Realization of finite-rate isothermal compression and expansion using optical feedback trap

John A. C. Albay, Pik-Yin Lai, and Yonggun Jun

We experimentally realize the finite-rate isothermal process of a Brownian particle in a breathing harmonic potential. For the compression process, finite-rate equilibration can be achieved by increasing and then decreasing the stiffness of the potential to the final stiffness according to the shortcuts-to-isothermal (ScI) protocol. On the other hand, the realization of the ScI expansion is experimentally impossible with optical tweezers due to the requirement of a negative stiffness. Here, we propose a simple and elegant method to resolve this problem and demonstrate the ScI expansion by using the optical feedback trap capable of creating an arbitrary spatiotemporal potential even with a negative stiffness. In addition, we check the thermodynamic energetics such as work, heat, and internal energy, which indeed obey stochastic thermodynamics. Our method provides the possibility of a Brownian heat engine with maximum efficiency but non-vanishing power.

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Characterization of non-linearities through mechanical squeezing in levitated optomechanics

Ashley Setter, Jamie Vovrosh, and Hendrik Ulbricht

We demonstrate a technique to estimate the strength of nonlinearities present in the trapping potential of an optically levitated nanoparticle. By applying a brief pulsed reduction in the trapping laser power of the system such as to squeeze the phase space distribution and then matching the time evolution of the shape of the phase space distribution to that of numerical simulations, one can estimate the strength of the nonlinearity present in the system. We apply this technique to estimate the strength of the Duffing nonlinearity present in the optical trapping potential.We would like to thank C. Timberlake for comments on this manuscript as well as M. Toroš, T. Georgescu, and M. Rashid for discussions. We also wish to thank the Leverhulme Trust and the Foundational Questions Institute (FQXi) for funding. A. Setter is supported by the Engineering and Physical Sciences Research Council (EPSRC) under the Center for Doctoral Training Grant No. EP/L015382/1. We also acknowledge support from the EU FET project TEQ (Grant Agreement No. 766900). In addition, the authors acknowledge the use of the IRIDIS High Performance Computing Facility and associated support services at the University of Southampton. All data supporting this study are openly available from the University of Southampton repository at https://doi.org/10.5258/SOTON/D0967. The code used to analyze the data is openly available at https://doi.org/10.5281/zenodo.1042526.

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Measurement of Van der Waals force using oscillating optical tweezers

Avijit Kundu, Shuvojit Paul, Soumitro Banerjee, and Ayan Banerjee

We employ oscillating optical tweezers as a probe to measure the surface forces between polystyrene and silica. Thus, we modulate a trapped polystyrene particle with an external sinusoidal force in close proximity (∼80 nm) of a silica surface. The particle motion is influenced by several factors which include an increased drag force according to Faxen's correction, a spurious force that comes into play due to the diffusion coefficient of the medium becoming position dependent, and finally, the London-Van der Waals (LVdW) force which becomes substantial when the particle approaches the surface. By accounting for the other forces from the analytically known results, we are able to directly quantify the LVdW force from the experimentally measured amplitude of the oscillating particle. Thereby, we determine the Hamaker constant H for the LVdW force between polystyrene and silica, and obtain a good agreement with the value reported in the literature. Our method is general in nature and can be extended toward probing other surface effects or other interaction forces using oscillating optical tweezers.

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Is it possible to enlarge the trapping range of optical tweezers via a single beam?

X. Z. Li, H. X. Ma, H. Zhang, M. M. Tang, H. H. Li, J. Tang, and Y. S. Wang

For optical tweezers, a tiny focal spot of the trapping beam is necessary for providing sufficient intensity-gradient force. This condition results in a limited small trapping range to guarantee stable trapping of the particle. Exploiting structured light, i.e., an optical vortex beam, the trapping range can be enlarged by adjusting its doughnut ring diameter. However, the trapped particle scarcely remains static due to the optical spanner action of the orbital angular momentum of the vortex beam. To enlarge the trapping range and simultaneously ensure stable trapping, we propose a beam, referred to as a mirror-symmetric optical vortex beam (MOV). Essentially, MOV is constructed by using two opposite optical spanners and a pair of static optical tweezers. The optical spanners attract the particle to the site of the static optical tweezers, which realizes long-range optical trapping. Through detailed force-field analysis, it is found that MOV could perform these setting functions. In experiments, yeast cells are manipulated in a long range of ∼25 μm, which is 3 times longer than that of the Gaussian beam. Further, the trapping range is easily adjusted by changing a parameter as desired. This technique provides versatile optical tweezers, which will facilitate potential applications for particle manipulation.
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Optimized rotation of an optically trapped particle for micro mixing

Mahmoud Hosseinzadeh, Faegheh Hajizadeh, Mehdi Habibi, Hossain Milani Moghaddam, and S. Nader S. Reihani
The angular momentum transferred by circularly polarized photons is able to rotate an optically trapped microparticle. Here, the optically rotating particle is introduced as an active micromixer to reduce the mixing time in a microfluidic system. To optimize the system for microfluidic application, the effect of several optical parameters such as spherical aberration and the numerical aperture of the objective on the rotation rate of a trapped particle is investigated. The results show that the optimized depth for the rotation of a particle is located close to the coverslip and can be changed by a fine adjustment of the refractive index of the immersion oil. By applying the obtained optimized optical parameters on a trapped particle at the interface of two fluids in a microchannel, the mixing length is reduced by a factor of ∼2.

