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Interview: Ying He

Hello Ying He, can you introduce yourself and your group?

I am a second-year Ph.D. student at the National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, China. Under the supervision of Prof. Xin Yu and Yufei Ma, I focus on exploring novel trace gas detection techniques, such as Tunable Diode Laser Absorption Laser Spectroscopy (TDLAS), Photoacoustic Spectroscopy (PAS), and Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS). Our group has worked on laser-based trace gas sensors for years. Prof. Ma brought the techniques to our group after his stay in Prof. Frank K. Tittle’s group at Rice University.

What are the principles of TDLAS/PAS/QEPAS, and what are potential applications?

PAS/QEPAS evolved from TDLAS, and the underlying principles are similar; it measures the absorption of light passing through a gas chamber. Wavelength-tunable light sources allow us to investigate the absorption spectroscopy. The fingerprints in the spectroscopy indicate the presence and the concentration of certain species. In order to achieve better signal-to-noise ratio, especially to avoid 1/f noise, we modulate the laser current in the kilohertz range, and demodulate the signal by using the harmonic detection method.

These techniques enable real-time trace gas monitoring and quantification. They could be applied in various fields, such as environmental monitoring, combustion field analysis, and medical diagnostics. They’re used, for example, in sensing atmospheric pollutants, measuring the concentration of methane in underground coal mines, identifying hazardous gases in automobile exhaust fumes, and analyzing the combustion field of rocket engines.

What is the role of the MFLI in your TDLAS/QEPAS systems? How does it make your life easier?

The MFLI is far more than just a lock-in amplifier – the test and measurement tools that come along with it are game-changers. Using the MFLI and LabOne, we can easily generate the modulated signal for the light source, demodulate the response of the photodetector/quartz tuning fork, and stream the results to the computer. In our QEPAS system, the MFLI is also used to obtain the frequency response of the quartz tuning fork, from which we fit the resonance frequency and Q-factor, using built-in functionality. This can be done very neatly with the Sweeper tool. The Sweeper sweeps any parameter of the drive signal in the experiment, not just the frequency; sweeping the amplitude and offset voltage is also extremely useful. We use the amplitude sweep to optimize the modulation depth in our experiments, and the offset voltage for wavelength tuning.

The MFLI’s oscilloscope is helpful when troubleshooting – I can examine any signal in the box, which helps to quickly identify the cause of any issues. I can also use the Plotter’s analysis tools to measure the peaks, noise floor, and signal statistics such as the average and standard deviation. All of these features help speed up the tuning process, and take the tediousness out of optimization. Thanks to the toolset, I don’t need to do any programming, and the amount of data saved for offline analysis is reduced significantly.

What have you achieved so far? What’s your favorite piece of work?

I have co-authored a few publications in some leading journals, such as Sensors and Actuators B, Applied Physics Letters, Optics Express, as well as proceedings in the Conference on Lasers and Electro-Optics (CLEO). My favorite piece is our recent report on Light-Induced Thermo-Elastic Spectroscopy (LITES) sensors. Conventionally, a quartz tuning fork in the QEPAS system functions as an acoustic wave detector. The detector is in contact with the gas under test. The gas may consist of corrosive elements, vapor, and other impurities, which can reduce the lifetime of the detector: the vapor and impurities can condense on the detector surface, which leads to instability and even failure. The LITES utilizes a novel mechanism to avoid this scenario, using light rather than acoustic waves. After passing through an isolated gas cell, the light is absorbed by the quartz tuning fork; this induces thermal elastic expansion, which in turn induces a piezoelectric signal in the tuning fork. This means that the turning fork doesn’t have to be in contact with the gases themselves, and can be placed outside of the gas cell. In our report, we experimentally demonstrate the function of LITES sensor for the first time. The detection sensitivity is better than conventional TDLAS and QEPAS by a factor of 5 to 20.

The LITES technique is nominated for ‘China’s Top 10 Optical Breakthroughs’ in 2019.

How did you come up with the LITES method?

Our group has conducted research on quartz-based gas sensors for years, so we have a lot of experience, and are constantly searching for novel approaches. Together with my supervisors, I did a thorough literature review on the use of quartz tuning forks, and found that they’re widely used in scanning probe microscopy (SPM), gyroscopes, and displacement sensors. It was reported in the SPM community that the incident radiation was able to actuate a quartz tuning fork, which was attributed to radiation pressure. We were inspired by the idea, and were curious whether it was possible to harness this effect for other uses. We collected some preliminary data and found that, in principle, it worked, but wasn’t in line with the radiation pressure theory. After a systematic investigation, we figured out that the response of the quartz tuning fork is better described by the light-thermo-elastic conversion mechanism. We proposed the LITES technique and did a benchmark against conventional TDLAS and QEPTAS techniques; we found that the LITES outperformed the conventional methods by a large margin.

What is your ongoing work about?

There are still open questions about the LITES technique. Our ultimate goal is to increase the detection limit of LITES. We’re working on enhancing its performance by optimizing the parameters overlooked in our previous work. We’re also considering how to adapt it to industry applications, which may result in a commercial product.

What are your interests outside of your lab?

I like listening to music, swimming, cycling, and other outdoor sports. I also like playing with aeromodelling, especially maneuvering fixed-wing planes.

Ying He

Ying He researches novel trace gas detection techniques at the Harbin Institute of Technology, China

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