H.X. Zhang

Micro-LED arrays: a tool for two-dimensional neuron stimulation

Stimulating neuron cells with light is an exciting new technology that is revolutionizing the neurosciences. To date, due to the optical complexity that is involved, photostimulation has only been achieved at a single site using high power light sources. Here we present a GaN based micro-light emitting diode (LED) array that can open the way to multi-site photostimulation of neuron cells. The device is a two-dimensional array of micrometre size LED emitters. Each emitter has the required wavelength, optical power and modulation bandwidth to trigger almost any photosensitizer and is individually addressable. We demonstrate micrometre resolution photoactivation of a caged fluorophore and photostimulation of sensitized living neuron cells. In addition, a complete system that combines the micro-LED array with multi-site electrophysiological recording based on microelectrode array technology and/or fluorescence imaging is presented.

Optical sectioning microscopes with no moving parts using a micro-stripe array light emitting diode

We describe an optical sectioning microscopy system with no moving parts based on a micro-structured stripe-array light emitting diode (LED). By projecting arbitrary line or grid patterns onto the object, we are able to implement a variety of optical sectioning microscopy techniques such as grid-projection structured illumination and line scanning confocal microscopy, switching from one imaging technique to another without modifying the microscope setup. The micro-structured LED and driver are detailed and depth discrimination capabilities are measured and calculated.

Matrix-Addressable Micropixellated InGaN Light-Emitting Diodes With Uniform Emission and Increased Light Output

Micropixellated InGaN light-emitting diodes (micro- LEDs) have a wide number of potential applications in areas including microdisplays, fluorescence-based assays and microscopy, and cell micromanipulation. Here, we present fabrication and performance details of matrix-addressable micro-LED devices which show significant improvements over their earlier counterparts. Devices with 64 x 64 micropixel elements, each of them having a 16-mum-diameter emission aperture on a 50-mum pitch, have been fabricated at blue (470 nm), green (510 nm), and UV (370 nm) wavelengths, respectively. Importantly, we have adopted a scheme of running n-metal tracks adjacent to each n-GaN mesa, so that resistance variation between the devices is reduced to below 8%, in contrast to the earlier fivefold resistance variation encountered. We have also made improvements to the spreading-layer formation scheme, resulting in significant increases in output power per element, improved current handling, and reduced turn-on voltages. These devices have been combined with a computer- driven programmable driver interface operating in constant- current mode, and representative microdisplay outputs are presented.

Efficient flip-chip InGaN micro-pixellated light-emitting diode arrays: promising candidates for micro-displays and colour conversion

Flip-chip InGaN micro-pixellated LED arrays with high pixel density and improved device performance are presented. The devices, with 64 × 64 elements, each of which have a 20 µm emission aperture on a 50 µm pitch, are fabricated with a matrix-addressable scheme at blue (470 nm) and UV (370 nm) wavelengths, respectively. These devices are then flip-chip bonded onto silicon mounts. Good emission uniformity across the LED array is demonstrated, which can be attributed to the introduced n-metal tracks adjacent to each n-GaN mesa and the p-contact lines running across parallel columns. More importantly, with a flip-chip configuration, the optical power output and the current-handling capability of these new devices are substantially enhanced, due to the improved heat dissipation capability and the increased light extraction efficiency. For instance, each pixel in the flip-chip blue (respectively UV) LED arrays can provide a maximum power density 43 W cm−2 (respectively 6.5 W cm−2) at an extremely high current density up to 4000 A cm−2 before breakdown. These flip-chip devices are then combined with a computer-programmable driver circuit interface to produce high-quality micro-scale displays. Other promising applications of these LEDs, such as colour conversion with quantum dots, are also demonstrated.

Microstripe-array InGaN light-emitting diodes with individually addressable elements

High-performance InGaN light-emitting diodes consisting of 120 side-by-side and individually addressable microstripe elements have been successfully fabricated. Each stripe in these devices is 24 mum in width and 3600 mum long, with a center-to-center spacing between adjacent stripes of 34 mum. The emission wavelengths demonstrated range from ultraviolet (UV) (370 nm) to blue (470 nm) and green (520 nm). The devices show good uniformity and performance due to finger-pattern n-electrodes running between adjacent stripes. In the case of the UV devices for example, turn-on voltages are around 3.5 V and continuous-wave output powers per stripe ~80 muW at 20 mA. A major feature of these devices is their ability to generate pattern-programmable emission, which offers applications in areas including structured illumination wide-field sectioning optical microscopy

Improved sectioning in a slit scanning confocal microscope

We describe a simple implementation of a slit scanning confocal microscope to obtain an axial resolution better than that of a point-scanning confocal microscope. Under slit illumination, images of a fluorescent object are captured using an array detector instead of a line detector so that out-of-focus light is recorded and then subtracted from the adjacent images. Axial resolution after background subtraction is 2.2 times better than the slit confocal resolution, and out-of-focus image suppression is calculated to attenuate with defocus faster by 1 order of magnitude than in the point confocal case.

The 310–340 nm ultraviolet light emitting diodes grown using a thin GaN interlayer on a high temperature AlN buffer

Previously, we reported that a thin GaN interlayer approach has been developed for growth of 340 nm ultraviolet light emitting diodes (UV-LEDs) with significantly improved performance. In this paper, more recent results on the further development of UV-LEDs with shorter wavelengths are reported, and the limitation of the wavelength of the UV-LEDs that can be pushed to, while retaining high device performance using the approach has been investigated. Transmission electron microscopy and device-performance data, including electrical and optical characteristics, indicated that the thin GaN interlayer approach can be effectively employed for growth of UV-LEDs to an emission wavelength approaching at least 300 nm. The approach should be taken into account in growth of UV-LEDs on sapphire substrates, as it provides a simple but effective growth method to achieve UV-LEDs with high performance. This paper also reports that a micro-LED array using the UV-LED wafer has been successfully fabricated, offering versatile micro-structured UV light sources for a wide range of applications.

CMOS-integrated flip-chip, micro-pixel InGaN LED arrays for on-chip microfluorimetry

4times16 arrays of micro-pixellated InGaN LEDs, each of diameter 72 mum, have been flip-chipped onto CMOS driver backplanes which also contain single-photon avalanche photodiodes. Pattern-programmable control is demonstrated in continuous and nanosecond modes. Such devices show promise as miniaturized excitation and detection systems for microfluorimetry studies.

Micro-pixel flip-chip AlInGaN LED arrays with high CW and nanosecond output power

Flip-chip AlInGaN micro-LED arrays with different wavelengths and pixel diameters have been fabricated, giving, per pixel, CW output power densities up to 32.5 W/cm2 at 20 mA and pulsed output of up to 150 pJ in 36 ns pulses.

Hybrid inorganic/organic micro-structured light-emitting diodes produced by self-aligned direct writing

By blending light-emitting polymers (LEPs) with photo-curable polymers and using self-aligned direct writing, LEP microstructures were fabricated directly on top of micro-pixellated InGaN light-emitting diodes, demonstrating a hybrid LED technology