Optical Emission Efficiency Research Articles

Polarization-Doped InGaN LEDs and Laser Diodes for Broad Temperature Range Operation.

This work reports on the possibility of sustaining a stable operation of polarization-doped InGaN light emitters over a particularly broad temperature range. We obtained efficient emission from InGaN light-emitting diodes between 20 K and 295 K and from laser diodes between 77 K and 295 K under continuous wave operation. The main part of the p-type layers was fabricated from composition-graded AlGaN. To optimize injection efficiency and improve contact resistance, we introduced thin Mg-doped layers of GaN (subcontact) and AlGaN (electron blocking layer in the case of laser diodes). In the case of LEDs, the optical emission efficiency at low temperatures seems to be limited by electron overshooting through the quantum wells. For laser diodes, a limiting factor is the freeze-out of the magnesium-doped electron blocking layer for temperatures below 160 K. The GaN:Mg subcontact layer works satisfyingly even at the lowest operating temperature (20 K).

Manipulating hyperbolic transient plasmons in a layered semiconductor

Anisotropic materials with oppositely signed dielectric tensors support hyperbolic polaritons, displaying enhanced electromagnetic localization and directional energy flow. However, the most reported hyperbolic phonon polaritons are difficult to apply for active electro-optical modulations and optoelectronic devices. Here, we report a dynamic topological plasmonic dispersion transition in black phosphorus via photo-induced carrier injection, i.e., transforming the iso-frequency contour from a pristine ellipsoid to a non-equilibrium hyperboloid. Our work also demonstrates the peculiar transient plasmonic properties of the studied layered semiconductor, such as the ultrafast transition, low propagation losses, efficient optical emission from the black phosphorus’s edges, and the characterization of different transient plasmon modes. Our results may be relevant for the development of future optoelectronic applications.

Facet-Defect Tolerant Bi-Doped Cs2AgxNa1-xInCl6 Nanoplatelets with a Near-Unity Photoluminescence Quantum Yield.

We report the colloidal synthesis of Bi-doped Cs2AgxNa1-xInCl6 double perovskite nanoplatelets (NPLs) exhibiting a near-unity photoluminescence quantum yield (PLQY), a record emission efficiency for nanoscale lead-free metal halides. A combination of optical spectroscopies revealed that nonradiative decay processes in the NPL were suppressed, indicating a well-passivated surface. By comparison, nanocubes with the same composition and surface ligands as the NPLs had a PLQY of only 40%. According to our calculations, the type of trap states arising from the presence of surface defects depends on their specific location: defects located on the facets of nanocubes generate only shallow traps, while those at the edges result in deep traps. In NPLs, due to their extended basal facets, most of the surface defects are facet defects. This so-called facet-defect tolerant behavior of double perovskites explains the more efficient optical emission of NPLs compared to that of nanocubes.

DIELECTRIC OPTICAL ANTENNA: A NEW CONCEPT FOR MICROWAVE FREQUENCIES

The dielectric optical antenna operating band is used to enhance bandwidth of operation band. The dielectric resonator antenna has high permittivity. The capabilities of antenna depend on made-up materials. Antenna range plays very important role for transmission of radio-signals. The properties of antenna play very important role in any technical or communicational devices. Current communication system requires high level portable communicational devices. Nanoantenna has high level configurable conductor power to help to change specific feature. Optical antenna has great capability to connect one network another network . The specific feature of optical antenna moving around development of our life to give convenient life. Only advanced generation technology has power to change our simple life to advanced technological life. In this research paper we have introduced new technical idea to develop our brain to gain high level efficiency to controlled our system with helps of antenna full usability for human being .The antenna characteristics depends on permittivity , dimension, properties limiting factors, geometrical properties, shape, model, wavelength scale and intensity of antenna aperture wavelength, which incident radiation relates to its design . In this paper we apply maxwell equations for free space propagation, as well as solve its amplitude of phase future plasmonic integrated circuits will be capable to extremely high range data processing at optical domain, the optical frequencies will be controlled by efficient optical emission of plasmonic antenna. In this project the new concept of microwave range arrays is applied to convert plasmonic optical antenna array, with nano-coupled plasmonic wave guide, as a spatial filter to absorb a specific wave length at particular specified incident angle. Nanoantenna transmits optical signals on nanometre scale. Optical nanoantenna are expected to radiate in THZ or GHZ frequency range. Optical DRA antenna has very unique and key application for such types of device based configuration .The optical DRA –antenna has unique spectral ability to optimize as well as design in nanometre scale.

