Qiufang Zhan

Dark focal spot shaping of hyperbolic-cosine-Gaussian beam

Dark focal spot shaping is investigated by vector diffraction theory in the focal region of a hyperbolic-cosine beam that contains one on-axis spiral optical vortex. Results show that a dark focal shape can be altered considerably by the decentered parameters in cosh parts of the beam and topological charge of the vortex. Many novel and interesting dark focal shapes may appear, including rhombic, quadrangular, cross-shaped, and foursquare dark foci. Some dark focal spot chains can also occur. In addition, the numerical aperture of the focusing system can also affect dark focal shapes remarkably, which may lead to dark focal spots that disappear or change focal shape due to the depolarization effect for a high numerical aperture. All of the above dark focal shapes can be used in an optical manipulation system to construct alterable optical traps.

Resolution Enhancement in High Numerical Aperture Optical System

Resolution enhancement requirement occurs in many optical systems, such as in imaging system, optical microscopy domain, optical lithography, and optical data storage. Some enhancement techniques are proposed, including higher numerical aperture (NA) and ring illumination. In this article, the resolution enhancement in high numerical aperture optical system is investigated by vector optics theory. It was found that the vector characteristics of the incident beam in high numerical aperture optical system affects resolution enhancement effect remarkably, and may weaken resolution enhancement effect for some cases, namely, the focus does not always decrease with increasing numerical aperture, in addition, the full width half maximum (FWHM) of focal spot differs for different direction for high numerical aperture and ring illumination, which means the resolution enhancement effect differs in different direction. Therefore, resolution enhancement of higher NA and ring illumination should be taken care of in practice use. KeywordsResolution enhancement; high numerical aperture; optical system

Concentric Two-Portion Radial Polarized Beam with Phase Shift

Focusing property and focal pattern of incident optical beam play an important role in many optical systems, including optical imaging system, optical sensing instruments, and analysis instrument. The focusing properties of concentric two-portion radial polarized beam are investigated theoretically. The phase shift of the center portion is π. Results show that the focal pattern can be altered considerably by adjusting the radius of dividing circle between the two concentric portions. With increasing radius, focal spot broadens along optical axis, and splits into two peaks, and then combine back into one peak again. In this evolution process, the local intensity minimum zone may also occur. For certain radius, the focal spot may shrink, which means super-resolution effect appears, and at the same time, focal depth can also be widened, which is very useful in many optical systems, for example increase resolution in imaging instruments.

Multiple Intensity Peaks Generation by a Radial Sine Phase Plate

Intensity distribution in focal region plays an important role in many optical systems, such as in optical tweezers, one intensity peak may act as one optical trap. In this article, the intensity in focal region induced by radial sine pure phase plate is investigated, and it was found that the phase plate can adjust focal intensity considerably. Especially, multiple intensity peaks can come into being and peak pattern can be altered by parameter in sine function of the phase plate, which may be used to construct multiple optical traps in one focusing system. KeywordsIntensity peak; radial sine phase plate; phase plate

Focal Pattern of Hyperbolic-Cosine-Gaussian Beam with Optical Vortex

Intensity distribution in focal region plays an important role in many optical systems, including optical tweezers, imaging system, and analysis instrument. The focal pattern of the hyperbolic-cosine-Gaussian beam with vortex was investigated theoretically in this article. The vortex includes one non-spiral vortex created by linear phase plate, and one spiral vortex created by spiral phase plate. By adjusting the parameters of the vortex, focal pattern evolves considerably, and some novel focal patterns may come into being, such as focal ring, two-peak pattern, four-peak pattern, which may be used to construct optical traps.

Alterable focal shift of concentric piecewise cylindrical vector beam

Focal shift plays an important in many optical fusing systems. In this article, focal shift of concentric piecewise cylindrical vector beam is investigated by means of vector diffraction theory in detail. The section of the beam consists of three concentric zones. The center circle zone and outer annular zone are radial polarized, and the inner annular zone is generalized polarized. In addition the wavefront phase distribution of the vector beam is linear function to radial coordinate. It is found that the parameter in phase distribution induces focal shift and can alter focal shift considerably. However, radii of the inner annular zone and polarization angle do affect focal shift very slightly. So the phase parameter can be used to alter big focal shift while the radii and polarization angle may be employed to adjust intensity distribution. In focusing system, the focal shift and intensity distribution may be controlled separately, which improves the application freedom of this kind of technique. Focal shift direction can also be altered by change the phase parameter.

Multiple foci generation and alteration by a radial sine phase filter

A radial sine phase filter is introduced in optical systems to control and alter the optical system focusing properties with incident Gaussian beam. It was found that the radial sine phase filter can induce tunable multiple foci in focal region, which means that the several field distances can be imaged clearly simultaneously with this kind of filter in imaging optical system. And the point of absolute maximum intensity does not coincide with the geometrical focus but shifts along the optical axis; this phenomenon is referred to as focal shift. Focal shift distance and direction can be altered by a sine parameter in the sine term of the phase distribution function. In addition, focal shift may be accompanied by an effective permutation of the focal point, namely, maximum intensity can jump from one position to other position for certain sine parameter, this effect is referred to as focal switch, which also can also be used to adjust imaging clearly field distance in-continuously. The radial sine phase filter can adjust and optimizing the focusing and imaging properties of the optical systems considerably, waist width and section shape of incident Gaussian beam also affect multiple foci.

Super-resolution in focusing system by three-zone pure phase plate

Superresolution is very important in imaging and optical storage systems, and has attracted much attention. In this article, concentric three-zone phase plate with 0, pi, 0 phase variation has been investigated numerically to show that this kind of phase plate can be used to obtain three dimensional superresolution. Focal depth, focal shift, full width half maximum, superresolution effect are listed for different radii of the phase zones, which paves the way for design of the phase plate, such as for purpose of radial superresolution with high focal depth in optical storage.