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Laser-Based Particle Manipulation and Trapping

Laser-based particle manipulation and trapping has captured the interest of experimentalists from all fields ranging from atomic physics to biology. Researchers can flexibly trap and manipulate single neutral particles within one or three dimensions using laser beams.

Radiation pressure forces enable this by elevating particles that are smaller compared to the size of the beam.

Optical Tweezers

Optic tweezers apply extremely minute forces to trap and manipulate small particles. They use a tightly focused laser beam, typically focused using a microscope objective. At its narrowest point – commonly referred to as the “beam waist” – an electric field gradient attracts dielectric particles towards it, creating an attractive force comprised of gradient force, scattering force caused by interaction of beam with environment, and gravity.

Tweezers can also transfer angular momentum from a beam onto trapped particles, which allows the system to rotate and position them – such as twisting DNA molecules (Figure 2).

Optic tweezers can be combined with various techniques, including fluorescence and label-free microscopy, to allow simultaneous manipulation and observation of single-molecule interactions within an integrated system. Our C-Trap instrument offers such functionality by combining high resolution optical tweezers with label-free fluorescence microscopy and advanced microfluidics all in one instrument.

Optic tweezers not only provide an efficient means for manipulating micron-sized particles, but they also offer real time data about their mechanical behaviour. This data includes real-time records of forces and extensions exerted on them through interferometry measurement techniques.

As an alternative to optical tweezers, focused laser beams can also be used in a similar manner without needing waveguides – this process is known as lensless evanescent optical trapping and has many uses including cell manipulation.

An optical tweezers setup typically includes a laser, beam expander and some optics to direct its location within the sample plane, microscope objective/condenser combination to create optical trap and position detector such as quadrant photodiode for measuring laser spot position.

Optic tweezers are most frequently used to hold and manipulate microscopic particles such as biological molecules, individual cells or subcellular organisms. They can be used to move, rotate, separate, stretch and join up to 2×10 microscopic objects simultaneously or independently using very light forces that fall below Brownian limits of solvent molecules – making them suitable for contact-free manipulation of living specimens.

Optical Binding

As recent developments in atom trapping have shown, similar forces can also be applied to manipulating neutral particles such as macrostyrene spheres. An intense electric field at the center of a laser beam polarizes particles much as an atom would and pushes them in the direction of propagation of laser beam. Furthermore, the force can be highly tuned as laser frequencies well below absorption resonance of sphere can be tuned; additionally it is not limited to its immediate vicinity but can expand over a wider volume through scattering.

Optics binding is a scattering process which extends an optical potential over a long distance in space, potentially even across multiple spatial dimensions. As seen below, one such instance can be observed through two-dimensional simulation. Here we see a two-dimensional region containing an isolated particle in its center and two external particles on either side. Optic potential generated by a trapped particle extends across each adjacent region and weakens in two further-apart arcs. This effect arises as a result of light scattering from external particles efficiently being transmitted via an induced polarization in the central particle, enabling external NPs to interact with trapped NPs via similar interactions as with 3LA.

As a result, external nanoparticles (NPs) can interact via light scattering to form optical binding between them, leading to dynamically coupling NPs through light scattering and trigger an optical binding effect. The dynamics behind this phenomenon is quite unpredictable and can be altered by changing either the angle of incidence between 3LA and NPs or modulating light polarization intensity.

Optically binding nanoparticles creates a dynamic swarming pattern of their movement. This phenomenon can be explained using various observations: for instance, the position of principal energy minima changes depending on both handedness of NPs and chirality of incident radiation (see Figure above). As such, energetically advantageous configurations become available, forcing right-handed NPs out while attractive forces attract left-handed ones toward them (this forces can either repel or attract each other depending on whether NPs have positive or negative charge).

Optical Trapping

Optical trapping involves using laser light beams directed at samples to attract, hold and manipulate particles. It was first demonstrated in 1970 when Arthur Ashkin trapped micron-sized latex spheres between two focsed counterpropagating beams of laser light (see “The Pressure of Laser Light,” SCIENTIFIC AMERICAN February 1972).

An optical trap instrument typically features an objective that serves as the trapping volume, and requires a strongly converging design in order to generate gradient forces, or Earnshaw forces, needed for stable trapping. Furthermore, this objective must be overfilled with trapping beam to avoid marginal (outer) rays from focused beam generating large scattering forces against trapped atoms; this also limits maximum achievable force/stiffness/geography parameters and trapping geometry limits.

To address these limitations, other techniques for creating single-beam traps were developed in the 1980s. One-dimensional approaches presented a major drawback when applied as trapping techniques: they lacked transverse cooling (i.e. slowing down peak velocity distribution to zero). This restriction severely limited their application range.

Recently, researchers have developed an entirely new generation of dual-beam trapping instruments capable of simultaneously performing slowing, cooling and trapping functions in one instrument. In these instruments, the trapping laser and fluorescence excitation beam are separated using either a mirror or electro-optic modulator; their switching frequencies usually exceed 10kHz for fast switching rates that ensure dye molecules never experience extended exposure to trapping beams that would compromise trapping performance.

Multi-beam systems also provide temperature control of both the sample stage and objective, which is crucial since thermal effects of samples contribute significantly to trapping force and stiffness. Furthermore, using high-powered trapping lasers may cause unwanted movement of the stage that needs to be reduced by using feedback loops to monitor and control its position.

Optical Imaging

Optic imaging is an indispensable method for studying particles and cell structures. It can be used to visualize single-molecule fluorescence or to capture images of proteins or cells, and when combined with optical trapping it provides valuable information about binding and dynamics without needing to bind cells to surfaces – giving researchers unprecedented clarity and resolution in studying complex biological systems.

By employing optical manipulation and trapping techniques, it is now possible to manipulate particles as diverse as atoms, large molecules, dielectric and absorbent microspheres ranging in size from several microns up to living cells – even whole living cell cultures! This ability has revolutionized numerous scientific fields ranging from materials science and nanotechnology through biology and medicine.

Svoboda and Block (31) created the inaugural stable atom trap in 1969 using two opposing moderately diverging Gaussian beams with moderate divergences, using photon radiation pressure force against gravity as well as side-to-side instabilities to achieve stability. They proved that photon radiation pressure force could effectively counter the downward force of gravity as well as side-to-side and vertical instability simultaneously.

This same principle was later employed to demonstrate a method for levitating particles using photon radiation pressure alone and provided other experimental capabilities, including particle guidance and separation, three-dimensional trapping, and ultracold collisions (48).

As its name implies, optical imaging uses a focused laser beam to hold and move submicroscopic objects such as atoms, nanoparticles and droplets like tweezers do. It can even be applied in air or vacuum conditions for levitation of particles in these environments.

Key to achieving this result lies in the interaction between the beam and surface of the trapped object, where light reflects off it to change direction upon exiting and Newton’s third law dictates this change should lead to it returning back into its original location, thus creating the trapping force.
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