Cryogenic Temperature Monitoring via Optical Pdms Sensors

PDMS micro-sensors coated onto an electrical wire are used to measure dynamic temperature variation at cryogenic range based on optical whispering-gallery mode (WGM) frequency shift principle. We designed a lab cryogenic cell via filling liquid nitrogen to create a stable low temperature down to 95K. The electrical wire is current carrying to simulate a working electrical/electronic component/device. The temperature variation due to Joule heating is monitored. The sensors are tested for their real time temperature monitoring capabilities and accuracy in the cryogenic temperature regime of 95 – 140K. INTRODUCTION Optical whispering-gallery mode (WGM) studies have attracted considerable attention over the past two decades (1-5). The small mode volume, localized to the surface, of optical WGM resonators (1,2) allows for considerable interaction with surrounding environment, and their intrinsically high quality (Q) factor enables WGM-based sensors to exhibit extremely high-resolution. Optical WGM sensors are being actively studied in a number of different detection and measurement applications. Examples included biological/chemical detection (3-8), humidity (9), pressure (10) and temperature (11-13) monitoring, to name a few. The intrinsic high Q-factor of WGM based sensors, in junction with the fact that the sensing principles are frequency, and not intensity based, enables high sensitivity and fine resolution measurement. Detection of single molecule and individual RNA viruses (4,5), pico-molar chemical residues (8), milli-newton force (10), and milli-kelvin temperature shifts (12) have been reported in the literature. Optical WGM temperature sensors were initially demonstrated at both room temperature (11, 12) and cryogenic (13) regimes using fused silica microspheres. He et al. (14) analyzed thermal compensation in WGM resonators coated with a layer of polydimethylsiloxane (PDMS). Several studies of PDMS coating on thermal sensing then followed (15-17). Li et al. (16) proposed the concept of on-chip sensing based on WGM resonance. The actual demonstration of on-chip dynamic temperature measurement was carried out by Frenkel et al. (17), in which a PDMS based WGM resonator was fabricated directly onto an electrical current-carrying wire and be used for dynamic temperature monitoring of the wire’s temperature due to Joule heating. WGM resonances of the coated PDMS composite sensors were determined jointly by the thermal effects of both the PDMS shell and core material. Recently, Ali et al. (18) considered polymeric microspheres to electrical field measurement. In this paper, we look to expand on our previous work with PDMS composite sensors at room temperature (17) to cryogenic temperature because of wide cryogenic applications. For example, it is paramount to determine the critical temperature of superconductivity. Because of the ease in fabrication and cost-effectiveness using PDMS coated sensors, it is worthy of exploring the feasibility and capability of PDMS based WGM resonators within the cryogenic temperature regime. EXPERIMENTAL SETUP The experimental setup used in this study is adapted from a previous study of the present authors (17). The setup consists of five key components: the working electric component, the WGM composite sensor, the purging chamber, the cryogenic working cell, and the data acquisition system. The working electric component, fabrication of PDMS coated WGM sensor, and data acquisition system were detailed in Frenkel et al. (17); the only notable difference, among these three components, is that the fiber taper used as the near-field coupling device was fabricated outside the purging system using the heat and pull method (12). The fabrication was done on a rail system so that it could be transported into the purging chamber without breaking the fragile fiber taper. The reason behind using the rail system instead of fabricating directly within the purging chamber was that the stepper motor could not be placed within the chamber making it more difficult to control the tension