Tuesday, 23 October 2012

Opticle tweezers can't pick up space ships

The concept of radiation pressure was considered by James Clerk Maxwell (1873) as he probed the consequences of his description of electromagnetic radiation. Optical tweezers (originally called "single-beam gradient force trap") are scientific instruments that use a highly focused laser beam to provide an attractive or repulsive force (typically on the order of piconewtons), depending on the refractive index mismatch to physically hold and move microscopic dielectric objects. Optical tweezers have been particularly successful in studying a variety of biological systems in recent years.
The detection of optical scattering and gradient forces on micron sized particles was first reported in 1970 by Arthur Ashkin, a scientist working at Bell Labs. Years later, Ashkin and colleagues reported the first observation of what is now commonly referred to as an optical tweezers: a tightly focused beam of light capable of holding microscopic particles stable in three dimensions.
In 1986, Arthur Ashkin and colleagues published a seminal paper in Optics Letters, ‘Observation of a single-beam gradient force optical trap for dielectric particles’ which outlined a technique for trapping micrometre-sized dielectric particles using a focused laser beam, a technology which is now termed optical tweezers. This paper provided a background in optical manipulation technologies and an overview of the applications of optical tweezers. It contains some recent work on the optical manipulation of aerosols and concludes with a critical discussion of where the future might lead this maturing technology.
The most basic form of an optical trap is a laser beam is focused by a high-quality microscope objective to a spot in the specimen plane. This spot creates an "optical trap" which is able to hold a small particle at its center. The forces felt by this particle consist of the light scattering and gradient forces due to the interaction of the particle with the light. Most frequently, optical tweezers are built by modifying a standard optical microscope. These instruments have evolved from simple tools to manipulate micron-sized objects to sophisticated devices under computer-control that can measure displacements and forces with high precision and accuracy.


In practice, optical tweezers are very expensive, custom-built instruments. These instruments usually start with a commercial optical microscope but add extensive modifications. In addition, the capability to couple multiple lasers into the microscope poses another challenge. High power infrared laser beams are often used to achieve high trapping stiffness with minimal photo-damage to biological samples. Precise steering of the optical trap is accomplished with lenses, mirrors, and acousto/electro-optical devices that can be controlled via computer. Figure 3 is meant to give an idea of the number of elements in such a system. In short, these are very complicated instruments that require a working knowledge of microscopy, optics, and laser techniques.
Tractor beams -- the ability to trap and move objects using laser light -- are the stuff of science fiction, but a team of NASA scientists has won funding to study the concept for remotely capturing planetary or atmospheric particles and delivering them to a robotic rover or orbiting spacecraft for analysis.

One experimental approach the team plans to study the optical vortex or "optical tweezers" method involves the use of two counter-propagating beams of light. The resulting ring-like geometry confines particles to the dark core of the overlapping beams. By alternately strengthening or weakening the intensity of one of the light beams in effect heating the air around the trapped particle researchers have shown in laboratory testing that they can move the particle along the ring's center. This technique, however, requires the presence of an atmosphere.



In 2011, researchers in China calculated that a type of laser called a Bessel beam, which puts out light in concentric rings, could be designed to make a particle inside the beam emit photons on the side facing away from the beam source. These photons should allow the particle to recoil towards the source. The effect is different from that employed in "optical tweezers" approaches, in which tiny objects can be trapped in the focus of a laser beam and moved around; this new force, the authors propose, would be one continuous pull toward the source. If such a Bessel beam were to encounter an object not head-on but at a glancing angle, the backward force can be stimulated. As the atoms or molecules of the target absorb and re-radiate the incoming light, the fraction re-radiated forward along the beam direction can interfere and give the object a "push" back toward the source. But nobody has so far managed to put the idea into practice.
optical solenoid beams, diffractionless solutions of the Helmholtz equation whose diffraction-limited in-plane intensity peak spirals around the optical axis, and whose wavefronts carry an independent helical pitch. Unlike other collimated beams of light, appropriately designed solenoid beams have the noteworthy property of being able to exert forces on illuminated objects that are directed opposite to the direction of the light's propagation. The light in the corkscrew can then be tilted at an angle that kicks the spheres backward even as the beam itself moves forward. Like a tennis player sprinting away from the net while deftly lobbing the ball back at an opponent, this tilt can potentially push an object all the way back to the beam’s source. Or it can be rotated to push forward. Physicist David Grier of New York University commented on this idea and says “You’d need a terawatt [or trillion-watt] laser to pull a person,” says Grier. Being struck by that much energy, though, would likely incinerate the person being pulled. “It would be a short trip.”
The prospect of using laser light to pull objects may be along way down the road as todays technology can only manage to hold nano sized particles. Nasa is funding three different methods to possibly retrieve rock samples for future exploration. I would like to hope that it might be possible for a tractor beam but large objects might prove very difficult. Even if the power was to increase the intensity of light will have a burning effect on the object it was trying to pull. Perhaps if light could change its property in the way it becomes solid, it might work. The idea of teleportation maybe not possible for earth to spaceship transportation but a simple netting matrix around an objet for pulling is abetter method of tractor beam. At this point in time laser technology hasn't got the pulling power...


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