Enhanced Light Emission Research Articles

Advanced hydrogenation process applied on Ge on Si quantum dots for enhanced light emission

For the development of photonic integrated circuits, it is mandatory to implement light sources on a Si-on-insulator (SOI) platform. However, point defects in the Si matrix and, e.g., at the Si/SiO2 interface act as nonradiative recombination channels, drastically limiting the performance of Si-based light emitters. In this Letter, we study how these defects can be healed by applying an advanced hydrogenation process, recently developed in photovoltaic research for the passivation of performance-limiting defects in Si solar cells. Upon hydrogenation, we observe an increase in the room temperature photoluminescence (PL) yield by a factor of more than three for defect-enhanced quantum dots (DEQDs) grown on float-zone Si substrates, revealing the potential of this technique to passivate detrimental defects. For DEQDs grown using SOI substrates, the PL yield enhancement even exceeds a factor of four, which we attribute to the additional passivation of defects originating from the substrate. The results for SOI substrates are of particular interest due to their relevance for future photonic integrated circuits.

Effects of electrochemical potentiostatic activation on carrier transport in AlGaN-based deep-ultraviolet light-emitting diodes

Electrochemical potentiostatic activation (EPA) is applied to AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs). The effects of EPA on DUV LEDs are disclosed by analyzing the temperature-dependent current-voltage characteristics. EPA-treated DUV LEDs show improved electrical optical properties with suppressed forward leakage currents, reduced operation voltages, smaller ideality factors, and enhanced light emission compared to the reference device. EPA treatment yields distinct differences in carrier conduction across the p-AlGaN layer. The limited conductivity of the p-AlGaN layer in a DUV LED causes space-charge-limited (SCL)-like conduction behavior in the voltage regime far exceeding the turn-on voltage. EPA treatment alleviates the SCL-like conduction, which infers the improved conductivity of the p-AlGaN layer. In the same vein, the occurrence of high-level injection is retarded in the EPA-treated device, which is also consistent with enhanced conductivity of the p-AlGaN layer that mitigates asymmetry between n- and p-type conductivities of a DUV LED. Thus, the improved performance of AlGaN-based DUV LED is attributed to the promoted charge transport, which is enabled by the EPA-induced enhanced p-type conductivity.

Probing energy dissipation in molecular-scale junctions via surface enhanced Raman spectroscopy: vibrational pumping and hot carrier enhanced light emission

Experimentally resolving the microscopic energy dissipation and redistribution pathways in a molecular-scale junction, the smallest possible nanoelectronic device, is of great current interest. Here we report measurements of the vibrational pumping and light emission processes in current-carrying molecular junctions using surface enhanced Raman spectroscopy. We show that the heating of vibrational modes exhibits distinct features when the molecular junctions are driven by electrical bias or optical power. We further discuss the hot carrier origin of the broadband continuum emission observed in the Raman scattering spectrum.

Zero-Dimensional Hybrid Organic-Inorganic Indium Bromide with Blue Emission.

Low-dimensional hybrid organic-inorganic metal halides have received increased attention because of their outstanding optical and electronic properties. However, the most studied hybrid compounds contain lead and have long-term stability issues, which must be addressed for their use in practical applications. Here, we report a new zero-dimensional hybrid organic-inorganic halide, RInBr4, featuring photoemissive trimethyl(4-stilbenyl)methylammonium (R+) cations and nonemissive InBr4- tetrahedral anions. The crystal structure of RInBr4 is composed of alternating layers of inorganic anions and organic cations along the crystallographic a axis. The resultant hybrid demonstrates bright-blue emission with Commission Internationale de l'Eclairage color coordinates of (0.19, 0.20) and a high photoluminescence quantum yield (PLQY) of 16.36% at room temperature, a 2-fold increase compared to the PLQY of 8.15% measured for the precursor organic salt RBr. On the basis of our optical spectroscopy and computational work, the organic component is responsible for the observed blue emission of the hybrid material. In addition to the enhanced light emission efficiency, the novel hybrid indium bromide demonstrates significantly improved environmental stability. These findings may pave the way for the consideration of hybrid organic In(III) halides for light emission applications.

