Optical tweezers gradient forces

In 1986, Ashkin and coworkers reported on the first successful single-beam gradient-force trap, or laser tweezers, for dielectric particles [21]. They were able to trap particles of glass, silica, and polystyrene (PS) in the range from 25 nm to 10 pm in water. Optical trapping techniques have since been integrated to a range of different... 

A. Ashkin, J.M. Dziedzic, J.E. Bjorkholm, S. Chu, Observation of a single beam gradient force optical trap for dielectric particles. Opt. Lett. 11, 288-290 (1986) M.J. Lang, S.M. Block, Resource letter LBOT-1 Laser-based optical tweezers. Am. J. Phys. 71, 201-215 (2003)... 

Fig. 2 Schematic diagram of laser optical tweezers (LOT) system. (Top panel) An infrared laser beam was steered to trap and move pm-size particles in the focal plane. At a sufficiently large laser intensity, the gradient force dominates over the scattering force. (Lower panel) Membrane tethers are extracted from live cells. Dielectric beads (0.5 pm in diameter) were conjugated with antibodies against integiins and attached to the cell membrane. LOT traps one bead and pulls it away from the cell, as indicated by the arrow. Thin membrane tethers extending from the beads to the cell body appear. Using conventional brightfield microscopy, the tether length and its diameter are measured which, in turn, provide an estimate of the plasma membrane tension.

As discussed earlier, the gradient forces induced by the optical tweezers can efficiently manipulate... 

Optical and magnetic tweezers manipulate a handle in the form of a bead attached to the end of a molecule such as DNA. Optical tweezers and traps exploit the restoring force that can be exerted on a dielectric microbead by the electric-field gradients at the focus of a laser beam. In the case of magnetic tweezers, a magnetic bead is manipulated between magnetic poles. 

Optical gradient forces are the fundamental physical phenomena responsible for optical tweezers. There exist two separate theories which accurately descrihe the origin of the optical gradient forces for two different physical... 

Similar spectral techniques as discussed for macroscopic tumour imaging can be employed for fluorescence microscopy. Confocal and two-photon-induced fluorescence microscopy [10.210], and imaging Fourier transform spectroscopy [10.211] are all valuable techniques for studies at the cellular level. Related to this field is the optical trapping of ceils with focused laser beams optical tweezers), which relies on gradient forces of the same kind as discussed in Sect. 9.8.5. Trapped cells and polymer strings can be manipulated in many ways to enable fundamental studies to be conducted [10.212]. 

The concept of the laser control of atomic motion, discussed in Chapters 5 and 6, was developed simultaneously and successfully for neutral microparticles, specifically with a view to creating optical tweezers. There are prospects for using the nonresonance gradient force produced by strong femtosecond laser pulses to control to some extent the motion of electrons and thus develop laser-induced electron optics in the future. It therefore seems quite natural to discuss, in conclusion, these problems related to the main subject matter of this book. 

In the photonic force microscope a small bead ( 1 pm) is optically confined by a focused laser beam, typically a Nd YAG laser with a wavelength of 1064 nm. These optical tweezers hold the partide with a weak force gradient so that very small forces (of the order of picoNewton) can be deteaed by its motion. Motion within this constraint is determined from the analysis of changes in the scattered laser light pattern. 

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