@article{Mallavarapu2023,

title = {TiO2 Metasurfaces with Visible Quasi-Guided Mode Resonances via Direct Imprinting of Aqueous Nanocrystal Dispersions},

author = {Akhila Mallavarapu and Chavez FK Lawrence and Brian Huang and Bryan O Torres Maldonado and Paulo Arratia and Cherie R Kagan},

url = {https://pubs.acs.org/doi/abs/10.1021/acsanm.3c03507},

doi = {10.1021/acsanm.3c03507},

year  = {2023},

date = {2023-09-07},

journal = {ACS Applied Nano Materials},

abstract = {We report a room temperature, environmentally benign, water-based, single-step direct nanoimprint process to pattern dielectric metasurfaces using aqueous titanium dioxide (TiO2) nanocrystal (NC) inks, which are free of polymer additives or nonaqueous solvents typically used in nanofabrication. The metasurfaces are composed of TiO2 NC structures with a high refractive index of 1.94 ± 0.02 at 543 nm. TiO2 NC metasurfaces are designed to resonate at visible wavelengths and are fabricated as two-dimensional nanopillar gratings atop waveguides. Guided mode resonances within the waveguide couple to the overlaying gratings and scatter into free space, forming high quality (Q) factor quasi-guided mode (QGM) resonances. Electric and magnetic QGM resonances are observed, and their environmental refractive index sensitivities (S) are measured to be 69.1 and 70.4 nm/RIU, respectively, with a figure of merit (FOM) = Q × S > 3000. The use of water-based inks and the room temperature processing allow integration of TiO2 NC metasurfaces on rigid and flexible, polymeric substrates.},

keywords = {dielectric metasurfaces, ink-lithography, lithography, metasurfaces, nanocrystal, nanoimprinting, optical metamaterials, optical properties, quasi-guided modes, sensors, sustainable manufacturing, TiO2 nanocrystals},

pubstate = {published},

tppubtype = {article}

}

@article{Choi2023,

title = {Surface Engineering of Metal and Semiconductor Nanocrystal Assemblies and Their Optical and Electronic Devices},

author = {Yun Chang Choi and Jaeyoung Lee and Jonah J. Ng and Cherie R. Kagan},

url = {https://doi.org/10.1021/acs.accounts.3c00147},

doi = {10.1021/acs.accounts.3c00147},

year  = {2023},

date = {2023-06-21},

urldate = {2023-06-21},

journal = {Accounts of Chemical Research},

volume = {56},

number = {13},

pages = {1791–1802},

abstract = {Colloidal nanocrystals (NCs) are composed of inorganic cores and organic or inorganic ligand shells and serve as building blocks of NC assemblies. Metal and semiconductor NCs are well known for the size-dependent physical properties of their cores. The large NC surface-to-volume ratio and the space between NCs in assemblies places significant importance on the composition of the NC surface and ligand shell. Nonaqueous colloidal NC syntheses use relatively long organic ligands to control NC size and uniformity during growth and to prepare stable NC dispersions. However, these ligands create large interparticle distances that dilute the metal and semiconductor NC properties of their assemblies. In this Account, we describe postsynthesis chemical treatments to engineer the NC surface and design the optical and electronic properties of NC assemblies. In metal NC assemblies, compact ligand exchange reduces the interparticle distance and drives an insulator-to-metal transition tuning the dc resistivity over a 1010 range and the real part of the optical dielectric function from positive to negative across the visible-to-IR region. Juxtaposing NC and bulk metal thin films in bilayers allows the differential chemical and thermal addressability of the NC surface to be exploited in device fabrication. Ligand exchange and thermal annealing densifies the NC layer, creating interfacial misfit strain that triggers folding of the bilayers and is used to fabricate, with only one lithography step, large-area 3D chiral metamaterials. In semiconductor NC assemblies, chemical treatments such as ligand exchange, doping, and cation exchange control the interparticle distance and composition to add impurities, tailor stoichiometry, or make entirely new compounds. These treatments are employed in longer studied II–VI and IV–VI materials and are being developed as interest in III–V and I–III–VI2 NC materials grows. NC surface engineering is used to design NC assemblies with tailored carrier energy, type, concentration, mobility, and lifetime. Compact ligand exchange increases the coupling between NCs but can introduce intragap states that scatter and reduce the lifetime of carriers. Hybrid ligand exchange with two different chemistries can enhance the mobility-lifetime product. Doping increases carrier concentration, shifts the Fermi energy, and increases carrier mobility, creating n- and p-type building blocks for optoelectronic and electronic devices and circuits. Surface engineering of semiconductor NC assemblies is also important to modify device interfaces to allow the stacking and patterning of NC layers and to realize excellent device performance. It is used to construct NC-integrated circuits, exploiting the library of metal, semiconductor, and insulator NCs, to achieve all-NC, solution-fabricated transistors.},