Coupled thermo–electrical analysis of highly scaled GaN micro-LEDs with meshed hybrid conductors

Display technology with ultrafine pixels for near-eye application is a rapidly growing field due to the recent emergence of augmented/mixed reality. With the constant demand for high energy efficiency, long lifetime, and high luminosity, micro-LED displays based on compound semiconductors are promising candidates for such applications. However, miniaturizing LEDs results in significant drawbacks in terms of their quantum efficiency, current injection efficiency, and heat extraction. With relatively low device resistance compared to that of liquid crystals or organic LEDs, micro-LEDs are also more susceptible to the effects of parasitic resistance. In this study, gallium nitride based micro-LED displays with very small pixels (5 μm pixel, 10 μm pitch) are fabricated to study the thermal–electrical effect of meshed hybrid conductors on the optical emission efficiency. In situ thermographic imaging with I–V measurement confirms a significant trade-off among heat transfer, electrical conductivity, and light extraction efficiency. An increase of 37.9% in the emission efficiency (2540 pixels per inch, 5 μm pixels, 1 mm2 display, at 49.2 °C) is achieved by optimizing the thermal and electrical conduction paths. This study experimentally confirms the importance of thermal management and multi-physics analysis in designing ultra-small-pixel micro-LEDs with high energy efficiency.

Strong Light–Matter Coupling in Carbon Nanotubes as a Route to Exciton Brightening

We show that strong light–matter coupling can be used to overcome a long-standing problem that has prevented efficient optical emission from carbon nanotubes. The luminescence from the nominally bright exciton state of carbon nanotubes is quenched due to the fast nonradiative scattering to the dark exciton state having a lower energy. We present a theoretical analysis to show that by placing carbon nanotubes in an optical microcavity the bright excitonic state may be split into two hybrid exciton–polariton states, while the dark state remains unaltered. For sufficiently strong coupling between the bright exciton and the cavity, we show that the energy of the lower polariton may be pushed below that of the dark exciton. This overturning of the relative energies of the bright and dark excitons prevents the dark exciton from quenching the emission. Our results pave the way for a new approach to band-engineering the properties of nanoscale optoelectronic devices.

Optical and electrical analysis of multi-electrode cylindrical dielectric barrier discharge (DBD) plasma reactor

Aiming to decrease the dielectric barrier discharge (DBD) capacitive current, a new type of multi-electrode cylindrical DBD plasma reactor was proposed in this work. Due to the novel multi-electrode design, the discharge current peak value of the newly designed plasma reactor decreased by 66.24%, from 16.26 mA for the traditional device to 5.49 mA. Besides, the number of the micro-discharges was found almost 2 times higher than that of the traditional device, indicating the generating of more discharge channels (from 37 to 89). The enhanced corona discharge formed at the electrode edges caused by fringe effect was considered as the main reason. The opti-electrical analysis was performed using multi-electrode plasma reactors with different electrode configurations. The changes of optical emission efficiency (Ieff), equivalent capacitance of dielectric layer (Cd) and the discharge gap (Cg) were investigated under different applied voltage, discharge length, interval between adjacent outer electrodes and electrode width. The experimental results indicated the efficient discharge length l and fringe effect were responsible for the variations of Ieff, Cd and Cg. Higher applied voltage resulted in a higher Ieff, but higher current at the same time. Besides, Cd gradually increased along with the increased applied voltage while Cg presented the decreased trend. Ieff increased along with the enhanced fringe effect caused by the shorter electrode width and smaller interval between adjacent outer electrodes, while decreased with the increased discharge length. Cd and Cg decreased with increased interval between outer electrodes, but increased with the enhanced discharge length and electrode width. The voltage distribution analysis indicated that the change of Ieff was mainly due to the change of the voltage across the gas gap Vg. This work can give a deeper and further understanding on the discharge characteristics of the proposed multi-electrode plasma reactor.

Understanding and improving the low optical emission of InGaAs quantum wells grown on oxidized patterned (001) silicon substrate

In 0.3 Ga 0.7 As quantum wells (QW) embedded in AlGaAs barriers and grown on oxidized patterned (001) silicon substrates by metalorganic chemical vapor deposition using the aspect ratio trapping method are studied. An appropriate method combining cathodoluminescence and high resolution scanning transmission electron microscopy characterization is performed to spatially correlate the optical and structural properties of the QW. A triple period (TP) ordering along the ⟨111⟩ direction induced by the temperature decrease during the growth to favor indium incorporation and aligned along the oxidized patterns is observed in the QW. Local ordering affects the band gap and contributes to the decrease of the optical emission efficiency. Using thermal annealing, we were able to remove the TP ordering and improve the QW optical emission by two orders of magnitude.

