Rotation of an optically trapped vaterite microsphere measured using rotational Doppler effect

Xinlin Chen

doi:10.1117/1.oe.57.3.036103

Abstract

The angular velocity of a vaterite microsphere spinning in the optical trap is measured using rotational Doppler effect. The perfectly spherical vaterite microspheres are synthesized via coprecipitation in the presence of silk fibroin nanospheres. When trapped by a circularly polarized beam, the vaterite microsphere is uniformly rotated in the trap center. The probe beams containing two Laguerre–Gaussian beams of opposite topological charge l = ± 7, l = ± 8, and l = ± 9 are illuminated on the spinning vaterite. By analyzing the backscattered light, a frequency shift is observed scaling with the rotation rate of the vaterite microsphere. The multiplicative enhancement of the frequency shift proportion to the topological charge has greatly improved the measurement precision. The reliability and practicability of this approach are verified through varying the topological charge of the probe beam and the trapping laser power. In consideration of the excellent measurement precision of the rotation frequency, this technique might be generally applicable in studying the torsional properties of micro-objects.

Highlights

The linear Doppler effect is a familiar concept, in which the relative motion between source and receiver causes a frequency shift
A laser beam carrying spin or orbital angular momentum (OAM) is illuminated on a roughness object rotating along the beam axis, and a Doppler frequency shift will be generated on the scattered light.[7]
The measurement precision depends on the full width at half maximum (FWHM) of the spectral peak

Summary

Introduction

The linear Doppler effect is a familiar concept, in which the relative motion between source and receiver causes a frequency shift This frequency shift scales with the linear velocity and is generally applicable in monitoring the translational motion.[1,2] Since first proposed by Yeh and Cummins[3] in 1964, just a few years after the invention of the laser, the laser linear Doppler technique has become an efficient tool for velocity measurement of surface,[4] fluid,[5] and atmospheric turbulence.[6] In recent years, a rotational analog to linear Doppler effect, namely rotational Doppler effect, has drawn great attention. This technique might be applicable in studying the torsional properties of micro-objects

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Conclusion