Lambertian Emission Research Articles

A Miniaturized and Highly Sensitive "Windmill" Three-Channel Fluorescence Detector for Simultaneous Detection of Various Mycotoxins.

A novel "windmill" three-channel light-emitting diode induced fluorescence detector (LED-IF) was proposed to maximize the excitation efficiency and fluorescence collection efficiency. Compared with the typical collinear arrangement, the fluorescence intensity of the three channels was increased by 7.85, 3.88, and 2.94 times, respectively. The compact shaping optical path was designed to obtain higher excitation efficiency and a lower background stray light effect caused by high divergence angle high-power ultraviolet (UV)-LEDs simultaneously, which increased the sensitivity of three channels by 4.6 to 5.7 times. It was found that using a photodiode (PD) with a flat window and a larger photosensitive surface can collect the Lambertian emission fluorescence in the flow cell more efficiently, increasing the signal-to-noise ratio of each channel 1.3 to 1.8 times. The limits of detection (LODs, 3 times peak-peak noise) of aflatoxin B2 (AFB2), ochratoxin (OTA), and zearalenone (ZEN) were 0.33, 1.80, and 28.2 ng/L, respectively. Finally, six mycotoxins were analyzed simultaneously by the detector coupling with HPLC. The results showed that the sensitivity of the detector was at the best level to date, which was better than that of the top commercial fluorescence detectors (FLDs). The developed detector has the advantages of having small volume, low cost, and long lifetime and being robust, which has wide application and market prospects.

92‐1: Invited Paper: Wide‐Color‐Gamut and Stable MicroLED Displays Using UV‐Pumped Cd‐Free Quantum Dots

Applied Materials, Inc. has reported a novel and manufacturable microLED display architecture, featuring UV‐A microLEDs, 4‐subpixel layout, pixel isolation wells filled with inkjetted Cd‐free R/G/B quantum dots (QDs), and a blanket UV blocker on top. A 1.37” smart‐watch active‐matrix display prototype was assembled using such method. Our QDs, including blue QD and the matrix materials exhibit excellent stability under UV excitation, as verified by accelerated lifetime test under higher UV flux and elevated temperature. The emission properties of our R/G/B QDs was further optimized, paving a path to achieve 99% DCI‐P3 color space coverage. Our display demonstrates a highly uniform Lambertian emission pattern with minimal color shift at high viewing angles, attributing to all emissions originating from QD embedded in the same polymer matrix. Furthermore, we can eliminate the QD photon‐brightening effect and demonstrate display with high thermal stability up to 90⁰C.

Study on Optical Positioning Using Experimental Visible Light Communication System

Visible light positioning systems (VLP) have attracted significant commercial and research interest because of the many advantages they possess over other applications such as radio frequency (RF) positioning systems. In this work, an experimental configuration of an indoor VLP system based on the well-known Lambertian light emission, is investigated. The corresponding results are also presented, and show that the system retains high enough accuracy to be operational, even in cases of low transmitted power and high background noise.

Surface lattice resonances for beaming and outcoupling green μ LEDs emission

Abstract Light-Emitting Diodes (LEDs) exhibit a typical Lambertian emission, raising the need for secondary optics to tailor their emission depending on specific applications. Here, we introduce plasmonic metasurfaces to InGaN green emitting quantum wells for LEDs to control their far-field emission directionality and enhance the collection efficiency. The proposed mechanism is based on surface lattice resonances (SLRs) and relies on the near-field coupling between the InGaN multiple quantum wells (MQWs) and periodic arrays of aluminum (Al) nanodisks. Fourier microscopy measurements reveal that the angular photoluminescence emission pattern depends on the lattice constant of the metasurfaces. We demonstrate that integrating Al metasurfaces in LED wafers can enhance the collected outcoupled light intensity by a factor of 5 compared to the same sample without metasurfaces. We have also performed numerical calculations of the far-field emission based on the reciprocity principle and obtained a very good agreement with the experimental data. The proposed approach controls the emission directionality without the need for secondary optics and it does not require post-etching of the GaN, which makes it a potential candidate to control and enhance the generated light from micro-LEDs.

