The displacement measurement of the levitated particle is essential in optical tweezers in vacuum. However, crosstalk between the radial axes often occurs and will deteriorate the measurement precision. Common methods are proposed to align the coordinate systems of the motion and measurement, but few have considered the polarizations of the trapping beam and possible crosstalk control. Here single SiO2 particles with a diameter of 200nm are trapped in the single-beam optical tweezers in vacuum. Balanced detectors and D-shaped mirrors are used to measure the particle's displacements. As expected, the crosstalk coefficient can be periodically changed with the control of the linear polarization of the trapping beam. When the polarization direction is along Y axis, the crosstalk on the displacement x from the other radial axis can reach to infinity. When the polarization direction is along X axis, the crosstalk is eliminated. For comparison, crosstalk elimination is also achieved by an inserted Dove prism to rotate the beam along the propagation axis. The crosstalk elimination by polarization control is simpler, but it needs linear-polarized beam and will slightly change the particle’s resonant frequencies. The crosstalk elimination by beam rotations will need at least two Dove prisms, but it is adapted to most common conditions and does not change the resonant frequencies. The research is useful for the feedback cooling and the precise measurement of the physical quantities in future.
In the traditional optical traps, the displacement of the trapped particle is usually detected by measuring the forward scattered (FS) light and back-scattered (BS) light. However, in the dual-beam fiber-optic traps, it is difficult to using the FS light and BS light to achieve the displacement detection. As mentioned in our previous work, the side-scattered light of the trapped particle can also be used for displacement detection and achieve a large linear range of the displacement detection. In this paper, we furtherly use a four-fiber bundle, rather than traditional quadrant photodiode (QPD) to collect the side-scattered light from trapped particle in the dual-beam fiber-optic trap. The power of the collected light in the cores of the fiber bundle is monitored by multiple detectors, and later operated for differential signals to denote the displacement of the particle. To achieve better displacement detection performance, we have analyzed the influence of core radius r, numerical aperture NA, and core interval Δ based on the simulations using a geometrical optics model. The results show that, high sensitivity and large linear range can be realized by optimizing these parameters. The proposed detection method is easy to integrate into a dual-beam fiber-optic trap, which provides a pathway for optical trapping systems with all fiber devises.
We present a method for measuring the van der Waals force between two microspheres based on photonic force microscopy. We trapped a microsphere as probe by optical tweezers. The restricted Brownian motion of Gaussian distribution could be found in this system. The vibration center of the probe was affected by the van der Waals force when a target microsphere was closer to the probe. We measured the vibration center of the probe at different separation between the pair of microspheres. Based on this, the measurement of the van der Waals force between the two microspheres was realized with a high precision. Our method can realize the direct measurement of van der Waals force without using the variation rules of it. This method results in a simple structure, would not damage the sample, and can be suitable for the surface of any shape. It is general and has a wide range of applications in other fields of micro-force measurement.
We presented an optical system that could measure the viscosity coefficient of liquid in a micro-area. The orbital rotation of a polystyrene microsphere was realized by a dual-beam fiber-optic trap with a transverse offset. The rotation rate increased with the viscosity coefficient of the environmental medium. On this basis, the viscosity coefficients of ethanol solutions with different concentrations were measured successfully. The volume of solution samples was less than 1 μL. This provides a basis for the viscosity measurement of rare liquid or enchylema, which is of great significance for biological applications such as cell characteristics and reaction dynamics.
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