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Nanoscale virtual potentials using optical tweezers

Avinash Kumar and John Bechhoefer

We combine optical tweezers with feedback to impose arbitrary potentials on a colloidal particle. The feedback trap detects a particle's position, calculates a force based on an imposed “virtual potential,” and shifts the trap center to generate the desired force. We create virtual harmonic and double-well potentials to manipulate particles. The harmonic potentials can be chosen to be either weaker or stiffer than the underlying optical trap. Using this flexibility, we create an isotropic trap in three dimensions. Finally, we show that we can create a virtual double-well potential with fixed well separation and adjustable barrier height. These are accomplished at length scales down to 11 nm, a feat that is difficult or impossible to create with standard optical-tweezer techniques such as time sharing, dual beams, or spatial light modulators.

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Gram-type differentiation of bacteria with 2D hollow photonic crystal cavities

R. Therisod, M. Tardif,   P. R. Marcoux, E. Picard, J.-B. Jager, E. Hadji, D. Peyrade, and R. Houdré

Fast and label-free techniques to analyze viruses and bacteria are of crucial interest in biological and bio-medical applications. For this purpose, optofluidic systems based on the integration of photonic structures with microfluidic layers were shown to be promising tools for biological analysis, thanks to their small footprint and to their ability to manipulate objects using low powers. In this letter, we report on the optical trapping of living bacteria in a 2D silicon hollow photonic crystal cavity. This structure allows for the Gram-type differentiation of bacteria at the single cell scale, in a fast, label-free, and non-destructive way.

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Optical trapping and axial shifting for strongly absorbing particle with single focused TEM00 Gaussian beam

Zhihai Liu, Jiaze Wu, Yu Zhang, Yaxun Zhang, Xiaoyun Tang, Xinghua Yang, Jianzhong Zhang, Jun Yang, and Libo Yuan

We propose and demonstrate a stable three-dimensional trap and manipulation of a micron-sized strongly absorbing particle in pure liquid glycerol by using a single tight focused TEM00 Gaussian beam. We employ a bottom-side bidirectional view observation system to observe the trapped particle. We use the light at 980 nm to trap the absorbing particle and the light at 532 nm to indicate the distribution of the temperature field around the trapped particle. The trapping position of the absorbing particle is related to the incident laser power; the lower the incident laser power, the longer the particle shift distance. Our approach provides full control over trapped absorbing particles and expands optical manipulation of strong absorbing particles into a liquid media.

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Patterning of graphene oxide with optoelectronic tweezers

Matthew B. Lim, Robert G. Felsted, Xuezhe Zhou, Bennett E. Smith, and Peter J. Pauzauskie

Optoelectronic tweezers (OET) offer a means for parallel trapping and dynamic manipulation of micro-scale particles using low-intensity light. Such capabilities can facilitate the formation of bulk materials with a precisely tailored microstructure. Here, we report the use of OET to vertically align, trap, and reposition sheets of graphene oxide (GO) in liquids, paving the way for textured and patterned graphene macroassemblies that could offer superior performance for applications in energy storage, catalysis, and electronic devices. Trapping can be achieved with low-power light from inexpensive digital projectors and diode lasers, making it simple for users to create and apply patterns while avoiding undesirable photothermal heating effects. To give users a quantitative idea of trap stiffness, we also present a theoretical framework for predicting the maximum achievable speed of a GO platelet in an OET trap.

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Extracting the potential-well of a near-field optical trap using the Helmholtz-Hodge decomposition

Mohammad Asif Zaman, Punnag Padhy, Paul C. Hansen, and Lambertus Hesselink

The non-conservative nature of the force field generated by a near-field optical trap is analyzed. A plasmonic C-shaped engraving on a gold film is considered as the trap. The force field is calculated using the Maxwell stress tensor method. The Helmholtz-Hodge decomposition is used to extract the conservative and the non-conservative component of the force. Due to the non-negligible non-conservative component, it is found that the conventional approach of extracting the potential by direct integration of the force is not accurate. Despite the non-conservative nature of the force field, it is found that the statistical properties of a trapped nanoparticle can be estimated from the conservative component of the force field alone. Experimental and numerical results are presented to support the claims.

