An Acoustic Add-Drop Filter in a Phononic Crystal for High-Sensitivity Detection of Methanol in Ethanol in the Liquid Phase

An add-drop filter based on linear waveguides coupled through a ring resonator in a two-dimensional phononic crystal is introduced and its operation in detecting methanol concentration in ethanol is numerically investigated. The filter, which operates at around 200 kHz, has a small footprint of approximately 71.2 mm-by-84.0 mm, corresponding to 9.6-by-12.0 wavelengths. Band analyses of the phononic crystal and waveguide, as well as frequency domain simulations of filter operation for different methanol concentrations in the ring resonator cells are carried out through finite-element method. When the ring resonator cells are filled with pure ethanol, the drop port output is maximized, whereas through and add port outputs are minimized at the resonance frequency of 212.3 kHz. The contrast ratio between the drop and through/add port outputs is close to 1.0 at this frequency. Although the speed of sound and density are close in the two materials, the sensitivity is calculated as 12.54 Hz/% in terms of molar fraction of methanol. The corresponding figure of merit, obtained by dividing the sensitivity by drop port output peak width, varies between approximately 4.0 and 5.0. Moreover, a linear shift in the drop port peak frequency as a function of methanol concentration is observed. The add-drop filter can be calibrated for concentration sensing in different binary liquid mixtures. Higher sensitivities can be expected when the contrast of acoustic parameters, i.e., speed of sound and density, are larger for the constituents of the mixture.

I. Introduction

Liquid concentration sensing applications of phononic crystals (PnCs) have recently been receiving attention [1]–[21]. They offer highly sensitive, label-free, low-cost, low-power, and repeatable means of liquid sensing. Moreover, they can operate at room temperature and do not require warm-up or recovery time. PnC liquid sensors mostly involve monitoring the variation of a particular resonant transmission peak leveraged by various mechanisms, such as defect states in one-dimensional (1D) [10]–[12], [14], [17], [20], two-dimensional (2D) [6], [7], [16], [21], or three-dimensional (3D) [19] PnCs, as well as cavity resonances [3], [5]. In addition, narrow band solid-liquid composite arrangements [15] and extraordinary acoustic transmission [1], [2], [4] are also employed for liquid concentration sensing. PnC-based interferometric liquid concentration sensors have also been proposed [8], [9], [13], [18]. To account for interfering ambient effects, such as temperature variations, differential operation in PnC based liquid concentration sensors can be achieved simply by measuring the interference effect of temperature [12].