keywords = {gold, ligand exchange, ligands, nanocrystal, nanoparticle assembly, Noble metal nanoparticles, optical metamaterials, optical properties, quantum dots, semiconductors, surface modification},

pubstate = {published},

tppubtype = {article}

}

@article{Cai2021,

title = {Chemical and Physical Properties of Photonic Noble-metal Nanomaterials},

author = {Yi-Yu Cai and Yun Chang Choi and Cherie R. Kagan},

url = {https://onlinelibrary.wiley.com/doi/10.1002/adma.202108104},

doi = {10.1002/adma.202108104},

year  = {2021},

date = {2021-12-12},

urldate = {2021-12-12},

journal = {Advanced Materials},

pages = {2108104},

abstract = {Colloidal noble metal nanoparticles are composed of metal cores and organic or inorganic ligand shells. These nanoparticles support size- and shape-dependent plasmonic resonances. They can be assembled from dispersions into artificial metamolecules which have collective plasmonic resonances originating from coupled bright and dark optical electric and magnetic modes that form depending on the size and shape of the constituent nanoparticles and their number, arrangement, and interparticle distance. Nanoparticles can also be assembled into extended two- and three-dimensional metamaterials that are glassy thin films or ordered thin films or crystals, also known as superlattices and supercrystals. The metamaterials have tunable optical properties that depend on the size, shape, and composition of the nanoparticles, and on the number of nanoparticle layers and their interparticle distance. Interestingly, strong light-matter interactions in superlattices form plasmon polaritons. Tunable interparticle distances allow designer materials with dielectric functions tailorable from that characteristic of an insulator to that of a metal, and serve as strong optical absorbers or scatterers, respectively. In combination with lithography techniques, these extended assemblies can be patterned to create subwavelength nanoparticle superstructures and form large-area 2D and 3D metamaterials that manipulate the amplitude, phase, and polarization of transmitted or reflected light.},

keywords = {chiral, metamolecule, nanoparticle assembly, Noble metal nanoparticles, optical metamaterials, plasmonic},

pubstate = {published},

tppubtype = {article}

}

@article{Guo2021,

title = {Broadband Circular Polarizers via Coupling in 3D Plasmonic Meta-Atom Arrays},

author = {Jiacen Guo and Ji-Young Kim and Shengsong Yang and Jun Xu and Yun Chang Choi and Aaron Stein and Christopher B. Murray and Nicholas A. Kotov and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/abs/10.1021/acsphotonics.1c00310},

doi = {10.1021/acsphotonics.1c00310},

year  = {2021},

date = {2021-04-13},

urldate = {2021-04-13},

journal = {ACS Photonics},

volume = {8},

issue = {5},

pages = {1286–1292},

abstract = {We report broadband circular polarizers achieved by engineering the electromagnetic coupling between 3D meta-atoms in large-area arrays. The 3D meta-atoms are composed of bulk Au/Au nanocrystal (NC) bilayer helical arms, tailored in their number, length, and curvature through a one-step patterning process and postfabrication chemical and thermal treatments. By tuning the meta-atom array periodicity, hybridization originating from dipole–dipole and quadrupole–quadrupole interactions broadens the chiral response. We demonstrate circular polarizers operating at wavelengths spanning 2.5 to 5 μm, with a maximal transmission difference between left- and right-hand circularly polarized light of 43%.},

keywords = {chiral, optical metamaterials, plasmonic},

pubstate = {published},

tppubtype = {article}

}

@article{Zhang2018,

title = {Nanoimprinted Chiral Plasmonic Substrates with Three-Dimensional Nanostructures},

author = {Mingliang Zhang and Victor Pacheco-Peña and Yao Yu and Wenxiang Chen and Nicholas J. Greybush and Aaron Stein and Nader Engheta and Christopher B. Murray and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/10.1021/acs.nanolett.8b03785},