Enhanced light extraction for red organic light emitting diodes via PDMS convex lens based on breath figure method

To inhibit the light loss of substrate mode at the substrate/air interface in organic light emitting diodes (OLEDs), regular porous polystyrene (PS) template was fabricated via the “Breath figure” method. The structure of porous mold was optimized by adjusting the PS solution concentration from 10 to 60 mg/ml and relative humidity from 60% to 80%. The optimal porous PS mold was achieved at 50 mg/ml PS solution under 70% relative humidity. Then, elastic micro-lens arrays based on transparent elastomer polydimethylsiloxane (PDMS) were obtained via a two-step transferring process from the porous PS mold. The projection experiment exhibits excellent focusing effect of the PDMS convex lens relative to the planar PDMS film. These PDMS micro-lens films were in situ fabricated onto the external face of the red OLED device to extract substrate mode light. By using the PDMS micro-lens array with a height of 1.2 μm and a diameter of 6.7 μm, the OLED device demonstrates an enhancement of 32.7%, 35.6%, 46.5%, and 27.6% for maximum luminance, current efficiency, power efficiency, and external quantum efficiency, respectively, without changing its color purity. The device with PDMS micro-lens exhibits a higher intensity than an ideal Lambertian emission pattern, especially at viewing angles from 30° to 60°. It suggests that the introduction of external micro-lens structure significantly improves the light outcoupling efficiency.

Unveiling light collection and pump enhancement from quantum wells with plasmonic metasurfaces using power dependent measurements

Low light extraction efficiency (LEE) is the greatest limiting factor for the brightness of reduced-size light-emitting diodes (LEDs) as it limits their emission intensity. In addition, LEDs have a Lambertian emission, which requires secondary optics to control the emission directionality. Plasmonic metasurfaces can introduce a way of manipulating the generated light from LEDs to enhance their LEE and steer the emitted light by reshaping the far-field emission. Here, we fabricate resonant plasmonic metasurfaces on top of a typical blue emitting wafer consisting of InGaN/gallium nitride quantum wells developed for commercial LED devices. The metasurface is separated from the InGaN quantum wells by p-GaN and indium-tin-oxide (ITO) layers with a cumulative thickness of 110 nm. Since this distance value is close to the emission wavelength in the corresponding medium, enhanced near-fields of localized plasmonic resonances do not reach the active region. Despite this, we observe a strong influence of the metasurfaces on the far-field photoluminescence emission from the quantum wells as demonstrated by Fourier imaging. Power-dependent excitation measurements of the samples allow us to retrieve the pump and light collection enhancement factors provided by the plasmonic metasurfaces. We demonstrate that the plasmonic metasurfaces can provide a pump enhancement factor of up to 4.1 and a collection enhancement factor of up to 3.2. We have also performed simulations based on the reciprocity principle and achieved a good qualitative agreement with the experimental results.

Visible Light Positioning With Lens Compensation for Non-Lambertian Emission

With greater demands for cost-effective, reliable, and highly accurate positioning, indoor wireless localisation using Visible Light Positioning (VLP) is a promising solution for future networks. One can expect VLP solutions to appear in all environments, from homes to industry; however, the existing literature primarily considers Visible Light Communication (VLC) sources with purely Lambertian emission patterns. To facilitate greater versatility within VLP solutions, this paper considers non-Lambertian sources. It evaluates practical Received Signal Strength Indicator (RSSI) data obtained during the Internet of Radio Light (IoRL) 5G Measurement Campaign conducted in a home environment using non-Lambertian Total Internal Reflection (TIR) lenses, which produce a halo lighting effect. The initial analysis explores the calibration of Lambertian source parameters against datasheet values leading to reductions in the average Positioning Error (PE) of 17% and 3% for averaged and individual RSSI measurement sets, respectively. While this highlights improvements from correct calibration, the Lambertian model proved to be unsuitable for non-Lambertian sources. In the absence of any existing non-Lambertian models, the authors proposed the Halo Lens Compensation (HLC) method to calibrate the considered non-Lambertian TIR sources correctly. The HLC further reduced PE in the calibrated results by 50% and 39%, with mean PE of 3.1 cm and 4.6 cm for averaged and individual RSSI measurement sets, respectively. In conclusion, for VLP using non-Lambertian sources, the existing Lambertian model is unsuitable. However, the proposed HLC is highly effective and achieves positioning accuracy comparable to existing literature using Lambertian sources.

