Bandwidth limits of luminescent solar concentrators as detectors in free-space optical communication systems

Optimizing Horticulture Luminescent Solar Concentrators via Enhanced Diffuse Emission Enabled by Micro-Cone Arrays

Optimizing the photon spectrum for photosynthesis concurrently with improving crop yields presents an efficient and sustainable pathway to alleviate global food shortages. Luminescent solar concentrators (LSCs), consisting of transparent host matrices doped with fluorophores, show excellent promise to achieve the desired spectral tailoring. However, conventional LSCs are predominantly engineered for photon concentration, which results in a limited outcoupling efficiency of converted photons. Here, we introduce a scheme to implement LSCs into horticulture (HLSC) by enhancing light extraction. The symmetry of the device is disrupted by incorporating microcone arrays on the bottom surface to mitigate total internal reflection. Both Monte Carlo ray tracing simulations and experimental results have verified that the greatest enhancements in converted light extraction, relative to planar LSCs, are achieved using microcone arrays (base width 50 μm, aspect ratio 1.2) with extruded and protruded profiles (85.15 and 66.55% improvement, respectively). Angularly resolved transmission measurements show that the HLSC device exhibits a broad angular radiation distribution. This characteristic indicates that the HLSC device emits diffuse light, which is conducive to optimal plant growth.

Fluorescent antenna based on Förster resonance energy transfer (FRET) for optical wireless communications

The use of fluorescent antennas in optical wireless communications (OWC) has been demonstrated previously, and it has been shown that it is an efficient method for enhancing receiver performance, providing both signal gain and a wide field of view (FoV). To achieve a high concentration gain at the receiver output, the selected fluorophores should have a high photoluminescence quantum yield (PLQY), limited overlap between their absorption and emission spectra, and emit light that can be efficiently detected. In addition, to support a high modulation bandwidth, the photoluminescence (PL) lifetime of the fluorophore needs to be short. In this paper, we propose a new fluorescent antenna architecture based on Förster resonance energy transfer (FRET). Our results show that, due to the photophysical interactions between the energy donor and energy acceptor, the use of FRET simultaneously increases PLQY and reduces PL lifetime. Additionally, employing FRET leads to an increased Stokes shift, ensuring that the emitted light has longer wavelengths, thus reducing self-absorption. This shift can also increase the efficiency with which the fluorescence is detected by a typical silicon (Si) photodetector. Consequently, our OWC results show that a new FRET-based antenna can achieve a significantly higher concentration gain and a wider transmission bandwidth than a conventional non-FRET antenna, leading to much higher data rates.

Multifunctional difluoroboron β-diketonate-based luminescent receiver for a high-speed underwater wireless optical communication system

The last decade has witnessed considerable progress in underwater wireless optical communication in complex environments, particularly in exploring the deep sea. However, it is difficult to maintain a precise point-to-point reception at all times due to severe turbulence in actual situations. To facilitate efficient data transmission, the color-conversion technique offers a paradigm shift in large-area and omnidirectional light detection, which can effectively alleviate the étendue limit by decoupling the field of view and optical gain. In this work, we investigated a series of difluoroboron β-diketonate fluorophores by measuring their photophysical properties and optical wireless communication performances. The emission colors were tuned from blue to green, and >0.5 Gb/s data transmission was achieved with individual color channel in free space by implementing an orthogonal frequency-division multiplexing (OFDM) modulation scheme. In the underwater experiment, the fluorophore with the highest transmission speed was fabricated into a 4×4 cm2 luminescent concentrator, with the concentrated emission from the edges coupled with an optical fiber array, for large-area photodetection and optical beam tracking. The net data rates of 130 Mb/s and 217 Mb/s were achieved based on nonreturn- to-zero on-off keying and OFDM modulation schemes, respectively. Further, the same device was used to demonstrate the linear light beam tracking function with high accuracy, which is beneficial for sustaining a reliable and stable connection in a dynamic, turbulent underwater environment.

