High Spatial Confinement Research Articles

Carrier tuning of 2D electron gas in field-effect devices based on Al2O3/ZnO heterostructures.

Two-dimensional electron gas (2DEG) formed at oxide heterointerfaces via atomic layer deposition (ALD) has attracted considerable interest toward fascinating electron-related physics and electronic device applications. The employment of oxide-based 2DEG in a confined channel in field-effect transistors (FETs) holds great promise for advanced electronic devices due to its high mobility, spatial confinement, and tunable conductivity. In this work, a 2DEG FET based on the Al2O3/ZnO heterostructure is fabricated with an optimized channel carrier density and oxide thickness. The carrier transport in the bulk and the oxide interface dominantly governed by percolation conduction, optical phonon scattering, and grain boundary scattering is comparatively studied through oxygen annealing and thickness engineering. A tunable carrier density from 4 × 1011 cm-2 to 2 × 1014 cm-2 is achieved with a maximum Hall mobility of ∼62 cm2 V-1 s-1. The electron distribution associated with the annealing process of the ZnO underlayer and the interface reaction during Al2O3 deposition are found to have an impact on the electrical characteristics of the devices. The fabricated Al2O3/ZnO-based 2DEG FET exhibits an on/off ratio over 108, a subthreshold swing of 224 mV dec-1, and a field-effect mobility of 5.7 cm2 V-1 s-1 which can be promising for advanced oxide thin film-containing device and system applications.

Six‐Photon Excited Self‐Trapped Excitons Photoluminescence in Lead‐Free Halide Perovskite

AbstractMultiphoton absorption (MPA) effects are suitable for biophotonic applications due to the long penetration depth and high spatial confinement. Compared with lead halide perovskites, relevant studies on MPA effects of lead‐free perovskites are still scarce, especially for the high‐order MPA effects. Herein, an all‐inorganic lead‐free Cs2TeCl6 perovskite crystal is reported that exhibits three‐ to six‐photon absorption (3PA to 6PA) effects over a wavelength range of 800–2200 nm. The results of steady and transient optical experiments indicate that the single‐ and multiphoton excited broadband orange emission in Cs2TeCl6 crystals is attributed to the self‐trapped excitons (STEs) recombination. What is more, the 6PA cross‐section (δ6) has been determined to be 1.87 × 10−174 cm12 s5 photon−5 (at 1980 nm). As far as it is known, the high‐order 6PA effect is unprecedented in lead‐free perovskites, which paves the way for the future development of lead‐free perovskite high‐order multiphoton application.

Experimental evidence of high spatial confinement of elastic energy in a phononic cantilever

We report on experimental high spatial confinement of elastic energy in a silicon phononic cantilever for which the quality factor of a higher-order flexural resonance is increased by a factor of 27 (from Q ∼ 80 to Q ∼ 2130) with the use of a three-row phononic crystal (PnC) strip. As shown by numerical simulations performed with the finite element method, the PnC both reduces anchor loss and confines elastic energy inside the cantilever. The PnC and the cantilever are fabricated with standard clean room techniques on a silicon on insulator substrate. Optical measurements of the out-of-plane displacements are performed with a laser scanning interferometer in a frequency range around 2 MHz.

Density-independent plasmons for terahertz-stable topological metamaterials

To efficiently integrate cutting-edge terahertz technology into compact devices, the highly confined terahertz plasmons are attracting intensive attention. Compared to plasmons at visible frequencies in metals, terahertz plasmons, typically in lightly doped semiconductors or graphene, are sensitive to carrier density (n) and thus have an easy tunability, which leads to unstable or imprecise terahertz spectra. By deriving a simplified but universal form of plasmon frequencies, here, we reveal a unified mechanism for generating unusual n-independent plasmons (DIPs) in all topological states with different dimensions. Remarkably, we predict that terahertz DIPs can be excited in a two-dimensional nodal line and one-dimensional nodal point systems, confirmed by the first-principle calculations on almost all existing topological semimetals with diverse lattice symmetries. Besides n-independence, the feature of Fermi velocity and degeneracy factor dependencies in DIPs can be applied to design topological superlattice and multiwalled carbon nanotube metamaterials for broadband terahertz spectroscopy and quantized terahertz plasmons, respectively. Surprisingly, high spatial confinement and quality factor, also insensitive to n, can be simultaneously achieved in these terahertz DIPs. Our findings pave the way for developing topological plasmonic devices for stable terahertz applications.

Microfluidic Assisted Flash Precipitation of Photocrosslinkable Fluorescent Organic Nanoparticles for Fine Size Tuning and Enhanced Photoinduced Processes.

