Manipulation of aerosols revolving in taper-ring optical traps

Active, controlled circular, and spin-rotational movement of optically trapped airborne micro-particles

We present a novel method for actively controlling circular and/or spin-rotational motion of an optically trapped airborne micro-particle. A 532-nm Gaussian laser beam is shaped into an elliptical ring by a pair of axicons and a cylindrical lens. The shaped beam is then focused into an elliptic cone that produces an optical trap. As the cylindrical lens is rotated, a torque is exerted on the trapped particle, resulting in circular or spin-rotational motion. We show examples of the circular-rotational movement as a function of laser power and the rotation rate of the cylindrical lens.

Optical-Trapping Laser Techniques for Characterizing Airborne Aerosol Particles and Its Application in Chemical Aerosol Study

We present a broad assessment on the studies of optically-trapped single airborne aerosol particles, particularly chemical aerosol particles, using laser technologies. To date, extensive works have been conducted on ensembles of aerosols as well as on their analogous bulk samples, and a decent general description of airborne particles has been drawn and accepted. However, substantial discrepancies between observed and expected aerosols behavior have been reported. To fill this gap, single-particle investigation has proved to be a unique intersection leading to a clear representation of microproperties and size-dependent comportment affecting the overall aerosol behavior, under various environmental conditions. In order to achieve this objective, optical-trapping technologies allow holding and manipulating a single aerosol particle, while offering significant advantages such as contactless handling, free from sample collection and preparation, prevention of contamination, versatility to any type of aerosol, and flexibility to accommodation of various analytical systems. We review spectroscopic methods that are based on the light-particle interaction, including elastic light scattering, light absorption (cavity ring-down and photoacoustic spectroscopies), inelastic light scattering and emission (Raman, laser-induced breakdown, and laser-induced fluorescence spectroscopies), and digital holography. Laser technologies offer several benefits such as high speed, high selectivity, high accuracy, and the ability to perform in real-time, in situ. This review, in particular, discusses each method, highlights the advantages and limitations, early breakthroughs, and recent progresses that have contributed to a better understanding of single particles and particle ensembles in general.

Regulating trapping energy for multi-object manipulation by random phase encoding

As known to all, optical tweezers depend intensely on trapping laser power. Therefore, the ability to separately regulate trapping power for each optical trap under a multi-object manipulation task empowers researchers with more flexibility and possibilities. Here, we introduce a simple strategy using complementary random binary phase design to achieve trapping energy assignment. The trap energy ratio can be expediently regulated by effective pixel numbers of the phase mask. We demonstrate the effectiveness and functionality of this approach by calibrating trap stiffness and directly measuring trapping power of each optical trap. In addition, we show the capability of rotating micro-beads with controlled speed and direction by supplying vortex beams with different energy ratios at specified positions. Our results imply that regulating the trap energy ratio will be of great significance in various applications, such as optical sorting and microfluidic scenarios.

Demonstration of a simple technique for controllable revolution of light-absorbing particles in air

The rotation of optically trapped particles is used in many applications for the realization of different micromechanical devices, such as micropumps, microrotors, and microgyroscopes, as well as for the investigation of particle interactions. Although for transparent micro-objects in both liquid media and vacuum, the rotation can easily be realized by transfer of the spin angular or orbital angular momentum from the light to the object. In the case of light-absorbing micro-objects in gaseous media, such transfers are insignificant in comparison with the thermal effects arising from the photo- and thermo-phoresis phenomena initiating the movement of trapped particles in a laser beam. Currently, proposed methods using a single focused laser beam, tapered-ring optical traps, or single and multiple bottle beams (BBs) have various limitations-for example, the inability to control the direction of the revolution of trapped particles or the low revolution frequency and small revolution angles. Here we propose a simple method for the realization of the revolution of airborne light-absorbing particles. The method is based on a combination of a circular diaphragm and a rotating cylindrical lens, enabling the generation of linear optical BBs. Our results show the flexibility and reliability of the proposed technique, allowing such laser traps to be used in various optical systems for the manipulation of micro-objects with different dimensions and shapes.

