Thin Emitter Research Articles

Graphene Thermal Infrared Emitters Integrated into Silicon Photonic Waveguides.

Cost-efficient and easily integrable broadband mid-infrared (mid-IR) sources would significantly enhance the application space of photonic integrated circuits (PICs). Thermal incandescent sources are superior to other common mid-IR emitters based on semiconductor materials in terms of PIC compatibility, manufacturing costs, and bandwidth. Ideal thermal emitters would radiate directly into the desired modes of the PIC waveguides via near-field coupling and would be stable at very high temperatures. Graphene is a semimetallic two-dimensional material with comparable emissivity to thin metallic thermal emitters. It allows maximum coupling into waveguides by placing it directly into their evanescent fields. Here, we demonstrate graphene mid-IR emitters integrated with photonic waveguides that couple directly into the fundamental mode of silicon waveguides designed to work in the so-called "fingerprint region" relevant for gas sensing. High broadband emission intensity is observed at the waveguide-integrated graphene emitter. The emission at the output grating couplers confirms successful coupling into the waveguide mode. Thermal simulations predict emitter temperatures up to 1000 °C, where the blackbody radiation covers the mid-IR region. A coupling efficiency η, defined as the light emitted into the waveguide divided by the total emission, of up to 68% is estimated, superior to data published for other waveguide-integrated emitters.

Resonant Fully Dielectric Metasurfaces for Ultrafast Terahertz Pulse Generation

AbstractMetasurfaces represent a new frontier in materials science paving for unprecedented methods of controlling electromagnetic waves, with a range of applications spanning from sensing to imaging and communications. For pulsed terahertz (THz) generation, metasurfaces offer a gateway to tuneable thin emitters that can be utilized for large‐area imaging, microscopy, and spectroscopy. In literature, THz‐emitting metasurfaces generally exhibit high absorption, being based either on metals or on semiconductors excited in highly resonant regimes. Here, the use of a fully dielectric semiconductor exploiting morphology‐mediated resonances and inherent quadratic nonlinear response is proposed. This system exhibits a remarkable 40‐fold efficiency enhancement compared to the unpatterned at the peak of the optimized wavelength range, demonstrating its potential as a scalable emitter design.

Blade emitters for atmospheric ionic thrusters

In the field of atmospheric ionic thrusters, the objective of this work is to evaluate the possibility of an alternative ionic emitter to the traditional thin wire emitter, in order to overcome the technical issues of the EHD technology related to the fragility of the wires and to make it more suitable to applications outside the laboratory. For the presented experiments, emitters in the form of metallic blades have been produced. These were tested while varying the geometric parameters of both the emitters themselves and of the thruster configuration. Through this measurement campaign, the electrical characteristics, as well as the feasibility and the performances of the new proposed solutions have been evaluated and compared with wire emitters. Results indicate that the blade emitters can work as alternative emitters, however the performance of the present prototypes does not reach that of wire emitters and therefore further research is needed in order to make them a valid alternative.

Electrically-driven ultrafast out-of-equilibrium light emission from hot electrons in suspended graphene/hBN heterostructures

Nanoscale light sources with high speed of electrical modulation and low energy consumption are key components for nanophotonics and optoelectronics. The record-high carrier mobility and ultrafast carrier dynamics of graphene make it promising as an atomically thin light emitter, which can be further integrated into arbitrary platforms by van der Waals forces. However, due to the zero bandgap, graphene is difficult to emit light through the interband recombination of carriers like conventional semiconductors. Here, we demonstrate ultrafast thermal light emitters based on suspended graphene/hexagonal boron nitride (Gr/hBN) heterostructures. Electrons in biased graphene are significantly heated up to 2800 K at modest electric fields, emitting bright photons from the near-infrared to the visible spectral range. By eliminating the heat dissipation channel of the substrate, the radiation efficiency of the suspended Gr/hBN device is about two orders of magnitude greater than that of graphene devices supported on SiO2 or hBN. We further demonstrate that hot electrons and low-energy acoustic phonons in graphene are weakly coupled to each other and are not in full thermal equilibrium. Direct cooling of high-temperature hot electrons to low-temperature acoustic phonons is enabled by the significant near-field heat transfer at the highly localized Gr/hBN interface, resulting in ultrafast thermal emission with up to 1 GHz bandwidth under electrical excitation. It is found that suspending the Gr/hBN heterostructures on the SiO2 trenches significantly modifies the light emission due to the formation of the optical cavity and showed a ∼440% enhancement in intensity at the peak wavelength of 940 nm compared to the black-body thermal radiation. The demonstration of electrically driven ultrafast light emission from suspended Gr/hBN heterostructures sheds the light on applications of graphene heterostructures in photonic integrated circuits, such as broadband light sources and ultrafast thermo-optic phase modulators.

