Journal of Quantitative Spectroscopy & Radiative Transfer 258 (2021) 107381 Contents lists available at ScienceDirect Journal of Quantitative Spectroscopy & Radiative Transfer journal homepage: www.elsevier.com/locate/jqsrt Improved backscattering detection in photonic force microscopy near dielectric surfaces with cylindrical vector beams Maria Grazia Donato a,1 , Francesco Patti a,b,1 , Rosalba Saija a,b , Maria Antonia Iatì a , Pietro G. Gucciardi a , Francesco Pedaci c , Giuseppe Strangi d,e,f , Onofrio M. Maragò a,∗ a CNR - IPCF, Istituto per i Processi Chimico - Fisici, Messina I - 98158, Italy b Dip. di Scienze Matematiche e Informatiche, Scienze Fisiche e Scienze della Terra, Università di Messina, Messina I - 98166, Italy c CBS Un.Montpellier, CNRS, INSERM, Montpellier 34090, France d Department of Physics, Case Western Reserve University, Cleveland 44106 - 7079, USA e Department of Physics and CNR - Nanotec, University of Calabria, Rende 87036, Italy f IIT Istituto Italiano di Tecnologia, Genova 16163, Italy a r t i c l e i n f o Article history: Received 15 July 2020 Revised 5 October 2020 Accepted 8 October 2020 Available online 9 October 2020 Keywords: Optical tweezers Cylindrical vector beams Back - focal plane interferometry T - matrix a b s t r a c t Cylindrical vector beams are used to improve back scattering detection in photonic force microscopy mea- surements near a dielectric surface. We compare back focal plane interferometry signals acquired on a quadrant photodiode when optical trapping a latex microparticle with gaussian, radial, and azimuthal beams. We find a consistent reduction of the interference pattern generated by the superposition of light backscattered by the trapped particle and backreflected by the dielectric surface. We contrast experimen- tal findings with a model based on light scattering theory in the T - matrix formalism. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Optical tweezers [1,2] (OT) are key photonic tools for the con- tactless manipulation of particles from single atoms and nanopar- ticles [3] to viruses and cells [4]. Among their several applications [5] the ability to accurately measure forces down to the femtonew- ton range [6–9] has been at the core of recent advances in bio- physics [10], soft - matter [11–14], and nanotechnology [3,15]. The evolution of optical tweezers into a scanning probe technique led to the implementation of photonic force microscopy (PFM) [9,16– 20], where an optically trapped probe in liquid is scanned over a (soft) sample or a three - dimensional environment to collect both morphology and force sensing detection through its thermal fluc- tuations. In OT a micro or nanoparticle is confined close to the focal spot of a tightly focused laser beam. The Brownian motion of the trapped particle is the origin of particle position fluctuations that are detected by video microscopy or by position sensitive devices, and it is used to calculate the trap spring constants, k i (i = x, y, z). ∗ Corresponding author. E-mail address: onofrio.marago@cnr.it (O.M. Maragò). 1 These authors contributed equally to this work. Any external force F ext,i inducing a further small displacement x i of the particle from the trap equilibrium position can be measured by using the Hooke’s law F ext,i = k i x i [2]. The precision of this kind of force measurement is crucially dependent on the quality of the signal arriving at the detector [21]. Typically, the detector col- lects the interference pattern due to the superposition of light for- ward - scattered by the trapped particle and unscattered incident light. This procedure requires that the sample chamber is transpar- ent and that all the space under and above it is available for mea- surements. Alternatively, a backscattering configuration [21,22] can be used, in which light is both directed to and back - collected from the sample by the same microscope objective. This detection scheme may be more difficult to implement with respect to the forward - scattered one [21], due to a lower signal and a more complex intensity pattern of the scattered field. However, this is a desirable configuration when optical forces are measured close to membranes [23] or to an opaque or reflective surface [24–27]. When trapping in front of an even weakly reflective surface, the interference, inside the sample chamber, between incident and re- flected beams is the origin of a standing wave, giving a periodic spatial distribution of trap positions [28]. This effect is stronger on particles having radius smaller than the trapping wavelength. Another possible source of modulation could be the superposi- tion, on the detector, of light backscattered by the trapped particle https://doi.org/10.1016/j.jqsrt.2020.107381 0022-4073/© 2020 Elsevier Ltd. All rights reserved.