Imaging multiple wavebands through a common aperture with transmissive optics brings new challenges for the optical system designer. Very few commonly available lens materials operate in both the visible and midwave infrared leading to complex lenses with many elements to correct for chromatic aberrations. NRL is developing new materials to fill that gap. These new materials enable greater flexibility for designers of lenses for advanced defense applications and potentially reduce the size, weight and cost of next-generation optics. This paper will discuss optical and physical properties of the new materials, progress in their development and the advantages of using NRL materials in transmissive optics designs for multiband applications.
We demonstrate meta-optic based accelerators that can off-load computationally expensive operations into high-speed and low-power optics. The key to these architectures are the new freedoms afforded by metasurfaces such as optical edge isolation, polarization discrimination, and the ability to spatially multiplex, and demultiplex, information channels. I will discuss how these freedoms can be utilized for accelerating optical segmentation networks and objection classifiers, both based on incoherent illumination. This approach could enable compact, high-speed, and low-power image and information processing systems for a wide range of applications in machine-vision and artificial intelligence.
We explore three types of advanced optical components for use in EO/IR systems. Freeform, GRIN, and meta- optics and combinations of all three used in various system designs are being studied to develop a trade space road map for future utilization. The emphasis is on SWAP benefits while maintaining or improving performance compared to existing systems.
NRL is developing new materials that transmit across wide wavelength ranges. MILTRAN is a new rugged optical ceramic that transmits visible through LWIR and is 3.5 times harder than ZnS. With a negative dn/dT, it is well suited as an internal lens element. NRL-series moldable glasses transmit SWIR through LWIR and may be bonded to each other in an adhesive-free thermal process. NRL-200-series glasses transmit visible through MWIR and expand the glass map for multispectral lens designs. These new materials enable greater flexibility for designers of lenses for advanced defense applications and potentially reduce the size, weight and cost of next-generation optics. This paper will discuss new optical materials, their properties and the advantages of using NRL materials in optics designs.
Photodetectors harnessing hot carrier generation on surface plasmon resonant nanoantennas are a promising avenue to achieving sub-bandgap imaging at room temperature. However, efficient extraction of plasmonic hot carriers under low-energy infrared (IR) excitation predicates careful design of Schottky interfaces. This work reports on the simulation-guided fabrication of Au (i) planar diodes and (ii) embedded IR nanoantennas interfaced with both n-/p-type Si and GaAs semiconductors in order to elucidate the impact of their electronic properties on photocurrent generation.
Targeted, sequential deposition of metals using localized surface plasmon resonance (LSPR) is a promising fabrication route for solar fuel catalysts and sensors. This work examines liquid-phase, reductive photodeposition of platinum (Pt) nanoparticles onto the longitudinal ends of gold nanorods (AuNR) under surface plasmon excitation. Reductive Pt nucleation is initiated by plasmonic hot electrons at the Au-liquid interface, whose sites are governed by the plasmon polarity. In this work, in situ spectroscopic monitoring of the photodeposition process permitted real-time feedback into AuNR surface functionalization with the Pt precursor, Pt growth kinetics under monochromatic AuNR LSPR excitation, and their evolving light-matter interactions. Energy dispersive spectroscopy (EDS) mappings show Pt deposition was localized toward the AuNR ends. Coordinated X-ray photoelectron spectroscopy (XPS) measurements with density functional theory (DFT) calculations of the Pt-decorated AuNR density of states (DOS) elucidated optoelectronic behavior. Catalytic photodeposition using plasmonic hot electrons provide an economical path towards targeted, hierarchal assembly of multi-metallic nanoarchitectures at ambient conditions with specified optoelectronic activity.
Surface plasmon resonant nanoantennas can confine incident energy onto two-dimensional (2D) transition metal dichalcogenides (TMD) to enhance efficiency of harmonic conversion to higher energies, which is otherwise limited by the intrinsic Å-scale interaction length. Second harmonic generation (SHG) from nanoantenna-decorated 2D TMD was heuristically examined with hyper Rayleigh scattering (HRS), multi-photon microscopy, electron energy loss spectroscopy (EELS), and discrete dipole computation. HRS experimentally quantified the frequency dependence of the second-order nonlinear susceptibility, χ (2) , for liquid-exfoliated WS2. Measured χ(2) fell within 21% of independent density functional theory (DFT) calculations, overcoming the known 100-1000x overestimation of microscopy approaches. EELS supported design of nanoantennas for integration with TMD. Overall SHG conversion efficiencies from chemical vapor-deposited (CVD) 4×105 nm2 MoS2 crystals on silicon dioxide were enhanced up to 0.025 % W-1 in the presence of by single 150 nm Au nanoshell monomers and dimers, ostensibly due to augmented local plasmonic fields.
