High-efficiency flexible multilevel photon sieves by single-step laser-based fabrication and optical analysis.

Over the past several decades, the need for high-resolution, high-efficiency, lightweight, high-contrast focusing optics has continued to increase due to their applications in fields such as astronomy, spectroscopy, free-space optical communications, defense, and remote sensing. In recent years, photon sieve planar diffractive optics, which are essentially Fresnel zone plates with the rings broken into individual "pinhole" apertures, have been developed on flexible, lightweight polyimide substrates. However, transmission efficiencies have continuously been very low (∼1%-11%) until this work, thus impeding the widespread use of photon sieves in practical applications. Here, we present flexible, lightweight, four- and eight-level phase photon sieves with 25.7% and 49.7% transmission efficiency, respectively, up to five times greater than that of any other photon sieve reported thus far. Additionally, these sieves were fabricated via a single step pulsed laser ablation method. The total time to fabricate a ∼3 cm2 photon sieve via the single-step fabrication was tens of seconds, giving the technique a significant advantage over traditional photolithography used to generate multilevel structures. Analytical analysis of the photon sieve was carried out via the finite-difference time-domain (FDTD) method and was in very good agreement with experimental results. We have also calculated via FDTD modeling the behavior of higher-level photon sieves for further enhanced efficiencies, and analytically show an estimated upper bound on photon sieve efficiency of 70% within the first focal plane null in the limit of increasing step number, and the data presented herein provide a relationship between efficiency and step number. Additionally, this process of multilevel diffractive lens fabrication can be extended to multilevel Fresnel zone plates, which have not previously been demonstrated by this process. The results presented in this work represent a new step in high-resolution diffractive optics, showing efficiencies suitable for widespread applications in addition to drastically reducing the cost and complexity of fabricating multilevel focusing elements.

Flexible binary phase photon sieves on polyimide substrates by laser ablation.

A binary phase diffractive optical element photon sieve is fabricated by direct laser ablation of a thin, flexible polyimide substrate with a nanosecond-pulsed ultraviolet laser. The binary phase photon sieve operates at 633 nm and was designed with 19 rings and a focal length of 400 mm. The total time to fabricate the photon sieves was tens of seconds. The surface properties of the laser-processed areas are examined, and the optical performance of the photon sieve is characterized and compared to FDTD simulations. By optimizing the laser fluence and travel distance between laser pulses, features with sub-wavelength surface roughness were achieved. The photon sieve showed good focusing ability with suppressed side-lobes. When the fractional area of photon sieve pinholes was made to approach 50%, the binary sieve diffraction efficiency approached 11%, matching the highest value reported in the literature for a photon sieve. Thus, this Letter demonstrates both high efficiency and lightweight diffractive optics suitable for space satellite and other applications, with capabilities for low cost and high throughput fabrication.

Fabrication of photon sieves by laser ablation and optical properties.

In this work, we demonstrate the feasibility and performance of photon sieve diffractive optical elements fabricated via a direct laser ablation process. Pulses of 50 ns width and wavelength 1064 nm from an ytterbium fiber laser were focused to a spot diameter of approximately 35 µm. Using a galvanometric scan head writing at 100 mm/s, a 30.22 mm2 photon sieve operating at 633 nm wavelength with a focal length of 400 mm was fabricated. The optical performance of the sieve was characterized and is in strong agreement with numerical simulations, producing a focal spot size full-width at half-maximum (FWHM) of 45.12 ± 0.74 µm with a photon sieve minimum pinhole diameter of 62.2 µm. The total time to write the photon sieve pattern was 28 seconds as compared to many hours using photolithography methods. We also present, for the first time to our knowledge in the literature, thorough characterization of the influence of angle of incidence, temperature, and illumination wavelength on photon sieve performance. Thus, this work demonstrates the potential for a high speed, low cost fabrication method of photon sieves that is highly customizable and capable of producing sieves with low or high numerical apertures.

Fully transparent photon sieve.

Regular photon sieve (PS) may only have up to ~25% transmission of light. The low transmission limits its applications in many fields such as satellite remote sensing when the reflected light incident on the PS is relatively weak. Binary PS was developed to overcome the low transmission problem of PS. However, binary PS which involves using different optical materials/thicknesses in different zones of the PS at a nanometer or micron scale, is not easy to manufacture. Therefore, in this study, we developed a fully transparent PS concept. We can use laser photolithography to simply make holes on a sheet of fully transparent material. With specifically designed optical thickness and PS-patterned pinholes, the transparent sheet can effectively focus light to its focal point. This concept is validated both by the finite-difference time domain (FDTD) modeling and by laboratory prototypes in this study.

Technique to separate lidar signal and sunlight.

Sunlight contamination dominates the backscatter noise in space-based lidar measurements during daytime. The background scattered sunlight is highly variable and dependent upon the surface and atmospheric albedo. The scattered sunlight contribution to noise increases over land and snow surfaces where surface albedos are high and thus overwhelm lidar backscatter from optically thin atmospheric constituents like aerosols and thin clouds. In this work, we developed a novel lidar remote sensing concept that potentially can eliminate sunlight induced noise. The new lidar concept requires: (1) a transmitted laser light that carries orbital angular momentum (OAM); and (2) a photon sieve (PS) diffractive filter that separates scattered sunlight from laser light backscattered from the atmosphere, ocean and solid surfaces. The method is based on numerical modeling of the focusing of Laguerre-Gaussian (LG) laser beam and plane-wave light by a PS. The model results show that after passing through a PS, laser light that carries the OAM is focused on a ring (called "focal ring" here) on the focal plane of the PS filter, very little energy arrives at the center of the focal plane. However, scattered sunlight, as a plane wave without the OAM, focuses at the center of the focal plane and thus can be effectively blocked or ducted out. We also find that the radius of the "focal ring" increases with the increase of azimuthal mode (L) of LG laser light, thus increasing L can more effectively separate the lidar signal away from the sunlight noise.