Design Rules for Active Control of Narrowband Thermal Emission Using Phase-Change Materials

We propose an analytical framework to design actively tunable narrowband thermal emitters at infrared frequencies. We exemplify the proposed design rules using phase-change materials (PCM), considering dielectric-to-dielectric PCMs (e.g. GSST) and dielectric-to-metal PCMs (e.g. $\mathrm{VO_2}$). Based on these, we numerically illustrate near-unity ON-OFF switching and arbitrarily large spectral shifting between two emission wavelengths, respectively. The proposed systems are lithography-free and consist of one or several thin emitter layers, a spacer layer which includes the PCM, and a back reflector. Our model applies to normal incidence, though we show that the behavior is essentially angle-independent. The presented formalism is general and can be extended to \textit{any} mechanism that modifies the optical properties of a material, such as electrostatic gating or thermo-optical modulation.

Optimization of electrical properties for performance analysis of p-Si/n-CdS/ITO heterojunction photovoltaic cell

There is always a photogenerated current in solar photovoltaics because of the movement of charge carriers in the photovoltaic device. Therefore, it is necessary and exciting to explore its electrical properties. Thus, in the present communication, the electrical properties (as electron/hole mobilities, the effect of electron affinity, series/shunt resistance, etc.) have been optimized for p-Si photovoltaic cells with thin n-CdS emitter and ITO window layer. In this context, the signature of charge carrier mobilities on the performance of the proposed configuration, i.e., the hole mobility of the p-Si (p-type Crystalline Silicon) absorber layer and the electron mobility of the n-CdS (n-type Cadmium Sulphide) emitter/buffer layer has been optimized. Under the variation in hole mobility from 50 cm2/Vs to 500 cm2/Vs and the electron mobility from 10 cm2/Vs to 100 cm2/Vs simultaneously, the observed 2D contour plots reveal the effective change in power conversion efficiency from 17.72 % to 18.25 %. Moreover, the effect of electrical parameters, i.e., series and shunt resistance, on the characteristic performance parameters such as power conversion efficiency (η), open-circuit voltage (Voc), short circuit current density (Jsc), and fill factor (FF). An increase in series resistance (0 Ωcm2 to 10 Ωcm2) deteriorates the performance of the proposed solar cell considerably (19.66% to 8.87%). Further, the effect of the electron affinity of the p-Si absorber layer on fill factor and open circuit voltage is observed.

Activation experiment and spectral response properties analysis of graded-bandgap AlGaAs/GaAs electron-injection cathode

A cesium/oxygen activation experiment was conducted on a gallium-arsenide-based electron-injection cathode in an ultra-high vacuum system connected to a spectral response measuring instrument. Before activation, high-temperature cleaning was performed at 650 °C to obtain an atomically clean surface. The cathode surface impurities before and after the experiment were characterized by X-ray photoelectron spectroscopy. Because of the cathode Ti/Pt/Au electrode properties, the sample could not be cleaned chemically. High-temperature cathode treatment before activation could not remove the oxides completely, which led to a low cathode emission capacity in the photocurrent and spectral response. The experimental quantum efficiency curve was fitted by a two-minima diffusion model, and parameters, such as the electron diffusion length and escape probability, which characterize the cathode performance, were calculated. According to the fitting results, the residual impurities on the cathode surface and the thin emitter layer resulted in a low probability of electron escape, which resulted in a low quantum efficiency.