Enhanced Emission Properties of Anodized Polar ZnO Crystals

Polar ZnO single crystals were microstructured in a controlled fashion by electrochemical etching. Surfaces with pyramids and inversed pyramids on oxygen and zinc faces, respectively, were received. Photoluminescence spectra of bulk and anodized ZnO samples were investigated at room and low temperatures. Cathodoluminescence images were also recorded from areas with different structures. A significant enhancement of light emission of the prepared microstructures was achieved after anodization. This allows to use such microstructures in light emitting devices and solar cells.

Hot-carrier enhanced light emission: The origin of above-threshold photons from electrically driven plasmonic tunnel junctions

Understanding the origin of above-threshold photons emitted from electrically driven tunnel junctions (ℏω>eVb with Vb being the applied voltage bias) is of current interest in nano-optics and holds great promise to create novel on-chip optoelectronic and energy conversion technologies. Here, we report experimental observation and theoretical analysis of above-threshold light emission from electromigrated Au tunnel junctions. We compare our proposed hot-carrier enhanced light emission theory with existing models, including blackbody thermal radiation, multi-electron interactions, and an interpretation involving finite temperature effects. Our study highlights the key role of plasmon-induced hot carrier dynamics in emitting above-threshold photons and the need to further explore the underlying mechanisms and optimization of upconversion effects in plasmonically active nanostructures.

Coupling of Germanium Quantum Dots with Collective Sub-radiant Modes of Silicon Nanopillar Arrays.

In this paper, we demonstrate the infrared photoluminescence emission from Ge(Si) quantum dots coupled with collective Mie modes of silicon nanopillars. We show that the excitation of band edge dipolar modes of a linear nanopillar array results in strong reshaping of the photoluminescence spectra. Among other collective modes, the magnetic dipolar mode with the polarization along the array axis contributes the most to the emission spectrum, exhibiting an experimentally measured Q-factor of around 500 for an array of 11 pillars. The results belong to the first experimental evidence of light emission enhancement of quantum emitters applying collective Mie resonances in finite nanoresonators and therefore represent an important contribution to the new field of active all-dielectric meta-optics.

Analysis of stress distribution in microfabricated germanium with external stressors for enhancement of light emission

In the field of silicon photonics, germanium (Ge) is an attractive material for monolithic light sources. Tensile strain is a promising means for Ge based light sources due to enhancing direct band gap recombination. We investigated strain engineering in Ge using silicon nitride (SiNx) stressors. We found that microfabricated Ge greatly improves the tensile strain because SiNx on the Ge sidewalls causes a large tensile strain in the direction perpendicular to the substrate. Tensile strain equivalent to an in-plane biaxial tensile strain of 0.8% at maximum was applied, and the PL emission intensity was improved more than five times at the maximum.

Microwave-assisted modulation of light emission intensity in alkali-pyrotechnic plumes

Creating next-generation pyrotechnic emitters capable of dynamically controllable light output requires a paradigm shift away from emission control via formulation. This work demonstrates the ability to modulate light emission intensity of a pyrotechnic flame with 2.46 GHz microwave energy within a multimodal cavity. Stoichiometric mixtures of three pyrotechnic systems are investigated: magnesium fuel with oxidizers of NaNO3, KNO3, or CsNO3. Using time-resolved visible and infrared emission spectroscopy and high-speed color videography, microwave illumination of flames is found to produce enhanced atomic photo emission from the alkali species. Emission increases of up to ~120% are demonstrated, which are predominantly in the visible and near-infrared wavelengths and have little effect on flame emission chromaticity. At near- and mid-IR wavelengths, gray body continuum enhancement is observed with moderate enhanced emission from CO2 and H2O bands. Sustained light emission from microwave illumination of combustion products long after pyrotechnic extinguishment was also demonstrated. A simplified model of microwave-enhanced visible and near-IR emission is presented and shown to be consistent with the observed trend of elevated emission enhancement for lower wavelength alkali transitions. Sensitivity analysis is performed which suggests a lower equilibrium population of electronically excited alkali atoms is primarily responsible for maximizing the degree of light emission enhancement of applied microwave fields, especially in extinguished low-temperature pyrotechnic plumes. These findings suggest microwave illumination of alkali-containing pyrotechnic flames may be a useful strategy to achieve dynamic control of light emission intensity.

