Optical Trapping Research Articles

Hybrid Paul-optical trap with large optical access for levitated optomechanics

We present a hybrid trapping platform that allows us to levitate a charged nanoparticle in high vacuum using either optical fields, radio-frequency fields, or a combination thereof. Our hybrid approach combines an optical dipole trap with a linear Paul trap while maintaining a large numerical aperture (0.77 NA). We detail a controlled transfer procedure that allows us to use the Paul trap as a “safety net” to recover particles lost from the optical trap at high vacuum. The presented hybrid platform adds to the toolbox of levitodynamics and represents an important step towards fully controllable “dark” potentials, providing control in the absence of decoherence because of photon recoil. Published by the American Physical Society 2024

Controlling the orientation of ellipsoidal nanoparticles using fractional vector beams

In the field of nanotechnology, achieving precise manipulation of ellipsoidal nanoparticles presents a significant challenge because it requires controlling five degrees of freedom, including three spatial dimensions (position in 3D space) and two angular dimensions (polar and azimuthal angles). In this work, we investigate both the optical forces and trapping potentials on an ellipsoidal nanoparticle produced by tightly focused fractional vector beams (FVBs). Unlike the integer vector beams (IVBs), which manipulate only three spatial dimensions of ellipsoidal particles, FVB with an initial phase not only provides spatial position control but also enables precise manipulation of spatial orientation. Moreover, by adjusting the topological index and initial phase of the incident FVBs, arbitrary orientations in the 3D space of ellipsoidal nanoparticles can be achieved. Our results may find interesting applications in microfluidics, biomedical engineering, and nanotechnology.

Compensating loss via non-Hermiticity in optically trapped and bounded particles

The non-Hermiticity in the optical trapping and binding originates from their open nature. Once the non-Hermiticity is sufficiently large such that it pushes the system across the exceptional point, the non-Hermitian force will provide an effective gain to the systems. In this scenario, the trapped particles acquire additional energies as a consequence of the non-Hermitian force field, which effectively serve as a gain. Conversely, these trapped or bound particles can also dissipate energy as a result of the damping effect experienced during oscillation within optical trapping or binding. People usually employ vacuum extraction to extend the lifetime of particles’ vibrational modes. However, low-pressure environments can induce instability in the systems. Here, we propose using the “non-Hermitian gain” to compensate for damping loss and enhance the quality factor (lifetime, Q-factor) of vibrational modes. Our study also takes into account the Brownian motion in optical trapping. Nevertheless, even after taking the Brownian motion into account, the Q-factors remained high. We further unveil the physical mechanism that can enhance or diminish non-Hermitian forces, such as increasing particle radius and refractive index and utilizing propagating or standing waves

Optical twin-vortex multi-trapping by Kolakoski lenses

In this work, we design and implement a new bifocal diffractive spiral lens within an optical tweezers system. The proposed diffractive optical element coined Kolokaski Kinoform Spiral Lens (KKSL), generates twin optical vortices along the propagation direction. The axial positions, as well as the diameters of the generated vortex beams, are correlated with the Kolakoski aperiodic sequence introduced in the design of the diffractive lens. The unique properties of KKSLs make it ideal for optical trapping applications, allowing independent multiple trapping and particle displacement within each optical vortex beam. We present both the focusing properties of KKSL and the experimental results of its integration into the optical tweezer setup.

Concurrently Probing the Mechanical and Electrical Characteristics of Living Cells via an Integrated Microdevice.

The mechanical and electrical properties of cells serve as critical indicators of their physiological and pathological state. Currently, distinct setups are required to measure the electrical and mechanical responses of cells. In addition, most existing methods such as optical trapping (OT) and atomic force microscopy (AFM) are labor-intensive, expensive, and low-throughput. Here, we developed a microdevice that integrates automated cell trapping, deformation, and electric impedance spectroscopy to overcome these limitations. Our device enables parallel aspiration of tens of trapped cells in a highly scalable manner by simply adjusting the applied pressures, allowing for rapid probing of the dynamic viscoelastic properties of cells. Furthermore, embedded microelectrodes enable concurrent investigations of the electrical impedance of the cells. Through testing on different cell types, our platform demonstrated superior capabilities in comprehensive cell characterization and phenotyping, highlighting its great potential as a versatile tool for single cell analysis, drug screening, and disease detection.