Maintaining superior crystallinity and conductivity in boron-doped nc-Si ultra-thin films by hydrogen plasma treatment

In view of using doped nanocrystalline Si (nc-Si) as the emitter layer in nc-Si/crystalline-Si heterojunction solar cells, a very thin (≤ 50 nm) emitter layer with high conductivity and superior crystallinity is optimum. In the present work, p-nc-Si:H thin film prepared in inductively coupled plasma chemical vapor deposition, without using additional H2-dilution in [SiH4 + B2H6 (1% in H2)]-plasma, retains conductivity ~10−1 S cm−1 and crystallinity ~68% at a low thickness (t) ~50 nm. The charge carrier transport through the heavily doped nc-Si network demonstrates reverse Meyer–Neldel characteristics for t ≥ 50 nm. However, with reduction to t < 50 nm, the evolution of the ultra-nanocrystalline dominated amorphous network, accompanied by accumulation of Si–H–Si platelet-like components in the grain boundary, leads to a drastic reduction in conductivity to 10−4 S cm−1. In this context, the post-deposition and short-time hydrogen plasma treatment (PSHPT) on the as-deposited ultra-thin (t ~30 nm) film becomes instrumental in substantially enhancing the crystallinity from 40% to 69% and increasing conductivity by two orders of magnitude to 10−2 S cm−1. The network modification by PSHPT proceeds via transformation of the surface and sub-surface weak bonds and is significant for the ultra-thin film compared to the thick film, due to retaining a superior surface-to-bulk ratio.

Atomic-Scale Control of Coherent Thermal Radiation

Controlling the temporal coherence of thermal radiation plays an important role in energy harvesting and thermal management. Conventional approaches for thermal radiation control rely on resonances in bulk thermal emitters, which usually have a large footprint and display little tunability. In this work, we explore the possibility to control coherent thermal emission with atomically thin emitters. We show that, despite the atomic thickness, the peak emissivity of these emitters could be as high as the bulk emitters. We identify several linear scaling laws for the radiative properties of atomic-scale systems, providing a pathway for controlling thermal radiation with atomic precision.

Hot electron emission from waveguide integrated lanthanum hexaboride nanoparticles

Recently, it has been shown that hot-electron photoemission in waveguide-integrated graphene can occur at peak optical power densities many orders of magnitude lower than multiphoton and strong field emission. In this work, we study how the deposition of low-work function lanthanum hexaboride nanoparticles can alter the behavior of hot-electron emission from graphene and thin gold waveguide-integrated hot electron emitters. This approach is promising, as the graphene enables an electrically conductive platform on which to deposit the nanoparticles, while still enabling interaction between the nanoparticles and incident photons. Despite nonideal coatings of LaB6 nanoparticles on the waveguide integrated graphene and gold, there is a nearly order of magnitude improvement over previous graphene-based hot-electron emitters. This hybrid approach demonstrates how a combination of integrated photonics and low-work function coatings can improve the performance of the emerging class of hot-electron emitters.

Light-Induced Al Plating on Si for Fabrication of an Ag-Free All-Al Solar Cell

Light-induced electroplating of Al as the front electrode on the n-type emitter of Si solar cells is proposed as a substitute for screen-printed Ag. The advantages and disadvantages of Al over Cu as the front electrode are discussed. The power of a green laser used for patterning of the SiNx antireflection coating is optimized. Conditions for removal of laser damage and contamination on the laser-patterned surface are identified. The effect of plating temperature and post-annealing temperature on Al morphology and resistivity are investigated. Several plating additives are explored to improve the morphology and resistivity of the Al film. Nicotinic acid produces the lowest resistivity of 3.1 μΩ-cm. However, the lowest contact resistivity between light-induced Al and Si is 69 mΩ-cm2 due to laser-induced damage to the emitter. The Al film spikes through the thin n-type emitter when annealed at 500 °C causing cell failure. The process reproducibility is also poor due to atmospheric moisture.

Study of resonant transport in InAs-based quantum hot electron transistors

A study of transport in a quantum hot electron transistor made of an InAs/AlSb heterostructure is reported. It exhibited that the quantum hot electron transistors with a thick emitter efficiently prevented the parasitic base currents compared with transistors with a thin emitter. The static characteristics of the fabricated devices demonstrated an enhancement of the current gain of 9 and a collector breakdown voltage of 1.5 V with thick-emitter designed transistors. In optimized devices, the current is dominated by fast resonant tunneling that is promising for their future development of as high frequency transistors.

