Far-field Emission Patterns Research Articles

Achieving Ideal Magnetic Light Emission with Electric-Type Emitters.

Optical magnetic dipole (MD) emission predominantly relies on emitters with significant MD transitions, which, however, rarely exist in nature. Here, we propose a strategy to transform electric dipole (ED) emission to a magnetic one by elegantly coupling an ED emitter to a silicon nanoparticle exhibiting a strong MD resonance. This emission mode transformation enables an artificially ideal magnetic dipole source with an MD purity factor of up to 99%. The far-field emission patterns of such artificial MD sources were experimentally measured, which unambiguously resolved their magnetic-type emission origin. This study opens the path to achieving ideal magnetic dipole emission with nonmagnetic emitters, largely extending the availability of magnetic light emitters conventionally limited by nature. Beyond the fundamental significance in science, we anticipate that this study will also facilitate the development of magnetic optical nanosource and enable potential photonic applications relying on magnetic light emission.

Individually adjustable emission from a quantum emitter embedded in a double bowtie-bullseye plasmonic nanoantenna

Single-photon sources have a wide range of applications in fields such as quantum computing and quantum communications. These applications require single-photon sources to possess both high emission directionality and a high Purcell factor. However, currently, there is no satisfactory solution that can meet both of these characteristics. Here, we present a novel hybrid plasmonic nanoantenna that addresses this challenge. The hybrid nanoantenna comprises a bullseye antenna with a double bowtie antenna placed at its center. This design enables the simultaneous achievement of high directionality and a high Purcell factor. Specifically, the Purcell factor can reach up to 10,000, and the emitted photons are predominantly concentrated in the 0° direction in the far-field emission pattern. To characterize the hybrid nanoantenna and investigate the influence of its dimensions on directionality and Purcell factor, we conducted Finite-Difference Time-Domain (FDTD) simulations. The results demonstrate promising findings for enhancing the performance of plasmonic antennas. Moreover, this platform holds potential for the development of efficient single-photon sources, thereby facilitating advancements in photonic integrated circuits and quantum devices.

Numerical analysis of photon absorption of gate-defined quantum dots embedded in asymmetric bull’s-eye optical cavities

Improving the photon–spin conversion efficiency without polarization dependence is a major challenge in realizing quantum interfaces gate-defined quantum dots (QDs) for polarization-encoded photonic quantum network systems. Previously, we reported the design of an air-bridge bull’s-eye cavity that enhances the photon absorption efficiency of an embedded gate-defined QD regardless of the photon polarization. Here, we numerically demonstrate that a further 1.6 times improvement in efficiency is possible by simply adjusting the distance of the substrate from the semiconductor slab where the bull’s-eye structure is formed. Our analysis clarifies that the upward-preferred coupling and narrow far-field emission pattern realized by substrate-induced asymmetry enable the improvement.

Cavity Mode-Matching InGaN Aperture-Emitting Device with a Nanoporous GaN Reflector via Ion Implantation

Cavity mode-matching InGaN resonant-cavity light-emitting diodes (RC-LEDs) with aperture size-dependent emission have been demonstrated through nitrogen ion (N+) implantation and electrochemical wet etching processes. The RC-LED structure consisted of 570 nm-depth ion-implanted regions and an embedded 17-pair nanoporous-GaN/n-GaN distributed Bragg reflector (DBR) outside the aperture, while there existed a full 20-pair DBR structure within the aperture regions. Shorter and mode-matching cavity lengths were fabricated, confirmed by transmission electron microscopy and angle-resolved photoluminescence spectra inside the aperture area without subjecting to the N+ implantation. By reducing the aperture sizes, higher current density and narrower far-field electroluminescence emission patterns were observed in the RC-LED with an aperture size of 40 μm-diameter. These results demonstrate that the cavity mode-matching RC-LEDs with optical/electrical confinements and controllable cavity length mismatch features can be applied in a wide range of optoelectronic-based device applications.

Multimode Lasing in All-Solution-Processed UV-Nanoimprinted Distributed Feedback MAPbI3 Perovskite Waveguides

