Integrated photonics is fast evolving and investigators are now exploring the shorter – visible – wavelength range to develop a broad variety of emerging appli­cations. These include chip-scale Lidar, AR/VR/MR goggles, holo­graphic displays, quantum information processing chips, and implantable opto­genetic probes in the brain. The one device critical to all these appli­cations in the visible range is an optical phase modulator, which controls the phase of a light wave. With a phase modulator, researchers can build an on-chip optical switch that channels light into different waveguide ports. With a large network of these optical switches, researchers could create sophis­ticated inte­grated optical systems that could control light propa­gating on a tiny chip or light emission from the chip.

But phase modulators in the visible range are very hard to make: there are no materials that are transparent enough in the visible spectrum while also providing large tuna­bility, either through thermo-optical or electro-optical effects. Currently, the two most suitable materials are silicon nitride and lithium niobate. While both are highly trans­parent in the visible range, neither one provides very much tuna­bility. Visible-spectrum phase modulators based on these materials are thus not only large but also power-hungry: the length of individual waveguide-based modu­lators ranges from hundreds of microns to several mm and a single modulator consumes tens of milliwatts for phase tuning. Researchers trying to achieve large-scale inte­gration have, up to now, been stymied by these bulky, energy-consuming devices.

Now, Columbia Engineering researchers announced that they have found a solution to this problem: they’ve developed a way based on micro-ring resonators to dramatically reduce both the size and the power consumption of a visible-spectrum phase modulator, from 1 milli­meters to 10 microns and from tens of milliwatts for π phase tuning to below 1 milliwatt. “Usually the bigger something is, the better. But integrated devices are a notable exception,” said Nanfang Yu, associate professor of applied physics and an expert in nano­photonics. “It’s really hard to confine light to a spot and manipulate it without losing much of its power. We are excited that in this work we’ve made a breakthrough that will greatly expand the horizon of large-scale visible-spectrum inte­grated photonics.” 

Conven­tional optical phase modulators operating at visible wavelengths are based on light propa­gation in waveguides. Yu worked with his colleague Michal Lipson, who is the leading expert on integrated photonics based on silicon nitride, to develop a very different approach. “The key to our solution was to use an optical resonator and to operate it in the strongly over-coupled regime,” said Lipson. Optical reso­nators are structures with a high degree of symmetry, such as rings that can cycle a beam of light many times and translate tiny refractive index changes to a large phase modu­lation. Resonators can operate under several different conditions and so need to be used carefully. For example, if operating in the “under coupled” or “critical coupled” regimes, a resonator will only provide a limited phase modulation and, more problema­tically, introduce a large amplitude variation to the optical signal. The latter is a highly undesirable optical loss because accu­mulation of even moderate losses from individual phase modulators will prevent cascading them to form a circuit that has a suffi­ciently large output signal.

To achieve a complete 2π phase tuning and minimal amplitude variation, the Yu-Lipson team chose to operate a micro-ring in the “strongly over-coupled” regime, a condition where the coupling strength between the microring and the bus waveguide that feeds light into the ring is at least 10 times stronger than the loss of the microring. “The latter is primarily due to optical scattering at the nanoscale roughness on the device sidewalls,” Lipson explained. “You can never fabri­cate photonic devices with perfectly smooth surfaces.”

The team developed several strategies to push the devices into the strongly over-coupled regime. The most crucial one was their invention of an adiabatic micro-ring geometry, where the ring smoothly transi­tions between a narrow neck and a wide belly, which are at the opposite edges of the ring. The narrow neck of the ring faci­litates the exchange of light between the bus waveguide and the microring, thus enhancing the coupling strength. The ring’s wide belly reduces optical loss because the guided light interacts only with the outer sidewall, not the inner sidewall, of the widened portion of the adiabatic microring, substan­tially reducing optical scattering at the sidewall roughness.

In a comparative study of adiabatic microrings and conventional microrings with uniform width fabri­cated side by side on the same chip, the team found that none of the conventional micro­rings satisfied the strong over-coupling condition – in fact, they suffered very bad optical losses – while 63 % of the adiabatic micro­rings kept operating in the strongly over-coupled regime. “Our best phase modu­lators operating at the blue and green colors, which are the most difficult portion of the visible spectrum, have a radius of only 5 microns, consume power of 0.8 milliwatt for π phase tuning, and intro­duce an amplitude variation of less than 10 %,” said Heqing Huang, a graduate student in Yu’s lab. “No prior work has demonstrated such compact, power-effi­cient, and low-loss phase modulators at visible wavelengths.”

The devices were designed in Yu’s lab and fabri­cated in the Columbia Nano Initiative cleanroom, at the Advanced Science Research Center NanoFabrication Facility at the Graduate Center of the City University of New York, and at the Cornell NanoScale Science and Technology Facility. Device charac­terization was conducted in Lipson’s and Yu’s labs. The researchers note that while they are nowhere near the degree of inte­gration of electronics, their work shrinks the gap between photonic and electronic switches substantially. “If previous modulator technologies only allow for inte­gration of 100 waveguide phase modulators given a certain chip footprint and power budget, now we can do that 100 times better and integrate 10,000 phase shifters on chip to realize much more sophis­ticated functions,” said Yu.

The Lipson and Yu labs are now colla­borating to demonstrate visible-spectrum Lidar consisting of large 2D arrays of phase shifters based on adiabatic microrings. The design strategies employed for their visible-spectrum thermo-optical devices can be applied to electro-optical modulators to reduce their footprints and drive voltages, and can be adapted in other spectral ranges and in other resonator designs beyond micro­rings. “Thus, our work can inspire future effort where people can implement strong over-coupling in a wide range of resonator-based devices to enhance light-matter inter­actions, for example, for enhancing optical non­linearity, for making novel lasers, for observing novel quantum optical effects, while suppressing optical losses at the same time,” Lipson said. (Source: Columbia U.)

Reference: G. Liang et al.: Robust, efficient, micrometre-scale phase modulators at visible wavelengths, Nat. Phot. 15, 908 (2021); DOI: 10.1038/s41566-021-00891-y

Link: Dept. of Applied Physics and Applied Mathematics, Columbia University, New York, USA