Sensitive carbon monoxide detection based on light-induced thermoelastic spectroscopy with a fiber-coupled multipass cell[Invited]

sensor based on light-induced thermoelastic spectroscopy(LITES)with a fiber-coupled multipass cell was demonstrated for carbon monoxide(CO)detection.The fiber-coupled structure has the merits of reducing optical interference and difficulty in optical alignment and increasing system robustness.A 1.57 nm continuous wave distributed feedback diode laser was used as the excitation source.A minimum detection limit of 9 ppm was obtained,and the calculated normalized noise equivalent absorption coefficient was 1.15×10^(-7)cm^(-1)•W•Hz^(-1/2).The reported CO-LITES sensor showed excellent linear concentration response and system stability.

Fabry-Perot-based phase demodulation of heterodyne light-induced thermoelastic spectroscopy

Fabry-Perot(F-P)-based phase demodulation of heterodyne light-induced thermoelastic spectroscopy(H-LITES)was demonstrated for the first time in this study.The vibration of a quartz tuning fork(QTF)was detected using the F-P interference principle instead of an electrical signal through the piezoelectric effect of the QTF in traditional LITES to avoid thermal noise.Given that an Fabry-Perot interferometer(FPI)is vulnerable to disturbances,a phase demodulation method that has been demonstrated theoretically and experimentally to be an effective solution for instability was used in H-LITES.The sensitivity of the F-P phase demodulation method based on the H-LITES sensor was not associated with the wavelength or power of the probe laser.Thus,stabilising the quadrature working point(Q-point)was no longer necessary.This new method of phase demodulation is structurally simple and was found to be resistant to interference from light sources and the surroundings using the LITES technique.

Light-Induced Charge Transfer at Interface Between Lipid-Free RhTSPc Film and p-Si(111)

The present paper covers the lipid-free rhodium tetrasulfonato-phthalocyanine (RhTSPc) films prepared on p-Si(111) by using Langmuir-Blodgett technique. Their surface photovoltage spectra were measured. It was found that there is a strong interaction at the interface between the RhTSPc film and p-Si (111) and that the surface photovoltaic effect of the film system is maximum when only one monolayer of RhTSPc molecules coats p-Si(111), which is similar to that of CuTSPc films on p-Si(111) reported previously. These results confirm that only the monolayer of dye molecules being adjacent to the semiconductor surface plays a key role in the light-induced interfacial charge transfer process.

Light-Induced Mid-Infrared Emission of Liquid Carbon Tetrachloride and Benzene

Light-induced infrared emission spectroscopy (LIRES) is a novel technique that permits to receive high-quality spectra in the mid-infrared region. Low-intensity visible light connected to a highly sensitive FTIR spectrometer is more advantageous for studying any samples, including biological samples without any damage. This technique permits obtaining unique information on the molecule structure via vibrational excitation fundamental frequencies, overtones, and combination modes. It also enables a direct observation of vibrational radiation transitions in vibrationally excited molecules as well as the channels of vibration energy redistribution, which is not allowed with any other method. In this work, the LIRES is being tested as a technique for studying of vibrationally-excited molecules of carbon tetrachloride and benzene in the liquid phase. On the other hand, using transparent liquids, we had tried to understand some of the physical phenomena that can drive emission in mid-IR. The characteristics of the infrared emission of both liquid species produced by different wavelength radiation from various types of light systems (100 - watt Xe-lamp and Nd:YAG laser;lambda = 1064 nm (8 mW) and lambda = 532 nm (4 mW)) are presented. We demonstrated that the IR-signal, as well as spectral properties of carbon tetrachloride and benzene, was dependent on the wavelength and power of excitation beam. Results obtained with different light sources show that the visible light produces a nonlinear IR-emission signal in transparent liquids. We believe that the visible light is the source of the nonlinear response and is producing the vibration excitation as well as photostimulated transformations of the molecules possessing the high activity for the nonlinear response.

