Simultaneous mapping of cathodoluminescence spectra and backscatter diffraction patterns in a scanning electron microscope.

Electron backscatter diffraction and cathodoluminescence are complementary scanning electron microscopy modes widely used in the characterisation of semiconductor films, respectively revealing the strain state of a crystalline material and the effect of this strain on the light emission from the sample. Conflicting beam, sample and detector geometries have meant it is not generally possible to acquire the two signals together during the same scan. Here, we present a method of achieving this simultaneous acquisition, by collecting the light emission through a transparent sample substrate. We apply this combination of techniques to investigate the strain field and resultant emission wavelength variation in a deep-ultraviolet micro-LED. For such compatible samples, this approach has the benefits of avoiding image alignment issues and minimising beam damage effects.

Hundred-meter Gb/s deep ultraviolet wireless communications using AlGaN micro-LEDs.

We demonstrate the use of deep ultraviolet (DUV) micro-light-emitting diodes (LEDs) for long-distance line-of-sight optical wireless communications. With a single 285 nm-emitting micro-LED, we have respectively achieved data rates greater than 6.5 Gb/s at a distance of 10 m and 4 Gb/s at 60 m. Moreover, we obtained >1 Gb/s data rates at a distance of 116 m. To our knowledge, these results are the highest data rates at such distances thus far reported using DUV micro-LEDs and the first demonstration of Gb/s communication at >100 m using any micro-LED-based transmitter.

Combining time of flight and photometric stereo imaging for 3D reconstruction of discontinuous scenes.

Time of flight and photometric stereo are two three-dimensional (3D) imaging techniques with complementary properties, where the former can achieve depth accuracy in discontinuous scenes, and the latter can reconstruct surfaces of objects with fine depth details and high spatial resolution. In this work, we demonstrate the surface reconstruction of complex 3D fields with discontinuity between objects by combining the two imaging methods. Using commercial LEDs, a single-photon avalanche diode camera, and a mobile phone device, high resolution of surface reconstruction is achieved with a RMS error of 6% for an object auto-selected from a scene imaged at a distance of 50 cm.

Synchronization-free top-down illumination photometric stereo imaging using light-emitting diodes and a mobile device.

Three dimensional reconstruction of objects using a top-down illumination photometric stereo imaging setup and a hand-held mobile phone device is demonstrated. By employing binary encoded modulation of white light-emitting diodes for scene illumination, this method is compatible with standard lighting infrastructure and can be operated without the need for temporal synchronization of the light sources and camera. The three dimensional reconstruction is robust to unmodulated background light. An error of 2.69 mm is reported for an object imaged at a distance of 42 cm and with the dimensions of 48 mm. We also demonstrate the three dimensional reconstruction of a moving object with an effective off-line reconstruction rate of 25 fps.

CMOS-integrated GaN LED array for discrete power level stepping in visible light communications.

We report a CMOS integrated micro-LED array capable of generating discrete optical output power levels. A 16 × 16 array of individually addressable pixels are on-off controlled through parallel logic signals. With carefully selected groups of LEDs driven together, signals suitable for discrete transmission schemes are produced. The linearity of the device is assessed, and data transmission using pulse amplitude modulation (PAM) and orthogonal frequency division multiplexing (OFDM) is performed. Error-free transmission at a symbol rate of 100 MSamples/s is demonstrated with 4-PAM, yielding a data rate of 200 Mb/s. For 8-PAM, encoding is required to overcome the baseline wander from the receiver, reducing the data rate to 150 Mb/s. We also present an experimental proof-of-concept demonstration of discrete-level OFDM, achieving a spectral efficiency of 3.96 bits/s/Hz.

Fabrication, characterization and applications of flexible vertical InGaN micro-light emitting diode arrays.

Flexible vertical InGaN micro-light emitting diode (micro-LED) arrays have been fabricated and characterized for potential applications in flexible micro-displays and visible light communication. The LED epitaxial layers were transferred from initial sapphire substrates to flexible AuSn substrates by metal bonding and laser lift off techniques. The current versus voltage characteristics of flexible micro-LEDs degraded after bending the devices, but the electroluminescence spectra show little shift even under a very small bending radius 3 mm. The high thermal conductivity of flexible metal substrates enables high thermal saturation current density and high light output power of the flexible micro-LEDs, benefiting the potential applications in flexible high-brightness micro-displays and high-speed visible light communication. We have achieved ~40 MHz modulation bandwidth and 120 Mbit/s data transmission speed for a typical flexible micro-LED.