In this work, we present an all-solution fabrication approach for external second-order 1D distributed feedback (DFB) gratings using soft UV-nanoimprint lithography (UV-NIL) above archetypical methylammonium lead iodide (MAPbI3) perovskite films. This high-throughput method can be carried out in an ambient environment and requires only slightly elevated temperatures as low as 70 °C, gentle imprint pressure, and the use of compatible UV-NIL resin. Under stripe-shaped optical excitation, we observe simultaneously occurring optical phenomena in our high-gain strong-scattering perovskite films, namely amplified spontaneous emission, random lasing, and 1D DFB lasing. In pursuit of distinguishing these mechanisms, we explore far-field emission patterns and output polarization. Additionally, the DFB lasing is hardly attenuated when a thin absorbing indium tin oxide (ITO) film, commonly used as an electrode in fully contacted electrical devices, is inserted between the perovskite film and the DFB grating. As a result, we reproducibly achieve single and multimode, low-threshold (below 100 μJ·cm–2), narrow linewidth (below 0.2 nm), and strongly polarized (extinction ratio above 50) optically pumped DFB lasing for MAPbI3 waveguides with and without an adjacent ITO layer. We believe that the proposed resonator integration approach can be extended toward complete electrically active devices, enabling an alternative integration scheme to achieve current-injection lasing.

High brightness and broad modulation bandwidth InGaN-based red micro-LEDs integrated with plasmonic gratings.

We propose red micro-LEDs integrated with plasmonic gratings, which demonstrate high efficiency and broad modulation bandwidth. The Purcell factor and external quantum efficiency (EQE) for an individual device can be improved up to 5.1 and 11%, respectively, due to the strong coupling between surface plasmons and multiple quantum wells. The cross talk effect between adjacent micro-LEDs can be efficiently alleviated as well, thanks to the high-divergence far-field emission pattern. Moreover, the 3-dB modulation bandwidth of the designed red micro-LEDs is predicted to be ∼ 528 MHz. Our results can be used to design high-efficiency and high-speed micro-LEDs for the applications of advanced light display and visible light communication.

Light Efficiency Investigation of Ultraviolet Light-Emitting Diodes Combined With Polarized Emission and Packaging Structure

Ultraviolet light-emitting diodes (UV-LEDs) have attracted extensive applications in resin curing, medical treatment, biochemical inspection, and disinfection. However, the UV-LEDs suffer from the main bottleneck of low light efficiency, which depends on the polarized emission and packaging structure of UV-LEDs simultaneously. In this work, we investigated the light efficiency of UV-LEDs combined with polarized emission and packaging structure. Four types of UV-LEDs (UVC-A, UVC-B, UVB, and UVC) without a lens (UV-Ref.), with a flat lens (UV-FL), and with a hemispherical lens (UV-HL) were fabricated, respectively. Through investigating the light output power, far-field emission pattern, and electrical field intensity distribution of UV-LEDs, the UV-HL packaging structure displays the highest emission intensity and light output power in the whole UV band. Compared with the UVC-A-Ref. and UVC-A-FL structures, the enhancement ratios of the light output power of UVC-A-HL structure are 28.9% and 46.5%, respectively. Furthermore, the light extraction efficiency of the TM-polarized mode in the UVC-A-HL structure is enhanced by 29.6% compared with the UVC-A-Ref. structure. The results indicate that the light extraction of UV-LEDs is greatly affected by the polarized emission and packaging structure. Our work opens an efficient guidance to enhance the light efficiency of various UV-LEDs with appropriate packaging structure.

Farfield pattern and guided-mode extraction analysis for highly directional emission from photonic-crystal based AlGaInP/InGaP MQW color-converters in thin-film geometry

This paper provides novel design guidelines for highly directional emission from PhC-based AlGaInP/InGaP MQW color converters (CC) in a thin-film geometry through an in-depth analysis of the measured azimuthal and spectrally resolved farfield emission patterns and a modal analysis based on coupled-mode theory and a herein-developed model for quantifying the spontaneous emission distribution between guided modes. The fabricated CC can exhibit directionality up to ∼5 times higher than that of Lambertian emitters close to normal incidence. We believe that all the novel insights set through our analyses help in properly controlling the emission directionality from photonic-crystal-based MQW CC at the display level, which would eliminate the need for additional external optics. This could pave the way for the use of MQW CC to achieve compact full-color microdisplays on a single wafer.

Experimental optimization of the fiber coupling efficiency of GaAs quantum dot-based photon sources

We present an efficient experimental method to optimize the combined extraction efficiencies and the far-field emission patterns of solid state-based single and entangled photon pair sources for efficient coupling to single mode fibers. This method is demonstrated for emitters based on droplet etched GaAs quantum dot nanomembranes attached to gallium phosphide solid immersion lenses using an adhesive layer of poly(methyl methacrylate). By varying the thickness of the latter, the optimization of both the extraction efficiency and the far-field emission pattern for single mode fiber coupling is facilitated. The applied method of far-field characterization is validated by benchmarking it against direct measurements of the single mode fiber coupling efficiency. Using this scheme, devices with a more than 150-fold enhanced free-space intensity compared to an unprocessed sample as well as a fiber coupling efficiency of 64% are achieved. In addition, the optimized device has been employed for on-demand generation of maximally entanglement photon pairs using two-photon excitation of the quantum dot bi-exciton exciton cascade. This universal approach for experimental optimization can be applied to other photonic nanostructures, including circular Bragg grating and micropillar cavities as well as monolithic microlenses.

