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

Tan Xu, Shangquan Wu, Zhaoxiang Jiang, Xiaoping Wu, and Qingchuan Zhang

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.

DOI

Characterization of optofluidic devices for the sorting of sub-micrometer particles

James White, Cyril Laplane, Reece P. Roberts, Louise J. Brown, Thomas Volz, and David W. Inglis

In this work, we investigate methods of fabricating a device for the optical actuation of nanoparticles. To create the microfluidic channel, we pursued three fabrication methods: SU-8 to molded polydimethylsiloxane soft lithography, laser etching of glass, and deep reactive ion etching of fused silica. We measured the surface roughness of the etched sidewalls, and the laser power transmission through each device. We then measured the radiation pressure on 0.5-µm particles in the best-performing fabricated device (etched fused silica) and in a square glass capillary.

DOI

Optimization of metallic nanoapertures at short-wave infrared wavelengths for self-induced back-action trapping

Chenyi Zhang, Jinxin Li, Jin Gyu Park, Yi-Feng Su, Robert E. Goddard, and Ryan M. Gelfand

This paper presents simulation results for double nanohole and inverted bowtie nanoapertures optimized to resonate in the short-wave infrared regime (1050 nm and 1550 nm). These geometries have shown great promise for trapping nanoparticles with applications in optical engineering, physics, and biology. Using a finite element analysis tool, we found that the outline length for inverted bowtie nanoapertures in a 100 nm thick gold film with a 20 nm gap dimension having an optimized transmission resonance for 1050 nm and 1550 nm optical wavelengths is 106.5 nm and 188.5 nm, respectively. With the same gap size, the radii of the circles for the double nanohole nanoapertures are 72 nm and 128 nm. The near-field enhancements of the two structures are almost the same, while the double nanohole geometries have a 20% larger full width at half-maximum than the inverted bowtie. Next, by studying the effect of changing the inner radii of the inverted bowtie corners, we found that the difference between 2 nm and 6 nm corner radii can blue-shift the optical resonance by up to 45 nm. As a result of not having any inner corners, the double nanohole structure requires less precise fabrication and therefore could potentially have a higher successful yield of nanoapertures during the manufacturing process. Lastly, we will show experimental results that confirm the optical resonance of the nanoapertures at 1550 nm. These results will enable better performance and signal-to-noise ratio in nanoaperture trapping for the short-wave infrared wavelength regime.

DOI

Fabricating fiber probes for optical tweezers by an improved tube etching method

Y. X. Liu, B. Zhang, N. Zhang, and Z. L. Liu

An improved tube etching method to fabricate high-quality fiber probes for optical tweezers by reserving a certain length of bare fiber to form a T-type composite structure was proposed and implemented. This method can overcome the impact of fiber types on the quality of probes in the conventional tube etching effectively. Based on the influence of gravity and diffusion on the motion of reactants, the analysis of formation mechanism was proposed for this method. This procedure retained the advantage of smooth surface in traditional tube etching but shortened the etching time. Our results also demonstrated that light transmittance of the probe fabricated by this method was improved by 6.8 times, resulting in a greater force in cells trapping. This work provided a way of designing and fabricating optical fiber tweezers with a high trapping efficiency.

DOI

Optical manipulation of Rayleigh particles by metalenses—a numerical study

Zhe Shen, Hongchao Liu, Shuang Zhang, Yao-Chun Shen, Baifu Zhang, and Saiyu Luo

Based on the focusing feature of a metalens, we numerically studied its application in optical manipulation of Rayleigh particles. Three types of metalenses—point focusing, line focusing, and line focusing with phase gradient—were designed. Simulation results using the finite-difference time-domain method showed that the incident optical beams could be focused into a spot or a line for stable particle trapping. Through engineering a gradient phase in the direction of the focal line, the proposed metalens can push the particles along the line. This provides a unique capability to move particles along a line without the need of any mechanical movement. Given its thin sheet structure and compactness, the proposed metalens can be easily integrated into microfluidic and optical tweezers systems, and it can find potential applications in optical sorting of biological cells.

DOI

Dual-laser-actuated operation of small size objects at a liquid interface

Xinbin Zhang, Yahui Kong, Jihong Yan, and Jie Zhao

This work focuses on the use, for the first time to our knowledge, of dual laser beams in photothermal-effect-based propulsion of small size objects at liquid interfaces. Compared with the single-laser mode, dual-laser-actuated operation turns out to be much more controllable with high quality, efficiency, and anti-interference capacity, which can be achieved through automated programming instead of through manual operation. A series of experiments were carried out to verify the principle, with the effects of laser power, laser-spot distance, and movement speed discussed in detail. The findings of this work might provide some insights into the development of intelligent macro/micro-operation systems for manipulating objects at different scales, such as drug particles and cells at liquid interfaces in the future.

