Emitter Arrays Research Articles

Improving the Response Speed of an Active-Current Controlled Field Emitter Arrays by Modifying the Controlled Current

Using an active-current control (ACC) component such as MOSFET in field emitter arrays (FEAs) is an important way to achieve stable emission current. However, the controlled current of ACC may reduce the response speed of FEAs due to the longer charging process, which limits its application. In this work, a general model for the field emission of MOSFET-controlled FEAs has been established to study the influence of controlled saturation current on its response speed. It is found that the response speed which decreases nearly linearly as the saturation current decreases can be reduced down by above 1 order of magnitude comparing to the intrinsic value. The field emission characteristics of a MOSFET-controlled ZnO nanowire FEAs under different saturation current and duty ratio have been investigated, in which the result is in consistency with the model. A solution using a controlled current which is larger than the induced current during the charging process and can rapidly reduce down to the field emission current has been proposed for balancing the stability and response speed in MOSFET-controlled FEAs. All the results provide a method for obtaining fast response ACC-FEAs, which should be useful in the application that requires pulsed driving FEAs.

Scalable Microfabrication of Multi-Emitter Arrays in Silicon for a Compact Microfluidic Electrospray Propulsion System.

The recent proliferation of SmallSats and their use in increasingly demanding applications require the development of onboard electric propulsion compatible with the power, mass, and volume constraints of these spacecraft. Electrospray propulsion is a promising technology for SmallSats due to its unique high efficiency and scalability across the wide power range of these platforms, for example, from a few watts available in a CubeSat to a few hundred watts in a MiniSat. The implementation of electrospray propulsion requires the use of microfabrication techniques to create compact arrays of thousands of electrospray emitters. This article demonstrates the microfabrication of multi-emitter electrospray sources of a scalable size for electrospray propulsion. In particular, a microfabrication and assembly process is developed and demonstrated by fabricating sources with arrays of 1, 64, and 256 emitters. The electrospray sources are tested in a relevant environment for space propulsion (inside a vacuum chamber), exhibiting excellent propulsive performance (e.g., absence of beam impingement in the extractor electrode, absence of hysteresis in the beam current versus propellant flow rate characteristic, proper operation in the cone-jet electrospraying mode, etc.) and nearly coincident output per emitter. Several design elements contribute to this performance: the even distribution of the propellant among all emitters made possible by the implementation of a network of microfluidic channels in the backside of the emitter array; the small dead volume of the network of microfluidic channels; the accurate alignment between the emitters and extractor orifices; and the use of a pipe-flow configuration to drive the propellant through closed conduits, which protects the propellant.

Direct thrust test and asymmetric performance of porous ionic liquid electrospray thruster

In order to meet the demand of CubeSats for low power and high-performance micro-propulsion system, a porous ionic liquid electrospray thruster prototype is developed in this study. 10 × 10 conical emitter arrays are fabricated on an area of 3.24 cm2 by computer numerical control machining technology. The propellant is 1-ethyl-3-methylimidazolium tetrafluoroborate. The overall dimension of the assembled prototype is 3 cm × 3 cm × 1 cm, with a total weight of about 15 g (with propellant). The performance of this prototype is tested under vacuum. The results show that it can work in the voltage range of ±2.0 kV to ±3.0 kV, and the maximum emission current and input power are about 355 μA and 1.12 W. Time of Flight (TOF) mass spectrometry results show that cationic monomers and dimers dominate the beam in positive mode, while a higher proportion of higher-order solvated ion clusters in negative mode. The maximum specific impulse is 2992 s in positive mode and 849 s in negative mode. The thrust is measured in two methods: one is calculated by TOF results and the other is directly measured by high-precision torsional thrust stand. The thrust (T) obtained by these two methods conforms to a certain scaling law with respect to the emission current (Iem) and the applied voltage (Vapp), following the scale of T ∼ IemVapp0.5, and the thrust range is from 2.1 μN to 42.6 μN. Many thruster performance parameters are significantly different in positive and negative modes. We speculate that due to the higher solvation energy of the anion, more solvated ion clusters are formed rather than pure ions under the same electric field. It may help to improve thruster performance if porous materials with smaller pore sizes are used as reservoirs. Although there are still many problems, most of the performance parameters of ILET-3 are good, which can theoretically meet the requirements of CubeSats for micro-propulsion system.