Highly concentrated dispersions of fluorescent organic nanoparticles (FONs), broadly used for optical tracking, bioimaging and drug delivery monitoring, are obtained using a newly designed micromixer chamber involving high impacting flows. Fine size tuning and narrow size distributions are easily obtained by varying independently the flow rates of the injected fluids and the concentration of the dye stock solution. The flash nanoprecipitation process employed herein is successfully applied to the fabrication of bicomposite FONs designed to allow energy transfer. Considerable enhancement of the emission signal of the energy acceptors is promoted and its origin is found to result from polarity rather than steric effects. Finally, we exploit the high spatial confinement encountered in FONs and their ability to encapsulate hydrophobic photosensitizers to induce photocrosslinking. An increase in the photocrosslinked FON stiffness is evidenced by measuring the elastic modulus at the nanoscale using atomic force microscopy. These results pave the way toward the straightforward fabrication of multifunctional and mechanically photoswitchable FONs, opening novel opportunities in sensing, multimodal imaging, and theranostics.

Nanoscale Tunable Optical Binding Mediated by Hyperbolic Metamaterials

Carefully designed nanostructures can inspire a new type of optomechanical interactions and allow surpassing limitations set by classical diffractive optical elements. Apart from strong near-field localization, a nanostructured environment allows controlling scattering channels and might tailor many-body interactions. Here we investigate an effect of optical binding, where several particles demonstrate a collective mechanical behavior of bunching together in a light field. In contrast to classical binding, where separation distances between particles are diffraction limited, an auxiliary hyperbolic metasurface is shown here to break this barrier by introducing several controllable near-field interaction channels. Strong material dispersion of the hyperbolic metamaterial along with high spatial confinement of optical modes, which it supports, allows achieving superior tuning capabilities and efficient control over binding distances on the nanoscale. In addition, a careful choice of the metamaterial slab’s thickness enables decreasing optical binding distances by orders of magnitude compared to free space scenarios due to the multiple reflections of volumetric modes from the substrate. Auxiliary tunable metamaterials, which allow controlling collective optomechanical interactions on the nanoscale, open a venue for new investigations including collective nanofluidic interactions, triggered biochemical reactions, and many others.

Multilayer Graphene Terahertz Plasmonic Structures for Enhanced Frequency Tuning Range

Graphene plasmonics has recently found a variety of applications in terahertz photonic devices. High spatial confinement and large frequency tunability are two key advantages of graphene plasmonics. Nevertheless, the frequency tuning range of plasmonic devices employing single-layer graphene is ultimately limited by its carrier density tuning range. Here, we demonstrate that the frequency tuning range of graphene-based plasmonic devices can be further extended by employing multilayer graphene structures. Both our experimental investigations and theoretical calculations show that the frequency tuning range of gate-controlled graphene plasmonic resonators can be significantly enhanced by employing two or three layers of stacked graphene, which is a result of the carrier distributions in multiple layers leading to higher total optical conductivity. However, contrary to the previous prediction, stacking even more graphene layers yields little additional benefit, as the interlayer charge screening effect leads to insignificant gate-induced carrier density in additional graphene layers. Our findings provide new insights for designing and optimizing graphene-based plasmonic structures for various photonic device applications, such as modulators, sensors, and detectors.

Hybrid Azo-fluorophore Organic Nanoparticles as Emissive Turn-on Probes for Cellular Endocytosis.

The development of fluorescent organic nanoparticles, serving as bioimaging agents or drug cargos, represents a buoyant field of investigations. Nevertheless, their ulterior fate and structural integrity after cell uptake remain elusive. Toward this aim, we have elaborated original photoactive organic nanoparticles (dTEM ∼ 35-50 nm wide) with an off-on signal upon cellular internalization. Such nanoparticles are based on the noncovalent association of red-emitting benzothiadiazole (BDZ) derivatives and azo dyes, acting as fluorescence quenchers. Upon varying the azo/BDZ ratio, we found that quantitative emission quenching could be obtained with only a 0.2:1 azo/BDZ ratio and originated from exergonic oxidative and reductive photoinduced electron transfer from the azo units (ΔelG0 = -0.21 and -0.29 eV, respectively). Such results revisited the origin of emission quenching, often confusedly ascribed to Förster resonance energy transfer. A nonlinear and sharp drop of the emission intensity with the increase in the azo unit density n was observed and presents comparable evolution to a n-1/3 mathematical law. Thorough biological examinations involving cancer cells prove a receptor-independent endocytosis pathway, leading to progressive cell lighting upon nanoparticle accumulation in the late endosomal/lysosomal compartments. Complete emission recovery of the initially quenched azo/BDZ nanosystems could be achieved by using mefloquine, which caused endosomal/lysosomal disruption, and release of their content in the cytoplasm. Such results demonstrate that the dotlike emission from endosomes actually stems from fully dissociated individual dyes and not integer nanoparticles. They conclude on the high spatial confinement promoted by organelles and finally question its severe impact on functional compounds or nanoparticles whose properties are strongly distance dependent.

