Nanolayer characterization through wavelength multiplexing of a microsphere resonator (original) (raw)
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Journal of the Optical Society of America B, 2003
A first-order perturbation theory similar to the one widely used in quantum mechanics is developed for transverse-electric and transverse-magnetic photonic resonance modes in a dielectric microsphere. General formulas for the resonance frequency shifts in response to a small change in the exterior refractive index and its radial profile are derived. The formulas are applied to three sensor applications of the microsphere to probe the medium in which the sphere is immersed: a refractive-index detector, an adsorption sensor, and a refractive-index profile sensor. When they are applied to a uniform change in the refractive index in the surrounding medium, the formulas give the same results that one would obtain from the exact resonance equations for the two modes. In the application to adsorption of a thin layer onto the sphere surface, the results are identical to the first-order terms in the exact formulas obtained for the adsorption layer. In the lastnamed example, a scheme is proposed for instantaneous measurement of the refractive-index profile near the sphere's surface.
2011 13th International Conference on Transparent Optical Networks, 2011
In this work we designed and tested a micro fluidic platform integrated with a tapered fiber for studying resonant properties of a variety of microspheres sizes from 3 μm to 25 μm with different index of refraction from 1.47 to 2.1 in water and air environments. Experiments are performed by reattaching the microspheres along the tapered fibers with the diameter varying from 1 μm to 7 μm. Although the quality factors of whispering gallery modes (WGMs) are usually degraded for compact cavities immersed in liquid due to the reduced index contrasts, we show that this problem can be solved by using cavities made from high index materials. We demonstrated loaded Q-factors approaching ~10 4 for coupling to WGMs in compact (as small as 4 -5 μm in diameter) barium titanate glass microspheres with refractive indices 1.9 and 2.1. The microfluidic platform developed in this work can be used for studying resonant optical forces exerted on compact microspheres in a liquid as well as for developing compact microsphere resonator sensor devices.
Journal of Applied Physics, 2010
A simple plane wave, ray-tracing approach was used to derive approximate equations for the dielectric microsphere whispering gallery mode ͑WGM͒ resonant wavenumber and quality factor, as dependent on the surrounding medium's refractive index. These equations are then used to determine the feasibility of a micro-optical sensor for species concentration. Results indicate that the WGMs are not sensitive enough to refractive index changes in the case of gas media. However, they can be sufficiently sensitive for measurements in liquids. Experiments were carried out to validate the analysis and to provide an assessment of this sensor concept.
Whispering gallery mode structure in polymer-coated lasing microspheres
Journal of the Optical Society of America B, 2017
Whispering gallery modes (WGMs) are frequently observed in the emission spectrum of a fluorophore coupled to a dielectric microsphere. If the fluorophore is excited strongly enough the WGMs can become lasing modes, producing a much higher signal-to-noise ratio and higher Q-factors. These favorable properties have led to recent demonstrations of biochemical sensing with lasing WGMs; however, as we show here, the underlying cavity structure that leads to lasing can be highly complicated and the lasing spectrum can result from a large number of closely spaced or overlapping modes. In silica spheres coated with a dye-doped polymer bilayer, hints of underlying complexity are already indicated by the skewed shape of the fluorescence WGMs. Under lasing conditions, a more complicated structure was observed with Q-factors over 10 times higher than observed in fluorescence, while transmission measurements showed a dense forest of resonances due to m-order degeneracy breaking from a lack of perfect spherical symmetry. Lasing preferentially occurs at wavelengths where these modes tended to be most densely spaced. The tapered part of the fiber to which the microsphere is attached can also have a strong effect on the lasing spectrum, leading to significant differences between the fluorescence and lasing resonances.
Broadband optical absorbance spectroscopy using a whispering gallery mode microsphere resonator
Review of Scientific Instruments, 2008
We demonstrate the ability to excite and monitor many whispering gallery modes ͑WGMs͒ of a microsphere resonator simultaneously in order to make broadband optical absorbance measurements. The 340 m diameter microsphere is placed in a microfluidic channel. A hemispherical prism is used for coupling the WGMs into and out of the microsphere. The flat surface of the prism seals the microfluidic channel. The slight nonsphericity in the microsphere results in coupling to precessed modes whose emission is spatially separated from the reflected excitation light. The evanescent fields of the light trapped in WGMs interact with the surrounding environment. The change in transmission observed in the precessed modes is used to determine the absorbance of the surrounding environment. In contrast to our broadband optical absorbance measurements, previous WGM sensors have used only a single narrow mode to measure properties such as refractive index. With the microfluidic cell, we have measured the absorbance of solutions of dyes ͑lissamine green B, sunset yellow, orange G, and methylene blue͒, aromatic molecules ͑benzylamine and benzoic acid͒, and biological molecules ͑tryptophan, phenylalanine, tyrosine, and o-phospho-L-tyrosine͒ at visible and ultraviolet wavelengths. The microsphere surface was reacted with organosilane molecules to attach octadecyl groups, amino groups, and fluorogroups to the surface. Both electrostatic and hydrophobic interactions were observed between the analytes and the microsphere surface, as indicated by changes in the measured effective pathlength with different organosilanes. For a given analyte and coated microsphere, the pathlength measurement was repeatable within a few percent. Methylene blue dye had a very strong interaction with the surface and pathlengths of several centimeters were measured. Choosing an appropriate surface coating to interact with a specific analyte should result in the highest sensitivity detection.
Optics Letters, 1996
We demonstrated the use of a near-field probe to map the evanescent field of an optical standing wave in a fused-silica whispering-gallery mode microresonator. The periodicity of the observed standing wave allows us to estimate accurately the radial mode number of the whispering-gallery mode resonance that is being excited. We find that the use of a fiber half-coupler to excite these resonances in fused-silica microspheres results in only the lowest radial mode numbers' being strongly excited, as predicted.
Modelling the response of whispering-gallery-mode optical resonators for biosensing applications
2009 11th International Conference on Transparent Optical Networks, 2009
We present a theoretical analysis of the response of whispering gallery modes for biosensing applications, studied numerically in microcylinders and semi-analytically in microspheres. The effect of single and multiple particles is calculated, simulating biological analytes of different sizes and polarizabilities attached to the microresonator surface. Besides whispering-gallery-mode frequency shifts, we find that also broadenings and splittings (from lifted rotational symmetry) appear due to particles attachment and/or the vicinity to a planar coupler. For a single analyte, both particle size and refractive index can be determined from the broadening and shift, opening the perspective to a new biosensing modality.