Directional Emission Research Articles

Optimizing Polarization Selective Unidirectional Photoluminescence from Phased‐Array Metasurfaces

AbstractMetasurface‐based optical elements offer a wide design space for miniature and lightweight optical applications. Typically, metasurface optical elements transform an incident light beam into a desired output waveform. Recent demonstrations of light‐emitting metasurfaces highlight the potential for directly producing desired output waveforms via metasurface‐mediated spontaneous emission. In this work, reciprocal finite‐difference time‐domain (FDTD) simulations and machine learning are used to enable the inverse design of highly unidirectional photoluminescent III‐Nitride quantum well metasurfaces capable of directive p‐, s‐, or combined p‐ and s‐ polarized emission at arbitrary angles. In comparison with previous intuition‐guided designs using the same quantum well architectures, the inverse design approach enables new polarization capabilities and experimentally demonstrated improvements in directivity of 54%. An analysis of ways in which the inverse design both validates and contradicts previous intuition‐guided design heuristics is presented. Ultimately, the combination of reciprocal simulations and efficient global optimization (EGO) grants remarkable improvements in emission directivity and results in full control over the polarization and momentum of emitted light, including simultaneous directional emission of s‐ and p‐polarized light.

Rethinking the environmental Kuznets curve hypothesis across 214 countries: the impacts of 12 economic, institutional, technological, resource, and social factors

Research over the past three decades has provided rich empirical evidence for the inverted U-shaped EKC theory, but current problems facing advancing climate mitigation actions require us to re-examine the shape of global EKC rigorously. This paper examined the N-shaped EKC in a panel of 214 countries with 12 traditional and emerging variables, including institutions and risks, information and communication technology (ICT), artificial intelligence(AI), resource and energy use, and selected social factors. The two-dimensional Tapio decoupling model based on N-shaped EKC to group homogeneous countries is developed to explore the inter-group heterogeneous carbon emission effects of each variable. Global research results show that the linear and cubic terms of GDP per capita are significantly positive, while the quadratic term is significantly negative, regardless of whether additional variables are added. This means the robust existence of an N-shaped EKC. Geopolitical risk, ICT, and food security are confirmed to positively impact per capita carbon emissions, while the impact of composite risk, institutional quality, digital economy, energy transition, and population aging are significantly negative. The impact of AI, natural resource rents, trade openness, and income inequality are insignificant. The inflection points of the N-shaped EKC considering all additional variables are 45.08 and 73.44 thousand US dollars, respectively. Combining the turning points and the calculated decoupling coefficients, all countries are categorized into six groups based on the two-dimensional decoupling model. The subsequent group regression results show heterogeneity in the direction and magnitude of the carbon emission impacts of most variables. Finally, differentiated carbon emission reduction strategies for countries in six two-dimensional decoupling stages are proposed.

Preliminary assessment of volatile organic compounds and hazardous gases dispersion at low winds: case study of Mellitah Gas Complex, Libya

Air pollution from industrial areas has become really worrying especially for city dwellers. The plume dispersion emitted from industrial sources is subject to several factors: temperature and emission rate velocity, wind speed and direction, source height, and atmospheric stability. This study aimed to evaluate the applicability of the dispersion coefficients correlated within a Gaussian plume approach to an industrial source in Libya (Mellitah Gas Complex) under low and moderate wind speeds. To this end, we have developed a specific code based on the Gaussian method to study the dispersion of (1) Volatile Organic Compounds (VOCs) from oil storage tanks and condensate storage tanks, and (2) sulfur oxides (SOx) and nitrogen oxides (NOx) emitted by the flaring process through three stacks of 80 m height. The emissions from multisource points and their dispersion have been predicted at calm wind conditions and the flammability and danger-prone toxic zones have been delimited around the studied site. The obtained results reveal that the emissions, particularly generated at low and moderate wind speeds, induce a dispersion with high concentration levels in the area surrounding the industrial site. The VOCs critical concentration region indicates a real risk of flammability at low-speed wind and stable atmospheric condition, from a height of 5 m above the ground. In fact, the VOCs concentration reach the Flammability Inferior Limit value of 0.018 m3 VOCs/m3 and these concentrations, appearing in the form of a plume, extend downstream to approximately 1000 m. The dispersion of NOx and SOx emissions downwind from the stacks are enhanced by wind speed; nevertheless, at 2 m height from the ground, the levels could exceed the limit value of 0.125 mg/m3, especially under the condition of unstable and very unstable atmospheric classes. From our findings, we recommend continuous monitoring campaigns inside and around the complex of Mellitah to ensure an environmentally secure zone that respects safety and health guidelines. Furthermore, enhanced simulations based on hourly weather conditions for extended area would be of great interest to accurately assess the air quality index in the region.