Resonant plasmon enhancement of light emission from CdSe/CdS nanoplatelets on Au nanodisk arrays.

Semiconducting nanoplatelets (NPLs) have attracted great attention due to the superior photophysical properties compared to their quantum dot analogs. Understanding and tuning the optical and electronic properties of NPLs in a plasmonic environment is a new paradigm in the field of optoelectronics. Here, we report on the resonant plasmon enhancement of light emission including Raman scattering and photoluminescence from colloidal CdSe/CdS nanoplatelets deposited on arrays of Au nanodisks fabricated by electron beam lithography. The localized surface plasmon resonance (LSPR) of the Au nanodisk arrays can be tuned by varying the diameter of the disks. In the case of surface-enhanced Raman scattering (SERS), the Raman intensity profile follows a symmetric Gaussian shape matching the LSPR of the Au nanodisk arrays. The surface-enhanced photoluminescence (SEPL) profile of NPLs, however, follows an asymmetric Gaussian distribution highlighting a compromise between the excitation and emission enhancement mechanisms originating from energy transfer and Purcell effects. The SERS and SEPL enhancement factors depend on the nanodisk size and reach maximal values at 75 and 7, respectively, for the sizes, for which the LSPR energy of Au nanodisks coincides with interband transition energies in the semiconductor platelets. Finally, to explain the origin of the resonant enhancement behavior of SERS and SEPL, we apply a numerical simulation to calculate plasmon energies in Au nanodisk arrays and emission spectra from NPLs in such a plasmonic environment.

Unveiling the Optical Emission Channels of Monolayer Semiconductors Coupled to Silicon Nanoantennas

Monolayers (MLs) of transition metal dichalcogenides (TMDs) such as WSe2 and MoSe2 can be placed by dry stamping directly on broadband dielectric resonators, which have the ability to enhance the spontaneous emission rate and brightness of solid-state emitters at room temperature. We show strongly enhanced emission and directivity modifications in room-temperature photoluminescence mapping experiments. By varying TMD material (WSe2 vs MoSe2) transferred on silicon nanoresonators with various designs (planarized vs nonplanarized), we experimentally separate the different physical mechanisms that govern the global light emission enhancement. For WSe2 and MoSe2, we address the effects of Mie resonances and strain in the monolayer. For WSe2, an important additional contribution comes from out-of-plane exciton dipoles. This paves the way for more targeted designs of TMD-Si nanoresonator structures for room-temperature applications.

Interface-engineering enhanced light emission from Si/Ge quantum dots

Si quantum dots (QDs) have a significant improvement in luminous efficiency compared with bulk Si, achieved by alleviating the forbiddance of no-phonon Γ–Γ radiative transition determined by the law of momentum conservation. Two divergent mechanisms have been proposed to account for the breakdown of momentum conservation in Si QDs, one is due to the space-confinement-induced spread of k-space wave functions associated with Heisenberg uncertainty principle Δr · Δk > 1/2, and the other is due to the interface-effect-induced intervalley mixing between indirect and direct bandgap states. Both mechanisms could cause a small overlap of the electron and hole wave functions in k-space and make vertical transitions allowed, which leads to the zero-phonon light emission. In this work, we unravel the hierarchical relationship between these two primary mechanisms in the process of zero-phonon light emission from indirect bandgap QDs, by performing semiempirical pseudopotential calculation including many-body interaction on the room-temperature luminescent properties of a series of Si, Ge, and Ge/Si core/shell QDs. We show that the space confinement mechanism is dominant in both Si and Ge indirect bandgap QDs, and the interface-induced intervalley coupling mechanism plays a minor role. While in Ge/Si core/shell QDs, the interface-induced intervalley coupling mechanism has a more pronounced contribution to enhanced light emission, implying one can further enhance light emission via engineering interface based on the intervalley coupling mechanism. Given this, we further engineer the Ge QD interface by bringing four motifs of Si/Ge multiple layers from previously inverse designed Si/Ge superlattices and core/shell nanowires for light emitters. We show that two out of four motifs always give rise to two orders of magnitude enhancement in light emission relative to the Ge and Si QDs. We demonstrate that the interface engineering can enhance light emission in indirect bandgap QDs substantially and regulate the intervalley coupling mechanism as the primary factor over the space confinement mechanism in breaking the momentum conservation law.