Quantitation of MRI sensitivity to quasi-monodisperse microbubble contrast agents for spatially resolved manometry.

PURPOSE: The direct in-vivo measurement of fluid pressure cannot be achieved with MRI unless it is done with the contribution of a contrast agent. No such contrast agents are currently available commercially, whilst those demonstrated previously only produced qualitative results due to their broad size distribution. Our aim is to quantitate then model the MR sensitivity to the presence of quasi-monodisperse microbubble populations. METHODS: Lipid stabilised microbubble populations with mean radius 1.2 ± 0.8 µm have been produced by mechanical agitation. Contrast agents with increasing volume fraction of bubbles up to 4% were formed and the contribution the bubbles bring to the relaxation rate was quantitated. A periodic pressure change was also continuously applied to the same contrast agent, until MR signal changes were only due to bubble radius change and not due to a change in bubble density. RESULTS: The MR data compared favourably with the prediction of an improved numerical simulation. An excellent MR sensitivity of 23 % bar(-1) has been demonstrated. CONCLUSION: This work opens up the possibility of generating microbubble preparations tailored to specific applications with optimised MR sensitivity, in particular MRI based in-vivo manometry.

Miniaturized optoelectronic tweezers controlled by GaN micro-pixel light emitting diode arrays.

A novel, miniaturized optoelectronic tweezers (OET) system has been developed using a CMOS-controlled GaN micro-pixelated light emitting diode (LED) array as an integrated micro-light source. The micro-LED array offers spatio-temporal and intensity control of the emission pattern, enabling the creation of reconfigurable virtual electrodes to achieve OET. In order to analyse the mechanism responsible for particle manipulation in this OET system, the average particle velocity, electrical field and forces applied to the particles were characterized and simulated. The capability of this miniaturized OET system for manipulating and trapping multiple particles including polystyrene beads and live cells has been successfully demonstrated.

A Vertically Integrated CMOS Microsystem for Time-Resolved Fluorescence Analysis.

We describe a two-chip micro-scale time-resolved fluorescence analyzer integrating excitation, detection, and filtering. A new 8×8 array of drivers realized in standard low-voltage 0.35-µm complementary metal-oxide semiconductor is bump-bonded to AlInGaN blue micro-pixellated light-emitting diodes (micro-LEDs). The array is capable of producing sample excitation pulses with a width of 777 ps (FWHM), enabling short lifetime fluorophores to be investigated. The fluorescence emission is detected by a second, vertically-opposed 16 × 4 array of single-photon avalanche diodes (SPADs) fabricated in 0.35-µm high-voltage CMOS technology with in-pixel time-gated photon counting circuitry. Captured chip data are transferred to a PC for further processing, including histogramming, lifetime extraction, calibration and background/noise compensation. This constitutes the smallest reported solid-state microsystem for fluorescence decay analysis, replacing lasers, photomultiplier tubes, bulk optics, and discrete electronics. The system is demonstrated with measurements of fluorescent colloidal quantum dot and Rhodamine samples.

A CMOS Time-Resolved Fluorescence Lifetime Analysis Micro-System.

We describe a CMOS-based micro-system for time-resolved fluorescence lifetime analysis. It comprises a 16 × 4 array of single-photon avalanche diodes (SPADs) fabricated in 0.35 µm high-voltage CMOS technology with in-pixel time-gated photon counting circuitry and a second device incorporating an 8 × 8 AlInGaN blue micro-pixellated light-emitting diode (micro-LED) array bump-bonded to an equivalent array of LED drivers realized in a standard low-voltage 0.35 µm CMOS technology, capable of producing excitation pulses with a width of 777 ps (FWHM). This system replaces instrumentation based on lasers, photomultiplier tubes, bulk optics and discrete electronics with a PC-based micro-system. Demonstrator lifetime measurements of colloidal quantum dot and Rhodamine samples are presented.