Strong Directional Emission Research Articles

Directional spontaneous emission in photonic crystal slabs

Abstract Spontaneous emission is one of the most fundamental out-of-equilibrium processes in which an excited quantum emitter relaxes to the ground state due to quantum fluctuations. In this process, a photon is emitted that can interact with other nearby emitters and establish quantum correlations between them, e.g., via super and subradiance effects. One way to modify these photon-mediated interactions is to alter the dipole radiation patterns of the emitter, e.g., by placing photonic crystals near them. One recent example is the generation of strong directional emission patterns – key to enhancing super and subradiance effects – in two dimensions by employing photonic crystals with band structures characterized by linear isofrequency contours and saddle points. However, these studies have predominantly used oversimplified toy models, overlooking the electromagnetic field’s intricacies in actual materials, including aspects like geometrical dependencies, emitter positions, and polarization. Our study delves into the interaction between these directional emission patterns and the variables mentioned above, revealing the untapped potential to fine-tune collective quantum optical phenomena.

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.

Bright Single Photon Emission from Quantum Dots Embedded in a Broadband Planar Optical Antenna

AbstractPhotonic structures capable of enhancing the light‐outcoupling efficiency of embedded epitaxial quantum dots (QDs) in a broad spectral range are attractive for the realization of bright sources of single photons and entangled photon pairs. In this work, a planar‐multilayer antenna is experimentally demonstrated for GaAs QDs embedded in AlGaAs membranes. The antenna consists of a metal (Au or Ag) reflector and a semi‐transparent metal (Ag) director, with a thin oxide layer (hafnium dioxide (HfO2) or aluminium oxide (Al2O3)) between metal and semiconductor layers. Simulations using the 3D finite‐difference time‐domain (FDTD) method indicate a maximal efficiency of 43% (for collection optics with a numerical aperture (NA) of 0.85), a wavelength‐dependent Purcell factor ranging from ≈0.7 to ≈1.45, strong directional emission characteristics, as well as a broad bandwidth of ≈30 nm. In the experiment, we find a Purcell factor compatible with the simulation results, a maximum collection efficiency as high as ≈19% and an ultralow multiphoton emission probability of 0.006 ± 0.005. The planar geometry, its compatibility with post‐growth tuning methods, facile fabrication, broad spectral bandwidth, and achieved performance make the presented structure competitive for solid‐state sources of quantum light.

Optical Bound States in Continuum in MoS2-Based Metasurface for Directional Light Emission.

High quality factor (Q-factor) and strong field localization in nanostructures is a newly emerged direction in nanophotonics. The bound states in the continuum (BIC) have been investigated in nanoparticles with infinite Q-factor. We report BIC in molybdenum disulfide (MoS2) based Mie nanoresonator suspended in air. The ultrathin nanodisk supports symmetry protected BIC, and the quasi-BIC (q-BIC) are exploited by breaking the symmetry of the structure. The strongly localized modes in our MoS2-based nanodisk sustain a similar magnetic field profile before and after symmetry breaking, unlike what has been previously reported in silicon-based structures. Strong directional emission is observed in BIC regime from a hybrid configuration with a resonator placed on the stacked metal-dielectric layers, which transform BIC to q-BIC and exploit highly directional light. The structure persists emission with small variations in normalized intensity at distorted symmetry. The giant Q-factor in q-BIC is highly desired for biosensing and optical filters.

Highly efficient light extraction and directional emission from large refractive-index materials with a planar Yagi-Uda antenna

Light emission is a basic process at the core of applications in optics and photonics, such as lighting, sensing and telecom. Despite extensive work on the design of light-matter interfaces, the outcoupling of electromagnetic radiation from nanoscale sources is still a challenge. Here, we show how a planar Yagi-Uda antenna based on thin-film optics can lead to more than 90% outcoupling efficiency and strong directional emission from materials with a large refractive index. Our findings are particularly relevant for semiconductor-based nanophotonic devices, which typically suffer from a large mismatch with respect to free-space and guided modes.

A random laser made of nematic liquid crystal doped with a laser dye

We report on random laser emission obtained in 5CB and E7 nematic liquid crystal (LC) mixtures doped with 1% weight to weight ratio of DCM laser dye. The LC cell was used as asymmetric planar waveguide were emission was collected from the edge of the sample. Variable stripe length method was utilized to estimate the gain and the losses coefficients. Both systems have shown the threshold energy fluence in order of several mJ/cm2. In both cases above Fredericks potential, significant increase of emission intensity was observed due to the increase of light scattering on liquid crystalline domains. Moreover the use of fifth order of diffraction grating covered with thin alumina film resulted in strong multimode and directional laser emission.

