Directional Emission Research Articles

Giant enhancement of optical nonlinearity from monolayer MoS2 using plasmonic nanocavity

Abstract 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

Abstract 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

Abstract 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.

Transparent energy-saving windows based on broadband directional thermal emission

Abstract Passive radiative cooling has emerged as a sustainable energy-saving solution, characterized by its energy-free operation and absence of carbon emissions. Conventional radiative coolers are designed with a skyward orientation, allowing for efficient heat dissipation to the cold heat sink. However, this design feature presents challenges when installed on vertical surfaces, as nearby objects obstruct heat release by blocking the cooler’s skyward view. Here, we introduce a directional radiative cooling glass (DRCG) designed to facilitate efficient heat dissipation through angular selective emission. The DRCG is constructed as a multilayer structure incorporating epsilon-near-zero materials, specifically Si3N4 and Al2O3, layered on an indium-tin-oxide thermal reflector. This innovative design restricts thermal emission to specific angular ranges, known as the Berreman mode. Additionally, the transparent layers enable a visible transmittance exceeding 84 %. Theoretical simulations validate the enhanced cooling performance of the DRCG, exhibiting a temperature reduction of over 1.5 °C compared with conventional glass in hot urban environments characterized by a nearby object temperature exceeding 60 °C and a sky view factor of 0.25. Furthermore, outdoor experiments demonstrate that employing the DRCG as a window enhances space-cooling performance by ∼1.5 °C. These findings underscore the potential of transparent energy-saving windows in mitigating the urban heat island effect.

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.

Directional emission in an on-chip acoustic waveguide

Integrated acoustic circuits leverage guided acoustic waves for applications ranging from radio frequency filters to quantum state transfer, biochemical sensing, and nanomechanical computing. In many applications, it is desirable to have a method for unidirectional acoustic wave emission. In this work, we demonstrate directional emission in an integrated single-mode, on-chip membrane waveguide, demonstrating over 99.9% directional suppression and reconfigurable directionality. This avoids both loss and unwanted crosstalk, allowing the creation of more complex and compact phononic circuits.

Photon energy dependence of photoneutron production from heavy targets

The double differential cross-sections (DDXs) of photoneutron production via the photonuclear reaction on tantalum, tungsten, and bismuth for 13 and 17 MeV linearly polarized photon beams were measured using the time-of-flight method at the NewSUBARU-BL01 facility. Polarized photons were obtained by the Laser Compton backscattering (LCS) technique. Two distinct components were observed on the spectra: the lowenergy component up to 4 MeV and the high-energy above 4 MeV. The angular distribution of the low-energy component was isotropic, whereas the high-energy was distributed anisotropically and influenced by the angle between direction of photon polarization and neutron emission, especially for 17 MeV photon energy. These phenomena were similar to those of 197Au target observed in the previous studies. The low-energy neutron distributions at 13 and 17 MeV photon energies were almost identical for all three targets. The DDX energy integration was calculated and compared among the three targets for two-photon energies. Given the horizontal polarization (plane parallel to the x-axis) of the incident photons, the emission angles of 90° on the x-axis and y-axis recorded the maximum and minimum photoneutron yield, respectively. The differences between these two positions were minor for 181Ta and natW at 13 MeV photon energy and noticeable for other cases. An underestimation of the TENDL nuclear data library was observed on the photoneutron DDXs compared to the experimental results for 181Ta and 209Bi.

Directional Land Surface Emissivity Retrieval From Combined MERSI/FY-3D/E and MODIS Data

Directional Land Surface Emissivity Retrieval From Combined MERSI/FY-3D/E and MODIS Data

Spatially oriented correlated emission based on selective drive of diatomic superradiance states