Highly transparent and stretchable organic light-emitting diodes with ultrathin metal films as double electrodes

Organic light-emitting diodes (OLEDs) with both high transparency and stretchability are of importance for next-generation wearable and deformable displays, which have not been studied in depth. In this work, highly flexible and transparent OLEDs (TrOLEDs) are achieved by using ultrathin metal films as both anode and cathode to fabricate stretchable TrOLEDs (S-TrOLEDs) based on a wrinkling strategy. The TrOLEDs show a transmittance value of 73% at 550 nm and a negligible micro-cavity effect with near Lambertian emission characteristics at a wide angle of view. Furthermore, almost symmetric electroluminescent properties are obtained from the anode and cathode, which is important for dual-side display. Two-dimensional S-TrOLEDs with random wrinkles show 50% stretchability and retain 80% of their initial luminance after 200 cycles of stretching. The effect of random wrinkles on the transmittance of S-TrOLEDs is researched and the transparency losses of the device are analyzed deeply. We hope that this work is meaningful for promoting the development of S-TrOLEDs in the display industries.

Resonance‐Based Directional Light Emission from Organic Light‐Emitting Diodes: Comparing Integrated Nanopatterns and Color Conversion Waveguide Gratings

Tailoring the angular emission pattern of organic light‐emitting diodes (OLEDs) is a promising approach to increase device performance in many applications. In miniaturized point‐of‐need sensor systems using organic light sources and photodetectors, directional illumination of specific sensing spots may enhance overall sensitivity by improved light utilization. Periodically nanopatterned waveguides forming photonic crystal slabs are oftentimes employed to obtain directional resonant light outcoupling from OLEDs. This work compares emission directionality for two types of devices utilizing the same polymer compounds as the emissive material: OLEDs comprising a nanopatterned bottom electrode and conventional OLEDs featuring a dedicated nanopatterned color conversion layer (CCL) for directional light outcoupling. Simulated and experimentally measured emission characteristics show that resonant outcoupling effects from a separate CCL are significantly stronger due to spatial separation from the lossy electrode layers as well as high optical contrast at the waveguide interfaces. While OLEDs with nanopatterned electrodes exhibit only small deviations from the Lambertian emission profile, the nanopatterned CCL induces high outcoupling peaks at specific viewing angles leading to increased emission directionality.

Outage and error performance analysis of dual hop hybrid RF-VLC system with wireless energy harvesting

The presented paper, investigates a dual hop hybrid radio frequency-visible light communication (RF-VLC) system with wireless energy harvesting at the relay. The information signal is transmitted via RF channel from the source (S) (located at some distant location) towards the destination (D). We assumed that there is no direct connectivity between S and D, thus a decode and forward relay (R) is used as an intermediate node between S and D. The source transmits the information signal towards the relay via RF channel. The received information signal at the relay is processed and converted into corresponding optical signal. The optical information signal is then forwarded to the destination. Parallelly, the relay employs the power splitting relaying (PSR) protocol with harvest-then cooperate (HTC) protocol for energy harvesting. The harvested energy is utilized to forward the optical information signal from relay to the user at the destination located inside a building’s room via VLC channel. We assumed that the RF and the VLC channels are modelled by generalized-K fading distribution and lambertian emission model respectively. The statistical characteristics are derived for the instantaneous signal to noise ratio of the individual channels using PSR protocol. Further, these statistics are used to derive the accurate closed form expressions for end to end outage probability and average bit error rate. Moreover, with the help of numerical results, end to end performance of the hybrid RF-VLC system under the influence of various system parameters such as shadowing parameters, power splitting factor, field of view angle of the detector and the distance between the source and the relay is analysed and verified.

Island-like random scattering layer for light extraction in organic light-emitting diodes with enhanced luminous efficiencies and angular spectral stability

Optical outcoupling efficiency of organic light-emitting diode (OLED) can be improved using nanoscale or sub-micrometer quasi-periodic structures. An embossed nano/micro-structure inserted between the substrate and anode is known to be effective in extracting the waveguide mode. However, the techniques to fabricate these nanoscale or sub-micrometer structures are usually complicated, and the light extraction is wavelength-oriented. Herein, we develop a simple process to enhance the performance of OLED. We texture the glass surface with shrinking Ag droplets inthe form of island structures and then deposit SiNx planarization film to form an island-like random scattering layer (IRSL), which changes the light path of emitting light in a way to reduce light attenuation and extract emission outward. Furthermore, IRSL has a low haze and a high optical transparency, both of which are beneficial to light output. The OLEDs constructed with IRSL as an internal light extraction structure exhibit a significant improvement of current and power efficiencies by 52% and 62%, respectively, compared to the planar OLED device. In addition, compared to the conventional pillar-structured RSL, the IRSL not only facilitates the extraction of light for a broad range of visible light region, but also favors better angular spectral stability and Lambertian emission.