Measurement of circular intensity differential scattering (CIDS) from single optically trapped biological particles

The circular intensity differential scattering (CIDS), which is the normalized Mueller matrix element -S14/S11, has been measured from single biological particles as a function of scattering angle. CIDS is valuable for its potential in detecting chiral particles that may include the helical structures of DNA or RNA molecules in biological samples, and as such is a potential method for detecting biological particles. Optical trapping is employed to levitate single particles within a custom-designed elliptical reflector for CIDS measurements. The advantage of optical levitation in light-scattering measurements is that single particles can be suspended in air with sufficient working distance to prevent interference from the suspending apparatus. To measure the phase function, the reflector is used to collect the angle-dependent scattering signals. We demonstrated that we can obtain two-dimensional angular optical scattering (TAOS) patterns that cover a wide angular range from single levitated particles. These TAOS patterns are generated using 532 nm illumination of left-handed and right-handed circular polarizations and recorded from trapped single particles (silica, English Oak, Ragweed, Mulberry, Glycine, and l-Aspartic acid).

Influence of astigmatic aberration on partially coherent Gaussian-Schell vortex beam focused by lensacon

Based on the mathematical framework of the partially coherent Gaussian Schell model vortex (PCGSMV) beam and the Huygens-Fresnel integral, we conducted a study to analyze the intensity distribution and depth of focus (DOF) of the PCGSMV beam as it propagates through a classical axicon. Our numerical results indicate the influence of factors such as the beam width, spatial degree of coherence, topological charge, and axicon base angle on the intensity distribution and DOF. In addition, we investigated the relationship between the beam spot size and propagation distance, as well as the effects of the spatial degree of coherence and propagation distance on the intensity profile. Our findings highlight the importance of the intensity values along the DOF for various applications, including the optical trapping and manipulation of micro particles.

Cylindrical Vector Beams with an MOPA Amplifier Based on Nonlinear Polarization Rotation Mode-Locking

CVBs (cylindrical vector beams) are widely used in optical imaging, optical trapping, material processing, etc. In this study, based on mode-selective couplers and passive mode-locking fiber technology, a cylindrical vector fiber amplifier with an MOPA (master oscillator power amplifier) structure was developed. In the experiment, the pre-amp stage reached 19.87 mW output power and a CVB output with a mode purity greater than 97%. The measured beam quality factor was M2 = 2.1. The CVB output power obtained by the first-amp stage was 152.4 mW, and the mode purity was greater than 92%. The measured beam quality factor was M2 = 1.99. The internal inhomogeneity and external effects of the isotropic LMA (large-mode-area) fiber led to a decrease in beam quality and mode purity. After amplification, the gain of the fundamental mode was higher and the power was greater, resulting in a greatly reduced mode purity. This CVB fiber amplifier yielded important research value in expanding the applications of high-power fiber lasers.

Micro-integrated crossed-beam optical dipole trap system with long-term alignment stability for mobile atomic quantum technologies

Quantum technologies extensively use laser light for state preparation, manipulation, and readout. For field applications, these systems must be robust and compact, driving the need for miniaturized and highly stable optical setups and system integration. In this work, we present a micro-integrated crossed-beam optical dipole trap setup, the µXODT, designed for trapping and cooling 87Rb. This fiber-coupled setup operates at 1064 nm wavelength with up to 2.5 W optical power and realizes a free-space crossed beam geometry. The µXODT precisely overlaps two focused beams (w0 ≈ 33 µm) at their waists in a 45° crossing angle, achieving a position difference of ≤3.4 µm and a 0.998 power ratio between both beams with long-term stability. We describe the design and assembly process in detail, along with optical and thermal tests with temperatures of up to 65 °C. The system’s volume of 25 ml represents a reduction of more than two orders of magnitude compared to typically used macroscopic setups while demonstrating exceptional mechanical robustness and thermal stability. The µXODT is integrated with an 87Rb 3D MOT setup, trapping 3 × 105 atoms from a laser-cooled atomic cloud, and has shown no signs of degradation after two years of operation.

