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Throughout nature, simple rules explain complex phenomena, such as the selective interaction of chiral objects with circularly polarized light. Here, we demonstrate chiroptical signals from gold nanorods, which are seemingly achiral structures. Shape anisotropy due to atomic-level faceting and rounding at the tips of nanorods, which are free of chiral surface ligands, induces linear-to-circular polarization modulation during second harmonic generation. The intrinsic nanorod chiroptical response is increased by plasmon-resonant excitation, which preferentially amplifies circularly polarized harmonic signals. This structure-plasmon interplay is uniquely resolved by polarization-resolved second harmonic generation measurements. The material's second-order polarizability is the product of the structure-dependent lattice-normal susceptibility and local surface plasmon field vectors. Synthetically scalable plasmon-supporting nanorods that amplify small circular dichroism signals provide a simple, assembly-free platform for chiroptical transduction., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

Photonic nanostructures that realize ultrafast switching of light polarization are essential to advancements in the area of optical information processing. The unprecedented flexibility of metasurfaces in light manipulation makes them a promising candidate for active polarization control. However, due to the lack of optical materials exhibiting a fast as well as large refractive index change, photonic metadevices capable of ultrafast polarization switching remain elusive. Here, an ultrathin nonlinear chiral meta-mirror consisting of an array of amorphous silicon (α-Si) split-ring resonators on top of a silver backplane is demonstrated as a feasible platform for picosecond all-optical polarization switching of near-infrared light at picojoule-per-resonator pump energies. This success was made possible by the high-quality-factor resonances of the proposed meta-atoms that enable the mirror to exhibit strong chiro- and enantioselectivity. Experimental results confirm that our meta-mirrors can be used to facilitate high-speed and power-efficient polarization-state modulators.

Conventional metallic mirrors flip the spin of a circularly polarized wave upon normal incidence by inverting the direction of the propagation vector. Altering or maintaining the spin state of light waves carrying data is a critical need to be met at the brink of photonic information processing. In this work, we report a chiral metamaterial mirror that strongly absorbs a circularly polarized wave of one spin state and reflects that of the opposite spin in a manner conserving the circular polarization. A circular dichroic response in reflection as large as ∼0.5 is experimentally observed in a near-infrared wavelength band. By imaging a fabricated pattern composed of the enantiomeric unit cells, we directly visualize the two key features of our engineered meta-mirrors, namely the chiral-selective absorption and the polarization preservation upon reflection. Beyond the linear regime, the chiral resonances enhance light-matter interaction under circularly polarized excitation, greatly boosting the ability of the metamaterial to perform chiral-selective signal generation and optical imaging in the nonlinear regime. Chiral meta-mirrors, exhibiting giant chiroptical responses and spin-selective near-field enhancement, hold great promise for applications in polarization sensitive electro-optical information processing and biosensing.

Nanostructured optical coatings with tailored spectral absorption properties are of interest for a wide range of applications such as spectroscopy, emissivity control, and solar energy harvesting. Optical metamaterial absorbers have been demonstrated with a variety of customized single band, multiple band, polarization, and angular configurations. However, metamaterials that provide near unity absorptivity with super-octave bandwidth over a specified optical wavelength range have not yet been demonstrated experimentally. Here, we show a broadband, polarization-insensitive metamaterial with greater than 98% measured average absorptivity that is maintained over a wide ± 45° field-of-view for mid-infrared wavelengths between 1.77 and 4.81 μm. The nearly ideal absorption is realized by using a genetic algorithm to identify the geometry of a single-layer metal nanostructure array that excites multiple overlapping electric resonances with high optical loss across greater than an octave bandwidth. The response is optimized by substituting palladium for gold to increase the infrared metallic loss and by introducing a dielectric superstrate to suppress reflection over the entire band. This demonstration advances the state-of-the-art in high-performance broadband metamaterial absorbers that can be reliably fabricated using a single patterned layer of metal nanostructures.

Metamaterials have dramatically expanded the range of available optical properties, enabling an array of new devices such as superlenses, perfect absorbers, and ultrafast switches. Most research has focused on demonstrating negative- and high-index metamaterials at terahertz and optical wavelengths. However, far less emphasis has been placed on low-loss near-zero-index metamaterials that exhibit unique properties including quasi-infinite phase velocity and infinite wavelength. Here, we experimentally demonstrate a free-standing metallodielectric fishnet nanostructure that has polarization-insensitive, zero-index properties with nearly ideal transmission at 1.55 μm. This goal was achieved by optimizing the metamaterial geometry to allow both its effective permittivity and permeability to approach zero together, which simultaneously produces a zero index and matched impedance to free space. The ability to design and fabricate low-loss, near-zero-index optical metamaterials is essential for new devices such as beam collimators, zero-phase delay lines, and transformation optics lenses.

Graphene, which has a linear electronic band structure, is widely considered as a semimetal. In the present study, we combine graphene with conventional metallic surface-enhanced Raman scattering (SERS) substrates to achieve higher sensitivity of SERS detection. We synthesize high-quality, single-layer graphene sheets by chemical vapor deposition (CVD) and transfer them from copper foils to gold nanostructures, i.e ., nanoparticle or nanohole arrays. SERS measurements are carried out on methylene blue (MB) molecules. The combined graphene nanostructure substrates show about threefold or ninefold enhancement in the Raman signal of MB, compared with the bare nanohole or nanoparticle substrates, respectively. The difference in the enhancement factors is explained by the different morphologies of graphene on the two substrates with the aid of numerical simulations. Our study indicates that applying graphene to SERS substrates can be an effective way to improve the sensitivity of conventional metallic SERS substrates.

We present a detailed comparison of surface-enhanced Raman spectroscopy (SERS) signals from metallic nanoparticle arrays and their complementary hole arrays. Using an analytical model for local field enhancement, we show that the SERS enhancements of the hole arrays are closely related to their transmission spectra. This trend is experimentally confirmed and characterized by a cos(4 )θ dependence of the SERS signal on the excitation polarization angle θ. The particle arrays, on the other hand, exhibit quite different behavior because of the existence of considerable evanescent modes in the near field. Their maximal local field gains appear at wavelengths generally much larger than their localized surface plasmonic resonant wavelengths.

Metamaterials offer a new approach to create surface coatings with highly customizable electromagnetic absorption from the microwave to the optical regimes. Thus far, efficient metamaterial absorbers have been demonstrated at microwave frequencies, with recent efforts aimed at much shorter terahertz and infrared wavelengths. The present infrared absorbers have been constructed from arrays of nanoscale metal resonators with simple circular or cross-shaped geometries, which provide a single band response. In this paper, we demonstrate a conformal metamaterial absorber with a narrow band, polarization-independent absorptivity of >90% over a wide ±50° angular range centered at mid-infrared wavelengths of 3.3 and 3.9 μm. The highly efficient dual-band metamaterial was realized by using a genetic algorithm to identify an array of H-shaped nanoresonators with an effective electric and magnetic response that maximizes absorption in each wavelength band when patterned on a flexible Kapton and Au thin film substrate stack. This conformal metamaterial absorber maintains its absorption properties when integrated onto curved surfaces of arbitrary materials, making it attractive for advanced coatings that suppress the infrared reflection from the protected surface.