Directional emission of a three-dimensional connection-type metamaterial.

Directional emission of electromagnetic waves plays an essential role in laser radar and free-space communication. For most directional antennas, bandwidth and miniaturization are a pair of contradictions due to their underlying interference mechanism. Connection-type metamaterials exhibit exotic electromagnetic response near zero-frequency, which relies on the global topology of mesh connectivity rather than resonance and thus has a broad working bandwidth. In this Letter, we investigate the broadband orientation-dependent coupling effect of a 3D double mesh metamaterial. Based on this effect, we achieve a broadband directional emission (relative bandwidth of 37.72%) using a compact structure (compared to twice working wavelength). Our work provides a novel, to the best of our knowledge, scheme to manipulate a long-wavelength wave and may pave the way to a miniaturized directional antenna.

Research on attitude correction algorithm for mobile wind lidars

The laser wind measurement technology is remarkable for detecting clear-sky wind fields. The Doppler beam swinging algorithm for wind lidars has been developed to obtain vertical wind profiles based on fixed observation methods. However, the Doppler frequencies are superposed due to the self-motions of lidars caused by carrier motions when lidars are used on motion carrier platforms. Meanwhile, the emission directions of laser beams are uncertain due to changes in carriers’ motion directions and tilts. Thus, a new wind measurement correction model must be studied with lidar attitudes. This study considers the influences of the motion velocities, the carrier’s tilt angles, and the laser beams’ yaw angles at the 0° azimuth angle on the measured results under lidar motions, a correction model of motion attitudes for mobile wind lidars was designed. Sensitivity simulation tests for motion attitude parameters were carried out, and the influences of different attitude parameters of the carrier on the measured results were investigated to evaluate and verify the effects of the correction model. Results indicated that the wind measurement correction model could correct data errors caused by the carrier’s motion and tilts. The motion velocities, carrier directions, and the yaw angles of the laser beams at the 0° azimuth angle had an essential influence on the wind velocity measurements. Besides, the carrier’s pitch angles and the roll angles, which did not influence the wind velocity measurements, only affected the altitudes of the wind field data. Furthermore, the pitch angles exerted more significant influences on the data altitudes than the roll angles.

The SMILE Effect in the Beam Propagation Direction Affects the Beam Shaping of a Semiconductor Laser Bar Array

Near-field bending of a laser diode bar (i.e., the SMILE effect) degrades the laser beam brightness, adversely affecting optical coupling and beam shaping. Previous reports mainly focused on the two-dimensional near-field bending of a laser diode bar. However, the near-field bending of a laser diode bar not only occurs in the laser bar growth direction, but also in the beam propagation direction. The present article proposes the three-dimensional near-field bending of a laser diode array, which is commonly known as the three-dimensional spatial SMILE effect. Through theoretical and simulated investigations, it has been found that a laser bar array not only deforms in the fast axis direction to cause the traditional two-dimensional SMILE effect but also experiences an additional deformation of approximately 2 μm in the laser emission direction simultaneously. Due to the SMILE effect in the beam propagation direction, not all emitters are aligned in a straight line, and some emitters experience defocusing during collimation. Consequently, there is an increase in the residual divergence angle and beam width, resulting in a degradation of the laser bar array’s beam quality. According to the theoretical calculations, ZEMAX simulations, and experimental results, for a FAC (fast axis collimation) with a focal length of 300 μm, the divergence angle of single emitter after collimating in the fast axis increases from 4.95 mrad to 6.46 mrad when the offsetting of the working distance between the incident beam waist and FAC lens increases from 0 μm to 2 μm.

