Gallium-nitride (GaN) based light-emitting diodes (LEDs) are highly-efficient sources for general purpose illumination. Visible light communications (VLC) uses these sources to supplement existing wireless communications by offering a large, licence-free region of optical spectrum. Here we report on progress in the development of micro-scale GaN LEDs (micro-LEDs), optimized for VLC. These blue-emitting micro-LEDs are shown to have very high electrical-to-optical modulation bandwidths, exceeding 800 MHz. The data transmission capabilities of the micro-LEDs are illustrated by demonstrations using on-off-keying (OOK), pulse-amplitude modulation (PAM) and orthogonal frequency division multiplexing (OFDM) modulation schemes to transmit data over free space at rates of 1.7, 3.4 and 5 Gbps, respectively.

Visible light communication (VLC) is a promising solution to the increasing demands for wireless connectivity. Gallium nitride micro-sized light emitting diodes (micro-LEDs) are strong candidates for VLC due to their high bandwidths. Segmented violet micro-LEDs are reported in this work with electrical-to-optical bandwidths up to 655 MHz. An orthogonal frequency division multiplexing-based VLC system with adaptive bit and energy loading is demonstrated, and a data transmission rate of 11.95 Gb/s is achieved with a violet micro-LED, when the nonlinear distortion of the micro-LED is the dominant noise source of the VLC system. A record 7.91 Gb/s data transmission rate is reported below the forward error correction threshold using a single pixel of the segmented array when all the noise sources of the VLC system are present.

Displays based on microsized gallium nitride light-emitting diodes possess extraordinary brightness. It is demonstrated here both theoretically and experimentally that the layout of the n-contact in these devices is important for the best device performance. We highlight, in particular, the significance of a nonthermal increase of differential resistance upon multipixel operation. These findings underpin the realization of a blue microdisplay with a luminance of 10 6 cd/m 2 .

Temperature-dependent trends in radiative and Auger recombination coefficients have been determined at different injection carrier concentrations using InGaN micro-light emitting diodes 40 μm in diameter. The differential lifetime was obtained first from the measured modulation bandwidth and was then employed to calculate the carrier concentration in the quantum well active region. When the temperature increases, the carrier concentration increases, but both the radiative and Auger recombination coefficients decrease. In addition, the temperature dependence of radiative and Auger recombination coefficients is weaker at a higher injection carrier concentration, which is strongly related to phase space filling

We report the transfer printing of blue-emitting micron-scale light-emitting diodes (micro-LEDs) onto fused silica and diamond substrates without the use of intermediary adhesion layers. A consistent Van der Waals bond was achieved via liquid capillary action, despite curvature of the LED membranes following release from their native silicon growth substrates. The excellence of diamond as a heat-spreader allowed the printed membrane LEDs to achieve optical power output density of 10 W/cm(2) when operated at a current density of 254 A/cm(2). This high-current-density operation enabled optical data transmission from the LEDs at 400 Mbit/s.

The aim of this study was to evaluate the water quality of the Cértima River basin (Central Portugal). For that purpose, surface water samples were collected in March, May and July 2003, at 10 selected sampling sites, and were analysed for physicochemical parameters, namely temperature, conductivity, pH, total suspended solids, dissolved oxygen, biochemical oxygen demand (BOD(5)), Kjeldahl nitrogen and total phosphorus. Results revealed an acceptable water quality during the spring season. Maxima of 64 mg dm(-3) for BOD(5), 39 mg dm(-3) for Kjeldahl nitrogen, and 5.2 mg dm(-3) for total phosphorus, were recorded during summer, indicating a significant degradation of the water quality in a river stretch located downstream of the town of Mealhada. These values, which did not comply with the objectives of minimum quality for surface waters prescribed by the Portuguese legislation, were related to domestic wastewater discharges and runoff waters from a cattle farm. Besides their effects on the middle stretch of the river, these pollution sources were the most likely cause of the high nutrient load in downstream waters, and thus may have a major impact on the trophic status of Pateira de Fermentelos, a sensitive wetland area located in the lower Cértima basin.

In this paper, we report the performance of an imaging multiple input multiple output (MIMO) visible light communication (VLC) system. The VLC transmitter consists of a two-dimensional, individually addressable Gallium Nitride micro light emitting diode (µLED) array. The receiver uses a two-dimensional avalanche photodiode (APD) array fabricated using complementary metal oxide semiconductor (CMOS). Using integrated CMOS-based LED drivers, a data rate greater than 1 Gbps was obtained at a link distance of 1 m with the system field of view (FOV) of 3.45 degree using four channels. At a reduced link distance of 0.5 m, a data rate of 7.48 Gbps was obtained using a nine channel MIMO system. This demonstrates the feasibility of compact MIMO systems which offer substantial data rates. Index Terms-Visible light communications, multiple input multiple output, VLC demonstrator, integrated VLC I. INTRODUCTION Visible light communications (VLC) systems, realized using solid-state lighting (SSL) devices, can offer high-speed data communication in addition to their primary purpose of illumination. With the white light emitting diodes (LEDs) expected to be the dominant illumination device in the home and office environment in the near future, the use of VLC systems will grow exponentially over the coming decades [1]. As a result, there has been significant research and commercial interest in VLC systems over the last ten years (see [2] for the detailed review). This is largely due to several key advantages that VLC offers in comparison to the existing radio frequency (RF) technology, including license-free operation, high available bandwidth, high spatial diversity, innate security, and controlled beam shaping. Phosphor based white LEDs have a low communication bandwidth (a few MHz) due to the long photoluminescence lifetimes of the phosphor [3]. The bandwidth of the blue LED itself is limited to 20-30 MHz. Using pre and post-equalisation for on-off keying (OOK) modulation, a data rate of 550 Mbps M anuscript

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