Quality factors of deformed dielectric cavities

An analysis is provided of the degradation that arises in the quality factor of a whispering gallery mode when a circular or spherical dielectric cavity is deformed. The large quality factors of such resonators are important to their use in applications such as sensors, wavelength filters or lasers. Yet a straightforward analysis of the effect of shape deformation on quality factors cannot be given because the underlying complex ray data demanded by a standard eikonal approximation frequently does not exist. In this paper we exploit an approach that has been successfully used elsewhere to describe the strong directional emission of such systems, based on a perturbative treatment of the relevant complex ray families. Applicable when the radial perturbation is formally of the order of a wavelength, the resulting approximation successfully describes changes to the quality factor using the ray geometry in a neighbourhood of a discrete set of escaping rays guiding the directions of maximum emitted intensity.

Directional Enhancement of Spontaneous Emission in Polymer Flexible Microcavities

We report on the control of spontaneous emission in flexible polymer 1D photonic crystal cavities fabricated by spin coating having a layer of poly(9,9-dioctylfluorenyl-2,7-diyl-co-1,4-benzo-(2,1′-3)-thiadiazole) (F8BT) as an active material. The optical properties of these full-polymer photonic crystals are systematically investigated by means of polarized angular-resolved transmittance and photoluminescence spectral measurements. We demonstrate strong directional emission enhancement when the emitter is located in the defect layer and resonantly coupled to the microcavity mode. The experimental results can be successfully reproduced with different theoretical optical models.

Comprehensive Numeric Study of Gallium Nitride Light-Emitting Diodes Adopting Surface-Plasmon-Mediated Light Emission Technique

The coupling of quantum well (QW) spontaneous emissions to surface plasmons (SPs) has been a promising technique to increase emission rate of LEDs. We carried out numeric investigations to explore the electromagnetic nature of these SP modes. It has been shown that the SP resonance frequency on a flat silver/GaN interface, and hence the corresponding emission enhancement factor can be easily tuned by altering the thickness of silver film and the separation between QW and metal. By using coupled SPPs, partial energy transfer across silver film can be achieved, where strong directional light emission can be re-emitted. We also utilized semiconductor simulation technique to investigate the internal operations of our proposed LED. We found that the internal inefficiency of the device might be attributed to the current crowding effect, poor carrier injection, as well as bad overlap of electron and hole wave functions inside the well. The combination of electromagnetic and semiconductor simulation techniques has been presented as a powerful tool in theoretical analysis of SP-mediated emission LED.

Lasing modes in a spiral-shaped dielectric microcavity

The resonance patterns and lasing modes in a spiral-shaped dielectric microcavity are investigated through passive and active medium calculations. We find that the high-Q resonance modes are whispering-gallery-like modes, and these resonance modes can be easily excited as lasing modes. We also find that the quasi-scarred resonance mode, which shows strong directional emission beams from the cavity boundary, can be excited with selectively applied external pumping. Through a spectral analysis of the time evolution of the light field, the competition between these lasing modes is discussed.

Strong Directional Emission Observed in Lasing Characteristics of a Pendant Drop Deformed by an Applied Electric Field

Strong Directional Emission Observed in Lasing Characteristics of a Pendant Drop Deformed by an Applied Electric Field

Lasing characteristics of a pendant drop deformed by an applied electric field

The lasing properties of an oval-shaped resonant cavity (ORC) with a continuously variable aspect ratio have been studied. The ORC was formed with a dye-doped pendant drop placed inside a variable static electric field. When the drop ORC was pumped by a nitrogen laser, lasing from the ORC was found to have strong directional emission characteristics and an intensity enhancement factor as great as 19.5. Calculated results of light rays escaping from ORC's by refraction are in good agreement with the experimental data.

New mechanism for electron-stimulated desorption of nonthermal halogen atoms from alkali-halide surfaces.

Fully angular-resolved kinetic-energy distributions of alkali and halogen atoms emitted due to electron-stimulated desorption have been measured for (100) and (110) alkali-halide surfaces. An unexpected strong directional emission of nonthermal halogen atoms along the axis of the investigated crystals has been found which is in contradiction to the predictions of the previously proposed Pooley model . We propose a new model involving diffusion of hot holes to the surface, and sudden localization of the excited hole at the surface of the crystal.

Angle-resolved kinetic energy distributions of atoms sputtered by electron bombardment of single crystal alkali halides

Angle-resolved kinetic energy distributions have been measured for atoms desorbed from (100) and (110) surfaces of KBr and the (100) surface of KCl due to electron bombardment. We found strong directional emission of nonthermal halogen atoms along the 〈100〉 axis of the investigated crystals rather than along the 〈110〉 axis as expected. Angular distributions of both halogen and alkali atoms emitted with thermal energies were found to be uniformly cosine with respect to the surface normal for all crystal faces. Fast hole diffusion to the bombarded surface and nonradiative decay of selftrapped surface excitons are proposed as essential elements in the explanation of these results.