Optical trapping describes the interaction between light and matter to manipulate micro-objects through momentum transfer. In the case of 3D trapping with a single beam, this is termed optical tweezers. Optical tweezers are a powerful and noninvasive tool for manipulating small objects, and have become indispensable in many fields, including physics, biology, soft condensed matter, among others. In the early days, optical trapping was typically accomplished with a single Gaussian beam. In recent years, we have witnessed rapid progress in the use of structured light beams with customized phase, amplitude, and polarization in optical trapping. Unusual beam properties, such as phase singularities on-axis and propagation invariant nature, have opened up novel capabilities to the study of micromanipulation in liquid, air, and vacuum. We summarize the recent advances in the field of optical trapping using structured light beams.
Airy beam is a kind of wavepacket existing in the form of photons, electrons, and plasmonics. Well known as diffraction-free beam, optical Airy beam tends to accelerate in transverse space with a parabolic trajectory, and exhibits self-healing property when partially blocked. Those properties have attracted a great deal of research interests and applications. Circular Airy beam, exhibiting cylindrically symmetric intensity pattern and abruptly autofocusing characteristics in the linear media, is a variant of Airy-like wave. Optical vortex, on the other hand, is a kind of phase singularity. We present to shape the autofocusing Airy beam with a vortex phase structure, which was realized through the binary amplitude modulation with a digital micromirror device (DMD). Each mirror on the DMD could be electronically addressed to situate at either of the two solid positional states corresponding to on and off. Shaping the light into a specific mode requires the calculation of the amplitude pattern for display on the DMD. By reshaping individual DMD pixels into giant pixels, the complex field of the vortex Airy beam could be encoded with a super-pixel method. The propagation property of the vortex Airy beam was investigated through numerical simulation for different topological charges. Furthermore, the propagation characteristics of this beam in free space were verified and discussed through the experiments. We anticipate that the proposed vortex Airy beam in particle trapping, biological field and optical communications. This method with DMD can also be used to generate other beams with different characteristics.
The micromechanical digital micromirror device (DMD) performs as a spatial light modulator to shape the light wavefront. Different from the liquid crystal devices, which use the birefringence to modulate the light wave, the DMD regulates the wavefront through an amplitude modulation with the digitally controlled mirrors switched on and off. The advantages of such device are the fast speed, polarization insensitivity, and the broadband modulation ability. The fast switching ability for the DMD not only enables the shaping of static light mode, but also could dynamically compensate for the wavefront distortion due to scattering medium. We have employed such device to create the higher order modes, including the Laguerre-Gaussian, Hermite-Gaussian, as well as Mathieu modes. There exists another kind of beam with shape-preservation against propagation, and self-healing against obstacles. Representative modes are the Bessel modes, Airy modes, and the Pearcey modes. Since the DMD modulates the light intensity, a series of algorithms are developed to calculate proper amplitude hologram for shaping the light. The quasi-continuous gray scale images could imitate the continuous amplitude hologram, while the binary amplitude modulation is another means to create the modulation pattern for a steady light field. We demonstrate the generation of the non-diffracting beams with the binary amplitude modulation via the DMD, and successfully created the non-diffracting Bessel beam, Airy beam, and the Pearcey beam. We have characterized the non-diffracting modes through propagation measurements as well as the self-healing measurements.
Time-sharing optical tweezers shares a single laser beam between several trap positions. The sharing rate or trap
switching frequency is an extremely important parameter in
time-sharing optical tweezers especially when it performs in
low frequency ranges. The effective stiffness of a novel time-sharing optical tweezers is investigated with different
switching frequencies. Our experimental results show that the larger the frequency is, the stiffer the trap is in low
frequency range from 5Hz to 50Hz.
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