Determining single nanoparticle morphology by laser induced optical alignment and Rayleigh scattering Markus Rademacher, 1 Jonathan Gosling, 1 Antonio Pontin, 1 Marko Toroˇ s, 2 Jence T. Mulder, 3 Arjan J. Houtepen, 3 and Peter F. Barker 1, a) 1) Department of Physics & Astronomy, University College London, London WC1E 6BT, United Kingdom 2) School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, United Kingdom 3) Optoelectronic Materials Section, Faculty of Applied Sciences, Delft University of Technology, 2629 HZ Delft, The Netherlands We demonstrate the measurement of nanoparticle shape by angularly resolved Rayleigh scattering of single optical levitated particles that are oriented in space via the trapping light in vacuum. This technique is applied to a range of particle geometries, from perfect spherical nanodroplets to octahedral nanocrystals. We show that this method can resolve shape differences down to a few nanometers and be applied in both low-damping environments, as demonstrated here, and in traditional overdamped fluids used in optical tweezers. The measurement of nanoparticle morphology is vi- tally important for aerosol science 1–3 , nanoparticle pro- duction 4 and even identification of airborne viruses 5–8 . For example, rapid measurement of the morphology of nanoparticles is of significance to determine their antibac- terial activity and toxicity 9 . Their geometry also deter- mines their optical properties, which is important for can- cer diagnosis and imaging 10 . This also governs their spe- cific absorption rate for in vivo applications of magnetic nanoparticle hyperthermia for cancer treatment 11–13 . Al- though accurate measurements can be carried out using electron microscopy or x-ray diffraction, light scatter- ing methods can rapidly characterize single particles and even large ensembles in solution. However, as nanopar- ticles are often subject to translational and rotational Brownian motion, only the averaged properties of the particles can be measured. While this is sometimes suffi- cient, scattering from an aligned particle contains much more information when strongly suppressing Brownian motion. More recently, the field of levitated optomechanics has greatly progressed. It offers enhanced control over the translational and rotational motion of single iso- lated nanoparticles, and the movement can be strongly damped, suppressing Brownian motion. Particles are held in optical, electric, or magnetic traps. In optical traps, the particles can be localised in position down to a few picometers 14–17 . This control has allowed these sys- tems to be brought into the quantum regime with cooling to the quantum ground state recently demonstrated 15–17 . Such experiments have paved the way for the next gen- eration of quantum applications ranging from quantum- limited sensing of gravity 18 to tests of the large-scale limit of quantum mechanics 19 . Knowledge of the shape, refrac- tive index, and other properties of the levitated nanopar- ticles is critical for current and future quantum experi- ments 20 . These include quantum metrology which aims a) p.barker@ucl.ac.uk to search for new physics beyond the standard model 21 and tests of quantum mechanics in this new mesoscopic regime 22 . A long-standing problem in levitated optomechanics is obtaining detailed information on the structure and geometry of the levitated nanometre-sized objects 23 .A basic procedure for determining the nanoparticle’s shape has been developed based on linewidth measurements of the directional damping values for different spherical nanoparticle cluster configurations 24 . While this tech- nique is helpful, it is not very sensitive to small changes in particle shape within the nanometer range and cannot be used in the over-damped regime where many optical traps operate. Despite the importance of the laser scattering behavior of optically trapped nanoparticles to levitated optome- chanics 25,26 , the measurement of the scattering pattern of an optically trapped object has not been used to deter- mine the shape and geometry of a single particle. In this study, we determine single nanoparticle morphology us- ing laser-induced optical alignment of levitated nanopar- ticles coupled with angularly resolved Rayleigh scatter- ing. We present the underlying laser Rayleigh scattering theory on which the characterization technique is based and demonstrate its application to a range of trapped and oriented symmetric top nanoparticles with different morphology. These results are compared with numerical simulations of the particle alignment. We demonstrate that this method effectively determines the geometry of the optically levitated particle, realizing a new tool for studying isolated nanoparticles. A key ingredient in this method is the ability to align the nanoparticle to the laboratory reference frame via the optical torque induced by the linearly polarized trapping light. This feature greatly simplifies the treatment of the light scattering and increases its sensitivity to particle shape because it allows us to minimize the orientational averaging. We consider light scattered from a linearly polarised laser along the y-axis as shown in Figure 1, and we measure the vertically polarised light intensity I V scat- arXiv:2209.10037v1 [physics.optics] 20 Sep 2022