Video microscopy-based accurate optical force measurement by exploring a frequency-changing sinusoidal stimulus

Optical tweezers are constantly evolving micromanipulation tools that can provide piconewton force measurement accuracy and greatly promote the development of bioscience at the single-molecule scale. Consequently, there is an urgent need to characterize the force field generated by optical tweezers in an accurate, cost-effective, and rapid manner. Thus, in this study, we conducted a deep survey of optically trapped particle dynamics and found that merely quantifying the response amplitude and phase delay of particle displacement under a sine input stimulus can yield sufficiently accurate force measurements. In addition, Nyquist-Shannon sampling theorem suggests that the entire recovery of the accessible particle sinusoidal response is possible, provided that the sampling theorem is satisfied, thereby eliminating the requirement for high-bandwidth (typically greater than 10 kHz) detectors. Based on this principle, we designed optical trapping experiments by loading a sinusoidal signal into the optical tweezers system and recording the trapped particle responses with 45 frames per second (fps) charge-coupled device (CCD) and 163 fps complementary metal-oxide-semiconductor (CMOS) cameras for video microscopy imaging. The experimental results demonstrate that the use of low-bandwidth detectors is suitable for highly accurate force quantification, thereby greatly reducing the complexity of constructing optical tweezers. The trap stiffness increases significantly as the frequency increases, and the experimental results demonstrate that the trapped particles shifting along the optical axis boost the transversal optical force.

Improving photophoretic trap volumetric displays [Invited]

Since the introduction of optical trap displays in 2018, there has been significant interest in further developing this technology. In an effort to channel interest in the most productive directions, this work seeks to illuminate those areas that, in the authors' opinion, are most critical to the ultimate success of optical trap displays as a platform for aerial 3D imaging. These areas include trapping, scanning, scaling, robustness, safety, and occlusion.

Design of bottle beam based on dual-beam for trapping particles in air

An optical system structure, consisting of a tunable bottle beam, was designed to capture micron absorbing particles in air. Using a 670 nm semiconductor laser as the light source, a bottle beam was formed with beam shaping elements, a double-axicon lens, and parabolic annular mirrors. Taking a particle of carbon nanoclusters as an example, the capturing effect of the bottle beam on a particle was analyzed. By adjusting the size of the bottle beam, the capture performance for particles of different radii could be optimized. When the optical power of the conical doughnut hollow beam was P=0.05  W, the composite bottle beam could capture a particle of carbon nanoclusters smaller than 237 μm.

Optical concatenation of a large number of beads with a single-beam optical tweezer

Optical tweezers consist of the spatial confinement of microscopic dielectric particles by the action of forces produced by the change in momentum of the photons of a highly focused laser beam that are deviated by the particle. In experiments that use a single laser beam, it is common to capture not only one but a few particles in the optical trap. However, to our knowledge, the formation of a long chain of beads optically confined with a single laser beam has never been reported. In this work, up to 73 silica spheres immersed in water are seen concatenated along the propagation direction of a 976-nm wavelength Gaussian laser of 300 mW of power. This long chain of beads is obtained when the laser is focused through an oil-immersion DIN microscope objective with 100× magnification and a numerical aperture of 1.25. When performing the same experiment using an infinity-corrected UplanFLN 100× objective with a numerical aperture of 1.3, the maximum number of concatenated beads is only 14. Our results suggest that the mechanisms responsible for the observed phenomena involve successive refocusing of the laser beam by each trapped sphere, optically induced dipole coupling (commonly referred to as optical binding), and aberrations generated by the DIN microscope objective.

Study on evolving phases of accelerating generalized polygon beams

Recently, accelerating beam is becoming a hotspot in optics research. In this paper, we studied the evolving phases of accelerating generalized polygon beams (AGPBs) and proposed a novel method to generate this beam family. An important discovery has been made about reconstructing AGPBs only by evolving low-frequency phases in high power region, which confirms the dominant role of phase terms in the AGPBs' evolution. We also succeeded controlling the size and quantity of AGPB's intensity peaks in an easy and direct manner by manipulating the evolving phases in low frequency. This result not only explains the self-healing property of AGPBs but also confirms that AGPBs can be a great candidate to function as an optical tweezer to trap and free microparticles and microcreatures for certain purpose.

Photophoretic trapping of multiple particles in tapered-ring optical field

We demonstrate the photophoretic trapping of more than several hundreds of absorbing particles by tapered-ring optical traps diffracted from a circular aperture. The experiments with different laser powers show the influence of air flow acting on particles. Three kinds of particles with different densities (about 1~7 g/cm(3)) and different shapes (spherical, non-spherical) can be trapped. The non-spherical particles (toner particles) disperse in optical field, while the spherical particles (ink droplets and iron particles) arrange as a straight line. More importantly, in the experiments of two counter-propagating tapered-ring beams, the agglomeration of particles is achieved and can help research the dynamics of aerosols.