Optical trapping forces exerted by a pulsed Hermite–Gaussian beam on a dielectric nanosphere

An analytical expression for the optical forces exerted by a pulsed Hermite–Gaussian (HG) beam on a nano-dielectric sphere has been derived. This allows us to investigate how the forces are affected by different beam modes (p,l) and the duration of the light pulse (τ). Our findings demonstrate that these factors significantly influence the strength of the forces, the number of regions where the sphere can be trapped, and the size of these trapping zones. This knowledge paves the way for precise control of these tiny particles with light, potentially leading to improved techniques for trapping and sorting nanoparticles.

On-The-Flight trapping, LIBS analysis and discrimination of single meteorite particles generated by laser ablation

BackgroundThousands of micrometeorites fall to the Earth on a daily basis. Most of these meteorites have a rocky composition, but others are mainly composed of iron and nickel. Due to their small size, often ca. 100 μm in diameter, the process of searching for, collecting, and identifying these samples is remarkably tedious. In this work, we introduce a minimally invasive methodology for evaluating the full elemental composition of micrometeorites using optical emission spectroscopy of single particles produced by laser ablation of bulk targets. ResultsBulk meteorite samples were directly ablated within an ablation cell. From few micrograms of ablated matrix, we originated dry aerosols consisting of multielemental particles which were representative of the sample chemistry. SEM images confirmed that the generated particles exhibited spherical geometry. Particles were first optically trapped in air and, then, analyzed by laser-induced breakdown spectroscopy (LIBS). LIBS spectra evidenced compositional differences among samples. For example, Campo de Cielo meteorite featured a high iron content due to its metallic nature whereas LIBS results for Jbilet Winselwan and NWA 869 suggested that pyroxene components dominated the composition of the samples. In contrast, NWA 13739 and Vaca Muerta contained high aluminum intensity, thus indicating that the feldspathic component was dominant, as then verified by XRD. The intra-sample compositional variability were quite satisfactory, as revealed by the RSD data, below 45 %. For quantitative analysis, the percentages of FeO, SiO2, Al2O3, Na2O, MgO, TiO2, CaO, K2O, MnO, SrO, Cr2O3, and Li2O were calculated using CF-LIBS. SignificanceThis work demonstrates the applicability of laser excitation of individual particles in an optical trap for the multielemental analysis of meteorites. The methodology provides a complete overview of the samples, is capable of classificating them according to their main phases and yield preliminary quantitative information about those phases. Therefore, we present a minimally destructive pathway to be used as the first step in the inspection of these delicate samples. If the particles represent nanostructures inherent to the meteorites, it would constitute a major step in the analysis of extraterrestrial material that may provide fundamental insights into the structure and composition of the original materials at the microscale, a topic that remains an active area of research worldwide.

ATP-induced reconfiguration of the micro-viscoelasticity of cardiac and skeletal myosin solutions.

We study the high-frequency, micro-mechanical response of suspensions composed of cardiac and skeletal muscle myosin by optical trapping interferometry. We observe that in low ionic strength solutions, upon the addition of magnesium adenosine triphosphate (MgATP2-), myosin suspensions radically change their micro-mechanics properties, generating a viscoelastic fluid characterized by a complex modulus similar to a suspension of worm-like micelles. This transduction of energy, from chemical to mechanical, may be related to the relaxed states of myosin, which regulate muscle contractility and can be involved in the etiology of many myopathies. Within an analogous generic mechanical response, cardiac and skeletal myosin suspensions provide different stress relaxation times, elastic modulus values, and characteristic lengths. These discrepancies probably rely on the dissimilar physiological functions of cardiac and skeletal muscle, on the different MgATPase hydrolysis rates of cardiac and skeletal myosins, and on the observed distinct cooperative behavior of their myosin heads in the super-relaxed state. In vitro studies like these allow us to understand the foundations of muscle cell mechanics on the micro-scale, and may contribute to the engineering of biological materials whose micro-mechanics can be activated by energy regulators.

