A Theoretical Framework for Optical Forces round a Fiber
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• Physics 16, s60
A brand new mannequin describes the vary of forces and torques that gentle in a fiber can impart on dielectric particles close by, even within the absence of helical gentle polarization.
Jinsheng Lu/Harvard College
Evanescent fields produced by gentle propagating by way of an optical fiber can work together with small dielectric particles positioned close to the fiber. The obvious interplay is a drive that arises from depth gradients, which may pull particles towards the fiber or push them away from it. However experiments have additionally detected a weaker set of uneven lateral and rotational forces, that are predicted to emerge when the sunshine within the fiber is helically polarized—it carries orbital angular momentum. In a brand new theoretical research, Jinsheng Lu of Harvard College and colleagues present that for dielectric particles bigger than the sunshine’s wavelength, resonant interactions with evanescent fields can induce such uneven forces even within the absence of helical gentle polarization [1]. Moreover, they discover that these forces might be a lot stronger than these seen for subwavelength particles.
The researchers contemplate a 2.7-µm-diameter optical fiber adjoining to a ten.6-µm sphere, each submerged in water. Within the fiber, they mannequin two counterpropagating gentle waves with completely different polarizations. The evanescent fields of those gentle waves can excite a “whispering-gallery mode” (WGM) within the microsphere, whose exact resonant wavelength differs for every polarization path. On account of this polarization-dependent resonance, the 2 enter gentle waves induce distinct WGMs with completely different section shifts. Even when the enter gentle waves should not helically polarized, these section shifts can impart a “artificial” helicity to the sunshine mode within the sphere.
From their mannequin, Lu and colleagues derive a complete framework for explaining how resonant interactions with the evanescent discipline trigger the microsphere to rotate, transfer alongside the fiber axis, and even orbit across the fiber. They are saying that the impact may result in new methods to optically manipulate microscopic objects in medical, sensing, or nanoengineering functions.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Journal primarily based in Bristol, UK.
References
- J. Lu et al., “Polarization-dependent forces and torques at resonance in a microfiber-microcavity system,” Phys. Rev. Lett. 130, 183601 (2023).
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