Electro-optical interfacial effects on a graphene/π-conjugated organic semiconductor hybrid system.

The influence of graphene and retinoic acid (RA) – a π-conjugated organic semiconductor – interface on their hybrid system is investigated. The physical properties of the interface are assessed via scanning probe microscopy, optical spectroscopy (photoluminescence and Raman) and ab initio calculations. The graphene/RA interaction induces the formation of a well-organized π-conjugated self-assembled monolayer (SAM) at the interface. Such structural organization leads to the high optical emission efficiency of the RA SAM, even at room temperature. Additionally, photo-assisted electrical force microscopy, photo-assisted scanning Kelvin probe microscopy and Raman spectroscopy indicate a RA-induced graphene doping and photo-charge generation. Finally, the optical excitation of the RA monolayer generates surface potential changes on the hybrid system. In summary, interface-induced organized structures atop 2D materials may have an important impact on both design and operation of π-conjugated nanomaterial-based hybrid systems.

Deep-UV emission at 219 nm from ultrathin MBE GaN/AlN quantum heterostructures

Deep ultraviolet (UV) optical emission below 250 nm (∼5 eV) in semiconductors is traditionally obtained from high aluminum containing AlGaN alloy quantum wells. It is shown here that high-quality epitaxial ultrathin binary GaN quantum disks embedded in an AlN matrix can produce efficient optical emission in the 219–235 nm (∼5.7–5.3 eV) spectral range, far above the bulk bandgap (3.4 eV) of GaN. The quantum confinement energy in these heterostructures is larger than the bandgaps of traditional semiconductors, made possible by the large band offsets. These molecular beam epitaxy-grown extreme quantum-confinement GaN/AlN heterostructures exhibit an internal quantum efficiency of 40% at wavelengths as short as 219 nm. These observations together with the ability to engineer the interband optical matrix elements to control the direction of photon emission in such binary quantum disk active regions offer unique advantages over alloy AlGaN quantum well counterparts for the realization of deep-UV light-emitting diodes and lasers.

Synthesis optimization of guest/host poly(styrene sulphonate) doped neodymium (III) films

In the present paper, we optimize the chemical synthesis of guest/host poly(styrene sulphonate) (PSS) doped with neodymium ions (Nd3+) using chemometric and univariate procedure for volume variables of acid form of PSS and neodymium chloride (NdCl3) confirmed by FTIR experiment. In optimization procedure, the exchange counter-ions H+ by Nd3+ increases, substantially, the luminescence quantum yield, up to 75%, to 4F3/2→4I11/2 transition of Nd3+. Results show the possibility of using the present material as an efficient optical emission material, in the near infrared range, by adding polymer mechanical properties.

Probing the intrinsic optical quality of CVD grown MoS2

Optical emission efficiency of two-dimensional layered transition metal dichalcogenides (TMDs) is one of the most important parameters affecting their optoelectronic performance. The optimization of the growth parameters by chemical vapor deposition (CVD) to achieve optoelectronic-grade quality TMDs is, therefore, highly desirable. Here, we present a systematic photoluminescence (PL) spectroscopic approach to assess the intrinsic optical and crystalline quality of CVD grown MoS2 (CVD MoS2). We propose the use of the intensity ratio between the PL measured in air and vacuum as an effective way to monitor the intrinsic optical quality of CVD MoS2. Low-temperature PL measurements are also used to evaluate the structural defects in MoS2, via defect-associated bound exciton emission, which well correlates with the field-effect carrier mobility of MoS2 grown at different temperatures. This work therefore provides a sensitive, noninvasive method to characterize the optical properties of TMDs, allowing the tuning of the growth parameters for the development of optoelectronic devices.

Ultralow Resistivity Ge:Sb heterostructures on Si Using Hydride Epitaxy of Deuterated Stibine and Trigermane.

