ABSTRACT
That light carries both a linear and an angular momentum in the direction of its propagation stems directly from Maxwell’s equations. In the early 1900s, Poynting used a mechanical analogy to establish that for circularly polarized light the ratio between the angular momentum and the optical energy flux was 1/ω, where ω is the angular frequency of the light (Poynting 1909). Interestingly, although this recognition is the precursor to all the work discussed in this chapter, Poynting himself thought that the resulting torques would be too small for experimental observation. The first macroscopic observation of optical angular momentum was in the 1930s when Beth succeeded in using circularly polarized light to set a quartz disk into rotation (Beth 1936). In that work, the quartz disk acted as a birefringent waveplate, converting the polarization state of the light from circular (with angular momentum) to linear (with no angular momentum) and hence transferring the optical angular momentum from the light to a macroscopic object. Within a modern description, we would associate the angular momentum arising from circular polarization with the photon spin, h. By taking the ratio of the spin to the energy of the photon we see the same result as Poynting, namely, ℏ/ℏω = 1/ω. The polarization state is usually described by σ, where σ = ± 1 for right- and left-handed circular polarization, σ = 0 for linear polarization, and intermediate values of σ correspond to elliptical states. We note that the magnitude of such angular momentum is limited to 1 ℏ per photon.