DOI

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

Zhikun Yang, Xinglei Lin, He Zhang, Xiaohui Ma, Yonggang Zou, Li Xu, Yingtian Xu, and Liang Jin

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.

DOI

All-fiber interferometer for displacement and velocity measurement of a levitated particle in fiber-optic traps

Wei Xiong, Guangzong Xiao, Xiang Han, Xinlin Chen, Kaiyong Yang, and Hui Luo

We propose an all-fiber interferometer based on laser Doppler velocimetry in a dual-beam fiber-optic trap to measure the displacement and velocity of a trapped particle. ABCD matrices are used to compute the contrast ratio of the interference. The influence of the reflectivity of the fiber end face is discussed. We have designed an optimized reflectivity based on the parameters of our setup. The antireflective coatings on the fiber end face are employed to achieve the given reflectivity. The displacement and velocity of the trapped microparticle are successfully measured by the period and frequency of the interference signal, respectively. The sensitivity of the displacement detection is 368 nm. By describing the miniaturization of the detection system, this paper provides a simple and practical scheme to achieve the integration of the entire optical trapping setup.

DOI

Microdeformation of RBCs under oxidative stress measured by digital holographic microscopy and optical tweezers

Jiaqi Liu, Lianqing Zhu, Fan Zhang, Mingli Dong, and Xinghua Qu

This paper utilized digital holographic microscopy and optical tweezers to study microdeformation of red blood cells (RBCs) dynamically under oxidative stress. RBCs attached with microbeads were stretched by dual optical tweezers to generate microdeformation. Morphology of RBCs under manipulation were recorded dynamically and recovered by off-axis digital holographic microscopy method. RBCs treated with H2O2 at different concentrations were measured to investigate the mechanical properties under oxidative stress. Use of optical tweezers and off-axis digital holographic microscopy enhanced measuring accuracy compared with the traditional method. Microdeformation of RBCs is also more consistent with the physiological situation. This proposal is meaningful for clinical applications and basic analysis of Parkinson’s disease research.

DOI

Geometrical-optics computer model of metal-knitted mesh for calculations of solar pressure on space deployable antenna reflectors

Yu. E. Geints, A. V. Klimkin, S. V. Latynsev, A. V. Ovchinnikov, I. V. Ptashnik, A. A. Solodov, A. M. Solodov, and E. N. Yakimov

A novel computer 3D model is presented for calculations of optical parameters (transmittance, reflectance, and absorbance) of a metal-knitted mesh textile as a structural element of deployable antenna reflectors for space satellites. The model is based on geometrical-optics ray tracing upon diffuse scattering of a broadband light source (Sun) at a complex knitted mesh structure with different inclinations to the radiative source. The proposed computer model is built for the special type of metal-wire textile (two-bar large void tricot) possessing extremely high transmittance and is verified by comparison with the experimental measurements of light scattering parameters of real antenna mesh samples of data-relaying satellites (Russian series “Loutch”). The model is used for calculations of solar radiation pressure exerted on a knitted mesh antenna reflector and gives the maximal pressure value of about 0.28  μN/m2.

DOI

Single-cell Raman spectroscopy reveals microsporidia spore heterogeneity in various insect hosts

Shenghui Huang, Xuhua Huang, Shengsheng Dai, Xiaochun Wang, and Guiwen Wang

Single-cell Raman spectroscopy was used to analyze the spore heterogeneity of 16 microsporidia strains from various insect hosts in order to better understand the basic biology of microsporidia. The Raman spectrum of a single spore revealed basic spore composition, and microsporidia spores in various hosts were found to be rich in trehalose. Principal component analysis and Raman intensity showed obvious heterogeneity in the trehalose, nucleic acid, and protein content of various spores; however, there was no correlation between various spore groups and host type. Trehalose content correlated with spore infectivity on Bombyx mori. Raman spectroscopy is an excellent tool for label-free investigation of intercellular molecular constituents, providing insight into the heterogeneity of microsporidia spores.