Plasmonic nanoparticles embedded in polymer films enhance optoelectronic properties of photovoltaics, sensors, and interconnects. This work examined optical extinction of polymer films containing randomly dispersed gold nanoparticles (AuNP) with negligible Rayleigh scattering cross-sections at particle separations and film thicknesses less than (sub-) to greater than (super-) the localized surface plasmon resonant (LSPR) wavelength, λLSPR. Optical extinction followed opposite trends in sub- and superwavelength films on a per nanoparticle basis. In ∼70-nm-thick polyvinylpyrrolidone films containing 16 nm AuNP, measured resonant extinction per particle decreased as particle separation decreased from ∼130 to 76 nm, consistent with trends from Maxwell Garnett effective medium theory and coupled dipole approximation. In ∼1-mm-thick polydimethylsiloxane films containing 16-nm AuNP, resonant extinction per particle plateaued at particle separations ≥λLSPR, then increased as particle separation radius decreased from ∼514 to 408 nm. Contributions from isolated particles, interparticle interactions and heterogeneities in sub- and super-λLSPR films containing AuNP at sub-λLSPR separations were examined. Characterizing optoplasmonics of thin polymer films embedded with plasmonic NP supports rational development of optoelectronic, biomedical, and catalytic activity using these nanocomposites.
Compact computational structure-function relations are needed to examine energy transfer between confined fields and carrier dynamics at heterostructure interfaces. This work used discrete dipole approximations to analyze quasiparticle excitation and dephasing at interfaces between metals and van der Waals materials. Simulations were compared with scanning transmission electron microscopy (STEM) for energy electron loss spectroscopy (EELS) at sub-nanometer resolution and femtosecond timescale. Artifacts like direct electron-hole pair generation were avoided. Comparing simulation with experiment distinguished quasiparticle energy transfer to hot carriers at the interface, and supported development of structure-function relations between interface morphology and emergent discrete and hybrid modes.
Improved fundamental understanding of resonant optical and electric interactions between noble metal nanoparticles and 2D materials, such as semiconductive molybdenum disulfide (MoS2), could benefit characterization of optoelectronic light harvesting schemes. Energy and damping of plasmon resonances of noble metal nanoparticle-decorated MoS2 were examined via parallel synthesis of (a) approximate discrete dipole (DDA) simulations and (b) near-field electron energy loss (EELS) and far-field optical transmission spectroscopies. Energy of localized surface plasmon resonance altered by MoS2 interactions was studied for gold nanospheres and silver nanoprisms. Augmented plasmon damping by injection of plasmon-excited electrons into the MoS2 was measured in EELS and represented by DDA. These techniques support rapid improvements in nanoparticle-2D material prototypes for photocatalysis and photodetection, for example.
Polymer films containing plasmonic nanostructures are of increasing interest for development of responsive energy, sensing, and therapeutic systems. A series of novel gold nanoparticle (AuNP)-polydimethylsiloxane (PDMS) films were fabricated to elucidate enhanced optical extinction from diffractive and scattering induced internal reflection. AuNPs with dramatically different scattering-to-absorption ratios were compared at variable interparticle separations to differentiate light trapping from optical diffraction and Mie scattering. Description of interfacial optical and thermal effects due to these interrelated contributions has progressed beyond Mie theory, Beer’s law, effective media, and conventional heat transfer descriptions. Thermal dissipation rates in AuNP-PDMS with this interfacial optical reflection was enhanced relative to films containing heterogeneous AuNPs and a developed thermal dissipation description. This heuristic, which accounts for contributions of both internal and external thermal dissipations, has been shown to accurately predict thermal dissipation rates from AuNP-containing insulating and conductive substrates in both two and three-dimensional systems. Enhanced thermal response rates could enable design and adaptive control of thermoplasmonic materials for a variety of implementations.
