Measurement of scattering intensity distribution of individual microparticles/nanoclusters based on laser levitation

The scattering measurement of particulates in gaseous medium is helpful to the understanding of light transmission, laser detection, combustion radiation and atmospheric environment. In order to explore the scattering characteristics of micron/nano -sized particles, this paper proposes to accurately measure the scattering intensity distribution of an individual micron-sized particle/nanocluster by combining laser levitation and scattering measurement methods. An experimental apparatus was first built based on the counter-propagated bi-Bessel beams levitation and scattering test systems. The microparticles/nanoclusters of various matters and sizes were then levitated and their stability was evaluated. Finally, the scattering intensity distribution of levitated particles within 2 π scattering angle was accurately measured at an angular resolution of 9.2″. The forces acting on particles under laser irradiation and the scattering intensity distribution of different particle parameters were simulated and calculated, and compared with experimental results. The influence of noises on the uncertainty of the scattering measurement system was analyzed in depth, including background light, laser beam, reflected light from the walls. The results show that for metallic magnesium and aluminum, whether single particles or clusters, the signal-to-noise ratio of scattering measurements within 2π angle is greater than 20 dB, with a maximum of 94.6 dB. For graphite nanoclusters, the signal-to-noise ratio in the backscattering direction is relatively poor. The influence of levitation instability on the scattering measurement results was estimated in detail, testifying that the influence of levitation instability in the test system on the scattering measurement is ignorable. Metallic magnesium, aluminum, and graphite particles can be stably levitated by the counter-propagated bi-Bessel beams, with a relative instability of less than 0.15. During the levitation, the photophoretic force plays a dominant role; The scattering intensity distribution of an individual micron-sized particle/nanocluster conforms to the scattering characteristics of Mie particles. Microparticles with large refractive index imaginary parts have stronger forward scattering characteristics. The larger the particle size parameter, the stronger the forward scattering effect becomes. The accurate measurement of the scattering intensity distribution of an individual microparticle confirms the versatility and reliability of the levitation scattering test system, providing a new research method for in-depth understanding of the scattering characteristics of substances.