January 2021
Spotlight Summary by Brynmor Davis
Photonic nanojet generation under converging and diverging beams
Photonic nanojets (PNJs) are the optical phenomenon responsible for a freshly watered plant getting burned in the sun. The foliage is damaged when small water droplets refract sunlight to produce very tight, very intense, optical foci behind the droplets. PNJs can be used more beneficially however, with the small bright fields finding applications in super-resolution imaging, particle trapping and data storage.
Photonic nanojets are most easily analyzed by considering plane wave illumination; but in laboratory practice, bright PNJs are often generated by placing a microsphere at the focus of a lens. In this case, unexpected behavior can occur due to the inevitable misalignments between the PNJ-generating particle and the lens focal point. Here Yousefi et al. present a systematic study showing important effects resulting from focused illumination and focal-spot misalignment. The location and size of the generated PNJ is explored as a function of particle diameter, illumination aperture, refractive index, and whether the particle is in the converging or diverging region of the focused illumination. In addition to quantifying general trends, this work identifies interesting specific phenomena. For example, it is well known that spherical particles with a refractive index greater than 2 will produce an internal (and therefore inacessible) PNJ; but here the authors show this can also occur for smaller refractive indices when the particle is positioned in the converging region of the illumination. Insights such as this will prove useful for anyone attempting to generate PNJs using focused illumination and spherical or cylindrical particles.
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Photonic nanojets are most easily analyzed by considering plane wave illumination; but in laboratory practice, bright PNJs are often generated by placing a microsphere at the focus of a lens. In this case, unexpected behavior can occur due to the inevitable misalignments between the PNJ-generating particle and the lens focal point. Here Yousefi et al. present a systematic study showing important effects resulting from focused illumination and focal-spot misalignment. The location and size of the generated PNJ is explored as a function of particle diameter, illumination aperture, refractive index, and whether the particle is in the converging or diverging region of the focused illumination. In addition to quantifying general trends, this work identifies interesting specific phenomena. For example, it is well known that spherical particles with a refractive index greater than 2 will produce an internal (and therefore inacessible) PNJ; but here the authors show this can also occur for smaller refractive indices when the particle is positioned in the converging region of the illumination. Insights such as this will prove useful for anyone attempting to generate PNJs using focused illumination and spherical or cylindrical particles.
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Igor Minin
05/26/2021 10:18 PM
The following statement is not correct:
"it is well known that spherical particles with a refractive index greater than 2 will produce an internal (and therefore inacessible) PNJ" - This is only true for large particles when the geometric optics approximation is applicable; Moreover, "... the authors show this can also occur for smaller refractive indices when the particle is positioned in the converging region of the illumination" - It is known that a particle with an index less than 2 can focus radiation at the boundary or inside the material. See overview https://doi.org/10.1364/OME.7.001820