Effects of detection-beam focal offset on displacement detection in optical tweezers

A high-resolution displacement detection can be achieved by analyzing the scattered light of the trapping beams from the particle in optical tweezers. In some applications where trapping and displacement detection need to be separated, a detection beam can be introduced for independent displacement detection. However, the detection beam focus possibly deviates from the centre of the particle, which will affect the performance of the displacement detection. In this paper, we detect the radial displacement of the particle by utilizing the forward scattered light of the detection beam from the particle. The effects of the lateral and axial offsets between the detection beam focus and the particle centre on the displacement detection are analyzed by the simulation and experiment. The results show that the lateral offsets will decrease the detection sensitivity and linear range and aggravate the crosstalk between the x-direction signal and y-direction signal of QPD. The axial offsets also affect the detection sensitivity, an optimal axial offset can improve the sensitivity of the displacement detection substantially. In addition, the influence of system parameters, such as particle radius a, numerical aperture of the condenser NAc and numerical aperture of the objective NAo on the optimal axial offset are discussed. A combination of conventional optical tweezers instrument and a detection beam provides a more flexible working point, allowing for the active modulation of the sensitivity and linear range of the displacement detection. This work would be of great interest for improving the accuracy of the displacement and force detection performed by the optical tweezers.Comment: 10 pages,11 figure

Analysis of Confinement in Dual Spherical-Tapered Ended Fiber Optical Trap

In dual-fiber optical traps, two counter-propagating laser beams emitted from opposing laser fibers trap and manipulate particles. We describe the operation and performance of a dual-fiber optical trap created using spherical-tapered ended fiber pigtailed to 1436 nm laser diodes. Compared with the dual flat ended fiber optical trap, the dual spherical-tapered ended fiber optical trap increased the axial stiffness from 0.44 pN/µm to 0.99 pN/µm, and increased the lateral stiffness from 1.68 pN/µm to 1.76 pN/µm. The dual-fiber optical trap fabricated by spherical-tapered ended fiber enhanced the trapping efficiency of the optical trap. It expanded the application range and reliability of the dual-fiber optical trap. Additionally, we integrated the dual-fiber optical trap into an optical chip, thereby improving the stability of the system

Spatio-temporal patterns of Ulva prolifera blooms and the corresponding influence on chlorophyll-a concentration in the Southern Yellow Sea, China

The world's largest macroalgal blooms (MABs) caused by the Mai prolifera outbreaks have occurred every summer since 2007 in the Southern Yellow Sea, China. Accumulating evidence showed that MABs may deteriorate the regional marine environment and influence the growth of some primary producers such as phytoplankton. In this study, we investigated the spatio-temporal patterns of U. prolifera green tides and chlorophyll-a concentration in the Southern Yellow Sea in 2015 using satellite images obtained from HI-1 CCD, MODIS, and GOCI. The correlation between the distributions of U. prolifera abundance and chlorophyll-a concentration was analyzed quantitatively by setting up a series of 5 x 5 km experimental grids, and we also discussed the possible mechanisms about the influence of U. prolifera blooms on the other floating microalgae. The results showed that the development of U. prolifera blooms in the Southern Yellow Sea in 2015 could be featured as "appearance - development - outbreak - decline - disappearance", while the concentration of chlorophyll-a showed "increase - sharp decline - slow recovery - stabilization" from April to August. We also found that the concentration of chlorophyll-a had the following relationships with U. proliferu temporally: (1) the concentration of chlorophyll-a increased with the growth of U. prolifera from April to mid-May; (2) the chlorophyll-a concentration decreased sharply with the dramatically increased coverage of U. prolifera in June; and (3) the chlorophyll-a concentration slowly recovered and finally stabilized as U. prolifera decreased in July. Generally, there was a negative correlation between the occurrence of U. prolifera and chlorophyll-a concentration in the Southern Yellow Sea, China. Our results showed that the outbreak of U. prolifera does have a certain impact on the growth and reproduction of planktonic microalgae, and it suggests that U. pro lifera blooms have potentially altered the ecological balance in the coastal waters of the Southern Yellow Sea. (C) 2018 Elsevier BM. All rights reserved