Compact structure-function simulations are needed to examine interactions between confined fields and carriers at interfaces of two-dimensional materials and metal contacts. This work used electron-source discrete dipole simulations of fields confined at metals interfaced with van der Waals materials to compare with measures using scanning transmission electron microscopy (STEM) for energy electron loss spectroscopy (EELS). Bright, dark, and hybrid modes at the interface were mapped at sub-nanometer resolution at resonant energies. Comparing simulation and measurement provided direct, femtosecond measures of confined field dephasing into carriers on topologically insulated surfaces for the first time.
Distinguishing contributions of physical and optical characteristics, and their interactions, to complicated features observed in spectra of nanocomposite plasmonic systems slows their implementation in optoelectronics. Use of vacuum, effective medium, or analytic approximations to compute such contributions are insufficient outside the visible spectrum (e.g., in energy harvesting) or for interfaces with complex dielectrics (e.g., semiconductors). This work synthesized discrete dipole computation of local physical/optical interaction with coupled dipole approximation of far-field Fano coupling to precisely distinguish effects of locally discontinuous dielectric environment and structural inhomogeneity on complicated spectra from a square lattice of gold nanospheres supported by complex dielectric substrates. Experimental spectra decomposition of resonant energies/bandwidths elucidated indium tin oxide affected surfaced plasmon resonance while silica affected diffractive coupled resonance features. Energy transport during plasmon decay was examined for each substrate under a variety of physical support configurations with the gold nanospheres. The compact, multi-scale approach can be adapted to arbitrary nanoantenna shapes (e.g., nanorings) interacting with various dielectrics (e.g., dichalcogenides). It offers >104-fold reduction in computation time over existing descriptions to accelerate the design and implementation of functional plasmonic systems.
Polymer thin films embedded with plasmonic gold nanoparticles (AuNPs) are of significant interest in biomedicine,
optics, photovoltaic, and nanoelectromechanical systems. Thin polydimethylsiloxane (PDMS) films containing 3-7
micron layers of AuNPs that were fabricated with a novel diffusive-reduction synthesis technique attenuated up to 85%
of incoming laser light at the plasmon resonance. Rapid diffusive reduction of AuNPs into asymmetric PDMS thin films
provided superior optothermal capabilities relative to thicker films in which AuNPs were reduced throughout. A photonto-
heat conversion of up to 3000°C/watt was demonstrated, which represents a 3-230-fold increase over previous AuNPfunctionalized
systems. Optical attenuation and thermal response increased in proportion to order of magnitude increases
in tetrachloroaurate (TCA) solution concentration. Optical and thermoplasmonic responses were observed with and
without an adjacent mesh support, which increased attenuation but decreased thermal response. Morphological, optical,
and thermoplasmonic properties of asymmetric AuNP-PDMS films varied significantly with diffusive TCA
concentration. Gold nanoparticles, networks, and conglomerates were formed via reduction as the amount of dissolved
TCA increased across a log10-scale. Increasing TCA concentrations caused polymer surface cratering, leading to a larger
effective surface area. This method, utilizing the diffusion of TCA into a single exposed partially cured PDMS interface,
could be used to replace expensive lithographic or solution synthesis of plasmon-functionalized systems.
Interest in the optical properties of plasmonic nanoparticles embedded in transparent polymers is expanding due to
potential uses in sustainability, biomedicine, and manufacturing. Geometric optics of polydimethylsiloxane (PDMS)
thin films containing uniformly or asymmetrically distributed polydisperse reduced gold nanoparticles (AuNPs) or
uniformly distributed monodisperse solution synthesized AuNPs were recently evaluated using a compact linear
algebraic sum. Algebraic calculation of geometric transmission, reflection, and attenuation for AuNP-PDMS films
provides a simple, workable alternative to effective medium approximations, computationally expensive methods, and
fitting of experimental data. Generally, transmission and reflection increased with AuNP isotropy and particle density, as
displayed on a novel ternary diagram. Irregular AuNP morphology and size distribution caused optical attenuation from
polydisperse films to increase in proportion to log10 increases in gold content, resulting in lower attenuation per gold
mass when compared to monodisperse AuNPs. Uniform monodisperse AuNP-PDMS films attenuated light in proportion
to gold content, with films attenuating 0.15 fractional units per 0.1 mass-percent AuNPs. Thin layers of concentrated
AuNPs attenuated light more efficiently. A 25 micron thick layer of 1.2 mass-percent AuNPs attenuated 0.5 fractional
units, the same number as a 130 micron thick 0.6 mass-percent film. Measured optical responses from asymmetric
AuNP-PDMS films with an adjacent back-reflector and pairs of uniformly distributed films were predictable within 0.04
units of linear algebraic estimates based on geometric optics. This approach allows for the summative optical responses
of a sequence of 2D elements comprising a 3D assembly to be analyzed.
