Optoelectronic thermometer based on microscale infrared-to-visible conversion devices
High-resolution temperature sensing, with both spatial and temporal precision, holds significant importance across various fields, including industrial manufacturing, environmental monitoring, and healthcare management.
Optical-based sensors are increasingly considered a promising option for temperature monitoring in biological diagnostics due to their advantages, such as non-intrusive measurements, resistance to electromagnetic interference, remote detection capabilities, and high-resolution output. These sensors utilize parameters like luminous intensity, wavelength, peak width, and decay duration to create various optical sensing techniques.
The upconversion mechanism, featured in these sensors, effectively reduces biological autofluorescence, enhances tissue penetration, and generates visible light signals that are easily detectable and recordable, rendering it a preferred tool for biological sensing applications.
In a recent publication in Light Science & Application titled “An Optoelectronic thermometer based on microscale infrared-to-visible conversion devices,” a team of researchers led by Dr. He Ding from the School of Optics and Photonics at the Beijing Institute of Technology, along with Prof. Xing Sheng from the Department of Electronic Engineering at Tsinghua University, and their collaborators, introduces an optoelectronic near-infrared-to-visible upconversion device. This device relies on specially designed semiconductor heterostructures, delivering a linear response, rapid dynamics, and low excitation power requirements.
The research thoroughly explores the temperature-dependent photoluminescence characteristics of the optoelectronic upconversion device, demonstrating its efficacy in thermal sensing applications.
The proposed temperature sensing approach relies on a fully integrated optoelectronic upconversion device, as illustrated in Figure 1a. This device comprises a double-junction photodiode based on low-bandgap gallium arsenide (GaAs) and a light-emitting diode (LED) based on large-bandgap indium gallium phosphide (InGaP), connected in a series configuration. Figure 1b presents a cross-sectional scanning electron microscopy (SEM) image of the device structure, which is formed on a GaAs substrate with a sacrificial interlayer.
These microscale devices, defined by lithography and epitaxially fabricated, possess dimensions of 300 x 300 square micrometers. They exhibit efficient near-infrared (NIR)-to-visible upconversion, characterized by a linear response and rapid dynamics, as previously demonstrated (Figure 1c).
When exposed to near-infrared light within the wavelength range of 770–830 nm, the optoelectronic upconversion device emits red light. As temperature increases, this red emission experiences a decrease in intensity and a redshift of the emission peak, shifting from 625 nm to 637 nm, as shown in Figure 2a.
(a) Spectra illustrating the excitation and upconverted photoluminescence (PL) emission at various temperatures ranging from 25 to 90 °C. (b) The peak wavelength and PL intensity of the upconverted red emission as a function of temperature, both calculated (represented by the dashed line) and observed (depicted as dots), with the shaded range indicating the standard deviation across 10 samples. (Image courtesy of the study’s authors).
Through a combination of material properties and structural design, a temperature sensitivity of 1.5 percent per degree Celsius for intensity and 0.18 nanometers per degree Celsius for the spectrum was determined (Figure 2b).
The study’s authors envision a multitude of applications for this robust optoelectronic upconversion optical thermometer. “We can achieve spatially resolved heat sensing by employing a large-area device array of optoelectronic upconversion devices” (Figure 3a). For instance, “we employ air cannons to generate a flow of hot air directed at the sample, which alters the upconversion emission and eventually extinguishes it. We can then capture the spatial distribution and real-time temperature fluctuations based on the correlation between emission intensity and temperature,” explained He Ding from the Beijing Institute of Technology.
Graph 3 depicts various applications of the upconversion device technology:
(a) On the left, we observe temperature mapping and spatially resolved photoluminescence (PL) responses from a device array subjected to nonuniform heating, while on the right, we have spatially resolved PL responses under the same conditions.
(b) To the left is an image of the fiber sensor utilized to measure exhalation temperature. On the right, dynamic temperature signals collected by the fiber sensor are presented, based on shifts in emission peak wavelength and variations in PL intensity during cyclic exhalation cycles. These results are compared with data simultaneously recorded by a thermocouple. The shaded grey areas indicate exhalation activities.
(c) The left side features an image of a behaving mouse with a fiber sensor and a thermocouple implanted in its brain for temperature monitoring. The right side illustrates dynamic temperature data generated by the fiber sensor within the mouse’s brain, determined through emission peak wavelength shifts and PL intensity variations, and compared to thermocouple measurements. The shaded gray region represents the period when the mouse is exposed to a high-temperature environment of around 40 ºC. (Image courtesy of the researchers).
“The upconversion device can be removed from the substrate it’s produced on and integrated with fiber optics to create light-guided heat sensors,” explained the researcher. Such optical-based technology, in contrast to tethered electrical sensors, is more suitable for use in environments with significant electromagnetic interference and is particularly adept at collecting data during magnetic resonance imaging (MRI). As a proof-of-concept demonstration, a portable system coupled with fiber optics can be effortlessly implemented for biomedical applications, including monitoring human exhalation patterns and deep tissue temperature within the mouse brain (Figures 3b and 3c),” noted Xing Sheng from Tsinghua University.
“The MRI-compatible implanted sensors combined with fiber optics offer both research and therapeutic potential, especially in the realm of deep-body temperature monitoring. These materials and device concepts represent a potent toolkit with extensive applications in both environmental and healthcare settings,” concluded Xing Sheng, while also offering a prediction for the future.