High-Q MgF_2 whispering gallery mode resonators for refractometric sensing in aqueous environment

Abstract

Abstract:We present our experiments on refractometric sensing with ultrahigh-Q, crystalline, birefringent magnesium fluoride (MgF 2 ) whispering gallery mode resonators. The difference to fused silica which is most commonly used for sensing experiments is the small refractive index of MgF 2 which is very close to that of water. Compared to fused silica this leads to more than 50% longer evanescent fields and a 4.25 times larger sensitivity. Moreover the birefringence amplifies the sensitivity difference between TM and TE type modes which will enhance sensing experiments based on difference frequency measurements. We estimate the performance of our resonators and compare them with fused silica theoretically and present experimental data showing the interferometrically measured evanescent field decay and the sensitivity of mm-sized MgF 2 whispering gallery mode resonators immersed in water. These data show reasonable agreement with the developed theory. Furthermore, we observe stable Q factors in water well above 1 × 10 8 .

Whispering gallery mode sensors

“…Variation of the refractive index contrast as a means to augment refractive index sensitivity necessitates appropriate choice of the resonator material. Crystalline MgF 2 disc resonators, with refractive index ~1.38, have recently been shown to give a sensitivity of 1.09 nm/RIU in aqueous environments by virtue of the long evanescent decay length of the WGM [64,142]. Comparable sensitivities have also been achieved using an integrated sapphire resonator [143]; however, sensitivities of 30, 570, and 700 nm/RIU have been reported in a microsphere resonator [19], a capillary-based optofluidic ring resonator [144,145], and a nanowire loop resonator [146], respectively.…”

Advances in optoplasmonic sensors – combining optical nano/microcavities and photonic crystals with plasmonic nanostructures and nanoparticles

“…For instance, Kuo et al [86] have exploited the high nonlinear susceptibility of gallium arsenide to generate a WGM second-harmonic spectrum ( Figure 8A), while Haigh et al [90] have enhanced Brillouin scattering by tuning of a magneto-optical yttrium iron garnet spherical cavity system. Recent materials/complexes of choice include diamond ( Figure 8B) [87], constrained germanium-tin alloy ( Figure 8C) [88], crystalline lithium niobate ( Figure 8D) [89], high-Q magnesium fluoride [91], or caesium lead halide perovskite-coated silica [92]. There are also magneto-and electrostrictive materials that suffer deformations from an applied field [93] and some isotropic crystalline platforms whose thermal characteristics enable nano-Kelvin thermometry, e.g.…”

Refractometry with Ultralow Detection Limit Using Anisotropic Whispering-Gallery-Mode Resonators

“…We calculate that for a WGMR with a 50 μm radius, with a noise level the same as here, we would have a detection limit below 1 × 10 −10 r.i.u.. One can also foresee applications for this technique in bioparticle sensing using the high-sensitivity of MgF 2 WGMR in aqueous environments [17]. The reactive effect of a single bioparticle at an optimum binding location on a 1-mm-diameter MgF 2 WGMR [3,5] will give a signalto-noise ratio of 1 for a single bioparticle of excess polarizability of ∼ϵ 0 × 1 × 10 −23 m 3 .…”

“…Unfortunately, this approach is not applicable to anisotropic resonators that possess some unique and useful advantages: first, resonators made from MgF 2 (which is anisotropic) have substantially enhanced sensitivity for aqueous sensing [17] and thus are highly desirable for many applications. Moreover, in an anisotropic resonator, it is possible to tune the frequency difference between orthogonal modes to a convenient value using temperature tuning: for the isotropic resonator, this difference is fixed by the geometry.…”