Four-color laser white illuminant demonstrating high color-rendering quality

Color Reproduction by Multi-Wavelength Bragg Diffraction of White Light

Accurate color reproduction is highly important in multiple industrial, biomedical and scientific applications. Versatile and tunable light sources with high color-rendering quality are very much in demand. In this study, we demonstrate the feasibility of multi-wavelength Bragg diffraction of light for this task. Tuning the frequencies and amplitudes of bulk acoustic waves in the birefringent crystal demonstrates high precision in setting the number, wavelengths and intensities of the monochromatic components necessary to reproduce a specific color assigned according to its coordinates in the CIE XYZ 1931 space. We assembled a setup based on multi-bandpass acousto-optic (AO) filtration of white light and verified the reproduced color balance in multiple experiments. The proposed approach delivers almost full coverage of the CIE XYZ 1931 space and facilitates building compact color reproduction systems (CRSs) for various purposes.

Fringe pattern analysis to evaluate light sources and sensors in digital photoelasticity

When experimental photoelasticity images are acquired, the spectral interaction between the light source and the sensor used affect the visual information of the fringe patterns in the produced images. Such interaction can lead to fringe patterns with an overall high quality, but also can lead to images with indistinguishable fringes, and bad stress field reconstruction. We introduce a strategy to evaluate such interaction that relies on measuring the value of four handcrafted descriptors: contrast, an image descriptor that accounts simultaneously for blur and noise, a Fourier-based descriptor to measure image quality, and image entropy. The utility of the proposed strategy was validated by measuring the selected descriptors on computational photoelasticity images, and the fringe orders achieved when evaluating the stress field, from 240 spectral configurations: 24 light sources and 10 sensors. We found that high values of the selected descriptors can be related to spectral configurations that lead to better stress field reconstruction. Overall, the results show that the selected descriptors can be useful to identify bad and good spectral interactions, which could help to design better protocols for acquiring photoelasticity images.

Flexible, Free-Standing Polymer Membranes Sensitized by CsPbX3 Nanocrystals as Gain Media for Low Threshold, Multicolor Light Amplification

Lead halide perovskite nanocrystals (NCs) are highly suitable active media for solution-processed lasers in the visible spectrum, owing to the wide tunability of their emission from blue to red via facile ion-exchange reactions. Their outstanding optical gain properties and the suppressed nonradiative recombination losses stem from their defect-tolerant nature. In this work, we demonstrate flexible waveguides combining the transparent, bioplastic, polymer cellulose acetate with green CsPbBr₃ or red-emitting CsPb(Br,I)₃ NCs in simple solution-processed architectures based on polymer-NC multilayers deposited on polymer micro-slabs. Experiments and simulations indicate that the employment of the thin, free-standing membranes results in confined electrical fields, enhanced by 2 orders of magnitude compared to identical multilayer stacks deposited on conventional, rigid quartz substrates. As a result, the polymer structures exhibit improved amplified emission characteristics under nanosecond excitation, with amplified spontaneous emission (ASE) thresholds down to ∼95 μJ cm^-2 and ∼70 μJ cm^-2 and high net modal gain up to ∼450 and ∼630 cm^-1 in the green and red parts of the spectrum, respectively. The optimized gain properties are accompanied by a notable improvement of the ASE operational stability due to the low thermal resistance of the substrate-less membranes and the intimate thermal contact between the polymer and the NCs. Their application potential is further highlighted by the membrane's ability to sustain dual-color ASE in the green and red parts of the spectrum through excitation by a single UV source, activate underwater stimulated emission, and operate as efficient white light downconverters of commercial blue LEDs, producing high-quality white light emission, 115% of the NTSC color gamut.

3D Laser Displays Based on Circularly Polarized Lasing from Cholesteric Liquid Crystal Arrays

3D laser displays play an important role in next-generation display technologies owing to the ultimate visual experience they provide. Circularly polarized (CP) laser emissions, featuring optical rotatory power and invariability under rotations, are attractive for 3D displays due to potential in enhancing contrast ratio and comfortability. However, the lack of pixelated self-emissive CP microlaser arrays as display panels hinders the implementation of 3D laser displays. Here, full-color 3D laser displays are demonstrated based on CP lasing with inkjet-printed cholesteric liquid crystal (CLC) arrays as display panels. Individual CP lasers are realized by embedding fluorescent dyes into CLCs with their left-/right-handed helical superstructures serving as distributed feedback microcavities, bringing in ultrahigh circular polarization degree values (g_em  = 1.6). These CP microlaser pixels exhibit excellent far-field color-rendering features and a relatively large color gamut for high-fidelity displays. With these printed CLC red-green-blue (RGB) microlaser arrays serving as display panels, proof-of-concept full-color 3D laser displays are demonstrated via delivering images with orthogonal CP laser emissions into one's left and right eyes. These results provide valuable enlightenment for the development of 3D laser displays.