Quantitative Analysis on the Field Strength in the Addressable Gated ZnO Nanowire Field Emitter Arrays: Model and Experiment

Addressable gated field emitter arrays (FEAs) have important applications in vacuum microelectronic devices. To fabricate high-performance device, a comprehensive understanding on the field emission characteristics of gated FEAs is necessary, which requires a quantitative analysis method. In this work, a general model based on Fowler-Nordheim (FN) theory has been established to fulfill this blank. It is found that nonlinear FN plot with positive and negative slopes could occur in gated FEAs, which its turning point is related to the proportion of anode and gate field. This provides a way for obtaining the field strengths applied by the anode and gate structures. Besides, the transconductance of the gated FEAs increases exponentially with the total surface field, which indicates that both the anode and gate field need to be increased for achieving a high transconductance device. As a demonstration, the addressable gated ZnO nanowire FEAs using multimicrosize pattern with radius as small as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.5 ~\mu \text{m}$ </tex-math></inline-formula> have been designed and fabricated. The relationship between the transconductance and anode voltage can be well fitted by using the model, which deduces the proportion of anode and gate field in the device to be about 3:1.

Comparative study on atomically heterogeneous surface with conical arrays of field emitters generated using plasma based low-energy ion beams

A comparative study of the field emission properties of conical arrays of atomically heterogeneous, self-organized, micro–submicro–nanodimensional structures, irradiated at normal incidence by high flux of 2 keV argon (flux=6.47×1015cm−2s−1) and krypton ions (flux=4.81×1015cm−2s−1) on copper substrates, without employing any external seeding, is presented. The variation in surface structural growths with ion beam fluence is investigated using scanning electron, atomic force, and transmission electron microscopy. The exposed surfaces are atomically heterogeneous due to the presence of embedded argon and krypton ions in the interstitial layers (≈nm) as observed from the x-ray photoelectron spectroscopy analysis. Kelvin probe force microscopy is employed to analyze the variation in local work function caused by surface deformities and implantation of inert gaseous ions. The conical arrays are naturally selected field emitter sources, and their field enhancement factor is calculated from the Fowler–Nordheim equations. The argon ion treated substrate at a fluence of 4.85×1018cm−2 gives rise to uniformly distributed structures and has a low turn-on voltage of 2.76 kV with an electron emission current of 0.58 nA. Among the krypton ion irradiated substrates, the sample irradiated at the highest fluence of 5.12×1018cm−2 produces self-organized conical arrays having uniform dimension, orientation, distribution, and even a higher electron emission current of 0.81 nA with a lower turn-on voltage of 2.12 kV. Thus, it may be concluded that krypton ion irradiation provides better generation of naturally selected arrays of field emitters.

Uncovering the morphological effects of high-energy Ga+ focused ion beam milling on hBN single-photon emitter fabrication.

Many techniques to fabricate complex nanostructures and quantum emitting defects in low dimensional materials for quantum information technologies rely on the patterning capabilities of focused ion beam (FIB) systems. In particular, the ability to pattern arrays of bright and stable room temperature single-photon emitters (SPEs) in 2D wide-bandgap insulator hexagonal boron nitride (hBN) via high-energy heavy-ion FIB allows for direct placement of SPEs without structured substrates or polymer-reliant lithography steps. However, the process parameters needed to create hBN SPEs with this technique are dependent on the growth method of the material chosen. Moreover, morphological damage induced by high-energy heavy-ion exposure may further influence the successful creation of SPEs. In this work, we perform atomic force microscopy to characterize the surface morphology of hBN regions patterned by Ga+ FIB to create SPEs at a range of ion doses and find that material swelling, and not milling as expected, is most strongly and positively correlated with the onset of non-zero SPE yields. Furthermore, we simulate vacancy concentration profiles at each of the tested doses and propose a qualitative model to elucidate how Ga+ FIB patterning creates isolated SPEs that is consistent with observed optical and morphological characteristics and is dependent on the consideration of void nucleation and growth from vacancy clusters. Our results provide novel insight into the formation of hBN SPEs created by high-energy heavy-ion milling that can be leveraged for monolithic hBN photonic devices and could be applied to a wide range of low-dimensional solid-state SPE hosts.