Directing valley-polarized emission of 3 L WS2 by photonic crystal with directional circular dichroism.

The valley degree of freedom that results from broken inversion symmetry in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) has sparked a lot of interest due to its huge potential in information processing. In this experimental work, to optically address the valley-polarized emission from three-layer (3 L) thick WS2 at room temperature, we employ a SiN photonic crystal slab that has two sets of holes in a square lattice that supports directional circular dichroism engendered by delocalized guided mode resonances. By perturbatively breaking the inversion symmetry of the photonic crystal slab, we can simultaneously manipulate s and p components of the radiating field so that these resonances correspond to circularly polarized emission. The emission of excitons from distinct valleys is coupled into different radiative channels and hence separated in the farfield. This directional exciton emission from selective valleys provides a potential route for valley-polarized light emitters, which lays the groundwork for future valleytronic devices.

Collectively enhanced Ramsey readout by cavity sub- to superradiant transition

When an inverted ensemble of atoms is tightly packed on the scale of its emission wavelength or when the atoms are collectively strongly coupled to a single cavity mode, their dipoles will align and decay rapidly via a superradiant burst. However, a spread-out dipole phase distribution theory predicts a required minimum threshold of atomic excitation for superradiance to occur. Here we experimentally confirm this predicted threshold for superradiant emission on a narrow optical transition when exciting the atoms transversely and show how to take advantage of the resulting sub- to superradiant transition. A π/2-pulse places the atoms in a subradiant state, protected from collective cavity decay, which we exploit during the free evolution period in a corresponding Ramsey pulse sequence. The final excited state population is read out via superradiant emission from the inverted atomic ensemble after a second π/2-pulse, and with minimal heating this allows for multiple Ramsey sequences within one experimental cycle. Our scheme is an innovative approach to atomic state readout characterized by its speed, simplicity, and highly directional emission of signal photons. It demonstrates the potential of sensors using collective effects in cavity-coupled quantum emitters.

HYPERION

We present ALMA deep observations of the [CII] 158 μm emission line and the continuum at 253 GHz and 99 GHz towards SDSS J0100+2802 at z ≃ 6.3, the most luminous quasi-stellar object (QSO) at z > 6. J0100+2802 belongs to the HYPERION sample of luminous QSOs at z ∼ 6 − 7.5. The observations have a 2.2 arcsec resolution in band 3 and a 0.9 arcsec resolution in band 6, and are optimized to detect extended emission around the QSO. We detect an interacting, tidally disrupted companion both in [CII], peaking at z ∼ 6.332, and in continuum, stretching on scales up to 20 kpc from the quasar, with a knotty morphology. The higher velocity dispersion in the direction of the companion emission and the complex morphology of the tidally stretched galaxy suggest a possible ongoing or future merger. For the newly detected companion, we derive the range of the dust mass, Mdust = (0.3 − 2.6)×107 M⊙, and of the star formation rate, SFR = [35 − 344] M⊙, obtained from the modelling of its cold dust spectral energy distribution. This shows that both the QSO and its companion are gas-rich and that a major merger may be at the origin of the boosted star formation. This close interacting companion is undetected by deep JWST imaging observations, showing the effectiveness of ALMA in detecting dust-obscured sources, especially in the vicinity of optically bright quasars. We also detect a broad blueshifted component in the [CII] spectrum, which we interpret as a gaseous outflow for which we estimate a mass outflow rate in the range Ṁout = (118 − 269) M⊙ yr−1. J0100+2802 was recently found to reside in a strong overdensity, however this close companion remained undetected by both previous higher resolution ALMA observations and by JWST-NIRCAM imaging. Our results highlight the importance of deep medium-resolution ALMA observations for the study of QSOs and their environment in the Epoch of Reionisation.