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Calcium effect on membrane of an optically trapped erythrocyte studied by digital holographic microscopy

Vahideh Farzam Rad, Rahim Tavakkoli, Ali-Reza Moradi, Arun Anand, and Bahram Javidi

The calcium level in blood affects the morphological and rheological properties of red blood cell (RBC) membranes. In this paper, we present an integrated optical system for a single cell study of hypercalcemia. The system consists of holographic optical tweezers and blinking optical tweezers, for photo-damage-free immobilization of the cells, combined with digital holographic microscopy, for quantitative analysis and live visualization of the cells. Digital holograms were recorded live, while the concentration of calcium ions in the buffer is gradually increased. Full morphometric data of RBCs were obtained by numerical reconstruction of the holograms. Morphological changes are expressed in terms of various parameters such as root mean square, skewness, and kurtosis of the cell membrane thickness distribution. We have observed dramatic changes of the cell morphology, which are attributed to the formation of calcium-induced hydrophobic aggregates of phospholipid molecules in the RBC membrane, resulting in a net change in membrane rigidity. Our experimental results are in agreement with previous biological studies of RBCs under the Ca2+ influence.

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Optofluidic trapping and delivery of massive mesoscopic matters using mobile vortex array

Jianxin Yang, Zongbao Li, Haiyan Wang, Debin Zhu, Xiang Cai, Yupeng Cheng, Mingyu Chen, Xiaowen Hu, and Xiaobo Xing

The realization of directional and controllable delivery of massive mesoscopic matters is of great significance in the field of microfluidics. Here, the mobile thermocapillary vortex array has achieved the enrichment and transport of massive mesoscopic matters in free or limited space. The ability of the vortex array to confine objects in the center ensures the controllability of particle trajectory. We also simulated the delivery process to reveal the stability of the mobile vortex. Owing to the distance between the vortex center and the heat source, the method provides the ability to protect trapped matters, including organisms and living cells. The mobile vortex array has opened the exciting possibilities of realizing that bridges the gap between remote optofluidics and lab on a chip.

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Mode conversion enables optical pulling force in photonic crystal waveguides

Tongtong Zhu, Andrey Novitsky, Yongyin Cao, M. R. C. Mahdy, Lin Wang, Fangkui Sun, Zehui Jiang, and Weiqiang Ding

We propose a robust scheme to achieve optical pulling force using the guiding modes supported in a hollow core double-mode photonic crystal waveguide instead of the structured optical beams in free space investigated earlier. The waveguide under consideration supports both the 0th order mode with a larger forward momentum and the 1st order mode with a smaller forward momentum. When the 1st order mode is launched, the scattering by the object inside the waveguide results in the conversion from the 1st order mode to the 0th order mode, thus creating the optical pulling force according to the conservation of linear momentum. We present the quantitative agreement between the results derived from the mode conversion analysis and those from rigorous simulation using the finite-difference in the time-domain numerical method. Importantly, the optical pulling scheme presented here is robust and broadband with naturally occurred lateral equilibriums and has a long manipulation range. Flexibilities of the current configuration make it valuable for the optical force tailoring and optical manipulation operation, especially in microfluidic channel systems.

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Measurement of laterally induced optical forces at the nanoscale

Fei Huang, Venkata Ananth Tamma, Mohsen Rajaei, Mohammad Almajhadi, and H. Kumar Wickramasinghe

We demonstrate the measurement of laterally induced optical forces using an Atomic Force Microscope (AFM). The lateral electric field distribution between a gold coated AFM probe and a single nano-aperture in a gold film is mapped by measuring the lateral optical force between the apex of the AFM probe and the nano-aperture. The fundamental torsional eigen-mode of an AFM cantilever probe was used to detect the laterally induced optical forces. We engineered the cantilever shape using focused ion beam milling to improve the detected signal to noise ratio. The measured distributions of lateral optical force agree well with electromagnetic simulations of the metal coated AFM probe interacting with the nano-aperture. This technique can be extended to simultaneously detect both lateral and longitudinal optical forces at the nanoscale by using an AFM cantilever as a multi-channel detector. This will enable simultaneous Photon Induced Force Microscopy detection of molecular responses with different incident field polarizations. The technique can be implemented on both cantilever and tuning fork based AFMs.

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Thermal fluctuation analysis of singly optically trapped spheres in hollow photonic crystal cavities

M. Tonin, F. M. Mor, L. Forró, S. Jeney, and R. Houdré

We report on the behaviour of singly optically trapped nanospheres inside a hollow, resonant photonic crystal cavity and measure experimentally the trapping constant using back-focal plane interferometry. We observe two trapping regimes arising from the back-action effect on the motion of the nanosphere in the optical cavity. The specific force profiles from these trapping regimes is measured.