Wide-field-of-view optical detectors for deep ultraviolet light communication using all-inorganic CsPbBr3 perovskite nanocrystals

Optical wireless communication (OWC) links suffer from strict requirements of pointing, acquisition, and tracking (PAT) between the transmitter and receiver. Extending the narrow field-of-view (FoV) of conventional light-focusing elements at the receiver side can relax the PAT requirements. Herein, we use all-inorganic CsPbBr3 nanocrystals (NCs) to extend various optical concentrators' FOV to 60°, regardless of the original FOV values of the concentrators. Given the robustness of UV light against communication channel misalignment, the used CsPbBr3 NCs provide another advantage of converting transmitted UVC light into a green color that matches the peak absorption of the widely available Si-based detectors. We evaluated the feasibility of the reported wide FoV optical detectors by including them in deep UV OWC systems, deploying non-return-to-zero on-off keying (NRZ-OOK) and orthogonal-frequency division multiplexing (OFDM) modulation schemes. The NRZ-OOK and OFDM schemes exhibit stable communication over the 60° FoV, providing data transmission rates of 100 Mb/s and 71.6 Mb/s, respectively, a unique capability to the reported design.

Three-dimensional echo light field analysis for dual-band laser active detection of a cat-eye optical system

Laser active detection technology utilizing the cat-eye effect provides rapid response, precise positioning, and long detection distances. However, current research mainly focuses on active detection within a single visible or near-infrared band, lacking quantitative analyses of the echo spot. In this paper, a four-interval theoretical model for dual band cat-eye target echo detection was constructed using matrix optics theory and Collins diffraction integration method. Dual-band echo detection experiments were conducted using 10.6 um far-infrared waves and 532 nm visible light waves, also the power, radius, and target-missing quantities of the echo spots were collected and quantitatively compared with the theoretical results. Results indicate that, due to the diffraction limit's effect on the distribution of the echo field, the echo power of far-infrared band detection is smaller than that of visible light band detection. The impact on the light spot caused by the positive and negative defocus values is asymmetric, with positive defocus having a lower impact on the echo spot than negative defocus at the same value. A weak positive defocus value that minimizes the radius of the echo spot and maximizes the echo power exists, with the value of weak positive defocus varying between detection bands. A linear relationship exists between the incident angle of the detection laser and the deviation of the echo spot. These findings provide a foundation for extracting working band details, predicting the motion trajectory of moving cat-eye targets, and achieving real-time tracking and detection recognition during laser active detection.

Capillary-based fluorescent antenna for visible light communications

The use of fluorescent optical antennas in visible light communications (VLC) systems can enhance their performance by selectively absorbing light from the transmitter and concentrating the resulting fluorescence, whilst preserving a wide field of view. In this paper, we introduce a new and flexible way of creating fluorescent optical antennas. This new antenna structure is a glass capillary which is filled with a mixture of epoxy and a fluorophore before the epoxy is cured. Using this structure, an antenna can be easily and efficiently coupled to a typical photodiode. Consequently, the leakage of photons from the antenna can be significantly reduced when compared to previous antennas created using microscope slides. Moreover, the process of creating the antenna is simple enough for the performance of antennas containing different fluorophores to be compared. In particular, this flexibility has been used to compare VLC systems that incorporate optical antennas containing three different organic fluorescent materials, Coumarin 504 (Cm504), Coumarin 6 (Cm6), and 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), when a white light-emitting diode (LED) is used as the transmitter. Results show that, since it only absorbs light emitted from the gallium nitride (GaN) LED, a fluorophore that hasn't previously been used in a VLC system, Cm504, can result in a significantly higher modulation bandwidth. In addition, the bit error rate (BER) performance at different orthogonal frequency-division multiplexing (OFDM) data rates of antennas containing different fluorophores is reported. These experiments show for the first time that the best choice of fluorophore depends on the illuminance at the receiver. In particular, when the illuminance is low, the overall performance of the system is dominated by the signal-to-noise ratio (SNR). Under these conditions, the fluorophore with the highest signal gain is the best choice. In contrast, when the illuminance is high, the achievable data rate is determined by the bandwidth of the system and therefore the fluorophore that results in the highest bandwidth is the best choice.

