Near-Unity Light-Matter Interaction in Mid-Infrared van der Waals Metasurfaces.

Accessing mid-infrared radiation is of great importance for a range of applications, including thermal imaging, sensing, and radiative cooling. Here, we study light interaction with hexagonal boron nitride (hBN) nanocavities and reveal strong and tunable resonances across its hyperbolic transition. In addition to conventional phonon-polariton excitations, we demonstrate that the high refractive index of hexagonal boron nitride outside the Reststrahlen band allows enhanced light-matter interactions in deep subwavelength (<λ/15) nanostructures across a broad 7-8 μm range.

Enhanced Infrared Photodiodes Based on PbS/PbCl Core/Shell Nanocrystals.

Improved passivation strategies to address the more complex surface structure of large-diameter nanocrystals are critical to the advancement of infrared photodetectors based on colloidal PbS. In this contribution, the performance of short-wave infrared (SWIR) photodiodes fabricated with PbS/PbCl (core/shell) nanocrystals vs their PbS-only (core) counterparts are directly compared. Devices using PbS cores suffer from shunting and inefficient charge extraction, while core/shell-based devices exhibit greater external quantum efficiencies and lower dark current densities.

Plasmonic nanoarcs: a versatile platform with tunable localized surface plasmon resonances in octave intervals.

The tunability of the longitudinal localized surface plasmon resonances (LSPRs) of metallic nanoarcs is demonstrated with key relationships identified between geometric parameters of the arcs and their resonances in the infrared. The wavelength of the LSPRs is tuned by the mid-arc length of the nanoarc. The ratio between the attenuation of the fundamental and second order LSPRs is governed by the nanoarc central angle.

Binary Superlattices of Infrared Plasmonic and Excitonic Nanocrystals.

Self-assembled superlattices of nanocrystals offer exceptional control over the coupling between nanocrystals, similar to how solid-state crystals tailor the bonding between atoms. By assembling nanocrystals of different properties (e.g.

Author Correction: Ultralow-loss polaritons in isotopically pure boron nitride.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Controlling dissolution of PbTe nanoparticles in organic solvents during liquid cell transmission electron microscopy.

We present direct visualization of the dynamics of oleic-acid-capped PbTe nanoparticles suspended in different organic solvents using liquid cell transmission electron microscopy. Liquid cell transmission electron microscopy is a powerful tool to directly observe the behavior of a variety of nanoparticles in liquids, but requires careful consideration and quantification of how the electron beam affects the systems being investigated. We find that etching and dissolution of PbTe nanoparticles occurs with a strong dependence on electron dose rate ranging from no perceivable effect on the nanoparticles with lower dose rates (50 e- Å-2 s-1) to complete dissolution within seconds or minutes at higher dose rates (100 and 200 e- Å-2 s-1).

Hybrid longitudinal-transverse phonon polaritons.

Phonon polaritons, hybrid light-matter quasiparticles resulting from strong coupling of the electromagnetic field with the lattice vibrations of polar crystals are a promising platform for mid-infrared photonics but for the moment there has been no proposal allowing for their electrical pumping. Electrical currents in fact mainly generate longitudinal optical phonons, while only transverse ones participate in the creation of phonon polaritons. We demonstrate how to exploit long-cell polytypes of silicon carbide to achieve strong coupling between transverse phonon polaritons and zone-folded longitudinal optical phonons.

Effects of a Lead Chloride Shell on Lead Sulfide Quantum Dots.

The size of a quantum-confined nanocrystal determines the energies of its excitonic transitions. Previous work has correlated the diameters of PbS nanocrystals to their excitonic absorption; however, we observe that PbS quantum dots synthesized in saturated dispersions of PbCl can deviate from the previous 1S-1S energy vs diameter curve by 0.8 nm.

Energy Level Alignment of Molybdenum Oxide on Colloidal Lead Sulfide (PbS) Thin Films for Optoelectronic Devices.

Interfacial charge transport in optoelectronic devices is dependent on energetic alignment that occurs via a number of physical and chemical mechanisms. Herein, we directly connect device performance with measured thickness-dependent energy-level offsets and interfacial chemistry of 1,2-ethanedithiol-treated lead sulfide (PbS) quantum dots and molybdenum oxide. We show that interfacial energetic alignment results from partial charge transfer, quantified via the chemical ratios of Mo relative to Mo.

Ultralow-loss polaritons in isotopically pure boron nitride.

Conventional optical components are limited to size scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called 'flat' optical components that beget abrupt changes in these properties over distances significantly shorter than the free-space wavelength. Although high optical losses still plague many approaches, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes in comparison to plasmon-polariton-based nanophotonics.

Aspect-ratio driven evolution of high-order resonant modes and near-field distributions in localized surface phonon polariton nanostructures.

Polar dielectrics have garnered much attention as an alternative to plasmonic metals in the mid- to long-wave infrared spectral regime due to their low optical losses. As such, nanoscale resonators composed of these materials demonstrate figures of merit beyond those achievable in plasmonic equivalents. However, until now, only low-order, phonon-mediated, localized polariton resonances, known as surface phonon polaritons (SPhPs), have been observed in polar dielectric optical resonators.

