Polarization, the path along which light’s electric field oscillates, is a key property of electromagnetic radiation. In this work, we motivate a mathematical framework—Matrix Fourier optics—that enables a simple description of light’s interaction with diffractive optics that spatially modify polarization. This formalism generalizes a large body of past work in metasurface polarization optics. We show how Matrix Fourier optics allows for the design of arbitrary polarization-analyzing metasurface gratings. These gratings can be used as the single polarization component in a compact full-Stokes polarization camera. We demonstrate practical, real-time polarization photography with this camera, which may find application in machine vision and remote sensing.
We present subwavelength chiral metasurfaces with freeform geometries obtained from an inverse design algorithm. These metasurfaces possess simultaneously a desired transmission, reflection and absorption spectrum at the wavelength of interest, a feature which is difficult to achieve for regular metasurfaces comprised of discrete shapes due to the limited degrees of freedom in the design. These absorptive metasurfaces could potentially be useful in display technologies by eliminating reflected ambient light, while transmitting the desired image. They have the potential to surpass the performance of traditional circular polarization analyzers and filters.
Polarization sensitive imaging can reveal the arrangement of tissue constituents without the need for resolving power necessary to directly visualize them. Several clinical applications for diseases in the pulmonary tract and the coronary artery can benefit from such capability. Unfortunately, robust and reliable endoscopic polarimetry is plagued by the unpredictability of the polarization state of light delivered to and collected from tissue through fiber optic catheters. We propose a nano-optic endoscope that uses polarization-sensitive metalenses to deliver and collection light of known polarization states to and from tissue, enabling measurement of tissue retardation and optic axis unambiguously.
Metasurfaces, nanophotonic arrays of subwavelength phase shifting elements, hold promise for the miniaturization of a variety of bulk optical elements. Owing to the flexibility with which their constituent elements may be engineered, metasurfaces allow for point-to-point polarization control on a subwavelength scale. Metasurfaces, then, represent an exciting new platform for polarization optics.
A single metasurface may combine many different polarization-dependent functionalities that would ordinarily be spread out over many optical elements. We describe how, through relatively simple optimization methods, a metasurface producing arbitrarily specified polarization states can be designed. This functionality is equivalent to a traditional diffraction grating with individual waveplate optics on each order; here, all the necessary polarization optics can be integrated into a flat, efficient, and ultrathin metasurface optical element. Moreover, such a metasurface can be used in a reverse configuration as a parallel snapshot polarimeter with no need for additional polarization optics. We present a detailed experimental characterization of this device in the visible spectral region and a comparison of the performance of the metasurface to a commercially available rotating waveplate polarimeter.
Metasurfaces can enable compact, miniaturized sensors for polarimetry and polarization imaging. We will conclude with a perspective on these possibilities and their implications for remote sensing. Metasurface polarization optics can overcome limitations of previous diffractive/grating based polarimetry schemes are potentially of significant interest to the imaging polarimetry community.
The strong optical chirality arising from certain synthetic metamaterials has important and widespread applications in polarization optics, stereochemistry and spintronics. Recently we have shown that strong intrinsic optical chirality can arise in planar high-index dielectric nanostructures whose thickness is greater than an optical wavelength, due to the excitation of magnetic dipoles that lie in the same plane as, but are orthogonal to, their electric counterparts. However these structures were comprised of a lossless dielectric, and incident light of the undesired helicity was diffracted instead of transmitted in the zeroth order. Here we explore the possibility of designing absorptive, subwavelength chiral metasurfaces with a desired transmission and absorption spectrum. We find that while the usual design process using discrete, well-known geometries can lead to structures with efficient contrast in transmission, it is extremely challenging to simultaneously engineer the reflection or absorption spectrum. We use topological optimization techniques to realize chiral metasurfaces comprised of freeform geometries, and show that they can exhibit a large intensity contrast in both transmission and absorption, depending on the helicity of incident circularly polarized light. These metasurfaces could be useful particularly for display technologies, and potentially overcome the inherent 50% transmission limit set by a regular circular polarization analyzer comprised of an absorptive linear polarizer and half-waveplate.
Metasurfaces, nanophotonic arrays of phase shifting elements, hold promise for the miniaturization of a variety of bulk optical elements, most notably lenses and imaging systems. Owing to the flexibility with which their constituent elements may be engineered, metasurfaces allow for point-to-point polarization control on a subwavelength scale. For this reason, metasurfaces represent an exciting new platform for polarization optics as well.
I will discuss how this functionality allows for a new perspective on diffractive optics which explicitly acknowledges the vectorial nature of light. This perspective motivates a theory of unitary polarization gratings; I will derive a few key results concerning these gratings. I will discuss and demonstrate how this perspective allows for the design of metasurfaces with new polarization functionalities.
I will describe how, through relatively simple optimization methods, a metasurface producing arbitrarily specified polarization states can be designed. This functionality is equivalent to a traditional diffraction grating with individual waveplate optics on each order; here, all the necessary polarization optics can be integrated into a flat, ultrathin optical element. Moreover, such a metasurface can be used in a reverse configuration as a parallel snapshot polarimeter with no need for additional polarization optics. I present a detailed experimental characterization of this device in the visible spectral region and a comparison of the performance of the metasurface to a commercially available rotating waveplate polarimeter.
Finally, I will discuss the extension of these concepts to compact polarization imaging systems and will provide a broad outlook on metasurfaces in polarization optics, polarization sensing systems, and polarization instrumentation more generally.