Analysis of Confinement in Dual Spherical-Tapered Ended Fiber Optical Trap

In dual-fiber optical traps, two counter-propagating laser beams emitted from opposing laser fibers trap and manipulate particles. We describe the operation and performance of a dual-fiber optical trap created using spherical-tapered ended fiber pigtailed to 1436 nm laser diodes. Compared with the dual flat ended fiber optical trap, the dual spherical-tapered ended fiber optical trap increased the axial stiffness from 0.44 pN/µm to 0.99 pN/µm, and increased the lateral stiffness from 1.68 pN/µm to 1.76 pN/µm. The dual-fiber optical trap fabricated by spherical-tapered ended fiber enhanced the trapping efficiency of the optical trap. It expanded the application range and reliability of the dual-fiber optical trap. Additionally, we integrated the dual-fiber optical trap into an optical chip, thereby improving the stability of the system

Optical Pulling Using Chiral Metalens as a Photonic Probe

Optical pulling forces, which can pull objects in the source direction, have emerged as an intensively explored field in recent years. Conventionally, optical pulling forces exerted on objects can be achieved by tailoring the properties of an electromagnetic field, the surrounding environment, or the particles themselves. Recently, the idea of applying conventional lenses or prisms as photonic probes has been proposed to realize an optical pulling force. However, their sizes are far beyond the scope of optical manipulation. Here, we design a chiral metalens as the photonic probe to generate a robust optical pulling force. The induced pulling force exerted on the metalens, characterized by a broadband spectrum over 0.6 μm (from 1.517 to 2.117 μm) bandwidth, reached a maximum value of −83.76 pN/W. Moreover, under the illumination of incident light with different circular polarization states, the longitudinal optical force acting on the metalens showed a circular dichroism response. This means that the longitudinal optical force can be flexibly tuned from a pulling force to a pushing force by controlling the polarization of the incident light. This work could pave the way for a new advanced optical manipulation technique, with potential applications ranging from contactless wafer-scale fabrication to cell assembly and even course control for spacecraft

Enhancing the performance of the counter-propagating dual-beam optical trap with the asymmetric configuration

The trapping stiffness and width are two important parameters to characterize a counter-propagating dual-beam optical trap. We present two types of asymmetric counter-propagating dual-beam optical trap with the different numerical aperture (NA) and trapping power to eliminate the multi-equilibrium positions when two foci of the optical trap are not coincided. Meanwhile, the asymmetric dual-beam trap with the different NA enhances the axial trapping width and stiffness over five and three times, respectively, higher than the standard dual-beam trap with the higher and same average NA. Besides, it increases the transverse trapping stiffness when two foci are not coincided. The asymmetric dual-beam optical trap will benefit the future applications for the study of precision measurement, basic physics and biomaterials

Giant enhancement of higher-order harmonics of an optical-tweezer phonon laser

Phonon lasers, as mechanical analogues of optical lasers, are unique tools for not only fundamental studies of phononics but also diverse applications such as acoustic imaging and force sensing. Very recently, by levitating a micro-size sphere in an optical tweezer, higher-order mechanical harmonics were observed in the phonon-lasing regime, as the first step towards nonlinear levitated optomechanics [Nat. Phys. 19, 414 (2023)]. However, both the lasing strengths and the quality factors of the observed harmonics are typically very low, thus severely hindering their applications. Here we show that, by applying a simple but powerful electronic control to such a levitated micro-sphere, three orders of magnitude enhancement are achievable in the brightness of the phonon lasers, including both the fundamental mode and all its higher-order harmonics. Also, giant improvements of their linewidth and frequency stability are realized in such an electro-optomechanical system, together with further improved higher-order phonon coherence. These results, as a significant step forward for enhancing and controlling micro-object phonon lasers, can be readily used for a wide range of applications involving nonlinear phonon lasers, such as acoustic frequency comb, ultra-sound sensing, atmospherical monitoring, and even bio-medical diagnosis of levitated micro-size objects