Laser regulation for variable color temperature lighting with low energy consumption by microlens arrays

The construction of a smart city puts forward new requirements for lighting systems, such as variable color temperature adapting to environment and low energy consumption. We introduce a variable color temperature laser lighting system that produces uniform light with minimum energy. The color temperature is controlled by tri-color RGB diode lasers, and uniform lighting is achieved by microlens arrays. Tri-color diode lasers with wavelengths of 650, 556, and 450 nm are used as the lighting sources, and the white light laser output is achieved by combining the three beams. The color temperature is controlled by changing the power ratio of each lighting source. Finally, the homogenization of laser energy is regulated by the microlens arrays, and the energy uniformity reaches 91.1%. Moreover, we do an experiment to compare LED street lighting and laser street lighting, finding that the street lighting system with this design can increase the energy utilization rate by 113.33%, and the color temperature of the car headlamps with this design can be changed according to the environment. Therefore, this laser lighting system is an effective solution for modern smart lighting systems and energy saving, which have vast application.

The reduction of the thermal quenching effect in laser-excited phosphor converters using highly thermally conductive hBN particles

Phosphor converters for solid state lighting applications experience a strong thermal stress under high-excitation power densities. The recent interest in laser diode based lighting has made this issue even more severe. This research presents an effective approach to reduce the thermal quenching effect and damage of laser-excited phosphor-silicone converters using thermally conductive hexagonal boron nitride (hBN) particles. Herein, the samples are analyzed by employing phosphor thermometry based on the photoluminescence decay time, and thermo-imaging techniques. The study shows that hBN particle incorporation increases the thermal conductivity of a phosphor-silicone mixture up to 5 times. It turns out, that the addition of hBN to the Eu\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{2+}$$\end{document}2+ doped chalcogenide-silicone converters can increase the top-limit excitation power density from 60 to 180 W cm\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{-2}$$\end{document}-2, thus reaching a 2.5 times higher output. Moreover, it is shown that the presence of hBN in Ce\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{3+}$$\end{document}3+ activated garnet phosphor converters, may increase the output power by up to 1.8 times and that such converters can withstand 218 W cm\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{-2}$$\end{document}-2 excitation. Besides, hBN particles are also found to enhance the stability of the converters chromaticity and luminous efficacy of radiation. This means that the addition of hBN particles into silicone-based phosphor converter media is applicable in a wide range of different areas, in particular, the ones requiring a high optical power output density.

Nanowire Waveguides and Lasers: Advances and Opportunities in Photonic Circuits

Due to their single-crystalline structures, comparatively large aspect ratios, tight optical confinement and smooth surfaces, nanowires have increasingly attracted research interests for both fundamental studies and technological applications in on-chip photonic devices. This class of nanostructures typically have cross-sections of 2~200 nm and lengths upwards of several micrometers, allowing for the bridging of the nanoscopic and macroscopic world. In particular, the lasing behaviors can be established from a nanowire resonator with positive feedback via end-facet reflection, making the nanowire a promising candidate in the next generation of optoelectronics. Consequently, versatile nanowire-based devices ranging from nanoscale coherent lasers, optical sensors, waveguides, optical switching, and photonic networks have been proposed and experimentally demonstrated in the past decade. In this article, significant progresses in the nanowire fabrication, lasers, circuits, and devices are reviewed. First, we focus on the achievements of nanowire synthesis and introduce the basics of nanowire optics. Following the cavity configurations and mode categories, then the different light sources consisting of nanowires are presented. Next, we review the recent progress and current status of functional nanowire devices. Finally, we offer our perspective of nanowires regarding their challenges and future opportunities in photonic circuits.

Optically Pumped Lasing in Microscale Light-Emitting Electrochemical Cell Arrays for Multicolor Displays

Laser displays, which offer wide achievable color gamut and excellent color rendering, have emerged as a promising next-generation display technology. Constructing display panels composed of pixelated microlaser arrays is of great significance for the actualization of laser displays in the flat-panel sector. Here, we report microscale light-emitting electrochemical cell (LEC) arrays that operate as both optically pumped lasers and electroluminescence devices, which can be applied as self-emissive panels for high quality displays. Optically pumped red, green, and blue laser emissions were achieved in individual circular microcells consisting of corresponding conjugated polymers and electrolytes, suggesting that the microstructures can act as resonators for coherent outputs. As-prepared microstructures possess a narrowed recombination region, which dramatically increases the current density by 3 orders of magnitude under pulsed operation, compared with the corresponding thin-film devices, representing a promising solution-processed device platform for electrical pumping. Under programmable electrical excitation, both static and dynamic displays were demonstrated with such microscale LEC arrays as display panels. The prominent performance of the demonstrated structures (microlaser arrays embedded in LEC devices) provide us deep insight into the concepts and device constructions of electrically driven laser displays.

Real-Time Sensing and Control of Integrative Horticultural Lighting Systems

Optical radiation, including light, plays a crucial role in the structural development of plants through photomorphogenesis and the response to environmental changes. However, plant sensitivity to optical radiation widely varies across species. While research efforts are currently underway to discover the fundamentals of plant physiology, light sources with preprogrammed light settings (light recipes) are offered to clients to expedite plant growth. Since horticultural lighting research is in its infancy, prescribed lighting conditions are not likely to address every plants’ needs in terms of the spatial and spectral distribution, intensity, and duration of the light sources. However, it is possible to imagine an intelligent horticultural lighting system that can diagnose plants through sensors, and adjust the light intensity, the spatial and spectral distribution for the specific plant species with active feedback. Such an advanced real-time horticultural lighting system would consist of sensors to detect physiological markers from plants and environmental factors and an artificial intelligence algorithm to adjust the output. While the underlying technology for a real-time optimization system exists, the implementation and training would require further research.

