Hair-Width LEDs Engineered to Emit Directional Light, Rivaling Short-Distance Lasers

Inside a modern data center, the distances between components are measured in meters or less, but the data rates are measured in terabits per second. At those speeds, electrical signaling between boards and racks runs into fundamental limits - resistance, capacitance, and heat. Optical interconnects, which use light pulses to carry data, have become the preferred solution for longer links. Lasers have dominated that role because they emit tightly collimated, coherent light that can be coupled efficiently into optical fibers. The problem with lasers is cost and complexity. For the shortest links - inside a server rack, between chips on a board - the expense and fragility of laser modules is hard to justify.

Micro-light-emitting diodes, or microLEDs, are far cheaper to make and more robust in operation. They have one significant disadvantage: they emit light in all directions, spreading their output across a wide cone rather than projecting it into a narrow beam. That inefficiency makes them poor candidates for applications where the light needs to travel a defined path into a fiber or detector.

New research published in Optica Express by engineers at UC Santa Barbara describes a modification to gallium nitride microLEDs that changes this. By engineering the structure of the device, the team reshaped how light exits the LED, achieving a degree of directionality that makes these tiny emitters viable alternatives to lasers for short-distance optical communication.

What makes gallium nitride unusual

Gallium nitride is the semiconductor that enabled blue and white LEDs - work for which Shuji Nakamura, co-author on this study, shared the 2014 Nobel Prize in Physics. The same material is now the basis for high-efficiency solid-state lighting and the displays in modern smartphones and televisions. UCSB has maintained one of the world's leading research programs in gallium nitride materials and devices, and the current work builds on that infrastructure.

The study was led by doctoral student Roark Chao, co-advised by Steven P. DenBaars and Jon A. Schuller, both also co-authors. The DenBaars/Nakamura group focuses on gallium nitride growth and optoelectronics; Schuller's group works on nanoscale photonics. The collaboration brought together expertise in making the material and understanding how it interacts with light at very small scales.

Engineering the light cone

A standard microLED emits in a Lambertian pattern - roughly hemispherical, with intensity falling off as the angle from normal increases. To couple that output efficiently into an optical fiber or a collimating lens, you need either to accept large losses or to use complex optics that add cost and size. Neither is attractive for high-volume manufacturing.

The UCSB team's approach modifies the microLED's physical structure so that the optical cavity formed by the device geometry preferentially directs light toward the surface normal - the direction perpendicular to the chip face. The specific architecture, detailed in the paper, uses features at a scale comparable to the wavelength of light to create interference effects that reshape the emission pattern.

Chao notes the practical framing of the work: "If you can engineer how the light comes out, those microLEDs can start to replace lasers in short-distance data communication." The devices the team studied are literally the size of a hair follicle - roughly 10 to 20 micrometers across - which is small enough to place in tight arrays for high-density applications.

The case for microLEDs over lasers

Lasers require a resonant optical cavity, precise electrical contacts, and thermal management to maintain stable single-mode emission. Fabricating and packaging them is expensive. For links that need to span a few meters at most, the cost of a laser module is hard to amortize against the marginal bandwidth improvement over a well-engineered LED solution.

MicroLEDs can be manufactured using processes compatible with high-volume semiconductor fabrication. If the directionality problem can be solved - which this work begins to address - they could dramatically reduce the cost per gigabit of short-reach optical interconnects. For data centers where thousands of such links run continuously, that cost difference accumulates quickly.

Display applications are a parallel opportunity. Current high-resolution microLED displays face efficiency challenges partly because not all emitted light exits the device in the intended direction. Engineering emission directionality at the device level could reduce power consumption or increase apparent brightness without changing the input power.

What remains to be demonstrated

The study demonstrates the optical engineering principle and measures the improvement in emission directionality. Translating that into a deployed product requires showing that the modified device maintains adequate brightness, efficiency, and operating lifetime under sustained data modulation conditions. MicroLEDs also switch more slowly than lasers at the very highest modulation speeds - though for many short-reach applications the required bandwidth is well within LED switching capability.

The work represents a step toward practical microLED-based optical links, but the path from laboratory demonstration to manufacturable module involves engineering challenges that the current paper does not fully address.