<p>Controlling light with subwavelength-designed metasurfaces (MSs) has allowed for the arbitrary creation of structured light by precisely engineered matter. We report on the purity and conversion efficiency of hybrid orbital angular momentum (OAM)-generating MSs. We use a recently reported method to design and fabricate meta-surfaces that exploit generalized spin-orbit coupling to produce vector OAM states with asymmetric OAM superpositions, e.g., 1 and 5, coupled to linear and circular polarization states, fractional vector OAM states with OAM values of 3.5 and 6.5, and also the common conjugate spin and OAM of ±1 as reported in previous spin-orbit coupling devices. The OAM and radial modes in the resulting beams are quantitatively studied by implementing a modal decomposition approach, establishing both purity and conversion efficiency. We find conversion efficiencies exceeding 75% and purities in excess of 95%. A phase-flattening approach reveals that the OAM purity can be very low due to the presence of undesired radial components. We characterize the effect and illustrate how to suppress the undesired radial modes.</p>
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.
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.
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.
Optical elements that couple the spin/orbital angular momentum (SAM/OAM) of light have found a range of applications in classical and quantum optics. The J-plate, which refers to the variable denoting the photon’s total angular momentum (TAM), is a metasurface device that allows converting arbitrary, orthogonal input SAM states into two unique OAM states. Using independent phase control of any orthogonal basis of polarization states, the J-plate permits the conversion of arbitrary polarizations into states with arbitrary OAM. Here, we present a further development: Cascaded J-plates provide for versatile combinations of OAM states on any orthogonal basis of spin states. J-plates operating on different polarization bases and imparting independent values of OAM are designed and experimentally demonstrated to generate multiple OAM channels with different polarization states. The generated OAM states are determined by the superposition of the OAM states of the individual J-plates while the generated SAM states are determined by the polarization basis of the last J-plate. Theoretically, there are maximum of 2^n channels of OAM and n×2^n channels of TAM that can be generated by n such cascaded J-plates. It is also demonstrated that cascaded J-plates may produce complex structured light. Cascading J-plates provides a new way to control the TAM of a laser beam. These results may find application in quantum and classical communication.
Metamaterials, with the ability of tailoring optical properties of materials, have been applied to holograms recently, which has shown the priorities of switchable polarization and multicolor image comparing with the conventional holograms. However, the current metasurface based multicolor holograms have suffered the problems of narrow band and low efficiency in phase modulation for gold and silver when their feature dimensions are in few tens of nanometers. Interestingly, aluminum with higher plasma frequency could yield surface plasmon resonance across a broader range of the spectrum ranging from visible to UV. Metasurfaces incorporating with the aluminum offer the unique opportunity to extend the working wavelength to cover the entire visible spectrum for the generation of full color meta-holograms.
Here we demonstrated a phase modulated multicolor meta-hologram that is polarization dependent and capable of producing images in red, green and blue colors. The metahologram is made of aluminum nanorods that are arranged in a two-dimensional array of pixels with surface plasmon resonance in the visible to UV range. The aluminum nanorod array is patterned on a 30 nm thick SIO2 spacer layer sputtered on top of a 130nm thick aluminum mirror. With proper design of the structure, we obtain resonances of narrow bandwidths to allow for implementation of multicolor scheme. Taking into account of the wavelength dependence of the diffraction angle, we can project images to specific locations with predetermined size and order. With tuning of aluminum nanorod size, we demonstrate that the image color can be continuously varied across the visible spectrum.
Selective excitation of specific multipolar resonances in matter can be of great utility in understanding the internal make-up of the underlying material and, as a result, in developing novel nanophotonic devices. Many efforts have been addressed on this topic. For example, the emission spectra related to the different multipolar transitions of trivalent europium can be modulated by changing the thickness of the dielectric spacer between the gold mirror and the fluorescent layer. In this talk, we reported the results about active control of the multipolar resonance in metadevices using the coherent control technique. In the coherent control spectroscopy system, the optical standing wave constructed from two counterpart propagation coherent beams is utilized as the excitation. By controlling the time delay between two ultrafast pulses to decide the location of metadivce as the electromagnetic field node or antinode node of standing wave, the absorption related to the specific multipolar resonance can be controlled. Using this technique, with the 30-nm-thick metadevice, the broadband controlling light with light without nonlinearity can be realized. The switching contrast ratios can be as high as 3:1 with a modulation bandwidth in excess of 2 THz. The active control of the high order and complex optical resonance related to the magnetic dipole, electric quadrupole, and toroidal dipole in the metamaterial is reported as well. This research can be applied in the all ultrafast all-optical data processing and the active control of the resonances of metadevice with high order multipolar resonance.