Mode Switching With Waveguide-Coupled Plasmonic Nanogratings

Active photonic elements enable a wide range of on-chip optical applications, including enhanced light emission, integrated optical sensing and spectroscopy, photonic signal routing, and optical computing. The ability to select between different optical channels forms the basis for many of these systems. Both spectral and spatial selectivity of optical modes are desirable and together offer flexibility in tailoring the optical response of the system. In this work, we experimentally demonstrate a platform for the active switching of hybrid plasmonic-photonic Fano resonances. Our structure consists of gold nanogratings sandwiched between two dielectric thin films. Diffractive coupling into each layer is modulated by varying the refractive index above the structure. By controlling the interaction of the substrate and superstrate modes through refractive index tuning, we experimentally demonstrate on/off switching of the narrow (FWHM = 30-40 nm) Fano resonance transparency windows with up to 85% resonance contrast, and tunability across a wide spectral range (600-850 nm). Full-wave electromagnetic simulations support these results and reveal that the tuning of these modes enables both spectral and spatial selectivity.

Enhanced light emission from gap plasmons in nano-strip MIM tunnel junctions

Electrical excitation of light using inelastic electron tunneling is a promising approach for the realization of ultra-compact on-chip optical sources with high modulation bandwidth. However, the practical implementation of these nanoscale light sources presents a challenge due to the low electron-to-photon transduction efficiencies. Here, we investigate designs for the enhancement of light generation and out-coupling in a periodic Ag-SiO2-Ag tunnel junction due to inelastic electron tunneling. The structure presents a unique advantage of a simple fabrication procedure as compared to the other reported structures. We achieve a resonant enhancement in the local density of optical states up to three orders of magnitude over vacuum for the periodic metal-insulator-metal tunnel junction. By efficiently coupling the gap plasmon mode and the lattice resonance, an enhanced radiative efficiency of ∼0.53 was observed, 30% higher as compared to the uncoupled structure.

New insights into the physics of rotating spokes in partially magnetized E×B plasmas

Regions of enhanced light emission rotating in the azimuthal direction (“rotating spokes”) have been observed in different types of partially magnetized E×B plasma devices such as magnetron discharges and Hall thrusters. A two-dimensional Particle-In-Cell Monte Carlo Collision (PIC MCC) model is used to study the formation of these rotating structures. The model shows that these current driven rotating structures are the results of a Simon–Hoh instability evolving into an ionization instability. The spoke is sustained by local electron heating induced by ∇B drift along a double layer separating the cathodic presheath from the plasma at a potential close to the anode potential. The PIC MCC simulations predict that spoke rotation can take place in the +E×B direction and in the −E×B direction depending on the magnetic field intensity.

Enhancing light emission of Si nanocrystals by means of high-pressure hydrogenation.

High-density Si nanocrystal thin film composed of Si nanocrystals and SiO2, or Si-NCs:SiO2, was prepared by annealing hydrogen silsesquioxane (HSQ) in a hydrogen and nitrogen (H2:N2=5%:95%) atmosphere at 1100°C. Conventional normal-pressure (1-bar) hydrogenation failed to enhance the light emission of the Si-NCs:SiO2 sample made from HSQ. High-pressure hydrogenation was then applied to the sample in a 30-bar hydrogen atmosphere for this purpose. The light emission of Si-NCs increased steadily with increasing hydrogenation time. The photoluminescence (PL) intensity, the PL quantum yield, the maximal electroluminescence intensity, and the optical gain were increased by 90%, 114%, 193% and 77%, respectively, after 10-day high-pressure hydrogenation, with the PL quantum yield as high as 59%, under the current experimental condition.