Stable Yellow Light-Emitting Devices Based on Ternary Copper Halides with Broadband Emissive Self-Trapped Excitons

Great successes have been achieved in developing perovskite light-emitting devices (LEDs) with blue, green, red, and near-infrared emissions. However, as key optoelectronic devices, yellow-colored perovskite LEDs remain challenging, mainly due to the inevitable halide separation in mixed halide perovskites under high bias, causing undesired color change of devices. In addition to this color-missing problem, the intrinsic toxicity and poor stability of conventional lead-halide perovskites also restrict their practical applications. We herein report the fabrication of stable yellow LEDs based on a ternary copper halide CsCu₂I₃, addressing the color instability and toxicity issues facing current perovskite yellow LED's compromise. Joint experiment-theory characterizations indicate that the yellow electroluminescence originates from the broadband emission of self-trapped excitons centered at 550 nm as well as the comparable and reasonably low carrier effective masses favorable for charge transport. With a maximum luminance of 47.5 cd/m² and an external quantum efficiency of 0.17%, the fabricated yellow LEDs exhibit a long half-lifetime of 5.2 h at 25 °C and still function properly at 60 °C with a half-lifetime of 2.2 h, which benefits from the superior resistance of CsCu₂I₃ to heat, moisture, and oxidation in ambient environmental conditions. The results obtained promise the copper halides with broadband light emission as an environment-friendly and stable yellow emitter for the LEDs compatible with practical applications.

Transmitter and receiver technologies for optical wireless

Providing a reliable link, with sufficient signal-to-noise ratio (SNR) and bandwidth to deliver high-capacity communications is a critical challenge for optical wireless (OW) communications and understanding and jointly optimizing the performance of the transmitter and receiver subsystems is a key part of this. At the transmitter a source of light, either a laser or a light-emitting diode, must be modulated with the communications signal. The resulting emission must be directed, using optics or steering systems, as required for the particular application, and must be within any safety levels set by relevant standards. The receiver is the most critical part of any optical link, as its design is a dominant factor in determining the received SNR, which determines the capacity and ultimately the utility of the link. A receiver must collect, filter and concentrate signal radiation, then detect and amplify the resulting electrical signal. This review surveys the state-of-the-art transmitter and receiver technologies. Details of design constraints are discussed, and potential future directions discussed. This article is part of the theme issue 'Optical wireless communication'.

Optimum resource allocation in optical wireless systems with energy-efficient fog and cloud architectures

Optical wireless communication (OWC) is a promising technology that can provide high data rates while supporting multiple users. The optical wireless (OW) physical layer has been researched extensively, however, less work was devoted to multiple access and how the OW front end is connected to the network. In this paper, an OWC system which employs a wavelength division multiple access (WDMA) scheme is studied, for the purpose of supporting multiple users. In addition, a cloud/fog architecture is proposed for the first time for OWC to provide processing capabilities. The cloud/fog-integrated architecture uses visible indoor light to create high data rate connections with potential mobile nodes. These OW nodes are further clustered and used as fog mini servers to provide processing services through the OW channel for other users. Additional fog-processing units are located in the room, the building, the campus and at the metro level. Further processing capabilities are provided by remote cloud sites. Two mixed-integer linear programming (MILP) models were proposed to numerically study networking and processing in OW systems. The first MILP model was developed and used to optimize resource allocation in the indoor OWC systems, in particular, the allocation of access points (APs) and wavelengths to users, while the second MILP model was developed to optimize the placement of processing tasks in the different fog and cloud nodes available. The optimization of tasks placement in the cloud/fog-integrated architecture was analysed using the MILP models. Multiple scenarios were considered where the mobile node locations were varied in the room and the amount of processing and data rate requested by each OW node was varied. The results help to identify the optimum colour and AP to use for communication for a given mobile node location and OWC system configuration, the optimum location to place processing and the impact of the network architecture. This article is part of the theme issue 'Optical wireless communication'.

Conversionless efficient and broadband laser light diffusers for high brightness illumination applications

Laser diodes are efficient light sources. However, state-of-the-art laser diode-based lighting systems rely on light-converting inorganic phosphor materials, which strongly limit the efficiency and lifetime, as well as achievable light output due to energy losses, saturation, thermal degradation, and low irradiance levels. Here, we demonstrate a macroscopically expanded, three-dimensional diffuser composed of interconnected hollow hexagonal boron nitride microtubes with nanoscopic wall-thickness, acting as an artificial solid fog, capable of withstanding ~10 times the irradiance level of remote phosphors. In contrast to phosphors, no light conversion is required as the diffuser relies solely on strong broadband (full visible range) lossless multiple light scattering events, enabled by a highly porous (>99.99%) non-absorbing nanoarchitecture, resulting in efficiencies of ~98%. This can unleash the potential of lasers for high-brightness lighting applications, such as automotive headlights, projection technology or lighting for large spaces.

