image: Schematic of microbubble sensors for in operando monitoring of phase transition dynamics. Inset: Variation of WGM spectra correlated to the mesoscopic structures of PNIPA, where the cyclic phase transition changing between hydrophilic state and hydrophobic state. The resonance shift of reference mode mainly arises from the temperature changes denoted as ∆λR, while the resonance shift of sensing mode ∆λS is caused by the thermo-optic effect ∆λT and intrinsic refractive index change ∆λn of PNIPA. view more
Credit: by Da-Quan Yang, Jin-hui Chen, Qi-Tao Cao, Bing Duan, Hao-Jing Chen, Xiao-Chong Yu, and Yun-Feng Xiao
High-Q microcavities have provided increasing opportunities for optical sensing and precision measurement in the past decades, with intriguing features of miniature size, non-invasiveness and fast response, etc. Currently, the optical microcavities sensing mainly focuses on the detecting existence and concentration of the analytes, or monitoring a single chemical/physical measurand (e.g., temperature, humidity, magnetic field et al.). In particular, the detection limit of the whispering gallery mode optical microcavity has reached levels of single molecules or single ions by employing mode-locking, optical spring, or plasmonic enhancement methods. However, it is challenging for optical microcavities to achieve the detection of complex dynamics in the physical/chemical reactions due to the multiple effects involved in these processes. The conventional schemes of microcavity sensing, for example, by monitoring the shift of a single resonance mode, usually cannot identify the mixed effects. Hence, in order to measure more complicated processes involving multi-physical quantities, new detection technology with straightforward signal deconvolution capability is required urgently.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Yun-Feng Xiao from Peking University, and co-workers have developed microcavity sensing technology to reveal transition dynamics of a phase-change material, where the existing multiple effects are decoupled by a self-referencing method. Experimentally, the transition dynamics of poly(N-isopropylacrylamide) (PNIPA) was monitored through a high-Q optofluidic microcavity, in which the integrated microfluidic channel allowed efficient coupling between the resonant optical field and the PNIPA solution. Benefiting from the proposed self-referencing strategy, the changes of temperature and refractive index of PNIPA during the phase transition were decoded from the transmission spectra of microcavity. In real-time measurements, the refractive index of PNIPA exhibited a hysteresis phenomenon in the cycles between hydrophilic and hydrophobic states, and the encircled area corresponded to the dissipated heat. Additionally, under thermal-equilibrium conditions, the refractive index of PNIPA on the heating power followed the rule of classical Boltzmann distribution and manifested the threshold of the phase transition.
The proposed self-referencing sensing strategy could identify the multiple effects by simultaneously analyzing the shifts of a reference mode and a sensing mode, which could be experimentally identified through mode broadening mechanism. They described the principle of the measurements in details:
“The resonance shift of reference mode mainly arises from the absorption of the heating light and the subsequent refractive index change by the thermo-optic effect. Thus, the wavelength shift of reference mode reflects the temperature change of measurement system. In contrast, the resonance shift of sensing mode is caused by the thermo-optic effect and intrinsic refractive index change of PNIPA. Therefore, the changes of temperature and refractive index can be decoupled and extracted separately by incorporating the shifts of these two modes in phase transition.”
“We would also point out that this work is a fundamental proof-of-principle study, which can be readily extended to probe the phase transition of other phase-change materials, and provide opportunities for exploring novel dynamic biochemical processes such as protein denaturation. In addition, this strategy combines microcavity photonics with microfluidics and phase change materials, in which not only the basic properties of phase change materials are well characterized with dual-mode self-referencing spectra, but also the functional photonic devices, such as optical switches and optical memories, can be constructed.” the researchers forecast.
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Journal
Light Science & Applications