Haynes-Shockley experiment analogs in surface and optoelectronics: Tunable surface electric field extracting nearly all photocarriers

Photocarriers predominantly recombine at semiconductor surfaces and interfaces, assuming high bulk carrier lifetime. Consequently, understanding the extraction of photocarriers via surfaces is critical to optoelectronics. Here, we propose Haynes-Shockley experiment analogs to investigate photocarrier surface extraction. A Schottky junction is used to tune the silicon near-surface electric field strength that varies over several orders of magnitude and simultaneously observe variations in broadband photocarrier extraction. Schottky barrier height and surface potential are both modulated. Work function tunable indium tin oxide (ITO) is developed to precisely regulate the barrier height and collect photocarriers at 0 V bias, thus avoiding the photocurrent gain effect. All experiments demonstrate >98% broadband internal quantum efficiency. The experiments are further extended to wave interference photonic crystals and random pyramids, paving a way to estimate the photogeneration rate of diverse surface light-trapping topologies by collecting nearly all photocarriers. The insights reported here provide a systematic experimental basis to investigate interfacial effects on photocarrier spatial generation and collection.

Enhancing UV photodetection performance of an individual ZnO microwire p-n homojunction via interfacial engineering

As a typical broad bandgap semiconductor, ZnO has received considerable attention for developing optoelectronic devices in ultraviolet wavelengths, but suffers from a lack of high-quality single-crystalline p-type ZnO. Herein, we report the realization of a homojunction ultraviolet photodetector, which involves a p-type Sb-doped ZnO microwire (ZnO:Sb MW) and n-type ZnO layer. The p-type conductivity of the as-synthesized ZnO:Sb MWs was evidenced using an individual wire field-effect transistor. Due to its good rectifying ability and excellent photovoltaic effect, the constructed p-ZnO:Sb MW/n-ZnO homojunction is able to work as an ultraviolet photodetector in self-biased and reversely biased manners. By appropriately engineering the band alignment of the p-ZnO:Sb/n-ZnO homojunction via a MgO interface modification layer, the optimized photodetector exhibits performance-enhanced ultraviolet detection capabilities, such as the light on/off ratio reaching up to 1.6 × 10⁸, responsivity of over 267 mA W^-1 and specific detectivity of approximately 1.2 × 10¹⁴ Jones upon 365 nm light illumination at 0 V. The detector also produces faster response with rise/recovery times of 102 μs/3.6 ms. This study not only employed a novel method to synthesize genuine p-type ZnO with excellent stability and reproducibility, but also opened up substantial opportunities for developing high-performance ZnO homojunction optoelectronic devices.

Study of using different colors of fluorescent fibers as optical antennas in white LED based-visible light communications

A fluorescent fiber can be used as an optical antenna in visible light communication (VLC) for simultaneous optical filtering and light concentration and therefore to build a compact receiver. Since its light concentration principle is based on fluorescence, it can exceed the étendue limit and achieve both a high concentration gain and a wide field-of-view (FOV). In addition, because the photoluminescence (PL) lifetime of the fluorophore is typically only several nanoseconds, it can support high-speed data transmissions. When a fluorescent fiber antenna is used in a white light-emitting diode (LED)-based VLC system, the transmission performance highly depends on how the absorption and emission spectra of the fluorophore are associated with both the spectrum of the LED and the responsivity of the silicon photodetector. In this paper, we analyze the performance of several different commercially available fluorescent fibers. We show that, when the data rate is low or the transmission distance is long, since the light emitted from a red fluorescent fiber is associated with high silicon responsivities, it can result in high signal-to-noise ratios (SNRs) at the receiver output and therefore lead to low transmission error rates. In contrast, when the data rate is high or the transmission distance is relatively short, the bandwidth dominates the overall performance and consequently the green fluorescent fiber has better performance since it only absorbs the light emitted from the blue LED rather than the light emitted from the yellow phosphor.

