Orbital angular momentum of light

The internal OAM is an origin-independent angular momentum of a light beam that can be associated with a helical or twisted wavefront.

The external angular momentum is a form of OAM, because it is unrelated to polarization and depends on the spatial distribution of the optical field (E).

Helical modes of the electromagnetic field are characterized by a wavefront that is shaped as a helix, with an optical vortex in the center, at the beam axis (see figure).

The second column is the optical phase distribution in a beam cross-section, shown in false colors.

The third column is the light intensity distribution in a beam cross-section (with a dark vortex core at the center).

, the wavefront is shaped as a single helical surface, with a step length equal to the wavelength

As mentioned in the Introduction, this expression corresponds to waves having a helical wavefront (see figure above), with an optical vortex in the center, at the beam axis.

can be created artificially using a variety of tools, such as using spiral phase plates, spatial light modulators and q-plates.

Although the wave plates themselves are efficient, they are relatively expensive to produce, and are, in general, not adjustable to different wavelengths of light.

A spatial light modulator operates in a similar way to diffraction gratings, but can be controlled by computer to dynamically generate a wide range of OAM states.

Theoretical work suggests that a series of optically distinct chromophores are capable of supporting an excitonic state whose symmetry is such that in the course of the exciton relaxing, a radiation mode of non-zero topological charge is created directly.

For example, a metamaterial composed of distributed linear polarizers in a rotational symmetric manner generates an OAM of order 1.

[10] To generate higher-order OAM wave, nano-antennas which can produce the spin-orbit coupling effect are designed and then arranged to form a metasurface with different topological charges.

[11] Consequently, the transmitted wave carries an OAM, and its order is twice the value of the topological charge.

Representative approaches include patterned ring resonators,[14] subwavelength holographic gratings,[15] Non-Hermitian vortex lasers,[16][17] and meta-waveguide OAM emitters.

Thus the SAM can be measured by transforming the circular polarization of light into a p- or s-polarized state by means of a wave plate and then using a polarizing beam splitter that will transmit or reflect the state of light.

Where a beam splitter could separate the two states of SAM, no device can separate the N (if greater than 2) modes of OAM, and, clearly, the perfect detection of all N potential states is required to finally resolve the issue of measuring OAM.

The path being collinear, these fringes are pure consequence of the relative phase structure of the source beam.

Each fringe in the pattern corresponds to one step through: counting the fringes suffices to determine the value of l. Computer-generated holograms can be used to generate beams containing phase singularities, and these have now become a standard tool for the generation of beams carrying OAM.

This generating method can be reversed: the hologram, coupled to a single-mode fiber of set entrance aperture, becomes a filter for OAM.

The phase of these optical elements results to be the superposition of several fork-holograms carrying topological charges selected in the set of values to be demultiplexed.

The position of the channels in far-field can be controlled by multiplying each fork-hologram contribution to the corresponding spatial frequency carrier.

[21] Other methods to measure the OAM of light include the rotational Doppler effect, systems based on a Dove prism interferometer,[22] the measure of the spin of trapped particles, the study of diffraction effects from apertures, and optical transformations.

[25] Research into OAM has suggested that light waves could carry hitherto unprecedented quantities of data through optical fibres.

[26] Further research into OAM multiplexing in the radio and mm wavelength frequencies has been shown in preliminary tests to be able to transmit 32 gigabits of data per second over the air.

The fundamental communication limit of orbital-angular-momentum multiplexing is increasingly urgent for current multiple-input multiple-output (MIMO) research.

[27] The DoF limit is universal for arbitrary spatial-mode multiplexing, which is launched by a planar electromagnetic device, such as antenna, metasurface, etc., with a predefined physical aperture.

OAM states can be generated in coherent superpositions and they can be entangled,[28][29] which is an integral element of schemes for quantum information protocols.

Photon pairs generated by the process of parametric down-conversion are naturally entangled in OAM,[30][31] and correlations measured using spatial light modulators (SLM).

[33] In 2019, a letter published in the Monthly Notices of the Royal Astronomical Society presented evidence that OAM radio signals had been received from the vicinity of the M87* black hole, over 50 million light years distant, suggesting that optical angular momentum information can propagate over astronomical distances.