| Optical tweezers is seen as a control means of the advent-tiny particles when it was born. 1986, Bell Labs in United States, led by Ashkin, successfully trapped and manipulated micron-sized glass in three-dimension by using strong focused laser beam for the first time, while the optical tweezers technology was applied in life science research.Non-contact manipulation of optical tweezers opened the door of non-destructive study about living cells. With the rapid development of nanotechnology and life sciences in the21 st century, optical tweezers, as a powerful tool in these two areas, will have a bright prospect to become an integral technical means of scientific research.The initial development of optical tweezers technology has greatly promoted the development of life sciences, accelerating the understanding pace of the life mystery. But as an emerging technology, its technical limitations are obvious. Ordinary optical tweezers can only trap the particles with high refractive index, and increasing laser power to increase the potential well force will damage living particles. However, because the central intensity of vortex beams is zero, the living particle will not be damaged when trapped by optical vortex. In addition, vortex beam can trap both high refractive index particles and low refractive index particles. The generation of vortex optical tweezers became a major breakthrough of the optical tweezers technology.An optical vortex(OV) is also known as a screw dislocation or phase singularity.In optical vortex, the phase of light is twisted like a helical structure(with phase term exp(ilφ)) around its axis of travel. Because of the twisting, the light waves at the axis itself cancel each other out, appearing as a ring of light with a dark hole. Allen found that the beam with a helical phase carries orbital angular momentum(OAM) of l each photon.Thereafter, the beam with orbital angular momentum began to gain wide attention and research, and shown its potential advantages in the laser biology, information transmission and processing.So far, optical vortices are studied by creating them in the lab in various ways: laser directly generating, spiral phase plates(SPPs), computer-generated holograms(CGH),mode converter, spatial light modulator,et al. Laguerre-Gaussian(LG) beam is a typical example of OV. In this work, we mainly fabricated fork-shaped diffraction gratings based on computer-generated hologram(CGH) through different methods, which can be used for the generation of LG beams.In the second chapter, we describes the theories of Gaussian beam and LaguerreGaussian beam, focusing on the helical phase wavefront structure and the orbital angular momentum of Laguerre-Gaussian beam, as well as its potential application in optical tweezers prospects(vortex optical tweezers).In the third chapter, we introduced the basic principle of CGH gratings fabrication and the LG beams generation through such gratings. Then in experiment, we fabricate the CGH gratings by printing transparent films, holographic imaging and etching gold mirror based on laser burning. Although printing transparent films is easy operated, the CGH gratings by this method feature low resolution, and cannot be applicable for intense laser pulse due to low damage threshold of the film. The CGH gratings based on holographic imaging feature high resolution, but also cannot withstand the high-energy femtosecond laser pulses. While the CGH grating fabricated by laser burning have a high damage threshold(up 1012W/cm2), but it features low resolution and rough grating groove, resulting in LG beams with poor mode.In order to overcome the shortcomings of above gratings, we will use a new method to fabricate CGH gratings. In the fourth chapter, the focus of our work, based on lithography technique, we fabricated amplitude CGH grating with high quality. Such grating features high resolution(up to 100 lines/mm), high damage threshold(can withstand the optical power up to hundreds of GW/cm2), fine smoothness(roughness with submicron),which can generate LG beams with perfect mode. And then, we used such CGH gratings to generate high power femtosecond optical vortex. However, the amplitude type CGH grating has low diffraction efficiency(less than 8%). In order to improve the diffraction efficiency, we further combine photolithography and wet-etching technique to fabricate the rectangular-groove phase CGH grating through wet-etching the glass with certain depth,and studied the relationship between the diffraction efficiency and incident angle, thus greatly improving the diffraction efficiency(up to 35%).The high-quality CGH gratings fabricated by lithography technique can be used to produce high power femtosecond optical vortex, which provide a powerful tool for the study of ”vortex optical tweezers”, remote atmospheric sensing etc. |
PDF Full Text Request