyan Zhi, Frank Marsiglio, Josh Silverstone, and Al Meldrum* Department of Physics, University of Alberta, T6G2G7 Edmonton, Canada ...

Research paper thumbnail of Structure of whispering gallery mode spectrum of microspheres coated with fluorescent silicon quantum dots

Journal of the Optical Society of America B, 2013

Whispering gallery modes (WGMs) in microspheres containing embedded fluorophores (e.g., organic d... more Whispering gallery modes (WGMs) in microspheres containing embedded fluorophores (e.g., organic dyes or quantum dots) may find refractometric sensing or microlasing applications. However, there have been relatively few investigations on the relationship between the intrinsic microsphere resonances and the WGMs observed in fluorescence spectra for emitters coupled to the microsphere. Here we find that an apparently simple fluorescence WGM spectrum can mask a much more complicated underlying microcavity mode structure and that the observed fluorescence spectra are controlled by the emitter linewidth. By examining the cavity structure, we also verify that an effective ensemble emitter linewidth can be extracted from the fluorescence data. Finally, spectral diffusion is suggested as a possible origin of the periodic fluorescence WGM spectra observed in many microsphere cavities, without which these resonances might be unobservable.

Research paper thumbnail of Silicon quantum dot coated microspheres for microfluidic refractive index sensing

Journal of the Optical Society of America B, 2013

ABSTRACT The electromagnetic resonances of optical microspheres—the so-called whispering gallery ... more ABSTRACT The electromagnetic resonances of optical microspheres—the so-called whispering gallery modes (WGMs)—can be used for refractometric sensing of surrounding fluids. Microspheres are attractive because they offer high sensitivity and can be utilized with fluorescent dyes or quantum dots. One issue with microspheres, however, is that they are difficult to integrate into microfluidic systems. Here, we develop a microfluidic structure that permits sensing applications using a single microsphere in a capillary. To achieve this, a microsphere formed on the end of a tapered fiber was first coated with fluorescent silicon quantum dots (QDs). The sphere was then inserted into a microcapillary and the fluorescence WGMs were monitored as different fluids were pumped through the channel. The sensitivity and detection limits for this sphere-in-a-capillary device were measured for several different QD film thicknesses and for two different microsphere sizes. Because of the relatively high-visibility mode structure, the sensitivity and detection limit can be defined by Fourier analysis of the free spectral range and WGM spectral shifts.

Research paper thumbnail of Refractometric sensitivity and thermal stabilization of fluorescent core microcapillary sensors: theory and experiment

Applied Optics, 2015

ABSTRACT Fluorescent-core microcapillaries (FCMs) present a robust basis for the application of o... more ABSTRACT Fluorescent-core microcapillaries (FCMs) present a robust basis for the application of optical whispering gallery modes toward refractometric sensing. An important question concerns whether these devices can be rendered insensitive to local temperature fluctuations, which may otherwise limit their refractometric detection limits, mainly as a result of thermorefractive effects. Here, we first use a standard cylindrical cavity formalism to develop the refractometric and thermally limited detection limits for the FCM structure. We then measure the thermal response of a real device with different analytes in the channel and compare the result to the theory. Good stability against temperature fluctuations was obtained for an ethanol solvent, with a near-zero observed thermal shift for the transverse magnetic modes. Similarly good results could in principle be obtained for any other solvent (e.g., water), if the thickness of the fluorescent layer can be sufficiently well controlled.

Research paper thumbnail of Refractometric Sensing with Silicon Quantum Dots Coupled to a Microsphere

Plasmonics, 2012

ABSTRACT Quantum dots (QDs) coupled to an optical microsphere can be used as fluorescent refracto... more ABSTRACT Quantum dots (QDs) coupled to an optical microsphere can be used as fluorescent refractometric sensors. The QD emission couples to the whispering gallery resonances of the microsphere, leading to sharp, periodic maxima in the fluorescence spectrum. Silicon QDs (Si-QDs) are especially attractive fluorophores because of their low toxicity and ease of handling. In this work, a thin layer of Si-QDs was coated onto the surface of a microsphere made by melting the end of a tapered optical fiber. Refractometric sensing experiments were conducted using two methods. First, the sphere was immersed directly into a cuvette containing methanol–water mixtures. Second, the sphere was inserted into a silica capillary and the solutions were pumped through the capillary channel. The latter method enables microfluidic operation, which is otherwise difficult to achieve with a microsphere. In both geometries, high-visibility (V = 0.83) modes were observed with Q factors up to 1,700. Using standard signal processing methods applied to the whispering gallery mode (WGM) spectrum, sensorgram-type measurements were conducted using single Si-QD-coated microspheres. The WGM resonances shifted as a function of the refractive index of the analyte solution, giving sensitivities ranging from ~30 to 100 nm/refractive index unit (RIU) for different microspheres and a detection limit on the order of 10−4 RIU.

Research paper thumbnail of Microcavity effects in ensembles of silicon quantum dots coupled to high-Q resonators

physica status solidi (a), 2011

Page 1. Microcavity effects in ensembles of silicon quantum dots coupled to high-Q resonatorsPabl... more Page 1. Microcavity effects in ensembles of silicon quantum dots coupled to high-Q resonatorsPablo Bianucci, Yanyan Zhi, Frank Marsiglio, Josh Silverstone, and Al Meldrum* Department of Physics, University of Alberta, T6G2G7 Edmonton, Canada ...

Research paper thumbnail of Structure of whispering gallery mode spectrum of microspheres coated with fluorescent silicon quantum dots

Journal of the Optical Society of America B, 2013

Whispering gallery modes (WGMs) in microspheres containing embedded fluorophores (e.g., organic d... more Whispering gallery modes (WGMs) in microspheres containing embedded fluorophores (e.g., organic dyes or quantum dots) may find refractometric sensing or microlasing applications. However, there have been relatively few investigations on the relationship between the intrinsic microsphere resonances and the WGMs observed in fluorescence spectra for emitters coupled to the microsphere. Here we find that an apparently simple fluorescence WGM spectrum can mask a much more complicated underlying microcavity mode structure and that the observed fluorescence spectra are controlled by the emitter linewidth. By examining the cavity structure, we also verify that an effective ensemble emitter linewidth can be extracted from the fluorescence data. Finally, spectral diffusion is suggested as a possible origin of the periodic fluorescence WGM spectra observed in many microsphere cavities, without which these resonances might be unobservable.

Research paper thumbnail of Silicon quantum dot coated microspheres for microfluidic refractive index sensing

Journal of the Optical Society of America B, 2013

ABSTRACT The electromagnetic resonances of optical microspheres—the so-called whispering gallery ... more ABSTRACT The electromagnetic resonances of optical microspheres—the so-called whispering gallery modes (WGMs)—can be used for refractometric sensing of surrounding fluids. Microspheres are attractive because they offer high sensitivity and can be utilized with fluorescent dyes or quantum dots. One issue with microspheres, however, is that they are difficult to integrate into microfluidic systems. Here, we develop a microfluidic structure that permits sensing applications using a single microsphere in a capillary. To achieve this, a microsphere formed on the end of a tapered fiber was first coated with fluorescent silicon quantum dots (QDs). The sphere was then inserted into a microcapillary and the fluorescence WGMs were monitored as different fluids were pumped through the channel. The sensitivity and detection limits for this sphere-in-a-capillary device were measured for several different QD film thicknesses and for two different microsphere sizes. Because of the relatively high-visibility mode structure, the sensitivity and detection limit can be defined by Fourier analysis of the free spectral range and WGM spectral shifts.