Micro-cone arrays enhance outcoupling efficiency in horticulture luminescent solar concentrators

Luminescent solar concentrators (LSCs) have shown the ability to realize spectral conversion, which could tailor the solar spectrum to better match photosynthesis requirements. However, conventional LSCs are designed to trap, rather than extract, spectrally converted light. Here, we propose an effective method for improving outcoupling efficiency based on protruded and extruded micro-cone arrays patterned on the bottom surface of LSCs. Using Monte Carlo ray tracing, we estimate a maximum external quantum efficiency (EQE) of 37.73% for our horticulture LSC (HLSC), corresponding to 53.78% improvement relative to conventional, planar LSCs. Additionally, structured HLSCs provide diffuse light, which is beneficial for plant growth. Our micro-patterned surfaces provide a solution to light trapping in LSCs and a foundation for the practical application of HLSCs.

Effects of Nanoscale Morphology on Optical Properties of Photoluminescent Polymer Optical Fibers

Bicomponent photoluminescent polymer optical fibers (PL-POFs) have been melt-spun and in-situ drawn to different extents. The results suggest that scattering in the sheath can effectively increase the photoluminescent dye excitation probability in the fiber core. The core/sheath PL-POFs are made of a semi-crystalline fluoropolymer sheath of low refractive index (RI) and an amorphous cycloolefin polymeric core of high RI, which is doped with a luminescent dye. The axial light emission, as well as the guiding attenuation coefficients of the core/sheath PL-POFs, have been measured using a side-illumination set-up. The incident blue laser is down-converted to red light, which is re-emitted and partially guided by the core. The axial light emission is measured at the fiber tip as a function of the distance of the illumination position to the integrating sphere. It is demonstrated that the presence of a semi-crystalline sheath significantly enhances the axial light emission and that it also lowers the attenuation coefficient, compared to the emission and guiding properties of PL core-only fibers. Additionally, the attenuation coefficient has been found to be lower in more strongly drawn PL-POFs. Wide-angle X-ray diffraction and small-angle X-ray scattering experiments reveal structural differences in differently drawn PL-POFs that can be linked to the observed differences in the optical properties.

A Superimposed QD-Based Optical Antenna for VLC: White LED Source

Visible light communication (VLC) is a versatile enabling technology for following high-speed wireless communication because of its broad unlicensed spectrum. In this perspective, white light-emitting diodes (LED) provide both illumination and data transmission simultaneously. To accomplish a VLC system, receiver antennas play a crucial role in receiving light signals and guiding them toward a photodetector to be converted into electrical signals. This paper demonstrates an optical receiver antenna based on luminescent solar concentrator (LSC) technology to exceed the conservation of etendue and reach a high signal-to-noise ratio. This optical antenna is compatible with all colors of LEDs and achieves an optical efficiency of 3.75%, which is considerably higher than the similar reported antenna. This antenna is fast due to the small attached photodetector—small enough that it can be adapted for electronic devices—which does not need any tracking system. Moreover, numerical simulation is performed using a Monte Carlo ray-tracing model, and results are extracted in the spectral domain. Finally, the fate of each photon and the chromaticity diagram of the collected photons’ spectra are specified.

Luminescent concentrators enable highly efficient and broadband photodetection

With luminescent concentrators, the high quantum yield luminescence emitted by embedded chromophores, featuring a broad absorption spectrum, can be well-tuned to match the peak response of integrated photodetectors. This integration can substantially enhance the device photoresponse all the way from deep UV to near-IR.

Total Internal Reflection Lens for Optical Wireless Receivers

This work considers the use of a freeform total internal reflection (TIR) lens for optical concentration and provides for the first time experimental results in the context of optical wireless communications (OWC). The lens is placed on a surface-mounted device (SMD) avalanche photodiode (APD) to minimize position tolerances and simplify assembly. The lens achieves a concentration ratio of go = 44.7 (16.5 dB) within the FOV center and exhibits an acceptance angle of ±5°. The TIR lens approach is validated by comparing eye diagrams and bit error ratios (BER) of a receiver with and without a TIR lens. For the measurements, non-return-to-zero (NRZ) on-off keying (OOK) signals are transmitted with a data rate of 1.289 Gbit/s.

