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<p>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 <i>l</i>. This index is continuous in nature and a non-trivial parameter to measure. In this study, Laguerre-Gaussian beams of differing <i>l</i> are generated through mode conversion using microscopic spiral phase plates. The value of <i>l</i> 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 &#xa0;×&#xa0; 10<sup>−7</sup> refractive index units. Our study interrogates samples of sucrose and haemoglobin, only 300 pL in volume, within a microfluidic channel.</p>

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Speckle-based measurement of the fractional azimuthal index of orbital angular momentum beams for refractive index sensing

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年5月8日

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  • Christopher Perrella,
  • Aman Anil Punse,
  • Anastasiia Zalogina,
  • Crispin Szydzik,
  • Megan Lim,
  • Andreas Boes,
  • Arnan Mitchell,
  • Kylie R. Dunning,
  • Kishan Dholakia

摘要

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 × 10−5. 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  ×  10−7 refractive index units. Our study interrogates samples of sucrose and haemoglobin, only 300 pL in volume, within a microfluidic channel.