Developments in the field of nanoplasmonics have the potential to advance applications from information processing and telecommunications to light-based sensing. Traditionally, nanoscale noble metals such as gold and silver have been used to achieve the targeted enhancements in light-matter interactions that result from the presence of localized surface plasmons (LSPs). However, interest has recently shifted to intrinsically doped semiconductor nanocrystals (NCs) for their ability to display LSP resonances (LSPRs) over a much broader spectral range, including the infrared (IR). Among semiconducting plasmonic NCs, spinel metal oxides (sp-MOs) are an emerging class of materials with distinct advantages in accessing the telecommunications bands in the IR and affording useful environmental stability. Here, we report the plasmonic properties of Fe3O4 sp-MO NCs, known previously only for their magnetic functionality, and demonstrate their ability to modify the light-emission properties of telecom-emitting quantum dots (QDs). We establish the synthetic conditions for tuning sp-MO NC size, composition and doping characteristics, resulting in unprecedented tunability of electronic behavior and plasmonic response over 450 nm. In particular, with diameter-dependent variations in free-electron concentration across the Fe3O4 NC series, we introduce a strong NC size dependency onto the optical response. In addition, our observation of plasmonics-enhanced decay rates from telecom-emitting QDs reveals Purcell enhancement factors for simple plasmonic-spacer-emitter sandwich structures up to 51-fold, which are comparable to values achieved previously only for emitters in the visible range coupled with conventional noble metal NCs.

We report an effective surface immobilization protocol for capture of Triggering Receptor Expressed on Myeloid Cells 2 (TREM2), a receptor whose elevated concentration in cerebrospinal fluid has recently been associated with Alzheimer's disease (AD). We employ the proposed surface functionalization scheme to design, fabricate, and assess a biochemical sensing platform based on plasmonic interferometry that is able to detect physiological concentrations of TREM2 in solution. These findings open up opportunities for label-free biosensing of TREM2 in its soluble form in various bodily fluids as an early indicator of the onset of clinical dementia in AD. We also show that plasmonic interferometry can be a powerful tool to monitor and optimize surface immobilization schemes, which could be applied to develop other relevant antibody tests.

Tetragonal tungsten bronze (TTB) materials are one of the most promising classes of materials for ferroelectric and nonlinear optical devices, owing to their very unique noncentrosymmetric crystal structure. In this work, a new TTB phase of LiNb6Ba5Ti4O30 (LNBTO) has been discovered and studied. A small amount of a secondary phase, LiTiO2 (LTO), has been incorporated as nanopillars that are vertically embedded in the LNBTO matrix. The new multifunctional nanocomposite thin film presents exotic highly anisotropic microstructure and properties, e.g., strong ferroelectricity, high optical transparency, anisotropic dielectric function, and strong optical nonlinearity evidenced by the second harmonic generation results. An optical waveguide structure based on the stacks of α-Si on SiO2/LNBTO-LTO has been fabricated, exhibiting low optical dispersion with an optimized evanescent field staying in the LNBTO-LTO active layer. This work highlights the combination of new TTB material designs and vertically aligned nanocomposite structures for further enhanced anisotropic and nonlinear properties.

Integration of nanoscale photonic and plasmonic components on Si substrates is a critical step toward Si‐based integrated nanophotonic devices. In this work, a set of unique complex 3D metamaterials with intercalated nanolayered and nanopillar structures with tunable plasmonic and optical properties on Si substrates is designed. More specifically, the 3D metamaterials combine metal (Au) nanopillars and alternating metal‐nitride (Au‐TiN and Au‐TaN) nanolayers, epitaxially grown on Si substrates. The ultrafine Au nanopillars ( d ≈ 3 nm) continuously grow throughout all the nanolayers with high epitaxial quality. Novel optical properties, such as highly anisotropic optical property, high absorbance covering the entire visible spectrum regime, and hyperbolic property in the visible regime, are demonstrated. Furthermore, a waveguide based on a silicon nitride (Si 3 N 4 ) ridge with a multilayer structure is successfully fabricated. The demonstration of 3D nanoscale metamaterial design integrated on Si opens up a new route toward tunable metamaterials nanostructure designs with versatile material selection for various optical components in Si integrated photonics.

Microstructural control in metal-dielectric hybrid metamaterials presents enormous opportunities in tailoring the physical properties including the magnetic and optical properties. Here, we demonstrate a strong tunability achieved in the microstructure of self-assembled ordered three-phase Au-BaTiO3-ZnO hybrid metamaterial along with its optical properties, grown by a pulsed laser deposition method. Varying the growth temperature, deposition frequency, and template thickness evolves the microstructure by tuning the Au and ZnO pillar geometry as well as the shape and size of the Au nanoparticles capping the ZnO nanowires. The three-phase hybrid metamaterials exhibit unique optical properties, including enhanced nonlinear optical properties, hyperbolic dispersion in the visible and near-infrared wavelength region, and tuned epsilon-near-zero (ENZ) wavelength upon varying the deposition parameters. This study suggests that the three-phase hybrid metamaterials present great potential in the microstructure and optical property tuning that can also be applied to other two-phase and three-phase nanocomposite systems.