Enhanced Light Emission by Magnetic and Electric Resonances in Dielectric Metasurfaces

AbstractAn enhanced emission of high quantum yield molecules coupled to dielectric metasurfaces formed by periodic arrays of polycrystalline silicon nanoparticles is demonstrated. Radiative coupling of the nanoparticles, mediated by in‐plane diffraction, leads to the formation of collective Mie scattering resonances or Mie surface lattice resonances (M‐SLRs), with remarkable narrow line widths. These narrow line widths and the intrinsic electric and magnetic dipole moments of the individual Si nanoparticles allow resolving electric and magnetic M‐SLRs. Incidence angle‐ and polarization‐dependent extinction measurements and high‐accuracy surface integral simulations show unambiguously that magnetic M‐SLRs arise from in‐ and out‐of‐plane magnetic dipoles, while electric M‐SLRs are due to in‐plane electric dipoles. Pronounced changes in the emission spectrum of the molecules are observed, with almost a 20‐fold enhancement of the emission in defined directions of molecules coupled to electric M‐SLRs, and a fivefold enhancement of the emission of molecules coupled to magnetic M‐SLRs. These measurements demonstrate the potential of dielectric metasurfaces for emission control and enhancement, and open new opportunities to induce asymmetric scattering and emission using collective electric and magnetic resonances.

Coupling Hexagonal Boron Nitride Quantum Emitters to Photonic Crystal Cavities.

Quantum photonics technologies require a scalable approach for the integration of nonclassical light sources with photonic resonators to achieve strong light confinement and enhancement of quantum light emission. Point defects from hexagonal boron nitride (hBN) are among the front runners for single photon sources due to their ultra-bright emission; however, the coupling of hBN defects to photonic crystal cavities has so far remained elusive. Here we demonstrate on-chip integration of hBN quantum emitters with photonic crystal cavities from silicon nitride (Si3N4) and achieve an experimentally measured quality factor (Q-factor) of 3300 for hBN/Si3N4 hybrid cavities. We observed 6-fold photoluminescence enhancement of an hBN single photon emission at room temperature. Our work will be useful for further development of cavity quantum electrodynamic experiments and on-chip integration of two-dimensional (2D) materials.

Rotating Spokes, Ionization Instability, and Electron Vortices in Partially Magnetized E×B Plasmas.

Regions of enhanced light emission rotating in the azimuthal direction are present in various E×B plasma devices. A kinetic model reveals that these "rotating spokes" are due to electron heating and enhanced ionization localized along a double layer at the interface between a region of large electric field and a quasi-equipotential region close to the anode potential. Electrons drifting along this interface are heated due to ∇B drift in the large electric field region. The formation of electron vortices due to the velocity shear in the double layer also contributes to electron heating. The possibility for spoke motion in the retrograde E×B direction as well as in the E×B direction, observed experimentally, can be reproduced and explained.

Theoretical Analysis of Strain‐Optoelectronic Properties in Externally Deformed Ge/GeSi Quantum Well Nanomembranes via Neutral Plane Modulation

For indirect‐bandgap germanium (Ge), strain‐engineering provides a controllable means to transform Ge into direct‐bandgap material with strongly enhanced light emission efficiency. Flexible electronics offer a powerful platform for introducing external strain into semiconductor nanomembranes (NMs) through deformation with flexible substrates. So far, the Ge/GexSi1−x alloy quantum well (QW) NMs have not been sufficiently investigated to comprehend how the external deformation tunes the wavelength and light‐emitting properties. Herein, a theoretical model based on the elasticity theory and k·p method is established to analyze the strain‐optoelectronic properties of the externally deformed Ge/GexSi1−x QW NMs via neutral plane (NP) modulation in both embedded and sticked form. Calculated results reveal that the elastic strain is linearly distributed, inducing tilt of the band edge similar to the electric field. Considering the fracture limit of the material, the NP layout determines the critical bending curvature that the NM can achieve. Large tunable range of wavelength Δλ of 1615 nm and enhanced transverse electric (TE)/transverse magnetic (TM) peak gain of 949/3778 cm−1 are predicted for r = 1000 nm (the distance between NP and QW) under the bending limit. Further calculations conclude that larger r allow for smaller bending curvature to achieve wider range of wavelength and more enhanced optical gain.