Demonstration of a White Laser with V2 C MXene-Based Quantum Dots

Multicolor photoluminescence over the full visible color spectrum is critical in many modern science and techniques, such as full-color lighting, displays, biological and chemical monitoring, multiband communication, etc., but the ultimate white lasing especially on the nanoscale is still a challenge due to its exacting requirements in the balance of the gain and optical feedback at different wavelengths. Recently, 2D transition metal carbides (MXenes) have emerged, with some superior chemical, physical, and environmental properties distinguishing them from traditional 2D materials. Here, a white laser with V₂ C MXene quantum dots (MQDs) is originally demonstrated by constructing a broadband nonlinear random scattering system with enhanced gain. The excitation-dependent photoluminescence of V₂ C MQDs is enhanced by passivation and characterized, and their localized nonlinear random scattering is realized by the generation of excitation-power-dependent solvent bubbles. With the optimized excitation, the blue, green, yellow, and red light is amplified and simultaneously lased. This work not only provides a kind of promising material for white lasers, but also a design strategy of novel photonics for further applications.

Laser-line-driven phosphor-converted extended white light source with uniform illumination

We report the development of laser-driven extended white light source designed as a light sheet for general illumination. This light sheet is made of two large diffused glass plates. A Ce:YAG phosphor layer was coated and sandwiched between the two diffusing sheets. The blue laser beam was first converted into a uniform laser line using a cylindrical lens, and a laser line was made incident parallel to the edges of the designed light sheet. The blue photons are waveguided inside the glass sheet via total internal reflection and scattered from the diffused surface. Some of the blue photons are absorbed by the Ce:YAG phosphor and down-converted into yellow light of longer wavelength. The white light emanating from the diffuse surface is the combined effect of yellow light with original blue light. The developed light sheet with the combination of laser line generation and total internal reflection is a unique and low-cost method for generating white light with uniform illumination. The details of the development of the light sheet and laser line generation are described. The experimental parameters, such as correlated color temperature and color coordinates, are reported.

A Wide-Area Coverage 35 Gb/s Visible Light Communications Link for Indoor Wireless Applications

Visible Light Communications (VLC) can provide both illumination and communications and offers a means to alleviate the predicted spectrum crunch for radio-frequency wireless communications. In this paper, we report a laser diode based white-light communications link that operates over a wide area and supports high data rates. The proposed system is a four-colour multiplexed high-speed VLC system that uses a microelectromechanical system (MEMS) mirror-based beam-steering. The system operates at record data-rates of more than 35 Gb/s (Bit Error Rate(BER) < 3.8 × 10⁻³) with a coverage area of 39 m² at a link distance of 4 m. To the best of our knowledge this is the fastest VLC demonstration reported thus far. The paper also addresses issues of eye-safety, showing data rates of  more than 10 Gb/s are feasible.

Full-color laser displays based on organic printed microlaser arrays

Laser displays, which exploit characteristic advantages of lasers, represent a promising next-generation display technology based on the ultimate visual experience they provide. However, the inability to obtain pixelated laser arrays as self-emissive full-color panels hinders the application of laser displays in the flat-panel sector. Due to their excellent optoelectronic properties and processability, organic materials have great potential for the production of periodically patterned multi-color microlaser arrays. Here, we demonstrate for the first time full-color laser displays on precisely patterned organic red-green-blue (RGB) microlaser matrices through inkjet printing. Individual RGB laser pixels are realized by doping respective luminescent dyes into the ink materials, resulting in a wide achievable color gamut 45% larger than the standard RGB space. Using as-prepared microlaser arrays as full-color panels, we achieve dynamic laser displays for video playing through consecutive beam scanning. These results represent a major step towards full-color laser displays with outstanding color expression.

In the next generation of display technology for portable devices, lasers could replace LEDs to achieve more vibrant colours. Here, Zhao et al. demonstrate a dynamic full-color display in which each pixel is made up of three printed organic microlasers to cover the RGB space.

Dual-Color Lasing Lines from EMPs in Diluted Magnetic Semiconductor CdS:NiI Structure

Have one ever seen a semiconductor that can issue two-color lasing lines? The diluted magnetic semiconductor (DMS) can do this. Here, we have observed dual lasing lines of 530 nm and 789 nm from a DMS structure of CdS:NiI, in which the excitonic magnetic polaron (EMP) and localized excitonic magnetic polaron (LEMP) are excitations out of ferromagnetic (NiS) _x nanocluster and NiI₂ nanoclusters within CdS lattice; both of them could lead to the collective EMP state at high excitation and therein produce coherent emission lines simultaneously. This occurrence is due to the superposition of EMP near CdS bandedge and the combination of the charge-transfer band of (NiI) _n cluster with the LEMP within CdS lattice by overcoming the strong electron correlation of NiI cluster in a DMS structure, evidenced also by ab initio calculation. This finding opens a way to understand the collective behaviour of spin-coupled excitons in DMS and to find novel applications in the spin-related quantum technology.

Towards a tailored indoor horticulture: a functional genomics guided phenotypic approach

As indoor horticulture gathers momentum, electric (also termed artificial) lighting systems with the ability to generate specific and tunable wavelengths have been developed and applied. While the effects of light quality on plant growth and development have been studied, authoritative and reliable sets of light formulae tailored for the cultivation of economically important plants and plant traits are lacking as light qualities employed across laboratories are inconsistent. This is due, at least in part, to the lack of molecular data for plants examined under electric lights in indoor environments. It has hampered progress in the field of indoor horticulture, in particular, the transition from small-scale indoor farming to commercial plant factories. Here, we review the effects of light quality on model and crop plants studied from a physiological, physical and biochemical perspective, and explain how functional genomics can be employed in tandem to generate a wealth of molecular data specific for plants cultivated under indoor lighting. We also review the current state of lighting technologies in indoor horticulture specifically discussing how recent narrow-bandwidth lighting technologies can be tailored to cultivate economically valuable plant species and traits. Knowledge gained from a complementary phenotypic and functional genomics approach can be harvested not only for economical gains but also for sustainable food production. We believe that this review serves as a platform that guides future light-related plant research.

