Computing at the Speed of Light

Today’s computers use electrons to shuffle information around, but this might change soon. Tomorrow’s computers might do the same with light, allowing them the same level of advancement that telecommunications has undergone in transitioning from electric signals (in metal wires) to optical signals (in fiber-optic cables).

Some recently-published research from the lab of Eric Mazur at Harvard has brought the cutting edge one step closer to this goal.

The trouble with using light to do computations is that it’s hard to direct light at very small scales. Traditional modern computers use upwards of 10 billion tiny signal-triggered switches called transistors to manage information. These tiny switches exchange information over even tinier wires all embedded within a microchip. In the face of the ever-present goal to do this optically rather than electronically, tiny fiber-optic cables have proven ill-equipped for connecting the tiny optical versions of transistors, so until now there’s been no way to miniaturize optical chips.

But that’s all now changed with the development of what researchers call “on-chip zero-index metamaterials.”

The prefix “meta-” comes from Greek, meaning “to go beyond.” While this could mean any material exhibiting properties going beyond nature, the name has come to represent a class of materials that bend light in strange ways. Folks with stronger eyeglass prescriptions can have thinner lenses made from special material with a high “index of refraction” which bends light more strongly. In a strange twist, metamaterials bend light in the direction opposite of conventional materials because their index is negative, or in the case of this new material, the index is zero.

Truly strange things happen when a material has a refractive index of zero. When light enters a zero-index material, it no longer oscillates over space, becoming an infinitely squished wave with crests and troughs infinitely far apart. The light also exhibits an infinite “phase velocity” meaning the crests and troughs of the light wave move infinitely fast. This doesn’t violate Einstein’s Laws of Relativity because the information contained in light travels instead at the “group velocity,” which is always below Einstein’s speed limit. These materials are ideal for transmitting light at tiny scales.

Zero-index materials have been manufactured in the past, though this is the first time such a material has been grown on a microchip. At the Nano scale (1 million times smaller than a millimeter), the material resembles a forest of gold-capped silicon trees on a ground of silica paved with gold, all flooded beneath a lake of epoxy resin, with another layer of gold on top.

According to Yang Li, a postdoctoral fellow in the Mazur Lab and first author on the paper, “regardless of how the material is shaped, electromagnetic energy (light) is produced in full transmission,” meaning it can effectively be used to transmit light within microchips.

Other applications are also on the horizon. The team hopes to use this new material to generate “superradiant lasers,” which can “operate without a cavity,” says Philip Muñoz, PhD student and co-author on the paper. In a conventional laser a cavity is a necessity, but it’s also a major source of imperfections. Superradiant lasers, because their beams are so pristine, will be able to hit much thinner targets.

Zero-index materials can also boost the efficiency of “nonlinear optics,” materials that change the wavelength of incoming light. Sometimes the light waves leaving nonlinear optics are out of phase and thus cancel out. The addition of zero-index materials can mitigate this problem. It is expected that the dual use of these materials will also have a role in future development of optics-based computing.