Tunable bandgap renormalization by nonlocal ultra-strong coupling in nanophotonics

In quantum optics, great effort is being invested in enhancing the interaction of quantum emitters with light. The different approaches include increasing the number of emitters, the laser intensity or the local photonic density of states at the location of an atom-like localized emitter. In contrast, solid-state extended emitters hold an unappreciated promise of vastly greater enhancements through their large number of vacant electronic valence states. However, the majority of valence states are considered optically inaccessible by a conduction electron. We show that, by interfacing three-dimensional (3D) solids with 2D materials, we can unlock the unoccupied valence states by nonlocal optical interactions that lead to ultra-strong coupling for each conduction electron. Consequently, nonlocal optical interactions fundamentally alter the role of the quantum vacuum in solids and create a new type of tunable mass renormalization and bandgap renormalization, which reach tens of millielectronvolts in the example we show. To present quantitative predictions, we develop a non-perturbative macroscopic quantum electrodynamic formalism that we demonstrate on a graphene–semiconductor–metal nanostructure. We find new effects, such as nonlocal Rabi oscillations and femtosecond-scale optical relaxation, overcoming all other solid relaxation mechanisms and fundamentally altering the role of optical interactions in solids. When interfacing a graphene layer with a thin solid emitter, the quantum plasmonic vacuum allows each solid electron to access all unoccupied valence states through the nonlocality of their light-matter interaction, creating ultra-strong coupling alongside mass and bandgap renormalization.

Luminogens for Aggregation-Induced Emission via Titanium-Mediated Double Nucleophilic Addition to 2,5-Dialkynylpyridines: Formation and Transformation of the Emitting Aggregates.

New luminogens for aggregation‐induced emission (AIE), which are characterized by a branched cross‐conjugated 2,6‐bis(1,2,2‐triarylvinyl)pyridine motif, have been synthesized exploiting the one‐pot Ti‐mediated tetraarylation of 2,6‐bis(arylethynyl)pyridines. Thin layer solid‐state emitters were prepared by spin‐coating of the luminogens, while AIE‐colloidal dispersions were investigated in terms of optical density and scattering behaviour. This has given insight into particle size distributions, time evolution of the aggregation and the influence of different functionalization patterns on the luminescence of molecular aggregates. In particular, a combination of extinction spectroscopy and dynamic light scattering is being proposed as a powerful method for investigating the dynamic aggregation process in AIE‐type colloids.

Effect of solar cell structure on the radiation resistance of an InP solar cell

Effects of electron irradiation-induced deep level defects have been studied on both n/p and p/n Indium Phosphide (InP) solar cells with very thin emitters. The simulation results reveal that the n/p structure offers a somewhat better short-circuit current and that the p/n structure renders an improved open-circuit voltage, not only before electron irradiation but also after 1 MeV electron irradiation with 5×1015 electrons per cm2 fluence. Further, the calculated findings highlight that the n/p solar cell structure is more resistant than that of a p/n structure.

To the Theory of Gyrotrons with Wide Emitters

The main trends in gyrotron development are escalation of the radiated power and increasing the frequency of coherent radiation. For both trends, it is beneficial to develop gyrotrons with wide emitters because this allows one to use cryomagnets with smaller inner bore sizes. For analyzing and optimizing the operation of gyrotrons with wide emitters, it is proposed to represent such emitters as a superposition of thin rings and analyze the properties of electron beams emitted by each of these rings. The present paper consists of two parts. In the first part, the peak values of the orbital velocities and their spread are determined in all fractions of an electron beam in a gyrotron with the standard and widened emitters; also, the effect of profiling the anode on characteristics of these electron beam fractions is considered. In the second part, the interaction efficiency of electron beams produced by thin emitter rings is described and the relationship between these efficiencies and orbital-to-axial velocity ratios in these beams is discussed.