Coherence and speckle contrast at the output of a stationary multimode optical fiber

We relate classic coherence properties of light at the output of a multimode optical fiber excited by a spatially coherent broadband source to speckle contrast measured by two different methods. Speckle contrast measured with an external diffuser is related to the effective number of modes, while that measured over the ensemble of random bends and twists of the fiber is related to the residual coherence defined as a spatial average of the modulus of the classic complex degree of coherence between pairs of widely separated points at the fiber output.

A White Random Laser

Random laser with intrinsically uncomplicated fabrication processes, high spectral radiance, angle-free emission, and conformal onto freeform surfaces is in principle ideal for a variety of applications, ranging from lighting to identification systems. In this work, a white random laser (White-RL) with high-purity and high-stability is designed, fabricated, and demonstrated via the cost-effective materials (e.g., organic laser dyes) and simple methods (e.g., all-solution process and self-assembled structures). Notably, the wavelength, linewidth, and intensity of White-RL are nearly isotropic, nevertheless hard to be achieved in any conventional laser systems. Dynamically fine-tuning colour over a broad visible range is also feasible by on-chip integration of three free-standing monochromatic laser films with selective pumping scheme and appropriate colour balance. With these schematics, White-RL shows great potential and high application values in high-brightness illumination, full-field imaging, full-colour displays, visible-colour communications, and medical biosensing.

Wide-gamut lasing from a single organic chromophore

The development of wideband lasing media has deep implications for imaging, sensing, and display technologies. We show that a single chromophore can be engineered to feature wide-gamut fluorescence and lasing throughout the entire visible spectrum and beyond. This exceptional color tuning demonstrates a chemically controlled paradigm for light emission applications with precise color management. Achieving such extensive color control requires a molecular blueprint that yields a high quantum efficiency and a high solubility in a wide variety of liquids and solids while featuring a heterocyclic structure with good steric access to the lone pair electrons. With these requirements in mind, we designed a lasing chromophore that encloses a lasing color space twice as large as the sRGB benchmark. This record degree of color tuning can in principle be adapted to the solid state by incorporating the chromophore into polymer films. By appropriately engineering the base molecular structure, the widest range of lasing wavelengths observed for a conventional gain medium can be achieved, in turn establishing a possible route toward high-efficiency light emitters and lasers with near-perfect chromaticity.

10-m 9.51-Gb/s RGB laser diodes-based WDM underwater wireless optical communication

The availability of the underwater wireless optical communication (UWOC) based on red (R), green (G) and blue (B) lights makes the realization of the RGB wavelength division multiplexing (WDM) UWOC system possible. By properly mixing RGB lights to form white light, the WDM UWOC system has prominent potentiality for simultaneous underwater illumination and high-speed communication. In this work, for the first time, we experimentally demonstrate a 9.51-Gb/s WDM UWOC system using a red-emitting laser diode (LD), a single-mode pigtailed green-emitting LD and a multi-mode pigtailed blue-emitting LD. By employing 32-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) modulation in the demonstration, the red-light, the green-light and the blue-light LDs successfully transmit signals with the data rates of 4.17 Gb/s, 4.17 Gb/s and 1.17 Gb/s, respectively, over a 10-m underwater channel. The corresponding bit error rates (BERs) are 2.2 × 10^-3, 2.0 × 10^-3 and 2.3 × 10^-3, respectively, which are below the forward error correction (FEC) threshold of 3.8 × 10^-3.

Tricolor R/G/B Laser Diode Based Eye-Safe White Lighting Communication Beyond 8 Gbit/s

White light generation by mixing red, green, and blue laser diodes (RGB LDs) was demonstrated with Commission International de l'Eclairage coordinates of (0.2928, 0.2981), a correlated color temperature of 8382 K, and a color rendering index of 54.4 to provide a maximal illuminance of 7540 lux. All the white lights generated using RGB LDs were set within the risk group-1 criterion to avoid the blue-light hazard to human eyes. In addition, the RGB-LD mixed white light was diffused using a frosted glass to avoid optical aberration and to improve the performance of the lighting source. In addition, visible light communication (VLC) by using RGB-LD mixed white-light carriers and a point-to-point scheme over 1 m was performed in the directly modulated 16-QAM OFDM data format. In back-to-back transmission, the maximal allowable data rate at 10.8, 10.4, and 8 Gbps was determined for R, G, and B LDs, respectively. Moreover, the RGB-LD mixed white light-based indoor wavelength-division multiplexing (WDM)-VLC system yielded a total allowable transmission data rate of 8.8 Gbps over 0.5 m in free space. Such a high-speed RGB-LD mixed WDM-VLC system without any channel interference can be used to simultaneously provide data transmission and white lighting in an indoor environment.

