Fabrication and testing of dual -wavelength photon sieves
A photon sieve (PS) is a diffractive lens composed of pinholes distributed in a similar way to the Fresnel zones in a Fresnel zone plate lens. In this research, multiple dual-wavelength PSs are designed, fabricated and tested to determine the advantages and disadvantages of combining the diffractive elements into a single lens from two photon sieves which were designed for different wavelengths.
Both single- and dual-wavelength PSs were fabricated by first electron beam depositing a 3 nm adhesion layer of nickel, followed by a 100 nm layer of silver on a glass substrate. In the optimized procedure, the metallized substrate was then coated both with PMMA and ZEP7000, a positive electron-beam lithography resist with excellent resistance to dry-etching. After patterning of the underlying polymer and metal films, the ZEP7000 and polymer were removed to result in the pattern of pinholes in the metal layer on the substrate that constituted the photon sieve.
Multiple single- and dual-wavelength PS designs were tested using a modified Perkin-Elmer 16U grating spectrometer with a WinCamD charge-coupled device (CCD) camera. The photon sieves were apodized and were designed to have a focal length of 50 mm, lens diameter of 3 mm, and a K-factor (ratio of hole spacing to diameter) of 1.2. The distributions of the two groups of pinholes for dual wavelength PSs were categorized as concentric and sectional designs. The point spread function (PSF) of each photon sieve was obtained using the CCD camera. The peak intensity and the full width half maximum (FWHM) of each PSF were used to evaluate the focusing ability of each photon sieve.
The measured focal lengths were in the range between 50.9 to 51.7 mm, i.e., very close to the nominal focal length of 50 mm. The FWHMs and peak intensities data demonstrated that the focusing ability of a dual-wavelength PS was degraded by about 10% and 50%, respectively, as compared to a single-wavelength PS using either polychromatic or monochromatic light. When switching between two monochromatic (500 or 600 nm) light sources, dual-wavelength PSs focused both wavelengths with limited performance, while the ability of a single-wavelength PSs to focus at the designed focal length was severely degraded at a wavelengths 100 nm or more away from the designed value (500 or 600 nm). The fact that PSs are highly dispersive for non-designed wavelengths would make them good candidates for a simple focusing filter lens for polychromatic light. Dual-wavelength PSs can be used for an imaging device that does not require the filtering of polychromatic light, but at the cost of intensity. The results from concentric dual-wavelength photon sieves also demonstrated that the pinholes located in the center zone of a photon sieve dominated the lens properties.