The nonconventional deuterated stibine (SbD3) compound has been used for the first time in combination with trigermane (Ge3H8) to produce hyper-doped Ge-on-Si films with carrier concentrations n > 10(20) cm(-3) and record-low resistivities ρ = 1.8 × 10(-4) Ω cm. The growth takes place on Ge and Ge1-xSix buffered Si(100) wafers at ultralow temperatures (∼330 °C) at which Sb diffusion is negligible, leading to extremely flat atomic profiles of the constituents. The Sb substitution in the Ge lattice is determined by RBS channeling and corroborated by high-resolution XRD, which also reveal a systematic increase in lattice constant vs concentration, as expected due to the incorporation of the larger Sb. High-resolution TEM illustrates defect-free monocrystalline structures with device-quality morphologies. The electrical characteristics of the samples are measured using Hall effect and resistivity measurements combined with contactless infrared ellipsometry and are found to be consistent with an extrapolation of the bulk Ge:Sb properties to the high carrier concentrations achieved in our films. The Sb/Ge ratio in the doped layers is approximately the same as that in the precursor reaction mixture, indicating a highly efficient Sb incorporation afforded by the compatible reactivity of the molecules employed in this study. The resultant films are attractive for next generation germanium technologies that require low-resistance n+ junctions or a Fermi level that approaches the direct gap minimum in the conduction band, which drastically enhances the optical emission efficiency of n-type Ge.

From Phase Separation to Nanocrystallization in Fluorosilicate Glasses: Structural Design of Highly Luminescent Glass-Ceramics

Tremendous enhancement of optical emission efficiency was achieved in fluorosilicate glasses by growing lanthanide doped fluoride nanocrystals embedded in oxide glass matrix. The formation mechanism of the microstructure was elucidated by combining solid-state NMR, scanning TEM, EDX map, and large-scale molecular dynamics simulations. The results reveal that the growth of fluoride nanocrystals in fluorosilicate glass was originated from fluoride phase separation. Atomic level structures of phase separation of fluoride-rich regions in oxyfluoride glasses matrix were observed from both EDX maps and MD simulations, and it was found that, while silicon exclusively coordinated by oxygen and alkali earth ions and lanthanide mainly coordinated by fluorine, aluminum played the role of linking the two fluoride glass and oxide glass regions by bonding to both oxygen and fluoride ions.

Well separated trion and neutral excitons on superacid treated MoS2 monolayers

Developments in optoelectronics and spin-optronics based on transition metal dichalcogenide monolayers (MLs) need materials with efficient optical emission and well-defined transition energies. In as-exfoliated MoS2 MLs, the photoluminescence (PL) spectra even at low temperature consist typically of broad, overlapping contributions from neutral, charged excitons (trions) and localized states. Here, we show that in superacid treated MoS2 MLs, the PL intensity increases by up to 60 times at room temperature. The neutral and charged exciton transitions are spectrally well separated in PL and reflectivity at T = 4 K, with linewidth for the neutral exciton of 15 meV, but both transitions have similar intensities compared to the ones in as-exfoliated MLs at the same temperature. Time resolved experiments uncover picoseconds recombination dynamics analyzed separately for charged and neutral exciton emissions. Using the chiral interband selection rules, we demonstrate optically induced valley polarization for both complexes and valley coherence for only the neutral exciton.

Nonlinear optical sub-bandgap excitation of ZnO-based photonic resonators

Zinc oxide (ZnO) is a versatile candidate for photonic devices due to its highly efficient optical emission. However, for pumping of ZnO photonic devices UV-sources are required. Here, we investigate the alternative usage of widely available pulsed near-infrared (NIR)-sources and compare the efficiency of linear and nonlinear excitation processes. We found that bulk ZnO, ZnO thin films grown by molecular beam epitaxy, and ZnO/SiO2 microdisk devices exhibit strong nonlinear response when excited with NIR pulses (λ ≈ 1060 nm). In addition, we show that the ZnO/SiO2 microdisks exhibit sharp whispering gallery modes over the blue-yellow part of the visible spectrum for both excitation conditions and high Q-factors up to Q = 4700. The results demonstrate that nonlinear excitation is an efficient way to pump ZnO photonic devices.