Cooperative subwavelength molecular quantum emitter arrays

Dipole-coupled subwavelength quantum emitter arrays respond cooperatively to external light fields as they may host collective delocalized excitations (a form of excitons) with super- or subradiant character. Deeply subwavelength separations typically occur in molecular ensembles, where in addition to photon-electron interactions, electron-vibron couplings and vibrational relaxation processes play an important role. We provide analytical and numerical results on the modification of super- and subradiance in molecular rings of dipoles including excitations of the vibrational degrees of freedom. While vibrations are typically considered detrimental to coherent dynamics, we show that molecular dimers or rings can be operated as platforms for the preparation of long-lived dark superposition states aided by vibrational relaxation. In closed ring configurations, we extend previous predictions for the generation of coherent light from ideal quantum emitters to molecular emitters, quantifying the role of vibronic coupling onto the output intensity and coherence.

Monitoring-induced entanglement entropy and sampling complexity

The dynamics of open quantum systems is generally described by a master equation, which describes the loss of information into the environment. By using a simple model of uncoupled emitters, we illustrate how the recovery of this information depends on the monitoring scheme applied to register the decay clicks. The dissipative dynamics, in this case, is described by pure-state stochastic trajectories and we examine different unravelings of the same master equation. More precisely, we demonstrate how registering the sequence of clicks from spontaneously emitted photons through a linear optical interferometer induces entanglement in the trajectory states. Since this model consists of an array of single-photon emitters, we show a direct equivalence with Fock-state boson sampling and link the hardness of sampling the outcomes of the quantum jumps with the scaling of trajectory entanglement.

Nonparaxial interference and diffraction under 3D spatial coherence.

The nonparaxial interference and diffraction by a planar array of emitters have been recently described in terms of the light energy confinement in Lorentzian wells, which are spatially structured by the geometric potential, activated in turn by the two-point correlation prepared at the array plane. Nevertheless, the use of nonplanar arrays of light emitters is of increasing interest in optical technology. Therefore, we extend the confinement model to include spatially structured Lorentzian wells by geometric potentials associated with nonplanar distributions of points. Such geometric potentials are activated by two-point correlations with 3D supports prepared at the nonplanar array. The theoretical analysis is supported and illustrated by numerical simulations.

Silicon optical phased array with a 180-degree field of view for 2D optical beam steering

Optical phased arrays (OPAs), the optical counterpart of phased arrays at radio frequencies, can electronically steer an optical beam without any moving parts. To achieve a 180° field of view (FOV), the array emitters should be spaced a half-wavelength apart or less. However, a conventional OPA based on a waveguide grating array suffers from strong cross talk between adjacent waveguides when the pitch is a half-wavelength or less. Here, we theoretically describe and experimentally demonstrate a two-dimensional aliasing-free beam steering regime for an integrated OPA with the entire 180° FOV. We achieve this by using a half-wavelength-pitch waveguide array combined with a trapezoidal slab grating as a single emitter. Our OPA also features a low sidelobe level of &lt; − 19 d B while the beam is steered from − 40 ∘ to + 40 ∘ , breaking the trade-off between FOV and beam quality. The chip-based OPA with a large beam steering range and high beam quality provides a promising route for a compact, solid-state, cost-effective, and high-performance light detection and ranging system, enabling a wide range of classical and quantum applications.

Stable and High Performance AlGaN Self-Aligned-Gate Field Emitter Arrays

AlGaN alloys are promising for field emission devices due to their low electron affinities. However, there have been limited demonstrations of AlGaN vacuum transistors so far. This letter combines a new self-aligned-gate (SAG) process and digital-etching tip sharpening to demonstrate three-terminal AlGaN SAG field emitter arrays (FEAs). These devices show a turn-on voltage of 19.5 V and an anode current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>A</i></sub> ) of 100 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at an overdrive voltage of 20 V, which are comparable with best Si devices. The AlGaN SAGFEAs can operate in DC mode at a fixed gate-emitter voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}_{\textit {GE}}$ </tex-math></inline-formula> ) with J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>A</i></sub> of 3–5 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for at least 5 hours without a significant degradation. The gate leakage does not increase after the long DC operation, suggesting high-performance and stable AlGaN vacuum transistors.

