- 2186 - Optical Laser Tweezers. - Optical tweezers have become an established tool in research fields ranging from biophysics to cell biology. Optical, or laser, tweezers use beams of light to hold and manipulate microscopically small objects such as biological molecules or even living cells.
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----------------------------- 2186 - Optical Laser Tweezers
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- Optical tweezers have become an established tool in research fields ranging from biophysics to cell biology. Optical, or laser, tweezers use beams of light to hold and manipulate microscopically small objects such as biological molecules or even living cells.
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- Optical tweezers are formed when a laser beam is tightly focused to a tiny region in space using a microscope objective as a lens. This region becomes an optical trap that can hold small objects in 3 dimensions.
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- Optical tweezers can also make accurate measurements of the tiny, sub-pico Newton forces exerted on the trapped objects. This allows researchers to study the diffusion dynamics , Brownian motion, of an object in a solvent. Optical tweezers can also be used to micro manipulate an object using well controlled forces. (Pico is 10^-12.)
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- The trapping force that holds an object in place in optical tweezers can be understood by considering how the object refracts light. Because it is tightly focused, the laser light is most intense at the center of the trap, which means that if the object moves slightly away from the center in a transverse direction, one part of the object will refract less light than the other.
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- As a result, the object refracts more light away from the center of the trap than towards it. Light carries momentum and the net effect of this refraction is a force that deflects some of this momentum away from the center of the trap. By Newton’s third law an equal and opposite force must act on the object, pushing it back towards the center of the trap.
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- A similar refraction-related effect also causes the object to push back in the opposite direction of the laser beam. The trapping is stable only if the force of the laser light scattering from the particle along the positive z–direction is compensated by a trapping force along the negative z-direction. To achieve this, a very tight focus is needed, with a significant fraction of the incident light coming in from large angles. This can be achieved using a lens with a very high aperture.
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- Force-sensing optical tweezers have the additional ability to track the motion of an object within the trap. The Brownian forces caused by an object being continuously bombarded by solvent molecules tend to displace the object from the center of the trap. Using interferometer measurements of the light refracted from the object, this displacement can be determined to nanometer accuracy, which allows the external forces to be measured at the sub-pico Newton level. (Pico is 10^-12, and nano- is 10^-9.)
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- Such external forces depend on the viscosity on the solvent and the properties of the trapped object. The trapped object can be pushed or pulled on other objects and the forces involved can be measured.
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- Optical tweezers have been used with great success in the field of single-molecule biophysics. They have helped researchers unravel the complex elasticity and folding dynamics of DNA, RNA, proteins and other long-chain “biopolymers”. In these experiments, the biopolymers are typically manipulated from both ends either by suspending them between an optical trap and a surface, or between multiple traps.
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- Optical tweezers have helped further our understanding of how “motor proteins” convert chemical energy into work. Such biological motors operate over distances of nanometers and with picoNewton forces. (Pico is 10^-12, and nano- is 10^-9.)
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- A plethora of mechanically-active enzymes have been studied in this way, including many involved in DNA metabolism .
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- Optical tweezers have also been used to study living biological cells. Initially, they were used mainly to sort, manipulate, push and pull cells in a qualitative manner. Today optical tweezers have been used to make quantitative measurements in or around live cells. They have been used to study the mechanics of the process by which a cell engulfs and ingests foreign particles.
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- The optical tweezers are revealing new capabilities while helping scientists understand quantum mechanics.
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- The quantum mechanics theories have led to some weird and counterintuitive conclusions. One of them is that quantum mechanics allows for a single object to exist in two different states of reality at the same time.
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- The technical name for this phenomenon is superposition. Superpositions have been observed for tiny objects like single atoms. We never see a superposition in our everyday lives. We do not see a cup of coffee in two locations at the same time. If we did it probably wasn’t coffee, maybe tequila?
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- To explain this observation, theoretical physicists have suggested that for objects larger than nanoparticles, containing about a billion atoms, superpositions collapse quickly to one or the other of the two possibilities. For larger objects the rate of collapse is faster , instantaneous, explaining why we never see the superposition of a coffee cup being in two states at once.
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- Exploring the fact that light can exert pressure on matter we find that the radiation pressure from even an intense laser beam is quite small. However, it is large enough to support a nanoparticle, countering gravity, effectively levitating it.
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- A nanoparticle held by an optic tweezers is well-isolated from its environment, since it is not in contact with any material support. Following these ideas it suggests ways to create and observe superpositions of a nanoparticle at two distinct spatial locations.
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- The nanoparticle does not sit still within the tweezers. Rather, it oscillates like a pendulum between two locations, with the restoring force coming from the radiation pressure due to the laser. Even in its lowest energy state, the particle moves around a little bit, just enough to satisfy the laws of quantum mechanics.
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- It was possible to cool an optically levitated nanoparticle to a hundredth of a degree above absolute zero by modulating the intensity of the optical tweezers. The effect was the same as that of slowing a child on a swing by pushing at the right times.
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- In 2016 researchers were able to cool to a ten-thousandth of a degree above absolute zero. This development allowed the experimental levitation of a nitrogen-defect-carrying nanodiamond. Using a magnetic field, researchers were able to achieve the physical coupling of the nitrogen atom and the crystal motion.
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- The race is now on to reach the ground state so that an object at two locations can be observed collapsing into a single entity. If the superpositions are destroyed at the rate predicted by the collapse theories, quantum mechanics as we know it will have to be revised.
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- Stay tuned, there is still more to learn.
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- November 26, 2018
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