doi = {10.1021/acs.nanolett.8b03785},

year  = {2018},

date = {2018-09-26},

journal = {Nano Letters},

volume = {18},

number = {11},

pages = {7389–7394},

abstract = {We report a large-area fabrication method to prepare chiral substrates patterned with arrays of multilayer, three-dimensional nanostructures using a combination of nanoimprint lithography and glancing angle deposition. Several structures are successfully fabricated using this method, including L-shaped, twisted arc and trilayer twisted Au nanorod structures, demonstrating its generality. As one typical example, arrays of L-shaped nanostructures, consisting of two layers of orthogonally oriented Au nanorods separated by a Ge dielectric layer in the thickness direction, exhibit giant optical chirality in the infrared region with an experimentally achieved g-factor as high as 0.38. Electromagnetic simulations show that the optical chirality results from plasmon hybridization between the two orthogonal Au segments. To demonstrate scalability, a 1 cm2 chiral substrate is fabricated with uniform chiral optical property. This method combines both high throughput and precise geometrical control and is therefore promising for applications of chiral metamaterials.},

keywords = {chiral, optical metamaterials},

pubstate = {published},

tppubtype = {article}

}

@article{Chen2018,

title = {Ultrasensitive, Mechanically Responsive Optical Metasurfaces via Strain Amplification},

author = {Wenxiang Chen and Wenjing Liu and Yijie Jiang and Mingliang Zhang and Naixin Song and Nicholas J. Greybush and Jiacen Guo and Anna K. Estep and Kevin T. Turner and Ritesh Agarwal and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/10.1021/acsnano.8b04889},

doi = {doi/10.1021/acsnano.8b04889},

year  = {2018},

date = {2018-09-24},

journal = {ACS Nano},

volume = {12},

number = {11},

pages = {10683–10692},

abstract = {Optical metasurfaces promise ultrathin, lightweight, miniaturized optical components with outstanding capabilities to manipulate the amplitude, phase, and polarization of light compared to conventional, bulk optics. The emergence of reconfigurable metasurfaces further integrates dynamic tunability with optical functionalities. Here, we report a structurally reconfigurable, optical metasurface constructed by integrating a plasmonic lattice array in the gap between a pair of symmetric microrods that serve to locally amplify the strain created on an elastomeric substrate by an external mechanical stimulus. The strain on the metasurface is amplified by a factor of 1.5–15.9 relative to the external strain by tailoring the microrod geometry. For the highest strain amplification geometry, the mechano-sensitivity of the optical responses of the plasmonic lattice array is a factor of 10 greater than that of state-of-the-art stretchable plasmonic resonator arrays. The spatial arrangement and therefore the optical response of the plasmonic lattice array are reversible, showing little hysteresis.},

keywords = {optical metamaterials, strain},

pubstate = {published},

tppubtype = {article}

}

@article{Zhang2018b,

title = {3D Nanofabrication via Chemo‐Mechanical Transformation of Nanocrystal/Bulk Heterostructures},

author = {Mingliang Zhang and Jiacen Guo and Yao Yu and Yaoting Wu and Hongseok Yun and Davit Jishkariani and Wenxiang Chen and Nicholas J. Greybush and Christian Kübel and Aaron Stein and Christopher B. Murray and Cherie R. Kagan},

url = {https://onlinelibrary.wiley.com/doi/10.1002/adma.201800233},

doi = {10.1002/adma.201800233},

year  = {2018},

date = {2018-04-15},

journal = {Advanced Materials},

volume = {30},

number = {22},

pages = {1800233},

abstract = {Planar nanocrystal/bulk heterostructures are transformed into 3D architectures by taking advantage of the different chemical and mechanical properties of nanocrystal and bulk thin films. Nanocrystal/bulk heterostructures are fabricated via bottom‐up assembly and top‐down fabrication. The nanocrystals are capped by long ligands introduced in their synthesis, and therefore their surfaces are chemically addressable, and their assemblies are mechanically “soft,” in contrast to the bulk films. Chemical modification of the nanocrystal surface, exchanging the long ligands for more compact chemistries, triggers large volume shrinkage of the nanocrystal layer and drives bending of the nanocrystal/bulk heterostructures. Exploiting the differential chemo‐mechanical properties of nanocrystal and bulk materials, the scalable fabrication of designed 3D, cell‐sized nanocrystal/bulk superstructures is demonstrated, which possess unique functions derived from nanocrystal building blocks.},

