Abstract
We propose planar, high numerical aperture (NA), low loss, focusing reflectors and lenses using subwavelength high contrast gratings (HCGs). By designing the reflectance and the phase of non-periodic HCGs, both focusing reflectors and lenses can be constructed. Numerical aperture values as high as 0.81 and 0.96 are achieved for a reflector and lens with very low losses of 0.3 and 0.2 dB, respectively. The design algorithm is also shown to be readily extended to a 2D lens. Furthermore, HCG optics can simultaneously focus the reflected and transmitted waves, with important technological implications. HCG focusing optics are defined by one-step photolithography and thus can be readily integrated with many devices including VCSELs, saturable absorbers, telescopes, CCDs and solar cells.
Highlights
Focusing reflectors and lenses are among the most fundamental components in applications involving manipulation of light, including imaging, communications, displays, sensing, solar cells and instrumentation
We demonstrate a unique capability of high contrast gratings (HCGs) reflectors – when a plane wave is incident upon the structure, both reflected and transmitted waves are simultaneously brought to a focus, i.e. a real image is formed on both sides of the element
The double focus and possibility to design with any reflection/transmission combinations will revolutionize the design of VCSELs or solid state lasers, where a single HCG reflector acts as both a cavity mirror and an external focusing element
Summary
Focusing reflectors and lenses are among the most fundamental components in applications involving manipulation of light, including imaging, communications, displays, sensing, solar cells and instrumentation. Because the lens must be transparent, the choice of material is limited and the highest refractive index is ~2 This limits the NA of conventional lenses. A reflector’s focusing capability arises from its aspherical shape, enabling high NA Both the lens and reflector require an aspheric shape and bulky thickness, presenting difficulties for standard microfabrication techniques. We propose novel planar focusing reflectors and lenses with both high NA and low loss, using subwavelength high contrast gratings (HCGs). An HCG is a subwavelength grating comprised of high index bars fully surrounded by low index medium [8] These structures can be fabricated with standard photolithography. We show that by designing the HCG in a non-periodic manner, we can achieve a phase distribution as in a typical lens, but in a planar device on the order of ~1 μm thick. We demonstrate a unique capability of HCG reflectors – when a plane wave is incident upon the structure, both reflected and transmitted waves are simultaneously brought to a focus, i.e. a real image is formed on both sides of the element
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