光强非线性响应下的光致热弹光谱光强修正

提出了基于光强非线性响应的光致热弹光谱光强修正方法,实现了激光光强的准确修正。控制DFB激光器工作于波长调制模式,设置激光调制频率为16369.75 Hz,通过光纤放大器增强其出射光强非线性项的幅值,光束经多次反射池后汇聚于石英音叉根部激发光热信号,采用数字锁相放大器解调得到对应的谐波信号,通过多项式拟合谐波信号背景,反演得到与浓度及光强分别对应的谐波信号。实验结果表明,当光强从22.03 mW变化至3.16 mW,谐波信号背景幅值与光强具有良好的线性关系,线性相关系数大于0.998,归一化后的谐波信号幅值波动小于0.37%。针对甲烷测量,系统在较大的浓度范围内具有良好的线性响应,谐波信号信噪比表明系统的最低检测限达0.22×10-6。该研究为光致热弹光谱的光强修正提供了一种新的方法,可有效提高LITES系统在长期测量应用中的稳定性。

A highly sensitive LITES sensor based on a multi-pass cell with dense spot pattern and a novel quartz tuning fork with low frequency

A highly sensitive light-induced thermoelectric spectroscopy(LITES)sensor based on a multi-pass cell(MPC)with dense spot pattern and a novel quartz tuning fork(QTF)with low resonance frequency is reported in this manuscript.An erbi-um-doped fiber amplifier(EDFA)was employed to amplify the output optical power so that the signal level was further enhanced.The optical path length(OPL)and the ratio of optical path length to volume(RLV)of the MPC is 37.7 m and 13.8 cm^(-2),respectively.A commercial QTF and a self-designed trapezoidal-tip QTF with low frequency of 9461.83 Hz were used as the detectors of the sensor,respectively.The target gas selected to test the performance of the system was acetylene(C2H2).When the optical power was constant at 1000 mW,the minimum detection limit(MDL)of the C2H2-LITES sensor can be achieved 48.3 ppb when using the commercial QTF and 24.6 ppb when using the trapezoid-al-tip QTF.An improvement of the detection performance by a factor of 1.96 was achieved after replacing the commer-cial QTF with the trapezoidal-tip QTF.

光致热弹光谱气体检测技术研究进展

光致热弹光谱是一种基于石英音叉热弹效应的新型气体检测技术,具有成本低、体积小、灵敏度高及光谱响应范围宽等优点,是目前一种重要的痕量气体检测方法。本文首先分析了基于光致热弹光谱的气体浓度测量原理,其次,从提高检测系统灵敏度的各种技术方法角度出发,介绍了近年来开发的提高石英音叉热弹光谱系统检测灵敏度的技术方法,从信号幅值、信噪比、最小检出限和归一化噪声等效吸收系数等方面,对系统的性能改进提升效果进行评估。最后,简要评述了光致热弹光谱在现场气体检测中的应用研究进展,对进一步提高检测系统灵敏度的方法进行了总结与展望。

基于圆柱腔的石英增强光谱传感技术研究(特邀)

设计了一种非一维共振腔的圆柱腔,利用入射声波与反射声波之间的共振增强效应来增大传感系统信号强度,并应用在面外入射的石英增强光声光谱技术(QEPAS)和石英增强光致热弹光谱技术(LITES)技术中。对添加圆柱腔前后的石英音叉摆幅和圆柱腔位置进行了理论模拟与优化。实验中,采用水汽为目标探测气体和输出波长1.395μm的半导体激光器为激发源,相比未添加圆柱腔的裸石英音叉系统,添加圆柱腔的QEPAS和LITES系统探测极限分别改善了2.32倍和1.27倍。

Light-induced frequency shifts for the lowest vibrational levels of ultracold Cs2 in the molecular pure long-range 0^-g state

The light-induced frequency shift(LIFS)of ultracold molecular ro-vibrational levels originates from the strong coupling of the atomic-scattering state and the bound-molecular state.In this paper,we present our experimental determination of the LIFSs of the lowest vibrational levels(v=0,1)in the purely long-range 0^-g state of ultracold cesium molecules.A high-resolution double photoassociation spectroscopy is developed,which serves as frequency ruler to measure the frequency shifts of the lowest molecular levels for Cs2.The experimental results are qualitatively consistent with the theoretical expectations.