All-optical radio-frequency modulation of Anderson-localized modes

All-optical modulation of light relies on exploiting intrinsic material nonlinearities. However, this optical control is rather challenging due to the weak dependence of the refractive index and absorption coefficients on the concentration of free carriers in standard semiconductors. To overcome this limitation, resonant structures with high spatial and spectral confinement are carefully designed to enhance the stored electromagnetic energy, thereby requiring lower excitation power to achieve significant nonlinear effects. Small mode-volume and high quality (Q)-factor cavities also offer an efficient coherent control of the light field and the targeted optical properties. Here, we report on optical resonances reaching Q - 10^5 induced by disorder on novel photonic/phononic crystal waveguides. At relatively low excitation powers (below 1 mW), these cavities exhibit nonlinear effects leading to periodic (up to - 35 MHz) oscillations of their resonant wavelength. Our system represents a test-bed to study the interplay between structural complexity and material nonlinearities and their impact on localization phenomena and introduces a novel functionality to the toolset of disordered photonics.

Real-Time Temperature Monitoring in Liver During Magnetite Nanoparticle-Enhanced Microwave Ablation With Fiber Bragg Grating Sensors: <italic>Ex Vivo</italic> Analysis

In this paper, we present the design and experiments of a fiber Bragg grating (FBG) sensing network for nanoparticle-enhanced microwave ablation (MWA). MWA is an emerging minimally invasive treatment for cancer care, making use of a miniaturized applicator; it is possible to deliver a cytotoxic thermal treatment in situ with high spatial confinement. Cells’ death is nearly instantaneous at 60 °C. The use of magnetite nanoparticles enhances the amount of ablated tissue, and reshapes the thermal distribution during the treatment. In order to measure the thermal maps (temperature as a function of space and time) instantaneously and in situ , a set of three arrays of five FBG sensors has been designed, mapping the thermal distribution of the tissue and estimating the ablated tissue in real time. Experiments carried out with different nanoparticle densities show that the extension of the ablated region is the highest for a density of 5 mg/mL. The results are displayed in the form of thermal maps, over the ablation plane.

Dynamic ejection behaviour of water molecules passing through a nano-aperture nozzle

ABSTRACTThis study used molecular dynamics (MD) simulation to investigate the passage of water molecules through a composite graphene/Au nano-nozzle. Our focus was on the degree to which system temperature, extrusion speed, and nozzle diameter affect jet dynamics and the associated transient phenomena. Our findings show that high pressure and spatial confinement cause the nanojet from a small nozzle diameter (1.0 nm) to bend and twist, whereas the jets from a nozzle with a diameter of 1.5 nm present columns of greater stability. At 100 K, the H2O nanojet froze at the outlet of the nozzle in the form of condensed icicles. At 500 K, the H2O nanojet formed a loose spray and gaseous clusters. High extrusion speed of 55.824 m/s produced recirculating flow downstream from the nanojet with the appearance of an erupting volcano, which further prompted the jet column to thicken. Lower extrusion speeds produced jets with flow velocity insufficient to overcome the capillary force at the outlet of the nozzle, which subsequently manifests as unstable fluctuations in the flow rate.HIGHLIGHTSWater molecules through a composite graphene/Au nano-nozzle forming a nanojet is investigated.High pressure and spatial confinement cause the nanojet from a small nozzle diameter (≤1.0 nm) to bend and twist.High extrusion speed (≧55.824 m/s) produced recirculating flow downstream from the nanojet.Figure abstract: Schematic of the H2O nano-jet through a nano-nozzle of graphene/Au

Ultrafast time-resolved measurement of energy transport at the metal-liquid interface

The nanoscale light-matter interaction at metallic interfaces has many important applications, especially when it is crucial to enhance the surface-to-volume ratio and to achieve high spatial energy confinement. Here, we report an ultrafast time-resolved measurement to study photo-excited transport at the metal-liquid interfaces of colloidal gold nanoparticles (AuNPs). By using the transient absorption spectroscopy method together with the stimulated emission depletion of fluorescence molecules, we simultaneously measured the perturbations of energy states on both sides of the interfaces within a nanoscale distance. Our measurement results showed the evidence of ultrafast coupling between AuNPs and their surrounding solvent molecules at the picosecond time scale. This method can be extended to study the energy transfer mechanisms at the various interfaces for biology, chemistry, or optoelectronics.

