Recently, Professor Dong Lei from Shanxi University and his team proposed a quartz-enhanced multi-heterodyne resonance photoacoustic spectroscopy technique.
They use a resonant quartz tuning fork that can sequentially extract frequency components resonating with the quartz tuning fork from multiple heterodyne tones, solving the bottleneck of traditional dual-comb spectroscopy.
Through this, the research group has achieved ultra-sensitive dual-comb spectroscopy independent of wavelength, and also updated the concept of photoacoustic dual-comb.
In the study, the superior performance of this technology was demonstrated by simultaneously tuning the dual-comb lines and the molecular rotation-vibration transitions and the quartz tuning fork resonance.
It has a wideband spectral acquisition capability of more than 1THz, a sensitivity of less than 100ppb, and a large dynamic range of 63dB, which is of great significance for applications such as gas sensing.Thanks to the small size and low cost of quartz tuning forks, quartz-enhanced multi-heterodyne resonance optoacoustic spectroscopy technology has a clear advantage in its applications.
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In the near future, it is expected that this technology will be widely used in fields such as environmental monitoring, health diagnostics, and industrial process testing.
Specifically:
Firstly, this technology can be used to monitor trace gas molecules in the atmospheric environment, and can further be extended to the field of carbon monitoring, thus meeting national strategic needs.
Secondly, this technology can also achieve non-invasive health diagnostics based on human exhaled breath. By expanding its wide-spectrum capabilities, it can perform quantitative analysis of the components of human exhaled breath for related medical diagnostics.Once again, this technology can be used to detect a variety of trace gases in industrial production, improving production efficiency and monitoring production safety.
The "grand unification" dilemma of photodetectors
It is reported that dual-comb spectroscopy is an emerging spectroscopic tool that uses the beat signal between two optical frequency combs with different repetition frequencies to down-convert the optical frequency information to the radio frequency band, which can overcome the shortcomings of photodetectors that cannot immediately respond to changes in the optical field intensity.
Dual-comb spectroscopy has the advantages of tunable laser absorption spectroscopy, while also possessing the main characteristics of traditional broadband spectroscopy, hence it can have a significant impact in various fields.However, there are still some challenges in dual-comb measurements at present.
Firstly, the dynamic range of dual-spectral spectrometers is limited by the individual photodetectors they are based on.
Since photodetectors can only withstand a limited amount of light power, under the condition of fixed light power, the power of each comb tooth will decrease as the tooth number increases, resulting in a lower signal-to-noise ratio.
Moreover, broadband photodetectors will receive all the noise within their detection bandwidth, which will reduce their final performance.
Secondly, the spectral resolution of dual-comb spectrometers is determined by the linewidth of the comb teeth and the time window of the measured time waveform.Ideally, the comb tooth spacing (repetition frequency) should match the desired spectral resolution.
Therefore, the tooth spacing must be adjustable over a wide range to cover all these spectral lines.
Thirdly, although the spectral span of dual combs can cover 14 octaves from terahertz to infrared to visible light, the photodetectors used to detect signals in different spectral regions are often based on different material systems.
For example, silicon detectors cover 400-1100nm, indium gallium arsenide detectors cover 800-1600nm, and mercury cadmium telluride detectors cover 3-10μm.
This means that optical frequency combs in specific spectral regions require different photodetectors. After realizing this issue, Dong Lei's research group decided to delve into the study."This unexpected event has made us feel extremely anxious."
At the beginning of the research, they conducted extensive surveys in the field of dual-comb optoacoustic spectroscopy, gaining a deep understanding of the current state of technology, cutting-edge developments, and potential challenges within the field.
They noticed that the current dual-comb optoacoustic spectroscopy is limited to non-resonant detection of acoustic waves, which not only leads to lower sensitivity but also restricts the dynamic range of detection.
To address this, the research team proposed the idea of using a quartz tuning fork to achieve resonant detection in dual-comb optoacoustic spectroscopy.For many years, the team has been deeply committed to the research in the field of optical gas sensing, and quartz-enhanced photoacoustic spectroscopy (QEPAS) is the main technical support for this course of study, which they happen to have a considerable accumulation in this area.
