In this talk, I give a brief overview of the evolution of metasurfaces. By appropriately adjusting the size, orientation, geometry and arrangement of subwavelength building-blocks (SBB) of metasurfaces, one can control/change the basic properties of incident light at will. The tremendous freedom of choice and orientations for SBBs open up new physics and lead to unprecedented functionalities such as dispersion engineering and polarization control capability. Owing to these unique features, metasurfaces have a potential to complement or even replace their conventional diffractive/refractive counterparts. Also, metasurfaces straightforward fabrication makes them an attractive platform for emerging technologies ranging from AR/MR/VR to LiDAR.
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.
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 present a new platform that realizes high performance metasurfaces in the visible spectrum. This platform is based on atomic layer deposition of titanium dioxide and allows molding incident light wavefront to desired shapes including holographic images, optical vortices, and Bessel beams. The focus of this work will be on the design and demonstration of planar metalenses. We report on our recent experimental realization of high numerical aperture metalenses with efficiency as high as 86%. These metalenses can focus light into a diffraction-limited spot and can be employed for imaging purposes to provide sub-wavelength imaging resolution. In addition, by the judicious design of metalens building blocks, one can achieve a multispectral chiral metalens (MCML) within a single metasurface layer. The MCML can simultaneously resolve chiral and spectral information of an object without the requirement of additional optical components such as polarizers, wave-plates, or even gratings. Using this MCML, we map the chiroptical properties of a macroscopic chiral biological specimen across the visible range. Finally, since many applications require polarization insensitive planar lenses, we discuss the experimental realization of such metalenses with numerical apertures as high as NA=0.85. These metalenses can focus incident light to a spot as small as ~0.6lambda with efficiencies up to 70%. The straightforward and CMOS-compatible fabrication process of this platform is promising for a wide range of optics-based applications in multidisciplinary science and technology.
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.
In this paper, we present our recent measurement of second harmonic generation (SHG) from silicon nanowires which are vertically aligned. The SHG shows a great enhancement due to the increase of the surface area which breaks the symmetry of silicon lattice and increase the surface SHG. A high SHG is also obtained in counter polarization for both S and P polarization excitation. An enhancement of 80 times is also observed. This huge enhancement opens the door for novel applications including frequency mixing and frequency generation for various novel nonlinear application of silicon based devices.
A simple and straightforward approach was devised to fabricate microspherical aggregates of alumina nanoparticles by self-assembly. Two different organosilanes were used as surfactant agents. Spherical aggregates were spin-coated on a silicon substrate. Spheres with diameters ranging from 100 nm to a few micrometers were thus assembled within an hour. The spatial distribution and the size distribution of the spheres are adjustable by changing the concentration of used organosilane. Effects of different organosilane agents, mixture molarity, and temperature on the size distribution of the spheres were also investigated.