Optical levitation traps particles in focused laser beams. The trap controls translational motion — center-of-mass oscillations along three axes. For spherical particles, that exhausts the interesting dynamics. For anisotropic particles — hexagonal prisms, for instance — there are also rotational degrees of freedom: spin around the long axis, precession, and the “coin-flip” mode where the prism tumbles end over end like a flipped coin.
The coin-flip mode is harder to control than spin. Spin responds directly to circular polarization. The coin-flip involves the prism's interaction with the field gradient, which depends on the particle's instantaneous orientation in a nonlinear way. Previous approaches modulated power or polarization — blunt instruments for a subtle degree of freedom.
Bykov and colleagues (arXiv:2602.20510) control the coin-flip by changing the wavelength of a secondary pump beam illuminating erbium-doped NaYF prisms. Different wavelengths excite different absorption cross-sections in the erbium ions, which changes the thermal distribution within the prism, which changes the radiation pressure torque asymmetry. The control is mediated by the internal spectroscopy of the particle, not by the external field geometry.
The binary modulation is clean enough to encode the ASCII message “hello” into the rotational frequency. At longer timescales, bimodal periodic dynamics appear — the Dzhanibekov effect, where intermediate-axis rotation flips spontaneously between two orientations.
The general observation: when external field parameters are too coarse to control an internal degree of freedom, the control can be routed through the particle's own spectroscopy. The internal structure becomes the actuator. The wavelength selects which internal mode to excite, and the internal mode selects which mechanical degree of freedom responds.