keywords = {chiral, multifunctional nanomaterials, optical metamaterials, synthesis},

pubstate = {published},

tppubtype = {article}

}

@article{Chen2018b,

title = {Angle-Independent Optical Moisture Sensors Based on Hydrogel-Coated Plasmonic Lattice Arrays},

author = {Wenxiang Chen and Gaoxiang Wu and Mingliang Zhang and Nicholas J. Greybush and Jordan P. Howard-Jennings and Naixin Song and F. Scott Stinner and Shu Yang and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/abs/10.1021/acsanm.8b00268},

doi = {abs/10.1021/acsanm.8b00268},

year  = {2018},

date = {2018-03-06},

journal = {ACS Applied Nano Materials},

volume = {1},

number = {3},

pages = {1430–1437},

keywords = {IOT, optical metamaterials, plasmonic},

pubstate = {published},

tppubtype = {article}

}

@article{Greybush2017,

title = {Plasmon Resonances in Self-Assembled Two-Dimensional Au Nanocrystal Metamolecules},

author = {Nicholas J. Greybush and Iñigo Libera and Ludivine Malassis and James M. Kikkawa and Nader Engheta and Christopher B. Murray and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/abs/10.1021/acsnano.6b08189},

doi = {10.1021/acsnano.6b08189},

year  = {2017},

date = {2017-02-12},

journal = {ACS Nano},

volume = {11},

number = {3},

pages = {2917–2927},

abstract = {We explore the evolution of plasmonic modes in two-dimensional nanocrystal oligomer “metamolecules” as the number of nanocrystals is systematically varied. Precise, hexagonally ordered Au nanocrystal oligomers with 1–31 members are assembled via capillary forces into polygonal topographic templates defined using electron-beam lithography. The visible and near-infrared scattering response of individual oligomers is measured by spatially resolved, polarized darkfield scattering spectroscopy. The response is highly sensitive to in-plane versus out-of-plane incident polarization, and we observe an exponentially saturating red shift in plasmon resonance wavelength as the number of nanocrystals per oligomer increases, in agreement with theoretical predictions. Simulations further elucidate the modes supported by the oligomers, including electric dipole and magnetic dipole resonances and their Fano interference. The single-oligomer sensitivity of our measurements also reveals the role of positional disorder in determining the wavelength and character of the plasmonic response. The progression of oligomer metamolecule structures studied here advances our understanding of fundamental plasmonic interactions in the transition regime between few-member plasmonic clusters and extended two-dimensional arrays.},

keywords = {nanoparticle assembly, optical metamaterials, plasmonic, templated assembly},

pubstate = {published},

tppubtype = {article}

}

@article{Zhang2016,

title = {High-strength magnetically switchable plasmonic nanorods assembled from a binary nanocrystal mixture},

author = {Mingliang Zhang and Daniel J. Magagnosc and Iñigo Liberal and Yao Yu and Hongseok Yun and Haoran Yang and Yaoting Wu and Jiacen Guo and Wenxiang Chen and Young Jae Shin and Aaron Stein and James M. Kikkawa and Nader Engheta and Daniel S. Gianola and Christopher B. Murray and Cherie R. Kagan},

url = {https://www.nature.com/articles/nnano.2016.235},

doi = {10.1038/nnano.2016.235},

year  = {2016},

date = {2016-11-07},

journal = {Nature Nanotechnology},

volume = {12},

pages = {228–232},

abstract = {Next-generation ‘smart’ nanoparticle systems should be precisely engineered in size, shape and composition to introduce multiple functionalities, unattainable from a single material1,2,3. Bottom-up chemical methods are prized for the synthesis of crystalline nanoparticles, that is, nanocrystals, with size- and shape-dependent physical properties4,5,6, but they are less successful in achieving multifunctionality7,8,9. Top-down lithographic methods can produce multifunctional nanoparticles with precise size and shape control2,3,10,11, yet this becomes increasingly difficult at sizes of ∼10 nm. Here, we report the fabrication of multifunctional, smart nanoparticle systems by combining top-down fabrication and bottom-up self-assembly methods. Particularly, we template nanorods from a mixture of superparamagnetic Zn0.2Fe2.8O4 and plasmonic Au nanocrystals. The superparamagnetism of Zn0.2Fe2.8O4 prevents these nanorods from spontaneous magnetic-dipole-induced aggregation, while their magnetic anisotropy makes them responsive to an external field. Ligand exchange drives Au nanocrystal fusion and forms a porous network, imparting the nanorods with high mechanical strength and polarization-dependent infrared surface plasmon resonances. The combined superparamagnetic and plasmonic functions enable switching of the infrared transmission of a hybrid nanorod suspension using an external magnetic field.},