Metal-Organic Frameworks in Mixed-Matrix Membranes for High-Speed Visible-Light Communication

Mixed-matrix membranes (MMMs) based on luminescent metal-organic frameworks (MOFs) and emissive polymers with the combination of their unique advantages have great potential in separation science, sensing, and light-harvesting applications. Here, we demonstrate MMMs for the field of high-speed visible-light communication (VLC) using a very efficient energy transfer strategy at the interface between a MOF and an emissive polymer. Our steady-state and ultrafast time-resolved experiments, supported by high-level density functional theory calculations, revealed that efficient and ultrafast energy transfer from the luminescent MOF to the luminescent polymer can be achieved. The resultant MMMs exhibited an excellent modulation bandwidth of around 80 MHz, which is higher than those of most well-established color-converting phosphors commonly used for optical wireless communication. Interestingly, we found that the efficient energy transfer further improved the light communication data rate from 132 Mb/s of the pure polymer to 215 Mb/s of MMMs. This finding not only showcases the promise of the MMMs for high-speed VLC but also highlights the importance of an efficient and ultrafast energy transfer strategy for the advancement of data rates of optical wireless communication.

A novel approach for designing efficient broadband photodetectors expanding from deep ultraviolet to near infrared

Broadband photodetection (PD) covering the deep ultraviolet to near-infrared (200-1000 nm) range is significant and desirable for various optoelectronic designs. Herein, we employ ultraviolet (UV) luminescent concentrators (LC), iodine-based perovskite quantum dots (PQDs), and organic bulk heterojunction (BHJ) as the UV, visible, and near-infrared (NIR) photosensitive layers, respectively, to construct a broadband heterojunction PD. Firstly, experimental and theoretical results reveal that optoelectronic properties and stability of CsPbI₃ PQDs are significantly improved through Er³⁺ doping, owing to the reduced defect density, improved charge mobility, increased formation energy, tolerance factor, etc. The narrow bandgap of CsPbI₃:Er³⁺ PQDs serves as a visible photosensitive layer of PD. Secondly, considering the matchable energy bandgap, the BHJ (BTP-4Cl: PBDB-TF) is selected as to NIR absorption layer to fabricate the hybrid structure with CsPbI₃:Er³⁺ PQDs. Thirdly, UV LC converts the UV light (200-400 nm) to visible light (400-700 nm), which is further absorbed by CsPbI₃:Er³⁺ PQDs. In contrast with other perovskites PDs and commercial Si PDs, our PD presents a relatively wide response range and high detectivity especially in UV and NIR regions (two orders of magnitude increase that of commercial Si PDs). Furthermore, the PD also demonstrates significantly enhanced air- and UV- stability, and the photocurrent of the device maintains 81.5% of the original one after 5000 cycles. This work highlights a new attempt for designing broadband PDs, which has application potential in optoelectronic devices.

A Proposal for Optical Antenna in VLC Communication Receiver System

Visible Light Communication (VLC) is an important emerging choice for high-speed wireless communication. In this perspective, light-emitting diodes as illuminators will be modulated to transmit data simultaneously. However, the receivers bring severe difficulties due to cost, response time, and sensitivity with a wide Field Of View (FOV). To avoid these problems, one approach is to apply a large area photodetector; however, this solution is slow and costly. Another method is to focus light on a fast photodetector by optical components, but the photodetector’s FOV decreases, resulting from the conservation of etendue. Another option is Luminescent Solar Concentrators (LSCs). This paper demonstrates a novel shape of LSC with advantages such as inexpensive, fast response time, small antenna area for VLC purposes with significant geometrical gain, FOV, and ultra-broad bandwidth. It does not require any complex tracking system and active pointing but, due to its tiny size, it can also be adapted in integrating and mobile devices. Numerical simulation is done using Monte-Carlo raytracing, and the results are demonstrated in the spectral domain. The optical efficiency of the proposed antenna is obtained at 1.058%, which is about 0.4% better than the efficiency levels reported in other works, and the geometric gain of the antenna is reported to be 44, which is significant.

