Tiny Resonators Modulate Visible Light

A visible-spectrum phase modulator

A visible-spectrum phase modulator fabricated by researchers at Columbia University measures just 20 microns across – making it far smaller than a grain of morning glory pollen (top) or one of the thousands of scales that cover a butterfly wing (bottom). [Image: Heqing Huang and Cheng-Chia Tsai/Columbia Engineering]

Optical phase modulators operating at visible wavelengths could potentially enable a raft of new applications for integrated photonics—from quantum information processing to light-based control of neurons. To date, however, such devices have been impractically large and power-hungry.

Researchers at Columbia University, NY, USA, have now designed a modulator scheme that they show can reduce both a device’s size and power consumption by at least an order of magnitude compared with existing technology (Nat. Photon., doi: 10.1038/s41566-021-00891-y). The scheme involves asymmetric micro-resonators operating in what is known as the strongly over-coupled regime.

A key missing element

The ability to modulate the phase of visible light waves will be key for much of tomorrow’s photonic technology. Modulators would enable integrated circuits to feature optical switches, large numbers of which could be used to control how light propagates through chips and is emitted by them.

Such switching is already done routinely using silicon circuits at near-infrared wavelengths, being exploited in, among other areas, high-speed cellular phone networks. But the thermo-optic and electro-optic effects used to modify a light wave’s phase, when exploited in materials that are compatible with CMOS processes, are weak at visible wavelengths. This means that modulators made from silicon-nitride waveguides, an established technology, are typically hundreds of micrometers long and require tens of milliwatts of power.

But while waveguides accumulate phase as a light wave propagates over extended distances, resonators require only a small area to achieve such accumulation. The idea here is to direct a laser beam slightly off-resonance at the device in question, and then use a small amount of heat or electric current to change the device’s refractive index such that the wave becomes resonant—at which point its phase shifts abruptly (circulating many times before leaving the device). However, resonators come with their own drawbacks, including the fact that resonance usually modulates a wave’s amplitude as well as its phase.

Ring-shaped micro-resonators

In the latest work, Optica Fellows Michal Lipson and Nanfang Yu, along with colleagues at Columbia, have shown how to largely overcome these problems by using ring-shaped micro-resonators with non-concentric inner and outer circumferences. The narrowest part of such “adiabatic” micro-rings enhances the coupling between the resonator and a waveguide supplying light. Meanwhile, the widest part helps decrease optical losses, as the light at that point experiences less scattering because it hugs only the device’s outer wall.

The researchers made their resonators from thin films of silicon nitride measuring just 10 to 20 µm across. Each resonator was placed in one of the two arms of a Mach–Zehnder interferometer, and the interferometer’s outputs were used to monitor the resonator’s phase and amplitude responses. The device was thermo-optically tuned by placing a micro-heater just above the resonator.

Order-of-magnitude improvements

Experiments demonstrated that the devices operate in the strongly over-coupled regime, which means that the rate of coupling between resonator and waveguide is at least ten times greater than the rate at which light decays from the ring. Exposing a resonator to green light (at 530 nm) and then heating it up, the researchers found they needed to increase the heat by a mere 0.85 mW to induce a phase shift of π. At the same time, they measured a roughly 10% variation in amplitude.

Lipson, Yu and colleagues compared this performance to that of modulators made from straight silicon-nitride waveguides some 200 to 500 µm long. They found that these control devices required about 20 mW of heat to achieve the same phase shift—more than 20 times that of the adiabatic resonators.

Effective at any color

Using simulations, the researchers also showed that a suitably designed adiabatic micro-ring could support a series of resonances across the entire visible spectrum, all of which would satisfy the strongly over-coupled condition. Such a device, they say, could therefore manipulate light “of any color in the visible region”—a “clear advantage,” the team argues, over non-resonant waveguide devices in terms of power efficiency and compactness.

What’s more, the Columbia team discovered that the reduced contact between light and device walls made the resonators more robust against fabrication imperfections than standard micro-rings. Comparing 52 adiabatic and 49 standard rings, all fabricated on the same chip, the group found that the former devices had a third of the optical losses of the latter, and were also less sensitive to variations in waveguide thickness and width. This, the team maintains, affords greater confidence in the amount of heat needed to achieve a given phase shift.

Applications in compact, power-stingy settings

The researchers say the most suitable applications of their technology are those involving small, light systems that consume little power—such as as augmented-reality goggles, quantum processing circuits, optical neural networks and neural probes for optogenetics.

They acknowledge that the adiabatic devices do have their drawbacks, such as limited rates of modulation and some remaining variation in amplitude. But they don’t think such problems will prove to be showstoppers. They argue, for example, that applications in the visible region typically don’t need modulation speeds as high as those of telecommunications. And it should be possible, according to the team, to reduce amplitude variation by further increasing coupling or decreasing losses.

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