Whole-channel acoustic energy and acoustophoretic efficiency frequency spectrum by the in-flow focusing method

The in-flow focusing method, introduced herein, enables rapid and straightforward measurements of the whole-channel acoustic energy of acoustic flow-through devices. The method is applied to assess the whole-channel acoustic energy and acoustophoretic efficiency frequency spectra of a bottom-actuated silicon-glass and a side-actuated glass chip. The input variables are the time-independent geometrical shape of the particle trajectories along the manipulation chamber, the channel cross-section geometry, the physical properties of the reference particles and fluid, and the volumetric flowrate. The outputs are the total acoustic energy, the acoustophoretic efficiency, as well as the acoustic energy density distribution along the channel, which provide a comprehensive overview of the performance of acoustically driven lab-on-a-chip setups. The acoustophoretic efficiency is an important parameter that relates the energy dissipation, and therefore the heat development in the device, to the useful acoustic energy in the channel. Unlike the current state-of-the-art methods such as particle image velocimetry, particle tracking velocimetry, and optical trapping, which are typically limited to small channel areas and are time-consuming, the in-flow focusing method can be applied to long channels and enables rapid measurements of the whole-channel acoustic energy. Published by the American Physical Society 2024

Sagnac-witnessed laser deflection is an ultra-sensitive acoustic detector

Laser-deflection-based acoustic sensing is known for high bandwidth but low sensitivity. By embedding the sensing laser within a Sagnac interferometer and incorporating split-beam detection—originally developed for optical trapping microscopy—we demonstrate sensitive acoustic detection in air with a 2 MHz bandwidth. In a direct comparison, our method far-exceeds performance metrics of a state-of-the-art, commercially-available, high-bandwidth microphone. In upcoming large-volume-bubble-chamber searches for dark matter, our method could replace traditional acoustic sensors confined to the chamber’s exterior where signals are weakest.

Optical trapping of lipid bodies reveals increased cytoplasm viscosity in Candida tropicalis post antimicrobial photodynamic therapy

Abstract Antimicrobial photodynamic therapy (aPDT) is a common treatment for large cell colonies, but its effectiveness is typically assessed through colony-forming unit counting, which lacks microscopic details about cell death. This study monitors the trap stiffness of optically trapped lipid bodies of Candida tropicalis of approximately 1 μm of radius following aPDT treatment. Methylene blue served as the photosensitizer at 10 μM concentration, with a lethal light dose of 60 J cm-². The results revealed a significant increase in viscosity after aPDT treatment. Additionally, image analysis confirmed substantial morphological changes indicative of cell death. These findings demonstrate the potential of optical tweezers as a non-invasive tool for assessing cellular health by providing both functional (viscosity) and morphological data on the response to aPDT.

Comparative Evaluation of Interaction Force Characteristics for the Lipopolysaccharide of Yersinia pseudotuberculosis and Antibodies by Optical Trapping and Atomic Force Microscopy

Optical tweezers and atomic force microscopy were used for comparative evaluation of the interaction force between the lipopolysaccharide of Yersinia pseudotuberculosis and monoclonal antibodies. This paper discusses the peculiarities of two methods which allow determining significant differences in the values of the measured force required to rupture the interaction of probe sensitized by lipopolysaccharide (polystyrene microsphere for optical tweezers and silicon nitride cantilever for atomic force microscopy) with substrate (glass and mica, respectively) covered with monoclonal antibodies. In atomic force microscopy, the cantilever slides along the substrate for some time after the piezo stage is brought to a stop, causing changes in the spatial structure of sensitins and, therefore, redistribution of multiple bonds between the lipopolysaccharide agglomerate and antibodies. No significant displacement of the microsphere along the substrate occurs when an optical tweezers setup is used, and, unlike atomic force microscopy, the breaking of a complex bond between lipopolysaccharide and complementary antibodies is recorded in the form of a single and short-term (1–2 ms) leap of the photodetector signal employing optical tweezers. The recorded values of the force required to rupture the interaction measured by both methods are relative and vary depending on the chosen experimental conditions. It is shown that the non-specific component of the force needed to break the interaction measured by atomic force microscopy is significantly higher than that determined with optical tweezers.

Geometry of Braided DNA Dictates Supercoiling Partition.