Vertical semiconductor deep ultraviolet light emitting diodes on a nanowire-assisted aluminum nitride buffer layer

Vertical light-emitting diodes (LEDs) have many advantages such as uniform current injection, excellent scalability of the chip size, and simple packaging process. Hitherto, however, technologically important semiconductor aluminum gallium nitride (AlGaN) deep ultraviolet (UV) LEDs are mainly through lateral injection. Herein, we demonstrate a new and practical path for vertical AlGaN deep UV LEDs, which exploits a thin AlN buffer layer formed on a nanowire-based template on silicon (Si). Such a buffer layer enables in situ formation of vertical AlGaN deep UV LEDs on Si. Near Lambertian emission pattern is measured from the top surface. The decent reflectivity of Si in the deep UV range makes such a configuration a viable low-cost solution for vertical AlGaN deep UV LEDs. More importantly, the use of such a thin AlN buffer layer can allow an easy transfer of device structures to other carrier wafers for vertical AlGaN deep UV LEDs with ultimately high electrical and optical performance.

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.

Multichannel optogenetics combined with laminar recordings for ultra-controlled neuronal interrogation

Simultaneous large-scale recordings and optogenetic interventions may hold the key to deciphering the fast-paced and multifaceted dialogue between neurons that sustains brain function. Here we have taken advantage of thin, cell-sized, optical fibers for minimally invasive optogenetics and flexible implantations. We describe a simple procedure for making those fibers side-emitting with a Lambertian emission distribution. Here we combined those fibers with silicon probes to achieve high-quality recordings and ultrafast multichannel optogenetic inhibition. Furthermore, we developed a multi-channel optical commutator and general-purpose patch-cord for flexible experiments. We demonstrate that our framework allows to conduct simultaneous laminar recordings and multifiber stimulations, 3D optogenetic stimulation, connectivity inference, and behavioral quantification in freely moving animals. Our framework paves the way for large-scale photo tagging and controlled interrogation of rapid neuronal communication in any combination of brain areas.

Throughput analysis of dual hop hybrid RF-VLC system with wireless energy harvesting

Abstract In this paper, we study a dual hop hybrid radio frequency-visible light communication (RF-VLC) system with wireless energy harvesting technique. In the proposed model the RF channel and the VLC channel are modelled by generalized-K distribution and lambertian emission model, respectively. A decode and forward relay is used as an intermediate node where power splitting relaying (PSR) protocol is employed to harvest energy from the received RF information signal. The source transmits the information signal towards the relay via RF channel. The relay then decodes the received signal and convert it into corresponding light signal. Simultaneously the energy harvesting circuit at relay harvests the energy from the received RF signal. The relay utilizes the harvested energy to aid the transmission of the light signal towards the user situated inside the building’s room at the destination. Further, the statistics of the instantaneous SNR of the hybrid RF-VLC system at the destination are derived by employing the PSR protocol to obtain an accurate closed form analytical expression for the system’s end-to-end ergodic capacity and achievable throughput in terms of Meijer’s G function. Further, with the help of numerical simulations, we analysed the end-to-end ergodic throughput performance of the hybrid system under the influence of varying channel parameters such as signal to noise ratio, power splitting factor, shadowing parameters, distance between source & relay and source transmitted power.

