Imaging from the visible to the longwave infrared wavelengths via an inverse-designed flat lens

Abstract

It is generally assumed that correcting chromatic aberrations in imaging requires optical elements. Here, we show that by allowing the phase in the image plane to be a free parameter, it is possible to correct chromatic variation of focal length over an extremely large bandwidth, from the visible (Vis) to the longwave infrared (LWIR) wavelengths using a single diffractive surface, i.e., a flat lens. Specifically, we designed, fabricated and characterized a flat, multi-level diffractive lens (MDL) with thickness £ 10µm, diameter ~1mm, and focal length = 18mm, which was constant over the operating bandwidth of l=0.45µm (blue) to 15µm (LWIR). We experimentally characterized the point-spread functions, aberrations and imaging performance of cameras comprised of this MDL and appropriate image sensors. We further show using simulations that such extreme achromatic MDLs can be achieved even at high numerical apertures (NA=0.81). By drastically increasing the operating bandwidth and eliminating several refractive lenses, our approach enables thinner, lighter and simpler imaging systems.

Achromatic diffractive lens limits

“…12 is seven times the CWL, so we can have an ADL that covers a very large spectral range of about 400-3850nm (VIS-MWIR). This type of broad bandwidth ADL is demonstrated in recent work [27,28]. In essence, this is originated from the ability of an ADL to operate at moderately high m values, whereas the implementation of such m values in AMLs is extremely challenging due to the deep subwavelength features that are needed.…”

“…If so, what are the limitations of ADLs? This question has been raised by several authors [26][27][28], but to our knowledge has not been properly answered yet. In this paper we provide an answer to this question.…”

“…In contrast to this, inverse-designed ADLs seem to suffer no such limitation. Recent papers have reported ADLs that operate over an extremely large wavelength bandwidth [27,28]. If so, what are the limitations of ADLs?…”

“…If what we say is true, how do we explain the many reports of moderate or even high-NA ADLs with near diffraction limited performance [27][28][29][30][31][32]? The answer is that the real limitation is not on NA but rather on the Fresnel number (FN), as will be explained in the paper.…”

Efficient Achromatic Broadband Focusing and Polarization Manipulation of a Novel Designed Multifunctional Metasurface Zone Plate

“…Fresnel zone plate (FZP) is a typical representative [1][2][3][4][5][6][7][8]. In particular, developed on the basis of FZP, the multi-level diffraction lens (MDL) has achieved an achromatic imaging function range from visible to long-wave infrared bands via a globally optimized numerical iterative algorithm [9,10]. However, it can not modulate the polarization state of incident light while focusing with wide spectrum achromatic [11][12][13].…”