Photon recoil propulsion
How it works
1) An unfocused, polarized laser beam (980 nm in the demonstration, ~100 mW, ~0.32 mW/µm²) illuminates a plasmonic directional antenna made of asymmetric gold nanorods (170 nm and 150 nm long, 60 nm wide). 2) The antenna absorbs and directionally scatters/re-emits photons, producing a strongly asymmetric scattering pattern. 3) Each scattered photon carries momentum; the asymmetry of the momentum flux yields a net recoil force that drives the structure in-plane (speeds up to 50 µm/s). 4) Vertically acting radiation pressure balances the repulsive electrostatic force from the charged surface, keeping the robot stably suspended above the substrate. 5) The direction of travel is locked perpendicular to the linear polarization axis; rotating the polarization rotates the robot, and circularly polarized light pulses break the twofold directional degeneracy via spin–momentum transfer, selecting the sense of motion. Polarization sequences are generated by electro-optic modulators (EOMs).
Problem solved
Controlling the motion of micro/nanoscale objects usually requires focused optical traps (optical tweezers) with precise mechanical beam steering, which increases system complexity and risks photodamage and excessive heating of the sample. Photon recoil propulsion allows an object to be propelled and steered with an unfocused, uniform beam by changing only its polarization, with reduced risk of photodamage.
Components
Asymmetric gold nanorods (e.g. 170 nm and 150 nm long, 60 nm wide) fabricated by helium-ion-beam milling from monocrystalline gold flakes. They scatter light directionally, producing an asymmetric photon-momentum flux and thus the recoil force.
A rigid, transparent silica disc (~0.92 µm diameter for the nanorobot, ~0.26 pg mass) that embeds the plasmonic antenna and serves as the payload-carrying body.
Official
Linear and circular polarization sequences generated by electro-optic modulators (EOMs). Linear polarization sets the motion axis (perpendicular to it), while circular pulses select the direction of travel via spin–momentum transfer.
Implementation
Plasmonic absorption produces local heating and temperature gradients that can perturb the environment.
The mechanism was demonstrated only in aqueous solution under controlled laboratory conditions; not shown in vivo or in complex media.
Motion is polarization-locked (perpendicular to linear polarization), so steering is indirect — via polarization sequences rather than free 2D navigation.
Speed drops when the robot carries larger bacterial clusters.
The robot stays suspended just above the substrate through a balance of radiation pressure and electrostatic repulsion, making it sensitive to surface conditions.
Evolution
Nichols and Hull (and independently Lebedev) experimentally confirm that light exerts pressure — the basis for the idea of momentum carried by photons.
Arthur Ashkin demonstrates acceleration and trapping of particles by radiation pressure, opening the field of optical manipulation.
Ashkin and colleagues demonstrate the single-beam gradient-force optical tweezers, the dominant method of optical micromanipulation.
Pakizeh and Käll demonstrate unidirectional ultracompact optical nanoantennas — the basis of the asymmetric directional scattering later used for recoil propulsion.
Bert Hecht's group presents light-driven plasmonic microdrones propelled by photon recoil — the first realization of this propulsion principle in micromachines.
Qin, Wu, Krueger and Hecht demonstrate a light-driven plasmonic microrobot for nanoparticle manipulation — the direct precursor of the nanoscale robotic cleaner.
Qin et al. present a miniaturized (sub-1 µm) nanorobot propelled by photon recoil from a single directional motor, capturing and transporting bacteria.
Hyperparameters (configurable axes)
Wavelength of the driving beam, tuned to the plasmonic resonance of the antenna.
Beam power and intensity set the magnitude of the recoil force and the level of local heating.
Linear polarization sets the motion axis; circular pulses select the direction (spin–momentum transfer).
The length difference of the nanorods sets the directionality of scattering and hence the recoil direction.