Experimental demonstration of mode selection in bridge-coupled metallo-dielectric nanolasers.

We experimentally demonstrate bridge-coupled metallo-dielectric nanolasers that can operate in the in-phase or out-of-phase locking modes at room temperature. By varying the length of the bridge, we show that the coupling coefficients can be realized in support of the stable operation of any of these two modes. Both coupled nanolaser designs have been fabricated and characterized for experimental validation. Their lasing behavior has been confirmed by the spectral evolution, light-in light-out characterizations, and emission linewidth narrowing. The operating mode is identified from the near-field and far-field emission pattern measurements. To the best of our knowledge, this is the first demonstration of mode selection in bridge-coupled metallo-dielectric nanolasers, which can serve as building blocks in nanolaser arrays for applications in imaging, virtual reality devices, and lidars.

Optimization of optical waveguide antennas for directive emission of light

Optical traveling wave antennas offer unique opportunities to control and selectively guide light into a specific direction, which renders them excellent candidates for optical communication and sensing. These applications require state-of-the-art engineering to reach optimized functionalities such as high directivity and radiation efficiency, low sidelobe levels, broadband and tunable capabilities, and compact design. In this work, we report on the numerical optimization of the directivity of optical traveling wave antennas made from low-loss dielectric materials using full-wave numerical simulations in conjunction with the particle swarm optimization algorithm. The antennas are composed of a reflector and a director deposited on a glass substrate, and an emitter placed in the feed gap between them serves as an internal source of excitation. In particular, we analyze antennas with rectangular- and horn-shaped directors made of either hafnium dioxide or silicon. The optimized antennas produce highly directional emissions due to the presence of two dominant guided TE modes in the director in addition to leaky modes. These guided modes dominate the far-field emission pattern and govern the direction of the main lobe emission, which predominately originates from the end facet of the director. Our work also provides a comprehensive analysis of the modes, radiation patterns, parametric influences, and bandwidths of the antennas, which highlights their robust nature.

On-chip unidirectional micro-nano-light sources based on multi-mode cesium lead halide perovskite nanowires

Unidirectional micro-nano-light sources have been widely applied in integrated photonics and optoelectronic devices. In this study, on-chip all-inorganic cesium lead halide perovskite nanowires are employed and excited as a bright light source to generate unidirectional light. Here, we directly image the angular emission patterns of perovskite nanowires by using fluorescence Fourier microscopy. The extinction ratio for the unidirectional emission is achieved up to 11 dB, and the detected minimum polar divergence angle reaches 8.6° in experiments. The emission direction also can be tuned flexibly within the range of 48° by changing the upper surrounding medium. Detailed calculations based on multi-mode expansion theory are used to reveal the inner physical mechanisms. Further analysis of the far-field emission patterns corresponds well with the experiment results. Our works investigating the perovskite nanowire light source emission from the perspective of Fourier space have potential applications in photon-detection and integrated photonics chips.

Simultaneous high Q/V-ratio and optimized far-field emission pattern in diamond slot-bridge nanobeam cavity

We present a design for a slot-bridge nanobeam cavity that supports ultrahigh Q/V and optimized far-field emission pattern, and operates at the zero-phonon transition (637nm) of the nitrogen-vacancy color centers in diamond. The slot-bridge cavity is formed by introducing a diamond bridge at the center of the air region of a slotted cavity. As a result of the boundary condition on the parallel component of the electric field, the electric field in the bridge region is the same as that in the slot. This results in a further reduction of the mode volume. Optimized slot-bridge cavities with a calculated Q factor of 6×106 and mode volume of Veff=0.28(λ/n)3have been realized. In comparison to nanobeam cavities, the mode volume is reduced by a factor of 139. The far-field pattern of a slot-bridge cavity is optimized by using the band folding method and by adding small air-slot extensions to the two closest air-holes surrounding the cavity. A high Q of 9.8×105 and a small mode volume of Veff=0.011(λ/n)3 is realized for this design. Therefore, this type of cavities would allow for the realization of efficient single-photon sources. Our results are important for fundamental studies, like cavity quantum electrodynamics, as well as applications in quantum information processing based on diamond where high quality-factor and ultrasmall mode volume are required.

Dielectric travelling wave antennas for directional light emission.

