Optical designers are keen to combine the high-intensity, directional output of a laser with the low cost and broad spectral output of an LED. These wishes can now be fulfilled, due to the development of a novel LED featuring a parabolic mirror, claims Bill Henry from InfiniLED.

Thanks to advancesin medical science, we hope that we can be cured from many of the conditions that proved fatal to our grandparents. But we know that these gains are not cheap – modern, cutting-edge medical treatments can be incredibly expensive, and may cost even more in future.

To try and prevent the costs from becoming exorbitant, the medical industry is developing an increasing number of portable diagnostic and point-of-care devices. These can be used at the patient’s bedside, enabling faster, cheaper treatment.

At the heart of many of these pieces of medical equipment is an optical system. Its role is to illuminate a sample, before recording the information obtained from light reflecting or re-emitted from this target. It is an approach that can monitor and diagnose an increasing range of conditions, including HIV and diabetes. The optical engineers designing these systems must decide how to control and manipulate the light, while dealing with optical aberrations, interference and losses. What’s more, they have to make a critical decision: What should they use as the light source? Get this wrong and the consequences can be significant.

Two options for the light source are the LED and the laser. They are often treated as separate species, serving applications that rarely overlap. Engineers tend to select an LED when they either need a broad spectral output, have to drive a device at a low current, are working to a tight budget, or must construct a highly portable product. But if they need to control the beam angle or employ a high light intensity, they select a laser. But the latter source has complications, such as wavelength drift and speckle. Another vitally important consideration facing the designer of any optical system is the étendue − in simple terms this is the ability of the system to effectively capture and use the light that is generated (a more formal definition is that, when viewed from the target area, the étendue is the product of the size of the light source and the angle it subtends). Often the étendue dictates system efficiency, making it critical to match the étendue of the system with the properties of the light source.

Unfortunately, engineers do not always heed this warning. Sometimes they are not able to, because the choice of source is dictated by other considerations, such as limits for power consumption, spectral output or emission wavelength. However, when these considerations take precedence, efficiency suffers, due to the low proportion of light captured within the system. Knock-on effects include additional costs and increased power requirements, both of which hinder the design of new and improved medical products.

A hybrid structure

At InifiniLED of Cork, Ireland, we have pioneered a new class of device that combines the merits of the LED and the laser. It’s called a MicroLED and its design can be optimised to match the étendue of a given system to ensure maximum performance.

Our novel emitter can aid the designers of bedside and point-of-care medical equipment delivering simple, fast and accurate tests. Such systems employ markers for diseases that can be identified using fluorescence measurement techniques – which are both highly sensitive and accurate. Up until now, the optical control required for such systems has confined these tests to primarily laboratory settings, but the efficiency, collimation and form factor of our MicroLEDs enables the miniaturisation of these systems. Armed with our devices, it will be possible to test for a wide range of analytes, thanks to the flexibility of the emitters – in both wavelength and illumination area – and our unique geometry that will allow for the integration of additional components with the chip, such as filters and polarisers. This will further reduce system complexity.

The high intensity collimated output of our MicroLED means that it is also suitable for free space and fibre-coupled data transmission. What’s more, it can also be used to make more efficient, tiny displays and LED print-heads.

All these applications are possible with an emitter that shares many of the attributes of a conventional LED, because it is built with standard LED materials and has the spectral profile, flexibility and reliability of this class of device. However, our MicroLED features a unique light-controlling structure, which is integrated onto the device during fabrication. This gives it characteristics associated with a laser, such as high light intensity and a collimated output. By modifying the design, it is possible to control the emission spectrum to target a particular figure for full-width half maximum.

Our device’s light-controlling structure is based on a parabolic reflector that surrounds the light-generating region and controls the emitted light at the site of light generation (see Figure 1). We have found that this approach is far more efficient than using external optics to control the properties of the light escaping from the chip. With conventional, high-performance LEDs, the large difference between the refractive index of GaN and air is a major impediment to high light extraction efficiency. But with our device, we are able to use this significant difference in refractive index as an asset: It enables incredibly low light loss associated with the total internal reflection processes.
 