Subsequently, through theoretical derivation and simulation calculations, the research group determined the parameters and plan to achieve resonant detection.
Specifically, they proposed a method: that is, by scanning the relative offset frequency of the acousto-optic modulator, to control the central frequency of the acoustic comb under the dual optical comb conversion.
In this way, a simple experimental setup can achieve the research objective. During this process, they continuously optimized the scanning method to ensure the resonant detection of each comb tooth by the quartz tuning fork.
Under the guidance of the above plan, they successfully built a quartz-enhanced multi-heterodyne resonant photoacoustic spectroscopy detection device.Subsequently, the research team collected relevant data regarding the key indicators such as linearity, dynamic range, resolution, and spectral range of the quartz-enhanced multi-heterodyne resonance photoacoustic spectroscopy system.
After completing the data collection, the team carefully examined each data point, excluding any potential errors and outliers, and performed analysis and processing on the data.
The experimental data showed that the quartz-enhanced multi-heterodyne resonance photoacoustic spectroscopy technique demonstrated superior performance in aspects such as broadband spectral acquisition, sensitivity, and large dynamic range.
Then, by discussing and interpreting the experimental results, they analyzed the sources of noise, noise levels, and signal-to-noise ratio expressions, and proposed a new scheme to enhance sensitivity.
In fact, despite the team's rich experimental experience, successfully improving the dual-comb photoacoustic spectroscopy technology still poses certain challenges.Especially when designing a resonant detection scheme, how to achieve the coupling of narrow bandwidth fork vibrations with the multiple heterodyne acoustic waves produced by broadband dual optical combs is a very challenging problem.
After several investigations and discussions, they finally proposed a plan to control the central frequency of the acoustic comb.
In addition, the team also encountered an accident in the experiment. Once, when collecting key spectral data, the experimental equipment suddenly failed, resulting in the loss of a large amount of valuable data.
"This accident made us feel very anxious and frustrated, but it also made us cherish every experimental opportunity more. Later, we spent a long time repairing the equipment and adopted more rigorous experimental operations to ensure that similar problems would not occur again," said Dong Lei.
Recently, the related paper was published in Light: Science & Applications (IF 19.4) with the title "Quartz-enhanced multiheterodyne resonant photoacoustic spectroscopy."Jiapeng Wang is the first author, and Dong Lei serves as the corresponding author.
For four consecutive years, they have been selected in the top 2% of global leading scientists and have been granted 22 invention patents.
Subsequently, they plan to continue optimizing the quartz-enhanced multiple-external-difference resonance photoacoustic spectroscopy technology, aiming to significantly improve its response speed and sensitivity.
Specifically: The detection speed of dual-comb photoacoustic spectroscopy is limited by the molecular relaxation rate, and it cannot achieve a rapid response at the microsecond level.To address this issue, they plan to detect the transient acoustic response of the tuning fork through the beat frequency process, thereby improving the system's response speed.
In addition, they also intend to apply the acoustic micro-resonator from the traditional quartz-enhanced photoacoustic spectroscopy to the existing quartz-enhanced multi-heterodyne resonance photoacoustic spectroscopy technique.
For example, it is used in positions such as the axis cavity, off-axis cavity, and single-tube cavity to better enhance the system's sensitivity.
Ultimately, they hope to significantly improve the system's response speed and sensitivity, promoting the application and development of quartz-enhanced multi-heterodyne resonance photoacoustic spectroscopy technology in more fields.
It is worth noting that Dong Lei graduated from Shanxi University with both his undergraduate and doctoral degrees, and now he is also employed here. It can be said that although he is a native of Henan, Shanxi has become his second hometown.It is also reported that Dong Lei's current H-index is 46, and he has been selected for the Elsevier "Highly Cited Chinese Researchers" list for two consecutive years, and has been included in the top 2% of the world's leading scientists list for four consecutive years.
So far, he has been authorized as the first inventor for 22 invention patents, among which 6 have been implemented and transformed in 4 different companies respectively.