Compact description of far-field optical interactions between plasmonic nanocomposites and adjacent media permits facile a priori design of devices for light manipulation. Limited tractability of nanoscale descriptions at device-architectures previously limited development of plasmonic devices. Optical interactions between nanocomposites and adjacent optical elements, a simple device, are describable using infinite linear algebraic sums. Influence of plasmonic absorption and non-linear phenomena on device performance are distinguishable from measured transmission, reflection, and attenuation (resonant and non-resonant losses) of nanocomposites featuring nanoparticles in multiple dimensions. Two- and threedimensional distributions of gold nanoparticles supported by silica and poly(dimethylsiloxane) substrates, respectively, are considered. A unique ternary map of transmission, reflection, and attenuation correlates far-field optical behavior to nanoparticle density and opacity of the adjacent element. Intuitive, visual specification of nanoparticle density and adjacent media needed to obtain a desired optical behavior is possible using the ternary map. The compact model and ternary map provide useful tools for the design and integration of plasmonic nanocomposites into photonic devices for sustainable energy and biomedical applications.
Rapid modeling of far-field Fano resonance supported by lattices of complex nanostructures is possible with the coupled
dipole approximation (CDA) using point, dipole polarizability extrapolated from a higher order discrete dipole
approximation (DDA). Fano resonance in nanostructured metamaterials has been evaluated with CDA for spheroids, for
which an analytical form of particle polarizability exists. For complex structures with non-analytic polarizability, such as
rings, higher order electrodynamic solutions must be employed at the cost of computation time. Point polarizability is
determined from the DDA by summing individual polarizable volume elements from the modeled structure. Extraction
of single nanoring polarizability from DDA permitted CDA analysis of nanoring lattices with a 40,000-fold reduction in
computational time over 1000 wavelengths. Maxima and minima of predicted Fano resonance energies were within 1%
of full volume elements using the DDA. This modeling technique is amenable to other complex nanostructures which
exhibit primarily dipolar and/or quadrupolar resonance behavior. Rapid analysis of coupling between plasmons and
photon diffraction modes in lattices of nanostructures supports design of plasmonic enhancements in sustainable energy
and biomedical devices.
Opto-electronic coupling of plasmonic nano-antennas in the near infrared water window in vitro and in vivo is of
growing interest for imaging contrast agents, spectroscopic labels and rulers, biosensing, drug-delivery, and optoplasmonic
ablation. Metamaterials composed of nanoplasmonic meta-atoms offer improved figures of merit in many
applications across a broader spectral window. Discrete and coupled dipole approximations effectively describe
localized and coupled resonance modes in nanoplasmonic metamaterials. From numeric and experimental results have
emerged four design principles to guide fabrication and implementation of metamaterials in bio-related devices and
systems. Resonance intensity and sensitivity are enhanced by surface-to-mass of meta-atoms and lattice constant. Fano
resonant coupling is dependent on meta-atom polarizability and lattice geometry. Internal reflection in plasmonic metaatom-
containing polymer films enhances dissipation rate. Dimensions of self-assembled meta-atoms depend on
balancing electrochemical and surface forces. Examples of these principles from our lab compare computation with
images and spectra from ordered metal-ceramic and polymeric nanocomposite metamaterials for bio/opto theranostic
applications. These principles speed design and description of new architectures for nanoplasmonic metamaterials that
show promise for bioapplications.
Comparing predicted and measured spectra from isolated and ordered nanoparticles (NPs) indicates that ordering NPs into lattices can blueshift the localized surface plasmon resonant (LSPR) spectral feature and increase its wavelength sensitivity to local changes in refractive index. This occurs at lattice constants at or above the resonant wavelength. Numerical analysis indicates its results from effects of diffractive modes on LSPR features that are distinct from Fano resonances, which arise separately due to coupling between diffractive modes and localized plasmons. Refractive index sensitivity of the aggregate LSPR peak from NPs in a square lattice (314 nm RIU−1) was 5.8-fold higher than a comparable peak from random NPs (54 nm RIU−1). Measured sensitivities of Fano resonance features in two ordered samples were 127% and 312%, respectively, of the highest LSPR sensitivity from a random assembly of NPs.
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