Directional Light Emission Research Articles

Asymmetrically Curved Hyperbolic Metamaterial Structure with Gradient Thicknesses for Enhanced Directional Spontaneous Emission.

We demonstrate hyperbolic metamaterials (HMMs) on a curved surface for an efficient outcoupling of nonradiative modes, which lead to an enhanced spontaneous emission. Those high-wavevector plasmonic modes can propagate along the curved structure and emit into the far field, realizing a directional light emission with maximal fluorescent intensity. Detailed simulations disclose a high Purcell factor and a spatial power distribution in the curved HMM, which agrees with the experimental result. Our work presents remarkable enhancing capability in both the Purcell factor and emission intensity, which could suggest a unique structure design in metamaterials for potential application in, e.g., high-speed optical sensing and communications.

Silicon nanocrystal-based photonic crystal slabs with broadband and efficient directional light emission

Light extraction from a thin planar layer can be increased by introducing a two-dimensional periodic pattern on its surface. This structure, the so-called photonic crystal (PhC) slab, then not only enhances the extraction efficiency of light but can direct the extracted emission into desired angles. Careful design of the structures is important in order to have a spectral overlap of the emission with extraction (leaky) modes. We show that by fabricating PhC slabs with optimized dimensions from silicon nanocrystals (SiNCs) active layers, the extraction efficiency of vertical light emission from SiNCs at a particular wavelength can be enhanced ∼ 11 times compared to that of uncorrugated SiNCs-rich layer. More importantly, increased light emission can be obtained in a broad spectral range and, simultaneously, the extracted light can stay confined within relatively narrow angle around the normal to the sample plane. We demonstrate experimentally and theoretically that the physical origin of the enhancement is such that light originating from SiNCs first couples to leaky modes of the PhCs and is then efficiently extracted into the surrounding.

Unique adjustable UC luminescence pattern and directional radiation of peculiar-shaped NaYF4: Yb3+/Er3+ microcrystal particle

A new design of peculiar-shaped β-NaYF4: 20% Yb3+/2% Er3+ hexagonal microcrystal (PSβHM) is proposed and its upconversion (UC) luminescence with adjustable color pattern is studied with near infrared excitation. Flower-like green UC luminescence emission pattern from blooming to withering process and intensive directional red emission are achieved by simply adjusting the focal point position of the excitation light. The mechanism that determines the unique UC luminescence phenomena are investigated systematically. The function of the internal light reflection and waveguide effect with annular microcavity is explored based on the structure characteristic of the hexagonal microplate. The current work may have great significant and potential applications in the development of optoelectronic device, color display, directional light and laser emission of microsystem.

Beaming and enhanced transmission through a subwavelength aperture via epsilon-near-zero media

We numerically validate and experimentally realize considerable funneling of electromagnetic energy through a subwavelength aperture that is covered with an epsilon-near-zero metamaterial (ENZ). The epsilon-near-zero metamaterial is composed of two layers of metasurfaces and operates at microwave frequencies. We demonstrate that the presence of the metamaterial at the inner and outer sides of the aperture not only lead to a significant enhancement in light transmission, but also cause a directional emission of light extracting from this hybrid system. In addition to these experimental results, we theoretically demonstrate the same concept in mid-IR region for a subwavelength gold aperture with indium tin oxide as an epsilon-near-zero material. Moreover, we found that using a dielectric spacer in-between the sunwavelength aperture and the ENZ medium, it is possible to red-shift the enhancement/directional frequency of the system.

An optical multimode fiber as pseudothermal light source

We report on a novel pseudothermal light source based on laser light coupled into an optical multimode fiber. The setup is simple, of low cost, exhibits inherently high directional light emission and allows for a flexible arrangement. By measuring the photon statistics and spatial two point intensity correlations in the far field we show that the setup exhibits all characteristics of a gaussian random source.

Directional emission from dye-functionalized plasmonic DNA superlattice microcavities

Three-dimensional plasmonic superlattice microcavities, made from programmable atom equivalents comprising gold nanoparticles functionalized with DNA, are used as a testbed to study directional light emission. DNA-guided nanoparticle colloidal crystallization allows for the formation of micrometer-scale single-crystal body-centered cubic gold nanoparticle superlattices, with dye molecules coupled to the DNA strands that link the particles together, in the form of a rhombic dodecahedron. Encapsulation in silica allows one to create robust architectures with the plasmonically active particles and dye molecules fixed in space. At the micrometer scale, the anisotropic rhombic dodecahedron crystal habit couples with photonic modes to give directional light emission. At the nanoscale, the interaction between the dye dipoles and surface plasmons can be finely tuned by coupling the dye molecules to specific sites of the DNA particle-linker strands, thereby modulating dye-nanoparticle distance (three different positions are studied). The ability to control dye position with subnanometer precision allows one to systematically tune plasmon-excition interaction strength and decay lifetime, the results of which have been supported by electrodynamics calculations that span length scales from nanometers to micrometers. The unique ability to control surface plasmon/exciton interactions within such superlattice microcavities will catalyze studies involving quantum optics, plasmon laser physics, strong coupling, and nonlinear phenomena.