Measurement of Optical Emission from Gesn Waveguides

The development of Si-based lasers has been a goal of the photonics industry for many years. A number of methods have been explored for achieving lasers on Si platforms, and a few attempts, such as the Si Raman laser, have achieved marginal success, but typically suffer from low efficiency. The silicon photonics problem requires the realization of high-efficiency lasers on CMOS-compatible platforms. One possible solution to this problem is to employ the use of direct band gap materials that can be grown on Si. Of course, the epitaxial growth of such materials has been extremely difficult, and the efforts have mostly concentrated on the use of III-V materials like InGaAs. These materials typically suffer from high defect densities when grown directly on silicon, making them unsuitable for use in lasers. Additionally, the introduction of Group III and Group V materials into a CMOS fabrication facility can cause severe problems in device production due the electronic properties of these atoms, which are typically used as dopants in Si. GeSn alloys present a possible path forward for the realization of Si-based lasers, as these materials are grown directly on Si or Ge-buffered Si and have a band structure that is more suited for efficient optical emission. A laser based on tensile strained Ge grown directly on Si has been demonstrated at room temperature, but the efficiency is limited by the indirect nature of the Ge band gap. More recently, demonstration of lasing at low temperature in GeSn (12.6% Sn) was achieved using optical pumping of a GeSn waveguide. While this represents a first step to the actualization of GeSn lasers, room temperature operation is a necessary condition for widespread adoption and incorporation into everyday devices. The approach for demonstration of lasing in GeSn and Ge-on-Si has typically relied on measuring the optical emission spectrum at very high resolution to see the narrowing of the emission linewidth and the presence of cavity modes while optically-pumping a waveguide with a pulsed laser via topside illumination. We have designed a new system for optically pumping waveguides by coupling into the waveguide edge, which increases the absorption length by orders of magnitude, thereby increasing optical emission intensity and relaxing the constraints under which lasing is achieved. We employ a CW laser and erbium-doped fiber amplifier at 1550 nm as the pump source, which serves to decrease the cost of the experiment and increase the pump power. The edge emission from the waveguide is sent into a Fabry-Perot cavity with an InGaAs detector, and the optical pump power is increased until coherence is achieved, which results in the observance of modes in the Fabry-Perot cavity. This measurement can be performed with much lower pump power than a measurement using a high-resolution spectrometer. In this work, we demonstrate the use of this method for measuring the optical emission from GeSn waveguides with Sn concentrations of 4-7%.

Effects of discharge voltage waveform on the discharge characteristics in a helium atmospheric plasma jet

We present here an analysis of the discharge characteristics of a He plasma jet operating under three different types of applied voltage waveform: (a) a μs-pulse voltage waveform with a slow voltage rise time, (b) ns-pulse, and (c) rectangular voltage waveforms with fast voltage rise time. Optical emission measurements show that the application of a voltage with a fast voltage rise time induces rapid discharge growth and, consequently, produces an abundance of energetic electrons, which in turn leads to high optical emission from the O atoms. We also estimate the optical emission efficiency of the O atom (ηo), which corresponds roughly to the production efficiency of the reactive O species. ηo increases with increasing applied voltage, and the highest value of ηo is obtained in the shortest pulse discharge, which was ignited by a ns-pulse voltage waveform with a fast voltage rise time and short pulse width.

Plasmonic nanopatch array for optical integrated circuit applications

Future plasmonic integrated circuits with the capability of extremely high-speed data processing at optical frequencies will be dominated by the efficient optical emission (excitation) from (of) plasmonic waveguides. Towards this goal, plasmonic nanoantennas, currently a hot topic in the field of plasmonics, have potential to bridge the mismatch between the wave vector of free-space photonics and that of the guided plasmonics. To manipulate light at will, plasmonic nanoantenna arrays will definitely be more efficient than isolated nanoantennas. In this article, the concepts of microwave antenna arrays are applied to efficiently convert plasmonic waves in the plasmonic waveguides into free-space optical waves or vice versa. The proposed plasmonic nanoantenna array, with nanopatch antennas and a coupled wedge plasmon waveguide, can also act as an efficient spectrometer to project different wavelengths into different directions, or as a spatial filter to absorb a specific wavelength at a specified incident angle.

Ultrafast dynamics of hot electrons and phonons in chemical vapor deposited graphene

The relaxation dynamics of photoexcited carriers in a chemical vapor deposited graphene transferred on quartz substrate has been investigated by using ultrafast optical-pump terahertz (THz)-probe spectroscopy. Terahertz transmission through graphene sample is reduced by optical pumping. The change of transmission decays exponentially after the optical pulse. We find the relaxation time is insensitive to the substrate temperature from 10 K to 300 K but increases sublinearly with pump fluence. We model the relaxation process involving electron-phonon coupling together with a set of rate equations to describe the transient responses of quasi-particles and optical phonons. The increases of the extracted carrier temperature and the measured relaxation time with pump fluence are associated with the fact that high pump fluence significantly increases the carrier temperature and broadens the carrier distribution. As a result, it leads to the reduction of optical phonon emission efficiency and the decrease of cooling rate as well.