DOI

Optimization of a spatial light modulator driven by digital video interface graphics to generate holographic optical traps

Deepak K. Gupta, B. V. R. Tata, and T. R. Ravindran

We propose a method to optimize spatial light modulators (SLMs) driven by digital video interface graphics in a holographic optical tweezers system. A method analogous to that used to optimize LCD televisions is used to optimize the properties of the graphics card through a diffraction-based experiment and develop a lookup table for the SLM. The optimization allows the SLM to function with its full phase modulation depth with improved diffraction efficiency. Further, we propose a simple and robust method to correct for the spatially varying phase response of the SLM to enhance its diffraction efficiency. The optimization results in an improvement of uniformity in the intensity and quality of the trap spots.

DOI

Nonparaxial propagation of abruptly autofocusing circular Pearcey Gaussian beams

Xingyu Chen, Dongmei Deng, Jingli Zhuang, Xiangbo Yang, Hongzhan Liu, and Guanghui Wang

We introduce a new class of abruptly autofocusing circular Pearcey Gaussian beams (AAFCPGBs) which tend to be abruptly autofocusing circular Pearcey beams with a small distribution factor, or Gaussian beams with a larger distribution factor. The nonparaxial propagation of the AAFCPGBs is investigated by numerical calculation. The radiation force of the AAFCPGBs exerted on a Rayleigh particle is analyzed in detail.

DOI

How light absorption modifies the radiative force on a microparticle in optical tweezers

Warlley H. Campos, Jakson M. Fonseca, Joaquim B. S. Mendes, Márcio S. Rocha, and Winder A. Moura-Melo

Reflection and refraction of light can be used to trap small dielectric particles in the geometrical optics regime. Absorption of light is usually neglected in theoretical calculations, but it is known that it occurs in the optical trapping of semi-transparent particles. Here, we propose a generalization of Ashkin’s model for the radiative force exerted on a spherical bead, including the contribution due to attenuation/absorption of light in the bulk of the particle. We discuss in detail the balance between refraction, reflection, and absorption for different optical parameters and particle sizes. Our findings contribute to the understanding of optical trapping of light-absorbing particles and may be used to predict whenever absorption is important in real experiments.

DOI

Experimental characterization and modeling of optical tweezer particle handling dynamics

Michael D. Porter, Brian Giera, Robert M. Panas, Lucas A. Shaw, Maxim Shusteff, and Jonathan B. Hopkins

We report a new framework for a quantitative understanding of optical trapping (OT) particle handling dynamics. We present a novel three-dimensional particle-based model that includes optical, hydrodynamic, and inter-particle forces. This semi-empirical colloid model is based on an open-source simulation code known as LAMMPS (large-scale atomic/molecular massively parallel simulator) and properly recapitulates the full OT force profile beyond the typical linear approximations valid near the trap center. Simulations are carried out with typical system parameters relevant for our experimental holographic optical trapping (HOT) system, including varied particle sizes, trap movement speeds, and beam powers. Furthermore, we present a new experimental method for measuring both the stable and metastable boundaries of the optical force profile to inform or validate the model’s underlying force profile. We show that our framework is a powerful tool for accurately predicting particle behavior in a practical experimental OT setup and can be used to characterize and predict particle handling dynamics within any arbitrary OT force profile.

DOI

Improving the throughput of automated holographic optical tweezers

Lucas A. Shaw, Samira Chizari, and Jonathan B. Hopkins

The purpose of this work is to introduce three improvements to automated holographic-optical-tweezers systems that increase the number and speed of particles that can be manipulated simultaneously. First, we address path planning by solving a bottleneck assignment problem, which can reduce total move time by up to 30% when compared with traditional assignment problem solutions. Next, we demonstrate a new strategy to identify and remove undesired (e.g., misshapen or agglomerated) particles. Finally, we employ a controller that combines both closed- and open-loop automation steps, which can increase the overall loop rate and average particle speeds while also utilizing necessary process monitoring checks to ensure that particles reach their destinations. Using these improvements, we show fast reconfiguration of 100 microspheres simultaneously with a closed-loop control rate of 6, and 10 Hz by employing both closed- and open-loop steps. We also demonstrate the closed-loop assembly of a large pattern in a continuously flowing microchannel-based particle-delivery system. The proposed approach provides a promising path toward automatic and scalable assembly of microgranular structures.