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents.

Imaging of Anomalous Internal Reflections of Hyperbolic Phonon-Polaritons in Hexagonal Boron Nitride.

We use scanning near-field optical microscopy to study the response of hexagonal boron nitride nanocones at infrared frequencies, where this material behaves as a hyperbolic medium. The obtained images are dominated by a series of "hot" rings that occur on the sloped sidewalls of the nanocones. The ring positions depend on the incident laser frequency and the nanocone shape.

Atomic-scale photonic hybrids for mid-infrared and terahertz nanophotonics.

The field of nanophotonics focuses on the ability to confine light to nanoscale dimensions, typically much smaller than the wavelength of light. The goal is to develop light-based technologies that are impossible with traditional optics. Subdiffractional confinement can be achieved using either surface plasmon polaritons (SPPs) or surface phonon polaritons (SPhPs).

Synthesis and Optical Properties of PbSe Nanorods with Controlled Diameter and Length.

The synthesis of PbSe nanorods with low branching (<1%), high aspect ratios (up to ∼16), and controlled lengths and diameters was demonstrated via the removal of water and oleic acid from the synthesis precursors. It was determined that the proper combination of reaction time and temperature allows for the control of PbSe nanorod length and diameter and therefore control over their electronic states, as probed through absorbance and photoluminescence measurements. Similar to PbSe nanowires, nanorods display higher Stokes shifts than for spherical nanocrystals due to intrananorod diameter fluctuations.

Sub-diffractional volume-confined polaritons in the natural hyperbolic material hexagonal boron nitride.

Strongly anisotropic media, where the principal components of the dielectric tensor have opposite signs, are called hyperbolic. Such materials exhibit unique nanophotonic properties enabled by the highly directional propagation of slow-light modes localized at deeply sub-diffractional length scales. While artificial hyperbolic metamaterials have been demonstrated, they suffer from high plasmonic losses and require complex nanofabrication, which in turn induces size-dependent limitations on optical confinement.

Impact of nanocrystal spray deposition on inorganic solar cells.

Solution-synthesized inorganic cadmium telluride nanocrystals (∼4 nm; 1.45 eV band gap) are attractive elements for the fabrication of thin-film-based low-cost photovoltaic (PV) devices. Their encapsulating organic ligand shell enables them to be easily dissolved in organic solvents, and the resulting solutions can be spray-cast onto indium-tin oxide (ITO)-coated glass under ambient conditions to produce photoactive thin films of CdTe.

Anisotropic absorption in PbSe nanorods.

We present absorption anisotropy measurements in PbSe nanostructures. This is accomplished via a new means of measuring absorption anisotropy in randomly oriented solution ensembles of nanostructures via pump-probe spectroscopy, which exploits the polarization memory effect. We observe isotropic absorption in nanocrystals and anisotropic absorption in nanorods, which increases upon elongation from aspect ratio 1 to 4 and is constant for longer nanorods.

Magneto-optical fingerprints of distinct graphene multilayers using the giant infrared Kerr effect.

The remarkable electronic properties of graphene strongly depend on the thickness and geometry of graphene stacks. This wide range of electronic tunability is of fundamental interest and has many applications in newly proposed devices. Using the mid-infrared, magneto-optical Kerr effect, we detect and identify over 18 interband cyclotron resonances (CR) that are associated with ABA and ABC stacked multilayers as well as monolayers that coexist in graphene that is epitaxially grown on 4H-SiC.

Control of PbSe nanorod aspect ratio by limiting phosphine hydrolysis.

The aspect ratio and yield of PbSe nanorods synthesized by the reaction of Pb-oleate with tris(diethylamino)phosphine selenide are highly sensitive to the presence of water, making it critical to control the amount of water present in the reaction. By carefully drying the reaction precursors and then intentionally adding water back into the reaction, the nanorod aspect ratio can be controlled from 1.1 to 10 and the yield from 1 to 14% by varying the water concentration from 0 to 204 mM.

Inorganic photovoltaic devices fabricated using nanocrystal spray deposition.

Soluble inorganic nanocrystals offer a potential route to the fabrication of all-inorganic devices using solution deposition techniques. Spray processing offers several advantages over the more common spin- and dip-coating procedures, including reduced material loss during fabrication, higher sample throughput, and deposition over a larger area. The primary difference observed, however, is an overall increase in the film roughness.

Enhanced open-circuit voltage of PbS nanocrystal quantum dot solar cells.

Nanocrystal quantum dots (QD) show great promise toward improving solar cell efficiencies through the use of quantum confinement to tune absorbance across the solar spectrum and enable multi-exciton generation. Despite this remarkable potential for high photocurrent generation, the achievable open-circuit voltage (Voc) is fundamentally limited due to non-radiative recombination processes in QD solar cells. Here we report the highest open-circuit voltages to date for colloidal QD based solar cells under one sun illumination.

Low-loss, extreme subdiffraction photon confinement via silicon carbide localized surface phonon polariton resonators.

Plasmonics provides great promise for nanophotonic applications. However, the high optical losses inherent in metal-based plasmonic systems have limited progress. Thus, it is critical to identify alternative low-loss materials.