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Trapping and rotating of a metallic particle trimer with optical vortex

Z. Shen, L. Su, X.-C. Yuan, and Y.-C. Shen

We have experimentally observed the steady rotation of a mesoscopic size metallic particle trimer that is optically trapped by tightly focused circularly polarized optical vortex. Our theoretical analysis suggests that a large proportion of the radial scattering force pushes the metallic particles together, whilst the remaining portion provides the centripetal force necessary for the rotation. Furthermore, we have achieved the optical trapping and rotation of four dielectric particles with optical vortex. We found that, different from the metallic particles, instead of being pushed together by the radial scattering force, the dielectric particles are trapped just outside the maximum intensity ring of the focused field. The radial gradient force attracting the dielectric particles towards the maximum intensity ring provides the centripetal force for the rotation. The achieved steady rotation of the metallic particle trimer reported here may open up applications such as the micro-rotor.

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Laser-driven gel microtool for single-cell manipulation based on temperature control with a photothermal conversion material

T. Hayakawa, M. Kikukawa, H. Maruyama, and F. Arai

We propose a laser-driven hybrid gel microtool for stable single-cell manipulation. The microtool is made of a microbead dyed with multiwalled carbon nanotubes (MWNT) and thermosensitive poly (N-isopropylacrylamide) gel coating. The gel adheres to cells at high temperatures but not at low temperatures. We can manipulate single cells without direct laser irradiation by adhering the cells to the gel on the microtool using the cell-adhesion property of the gel. The microtool is heated by trapping it with optical tweezers to make its surface cell-adhesive during the manipulation. Furthermore, we can control the optical heating property of the microtool by dyeing the microbeads with MWNT ink. The laser-heating-induced temperature increase of the microtool can be controlled from 4.2 °C to 23.5 °C by varying the concentration of MWNT ink. We succeeded in fabricating the proposed microtool and demonstrated single-cell transportation using the microtool without direct laser irradiation of the cell.

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Optically assisted trapping with high-permittivity dielectric rings: Towards optical aerosol filtration

Rasoul Alaee, Muamer Kadic, Carsten Rockstuhl, and Ali Passian

Controlling the transport, trapping, and filtering of nanoparticles is important for many applications. By virtue of their weak response to gravity and their thermal motion, various physical mechanisms can be exploited for such operations on nanoparticles. However, the manipulation based on optical forces is potentially most appealing since it constitutes a highly deterministic approach. Plasmonic nanostructures have been suggested for this purpose, but they possess the disadvantages of locally generating heat and trapping the nanoparticles directly on the surface. Here, we propose the use of dielectric rings made of high permittivity materials for trapping nanoparticles. Thanks to their ability to strongly localize the field in space, nanoparticles can be trapped without contact. We use a semi-analytical method to study the ability of these rings to trap nanoparticles. The results are supported by full-wave simulations. Application of the trapping concept to nanoparticle filtration is suggested.

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Interplay between optical, viscous, and elastic forces on an optically trapped Brownian particle immersed in a viscoelastic fluid

P. Domínguez-García, László Forró, and Sylvia Jeney

We provide a detailed study of the interplay between the different interactions which appear in the Brownian motion of a micronsized sphere immersed in a viscoelastic fluid measured with optical trapping interferometry. To explore a wide range of viscous, elastic, and optical forces, we analyze two different viscoelastic solutions at various concentrations, which provide a dynamic polymeric structure surrounding the Brownian sphere. Our experiments show that, depending on the fluid, optical forces, even if small, slightly modify the complex modulus at low frequencies. Based on our findings, we propose an alternative methodology to calibrate this kind of experimental set-up when non-Newtonian fluids are used. Understanding the influence of the optical potential is essential for a correct interpretation of the mechanical properties obtained by optically-trapped probe-based studies of biomaterials and living matter.

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Suppression of photothermal convection of microparticles in two dimensional nanoplasmonic optical lattice

Yi-Chung Chen, Gilad Yossifon and Ya-Tang Yang
Photothermal convection has been a major obstacle for stable particle trapping in plasmonic optical tweezer at high optical power. Here, we demonstrate a strategy to suppress the plasmonic photothermal convection by using vanishingly small thermal expansion coefficient of water at low temperature. A simple square nanoplasmonic array is illuminated with a loosely Gaussian beam to produce a two dimensional optical lattice for trapping of micro particles. We observe stable particle trapping due to near-field optical gradient forces at elevated optical power at low temperature. In contrast, for the same optical power at room temperature, the particles are convected away from the center of the optical lattice without their accumulation. This technique will greatly increase usable optical power and enhance the trapping capability of plasmonic optical tweezer.

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