Switchable unidirectional emissions from hydrogel gratings with integrated carbon quantum dots

Directional emission of photoluminescence despite its incoherence is an attractive technique for light-emitting fields and nanophotonics. Optical metasurfaces provide a promising route for wavefront engineering at the subwavelength scale, enabling the feasibility of unidirectional emission. However, current directional emission strategies are mostly based on static metasurfaces, and it remains a challenge to achieve unidirectional emissions tuning with high performance. Here, we demonstrate quantum dots-hydrogel integrated gratings for actively switchable unidirectional emission with simultaneously a narrow divergence angle less than 1.5° and a large diffraction angle greater than 45°. We further demonstrate that the grating efficiency alteration leads to a more than 7-fold tuning of emission intensity at diffraction order due to the variation of hydrogel morphology subject to change in ambient humidity. Our proposed switchable emission strategy can promote technologies of active light-emitting devices for radiation control and optical imaging.

Assessing the Mediating Role of E-Techniques in the Relationship Between Information Systems and Carbon Reduction Strategies During the Construction Phase of Chinese Construction Projects

In recent years, information systems have become increasingly popular for supporting sustainable operations, especially in carbon reduction efforts. Due to the need to minimize carbon emissions and adopt green practices, organizations are increasingly using information systems to achieve these aims. By using e-techniques as a mediator and taking into account the moderating effects of project orientation and project complexity, this quantitative study seeks to clarify the connection between information systems and carbon reduction approaches. e-techniques connect information systems to carbon reduction efforts. The results imply that project emphasis and complexity affect information systems and carbon reduction approaches. To test these hypotheses, 470 Chinese construction companies that use information systems to reduce carbon emissions were studied. Structural equation modeling determined variable relationships and mediating and moderating factors. This quantitative investigation shows that information systems help companies minimize carbon emissions. This suggests that information systems are highly effective for increasing sustainability in this context. It also emerged that electronic procedures mediate the interaction between information systems and carbon-reduction activities. Project direction and complexity operate as a moderating element, affecting the relationship between information systems and carbon reduction initiatives. The study explores how project direction and complexity impact carbon emission reduction information system efficacy.

Thermophotovoltaic emitter design with a hyper-heuristic custom optimizer enabled by deep learning surrogates

Micro/nanostructures hold promise for use as emitters to boost the efficiency of thermophotovoltaic (TPV) systems. For example, periodic gratings may alter the spectrum of irradiation on the photovoltaic cell to better match the spectral response of the cell. Photons with energies slightly higher than the bandgap of the semiconductor are the most desired as they generate electron-hole pairs with minimal thermalization losses. This prompts the use of gratings as selective emitters, and the choice of grating geometry has been an active research topic. Even for a one-dimensional (1D) grating, millions of possible geometries exist, and each grating requires computationally intensive full-wave simulation, e.g., the rigorous coupled-wave analysis (RCWA), to calculate the spectral, directional emissivity. Since optimization algorithm performance is problem-dependent, in this work, a hyper-heuristic search enabled by a fully connected neural network surrogate of the native RCWA is employed to select an algorithm for a specific grating design problem. A comparison with existing untuned algorithms for an ideal emitter problem demonstrates that the hyper-heuristically generated algorithm yields superior performance. This algorithm is then employed for the optimization of the emitter for a full TPV system comprising a heated 1D tungsten binary grating paired with a 300 K InGaSb cell. The system is optimized for maximum power or efficiency at 2000 K and 1500 K, respectively, and the grating properties for the optimized cases are analyzed.

Giant enhancement of optical nonlinearity from monolayer MoS2 using plasmonic nanocavity.