We present a method to simultaneously engineer the energy-momentum dispersion and the local density of optical states (LDOS). Using vertical symmetry breaking in high-contrast gratings, we enable the mixing of modes with different parities, thus producing hybridized modes with controlled dispersion. By tuning geometric parameters, we control the coupling between Bloch modes, leading to flatband, M- and W-shaped dispersion as well as Dirac dispersion. This dispersion-engineering leads to tailored LDOS and we experimentally demonstrate a two order of magnitude enhancement of photoluminescence from weak emitters-defects in silicon-via optical modes with adjustable angle of emission. This vertical symmetry-breaking method could readily be used in various photonic crystals and metasurfaces devices and opens up a new way to strongly boost light emission on-chip and to steer it to arbitrary directions.

Due to the scarcity of circular polarization light sources, linear-to-circular polarization conversion is required to generate circularly polarized light for a variety of applications. Despite significant past efforts, broadband linear-to-circular polarization conversion remains elusive particularly in the terahertz and midinfrared frequency ranges. Here we propose a novel mechanism based on coupled mode theory, and experimentally demonstrate at terahertz frequencies that highly efficient (power conversion efficiency approaching unity) and ultrabroadband (fractional bandwidth up to 80%) linear-to-circular polarization conversion can be accomplished by the judicious design of birefringent metasurfaces. The underlying mechanism operates in the frequency range between well separated resonances, and relies upon the phase response of these resonances away from the resonant frequencies, as well as the balance of the resonant and nonresonant channels. This mechanism is applicable for any operating frequencies from microwave to visible. The present Letter potentially opens a wide range of opportunities in wireless communications, spectroscopy, and emergent quantum materials research where circularly polarized light is desired.

We experimentally demonstrated that the lifetime of quantum emitters with strongly mixed electric dipole (ED) and magnetic dipole (MD) transitions can directly probe the combined electro-magnetic local density of optical states. © 2019 The Author(s)

The temperature‐dependent operation of high efficiency Ge quantum dot (QD) photodetectors (PDs) is reported, that shows spectral responsivity of 1.2 A W−1, internal quantum efficiency (IQE) of 228% and signal‐to‐noise ratio (SNR) equal to 7 × 106 at a wavelength of 640 nm for 12 μW of incident power. The performance of these photodetectors can be improved by reducing the operating temperature, especially at low incident power. For instance, at 10 nW of 640 nm illumination power, lowering temperature from 300 to 100 K improves SNR from 2 × 104 to 2 × 105 and specific detectivity D* from 1.2 × 1011 to 2 × 1013 cm Hz1/2 W−1. This enhanced performance is attributed to saturation of the charging process within the QD layer, that leads to longer hole lifetimes and IQE exceeding 22 000%. Also, the near‐infrared performance of these PDs is reported, finding that below 200 K there is a significant near‐IR photocurrent (three orders of magnitude larger than the dark current at 1100 nm and two orders of magnitude larger than the dark current at 1300–1550 nm, where only the Ge QDs contribute to optical absorption), leading to operational PDs, albeit at lower D*.

We experimentally demonstrate that the radiative decay rate of a quantum emitter is determined by the combined electric and magnetic local density of optical states (LDOS). A Drexhage-style experiment was performed for two distinct quantum emitters, divalent nickel ions in magnesium oxide and trivalent erbium ions in yttrium oxide, which both support nearly equal mixtures of isotropic electric dipole and magnetic dipole transitions. The disappearance of lifetime oscillations as a function of emitter-interface separation distance confirms that the electromagnetic LDOS refers to the total mode density, and thus similar to thermal emission, these unique electronic emitters effectively excite all polarizations and orientations of the electromagnetic field.

Germanium (Ge) quantum dot (QD) photodetectors (PDs) were fabricated on Ge substrates exhibiting a broadband, visible to near-infrared (near-IR) photoresponse in the λ = 400–1550 nm range. Room-temperature responsivities (Rsp) up to 1.12 A/W and internal quantum efficiency IQE = 313% were obtained, superior to conventional silicon and germanium photodiodes. Noise analysis was performed at visible λ = 640 nm and telecom λ = 1550 nm wavelengths, both yielding room-temperature specific detectivity D* ≃ 2 × 1010 cm Hz1/2 W−1. Lowering the operating temperature and incident power led to sharply enhanced performance, with D* = 1.1 × 1012 cm Hz1/2 W−1 and IQE = 1000% at T = 100 K for an incident power of 10 nW at λ = 1550 nm. Based on their simple fabrication and silicon technology compatibility, these Ge QD PDs represent a promising alternative for broadband, high-performance visible to near-IR detection.

