C. Griffin

Reflection/transmission confocal microscopy characterization of single-crystal diamond microlens arrays

Using the method of photoresist reflow and inductively coupled plasma dry etching, we have fabricated microlens arrays in type-IIa natural single-crystal diamond, with diameters down to 10 μm. The surface profile of the microlenses was characterized by atomic force microscopy and was found to match well with a spherical shape, with a surface roughness of better than 1.2 nm. To characterize the optical properties of these diamond microlens arrays, a laser scanning reflection/transmission confocal microscopy technique has been developed. This technique enabled the surface profile of the microlenses to be measured simultaneously with optical parameters including focal length and spot size, opening up an application area for confocal microscopy.

Spectral conversion of InGaN ultraviolet microarray light-emitting diodes using fluorene-based red-, green-, blue-, and white-light-emitting polymer overlayer films

We report the fabrication of hybrid organic/inorganic semiconductor light-emitting devices that operate across the entire visible spectrum. The devices are based on a series of blue-, green-, and red-light-emitting polyfluorene materials that convert the emission from an array of micron-sized ultraviolet InGaN light-emitting diodes. We also demonstrate white-light-emitting versions of these hybrid devices by employing single films of carefully adjusted polyfluorene blends in which cascade energy transfer occurs between the constituent materials. The spectral and operating characteristics of the devices are described in detail. Such organic emission layer/inorganic light-emitting diode (LED) array based devices may provide a promising route to the fabrication of low-cost full-color microdisplays and other instrumentation devices.

CMOS driven micro-pixel LEDs integrated with single photon avalanche diodes for time resolved fluorescence measurements

We describe a single chip approach to time resolved fluorescence measurements based on time correlated single photon counting. Using a single complementary metal oxide silicon (CMOS) chip, bump bonded to a 4 × 16 array of AlInGaN UV micro-pixellated light-emitting diodes, a prototype integrated microsystem has been built that demonstrates fluorescence excitation and detection on a nanosecond time scale. Demonstrator on-chip measurements of lifetimes of fluorescence colloidal quantum dot samples are presented.

New light from hybrid inorganic-organic emitters

We present the highlights of a research programme on hybrid inorganic?organic light emitters. These devices combine recent developments in III?V nitride technology (including UV emitting micro-arrays and specifically tailored quantum wells) with conjugated polymers to access the entire visible spectrum. Two types of devices are studied, those based on down conversion of the quantum well emission by radiative transfer and those based on non-radiative resonant energy transfer. The spectral and operating characteristics of the devices are described in detail. Selectable colour micro-arrays and bar emitters are demonstrated. The nature of the non-radiative energy transfer process has also been studied and we find transfer efficiencies of up to 43% at 15?K, with a 1/R2 dependence on the distance between quantum well and polymer layer, suggesting a plane?plane interaction. The relative importance of the non-radiative resonant energy transfer process increases with temperature to be up to 20 times more efficient, at 300?K, than the radiative transfer process.

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.

Fabrication of natural diamond microlenses by plasma etching

Advantageous properties including optical transparency, high thermal conductivity, and high carrier mobility make natural diamond an attractive choice for a range of optical and electrical devices. However, its hardness and chemical inertness provide a significant challenge for device processing. We demonstrate the ability to etch natural type IIa diamond using inductively coupled plasma etching with a significant etch rate of 228nm∕min. The etched surfaces were characterized by atomic force microscopy and found to have a root-mean-square roughness of below 3 nm. Using the photoresist reflow technique, refractive microlens arrays, with diameters ranging from 10 to 100 μm, were fabricated on the same diamond substrates. The lenses were characterized by confocal microscopy, which showed that their focal lengths, ranging from 5 to 500 μm, were in excellent agreement with the predicted values, demonstrating the high fidelity of the fabrication process.

Individually-addressable flip-chip AlInGaN micropixelated light emitting diode arrays with high continuous and nanosecond output power

Micropixelated blue (470 nm) and ultraviolet (370 nm) AlInGaN light emitting diode (micro-LED) arrays have been fabricated in flip-chip format with different pixel diameters (72 microm and 30 microm at, respectively, 100 and 278 pixels/mm(2)). Each micro-LED pixel can be individually-addressed and the devices possess a specially designed n-common contact incorporated to ensure uniform current injection and consequently uniform light emission across the array. The flip-chip micro-LEDs show, per pixel, high continuous output intensity of up to 0.55 microW/microm(2) (55 W/cm(2)) at an injection current density of 10 kA/cm(2) and can sustain continuous injection current densities of up to 12 kA/cm(2) before breakdown. We also demonstrate that nanosecond pulsed output operation of these devices with per pixel onaxis average peak intensity up to 2.9 microW/microm(2) (corresponding to energy of 45pJ per 22ns optical pulse) can be achieved. We investigate the pertinent performance characteristics of these arrays for micro-projection applications, including the prospect of integrated optical pumping of organic semiconductor lasers.

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.

A Microsystem for Time-Resolved Fluorescence Analysis using CMOS Single-Photon Avalanche Diodes and Micro-LEDs

Although microfluidics and microarray technologies are revolutionizing the throughput, sensitivity and cost in many areas of biodiagnostics, they are still reliant on bulky and expensive fluorescence analysis instrumentation. Conventional fluorescence intensity measurements are prone to misinterpretation due to illumination and fluorophore concentration non-uniformities. Thus, there is a growing interest in time-resolved fluorescence detection, whereby the characteristic fluorescence decay time-constant (or lifetime) in response to an impulse excitation source is measured.

Beam divergence measurements of InGaN∕GaN micro-array light-emitting diodes using confocal microscopy

The recent development of high-density, two-dimensional arrays of micrometer-sized InGaN∕GaN light-emitting diodes (micro-LEDs) with potential applications from scientific instrumentation to microdisplays has created an urgent need for controlled manipulation of the light output from these devices. With directed light output these devices can be used in situations where collimated beams or light focused onto several thousand matrix points is desired. In order to do this effectively, the emission characteristics of the devices must be fully understood and characterized. Here we utilize confocal microscopy to directly determine the emission characteristics and angular beam divergences from the individual micro-LED elements. The technique is applied to both top (into air) and bottom (through substrate) emission in arrays of green (540nm), blue (470nm), and UV (370nm) micro-LED devices, at distances of up to 50μm from the emission plane. The results are consistent with simple optical modeling of the expected ...