We present a combined experimental and numerical study of the far-field emission properties of optical travelling wave antennas made from low-loss dielectric materials. The antennas considered here are composed of two simple building blocks, a director and a reflector, deposited on a glass substrate. Colloidal quantum dots placed in the feed gap between the two elements serve as internal light source. The emission profile of the antenna is mainly formed by the director while the reflector suppresses backward emission. Systematic studies of the director dimensions as well as variation of antenna material show that the effective refractive index of the director primarily governs the far-field emission pattern. Below cut off, i.e., if the director's effective refractive index is smaller than the refractive index of the substrate, the main lobe results from leaky wave emission along the director. In contrast, if the director supports a guided mode, the emission predominately originates from the end facet of the director.

Birefringent whispering gallery cavities designed by linear transformation optics.

It was reported that whispering gallery cavities designed by conformal transformation optics can support high-Q resonant modes with emission directionality. Intrinsically, these cavities have gradient index profiles implementing conformal mappings in physical space. In this paper, using the linear coordinate transformation, we propose another design scheme of whispering gallery cavities with (piecewise-) homogeneous, anisotropic index profile. We numerically show that so-designed cavities are also able to support high-Q whispering gallery modes with directional far-field emission patterns. We verify such characteristics by using a phase space representation (called the Poincaré Husimi function) of the intracavity wave function.

Unidirectional emission and nanoparticle detection in a deformed circular square resonator.

We propose a novel deformed square resonator which has four asymmetric circular sides. Photons leak out from specific points, depending on the interplay between stable islands and unstable manifolds in phase space. By carefully breaking the mirror reflection symmetry, optical modes with strong chirality approaching 1 and unidirectional emission can be achieved simultaneously. Upon binding of a nanoparticle, the far-field emission pattern of the deformed microcavity changes drastically. Due to the EP point of the degenerate mode pairs in the deformed cavity, chirality-based far-field detection of nanoparticles with ultra-small size can be realized.

Experimental investigation of the far-field emission pattern of microdisk laser modes

We studied a far-field emission pattern for microlasers with InGaAs/GaAs quantum well-dots in the active region. Angular-resolved electroluminescence spectra measurement revealed various far-field patterns depending on current and resonance mode.

Weakly deformed optical microdisks: A third-order perturbation theory for transverse-magnetic modes

In the past years weakly deformed optical microdisks have become a focus for fundamental and applied research with lots of interesting new findings. A commonly used method to study such cavities is a perturbation theory based on weak boundary deformations (Dubertrand et al 2008 Phys. Rev. A 77, 013 804). In this paper we extent the perturbation theory to the third order which allows us to improve its accuracy significantly. We discuss various example systems in regard of Q-spoiling, frequency splitting, and far-field emission pattern. The results from the perturbation theory are in a very good agreement to full numerical simulations.

Doubly resonant second-harmonic generation of a vortex beam from a bound state in the continuum

Second-harmonic generation in nonlinear materials can be greatly enhanced by realizing doubly resonant cavities with high quality factors. However, fulfilling such doubly resonant condition in photonic crystal (PhC) slab cavities is a long-standing challenge, because of the difficulty in engineering photonic bandgaps around both frequencies. Here, by implementing a second-harmonic bound state in the continuum (BIC) and confining it with a heterostructure design, we show the first doubly resonant PhC slab cavity with 2.4 × 10 − 2 W − 1 intrinsic conversion efficiency under continuous-wave excitation. We also report the confirmation of highly normal-direction concentrated far-field emission pattern with radial polarization at the second harmonic frequency. These results represent a solid verification of previous theoretical predictions and a cornerstone achievement, not only for nonlinear frequency conversion but also for vortex beam generation and prospective nonclassical sources of radiation.

Performance improvement of ultraviolet-A multiple quantum wells using a vertical oriented nanoporous GaN underlayer

Well-aligned, lateral and vertical oriented nanoporous GaN was fabricated using the electrochemical etching procedure and its influence on the optical characteristics of ultraviolet-A multiple quantum well (MQW) structure was investigated. We used a MQW structure with a V-defect and n-Al0.1Ga0.9 N layer, which greatly improved the uniformity of vertical electrochemical etching. Compared to the as-grown MQW structure, the lateral and vertical oriented nanoporous MQW structures have 3.8-fold and 8.1-fold photoluminescence intensity enhancement and the full width at half maximum has been narrowed from 18.4 nm to 7.9 nm and 2.8 nm, respectively. The vertical oriented nanoporous MQW structure has a rectangular far-field emission pattern with uniform forward light distribution and the view angle of 85% intensity is 50°. This study provides an effective method for improving the light output and controlling the emission angle of GaN based light emitting devices, as well as a method for preparing well-aligned nanopores in semiconductors.