The particle-on-mirror nanocavity, supporting multiple plasmonic resonances, provides an ideal platform to efficiently boost the nonlinear optical processes at the nanoscale. Here, we report on the enhancement of the second (SHG) and third-harmonic generations (THG) from the monolayer MoS2 using a multi-resonant Au nanosphere dimer-on-mirror nanocavity (DoMN). The strong plasmon hybridization between the dimer and underlying Au substrate leads to the emergence of two distinct cavity modes, which are intentionally aligned with the SH and TH frequencies, rendering a 15- and 68-fold enhancement for the SHG and THG of the monolayer MoS2, respectively. Further theoretical analysis yields that these remarkable nonlinearity enhancements are also ascribed to the amplification of nonlinear source because of the excellent spatial mode overlap and the high directivity of nonlinear emission enabled by the cavity modes. Our results pave the way for the implementation of low-cost, and highly efficient nonlinear photonics devices integrated with plasmonic nanocavities.

The Effect on Emissions of using Alternative Fuels in Turbo-Charged Diesel Engines

The paper outlines the Emission and Biofuel Directives relating to CO2 emissions in cars and the fiscal penalties incurred by manufacturers failing to meet targets. The effect on emissions of the biofuel directives are quantified through a test programme which measures emissions from a turbo-charged diesel engine using a range of biodiesel fuels, using standard diesel fuel to obtain a comparison. The tests measure O2 , NOx , THC and CO emissions data as well as CO2 . The results are difficult to quantify but , with the exception of corn oil show an increase in CO2 emissions. Corn oil also shows a reduction in other emissions. The discussion considers other mechanisms which can reduce overall global automotive emissions including range extenders for electric vehicles.

Ultra-broadband directional thermal emission.

Directional control of thermal emission over its broad wavelength range is a fundamental challenge. Gradient epsilon-near-zero (ENZ) material supporting Berreman mode has been proposed as a promising approach. However, the bandwidth is still inherently limited due to the availability of ENZ materials covering a broad bandwidth and additional undesired omnidirectional modes in multilayer stacking with increased thickness. Here, we show that broadband directional thermal emission can be realized beyond the previously considered epsilon-near-zero and Berreman mode region. We then establish a universal approach based on effective medium theory to realizing ultra-broadband directional thermal emitter. We numerically demonstrate strong (emissivity >0.8) directional (80 ± 5°) thermal emission covering the entire thermal emission wavelength range (5-30 μm) by using only two materials. This approach offers a new capability for manipulating thermal emission with potential applications in high-efficiency information encryption, energy collection and utilization, thermal camouflaging, and infrared detection.

Nonlinear nanoresonators for Bell state generation

Entangled photon states are a fundamental resource for optical quantum technologies and investigating the fundamental predictions of quantum mechanics. Up to now such states are mainly generated in macroscopic nonlinear optical systems with elaborately tailored optical properties. In this theoretical work, we extend the understanding on the generation of entangled photonic states toward the nanoscale regime by investigating the fundamental properties of photon-pair generation in sub-wavelength nonlinear nanoresonators. Taking materials with Zinc-Blende structure as an example, we reveal that such systems can naturally generate various polarization-entangled Bell states over a very broad range of wavelengths and emission directions, with little to no engineering needed. Interestingly, we uncover different regimes of operation, where polarization-entangled photons can be generated with dependence on or complete independence from the pumping wavelength and polarization, and the modal content of the nanoresonator. Our work also shows the potential of nonlinear nanoresonators as miniaturized sources of biphoton states with highly complex and tunable properties.

Lithography-free directional control of thermal emission.

