In recent years, there have been significant advancements in nanoscale optical thermometry techniques, particularly in key technological fields such as materials science, biophotonics, and semiconductor engineering. These technologies enable precise temperature detection at the microscopic scale, which is of tremendous value for both scientific research and practical applications. Traditional temperature sensing methods, including quantum dots, fluorescent dyes, and nanoparticles, have garnered extensive research interest. Notably, quantum defects in solid materials, such as nitrogen-vacancy (NV) centers and silicon-vacancy (SiV) centers in nanodiamonds, have gained attention for their outstanding stability, biocompatibility, and suitability across a wide temperature range. These defects can even maintain their performance under extreme conditions. However, the performance of these sensors typically relies on complex optical detection techniques like optically detected magnetic resonance (ODMR) or optical analysis of zero-phonon lines (ZPL). ODMR techniques may be affected by microwave heating and magnetic field noise, while ZPL analysis requires spectral resolution, which limits measurement speed and temperature accuracy. These challenges have driven researchers to explore simpler and more efficient methods for temperature measurement.

To this end, the research team led by Professors Qinghai Song and Yu Zhoufrom Harbin Institute of Technology (Shenzhen) has utilized silicon carbide quantum defects to achieve real-time, high-sensitivity temperature sensing (1.06% K⁻¹). This was accomplished by exciting the sample and monitoring the intensity ratio of anti-Stokes and Stokes emissions, taking advantage of the strong temperature dependence of the intensity ratio. Additionally, as the experiment only involves the generation and transmission of optical signals, it is characterized by non-contact and non-interference, ensuring measurement accuracy. Relevant research results were recently published in Photonics Research, Volume 12, Issue 8, 2024. [Chengying Liu, Haibo Hu, Zhengtong Liu, Shumin Xiao, Junfeng Wang, Yu Zhou, Qinghai Song, "All-optical nanoscale thermometry with silicon carbide color centers," Photonics Res. 12, 1696 (2024)]

Color centers are quantum light sources formed by defects in solid crystal structures. When a color center is excited by a laser field, the energy is released in the form of photons due to the transition of internal electrons. In this work, the authors utilized a silicon carbide sample with two types of color centers: the 4H-SiC silicon-vacancy color centers and the divacancy color centers. By using a 980 nm continuous-wave laser to simultaneously excite both kinds of color centers, researchers obtained the intensity ratio of anti-Stokes and Stokes fluorescence. By varying the temperature and leveraging the sensitive change in intensity ratio with temperature, they proposed an experimental scheme for real-time temperature sensing.

Figure 1(a) is a schematic diagram of exciting the sample, where a 980 nm continuous-wave laser excites both the 4H-SiC divacancy and silicon vacancy. The zero-phonon line (ZPL) emission range of the divacancy center typically spans from 1038 nm to 1133 nm. This emission, with photon energy lower than the excitation laser, is referred to as Stokes emission, and at room temperature, phonon sideband emission usually dominates. In contrast, photons emitted by the silicon vacancy in the sample have energy greater than that of the excitation laser, and this emission is known as anti-Stokes emission. In the anti-Stokes fluorescence mechanism, the laser photon energy is low, requiring the absorption of phonon energy to complete the transition. Since the phonon density is closely related to temperature, there is a strong connection between anti-Stokes and Stokes emission and temperature, which is the fundamental mechanism for temperature sensing. Figure 1(b) is a schematic diagram of the optical setup for the experiment, which involves setting up a confocal microscopy system for optical excitation of the sample and collection of fluorescence signals.

Figure 1: Characterization of anti-Stokes and Stoke emission from two types of color centers in 4H-SiC: (a) Excitation and emission schematic: a 980 nm laser is employed to excite silicon vacancies and divacancies simultaneously. The energy level diagram depicts phonon absorption and emission processes during anti-Stokes and Stokes emissions. (b) Schematic of the optical setup. (c) Photoluminescence spectrum: the emission peaks associated with silicon vacancies (black) and divacancies excited by a 980 nm laser are illustrated. (d) Microwave system diagram. (e) ODMR spectrum for silicon-vacancy centers under anti-Stokes excitation. (f) ODMR spectrum for PL5 and PL6 across a magnetic field (0–30 Gauss).

Figure 2(b) shows the schematic diagram and photo of the temperature control setup used in the experiment. A single-stage TEC element was used to heat the sample, which was attached to a ceramic piece, and the temperature was raised by applying a voltage to the TEC module. By increasing the voltage from 0V to 16V, the temperature was raised from 296K to 463K, and the change in fluorescence intensity of the silicon vacancy and divacancy color centers with temperature was measured during this process. To ensure the reliability of the results, temperature sensing was achieved by measuring the change in the ratio of anti-Stokes to Stokes fluorescence intensity with temperature changes. Figure 2(d) presents a fitted graph showing the change in intensity ratio with temperature changes, which exhibits a high dependence on temperature, reflecting the fundamental principle of temperature sensing. From this, a relative sensitivity of 1.06% K⁻¹ for the temperature sensing was extracted.

This research achieved a breakthrough in nanoscale temperature sensing technology by using the intensity ratio of anti-Stokes and Stokes fluorescence from solid-state quantum defects to indicate temperature changes. This approach overcomes the limitations of inconvenient setups, heating effects from the microwave, and the complexity of spectral analysis. It addresses the drawbacks of traditional temperature sensing methods based on defects, which require dual laser excitation, microwave introduction, and complex spectral analysis. Furthermore, given the extensive application of silicon carbide materials in the integrated circuit and semiconductor industries, this method provides a new perspective for integrating optical temperature sensing technology into semiconductor manufacturing.

Figure 2: Characterization of anti-Stokes and Stokes emission: (a) Schematic illustration and a photo of the temperature change device. The sample is attached to a ceramic piece and heated by applying voltage to the TEC module. (b)Temperature dependence of anti-Stokes fluorescence from silicon-vacancy. (c) Change of Stokes fluorescence with temperature. (d) Temperature dependence of anti-Stokes to Stokes PL ratio.