Diagnosis of peripheral lung nodules through transbronchial biopsy is highly prone to sampling errors due to the inability of current techniques to accurately locate and/or sample lesions. Volumetric optical imaging techniques such as optical coherence tomography (OCT) have the potential to address this issue, however, current imaging catheter designs cannot achieve sufficiently high-resolution, or diffraction-limited imaging; focusing elements bear spherical aberrations and multilayered structures with asymmetric curvatures in the optical path cause astigmatism. In this work, we propose a new class of optical imaging catheters – termed nano-optic endoscope – that use metalenses to achieve diffraction-limited endoscopic imaging at greatly extended depth-of-focus through negating non-chromatic aberrations and chromatic dispersion engineering. A metalens consists of a 2-dimentional array of subwavelength-spaced scatterers with specific geometric parameters and distribution that locally shift the phase of the incident light and modify its wavefront. The metalens ability to arbitrarily and accurately modify the phase allows the nano-optic endoscope to overcome spherical aberrations and astigmatism. Remarkably, the tailored chromatic dispersion of the metalens in the context of spectral interferometry is utilized to maintain high-resolution imaging beyond the input field Rayleigh range, overcoming the compromise between transverse resolution and depth-of-focus. Endoscopic imaging is demonstrated ex vivo in resected human airway specimens and in vivo in sheep airways. Fine pathology such as irregular glandular pattern, the hallmark of adenocarcinoma, is readily visualized in high-resolution images captured by the nano-optic endoscope. The versatility and design flexibility of the nano-optic endoscope significantly elevate endoscopic imaging capabilities that will likely impact clinical applications.
Acquisition of high-resolution images from within internal organs using endoscopic optical imaging has several clinical applications. In particular, endoscopic optical coherence tomography (OCT) capable of visualizing tissue microstructures is emerging as a promising tool for detection, diagnosis, and monitoring of disease in luminal organs. However, difficulties associated with optical aberrations and the trade-off between transverse resolution and depth-of-focus significantly limits the scope of applications. This work presents a new class of optical imaging catheters termed nano-optic endoscopes that address the difficulties associated with current endoscopic imaging catheters. We incorporate a metalens with the ability to modify the phase of incident light at sub-wavelength level into the design of an OCT catheter to achieve near diffraction-limited imaging through negating non-chromatic aberrations. A metalens consists of a 2-dimentional array of subwavelength-spaced scatterers with specific geometric parameters and distribution that locally shift the phase of the incident light and modify its wavefront. The metalens ability to arbitrarily and accurately modify the phase allows the nano-optic endoscope to overcome spherical aberrations and astigmatism, the essential barriers to diffraction-limited imaging. Remarkably, the tailored chromatic dispersion of the metalens in the context of spectral interferometry is utilized to maintain high-resolution imaging beyond the input field Rayleigh range, overcoming the compromise between transverse resolution and depth-of-focus. Endoscopic imaging is demonstrated in resected human and swine airway specimens and in sheep airways in vivo. The versatility and design flexibility of the nano-optic endoscope significantly elevate endoscopic imaging capabilities that will likely impact clinical applications.
We proposed a waveguide display design based on polarization dependent metagratings. By encoding the left and right half of field of view (FOV) in two orthogonal polarization channels, we achieved an overall horizontal FOV of 67° at 460 nm using a single waveguide, which is 70% larger than that achieved with conventional diffractive gratings. Metagratings that selectively diffract out TE or TM polarized light are designed and simulated using rigorous coupled wave analysis (RCWA). High polarization selectivity is achieved, with minimal crosstalk between the two channels. The transmission spectrum at normal incidence is calculated to assess the see-through effect. Remaining challenges such as fabrication and efficiency issues are discussed. The concept of multiplexing information in the polarization domain enables wide FOV waveguide displays for future AR devices.
We proposed a 2D/3D switchable display design based on polarization-dependent metasurfaces. Metasurfaces are ultrathin planar optical devices patterned with subwavelength nanostructures. We design the metasurfaces such that they can simultaneously deflect right-hand circularly polarized (RCP) light to an angle and transmit left-hand circularly polarized (LCP) light to the normal direction. Combined with an active polarization rotator, the device can be switched between high resolution 2D display mode and multiview 3D display mode. Proof-of-principle metasurface designs are demonstrated. The far field radiation patterns in the 2D and 3D mode are simulated and analyzed. The effects of spectral bandwidth and beam directionality are also discussed. Compared with liquid crystal lenses, which is the key element in previous 2D/3D switchable displays, metasurfaces 1) deliver more precise phase profile control, thus less aberrations and higher image quality; 2) offer additional degrees of freedom in polarization manipulation; and 3) can be adapted to much smaller sizes.
Using immersion lenses is a common approach to enhance the resolving power in various fields of optics such as microscopy and lithography. However, conventional immersion lenses are bulky, high-cost and are typically designed for only a few specific immersion liquids. The development of meta-surfaces provides a promising approach to manipulate light in a compact configuration, enabling many optical devices such as polarizers, waveplates and lenses. These are mainly focused in the near-infrared or the long-wavelength region of the visible spectrum due to fabrication challenges and intrinsic losses of materials used. Here, we demonstrate oil immersion planar lenses with a numerical aperture of 1.1 at visible wavelengths. The lenses provide diffraction-limited focal spots with Strehl ratios higher than 0.9 and 0.8 at their design wavelengths of 532 nm and 405 nm, respectively. Fabrication is based on an atomic-layer deposition (ALD) of TiO2. The loss of TiO2 in the visible is negligible and the surface roughness is well-controlled due to the precise monolayer growth of the TiO2 film. By applying the lens (designed at 532 nm) in a confocal scanning microscopy setup, we are able to achieve high-quality images with sub-wavelength resolution. It should be noted that this lens can be efficiently tailored for any liquid. We demonstrate another design for water-immersion lenses, which are highly applicable to super-resolution bio-imaging applications. The compactness and design flexibility of this platform is highly promising for widespread applications in imaging and spectroscopy.