The new device modulates visible light, without darkening it, with the smallest footprint and lowest power consumption

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Newswise – New York, NY – November 22, 2021 – Over the past decades, researchers have moved from using electrical currents to manipulating near infrared light waves for telecommunications applications such as 5G networks to broadband, on-chip biosensors, and driverless cars. This area of ​​research, known as integrated photonics, is evolving rapidly and researchers are now exploring the shorter (visible) wavelength range to develop a wide variety of emerging applications. These include Chip Scale Light Sensing and Telemetry (LIDAR), Augmented / Virtual / Mixed Reality (AR / VR / MR) Glasses, Holographic Displays, Chip Processing Chips quantum information and optogenetic probes implantable in the brain.

The only device essential to all of these applications in the visible domain is an optical phase modulator, which controls the phase of a light wave, in the same way that the phase of radio waves is modulated in wireless computer networks. 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 capable of controlling the propagation of light on a tiny chip or the emission of light from the chip.

But phase modulators in the visible are very difficult to achieve: there are no sufficiently transparent materials in the visible while offering great tunability, whether by thermo-optical or electro-optical effects. Currently, the two most suitable materials are silicon nitride and lithium niobate. Although both are highly transparent in the visible range, neither offer much possibility of tuning. Visible spectrum phase modulators based on these materials are therefore not only bulky but also power hungry: the length of individual waveguide-based modulators varies from hundreds of microns to several millimeters, and a single modulator consumes tens. milliwatts for phase adjustment. Researchers trying to achieve large-scale integration – integrating thousands of devices on a single microchip – have so far been blocked by these bulky and power-hungry devices.

Today, researchers at Columbia Engineering announced that they have found a solution to this problem: they have developed a means based on micro-ring resonators to drastically reduce both the size and the power consumption of a modulator. phase to visible spectrum, from one millimeter to 10 microns, and tens of milliwatts for phase tuning to less than one milliwatt. The study was published today by Photonics of nature.

“Usually the bigger something the better. But integrated devices are a notable exception,” said Nanfang Yu, associate professor of applied physics, team co-principal investigator (PI) and nanophotonics expert. . “It is really difficult to confine light to one place and manipulate it without losing a lot of its power. We are delighted that in this work we have made a breakthrough that will significantly expand the horizon of visible spectrum integrated photonics. in large scale.

Conventional optical phase modulators operating at visible wavelengths are based on the propagation of light in waveguides. Yu worked with his colleague Michal Lipson, who is the leading expert in integrated silicon nitride-based photonics, to develop a very different approach.

“The key to our solution was to use an optical resonator and run it in the so-called ‘highly over-coupled’ regime,” said Lipson, team co-PI and Eugene Higgins professor of electrical engineering and professor of application physics.

Optical resonators are structures with a high degree of symmetry, such as rings that can cycle a beam of light multiple times and translate tiny changes in refractive index into large phase modulation. Resonators can operate under several different conditions and should therefore be used with caution. For example, if operating in the “under-coupled” or “critical-coupled” regimes, a resonator will only provide limited phase modulation and, more problematically, introduce a large amplitude variation to the optical signal. The latter is a highly undesirable optical loss because the accumulation of even moderate losses from individual phase modulators will prevent them from being cascaded to form a circuit which has a sufficiently large output signal.

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

The team developed several strategies to push the devices into the heavily over-coupled regime. Most crucial was their invention of an adiabatic micro-ring geometry, in which the ring passes smoothly between a narrow neck and a wide belly, which lie on opposite edges of the ring. The narrow neck of the ring facilitates the exchange of light between the bus waveguide and the micro-ring, thus improving the coupling force. The broad belly of the ring reduces optical loss as the guided light only interacts with the outer side wall, not the inner side wall, of the enlarged portion of the adiabatic micro-ring, significantly reducing optical scattering at the level roughness of the side wall.

In a comparative study of adiabatic micro-rings and conventional micro-rings of uniform width fabricated side-by-side on the same chip, the team found that none of the conventional micro-rings met the condition of strong over-coupling. very poor optical losses, while 63% of the adiabatic micro-rings continued to operate in the highly over-coupled regime.

“Our best phase modulators working in the colors blue and green, which is the most difficult part of the visible spectrum, have a radius of only five microns, consume 0.8 mW of power for π phase adjustment and introduce variation. amplitude less than 10%, ”said Heqing Huang, Yu’s lab graduate student and first author of the paper.“ No previous work has demonstrated such compact, energy-efficient, low-loss phase modulators at visible wavelengths. ”

The devices were designed in Yu’s lab and fabricated in the Columbia Nano Initiative’s cleanroom, the nanofabrication facility at the Advanced Science Research Center at the Graduate Center of New York City University, and the science facility. and Cornell NanoScale technology. The characterization of the device was carried out in the laboratories of Lipson and Yu.

The researchers note that although they are far from the degree of integration of electronics, their work significantly narrows the gap between photonic and electronic switches. “If previous modulator technologies only allowed the integration of 100 waveguide phase modulators based on a certain chip footprint and power budget, now we can do that 100 times better and integrate 10,000 on-chip phase shifters to achieve much more sophisticated functions, ”said Yu.

Lipson and Yu laboratories are now collaborating to demonstrate a visible spectrum LIDAR made up of large 2D arrays of phase shifters based on adiabatic micro-rings. The design strategies employed for their visible spectrum thermo-optic devices can be applied to electro-optic modulators to reduce their footprints and control voltages, and can be adapted in other spectral ranges (e.g., ultraviolet, telecom, medium infrared and terahertz) and in other designs of resonators beyond micro-rings.

“So our work may inspire future efforts where people can implement strong over-coupling in a wide range of resonator-based devices to improve light-matter interactions, for example, to improve optical non-linearity, to to manufacture new lasers, to observe new quantum optical effects, while suppressing optical losses at the same time, ”said Lipson.

About the study

The study is entitled “Efficient and robust micron scale phase modulators at visible wavelengths.

The authors are: Guozhen Liang1,??, Heqing Huang1,??, Aseema Mohanty2.3, Min Chul Shin2, Xingchen Ji2, Michael Joseph Carter1, Sajan Shrestha1, Michal Lipson2*, and Nanfang Yu1*
1Department of Applied Physics and Applied Mathematics, Columbia Engineering
2Department of Electrical Engineering, Columbia Engineering
3Department of Electrical and Computer Engineering, Tufts University

This work was supported by the National Science Foundation (grant # QII-TAQS-1936359 (HH and NY) and # ECCS-2004685 (SS and NY)), the Defense Advanced Research Projects Agency (grant # HR00111720034 (GL , AM, MCS, XJ, ML and NY)), and the Air Force Office of Scientific Research (grant n ° FA9550-14-1-0389 (SS and NY) and n ° FA9550-16-1-0322 (NY )). Aseema Mohanty is supported by a Clare Boothe Luce Chair from the Henry Luce Foundation. Min Chul Shin acknowledges the support of the Facebook Fellowship Award 2020. Michael Joseph Carter is supported by the 2018 US Department of Defense SMART Fellowship Program.

GL, HH, ML, and NY are listed as inventors in U.S. Non-Provisional Patent Application No. 16/838,714, which relates to the technology reported in this article and claims priority over U.S. Provisional Application No. 62/838 084 and 62 / 828.261 filed by Columbia University. The other authors declare no competing interests.

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