Highly sensitive CO_(2)-LITES sensor based on a self-designed low-frequency quartz tuning fork and fiber-coupled MPC

A highly sensitive carbon dioxide(CO_(2))sensor based on light-induced thermoelastic spectroscopy(LITES)utilizing a selfdesigned low-frequency quartz tuning fork(QTF)and a fiber-coupled multipass cell(MPC)is reported in this paper.The QTF with a low resonant frequency of 8675 Hz and a high Q factor of 11,675.64 was used to improve its energy accumulation time and the sensor’s signal level.The MPC with the fiber-coupled structure and optical length of 40 m was adopted to significantly increase the gas absorbance and reduce the optical alignment difficulty as well as improve the robustness of the sensor system.A distributed feedback(DFB),near-infrared diode laser with an emission wavelength of 1.57μm was used as an excitation source.The experimental results showed that this CO_(2)-LITES sensor had an excellent linear response to CO_(2) concentrations.The minimum detection limitation(MDL)of this CO_(2)-LITES sensor was obtained to be 445.91 ppm,and it could be improved to 47.70 ppm(parts per million)when the integration time of the system reached 500 s.Further improvement methods for the detection performance of such sensors were also discussed.

基于光致热弹光谱的硫化氢痕量气体高灵敏探测(特邀)

光致热弹光谱(LITES)技术是近年来发展迅速的一种新颖痕量气体检测技术,该技术以体积小巧、成本低廉且无波长选择性的音叉式石英晶振替代成本高、探测波段窄的光电探测器作为光电换能器,通过探测激光与目标气体相互作用后光强的变化量实现目标气体浓度的反演。LITES技术具有探测灵敏度高、响应时间短、无波长选择性等优点。本文以下水道中的硫化氢气体为测量目标,开展了基于LITES技术的痕量气体探测系统的研究。以输出波长为1.582μm的近红外连续波分布反馈单纵模二极管激光器作为激发光源,采用激光器波长调制和二次谐波探测技术,首先研究了激光波长调制深度对LITES系统产生的信号幅度的影响,而后详细研究了气体压强及环境压强对装置性能的影响。此外,为进一步提升装置探测灵敏度,有效光程长度为14.5 m的Herriott多通池被装配在激光器和作为光电探测器的音叉式石英晶振之间,从而使传感器在积分时间为300 ms时,获得4.87×10^(-7)的最低探测极限,当积分时间延长至52 s时其探测灵敏度可达7.78×10^(-8)。在完成装置各项参数优化之后开展了下水道中硫化氢气体的实测研究,结果显示,该系统完全可满足下水道臭气监测分析等领域的应用需求。

Recent progress in online detection methods of bioaerosols

The aerosol transmission of coronavirus disease in 2019,along with the spread of other respiratory diseases,caused significant loss of life and property;it impressed upon us the importance of real-time bioaerosol detection.The complexity,diversity,and large spatiotemporal variability of bioaerosols and their external/internal mixing with abiotic components pose challenges for effective online bioaerosol monitoring.Traditional methods focus on directly capturing bioaerosols before subsequent time-consuming laboratory analysis such as culture-based methods,preventing the high-resolution time-based characteristics necessary for an online approach.Through a comprehensive literature assessment,this review highlights and discusses the most commonly used real-time bioaerosol monitoring techniques and the associated commercially available monitors.Methods applied in online bioaerosol monitoring,including adenosine triphosphate bioluminescence,laser/light-induced fluorescence spectroscopy,Raman spectroscopy,and bioaerosol mass spectrometry are summarized.The working principles,characteristics,sensitivities,and efficiencies of these real-time detection methods are compared to understand their responses to known particle types and to contrast their differences.Approaches developed to analyze the substantial data sets obtained by these instruments and to overcome the limitations of current real-time bioaerosol monitoring technologies are also introduced.Finally,an outlook is proposed for future instrumentation indicating a need for highly revolutionized bioaerosol detection technologies.