Mechanically reconfigurable architectured graphene for tunable plasmonic resonances

Graphene nanostructures with complex geometries have been widely explored for plasmonic applications, as their plasmonic resonances exhibit high spatial confinement and gate tunability. However, edge effects in graphene and the narrow range over which plasmonic resonances can be tuned have limited the use of graphene in optical and optoelectronic applications. Here we present a novel approach to achieve mechanically reconfigurable and strongly resonant plasmonic structures based on crumpled graphene. Our calculations show that mechanical reconfiguration of crumpled graphene structures enables broad spectral tunability for plasmonic resonances from mid- to near-infrared, acting as a new tuning knob combined with conventional electrostatic gating. Furthermore, a continuous sheet of crumpled graphene shows strong confinement of plasmons, with a high near-field intensity enhancement of ~1 × 104. Finally, decay rates for a dipole emitter are significantly enhanced in the proximity of finite-area biaxially crumpled graphene flakes. Our findings indicate that crumpled graphene provides a platform to engineer graphene-based plasmonics through broadband manipulation of strong plasmonic resonances.

Light-activated protein interaction with high spatial subcellular confinement

Methods to acutely manipulate protein interactions at the subcellular level are powerful tools in cell biology. Several blue-light-dependent optical dimerization tools have been developed. In these systems one protein component of the dimer (the bait) is directed to a specific subcellular location, while the other component (the prey) is fused to the protein of interest. Upon illumination, binding of the prey to the bait results in its subcellular redistribution. Here, we compared and quantified the extent of light-dependent dimer occurrence in small, subcellular volumes controlled by three such tools: Cry2/CIB1, iLID, and Magnets. We show that both the location of the photoreceptor protein(s) in the dimer pair and its (their) switch-off kinetics determine the subcellular volume where dimer formation occurs and the amount of protein recruited in the illuminated volume. Efficient spatial confinement of dimer to the area of illumination is achieved when the photosensitive component of the dimerization pair is tethered to the membrane of intracellular compartments and when on and off kinetics are extremely fast, as achieved with iLID or Magnets. Magnets and the iLID variants with the fastest switch-off kinetics induce and maintain protein dimerization in the smallest volume, although this comes at the expense of the total amount of dimer. These findings highlight the distinct features of different optical dimerization systems and will be useful guides in the choice of tools for specific applications.

Tuning Infrared Plasmon Resonance of Black Phosphorene Nanoribbon with a Dielectric Interface

We report on the tunable edge-plasmon-enhanced absorption of phosphorene nanoribbons supported on a dielectric substrate. Monolayer anisotropic black phosphorous (phosphorene) nanoribbons are explored for light trapping and absorption enhancement on different dielectric substrates. We show that these phosphorene ribbons support infrared surface plasmons with high spatial confinement. The peak position and bandwidth of the calculated phosphorene absorption spectra are tunable with low loss over a wide wavelength range via the surrounding dielectric environment of the periodic nanoribbons. Simulation results show strong edge plasmon modes and enhanced absorption as well as a red-shift of the peak resonance wavelength. The periodic Fabry-Perot grating model was used to analytically evaluate the absorption resonance arising from the edge of the ribbons for comparison with the simulation. The results show promise for the promotion of phosphorene plasmons for both fundamental studies and potential applications in the infrared spectral range.

Single-Molecule Imaging Using Atomistic Near-Field Tip-Enhanced Raman Spectroscopy.

Advances in tip-enhanced Raman spectroscopy (TERS) have demonstrated ultrahigh spatial resolution so that the vibrational modes of individual molecules can be visualized. The spatial resolution of TERS is determined by the confinement of the plasmon-induced field in the junction; however, the conditions necessary for achieving the high spatial confinement required for imaging individual molecules are not fully understood. Here, we present a systematic theoretical study of TERS imaging of single molecules, using a hybrid atomistic electrodynamics-quantum mechanical method. This approach provides a consistent treatment of the molecule and the plasmonic near field under conditions where they cannot be treated separately. In our simulations, we demonstrate that TERS is capable of resolving intricate molecule vibrations with atomic resolution, although we find that TERS images are extremely sensitive to the near field in the junction. Achieving the atomic resolution requires the near field to be confined within a few ångstroms in diameter and the near-field focal plane to be in the molecule plane. Furthermore, we demonstrate that the traditional surface selection rule of Raman spectroscopy is altered due to the significant field confinement that leads to significant field-gradient effects in the Raman scattering. This work provides insights into single-molecule imaging based on TERS and Raman scattering of molecules in nanojunctions with atomic dimensions.