Figure 1: Schematic of a MicroLED emitter

Reflections guide light from the sidewalls of a parabolic structure and focus it towards the extraction surface. Accurate shaping of the sidewalls ensures that a high proportion of the light reaching the exit surface is propagating perpendicular to the interface, leading to minimal back-reflections, high light extraction efficiency and a controlled beam profile. Extracting most of the light through a single surface simplifies the optical design of the full system. In addition, it reduces the number of processing steps to make this chip. With our approach, we differ from that employed by many LED manufacturers – they roughen the surface of the devices they make to overcome back reflections and trapping losses. Although this step increases the amount of light escaping from the chip, it also results in uncontrolled light emitted in all directions.  With our approach, light extraction efficiency (power efficiency) through a single face of the MicroLED can be up to 50 percent – that’s four times higher than that extracted through a single surface of a standard LED chip. This translates to a tremendous increase in the amount of light that can be used within a system.

One key attribute of the MicroLED is that it enables light generation to be controlled at the source. The active area of this device can be tailored to a specific application, and if the area of interest is 500 µm2 or less, a single pixel can be employed. This approach ensures optimal power efficiency, because minimal light is wasted. Note that if the target area is larger, a cluster of emitters can be used that are packed closely together and driven in parallel.

Device characteristics

Our MicroLED’s far-field emission profile has a full-width half-maximum of typically ±30° – half that of the radiation pattern from a standard LED. We continue to make improvements in the control of light that exits these devices, and it is now possible for us to provide an emission profile with a divergence angle of less than ±3°. To improve the performance of our MicroLEDs, they are designed to be housed in a flip-chip package, with the light-generating layer positioned close to the heat-sink. By considering thermal management in the design, these emitters can be driven at high current densities without significant internal heating, enabling them to be used for applications demanding very high light intensity. For example, an output of 1 mW can be produced by a single MicroLED pixel with a 20 µm-wide emission area. This equates to a power density exceeding 300 W/cm2, which is common for lasers, but not LEDs. The MicroLED not only operates in this regime, but can do so over a wide range of wavelengths.

Thanks to the small active area of the MicroLED, it has a low device capacitance, so can switch at very high speeds. This makes the device optimal for optical communication applications. Measurements performed at Tyndall National Institute have revealed data transfer rates in our green MicroLEDs in excess of 500 Mbit/s. This result sets a new record for transfer rates using a green LED-based source, which is attractive for plastic optical fibre data transmission, thanks to low attenuation at these wavelengths that enables longer transmission distances.  Switching speeds of less than a nanosecond have also been observed, and we anticipate transfer rates of over 1 Gbit/s following appropriate optimisation.

One-dimensional and two-dimensional arrays can be formed with our MicroLEDs. Collimation at the source means that it is possible to distinguish the light emitted from individual pixels, and there is minimal cross-talk between them. The packaging technology selected for the source determines how close the LEDs can be packed together; a high density of MicroLEDs is possible with a range of packaging techniques, including direct bonding to CMOS or appropriate heat-sinked packages.

Figure 2: Far-field emission pattern from a MicroLED pixel directly from the chip (blue). The equivalent spectrum from a standard LED is shown in black

Targeting applications

By producing single MicroLEDs, plus clusters and arrays of these emitters, we are able to target many applications. Single pixels, which have an emitter diameter of 5-20 µm and offer high power efficiency, can be used for scanning and position sensing. These tiny devices are cheaper and more frugal than lasers, and when combined with integrated components, can produce patterns, shapes or images in the area of interest. Combining the MicroLEDs to form clusters enables the illumination of larger areas. Using such a source for machine vision or detection can increase system lifetime. This is possible by producing more useable and less stray light. By optimising the size of the emission area and the emission angle, all the light can be collected within the system. When MicroLEDs are deployed, component count can fall, because there is no need to mask and shape the light, and this ultimately leads to a simpler, cheaper system.

Addressable arrays can also be formed with MicroLEDs (see Figure 3 for example). Two-dimensional variants could be used for pico-projectors or near-to-the-eye displays. The addressable array takes on the roles of both the light source and the imaging engine, eliminating the need for a liquid crystal display or digital mirror device, leading to a lower system cost and trimmed power consumption.Whether these MicroLEDs are used by themselves, or in clusters or arrays, they have similar drive characteristics to LEDs. This means that they do not require complex control electronics or heat sinking, and effective operating lifetimes are 50,000 hours. Manufacture of these devices uses LED-type materials and processing, so the MicroLED benefits from the economies of scale available within this industry. This enables the light source to be affordable, giving it every opportunity for success in applications benefitting from efficient delivery of light by an optical system.

 
Figure 3: Individually addressable MicroLED pixels as presented at NIP28 conference