In recent years, researchers have made strides in embedding light-controlling elements on microchips. Even so, the photonics field has yet to see a functional on-chip optical isolator—one that transports light unidirectionally.

Recently, however, a team of researchers out of the Illinois Quantum Information Science and Technology (IQUIST) published a paper that describes how they used on-chip optical isolation in a soundwave-based, light-controlling photonic circuit.

Fabricated using lithium niobate, these on-chip optical isolators are designed for the 780 nm and 1550 nm wavelengths. Image used courtesy of Ogulcan Orsel and the University of Illinois Urbana-Champaign

What exactly are optical isolators? And what are the implications of this new study for quantum computing?

An Overview of Optical Isolators

Because it can be difficult to control light in photonic applications, designers often incorporate additional devices to ensure unidirectional light transfer. One such device is the optical isolator.

Optical isolators enable light transmission in a single direction, consequently eliminating multidirectional light transfer. Light can be reflected, refracted, or absorbed by objects during transmission, causing several issues, including interference and equipment malfunction.

Example of an optical isolator. Image from Geek3 [CC BY-SA 4.0]

These devices rely on three key components: an input polarizer, Faraday rotator, and output polarizer.

Optical isolators offer two basic modes of operation, forward and backward modes. In the former, the input polarizer receives light waves, filters them, and releases linearly polarized light to the Faraday rotator. This polarized light passes through the Faraday rotator, rotating the plane of polarization (POP) by 45° to the input signal. The output polarizer maintains this rotation and releases a 45° polarized light signal.

Conversely, the backward mode begins with the output polarizer, which receives the light and polarizes it at 45°. When the light travels through the Faraday rotator, it experiences a 45° POP rotation in the positive direction. At this point, the total rotation to the input polarizer is 90°.  

The entire process ends with the optical isolator releasing a perpendicular POP-based light for reflection or absorption.

Operation of an optical isolator. Image used courtesy of Thor Labs

The Challenges of Miniaturizing Optical Isolators

Researchers have explored several approaches to miniaturize optical isolators for a host of applications. But the goal of shrinking these isolators to chip size has been hindered by a few factors.

For one, magneto-optical materials do not comply with conventional chip fabrication techniques. As a result, some researchers have attempted to physically modify conventional semiconductors (such as silicon) by wafer-bonding yttrium iron garnet thin-films, but this proved inefficient. During past attempts to shrink optical isolators, researchers have also noted the adverse effects of magnetic fields on atoms and the inefficiency of small-scaled isolator components. 

Even so, optical isolators are crucial components of photonic devices. They minimize optical signal interference and optimize device performance in the process. 

Shrinking Optical Isolators Might Shrink Quantum Tech

Recent research by a team at IQUIST demonstrates an efficient way to achieve on-chip optical isolation.

This study is said to provide high-performance light isolation and directionality control. According to the publication, the approach may also offer better results than existing on-chip alternatives and is compatible with atom-based sensors.

Researchers faced several challenges in developing on-chip optical isolators, including:

• Choosing the material
• Ensuring compatibility with atomic sensors
• Preventing malfunctions common in conventional optical isolator-based components
• Mitigating magnetic field interference with atoms

The research team side-stepped these roadblocks by harnessing the capabilities of a non-magnetic isolator that adapts to several light wavelengths. 

Microscopic view of the on-chip isolator. Image used courtesy of Sohn et al.

The on-chip photonic isolator includes an adjacent ring resonator and a waveguide. The waveguide channels incoming light into the resonator, which blocks the light flow. The team ensured the continuous flow of light by applying sound waves to the ring. The acoustics help to capture light moving backward and transmit the light forward. These features helped the team address the challenges of miniaturizing optical isolators. 

A key benefit of this research is that it also opens doors to miniaturize quantum technology. Light control is essential for quantum technology; the implications of on-chip optical isolation may be key to reducing the footprint of this historically-large technology.