Effect of laser speckle on light from laser diode-pumped phosphor-converted light sources

Laser diode (LD) pumped white light sources are being developed as an alternative to light-emitting diode-pumped sources for high efficiency and/or high brightness applications. While several performance metrics of laser-pumped phosphor-converted light sources have been investigated, the effect of laser speckle has not been sufficiently explored. This paper describes our experimental studies on how laser speckle affects the behavior of light from laser-excited phosphor lamps. A single LD pumping a phosphor plate was the geometry explored in this work. Overall, our findings are that the down-converted light did not exhibit any speckle, whereas speckle was present in the residual pump light but much reduced from that in direct laser light. Furthermore, a thicker coating of small-grained phosphors served to effectively reduce speckle through static pump light diffusion in the phosphor coating. Our investigations showed that speckle is not of concern in illumination from LD-pumped phosphor-converted light sources.

Growth and development of Arabidopsis thaliana under single-wavelength red and blue laser light

Indoor horticulture offers a sensible solution for sustainable food production and is becoming increasingly widespread. However, it incurs high energy and cost due to the use of artificial lighting such as high-pressure sodium lamps, fluorescent light or increasingly, the light-emitting diodes (LEDs). The energy efficiency and light quality of currently available horticultural lighting is suboptimal, and therefore less than ideal for sustainable and cost-effective large-scale plant production. Here, we demonstrate the use of high-powered single-wavelength lasers for indoor horticulture. They are highly energy-efficient and can be remotely guided to the site of plant growth, thus reducing on-site heat accumulation. Furthermore, laser beams can be tailored to match the absorption profiles of different plant species. We have developed a prototype laser growth chamber and demonstrate that plants grown under laser illumination can complete a full growth cycle from seed to seed with phenotypes resembling those of plants grown under LEDs reported previously. Importantly, the plants have lower expression of proteins diagnostic for light and radiation stress. The phenotypical, biochemical and proteome data show that the single-wavelength laser light is suitable for plant growth and therefore, potentially able to unlock the advantages of this next generation lighting technology for highly energy-efficient horticulture.

Ultrabroad linewidth orange-emitting nanowires LED for high CRI laser-based white lighting and gigahertz communications

Group-III-nitride laser diode (LD)-based solid-state lighting device has been demonstrated to be droop-free compared to light-emitting diodes (LEDs), and highly energy-efficient compared to that of the traditional incandescent and fluorescent white light systems. The YAG:Ce³⁺ phosphor used in LD-based solid-state lighting, however, is associated with rapid degradation issue. An alternate phosphor/LD architecture, which is capable of sustaining high temperature, high power density, while still intensity- and bandwidth-tunable for high color-quality remained unexplored. In this paper, we present for the first time, the proof-of-concept of the generation of high-quality white light using an InGaN-based orange nanowires (NWs) LED grown on silicon, in conjunction with a blue LD, and in place of the compound-phosphor. By changing the relative intensities of the ultrabroad linewidth orange and narrow-linewidth blue components, our LED/LD device architecture achieved correlated color temperature (CCT) ranging from 3000 K to above 6000K with color rendering index (CRI) values reaching 83.1, a value unsurpassed by the YAG-phosphor/blue-LD counterpart. The white-light wireless communications was implemented using the blue LD through on-off keying (OOK) modulation to obtain a data rate of 1.06 Gbps. We therefore achieved the best of both worlds when orange-emitting NWs LED are utilized as "active-phosphor", while blue LD is used for both color mixing and optical wireless communications.

Green Stimulated Emission Boosted by Nonradiative Resonant Energy Transfer from Blue Quantum Dots

Thanks to their tunability and versatility, the colloidal quantum dots (CQDs) made of II-VI semiconductor compound offer the potential to bridge the "green gap" in conventional semiconductors. However, when the CQDs are pumped to much higher initial excitonic states compared to their bandgap, multiexciton interaction is enhanced, leading to a much higher stimulated emission threshold. Here, to circumvent this drawback, for the first time, we show a fully colloidal gain in green enabled by a partially indirect pumping approach assisted by Förster resonance energy transfer process. By introducing the blue CQDs as exciton donors, the lasing threshold of the green CQDs, is reduced dramatically. The blue CQDs thus serve as an energy-transferring buffer medium to reduce excitation energy from pumping photons in a controlled way by injecting photoinduced excitons into green CQDs. Our newly developed colloidal pumping scheme could enable efficient CQD lasers of full visible colors by a single pump source and cascaded exciton transfer. This would potentially pave the way for an efficient multicolor laser for lighting and display applications.

Electrically switchable organo-inorganic hybrid for a white-light laser source

We demonstrate a spectrally discrete white-light laser device based on a photonic bandgap hybrid, which is composed of a soft photonic crystal; i.e., a layer of dye-doped cholesteric liquid crystal (CLC), sandwiched between two imperfect but identical, inorganic multilayer photonic crystals. With a sole optical pump, a mono-, bi-, or tri-chromatic laser can be obtained and, through the soft photonic crystal regulated by an applied voltage, the hybrid possesses electrical tunability in laser wavelength. The three emitted spectral peaks originate from two bandedges of the CLC reflection band as well as one of the photonic defect modes in dual-mode lasing. Thanks to the optically bistable nature of CLC, such a white-light laser device can operate in quite an energy-saving fashion. This technique has potential to fulfill the present mainstream in the coherent white-light source.