Undamped Rabi oscillations due to polaron-emitter hybrid states in a nonlinear photonic waveguide coupled to emitters

The collective dynamics of two non-interacting two-level emitters, which are coupled to a structured wave guide that supports two-photon bound states, is investigated. Tuning the energy of the two emitters such that they are in resonance with the two-photon bound state energy band, we identify parameter regimes where the system displays fractional populations and essentially undamped Rabi oscillations. The Rabi oscillations, which have no analog in the single-emitter dynamics, are attributed to the existence of a collective polaron-like photonic state that is induced by the emitter-photon coupling. The full dynamics is reproduced by a two-state model, in which the photonic polaron interacts with the state $|e,e,\text{vac} \rangle$ (two emitters in their excited state and empty wave guide) through a Rabi coupling frequency that depends on the emitter separation. Our work demonstrates that emitter-photon coupling can lead to an all-to-all momentum space interaction between two-photon bound states and tunable non-Markovian dynamics, opening up a new direction for emitter arrays coupled to a waveguide.

(Invited, Digital Presentation) Circularly Polarized Photoluminescence from Nanostructured Arrays of Light Emitters

Metasurfaces offer compact routes to spatial and polarization control of luminescence from nearby emitters. The most common integration strategy is to coat a patterned metamaterial or metasurface with a film of light emitting material. For example, structures such as arrays of Au nanorods coated with achiral light emitters exhibit circularly polarized photoluminescence at specific outcoupled angles. However, the degree of circular polarization in these systems is limited to relatively low values, and does not coincide with the angles with high photoluminescence intensity.This talk will discuss an alternative strategy, where the light emitters are patterned instead. We show that this structure offers several advantages: rather than averaging over the contributions of emitters in many different locations, this system exhibits highly directional photoluminescence with high degrees of circular polarization. Most importantly, the photoluminescence intensity is high at the same angles where the degree of circular polarization is high.These patterned light-emitting nanostructures are formed using direct-write electron beam lithography on semiconductor nanocrystals. This versatile method is capable of forming structures with aspect ratios greater than 2 and feature sizes as small as 30 nm, photoluminescence is retained after patterning, and the system is robust to multiple patterning steps.

Scanning anode field emission microscopy of a single Si emitter

Emitter tip radius nonuniformity results in exponential variations in emission current and a relatively low array utilization. Here, we provide a method of mapping the current and field-factor from a single emitter over a small area using a scanning anode field emission microscope. A dull W probe is used as the anode, and an array of emitters is fabricated on silicon (Si) wafers. We use a relatively wide spaced (100 μm pitch) emitter array with each emitter having an integrated Si pillar. Current-voltage characteristics are used to extract the field-factor and to experimentally demonstrate the mapping of the currents and field-factor of a single emitter. From emission spot sizes, the emission half-angles are measured to be &amp;lt;14° at anode voltages 2.5 kV and a minimum resolvable feature of 2–3 μm at 1.8 kV. We also determine the field-factor dependence with the distance between the anode and the emitter, where limiting the current becomes essential to prevent early burn-out of the emitter that could reduce the current. We also simulated the maximum currents tolerated by the pillar to assess the thermal effects on the emitter. Finite element modeling confirms the experimental trend in the field-factor with the distance between the anode and the emitter tip, resulting in a value of approximately 105 cm−1 for an emitter tip radius of 5 nm and an emitter-anode distance of 50 μm.

Nonreciprocal Single-Photon Band Structure.

We study a single-photon band structure in a one-dimensional coupled-resonator optical waveguide that chirally couples to an array of two-level quantum emitters (QEs). The chiral interaction between the resonator mode and the QE can break the time-reversal symmetry without the magneto-optical effect and an external or synthetic magnetic field. As a result, nonreciprocal single-photon edge states, band gaps, and flat bands appear. By using such a chiral QE coupled-resonator optical waveguide system, including a finite number of unit cells and working in the nonreciprocal band gap, we achieve frequency-multiplexed single-photon circulators with high fidelity and low insertion loss. The chiral QE-light interaction can also protect one-way propagation of single photons against backscattering. Our work opens a new door for studying unconventional photonic band structures without electronic counterparts in condensed matter and exploring its applications in the quantum regime.

Tunable Broadband Photoresponse Electron‐Emission Nanostructure: Metasurface‐Extended Spectrum‐Selecting and Plasmon‐Enhanced Emitting

Exploring novel nanotechnology schemes for tunable broadband vacuum photodetection devices is of great significance. Herein, a strategy to combine a field‐emission cold cathode with plasmonic nanostructures to form a plasmon‐enhanced field‐emission metasurface array is proposed. Mo nanostructures with excellent emission properties act as field‐emission cold cathode materials. Au nanospheres are applied to enhance the photocurrent of Mo nanopyramid emitters and to generate tunable resonance responses by plasmonic couplings. The response frequency and polarization can be selective in a broadband range from visible to near‐infrared, which originates from the periodic couplings of the Mo–Mo microcavities and the Au–Mo interfaces in the designed metasurface emitter arrays. The amplitude of photodetection signals is experimentally demonstrated to be tunable not only by the geometrical dimensions of the Mo nanopyramid and the number of decorated Au plasmons, but also by the applied external electrostatic fields. This plasmon‐enhanced photoresponse field‐emission process is attributed to plasmon‐mediated electron emission. Herein, a route to tunable broadband photoresponse electron‐emission nanostructures at room temperature (≈300 K) and low energy consumption for photocathode and vacuum device applications is provided in the results.

