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Applied Optics Final Revision

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1Thermal Characterization of Liquid Core Optical Ring Resonator Sensors Jonathan D. Suter, Ian M. White, Hongying Zhu, and Xudong Fan Department of Biological Engineering, University of Missouri-Columbia 240D Life Sciences Center, 1201 E. Rollins Street, Columbia, Missouri 65211 Abstract The liquid core optical ring resonator (LCORR) has recently shown promise as a high-sensitivity label-free lab-on-a-chip bio/chemical sensor. We investigate experimentally and theoretically the temperature dependence of the LCORR to establish a noise baseline, which will enable us to implement a temperature stabilization mechanism to reduce the thermally-induced noise and to improve the sensor detection limit. Our studies involve analysis of the thermo-optic and thermo-mechanical effects of fused silica and aluminosilicate glass as they impact LCORR performance. Both thick-walled and thin-walled LCORRs are investigated to elucidate the contribution of water in the core to the thermal response of the LCORRs. Theoretical calculations based on Mie theory are used to verify the experimental observations. OCIS codes: 230.5750, 140.4780, 170.4520, 290.402021. Introduction The liquid core optical ring resonator (LCORR) sensor is a newly developed capillary-based ring resonator that can provide tremendous advantages in terms of performance and practicality in a broad range of applications [1,2]. Ring resonator sensors have been heavily investigated for sensing applications in recent years [3-8]. They can quickly deliver quantitative and kinetic information without the complication of fluorescent labeling. Due to the label-free operating mechanism, they are simple to employ and can be relatively low-cost. The LCORR represents a recent advancement in the architecture of ring resonator sensors because of its inherent ability to effectively deliver the sample to the sensor head without sacrificing performance or increasing complexity [1,2]. Figure 1 illustrates the concept of the capillary-based LCORR sensor array (Fig. 1 (a)) and its sensing mechanism (Fig. 1 (b)). The capillary has an outer diameter of 50-100 µm and a wall thickness of around 3-5 µm. The cross-section of the capillary forms a ring resonator that supports the circulation of photons in the form of whispering gallery modes (WGM), as shown in Fig. 1 (b). Using thin-walled capillaries (i.e., a few micrometers), significant evanescent exposure can be produced in the interior of the LCORR, as illustrated in Fig. 2, which can be utilized for bio/chemical sensing when aqueous analytes are conducted through the capillary core. The circulation of the light enables repetitive light-analyte interaction, thus resulting in an effective sensing length much longer than the LCORR circumference. Such strong light-analyte interaction significantly improves sensor detection limit, which, together with sub-millimeter dimensions and the excellent fluid handling capabilities of the capillary, makes the LCORR a promising candidate for integration with advanced microfluidic techniques onto an arrayed lab-on-a-chip sensing system.3 In addition to sensor development, the LCORR can also be used in capillary electrophoresis and chromatography. Compared to current capillary technology, the LCORR adds the function of on-capillary detection that permits sensitive, non-invasive, and quantitative measurement of analytes flowing in the core in real-time. This on-capillary detection capability will be useful to extract flow profile information in capillary-based separation techniques. Measurements with the LCORR can be made using the effective refractive index change induced by the binding of analyte to the interior surface or by changes in the bulk solution in the core, which is manifested as a shift in the WGM spectral position [1-3]. Currently, LCORRs have been manufactured successfully, and the refractive index sensitivity has been characterized [1,2]. A refractive index sensitivity of 16 nm/RIU (refractive index units) has been demonstrated for LCORRs [2]. This is on the same order as demonstrations for microsphere ring resonators [8]. Thus, it is anticipated that LCORRs will have a detection limit similar to that of conventional microsphere-based sensors, which have shown a potential refractive index detection limit as low as 10-7 RIU, and a protein detection limit of 1-10 pm/mm2 [4,5,8]. Unfortunately, this potential is limited by the thermal noise. As with other types of label-free optical sensors, the sensing signal is inevitably convoluted by thermal effects. Thermal fluctuations produce changes in the refractive properties of both the LCORR material and the liquid solution, and will also lead to changes in the LCORR size due to thermal expansion, both of which generate undesirable spectral variations in the WGM. The resulting spectral changes are added to the sensing signal as a noise term. In fact, in the case of the LCORR, thermal noise is by far the dominant term, and thus without thermal control, this term is essentially the noise baseline. The two coefficients that define the magnitude of the noise term are the thermo-optic coefficient and the thermal expansion coefficient, which produce noise by affecting the resonant4condition of the device. The thermo-optic properties of ring resonator materials have been leveraged in past studies to produce highly-tunable filters and other optoelectronic devices [9-10]. With filters as well as sensors, however, this tuning effect must be precisely controlled to prevent signal corruption. A degree of thermal noise is present in these devices that is determined by characteristic thermal sensitivity and limited by the degree of temperature control available. Therefore, detailed thermal characterization of the LCORR is needed to establish the noise baseline as a foundation for further improvements. Furthermore, for many lab-on-a-chip applications, in-situ temperature monitoring is highly desirable. A well-calibrated LCORR can also function as a sensitive thermometer capable of measuring the temperature along the LCORR. In this article, we conduct both experimental and theoretical investigations to elucidate the thermal expansion and thermo-optic effects present in two commonly used capillary materials, fused silica and aluminosilicate glass. In particular, we study thick-walled and thin-walled LCORRs, where in the latter case the thermo-optic contribution from water in the liquid core


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