DOI

Plasmonic nano-tweezer based on square nanoplate tetramers

Qijian Jin, Li Wang, Sheng Yan, Hua Wei, and Yingzhou Huang

The research fields of trapping nanoparticles have experienced a huge development in recent years, which mainly benefits from the unique field enhancement in plasmonic nanomaterials. Since the large field enhancement originates from the excited localized surface plasmon at the metal surface, exploring novel metal nanostructures with high trapping efficiency is always the main goal in this field. In this work, the plasmonic trapping of nanoparticles based on the gold periodic square tetramers (PST) was investigated through full-wave simulations using the finite-difference time-domain (FDTD) method. The electric field and surface charge distributions on the surface of PST indicate that both the trapping position and efficiency are influenced by orientations of the square nanoplates. The maximum electromagnetic enhancement is achieved when all square nanoplates rotate 45° along the 𝑧 axis. Therefore, the gradient force and trapping potential of this PST with optimal orientation were further studied, and the results indicate that a dielectric nanoparticle of 15 nm radius can be stably captured. Furthermore, the calculation results show that the plasmonic trapping with this PST exhibits strong polarization dependence. It is easy to change the trapping position and the field intensity by tuning the polarization of the incident wave. Our work enables a deeper understanding of this kind of plasmonic trapping and could have potential applications in biomedical research and life science.

DOI

Optical tweezers for trapping in a microfluidic environment

R. Kampmann, S. Sinzinger, and J. G. Korvink

Optical tweezers use the force from a light beam to implement a precise gripping tool. Based purely on an optical principle, it works without any bodily contact with the object. In this paper we describe an optical tweezers that targets an application within the framework of nuclear magnetic resonance (NMR) spectroscopy of small objects, which are embedded inside a microfluidic channel that will be integrated in a micro-NMR detector. In the project’s final stages, the whole system will be installed within the wide bore of a superconducting magnet. The aim is to precisely maintain the position of the object to be measured, without the use of susceptibility disturbing materials or geometries. In this contribution we focus on the design and construction of the tweezers. For the optical force simulation of the system we used a geometrical optics approach, which we combined with a ray fan description of the output beam of an optical system. By embedding both techniques within an iterative design process, we were able to design efficient optical tweezers that met the numerous constraints. Based on details of the constraints and requirements given by the application, different system concepts were derived and studied. Next, a highly adapted and efficient optical trapping system was designed and manufactured. After the components were characterized using vertical scanning interferometry, the system was assembled to achieve a monolithic optical component. The proper function of the optical tweezers was successfully tested by optical trapping of fused silica particles.

DOI

Determining the size and refractive index of homogeneous spherical aerosol particles using Mie resonance spectroscopy

L. J. Nugent Lew, Michelle V. Ting, and Thomas C. Preston
Methods for determining the size and refractive index of single, homogeneous, micrometer-sized aerosol particles using Mie resonance spectroscopy are studied using measurements from optically trapped particles and light-scattering calculations based on Mie theory. We consider both single-particle broadband light scattering and cavity-enhanced Raman scattering (CERS) and demonstrate that, when resonances observed in either type of spectroscopy are fitted using Mie theory, the accuracy of the best fits are similar. However, broadband measurements can yield more resonances than CERS, thus reducing the uncertainty in the retrieved parameters of best fit and increasing the range of particles that can be characterized. Resonance fitting methods are also compared to methods that fit the entire Mie scattering spectrum. Through calculations, it is shown that measured scattering spectra are sensitive to small changes in how light is collected, while Mie resonance positions are much less sensitive. This means that additional parameters are required to accurately fit entire light-scattering spectra using Mie theory, but these parameters are not needed to accurately determine Mie resonance positions.

DOI

Aberration correction in holographic optical tweezers using a high-order optical vortex

Yansheng Liang, Yanan Cai, Zhaojun Wang, Ming Lei, Zhiliang Cao, Yue Wang, Manman Li, Shaohui Yan, Piero R. Bianco, and Baoli Yao

Holographic optical tweezers are a powerful optical trapping and manipulation tool in numerous applications such as life science and colloidal physics. However, imperfections in the spatial light modulator and optical components of the system will introduce detrimental aberrations to the system, thereby degrading the trapping performance significantly. To address this issue, we develop an aberration correction technique by using a high-order vortex as the correction metric. The optimal Zernike polynomial coefficients for quantifying the system aberrations are determined by comparing the distorted vortex and the ideal one. Efficiency of the proposed method is demonstrated by comparing the optical trap intensity distribution, trap stiffness, and particle dynamics in a Gaussian trap and an optical vortex trap, before and after aberration corrections.

DOI