Photonic microresonators are promising building blocks for this purpose because they can reproduce several key features of biological neurons using light alone. When properly designed, these tiny optical devices can respond selectively to inputs, emit brief pulses, and become temporarily less responsive, or entirely unresponsive, after firing. These behaviors arise naturally from the interplay between light and the material properties of the resonator, without requiring external circuitry.
However, conventional microresonators have an important limitation: they respond in essentially the same way when light travels through them in one direction or the other. Biological neurons, in contrast, have a strong directional character. Signals typically travel from the input side of a neuron to its output, whereas propagation in the reverse direction is restricted. This asymmetry is important because it influences how neurons interact and helps to prevent unwanted feedback from spreading through a network.
In practice, this has direct consequences for photonic neural networks. A microresonator that emits a spike in response to an input can also send an uncontrolled signal back toward the previous node, because the forward and backward responses are intrinsically linked. Programmable directionality could break this link: a photonic neuron could fire when driven from one direction while remaining inactive when driven from the other, and its backward signal could be independently suppressed or deliberately enhanced. Such control could reduce unwanted crosstalk while turning backward signals into a resource for controlling interactions between neighboring neurons.
The authors of this article have provided a solution in the form of a device called DRUM, Dynamically Reconfigurable Unified Microresonator. This device comprises a silicon ring-shaped waveguide connected to two side lobes, each of which incorporates two microheaters (see Figure 1(a)). The flow of small currents through these heaters controls how light is exchanged between the two directions in which it can circulate inside the ring. In an ordinary microresonator, by contrast, this property is fixed once the chip is fabricated. That control is enough to change how the device behaves. Driven from one direction, the ring accumulates enough energy to break a steady laser beam into a train of optical spikes. Driven from the other, it remains quiet even when the incoming power is pushed well beyond the typical firing threshold. The same physical component can be operated in three distinct regimes: silent, excitable, or hypersensitive, without altering its geometry, allowing a single element to perform roles that would otherwise require different devices.
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