The toroidal dipole moments of natural molecules are hard to be detected so the artificial toroidal materials made by metamaterial attract more attentions. Metamaterial, the sub-wavelength artificial structures, can modulate reflection or transmission of light. The toroidal metamaterial can not only amplify the toroidal moment but also repress the electric and magnetic dipole so it can be used to study the properties of toroidal dipole moment. However, there are many limitations for the experiments, such as the lateral light is necessary to excite the toroidal response. Most of the toroidal dipole moments oscillate perpendicularly to the substrate, therefore it is difficult to couple it with other dipole moments and could be only excited in the microwave region. In this paper, we design a toroidal metamaterial consisting of dumbbell-shaped aperture and vertical split ring resonator (VSRR) vertically. The toroidal dipole moment of our metamaterial is excited in the optical region. The arrangement of our nanostructures is vertical instead of planar annular arrangement to reduce the size of the unit cell and increase the density of the toroidal dipole moment. Moreover, the direction of toroidal dipole moment is parallel to the substrate which can be used for the study of the coupling effect with other kinds of dipolar moments.
Split-ring resonator (SRR), one kind of building block of metamaterials, attracts wide attentions due to the resonance excitation of electric and magnetic dipolar response. The fundamental plasmonic properties and potential applications in novel three dimensional vertical split-ring resonators (VSRRs) are designed and investigated. The resonant properties arose from the electric and magnetic interactions between the VSRR and light are theoretically and experimentally studied. Tuning the configuration of VSRR unit cells is able to generate various novel coupling phenomena in VSRRs, such as plasmon hybridization and Fano resonance. The magnetic resonance plays a key role in plasmon coupling in VSRRs. The VSRR-based refractive-index sensor is demonstrated. Due to the unique structural configuration, the enhanced plasmon fields localized in VSRR gaps can be lifted off from the dielectric substrate, allowing for the increase of sensing volume and enhancing the sensitivity. We perform a VSRR based metasurface for light manipulation in optical communication frequency. By changing the prong heights, the 2π phase modulation can be achieved in VSRR for the design of metasurface which can be used for high areal density integration of metal nanostructures and optoelectronic devices.
Toroidal dipole moments, the third kind of fundamental dipole moment, have unusual electromagnetic properties
different from the electric and magnetic multipoles. We fabricate a new type of 3D plasmonic toroidal metamaterial by
using mutual coupling between dumbbell-shaped gold apertures with vertical split-ring resonators (VSRRs) at optical
frequency. The radiated power of multipole moments are calculated and analyzed to improve the meta-system is
dominated by the toroidal dipole moment. This result paves a way for practical application on metamaterial based
devices, such as biosensor and lasing spaser.
Holograms, the optical devices to reconstruct pre-designed images, have been evolved dramatically since the advances in today’s nanotechnology [1-4]. Metamaterials, the sub-wavelength artificial structures with tailored refraction index, enable us to design the meta-hologram working in arbitrary frequency region. Here we demonstrated the first reflective type, dual image and high efficient meta-hologram with the incident angle as well as the coherence of incident wave insensitivity in visible region at least from λ = 632.8 nm to λ = 850 nm. The meta-hologram is composed of 50-nm-thick gold cross nano-antenna coupled with 130-nm-thick gold mirror with a 50-nm-thick MgF<sub>2</sub> as spacer. It shows different images “RCAS” and “NTU” with high image contract under x- and y-polarized illumination, respectively. Making use of the characteristic of meta-materials, these optical properties of proposed meta-hologram can be transferred to arbitrary electromagnetic region by scale-up the size of the unit cell of meta-hologram, leading to more compact, efficient and promising electromagnetic components.
Split ring resonator (SRR) has attracted wide attentions since the discovery of negative refraction in 2002. Here, we
designed and fabricated vertical SRR (VSRR) arrays and toroidal metamolecule by using double exposure e-beam
lithography with precise alignment technique, and their resonance behaviors are subsequently studied in optical region.
The fundamental resonance properties of VSRR are studied as well as the plasmon coupling in a VSRR dimer structure
by changing the gap distance between SRRs. In addition, we proposed a three-dimensional toroidal structure composed a
VSRR with a dumbbell structure that supported a toroidal resonance under normal incidence with broadband working
frequency. Such toroidal metamaterial confines effectively the electric as well as magnetic energy paving a way for
promising applications in the field of plasmonics, such as integrated 3D plasmonic metamaterials, plasmonic biosensor
and lasing spaser.
The toroidal as well as magnetic spectral responses at optical frequencies by integrating four gold U-shaped split-ring
resonators (SRRs) are numerically studied. We study two kinds of toroidal structures; the first one is consisted of four-up
U-shaped SRRs. The second kind, two of the four U-shaped SRRs is reversed showing two-up-two-down configuration.
By reversing two SRRs of toroidal structure, their toroidal resonance and magnetic resonance are also reversed between
higher and lower ones. The optical properties of toroidal resonance are also investigated in this paper.