One photon to flip them all: New optical switch approaches peak energy efficiency

Working with colleagues from several academic institutions and within the EU-funded project, PoLLoC, we have created an extremely efficient, ultrafast all-optical switch. In the journal Nature, we detail¹ how it brings power consumption nearly to the lowest possible limit.

The switch could in future enable us to create devices that run on light rather than electronics. Such photonics devices could excel both in terms of speed and energy efficiency. Our results could also contribute to the nascent field of quantum optical communication, to link quantum computers together by transmitting data between them via optical signals.

Laser switching

Our optical switch uses a very weak laser beam (control beam) to turn another, brighter laser beam on and off. To do the trick, we exploit a microcavity—a 35 nanometer thin sheet of an organic semiconducting polymer, sandwiched between highly reflective inorganic materials.

The microcavity is built in a way to keep incoming light trapped inside for as long as possible, for it to remain coupled with the cavity’s material. When that light has the right characteristics, such as the correct wavelength, it can switch the brighter laser on and off using just a few photons. A photon is nature’s smallest unit of light energy—meaning it’s the ultimate rock-bottom when it comes to energy consumption. It doesn’t get more power-saving than that.

Even running on minimal power, our switch can change between “0” and “1” states faster than a picosecond. It would translate into a trillion switching operations per second—more than a hundredfold speed-up with respect to the fastest commercial circuits in today’s computers.

The switch works at room temperature, so it’s easy to scale it up to real-world use in logic circuits.

And it’s more than just a switch: When used in amplifier mode, it can increase the signal strength (intensity) of an incoming laser beam by a factor of up to 23,000.

Building up on previous success

The switch, created at the IBM Research Europe lab in Zurich, is based on our previous results on exciton-polaritons from two years ago that have paved the way to the world’s first ultrafast all-optical exciton-polariton transistor. Exciton-polaritons, which also form the basis of the new switch, belong to a class of short-lived entities called quasiparticles.

These entities can arise in a solid-state device when light couples so strongly with the bound electron-hole pairs (excitons) in its material that they become inseparable. It is these hybrid quasiparticles that allow us to set the “0” and “1” states in our switch. The “1” state corresponds to a large number of quasiparticles in the lowest, ground energy state, and the “0” state corresponds to none or just a few quasiparticles in this state.

By shining a “pump” laser on the device, we first create a large number of quasiparticles with energies higher than that of the ground state. To trigger the switching response, we then use a seeding or “control” laser. Its role is to create a few quasiparticles with the ground state energy. The “seeding” of the ground state triggers an avalanche process that pushes the more energetic quasiparticles down to the ground state. That relaxation sets the switch to the “1” state.

In addition to the seeding, vibrations of the semiconducting polymer’s molecules can help the relaxation process, making it much faster and more efficient. The trick is to match the energy gap between the higher-energy states and the ground state to the energy of one specific type of molecular vibration in the polymer.

In 2019, we optimized our all-optical room temperature transistor for fast switching, focusing less on its energy expenditure. This time, our collaborators and we have taken a closer look at the setup in search for maximum efficiency.

As we needed just a few photons, we’ve achieved switching around 1 attojoule—a tiny amount of energy. Most modern electrical transistors take tens of attojoules to switch, so replacing electricity with light could lower consumption by a factor of 10.

While there are electrical switches (transistors) that work at attojoule energy levels using single electrons, those aren’t nearly as fast and most don’t work at room temperature.

To help cut the energy consumption further, we also carefully chose the laser’s wavelength and the angle at which we shone the pump laser on the switch. That increased the efficiency of the pump process and avoided energy loss through absorption that would trigger molecular vibration and heat up the microcavity.

A single photon is enough to flip our switch on and off, but we do require a few photons for a sufficiently large signal-to-noise ratio.

There’s still some work ahead to lower the overall power consumption of the device, currently dominated by the pump laser that serves to create the bath of energetic quasiparticles. For that, we are considering using perovskite supercrystal materials instead of the organic polymer. They could be excellent candidates given their strong light-matter coupling that in turn leads to a powerful collective quantum response in the form of superfluorescence.

The latest switch is yet another addition to the growing toolkit of all-optical components we have assembled over the last few years. It also includes a low-loss silicon waveguide for visible light that would shuttle the optical signals back and forth between transistors.

The future just keeps getting brighter for optical circuits.