High‐Efficiency Top‐Emitting Green Perovskite Light Emitting Diode with Quasi Lambertian Emission

AbstractMetal halide perovskite light‐emitting diodes (PeLEDs) are regarded as alternative candidates for next‐generation display technologies due to their high efficiency, superior color purity, tunable bandgap. However, the research on transparent and top‐emitting PeLED with Lambertian emission profile significantly lags behind due to optical microcavity effect, which has become one of the main obstacles for potential practical display applications. Here, strategies are developed to suppress the microcavity effect by enhancing the transmission of the semitransparent electrode and extending the optical cavity length. Besides, a nanopatterned structure is incorporated to suppress the surface plasma and waveguide mode effect. Based on the above combination strategies, a transparent PeLED yields a record total external quantum efficiency (EQE) of 16.1% with Lambertian emission by depositing high refractive index molybdenum oxide on the semitransparent cathode. Besides, a high EQE of 13.6% of a top‐emitting device with quasi‐Lambertian emission is achieved by stretching the optical distance between two reflective layers and enhancing the transmission of the semitransparent electrode. The nanopatterned structure enables suppressed waveguide mode and surface plasmon polariton dilapidation of top‐emitting PeLEDs. This work paves a path to design transparent and top‐emitting PeLEDs for potential applications in both traditional and novel transparent displays.

Lambertian Extraction of Light from Organic Light-Emitting Devices Using Randomly Dispersed Sub-Wavelength Pillar Arrays.

A simple light extraction method is reported to minimize the variation in emission spectrum with viewing angle by embedding a light extraction layer, consists of randomly dispersed nano-pillars on a substrate. Nano-imprint lithography technique was employed to fabricate the nano-pillars using ultra-violet curable acrylate. An organic light emitting diode device employing the imprint technology showed 46% enhancement in light extraction efficiency in the forward direction with Lambertian emission pattern and no color change with viewing angle. Moreover, the emitted light does not show any specific periodic patterns.

Physical layer security analysis of a dual-hop hybrid RF-VLC system

Abstract In this paper, we investigate the secrecy performance of a dual-hop hybrid radio frequency–visible light communication (RF–VLC) system in which we consider a single source antenna (S) transmitting data signals via RF channel through decode and forward (DF) relay (R) toward an authorized receiving antenna (D) at the destination via VLC channel. In this system, we assume that the eavesdropper (E) attempts to eavesdrop the RF channel (S,R) during transmission of the signal. The relay decodes the incoming RF signal into corresponding optical signal and retransmits toward destination via VLC channel (R,D). The RF and VLC channels of the hybrid system are modelled using generalized-K fading distribution and generalized Lambertian radiant intensity distribution respectively. Also the receiver is assumed to be mobile and its mobility is defined by random waypoint (RWP) model. The channels statistics are used to derive the exact analytical expressions for average secrecy capacity and intercept probability in terms of Meijer’s G-function. The derived closed form analytical expressions are the efficient tool to illustrate the impact of variation in different channel parameters on the secrecy performance, namely, shadowing parameters of RF channel, field of view (FOV), semiangle, and the order of Lambertian emission of the VLC channel. Further, numerical analysis is carried out to support the mathematical analysis of the RF–VLC hybrid system.

An Accurate Ranging Algorithm Based on Received Signal Strength in Visible Light Communication

In this paper, the line of sight (LOS) propagation model in visible light communication for a receiver with tilting plane is precisely derived. In addition, a relatively simple and reasonably accurate model of LOS propagation is proposed to employ received signal strength algorithm. Then, the maximum likelihood estimator is applied in range estimation where a new shot noise (RSN) source as a function of received signal power, is considered. Moreover, the Cramer-Rao lower bound is derived in closed form by considering RSN and tilting plane. Next, the influence of the Lambertian emission order on the channel model is investigated, theoretically and the optimum value of the Lambertian emission order is presented. Finally, performance is evaluated numerically.

Realistic performance analysis for visible light communication with random receivers.

In previous studies, the receivers in indoor visible light communication (VLC) were usually fixed. However, in reality, the receivers in VLC have random locations and orientations. Therefore, it is important to consider these random factors for performance analysis in VLC. In this paper, we consider a typical VLC system with a fixed transmitter and a random receiver. Two types of receivers are investigated: (1) those with random location and (2) those with random orientation. Based on the established system model, the statistical characteristics of the channel were obtained, and closed-form expressions of the average channel capacity and the outage probability were derived, respectively. Finally, numerical results verified the accuracy of derived theoretical expressions. Moreover, the effects of the nominal optical intensity, the dimming target, the transmitter height, the receiver zone's radius, the outage threshold, and the Lambertian emission order on system performance were also provided.