Depth-resolved measurement of optical radiation-pressure forces with optical coherence tomography

Abstract

A weakly focused laser beam can exert sufficient radiation pressure to manipulate microscopic particles over a large depth range. However, depth-resolved continuous measurement of radiation-pressure force profiles over an extended range about the focal plane has not been demonstrated despite decades of research on optical manipulation. Here, we present a method for continuous measurement of axial radiation-pressure forces from a weakly focused beam on polystyrene micro-beads suspended in viscous fluids over a depth range of 400 μm, based on real-time monitoring of particle dynamics using optical coherence tomography (OCT). Measurements of radiation-pressure forces as a function of beam power, wavelength, bead size, and refractive index are consistent with theoretical trends. However, our continuous measurements also reveal localized depth-dependent features in the radiation-pressure force profiles that deviate from theoretical predictions based on an aberration-free Gaussian beam. The combination of long-range radiation pressure and OCT offers a new mode of quantitative optical manipulation and detection with extended spatial coverage. This may find applications in the characterization of optical tractor beams, or volumetric optical manipulation and interrogation of beads in viscoelastic media.

Photonic force optical coherence elastography for three-dimensional mechanical microscopy

“…Second, we assumed that the mechanical response of the 0.1-μm beads was negligible. This assumption was supported by both theoretical predictions and experimental observation of F rad 44 , and experimental observation of OCT scattering intensity (Supplementary Methods). Third, we assumed that the photothermal response in our single-concentration samples was transversely uniform within the volume considered for photothermal fitting.…”

“…In this case, PF-OCE offers over 60× improvement in volumetric throughput over the current state-of-the-art OT-based microrheology techniques. Future studies will also address quantitative reconstruction of the local complex shear modulus from the measured PF-OCE bead mechanical responses by utilizing OCT-based depth-resolved measurement of radiation-pressure force 44 . Additionally, such studies may warrant bead sizes smaller than the 3-µm ones used here; the NA of the PF forcing beam could be appropriately increased to optimize forces and the corresponding displacements (Fig.…”

“…The use of low-NA radiation pressure to apply transversely localized axial force over an extended depth range can be advantageous for volumetric data acquisition, but comes with the challenges that a low-NA beam exerts significantly lower force than high-NA OTs, and that historically, studies on isolating the effects of photonic radiation pressure have been hindered by accompanying photothermal responses 11 , 42 , 43 . OCT has been used previously to monitor the radiation-pressure-induced trajectories of beads in liquid media 44 ; but this method has yet to be applied to solid viscoelastic materials.…”

Microrheological quantification of viscoelastic properties with photonic force optical coherence elastography

“…Using optical coherence tomography (OCT) to monitor the dynamics of accelerated beads in real-time, we recently performed the 'OCT-version' of Ashkin's seminal experiment and measured depth-resolved radiation-pressure force profile from a low-NA Gaussian beam on polystyrene micro-beads in various viscous fluids [15]. We found that radiation-pressure force on the order of 0.1 pN/mW could be achieved depending on the characteristics of the forcing beam as well as the properties of the beads and the medium.…”

“…In this paper, we combined our PF-OCE approach [16] with depth-resolved radiationpressure force measurement [15] to quantitatively reconstruct microscopic complex shear modulus around each probed bead in polyacrylamide (PAM) gels, and compare our quantitative PF-OCE results to shear rheometry. In order to obtain reliable measurements of bead mechanical response for quantitative PF-OCE, we analyzed measurement noise at key processing steps and identified data exclusion criteria based on three noise-related parameters.…”