Broadband point-spread function engineering via a freeform diffractive microlens array

Diffraction efficiency analysis of dual-layer diffractive elements with oblique incident angles

This paper proposes a method that combines the effective area method and the oblique factors to analyze and design dual-layer diffractive optical elements at large incident angles. The method considers the effects of shadow and shield on the diffraction efficiency, as well as the correction of phase delay due to oblique incidence. The relationships among the diffraction efficiency, incident wavelengths, incident angles and period widths were analyzed with our method. A detailed comparison of the proposed method with the scalar diffraction theory is presented. The method was validated by numerical simulations of vector diffraction theory and showed similar diffraction efficiency distributions at different wavelengths and incident angles. The method provides a simple and efficient way to design and apply DLDOEs.

Multilevel diffractive lens in the MWIR with extended depth-of-focus and wide field-of-view

Optics in the mid-wave-infra-red (MWIR) band are generally heavy, thick and expensive. Here, we demonstrate multi-level diffractive lenses; one designed using inverse design and another using the conventional propagation phase (the Fresnel zone plate or FZP) with diameter = 25 mm and focal length = 25 mm operating at λ=4μm. We fabricated the lenses by optical lithography and compared their performance. We show that the inverse-designed MDL achieves larger depth-of-focus and better off-axis performance when compared to the FZP at the expense of larger spot size and reduced focusing efficiency. Both lenses are flat with thickness ≤0.5 mm and weigh ≤3.63 g, which are far smaller than their conventional refractive counterparts.

Refractive-type varifocal liquid-crystal Fresnel lenses for smart contacts

We demonstrate the implementation of a low-power, low-profile, varifocal liquid-crystal Fresnel lens stack suitable for tunable imaging in smart contact lenses. The lens stack consists of a high-order refractive-type liquid crystal Fresnel chamber, a voltage-controlled twisted nematic cell, a linear polarizer and a fixed offset lens. The lens stack has an aperture of 4 mm and thickness is ∼980 µm. The varifocal lens requires ∼2.5 V_RMS for a maximum optical power change of ∼6.5 D consuming electrical power of ∼2.6 µW. The maximum RMS wavefront aberration error was 0.2 µm and the chromatic aberration was 0.008 D/nm. The average BRISQUE image quality score of the Fresnel lens was 35.23 compared to 57.23 for a curved LC lens of comparable power indicating a superior Fresnel imaging quality.

Broadband polarization-insensitive metalens integrated with a charge-coupled device in the short-wave near-infrared range

The performance of a charge-coupled device is important in detection accuracy for terminal sensitivity of a short-wave near-infrared spectrometer. The sizes of pixel pitch and pixel itself are reducing with the development of CCD technologies. However, the fill factor of CCD pixels has not been significantly improved due to the limits of the shift registers, which makes a lower utilization rate of incident light of CCD. In recent years, the advance of metasurface optics provides a new solution for solving this problem. In this paper, a polarization-insensitive metalens is experimentally demonstrated to increase the fill factor of short-wave near-infrared CCD pixels by 4 times, and the simulated results show that the designed metalens has an excellent optical crosstalk (≤0.8%). It proves that the fill factor of CCD pixels can be further improved by the proposed approach which would pave the way for the overall integration of metalens array and photodetectors, as well as the development of CCD miniaturization and lightweight.

Freeform optics: introduction

This feature issue of Optics Express highlights 28 state-of-the-art articles that capture a snapshot of the recent developments in the field of freeform optics. As an introduction, the editors provide an overview of all published articles, which cover a broad range of topics in freeform optics. The wide variety of applications presented here demonstrates that freeform optics is a growing and vibrant field with many more innovations to come.