Abstract
Cascade conical refraction occurs when a beam of light travels through two or more biaxial crystals arranged in series. The output beam can be altered by varying the relative azimuthal orientation of the two biaxial crystals. For two identical crystals, in general the output beam comprises a ring beam with a spot at its centre. The relative intensities of the spot and ring can be controlled by varying the azimuthal angle between the refracted cones formed in each crystal. We have used this beam arrangement to trap one microsphere within the central spot and a second microsphere on the ring. Using linearly polarized light, we can rotate the microsphere on the ring with respect to the central sphere. Finally, using a half wave-plate between the two crystals, we can create a unique beam profile that has two intensity peaks on the ring, and thereby trap two microspheres on diametrically opposite points on the ring and rotate them around the central sphere. Such a versatile optical trap should find application in optical trapping setups.
Highlights
Conical refraction of light was predicted by Hamilton and demonstrated by Lloyd in one of the earliest theoretical predictions that was followed by an experimental demonstration [1, 2]
We have shown how conical refraction of linearly polarized light beam can be used to control the angular position of a trapped microsphere, thereby adding to the panoply of techniques for optical manipulation studies [20]
The irradiance distribution in the focal image plane (FIP) plane is imaged to the sample plane of the microscope with demagnification determined by the lens L2 and the objective L3
Summary
Conical refraction of light was predicted by Hamilton and demonstrated by Lloyd in one of the earliest theoretical predictions that was followed by an experimental demonstration [1, 2]. This cone is refracted into a ring-shaped beam that propagates in the same direction as the incident beam. Recent reviews of the field of optical trapping describe the large range of areas to which trapping has been applied and suggest future developments [10, 11]. We have shown how conical refraction of linearly polarized light beam can be used to control the angular position of a trapped microsphere, thereby adding to the panoply of techniques for optical manipulation studies [20]. In this study, using cascade conical diffraction, we can form a unique optical profile consisting of a ring-shaped beam with a central Gaussian spot which offers new functionality for optical trapping and manipulation
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