A 0.55 THz Near-Field Sensor With a $\mu \text{m}$ -Range Lateral Resolution Fully Integrated in 130 nm SiGe BiCMOS

This paper presents a room-temperature operating superresolution terahertz (THz) planar near-field solid-state sensor breaking the diffraction limit. Contrary to classical superresolution imagers implemented in the optical domain, the sensor features inbuilt illumination, evanescent-field sensing, and detection on a single chip, eliminating the need for any external optics. Both metallic and dielectric objects can be imaged with the sensor and mapped into a monotonic sensor response. It is operated around 533–555 GHz in 130 nm SiGe HBT technology and is capable of resolving structural details with a $\mu \text{m}$ -range resolution and a high response of up to 21.7 $\mu \text{A}$ with no amplification stages in the readout path. Here, the stop-band characteristic of a differentially driven 3-D split-ring resonator with high spatial confinement of evanescent surface fields is exploited as an object-tunable transmission modulator inserted in-plane between a tunable 3-push Colpitts oscillator and a broadband power detector. A separate antenna-coupled oscillator breakout provides a radiated power of up to 45 $\mu \text{W}$ (−13.4 dBm). This paper further demonstrates the 2-D scanned superresolution image with a remarkable SNR of up to 40 dB for the sensor operating at dc.

Surface‐plasmon wavefront and spectral shaping by near‐field holography

AbstractSurface‐plasmon‐polariton waves are two‐dimensional electromagnetic surface waves that propagate at the interface between a metal and a dielectric. These waves exhibit unusual and attractive properties, such as high spatial confinement and enhancement of the optical field, and are widely used in a variety of applications, such as sensing and subwavelength optics. The ability to precisely control the spatial and spectral properties of the surface‐plasmon wave is required in order to support the growing interest in both research and applications of plasmonic waves, and to bring it to the next level. Here, we review the challenges and methods for shaping the wavefront and spectrum of plasmonic waves. In particular, we present the recent advances in plasmonic spatial and spectral shaping, which are based on the realization of plasmonic holograms for the optical nearfield. image

Nanoscopic structural rearrangements of the Cu-filament in conductive-bridge memories.

The electrochemical reactions triggering resistive switching in conductive-bridge resistive random access memory (CBRAM) are spatially confined in few tens of nm(3). The formation and dissolution of nanoscopic Cu-filaments rely on the displacement of ions in such confined volume, and it is driven by the electric field induced ion migration and nanoscaled redox reactions. The stochastic nature of these fundamental processes leads to a large variability of the device performance. In this work, a combination of two- and three-dimensional scanning probe microscopy (SPM) techniques are used to study the conductive filament (CF) formation, rupture and its nanoscopic structural rearrangements. The high spatial confinement of our approach enables to locally induce RS in a confined area and image it in 3D. A conical shape of the CF is consistently observed, indicating that the ion migration is the rate limiting step in the filament formation when using high quality dielectrics as switching layers. The sub-10 nm electrical contact size of the AFM tip is used to study the filament's dissolution and detect the hopping conduction of Cu during the CF rupture. We consistently observe a tunnel gap formation associated with the tip-induced filament reset. Finally, aiming to match the fundamental understanding with the integrated device operations, we apply scalpel SPM to failed memory cells and directly observe the appearance of filament multiplicity as a major source of failures and variability in CBRAM.

Rigorous design of an ultra-high Q/V photonic/plasmonic cavity to be used in biosensing applications

A hybrid device based on a 1D PhC dielectric cavity vertically coupled to a plasmonic slot is proposed for use in biosensing applications. Under efficient coupling conditions between the Bloch mode in the 1D PhC dielectric cavity and the surface plasmon polaritons mode in the metal slot, an ultra-high Q/V ratio (~107(λ/n)−3) has been achieved with a remarkable resonance transmission T (=47%), due to high spectral and spatial confinement in the cavity. The rigorous design process of the cavity, including the influence of geometrical and physical parameters on its performance, has been carried out using the 3D Finite Element Method. A strong light-matter interaction was observed, making the photonic-plasmonic cavity suitable for biosensing and, in particular, for optical trapping of living matter at nanoscale, such as proteins and DNA sections, as required in several biomedical applications.