During DNA replication, the replisome must rotate relative to the DNA substrate, generating supercoiling that must be partitioned in front of or behind the replisome. Supercoiling partitioned behind the replisome may intertwine (or braid) daughter DNA molecules and restrict chromosome segregation. Supercoiling partitioning and torsional resistance at the replisome should depend on the geometry of the two daughter DNA molecules, determined by their end separations. However, experimental investigation of DNA braiding under well-defined DNA geometry has proven challenging. Here, we present methods to engineer braiding substrates of defined geometry, from minimal to significant end separations. We then directly measured the torque required to braid these substrates using an angular optical trap (AOT) and found that the torque required to initiate the braiding during the first 0.5 turn critically depends on the end separation. Once braiding started, we found that the subsequent effective twist persistence length of DNA braiding is about 20-30 nm, insensitive to the end separations. Our work highlights the crucial role of braiding geometry in dictating supercoiling partitioning and torque build-up during replication. It suggests that dynamic modulation of end separation on the daughter DNA molecules could serve as a mechanism to regulate replication progression in vivo .

Experimental realization of multiple frequency photoassociation in an optical dipole trap.

The generation of cold molecules is an important topic in the field of cold atoms and molecules and has received relevant advanced research attention in ultracold chemistry, quantum computation, and quantum metrology. With a high atomic phase space density, optical dipole traps have been widely used to prepare, trap, and study cold molecules. In this work, Rb2 molecules were photoassociated in a magneto-optical trap to obtain a precise rovibrational spectrum, which provided accurate numerical references for the realization of multiple frequency photoassociation. By meeting the harsh requirements of photoassociation in optical dipole traps, the cold molecule photoassociation process was well explored, and different photoassociation resonances were simultaneously addressed in a single optical dipole trap. This method can be universally extended to simultaneously photoassociate cold molecules with different internal states or atomic species in a single optical dipole trap, thus advancing generous cold molecule studies such as cold molecule collision dynamics.

Large-scale fabrication of meta-axicon with circular polarization on CMOS platform

Abstract Metasurfaces, consisting of arrays of subwavelength structures, are lightweight and compact while being capable of implementing the functions of traditional bulky optical components. Furthermore, they have the potential to significantly improve complex optical systems in terms of space and cost, as they can simultaneously implement multiple functions. The wafer-scale mass production method based on the CMOS (complementary metal oxide semiconductor) process plays a crucial role in the modern semiconductor industry. This approach can also be applied to the production of metasurfaces, thereby accelerating the entry of metasurfaces into industrial applications. In this study, we demonstrated the mass production of large-area meta-axicons with a diameter of 2 mm on an 8-inch wafer using DUV (Deep Ultraviolet) photolithography. The proposed meta-axicon designed here is based on PB (Pancharatnam–Berry) phase and is engineered to simultaneously modulate the phase and polarization of light. In practice, the fabricated meta-axicon generated a circularly polarized Bessel beam with a depth of focus (DoF) of approximately 2.3 mm in the vicinity of 980 nm. We anticipate that the mass production of large-area meta-axicons on this CMOS platform can offer various advantages in optical communication, laser drilling, optical trapping, and tweezing applications.

Probing the Nanonewton Mitotic Cell Deformation Force by Ion-Resonance-Enhanced Photonics Force Microscopy.

Mechanical forces are essential for regulating dynamic changes in cellular activities. A comprehensive understanding of these forces is imperative for unraveling fundamental mechanisms. Here, we develop a microprobe capable of facilitating the measurement of biological forces up to nanonewton levels in living cells. This probe is designed by coating the core of anatase titania particles with amorphous titania and silica shells and an upconversion nanoparticles (UCNPs) layer. Leveraging both antireflection and ion resonance effects from the shells, the optically trapped probe attains a maximum lateral optical trap stiffness of 14.24 pN μm-1 mW-1, surpassing the best reported value by a factor of 3. Employing this advanced probe in a photonic force microscope, we determine the elasticity modulus of mitotic HeLa cells as 1.27 ± 0.3 kPa. Nanonewton probes offer the potential to explore 3D cellular mechanics with unparalleled precision and spatial resolution, fostering a deeper understanding of the underlying biomechanical mechanisms.