Universality of Dicke superradiance in arrays of quantum emitters

Dicke superradiance is an example of emergence of macroscopic quantum coherence via correlated dissipation. Starting from an initially incoherent state, a collection of excited atoms synchronizes as they decay, generating a macroscopic dipole moment and emitting a short and intense pulse of light. While well understood in cavities, superradiance remains an open problem in extended systems due to the exponential growth of complexity with atom number. Here we show that Dicke superradiance is a universal phenomenon in ordered arrays. We present a theoretical framework – which circumvents the exponential complexity of the problem – that allows us to predict the critical distance beyond which Dicke superradiance disappears. This critical distance is highly dependent on the dimensionality and atom number. Our predictions can be tested in state of the art experiments with arrays of neutral atoms, molecules, and solid-state emitters and pave the way towards understanding the role of many-body decay in quantum simulation, metrology, and lasing.

Dual Polarization and Bi-Directional Silicon-Photonic Optical Phased Array With Large Scanning Range

We propose a large scanning range silicon optical phased array (OPA), which is polarization multiplexed and bi-directional with only one waveguide grating antenna array. Mach-Zehnder interferometers (MZIs) and polarization splitter-rotator (PSR) are utilized to select the polarization states and propagation directions of the input light to the waveguide grating emitter array. By optimizing the parameters of the waveguide grating, the longitudinal far-field steering range is increased by four times compared to that of the conventional ones among the same wavelength range. The simulation results show that the 54.5° longitudinal scanning range could be realized with the tuning wavelength ranging from 1500 nm to 1600 nm. The high wavelength tuning efficiency is up to 0.545 °/nm. The beam steering range in the lateral direction is 77.8° via phase tuning.

Self-ordering of individual photons in waveguide QED and Rydberg-atom arrays

The scattering between light and individual saturable quantum emitters can induce strong optical nonlinearities and correlations between individual light quanta. Typically, this leads to an effective attraction that can generate exotic bound states of photons, which form quantum mechanical precursors of optical solitons, as found in many optical media. Here, we study the propagation of light through an optical waveguide that is chirally coupled to three-level quantum emitters. We show that the additional laser-coupling to a third emitter state not only permits to control the properties of the bound state but can even eliminate it entirely. This makes it possible to turn an otherwise focussing nonlinearity into a repulsive photon-photon interaction. We demonstrate this emerging photon-photon repulsion by analysing the quantum dynamics of multiple photons in large emitter arrays and reveal a dynamical fragmentation of incident uncorrelated light fields and self-ordering into regular trains of single photons. These striking effects expand the rich physics of waveguide quantum electrodynamics into the domain of repulsive photons and establish a conceptually simple platform to explore optical self-organization phenomena at the quantum level. We discuss implementations of this setting in cold-atom experiments and propose a new approach based on arrays of mesoscopic Rydberg-atom ensembles.

Design of a sparse array for a one-dimensional non-uniform optical phased array

To effectively improve the far-field scanning range of an optical phased array (OPA), we propose a genetic algorithm using double fitness functions to optimize the array element arrangement of a one-dimensional non-uniform OPA and simulate a one-dimensional OPA with different array element numbers. The results show that the non-uniform OPA with more array elements exhibits an improved grating lobe suppression effect, and the optimized antenna array pattern exhibits improved comprehensive performance upon employing the double fitness function of grating lobe suppression and beam steering. Considering 128 array elements as an example, the sidemode suppression ratio (SMSR) exhibits a 2.8-dB improvement in the steering process, which verifies the importance of incorporating the novel fitness function of steering optimization. In addition, we further analyze the influence of manufacturing errors such as emission intensity and array position on the SMSR; it is found that the OPA obtained by simulation is sufficiently robust. Our research lays a theoretical foundation for the development of a one-dimensional non-uniform OPA sparse array.