Blackbody radiation is incoherent and omnidirectional, whereas various novel applications in renewable energy require a degree of directional control of a thermally emitted beam. So far, such directional control has required nano-structuring the surface of a thermally emitting material, typically by forming diffraction gratings. This, however, necessitates lithography and usually results in polarization-dependent properties. Here, we derive analytical conditions for highly directional thermal emission using a planar 3-layer structure analogous to a Salisbury screen. We present design rules for maximizing the directionality of such structures. Notably, these design rules pertain to both linear polarizations, thus generalizing the principles of a grating for unpolarized light. We show that the key requirement to achieve such performance is ultra-high quality factor resonances in materials supporting phonon polaritonic modes, as those found in low-dimensional materials. We propose a realistic device based on hexagonal Boron Nitride and predict performances comparable to lithography-based nano-structures.

Fractal acoustic metamaterials with near-zero index and negative properties

A subwavelength two-dimensional fractal acoustic metamaterial was designed based on the Hilbert fractal curve and spatial curl concept. The equivalent acoustic parameters of the fractal acoustic metamaterial were extracted by the equivalent parameter method, and the acoustic metamaterial exhibited near-zero density and negative density properties at specific frequency points. The analysis based on the finite element method shows that the acoustic prism constructed by using the near-zero density property of this acoustic metamaterial can realize the single-channel acoustic unidirectional transmission and multichannel acoustic unidirectional transmission effect, the bidirectional acoustic conduction and multidirectional acoustic conduction effect, and the bidirectional sound insulation effect. We also investigated the effect of embedding rigid scatterers inside the prism on the acoustic one-way transmission performance. Based on the acoustic direction selection mechanism of the near-zero refractive index acoustic metamaterial, arbitrary modulation of the plane acoustic wave emission direction can be achieved by changing the angle of the acoustic prism emission interface. In addition, the negative density property of the acoustic metamaterial is used to design an acoustic triangular prism to realize the acoustic negative refraction phenomenon. next, we discuss the trend of the influence of the geometrical parameters of the metamaterial on its effective properties, where the modulating wave frequency gradually moves to lower frequencies as the geometrical parameters of the metamaterial structural units increase. Finally, we fabricated the corresponding experimental samples of acoustic prisms using 3d printing technology and conducted experimental tests. The results show that the measured acoustic pressure field distribution matches well with the analytical results of the finite element method, which proves the accuracy of the results and the reasonableness of the structure, indicating that acoustic prisms composed of this acoustic super-material are expected to be used in ultrasonic imaging, acoustic energy rectification and other fields.

Low-frequency directional wideband transducer based on lead-free piezoelectric ceramics

In recent years, lead-free piezoelectric ceramic devices have been extensively researched and applied in the high-frequency range, specifically above 20 kHz, for airborne acoustics applications. However, few applications have been reported on low-frequency underwater acoustic transducers, which are crucial in the fields of underwater acoustic communication, ocean resource development, etc., because this imposes more stringent requirements for lead-free piezoelectric ceramics in terms of size, mechanical coupling coefficient, piezoelectric coefficients, and temperature stability. This paper explores the application of high-performance potassium sodium niobate based lead-free piezoelectric ceramics in a low-frequency directional wideband transducer. It covers design, fabrication, and performance testing of the transducer. The transducer measures Φ236 × 83 mm2 and weighs 4.45 kg, which achieves wideband emission (3500–8200 Hz) and directional sound emission with a front-to-back sound pressure ratio exceeding 6 dB across a significant frequency range. Both the theoretical analysis and the experimental results confirm the competitiveness application of the lead-free piezoelectric ceramics in the field of low-frequency underwater acoustic transducers applications.

Directional spectral emissivity characterization and modeling of laser-patterned steel surfaces

We present preliminary results on the fabrication of patterned surfaces by Direct Laser Interference Patterning and the characterization and theoretical interpretation of their infrared emissivities. The upgraded experimental method is capable of studying the full directional emission of samples under a controlled atmosphere at high temperatures. The effects of surface patterning can be quantitatively studied and modeled using a numerical method based on rigorous coupled-wave analysis (RCWA), a technique usually employed for periodic surfaces. The results show that laser interference patterning is capable of modifying the infrared emission of metallic materials, and that these changes can be accurately measured and numerically reproduced.