All-inorganic halide-perovskite polymer-fiber-photodetector for high-speed optical wireless communication

The use of optical carrier frequencies will enable seamless data connection for future terrestrial and underwater internet uses and will resolve the technological gap faced by other communication modalities. However, several issues must be solved to propel this technological shift, which include the limitations in designing optical receivers with large detection areas, omnidirectionality, and high modulation bandwidth, mimicking antennas operating in the radio-frequency spectrum. To address this technological gap, herein, we demonstrate halide-perovskite-polymer-based scintillating fibers as a near-omnidirectional detection platform for several tens-to-hundreds of Mbit/s optical communication in both free space and underwater links. The incorporation of all-inorganic CsPbBr₃ nanocrystals by engineering the nanocrystal concentration in an ultraviolet-curable polymer matrix ensures a high photoluminescence quantum yield, Mega-Hertz modulation bandwidth and Mbit/s data rate suitable to be used as a high-speed fibers-based receiver. The resultant perovskite polymer-based scintillating fibers offer flexibility in terms of shape and near-omnidirectional detection features. Such fiber properties also introduce a scalable detection area which can resolve the resistance-capacitance and angle-of-acceptance limits in planar-based detectors, which conventionally impose a trade-off between the modulation bandwidth, detection area, and angle of view. A high bit rate of 23 Mbit/s and 152.5 Mbit/s was achieved using an intensity-modulated laser for non-return-to-zero on-off-keying (NRZ-OOK) modulation scheme in free-space and quadrature amplitude modulation orthogonal frequency-division multiplexing (QAM-OFDM) modulation scheme in an underwater environment, respectively. Our near-omnidirectional optical-based antenna based on perovskite-polymer-based scintillating fibers sheds light on the immense possibilities of incorporating functional nanomaterials for empowering light-based terrestrial- and underwater-internet systems.

Reflection based coupling efficiency enhancement in a fluorescent planar concentrator for an optical wireless receiver

Fluorescent planar concentrators have been proposed as optical concentrators that can have both a wide field of view and a high optical gain stemming from a large collection area for optical wireless communications. However, the fluorescent concentrators with such a large collection area often lead to a low light coupling efficiency due to the edge coupling mechanism leading to a considerable optical power loss. In this work, an analysis of the light coupling efficiency enhancement in the electrical power gain is presented. In particular, a practical method to improve the coupling efficiency by introducing edge and back reflection using Lambertian-, specular-, and retro-reflectors is presented. It is demonstrated that by choosing the optimal reflectors, the received signal strength can be improved by more than a factor of two. Also demonstrated with the proposed method is a data rate more than 1.12 Gbps with bit error rate less than 3.8 × 10^-3 using a DC-biased optical orthogonal frequency division multiplexing. This is, to the best of our knowledge, the first Gbps class demonstration using a commercial fluorescent planar concentrator.

Melt-Spun Photoluminescent Polymer Optical Fibers for Color-Tunable Textile Illumination

The increasing interest in luminescent waveguides, applied as light concentrators, sensing elements, or decorative illuminating systems, is fostering efforts to further expand their functionality. Yarns and textiles based on a combination of distinct melt-spun polymer optical fibers (POFs), doped with individual luminescent dyes, can be beneficial for such applications since they enable easy tuning of the color of emitted light. Based on the energy transfer occurring between differently dyed filaments within a yarn or textile, the collective emission properties of such assemblies are adjustable over a wide range. The presented study demonstrates this effect using multicolor, meltspun, and photoluminescent POFs to measure their superimposed photoluminescent emission spectra. By varying the concentration of luminophores in yarn and fabric composition, the overall color of the resulting photoluminescent textiles can be tailored by the recapturing of light escaping from individual POFs. The ensuing color space is a mean to address the needs of specific applications, such as decorative elements and textile illumination by UV down-conversion.