Binary Phase Zone Plates Research Articles

Creating axial bifocal spots by solving the binary phase zone plate in inverse in 4Pi focusing system

We propose an approach to obtain bifocal spots with variable axial distance in 4Pi focusing system. The approach is to inversely solve the radii of a binary phase zone plate (BPZP) according to the conceived 4Pi focal field distribution. The ring radius of the BPZP solved in reverse has an analytic expression by the Richards-Wolf vector diffraction theory, which is directly related to the axial offset distance of the bifocal spots. With the BPZP designed by the analytic expression to modulate the incident radially polarized beam, bifocal spots with variable axial distance can be achieved in the 4Pi focusing region, and two focal spots have the same size. Moreover, some special focal fields, such as optical chain and optical cage, can also be created by selecting suitable axial offset distance to design the BPZP for modulating radially polarized beam. In addition, axial multi-spherical spots can also be generated by superposition of a BPZP and an annular obstruction.

Axial multifocal binary-phase zone plate for high numerical aperture focusing

Binary-phase zone plates (BPZPs) are proposed for generating two foci along the optical axis of a high numerical aperture objective. The radii of BPZPs are given by an analytical expression, in which the axial shifting distance of two foci is included. By changing the axial shifting distance, a set of BPZPs can be designed for achieving tunable two foci in the optical axial direction of the objective. Theoretical analysis and simulation results prove that the tunable axial two foci can be realized successfully, and furthermore, the intensity distribution of focal spots with special shapes (optical needle and optical bubble) can also be created by using BPZPs with suitable axial shifting distance. In addition, axial multifocal spots array can also be realized by superposition of multiple BPZPs with different axial shifting distances.

Self-similarity in radial Walsh filters and axial intensity distributions in the far-field diffraction pattern

Pupil plane filtering by radial Walsh filters is a convenient technique for tailoring the axial intensity distribution near the focal plane of a rotationally symmetric imaging system. Radial Walsh filters, derived from radial Walsh functions, form a set of orthogonal phase filters that take on values either 0 or π phase, corresponding to +1 or -1 values of the radial Walsh functions over prespecified annular regions of the circular filter. Order of these filters is given by the number of zero-crossings, or equivalently phase transitions within the domain over which the set is defined. In general, radial Walsh filters are binary phase zone plates, each of them demonstrating distinct focusing characteristics. The set of radial Walsh filters can be classified into distinct groups, where the members of each group possess self-similar structures. Self-similarity can also be observed in the corresponding axial intensity distributions. These observations provide valuable clues in tackling the inverse problem of synthesis of phase filter in accordance with prespecified axial intensity distributions. This paper reports our observations on self-similarity in radial Walsh filters of various orders and corresponding axial intensity distributions.

Subwavelength focusing of laser light by microoptics

Near-field scanning optical microscope (NSOM) measurements revealed that a linearly polarized Gaussian beam of wavelength λ = 532 nm focused with a binary zone plate (ZP) of focal length λ, radius 7 μm, and groove depth 510 nm, fabricated in hydrogen silsesquioxane, produces a focal spot of size FWHM = (0.44 ± 0.02)λ, with the side lobes being lower than 10% of the intensity peak in the focus. Replacing the incident 532 nm wavelength with a 633 nm wavelength resulted in a 1.8 times shorter focal length and a tighter (in terms of wavelengths) focal spot of FWHM = (0.40 ± 0.02)λ. This value is smaller than the scalar diffraction-limited size in vacuum, FWHM = 0.51λ. This is the smallest focal spot so far experimentally obtained for a binary phase ZP and the root-mean-square deviation of the experimental curve from a FDTD simulation is 5%. We show that the metallic pyramid-shaped cantilever with a 100-nm-hole in the tip that is used in the NSOM is only able to detect the transverse electric field component. The FDTD simulation showed such a cantilever to be over 3 times more sensitive to the transverse electric field component than to the longitudinal one. Using the Richards–Wolf (RW) formulae, the near-focus intensity distribution can be calculated with 6% error for focal lengths larger than 4λ. It is usually assumed that the Debye theory and the RW formulae are only valid for focal lengths much larger than the incident wavelength. By FDTD simulation, we showed that when illuminating the ZP by a radially (rather than linearly) polarized beam, a decrease in the focal spot transverse size did not result in a substantially reduced total volume of focus (4%).

