Photophoretic Trapping: Fundamentals, Advances and Future Directions

Abstract

Photophoretic forces, several orders of magnitude stronger than radiation pressure, enable particle trapping at remarkably low optical intensities and have opened pathways to applications in aerosol science, free-space 3D volumetric displays, and even deployment of lightweight payloads in space. In this review, we provide a comprehensive explanation of the underlying physics of photophoretic forces and how they facilitate stable three-dimensional manipulation of absorbing particles. We examine the experimental configurations that enable robust trapping, and we detail the physical parameters that govern the magnitude and behavior of photophoretic forces in these geometries. The rich dynamical phenomena exhibited by photophoretically trapped particles are discussed alongside current and emerging applications and possible future research directions. This review thus attempts to systematically unify the theoretical, experimental, and application-oriented aspects of photophoretic trapping, with the aim of advancing and strengthening research in this rapidly developing field.

Figures (8)

• Figure 1: (a) Schematic of a Crookes radiometer (b) and (c) show schematic representations of the photophoretic forces due to a temperature gradient ($\Delta T$) and an accommodation coefficient gradient ($\Delta \alpha$), respectively. The left arrows indicate the direction of the $\Delta T$ photophoretic force in (a) and $\Delta \alpha$ in (b). The central arrow represents the direction of the light beam incident on the particle.
• Figure 2: Experimental configurations and corresponding intuitive mechanisms for photophoretic trapping in air: (a, b) horizontal trapping arrangement with its underlying mechanism, where $F_C$ denotes the centripetal force acting on the particle lin2014optical © 2014 AIP Publishing LLC; (c, d) vertical trapping configuration and the associated trapping process pahi2025light © 2025 IOP Publishing Ltd, and (e, f) counter-propagating dual-vortex beam setup together with the proposed mechanism. In this case, $F_{ph}$ represents the photophoretic force exerted by the forward and backward vortex beams (indicated by the red and blue arrows, respectively), while the circular rings illustrate the sense of rotation of the forward and backward vortex beams (red and blue, respectively) shvedov2009optical © 2009 Optica Publishing Group.
• Figure 3: Parameter Dependence of the Photophoretic Force (a) Polarisation dependence of photophoretic forceshvedov2012polarization © 2012, AIP Publishing LLC (b) Power vs position along beam direction in a vertical trapping configurationpahi2024study (c) Power vs position along beam direction in a horizontal trapping configurationlin2014optical © 2014, AIP Publishing LLC (d) Variation of $\Delta T$ photophoretic force with ambient pressure for particles of different diametershorvath2014photophoresis © 2014 Hosokawa Powder Technology Foundation (e) Variation of $\Delta \alpha$ photophoretic force with ambient pressure for particles of different diametershorvath2014photophoresis. © 2014 Hosokawa Powder Technology Foundation (f,g) shows temporal dependence of photophoretic forcechen2018temporal. © 2018, American Physical Society
• Figure 4: (a) Intensity distribution of dark-hollow beam, made by superimposing two Bessel beams porfirev2015dark © 2015 Optical Society of America. (b) Experimental photography of a single optical bottle beam, where the two bottle necks are located near the planes 2 and 4. At the bottom, snapshots of the transverse intensity patterns of the bottle beam (contrast-enhanced) taken at planes 1–4 are marked. zhang2011trapping © 2011 Optical Society of America; (c) Experimental configuration of generating a confocal-beam trap using a hollow beam, which was generated through axicons. gong2016optical © 2016 AIP Publishing LLC. (d) Optical lattice structure profile for trapping multiple particles. shvedov2012optical (e) Left: Generation of taper-ring optical field using aperture and lens. Right: Taper-ring light distribution of one transverse section. liu2013manipulation © 2014 Optical Society of America (f) Numerical 3D modelling of the speckle intensity distribution for a small volume 5 $\mu m$$\times$5 $\mu m$$\times$ 15 $\mu m$ inside the trapping region with an average speckle size of 2 $\mu m$ in diameter. shvedov2010laser © 2010 IOP Publishing Ltd (g) Trapping locations of the trapped absorbing particles in Gaussian (left) and superposition of Gaussian + Hermite-Gaussian (HG) (right) beams after determining the radial and axial positions of the trapped particles. Particles are not drawn to scale, though their size ratio has been maintained.sil2017dual © 2017 IOP Publishing Ltd (h) Normalized trapped particle position distribution over a transverse plane of the specific beam size for three different speckle patterns generated from a multi-mode fiber. sil2024ultrastable © 2024 American Chemical Society
• Figure 5: Variation of trap stiffness with laser power: (a) intensity modulation using an AOM (red curve shows stiffness as a function of trapping intensity)lin2017measurement © 2017 Optical Society of America, (b) spatial modulation via a multi-sine signal (red and black lines correspond to particles of different masses) sil2020study © 2020 AIP Publishing LLC, (c) flight-based method, where the particle transitions between adjacent traps when one of the counter-propagating beams is switched off zhu2024measurement © 2024 AIP Publishing LLC, and (d) trapping of absorbing particles in an optical bottle (OB) beam generated by dynamic holography (with axial and transverse stiffness versus power shown in the top and bottom panels for two OBs, respectively) xu2025dynamic © 2025 Optica Publishing Group & Chinese Laser Press. While (a) and (b) show an increasing stiffness with power, both (c) and (d) exhibit stiffness that remains independent of laser power.