Hyperbolic metamaterials for dispersion-assisted directional light emission.

A novel method is presented to outcouple high spatial frequency (large-k) waves from hyperbolic metamaterials (HMMs) without the use of a grating. This approach relies exclusively on dispersion engineering, and enables preferential power extraction from the top or from the side of a HMM. Multilayer (ML) HMMs are shown to be better suited for lateral outcoupling, while nanowire HMMs are the most convenient choice for top outcoupling. A 6-fold increase in laterally extracted power is predicted for a dipole-HMM system with a Ag/Si ML operating at λ = 530 nm, when metallic filling ratio is changed from an unoptimized to the optimized one. This new design concept supports the cost-effective mass production of high-speed HMM optical transmitters.

Inverse problem for light emission from weakly deformed microdisk cavities

Microdisk cavities with a smoothly deformed boundary allow for highly directional light emission. The conventional direct problem is to determine the far-field emission pattern of an optical mode for a given cavity shape by solving Maxwell's equations. We introduce the inverse problem of finding the cavity shape for which the far-field intensity pattern of the optical mode best fits a desired profile. We find an optimal solution within a second-order perturbation theory for weakly deformed cavities with a mirror reflection symmetry. Surprisingly, in this case the optimal solution is unique for specified mode numbers and refractive index. The perturbative results are confirmed by full numerical simulations.

Optics of an individual organic molecular mesowire waveguide: directional light emission and anomalous refractive index

We report on experimental investigations performed on an isolated organic mesowire waveguide resting on a glass substrate. The waveguide was made of diaminoanthraquinone (DAAQ) molecular aggregates. First, we show directional emission of light from distal ends of the DAAQ waveguide. For a given mesowire geometry, operating in passive or photoluminescence regimes, we quantified the emission angles by combining multi-wavelength Fourier-plane optical microscopy and photoluminescence micro-spectroscopy. We found light emission in the photoluminescence regime to be more directional in nature compared to the passive waveguiding regime, which was supported by three-dimensional finite-difference time-domain (FDTD) simulations. Second, we measured the anomalous behaviour of refractive index as a function of emission wavelength using the spectra of directionally emitted light. Third, by using spatial-filtered collection optics, we observed and quantified single-excitation dual-channel directional, active emission from DAAQ mesowire. The results discussed herein has implication not only in understanding some fundamental aspects of exciton-polariton mediated directional light emission, but also in applications such as organic optical antennas and photonic couplers.

Directional Light Extinction and Emission in a Metasurface of Tilted Plasmonic Nanopillars.

Plasmonic optical antennas and metamaterials with an ability to boost light-matter interactions for particular incidence or emission angles could find widespread use in solar harvesting, biophotonics, and in improving photon source performance at optical frequencies. However, directional plasmonic structures have generally large footprints or require complicated geometries and costly nanofabrication technologies. Here, we present a directional metasurface realized by breaking the out-of-plane symmetry of its individual elements: tilted subwavelength plasmonic gold nanopillars. Directionality is caused by the complex charge oscillation induced in each individual nanopillar, which essentially acts as a tilted dipole above a dielectric interface. The metasurface is homogeneous over a macroscopic area and it is fabricated by a combination of facile colloidal lithography and off-normal metal deposition. Fluorescence excitation and emission from dye molecules deposited on the metasurface is enhanced in specific directions determined by the tilt angle of the nanopillars. We envisage that these directional metasurfaces can be used as cost-effective substrates for surface-enhanced spectroscopies and a variety of nanophotonic applications.

Coexistence of Scattering Enhancement and Suppression by Plasmonic Cavity Modes in Loaded Dimer Gap-Antennas.

Plasmonic nanoantenna is of promising applications in optical sensing and detection, enhancement of optical nonlinear effect, surface optical spectroscopy, photoemission, etc. Here we show that in a carefully-designed dimer gap-antenna made by two metallic nanorods, the longitudinal plasmon antenna mode (AM) of bonding dipoles can compete with the transverse plasmonic cavity modes (CMs), yielding dramatically enhanced or suppressed scattering efficiency, depending on the CMs symmetry characteristics. More specifically, it is demonstrated that an appropriately loaded gap layer enables substantial excitation of toroidal moment and its strong interaction with the AM dipole moment, resulting in Fano- or electromagnetically induced transparency (EIT)-like profile in the scattering spectrum. However, for CMs with nonzero azimuthal number, the spectrum features a cumulative signature of the respective AM and CM resonances. We supply both detailed near-field and far-field analysis, showing that the modal overlap and phase relationship between the fundamental moments of different order play a crucial role. Finally, we show that the resonance bands of the AM and CMs can be tuned by adjusting the geometry parameters and the permittivity of the load. Our results may be useful in plasmonic cloaking, spin-polarized directional light emission, ultra-sensitive optical sensing, and plasmon-mediated photoluminescence.