We present our work on high performance germanium (Ge) quantum dot (QD) photodetectors (PDs), fabricated on Si and Ge substrates, that operate via tunneling transport through a QD-containing active layer and feature high internal photoconductive gain. In the lambda = 400-1100 nm range, the PDs fabricated on Si substrates exhibit room-temperature spectral responsivity (Rsp) up to 4 A/W and internal quantum efficiency (IQE) up to 700%. At lambda = 640 nm and 12 mu W of incident power, signal-to-noise ratio (SNR) of 7 x 106 and specific detectivity (D*) of 1.2x10(11) cmHz(1/2)W(-1) are obtained. The PDs demonstrate 3 dB bandwidths (f(3dB)) up to 10 MHz, corresponding to response times of similar to 40 ns. When operated at 100 K temperature the performance improves, especially at low incident power, where at 10 nW D* increases to 2 x 10(13) cmHz(1/2)W(-1), due to IQE in excess of 22000%. In order to extend the photoresponse into the near-infrared (near-IR), PDs were fabricated on Ge substrates, yielding room-temperature R-sp = 1.5 A/W, IQE = 134% and f3dB = 10 kHz at the lambda = 1550 nm telecommunication wavelength. Significant improvement to f(3dB) is expected in PDs employing thinner QD-containing layers. Lowering temperature to 50 K and incident power to 10 nW yield D* = 10(13) cmHz(1/2)W(-1), resulting from IQE exceeding 60000%. Based on the above figures of merit, as well as their Si technology compatibility, our Ge QD PDs appear promising for high-performance photodetectors working in the visible and near-IR.

High-speed control of polarization may lead to ultrafast modulators and help explore polarization-dependent ultrafast dynamics in matter. Now, femtosecond polarization switching is realized through intraband optical excitation in an ultrathin semiconductor layer.

The degree of optical spatial coherence-a fundamental property of light that describes the mutual correlations between fluctuating electromagnetic fields-has been proven challenging to control at the micrometer scale. We use surface plasmon polaritons-evanescent waves excited on both surfaces of a thin metal film-as a means to mix the random fluctuations of the incident electromagnetic fields at the slit locations of a Young's double-slit interferometer. Strong tunability of the complex degree of spatial coherence of light is achieved by finely varying the separation distance between the two slits. Continuous modulation of the degree of spatial coherence with amplitudes ranging from 0 to 80% allows us to transform totally incoherent incident light into highly coherent light and vice versa. These findings pave the way for alternative methods to engineer flat optical elements with multifunctional capabilities beyond conventional refractive- and diffractive-based photonic metasurfaces.

Optical interferometry has empowered an impressive variety of biosensing and medical imaging techniques. A widely held assumption is that devices based on optical interferometry require coherent light to generate a precise optical signature in response to an analyte. Here we disprove that assumption. By directly embedding light emitters into subwavelength cavities of plasmonic interferometers, we demonstrate coherent generation of surface plasmons even when light with extremely low degrees of spatial and temporal coherence is employed. This surprising finding enables novel sensor designs with cheaper and smaller light sources, and consequently increases accessibility to a variety of analytes, such as biomarkers in physiological fluids, or even airborne nanoparticles. Furthermore, these nanosensors can now be arranged along open detection surfaces, and in dense arrays, accelerating the rate of parallel target screening used in drug discovery, among other high volume and high sensitivity applications.

Optical interferometry has enabled quantification of the spatial and temporal correlations of electromagnetic fields, which laid the foundations for the theory of optical coherence. Despite significant advances in fundamental theories and applications, the measurement of nanoscale coherence lengths for highly incoherent optical fields has remained elusive. Here, we employ plasmonic interferometry (that is, optical interferometry with surface plasmons) to characterize the spatial degree of coherence of light beams down to subwavelength scales, with measured coherence lengths as low as ∼330 nm for an incident wavelength of 500 nm. Furthermore, we demonstrate a compact coherence meter that integrates this method with an image sensor. Precise determination of spatial coherence can advance high-resolution imaging and tomographic schemes, and provide an experimental platform for the development and testing of optical coherence theories at the nanoscale.

We experimentally unveiled up to the sixth-order surface plasmon contributions to hole-groove plasmonic interferometry using discrete Fourier transform. This method is further extended to double-slit plasmonic structures to deconvolve competing interference effects from asymmetric interfaces.

Modulation is a cornerstone of optical communication, and as such, governs the overall speed of data transmission. Currently, the two main strategies for modulating light are direct modulation of the excited emitter population (for example, using semiconductor lasers) and external optical modulation (for example, using Mach–Zehnder interferometers or ring resonators). However, recent advances in nanophotonics offer an alternative approach to control spontaneous emission through modifications to the local density of optical states. Here, by leveraging the phase-change of a vanadium dioxide nanolayer, we demonstrate broadband all-optical direct modulation of 1.5 μm emission from trivalent erbium ions more than three orders of magnitude faster than their excited state lifetime. This proof-of-concept demonstration shows how integration with phase-change materials can transform widespread phosphorescent materials into high-speed optical sources that can be integrated in monolithic nanoscale devices for both free-space and on-chip communication.