General design formalism for highly efficient flat optics for broadband applications

Tailored chromatic diffraction efficiency of double-layer single-relief blazed gratings

"Wavelength selectivity" is an important intrinsic property of diffractive optical elements that offers significant application potential. Here, we focus on tailored wavelength selectivity, the controlled efficiency distribution into different specific diffraction orders for selected wavelengths or wavelength ranges from UV to IR using interlaced double-layer single-relief blazed gratings composed of two materials. Dispersion characteristics of inorganic glasses, layer materials, polymers, nanocomposites, and high-index liquids are taken into account to investigate the impact of especially intersecting or partially overlapping dispersion curves on diffraction efficiency in different orders, providing a guideline for material choice depending on the required optical performance. By selecting appropriate combinations of materials and adjusting the grating depth, a wide variety of small or large wavelength ranges can be assigned to different diffraction orders with high efficiency that can be beneficially applied to wavelength selective functions in optical systems also including imaging or broadband lighting applications.

Combined diffractive optical elements with adjustable optical properties controlled by a relative rotation: tutorial

A pair of adjacent transmissive diffractive optical elements (DOEs) forms a combined DOE with tunable optical properties, as, for example, a diffractive lens with an adjustable focal length. The optical properties are controlled by a relative movement of the two DOEs, such as a translation or a rotation around the optical axis. Here we discuss various implementations of this principle, such as tunable diffractive lenses, axicons, vortex plates, and aberration correction devices. We discuss the limits of the tuning range and of diffraction efficiency. Furthermore, it is demonstrated how chromatic aberrations can be suppressed by using multi-order DOEs.

A theoretical framework for general design of two-materials composed diffractive fresnel lens

Near 100% of diffractive efficiency for diffractive optical elements (DOEs) is one of the most required optical performances in broadband imaging applications. Of all flat DOEs, none seems to interest researchers as much as Two-Materials Composed Diffractive Fresnel Lens (TM-DFL) among the most promising flat DOEs. An approach of the near 100% of diffractive efficiency for TM-DFL once developed to determine the design rules mainly takes the advantage of numerical computation by methods of mapping and fitting. Despite a curved line of near 100% of diffractive efficiency can be generated in the Abbe and partial dispersion diagram, it is not able to analytically elaborate the relationship between two optical materials that compose the TM-DFL. Here, we present a theoretical framework, based on the fundaments of Cauchy's equation, Abbe number, partial dispersion, and the diffraction theory of Fresnel lens, for obtaining a general design formalism, so to perform the perfect material matching between two different optical materials for achieving the near 100% of diffractive efficiency for TM-DFL in the broadband imaging applications.

Glued diffraction optical elements with broadband and a large field of view

High diffraction efficiency is an important requirement for hybrid diffractive-refractive optical systems with a wide field of view. The issue is that diffractive optical elements cannot maintain high diffraction efficiency across a designed waveband and range of incident angles simultaneously. Glued diffractive optical elements (GDOEs) consist of two single-layer diffractive elements, and optical adhesives are presented to address the problem. Two diffractive optical elements are glued together to reduce the straylight scattered into unwanted diffraction orders. The parameters of diffractive optical elements are optimized to achieve broadband high diffraction efficiency and modulation transfer function over a wide-incident-angle range. The GDOEs enable the system to realize a diffraction efficiency of over 90% when the incident angle is no more than 58°. Through gluing two single-layer diffractive optical elements together, we can minimize the inner reflection and refraction. Diffraction efficiency losses can be compensated by the optical adhesives layer, and image quality can be improved. Our design method could make possible the use of diffraction elements in different kinds of optical systems.

Impact of fabrication errors and refractive index on multilevel diffractive lens performance

Multilevel diffractive lenses (MDLs) have emerged as an alternative to both conventional diffractive optical elements (DOEs) and metalenses for applications ranging from imaging to holographic and immersive displays. Recent work has shown that by harnessing structural parametric optimization of DOEs, one can design MDLs to enable multiple functionalities like achromaticity, depth of focus, wide-angle imaging, etc. with great ease in fabrication. Therefore, it becomes critical to understand how fabrication errors still do affect the performance of MDLs and numerically evaluate the trade-off between efficiency and initial parameter selection, right at the onset of designing an MDL, i.e., even before putting it into fabrication. Here, we perform a statistical simulation-based study on MDLs (primarily operating in the THz regime) to analyse the impact of various fabrication imperfections (single and multiple) on the final structure as a function of the number of ring height levels. Furthermore, we also evaluate the performance of these same MDLs with the change in the refractive index of the constitutive material. We use focusing efficiency as the evaluation criterion in our numerical analysis; since it is the most fundamental property that can be used to compare and assess the performance of lenses (and MDLs) in general designed for any application with any specific functionality.

Structured metal double-blazed dispersion grating for broadband spectral efficiency achromatization

The majority of spectrometers use reflective dispersion gratings with a metal-coated blazed grating profile for spectral decomposition. They achieve high diffraction efficiency at the design wavelength, which decays considerably in the adjacent longer and shorter wavelength ranges. We introduce a structured metal double-blazed grating with a high diffraction efficiency for a broadband spectral range, consisting of a sawtooth-like structured metal surface filled with a first dielectric transparent material. The planarized upper surface is covered with a second blazed profile of a different transparent material. We present a systematical theoretical analysis of the diffraction efficiency in reflection geometry, based on a scalar approach involving fundamental dispersion parameters such as Abbe numbers and relative partial dispersions of the materials. We find material combinations reducing the profile heights down to 1-2 µm.