Directional Light Emission in a Single NaYF4 Microcrystal via Photon Upconversion

Directional Light Emission in a Single NaYF₄ Microcrystal via Photon Upconversion

Enhanced transmission and beaming of light from a photonic crystal waveguide via two collimation lenses

Directional emission of light passing through a subwavelength photonic crystal waveguide (PCW) with the help of two collimation lenses is researched. The collimation lens is another photonic crystal with a self-collimation effect. Four different configurations of PCW and collimation lens are studied by the finite-difference time-domain (FDTD) method. Simulation results show that this collimation lens method can not only achieve a high transmission and low divergence light beam but also provide a wide band width of Δω/ω=5% (where, ω is the angular frequency), which is better than the mechanism of interference of surface modes.

Stand-off biodetection with free-space coupled asymmetric microsphere cavities.

Asymmetric microsphere resonant cavities (ARCs) allow for free-space coupling to high quality (Q) whispering gallery modes (WGMs) while exhibiting highly directional light emission, enabling WGM resonance measurements in the far-field. These remarkable characteristics make “stand-off” biodetection in which no coupling device is required in near-field contact with the resonator possible. Here we show asymmetric microsphere resonators fabricated from optical fibers which support dynamical tunneling to excite high-Q WGMs, and demonstrate free-space coupling to modes in an aqueous environment. We characterize the directional emission by fluorescence imaging, demonstrate coupled mode effects due to free space coupling by dynamical tunneling, and detect adsorption kinetics of a protein in aqueous solution. Based on our approach, new, more robust WGM biodetection schemes involving microfluidics and in-vivo measurements can be designed.

Whispering gallery mode emission from a composite system of J-aggregates and photonic microcavity

We report on development and characterization of Whispering Gallery Modes spherical microcavities integrated with organic dye molecules in a J-aggregate state. The microcavities are studied using micro-photoluminescence spectroscopy, and fluorescence lifetime imaging confocal microscopy. Directional emission of light from the microcavity is also experimentally demonstrated and attributed to the photonic jets generated in the microsphere.

Pump-Controlled Directional Light Emission from Random Lasers

The angular emission pattern of a random laser is typically very irregular and difficult to tune. Here we show by detailed numerical calculations that one can overcome the lack of control over this emission pattern by actively shaping the spatial pump distribution. We demonstrate, in particular, how to obtain customized pump profiles to achieve highly directional emission. Going beyond the regime of strongly scattering media where localized modes with a given directionality can simply be selected by the pump, we present an optimization-based approach which shapes extended lasing modes in the weakly scattering regime according to any predetermined emission pattern.

Fabrication and Characterization of Light-Emitting Diodes Comprising Highly Ordered Arrays of Emissive InGaN/GaN Nanorods

A simple approach to fabricating ordered InGaN/GaN nanorod arrays light-emitting diodes (LEDs) with strongly directional light emission is reported. The far field radiation pattern of the nanorod arrays LEDs shows preferential emission in a ±10° cone about the surface normal and contained other features arising from diffraction associated with the periodicity of the nanorod array. The output power per unit in-plane emissive area at 300-mA drive current is three times larger than that of an equivalent planar LED.

Directional Emission from Plasmonic Yagi–Uda Antennas Probed by Angle-Resolved Cathodoluminescence Spectroscopy

Optical nanoantennas mediate optical coupling between single emitters and the far field, making both light emission and reception more effective. Probing the response of a nanoantenna as a function of position requires accurate positioning of a subwavelength sized emitter with known orientation. Here we present a novel experimental technique that uses a high-energy electron beam as broad band point dipole source of visible radiation, to study the emission properties of a Yagi-Uda antenna composed of a linear array of Au nanoparticles. We show angle-resolved emission spectra for different wavelengths and find evidence for directional emission of light that depends strongly on where the antenna is excited. We demonstrate that the experimental results can be explained by a coupled point dipole model which includes the effect of the dielectric substrate. This work establishes angle-resolved cathodoluminescence spectroscopy as a powerful technique tool to characterize single optical nanoantennas.

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.

Controlling the Directional Emission of Light by Periodic Arrays of Heterostructured Semiconductor Nanowires

We demonstrate experimentally the directional emission of light by InAsP segments embedded in InP nanowires. The nanowires are arranged in a periodic array, forming a 2D photonic crystal slab. The directionality of the emission is interpreted in terms of the preferential decay of the photoexcited nanowires and the InAsP segments into Bloch modes of the periodic structure. By simulating the emission of arrays of nanowires with the emitting segments located at different heights, we conclude that the position of this active region strongly influences the directionality and efficiency of the emission. Our results will help to improve the design of nanowire based LEDs and single photon sources.