Vortex Beams Meet Scattering Media

Vector vortex beam before and after scattering process in a latex bead solution. (Source: I. Gianini, Sapienza U. Roma)

Propagate light through any kind of medium – be it free space or bio­logical tissue – and light will scatter. Robustness to scat­tering is a common requirement for communi­cations and for imaging systems. Structured light, with its use of projected patterns, is resistant to scattering, and has therefore emerged as a versatile tool. In particular, modes of structured light carrying orbital angular momentum (OAM) have attracted signi­ficant attention for appli­cations in bio­medical imaging.

OAM is an internal property of light conferring a charac­teristic doughnut shape to the spatial profile. The polari­zation profile of OAM modes of light can also be structured. Super­impose two OAM modes, and you can get a vector vortex beam (VVB) charac­terized by a doughnut intensity distri­bution in the beam cross-section, and with spatially variant polarization. VVBs are considered suitable and advan­tageous for quantum appli­cations in medical tech­nology. An international team of researchers prepared now a compre­hensive study of VVB trans­mission in scattering media.

The team is colla­borating under the aegis of the European Union’s FET-OPEN project Cancer Scan, which proposes to develop a radically new unified techno­logical concept of biomedical detection deploying new ideas in quantum optics and quantum mechanics. The new concept is based on unified transmission and detection of photons in a three-dimen­sional space of orbital angular momentum, entanglement, and hyper­spectral charac­teristics. Theo­retically, these elements can contribute to developing a scanner that can screen for cancer and detect it in a single scan of the body, without any risk of radiation. The team imple­mented a flexible platform to generate VVBs and Gaussian beams, and investigated their propa­gation through a medium that mimics the features of biological tissue. They demons­trate and analyze the degra­dation of both the spatial profile and polari­zation pattern of the different modes of light.

For both Gaussian beams and VVBs, the scientists remark that spatial profiles undergo an abrupt change as the concen­tration of the medium increases beyond 0.09%: a sudden swift decrease in contrast. They observe that the change is due to the presence of a uniform background caused by the scattered components of the beams. Investigating the polari­zation profiles, they found that VVB behavior is quite different from that of the Gaussian beams. Gaussian beams present a uniform polari­zation pattern that is unaffected by the scattering process. In contrast, VVBs present a complex distri­bution of polarization on the trans­verse plane. The team observed that a portion of the VVB signal becomes completely depo­larized when it passes through scattering media, but a portion of the signal preserves its structure.

These insights into how interaction with scattering media can affect the behavior of structured OAM light represent a step forward in exploring how it may interact with biological tissue. The team hopes that their comprehensive study will stimulate further investigation into the effects of light-scattering tissue-mimicking media, to advance the quest for innovative biomedical detection technology. (Source: SPIE)

Reference: I. Gianani et al.: Transmission of vector vortex beams in dispersive media, Adv. Phot. 2, 036003 (2020); DOI: 10.1117/1.AP.2.3.036003

Link: Quantum Information Lab, Dipt. di Fisica, Sapienza Univ. di Roma, Rome, Italy

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