Resonant optothermoacoustic detection of optical absorption

Technology This technology is an improvement to optothermal spectroscopy and photoacoustic spectroscopy (PAS). The proposed method of resonant optothermoacoustic detection (ROTA) is based upon the following processes:
(1) excitation of a fluid region or object under study by a modulated optical radiation, where the energy input frequency is equal to the resonant mechanical frequency of the detector
(2) energy transfer from the optically excited region or object to the sensing element via thermal conductivity, molecular diffusion, or free molecular motion
(3) periodic heating and the related thermal expansion of the sensing element
(4) excitation of resonant mechanical vibrations in a detector (preferably a piezoelectric crystal) being the same as the sensing element or having a rigid mechanical connection to the sensing element
(5) measuring the vibration amplitude of the detector, preferably by means of its piezoelectric response. Other possible options include interferometric and capacitive measurements of the detector vibrations

Advantages Advantages of the ROTA detection over conventional optothermal spectroscopy include:
• possibility to operate the sensor at higher modulation frequency compared to pyroelectric or other purely thermal detectors, thus reducing noise
• potentially extended interaction length
• larger flexibilities in terms of materials interacting with molecules and radiation and sensor element geometries

Advantages of the ROTA detection over conventional PAS or quartz-enhanced PAS (QEPAS) include:
• less signal dependence on the V-T relaxation processes in the gas under study
• possibility of operating at lower pressures for better spectral resolution in gases, including sub-Doppler resolution

Potential Applications The inventor has identified two classes of independent applications.

1. Detecting of relatively weak absorption in environment, gaseous and, possibly, liquid. This includes absorption of gases and separate components of gases, components of liquids or solutes, such as: a. gas sensing applications, identical to QEPAS b. analysis of gaseous components in chemical industry c. environmental control and other as required d. water quality control e. testing of other liquids for some impurities f. monitoring in biomedical and pharmaceutical applications

2. Detecting relatively strong absorption by very small objects at the sensor fiber or near it. Application for detection of microscopic particles, less labor-intense than microscopy. a. air quality control b. detection and analysis of solid microparticles in gases c. R&D in the area of nanotechnology

This invention is also complimentary to current approaches (i.e. multipass optical cells, PAS, optothermal spectroscopy) that measure weak optical absorption in small fluid samples or microscopic objects. Applications of these approaches include:
• Greenhouse and industrial gas emissions detection
• Detection of drugs, explosives, hazardous compounds (security)

Inventor(s): Anatoliy Kosterev

Type of Offer: Licensing

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