Laser-driven phosphor-converted white light source for solid-state illumination

Energy efficiency and lighting quality considerations are driving research into laser-pumped white light sources. Laser diodes as pump sources for downconversion phosphors promise freedom from "droop" that adversely affects the efficiency of light-emitting diodes (LEDs). High-intensity laser diode-pumped light sources for applications such as search lights and automobile headlights have been demonstrated recently. Our paper describes the design and construction of a domestic/office-type solid-state luminaire driven by light from an integrated violet laser-diode module. A trichromatic phosphor made from a blend of separate europium-containing rare-earth phosphors was used as the downconversion medium. Mechanical and optical design of the reflector and the phosphor plate are described. Characteristics of both the pump light and the downconverted light are also described. Our studies also looked at the variation of chromaticity coordinates with variation in pump power and the effect of laser speckle on the lamp's light output. Finally, there is a brief discussion of energy conversion efficiency and longevity considerations, comparing pumping with LEDs versus pumping with laser diodes.

High luminous flux from single crystal phosphor-converted laser-based white lighting system

The efficiency droop of light emitting diodes (LEDs) with increasing current density limits the amount of light emitted per wafer area. Since low current densities are required for high efficiency operation, many LED die are needed for high power white light illumination systems. In contrast, the carrier density of laser diodes (LDs) clamps at threshold, so the efficiency of LDs does not droop above threshold and high efficiencies can be achieved at very high current densities. The use of a high power blue GaN-based LD coupled with a single crystal Ce-doped yttrium aluminum garnet (YAG:Ce) sample was investigated for white light illumination applications. Under CW operation, a single phosphor-converted LD (pc-LD) die produced a peak luminous efficacy of 86.7 lm/W at 1.4 A and 4.24 V and a peak luminous flux of 1100 lm at 3.0 A and 4.85 V with a luminous efficacy of 75.6 lm/W. Simulations of a pc-LD confirm that the single crystal YAG:Ce sample did not experience thermal quenching at peak LD operating efficiency. These results show that a single pc-LD die is capable of emitting enough luminous flux for use in a high power white light illumination system.

4-Gbit/s visible light communication link based on 16-QAM OFDM transmission over remote phosphor-film converted white light by using blue laser diode

Visible Light Communication (VLC) as a new technology for ultrahigh-speed communication is still limited when using slow modulation light-emitting diode (LED). Alternatively, we present a 4-Gbit/s VLC system using coherent blue-laser diode (LD) via 16-quadrature amplitude modulation orthogonal frequency division multiplexing. By changing the composition and the optical-configuration of a remote phosphor-film the generated white light is tuned from cool day to neutral, and the bit error rate is optimized from 1.9 × 10(-2) to 2.8 × 10(-5) in a blue filter-free link due to enhanced blue light transmission in forward direction. Briefly, blue-LD is an alternative to LED for generating white light and boosting the data rate of VLC.

Phosphorous Diffuser Diverged Blue Laser Diode for Indoor Lighting and Communication

An advanced light-fidelity (Li-Fi) system based on the blue Gallium nitride (GaN) laser diode (LD) with a compact white-light phosphorous diffuser is demonstrated for fusing the indoor white-lighting and visible light communication (VLC). The phosphorous diffuser adhered blue GaN LD broadens luminescent spectrum and diverges beam spot to provide ample functionality including the completeness of Li-Fi feature and the quality of white-lighting. The phosphorous diffuser diverged white-light spot covers a radiant angle up to 120^o with CIE coordinates of (0.34, 0.37). On the other hand, the degradation on throughput frequency response of the blue LD is mainly attributed to the self-feedback caused by the reflection from the phosphor-air interface. It represents the current state-of-the-art performance on carrying 5.2-Gbit/s orthogonal frequency-division multiplexed 16-quadrature-amplitude modulation (16-QAM OFDM) data with a bit error rate (BER) of 3.1 × 10⁻³ over a 60-cm free-space link. This work aims to explore the plausibility of the phosphorous diffuser diverged blue GaN LD for future hybrid white-lighting and VLC systems.

A monolithic white laser

Monolithic semiconductor lasers capable of emitting over the full visible-colour spectrum have a wide range of important applications, such as solid-state lighting, full-colour displays, visible colour communications and multi-colour fluorescence sensing. The ultimate form of such a light source would be a monolithic white laser. However, realizing such a device has been challenging because of intrinsic difficulties in achieving epitaxial growth of the mismatched materials required for different colour emission. Here, we demonstrate a monolithic multi-segment semiconductor nanosheet based on a quaternary alloy of ZnCdSSe that simultaneously lases in the red, green and blue. This is made possible by a novel nanomaterial growth strategy that enables separate control of the composition, morphology and therefore bandgaps of the segments. Our nanolaser can be dynamically tuned to emit over the full visible-colour range, covering 70% more perceptible colours than the most commonly used illuminants.

Going beyond 4 Gbps data rate by employing RGB laser diodes for visible light communication

With increasing interest in visible light communication, the laser diode (LD) provides an attractive alternative, with higher efficiency, shorter linewidth and larger bandwidth for high-speed visible light communication (VLC). Previously, more than 3 Gbps data rate was demonstrated using LED. By using LDs and spectral-efficient orthogonal frequency division multiplexing encoding scheme, significantly higher data rates has been achieved in this work. Using 16-QAM modulation scheme, in conjunction with red, blue and green LDs, data rates of 4.4 Gbps, 4 Gbps and 4 Gbps, with the corresponding BER/SNR/EVM of 3.3 × 10⁻³/15.3/17.9, 1.4 × 10⁻³/16.3/15.4 and 2.8 × 10⁻³/15.5/16.7were obtained over transmission distance of ~20 cm. We also simultaneously demonstrated white light emission using red, blue and green LDs, after passing through a commercially available diffuser element. Our work highlighted that a tradeoff exists in operating the blue LDs at optimum bias condition while maintaining good color temperature. The best results were obtained when encoding red LDs which gave both the strongest received signal amplitude and white light with CCT value of 5835K.

