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

Abstract

The scattering measurement of particulates in gaseous medium is helpful in understanding light transmission, laser detection, combustion radiation and atmospheric environment. In order to explore the scattering characteristics of micron-/nano-sized particles, this paper proposes a method of accurately measuring the scattering intensity distribution of a single micron-sized particles/nanoclusters by combining laser levitation and scattering measurement. An experimental apparatus is first built based on the counter-propagated bi-Bessel beams levitation system and scattering test system. The microparticles/nanoclusters of various matters and sizes are then levitated and their stabilities are evaluated. Finally, the scattering intensity distribution of levitated particles within 2π scattering angle is accurately measured with an angular resolution of 9.2″. The forces acting on particles under laser irradiation and the scattering intensity distribution of different particle parameters are simulated and calculated, and compared with experimental results. The influence of noise on the uncertainty of the scattering measurement system is analyzed in depth, including background light, laser beam, and reflected light from the walls. The results show that the signal-to-noise ratio of scattering measurement for metallic magnesium and aluminum, whether single particles or clusters, are both greater than 20 dB and their maximum values are both 94.6 dB in a range of 2π angle. 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 is estimated in detail, verifying 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 a single micron-sized particles and nanoclusters both conform 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 a single microparticles 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.