<sec>In recent years, the radiative properties of atomic systems have been a hot topic in the research fields of quantum optics and quantum information. With the continuous development of nanophotonics, quantum antennas have become an important model for studying atomic radiation. In order to investigate these phenomena in depth, we investigate a system composed of two two-level atoms, and study the two-photon emission phenomenon of diatomic system under conditions of driving directional tunable laser field, interatomic dipole-dipole interaction, and spontaneous emission coherence.</sec><sec>In this study, we diagonalize the atomic Hamiltonian to obtain the eigenvalues and entangled states of the system (symmetric and asymmetric states of two atoms), and use the rotating wave approximation to rotate the system into the laser frame. The evolution of the system is characterized mainly by the evolution of symmetric and asymmetric state, as well as the evolution of coherent terms. In our studies it is found that for identical atoms, certain laser directions and geometric configurations can exclusively drive the superradiant and subradiant states of atoms, which can enhance the first-order interference effect of the atoms and markedly increase the probability of two-photon emission in a specific detection direction. When the superradiant state of the atom is solely driven, there will be no coupling between the superradiant state and subradiant state, resulting in a correlation function angular distribution that is symmetric along the direction perpendicular to atomic axis. Further adjusting the laser direction causes the atomic interference patterns to shift, and the system will exhibit two-photon emission characteristics on one side or both sides.</sec><sec>For nonidentical atomic systems, due to detuning between the two atoms, the laser cannot drive the superradiant state or subradiant state individually, and the influence of changing the laser direction on the coupling strength diminishes with the increase of detuning between the atoms. When the laser is in resonance with one of the atoms, due to the atomic interactions, the other atom can achieve the strongest coherent effect without resonating with the laser. This research reveals that atomic detuning is crucial for the correlation values and angular distribution of the correlation function. By adjusting the atomic detuning and laser direction, the system can display highly directed one-sided two-photon emission characteristics. However, different dissipation rates will lead the probability of two-photon emission to decrease. Our studies can achieve highly directional two-photon emission on one side or both sides, which provides a theoretical basis for studying the two-photon emission of nanoantennas.</sec>

A New Canopy Emissivity Model for Sparsely Vegetated Surfaces Incorporating Soil Directional Emissivity and Topography

A New Canopy Emissivity Model for Sparsely Vegetated Surfaces Incorporating Soil Directional Emissivity and Topography

Directional emission properties of thin film microdisk

Based on the directional emission effect of semiconductor deformed microcavities, the fabrication of deformed microcavities in isotropic thin films will provide a new solution for multifunctional and highly integrated photonic active chips. Because the Limacon shaped microcavity has become one of the important configurations of single-mode, low threshold on-chip lasers, the directional emission properties of microdisks fabricated in thin film are investigated. Taking the TE<sub>20,1</sub> mode existing in the Z-cut lithium niobate thin film microdisk for example, according to two-dimensional wave optics theory, the mode distribution, quality factor <i>Q</i>, and directional emission factor <i>D</i> of microdisk variations with deformation factor <inline-formula><tex-math id="M5">\begin{document}$\varepsilon $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231754_M5.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231754_M5.png"/></alternatives></inline-formula> are respectively analyzed through using the wave optics module of COMSOL. Adopting classical scattering theory, Poincaré surfaces of sections under different deformation factors are simulated by optimizing the Dynamical Billards.jl library in Julia. In the simulation realized by Julia, 200 particle collisions are used 200 times to simulate 200 reflections of rays and finally PSOS images are obtained. Simulation results reveal that when the azimuthal quantum number of the light wave mode remains unchanged, although the shape of the microdisk varies, the ratio of the resonant wavelength inside the microdisk to the circumference of the microdisk is approximately a constant, which can predict the microdisk size and resonant wavelength estimation of microcavities. The corresponding PSOS shows that when <inline-formula><tex-math id="M6">\begin{document}$\varepsilon > 0.45$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231754_M6.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231754_M6.png"/></alternatives></inline-formula>, the entire region is covered by chaotic sea area, therefore <inline-formula><tex-math id="M7">\begin{document}$\varepsilon $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231754_M7.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20231754_M7.png"/></alternatives></inline-formula> values of 0, 0.16, 0.24, 0.28, 0.45 are selected to simulate the TE<sub>20,1</sub> mode distribution, far-field radiation flux angle distribution, and PSOS. Theoretical simulation results show that when the deformation factor is greater than 0.24, the microdisk has good unidirectional lasing property, with a <i>Q</i> factor greater than 10<sup>5</sup>. When the deformation factor is greater than 0.4, the PSOS is almost occupied by the chaotic sea area, with a <i>Q</i> factor below 10<sup>3</sup>. Therefore, when the deformation factor of the limacon microdisk in the thin film can be chosen between 0.24 and 0.4, under which circumstance the microdisk not only carries high quality factor (about 10<sup>3</sup>–10<sup>5</sup>), but also forms high laser directionality (about 6.45–8.32). The theoretical simulation results will provide a certain theoretical reference for conducting the experimental research of thin film deformation microcavities.

