Speckle-based measurement of the fractional azimuthal index of orbital angular momentum beams for refractive index sensing.
basic_science · Level V
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- Record sourced from PubMed, PMID 42103710.
- Also identified by DOI 10.1038/s41467-026-72281-3.
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Abstract
Light beams possessing orbital angular momentum (OAM) have gained significant interest in areas such as optical manipulation, quantum entanglement, and super-resolved imaging. The OAM per photon of such beams, typically Laguerre-Gaussian beams, is directly proportional to the azimuthal index l. This index is continuous in nature and a non-trivial parameter to measure. In this study, Laguerre-Gaussian beams of differing l are generated through mode conversion using microscopic spiral phase plates. The value of l depends on the refractive index of the medium surrounding the spiral phase plates. Utilising laser speckle, we demonstrate an ultra-precise measurement of the azimuthal index of the generated beams to a precision of 2 × 10<sup>-5</sup>. In turn, this enables an ultra-precise measurement of the refractive index of the medium surrounding the spiral phase plates, with a best measured precision of 6.4 × 10<sup>-7</sup> refractive index units. Our study interrogates samples of sucrose and haemoglobin, only 300 pL in volume, within a microfluidic channel.