Experimental Verification of a Bigrating Beam Rider

An optical beam rider making use of a light sail comprising two opposing diffraction gratings is experimentally demonstrated for the first time. We verify that the illuminated space-variant grating structure provides an optical restoring force, exhibiting stable oscillations when the bigrating is displaced from equilibrium. We further demonstrate parametric cooling by illuminating the sail with synchronized light pulses. This experiment enhances the technical feasibility of a laser-driven light sail based on diffractive radiation pressure.

Figure 1

Transverse restoring force vs displacement of the bigrating. Laser of radial size $w_0$ illuminating (a) panel $A$ ($x<0$) and (b) panel $B$ ($x>0$). (c) Position-dependent force profile calculated from Eq. (1) (solid line) and the “tanh” model (dashed line). Potential energy landscape (dotted line). (Inset) Integrated beam profile of measured beam $\int g(x,y)dy$.

Figure 2

(a) Diffractive beam rider mounted on the pendulum under unpolarized white light. (b) A panel of the diffractive beam rider under a polarization microscope. (c) Director orientation in the cycloidal diffractive wave plates of opposite sign.

Figure 3

Partial top view of torsional pendulum with arm of length $R$ that pivots from equilibrium owing to radiation pressure on a bigrating from a forcing laser. Angular displacement $\varphi$ is obtained from the recorded position on a screen of a tracking laser reflected from a mirror at the pivot.

Figure 4

Parametric cooling of a bigrating using square wave laser modulation at twice the oscillation period. Measured (solid line) and numerically modeled (dotted line) oscillations agree. The oscillation envelope (dashed line) shows exponential decay, with decay time ${\tau }^{\prime \prime }$. (Inset) Radiation pressure force $\widetilde{F_x}(t)$ synchronized to the oscillation angle $\varphi (t)$.

Figure 5

Parametric cooling decay time ${\tau }^{\prime \prime }$ as a function of the transverse radiation pressure force $F_0$.