Observation of polarized gain from aligned colloidal nanorods

In recent years, colloidal semiconductor nanorods have attracted great interest for polarized spontaneous emission. However, their polarized gain has not been possible to achieve so far. In this work we show the highly polarized stimulated emission from the densely packed ensembles of core-seeded nanorods in a cylindrical cavity. Here CdSe/CdS dot-in-rods were coated and aligned on the inner wall of a capillary tube, providing optical feedback for the nanorod gain medium. Results show that the polarized gain originates intrinsically from the aligned nanorods and not from the cavity and that the optical anisotropy of the nanorod ensemble was amplified with the capillary tube, resulting in highly polarized whispering gallery mode lasing. The highly polarized emission and lasing, together with easy fabrication and flexible incorporation, make this microlaser a promising candidate for important color conversion and enrichment applications including liquid crystal display backlighting and laser lighting.

Towards a 100 Gb/s visible light wireless access network

Potential visible light communication (VLC) data rates at over 10 Gb/s have been recently demonstrated using light emitting diodes (LEDs). The disadvantage is, LEDs have an inherent trade-off between optical efficiency and bandwidth. Consequently, laser diodes (LDs) can be considered as a very promising alternative for better utilization of the visible light spectrum for communication purposes. This work investigates the communication capabilities of off-the-shelf LDs in a number of scenarios with illumination constraints. The results indicate that optical wireless access data rates in the excess of 100 Gb/s are possible at standard indoor illumination levels.

Study on the correlations between color rendering indices and the spectral power distribution

The intrinsic spectrally resolved sensitivity (ISRS) of color rendering indices (CRIs) is investigated by using spectral loss simulations. It is demonstrated that R(a) exhibits large sensitivities around 444, 480, 564, and 622 nm, while for R(9) the sensitivity peaks are around 461, 581 and 630 nm, which all shift slightly with the correlated color temperature. If considering the ISRS as a bridge between the spectral power distribution of LED and its CRI, one could obtain a high CRI by minimizing the deviation between the shapes of the illuminant spectrum and the reference spectrum, both after modulations by the ISRS as a weighting function. This approach, recommended as a guideline for the spectra design aiming at a high CRI, is described and justified in depth via a mathematical model. This method is spectra-oriented and could largely facilitate the spectra design.

Multi-colour nanowire photonic crystal laser pixels

Emerging applications such as solid-state lighting and display technologies require micro-scale vertically emitting lasers with controllable distinct lasing wavelengths and broad wavelength tunability arranged in desired geometrical patterns to form “super-pixels”. Conventional edge-emitting lasers and current surface-emitting lasers that require abrupt changes in semiconductor bandgaps or cavity length are not a viable solution. Here, we successfully address these challenges by introducing a new paradigm that extends the laser tuning range additively by employing multiple monolithically grown gain sections each with a different emission centre wavelength. We demonstrate this using broad gain-bandwidth III-nitride multiple quantum well (MQW) heterostructures and a novel top-down nanowire photonic crystal nanofabrication. We obtain single-mode lasing in the blue-violet spectral region with a remarkable 60 nm of tuning (or 16% of the nominal centre wavelength) that is determined purely by the photonic crystal geometry. This approach can be extended to cover the entire visible spectrum.

Dynamical color-controllable lasing with extremely wide tuning range from red to green in a single alloy nanowire using nanoscale manipulation

Multicolor lasing and dynamic color-tuning in a wide spectrum range are challenging to realize but critically important in many areas of technology and daily life, such as general lighting, display, multicolor detection, and multiband communication. By exploring nanoscale growth and manipulation, we have demonstrated the first active dynamical color control of multicolor lasing, continuously tunable between red and green colors separated by 107 nm in wavelength. This is achieved in a purposely engineered single CdSSe alloy nanowire with composition varied along the wire axis. By looping the wide-gap end of the alloy nanowire through nanoscale manipulation, two largely independent (only weakly coupled) laser cavities are formed respectively for the green and red color modes. Our approach simultaneously overcomes the two fundamental challenges for multicolor lasing in material growth and cavity design. Such multicolor lasing and continuous color tuning in a wide spectral range represents a new paradigm shift and would eventually enable color-by-design and white-color lasers for lighting, illumination, and many other applications.

Optimization of laser-based white light illuminants

The authors build on previous experience in the optimization of white-light sources based on combinations of narrow-band spectra. They extend those concepts by using delta-function spectra to study the prospects of future optimal laser-based sources. The optimization process is based on a trade-off between the color rendering properties and the luminous efficacy of the radiation. Optimal solutions for four, five and six delta-function spectra with correlated color temperatures in the 3000 to 5500 K range are presented and analyzed. White-light sources with these properties would likely find wide acceptance in numerous lighting applications.