Towards Fast and Accurate 3D Direct Emitter Tracking via Laplace Approximation

Towards Fast and Accurate 3D Direct Emitter Tracking via Laplace Approximation

Research Progress of Thermal Emission Direction Control (Invited)

Research Progress of Thermal Emission Direction Control (Invited)

Distinctive Features of Mesa‐Type Metal/Dielectric Surface Grating Structures Assisting Resonant Enhancement of Thermal Emission at Longitudinal–Optical Phonon Energy

Previously, a longitudinal optical (LO) phonon resonant emission (LORE) at 8.5 THz from Au‐GaAs surface microstructures at 630 K is reported. This emission ascribed to thermally generated electric dipoles by the coherently vibrating charges at Au/GaAs/Au interfaces shows no dependence of the emission photon energy on the emission direction, which is different from the property of surface phonon polaritons. Herein, the advantage of surface grating structures with high mesa and narrow window width is shown. The experimental emission properties of samples with 1.4–6.0 μm window width and mesa height up to 2.7 μm and finite difference time domain simulations reveal the increase in mesa height increases the electric dipole moment, while it augments the reabsorption of LORE and the background radiation subject to Planck's law for a wide window width of 6 μm. Reabsorption and the background emission can be reduced by adopting high mesa and narrow windows, which is due to the reduction of both the wire‐grid polarizer function and the distribution of the electric field beyond the LO‐phonon coherence inside the GaAs wafer. Theoretical simulation results suggest that a structure with a window width of 0.75 μm and a mesa height of 2 μm is an effective countermeasure.

Color Routing of the Emission from Magnetic and Electric Dipole Transitions of Eu3+ by Broken-Symmetry TiO2 Metasurfaces.

Manipulation of magnetic dipole emission with resonant photonic nanostructures is of great interest for both fundamental research and applications. However, obtaining selective control over the emission properties of magnetic dipole transitions is challenging, as they usually occur within a manifold of spectrally close emission lines associated with different spin states of the involved electronic levels. Here we demonstrate spectrally selective directional tailoring of magnetic dipole emission using designed photonic nanostructures featuring a high quality factor. Specifically, we employ a hybrid nanoscale optical system consisting of a Eu3+ compound coupled to a designed broken-symmetry TiO2 metasurface to demonstrate directional color routing of the compound's emission through its distinct electric and magnetic-dominated electronic transition channels. Using low numerical aperture collection optics, we achieve a fluorescence signal enhancement of up to 33.13 for the magnetic-dominated dipole transition at 590 nm when it spectrally overlaps with a spectrally narrow resonance of the metasurface. This makes the, usually weak, magnetic dipole transition the most intense spectral line in our recorded fluorescence spectra. By studying the directional emission properties for the coupled system using Fourier imaging and time-resolved fluorescence measurements, we demonstrate that the high-quality-factor modes in the metasurface enable free-space light routing, where forward-directed emission is established for the magnetic-dominated dipole transition, whereas the light emitted via the electric dipole transition is mainly directed sideways. Our results underpin the importance of magnetic light-matter interactions as an additional degree of freedom in photonic and optoelectronic systems. Moreover, they facilitate the development of spectrometer-free and highly integrated nanophotonic imaging, sensing, and probing devices.

Tunable Epsilon‐and‐Mu‐Near‐Zero Metacomposites

AbstractEpsilon‐and‐mu‐near‐zero (EMNZ) metamaterials have garnered significant attention in near‐zero‐parameter metamaterial research for their exceptional ability to attain concurrent near‐zero permittivity and permeability. Nowadays achieving EMNZ properties through the use of metacomposites remains a novel endeavor. Presented here is an innovative approach of near‐zero‐parameter metacomposites, illustrating excellent and tunable EMNZ properties in the radio frequency regime. The self‐organization approach is applied to construct the conductive 3D network and the circuits, serving as the underlying framework for achieving EMNZ properties. Near‐zero permeability is effectively maintained while permittivity reaches epsilon‐near‐zero frequency regime. Efficient manipulation of electromagnetic parameters is initially realized via adjusting component content in metacomposites. Significantly, an excellent EMNZ property is observed as carbon content reaches 15 wt.% at 915 MHz. Through both numerical simulations and experimental testing, the PGC metacomposites have exhibited tunneling effects and directional emission characteristics, confirming their EMNZ properties. Besides, the Lego‐like adjustment facilitates the achievement of EMNZ property and advances the EMNZ frequency point to 700 MHz, expanding the EMNZ range. Furthermore, thanks to the remarkable excitation effect of photo‐induced adjustment, the metacomposite with low‐carbon content also achieves extraordinary EMNZ properties. This research offers promising self‐organized EMNZ metacomposites and lays the foundation for future endeavors in precisely adjusting near‐zero parameters.