Thin and low light-induced degradation passivated emitter and rear cells based on boron-aluminum pre-gettering

In the present work, B/Al pre-gettering technique was proposed to suppress the light-induced degradation (LID) of thin (150 μm) monocrystalline passivated emitter and rear cell (PERC). B/Al co-doping technique raised the acceptor concentration in p + doping area by more than one magnitude, contributing to the superior gettering effect with the interstitial iron concentration decreased from 9.58e10 cm−3 to a minimal value of 4.93e9 cm−3. By employing pre-gettering process in the thin PERC solar cells fabrication, LID was suppressed dramatically, with the efficiency loss improvement of 0.63% and 0.44% for the maximum degradation and ultimate degradation, respectively. Accordingly, although B/Al pre-gettering degraded the electrical performance slightly, the ultimate efficiency after LID was 0.06% higher than the samples without pre-gettering, reaching 20.68%. The present work will be helpful to deepen the understanding of B/Al gettering effect as well as suppress LID induced by iron atoms.

A polydimethylsiloxane-coated metal structure for all-day radiative cooling

Radiative cooling is a passive cooling strategy with zero consumption of electricity that can be used to radiate heat from buildings to reduce air-conditioning requirements. Although this technology can work well during optimal atmospheric conditions at night, it is essential to achieve efficient cooling during the daytime when peak cooling demand actually occurs. Here we report an inexpensive planar polydimethylsiloxane (PDMS)/metal thermal emitter thin film structure, which was fabricated using a fast solution coating process that is scalable for large-area manufacturing. By performing tests under different environmental conditions, temperature reductions of 9.5 °C and 11.0 °C were demonstrated in the laboratory and an outside environment, respectively, with an average cooling power of ~120 W m–2 for the thin film thermal emitter. In addition, a spectral-selective structure was designed and implemented to suppress the solar input and control the divergence of the thermal emission beam. This enhanced the directionality of the thermal emissions, so the emitter’s cooling performance was less dependent on the surrounding environment. Outside experiments were performed in Buffalo, New York, realizing continuous all-day cooling of ~2–9 °C on a typical clear sunny day at Northern United States latitudes. This practical strategy that cools without electricity input could have a significant impact on global energy consumption. Radiative cooling can reduce air-conditioning requirements. In this study, the authors demonstrated an inexpensive thermal emitter that provided continuous daytime cooling up to 9 °C outdoors on a clear, sunny New York day.

Enhanced Field-Emission Properties of Sol–Gel-Derived Nanostructured $$\hbox {SnO}_{2}$$ SnO 2 :F Thin Film for Vacuum Microelectronics

Nanocrystalline fluorine-doped tin oxide thin film is synthesized by sol–gel-dip-coating technique. Hydrofluoric acid is used as the F-source, which has performed a simultaneous dual activity: (1) it has improved the conductivity by F-doping and (2) has increased the surface roughness via mild etching, both of which have improved the field-emission performance of the doped film over the undoped film. Electron microscopic image of $$\hbox {SnO}_{2}$$ :F film has revealed highly sharp emitter tips at the film surface, which has provided a very high geometrical field enhancement factor ( $$\beta \sim 760$$ ) and a very low turn-on field ( $$\sim $$ 4.8 V $$\upmu \hbox {m}^{-1})$$ . The field-emission process follows the Fowler–Nordheim tunnelling of cold-electron emission. This simple and cost-effective approach can be very useful for surface nanostructuring of thin film-based field emitters with improved field-emission characteristics for vacuum microelectronics applications.

Extracting scaled barrier field from experiments with conducting large-area field emitters: Considerations by inclusion of the dependence between area of emission and the applied field

With a large-area field electron emitter (LAFE), the area of emission is expected to be dependent of the applied field. One possible explanation for this behavior is the statistical distribution of the local field enhancement factors (FEFs), as a consequence of an irregular surface's morphology of the LAFE. In this paper, the authors present a simple and more general theory for extracting the scaled barrier field, f, by considering the dependence of the formal area of emission of conducting LAFEs with an applied field. In our model, the local FEFs of LAFE sites are exponentially distributed, which is consistent with thin film electron emitters. As a byproduct of technological relevance, our results show that general effective f values extracted from linear Fowler–Nordheim plots are outside of the “experimentally reasonable” range of values for physically orthodox emission, when the area of emission varies significantly with the applied field. Thus, a more general criterion for detecting and interpreting nonorthodox field emission is proposed and can be applied to any distribution of local FEFs in conducting LAFEs.