keywords = {nanoparticle assembly, optical metamaterials},

pubstate = {published},

tppubtype = {article}

}

@article{Urbas2016,

title = {Roadmap on optical metamaterials},

author = {Augustine M Urbas and Zubin Jacob and Luca Dal Negro and Nader Engheta and A D Boardman and P Egan and Alexander B Khanikaev and Vinod Menon and Marcello Ferrera and Nathaniel Kinsey and Clayton DeVault and Jongbum Kim and Vladimir Shalaev and Alexandra Boltasseva and Jason Valentine and Carl Pfeiffer and Anthony Grbic and Evgenii Narimanov and Linxiao Zhu and Shanhui Fan and Andrea Alù and Ekaterina Poutrina and Natalia M Litchinitser and Mikhail A Noginov and Kevin F MacDonald and Eric Plum and Xiaoying Liu and Paul F Nealey and Cherie R Kagan and Christopher B Murray and Dorota A Pawlak and Igor I Smolyaninov and Vera N Smolyaninova and Debashis Chanda},

url = {https://iopscience.iop.org/article/10.1088/2040-8978/18/9/093005},

doi = {10.1088/2040-8978/18/9/093005},

year  = {2016},

date = {2016-08-09},

journal = {Journal of Optics},

volume = {18},

number = {9},

pages = {093005},

abstract = {Optical metamaterials have redefined how we understand light in notable ways: from strong response to optical magnetic fields, negative refraction, fast and slow light propagation in zero index and trapping structures, to flat, thin and perfect lenses. Many rules of thumb regarding optics, such as μ = 1, now have an exception, and basic formulas, such as the Fresnel equations, have been expanded. The field of metamaterials has developed strongly over the past two decades. Leveraging structured materials systems to generate tailored response to a stimulus, it has grown to encompass research in optics, electromagnetics, acoustics and, increasingly, novel hybrid material responses. This roadmap is an effort to present emerging fronts in areas of optical metamaterials that could contribute and apply to other research communities. By anchoring each contribution in current work and prospectively discussing future potential and directions, the authors are translating the work of the field in selected areas to a wider community and offering an incentive for outside researchers to engage our community where solid links do not already exist.},

keywords = {optical metamaterials},

pubstate = {published},

tppubtype = {article}

}

@article{Chen2015,

title = {Large-Area Nanoimprinted Colloidal Au Nanocrystal-Based Nanoantennas for Ultrathin Polarizing Plasmonic Metasurfaces},

author = {Wenxiang Chen and Mykhailo Tymchenko∥ and Prashanth Gopalan and Xingchen Ye and Yaoting Wu and Mingliang Zhang and Christopher B. Murray and Andrea Alu and Cherie R. Kagan},

url = {https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b02647},

doi = {10.1021/acs.nanolett.5b02647},

year  = {2015},

date = {2015-07-10},

urldate = {2015-07-10},

journal = {Nano Letters},

volume = {15},

number = {8},

issue = {8},

pages = {5254–5260},

abstract = {We report a low-cost, large-area fabrication process using solution-based nanoimprinting and compact ligand exchange of colloidal Au nanocrystals to define anisotropic, subwavelength, plasmonic nanoinclusions for optical metasurfaces. Rod-shaped, Au nanocrystal-based nanoantennas possess strong, localized, plasmonic resonances able to control polarization. We fabricate metasurfaces from rod-shaped nanoantennas tailored in size and spacing to demonstrate Au nanocrystal-based quarter-wave plates that operate with extreme bandwidths and provide high polarization conversion efficiencies in the near-to-mid infrared.},

keywords = {gold, nanocrystal, nanoimprinting, optical metamaterials, optical properties, plasmonic},

pubstate = {published},

tppubtype = {article}

}