The promise of diffractive optics in maskless lithography

For photon-based maskless lithography, a system based on the incoherent addition of scanned, multiplexed, on-axis focal spots from an array of diffractive-optical elements, such as binary-phase zone plates, has advantages over systems based on partially coherent projection of the image of a micromechanical mirror array through a large refractive or reflective lens. We have fabricated binary-phase zone-plate arrays with numerical apertures up to 0.9 with near-ideal performance, and used them to achieve high quality lithography of arbitrary patterns. Using a wavelength of 400 nm we achieve k 1 factors as low as 0.32, without the use of any resolution enhancement techniques. Sufficient contrast is achieved in dense patterns despite the presence of diffracted orders other than +1. Advantages of our diffractive-optical approach include: the feasibility of wavefront engineering by using elements other than binary-phase zone plates; the simplicity of the planar processes needed to manufacture large arrays of high-NA diffractive-optical elements; and the ease of wavelength scaling, with the promise of ultimately approaching the limits of the lithographic process.

The promise of diffractive optics in maskless lithography

For photon-based maskless lithography, a system based on the incoherent addition of scanned, multiplexed, on-axis focal spots from an array of diffractive-optical elements, such as binary-phase zone plates, has advantages over systems based on partially coherent projection of the image of a micromechanical mirror array through a large refractive or reflective lens. We have fabricated binary-phase zone-plate arrays with numerical apertures up to 0.9 with near-ideal performance, and used them to achieve high quality lithography of arbitrary patterns. Using a wavelength of 400 nm we achieve k 1 factors as low as 0.32, without the use of any resolution enhancement techniques. Sufficient contrast is achieved in dense patterns despite the presence of diffracted orders other than +1. Advantages of our diffractive-optical approach include: the feasibility of wavefront engineering by using elements other than binary-phase zone plates; the simplicity of the planar processes needed to manufacture large arrays of high-NA diffractive-optical elements; and the ease of wavelength scaling, with the promise of ultimately approaching the limits of the lithographic process.

The case for diffractive optics in maskless lithography

For photon-based maskless lithography we show that a system based on the incoherent addition of scanned, multiplexed, and on-axis focal spots from an array of diffractive-optical elements, such as binary-phase zone plates, has significant advantages over systems based on partially coherent projection of the image of a micromechanical mirror array through a large refractive or reflective lens. We show that zone-plate arrays with numerical apertures up to 0.9 can be manufactured with near-ideal performance, and used to achieve high-quality lithography of arbitrary patterns. Using a wavelength of 400 nm we demonstrate k1 factors as low as 0.32, without the use of any resolution-enhancement techniques; that sufficient contrast is achieved in dense patterns despite the presence of diffracted orders other than +1; and that a process-latitude greater than 10% is achieved for 150 nm lines and spaces. Advantages of our diffractive-optical approach include: the feasibility of wave-front engineering by using elements other than binary-phase zone plates; the simplicity of the processes needed to manufacture large arrays of high-numerical-aperture diffractive-optical elements; and the ease of wavelength scaling, with the promise of ultimately approaching the limits of the lithographic process.

Binary phase zone plate designs using calculations of far-field distributions

Binary phase Fresnel zone plate arrays have been used recently to generate circular top hat intensity profiles for high intensity laser irradiation of targets. A simple analysis of these binary phase zone plates using diffraction theory is presented and used both to optimise the uniformity of the focal spot by modifying the Fresnel zone structure and to produce noncircular foci.