Silicon photonics uses photons to detect processes and transmit information more efficiently than electrical signals, and yet have low manufacturing costs as a result of using conventional silicon-integrated-circuit processes. The same processes have made transistors, microchips, and integrated circuits from silicon to be small, affordable, and energy-efficient devices that could execute complex tasks.
Silicon photonics refers to the application of photonic systems using silicon as an optical medium. The silicon material used in such photonic systems is designed with sub micrometer precision and is deployed into the microphotonic components. Silicon photonics combines technologies such as complementary metal oxide semiconductor (CMOS), micro-electro-mechanical systems (MEMS) and 3D Stacking.
The propagation of light through silicon devices is governed by a range of nonlinear optical phenomena including the Kerr effect, the Raman effect, two-photon absorption and interactions between photons and free charge carriers. The presence of nonlinearity is of fundamental importance, as it enables light to interact with light thus permitting applications such as wavelength conversion and all-optical signal routing, in addition to the passive transmission of light.
Silicon waveguides are also of great academic interest, due to their unique guiding properties, they can be used for communications, interconnects, biosensors, and they offer the possibility to support exotic nonlinear optical phenomena such as soliton propagation.
Recent advancements in Silicon Photonics enable tighter monolithic integration of many optical functions within a single device is leading to higher yield, high reliability, and low costs common in the electronics industry. Along with the waveguides came optical modulators, oscillators, and resonators that could precisely modify the properties of light. On-chip spectrometers have become available that can extract molecular and atomic signals from incident light on a device that is smaller than a fingernail.
Silicon waveguide technology
Waveguides are components used to route light signals on a chip. The high refractive index of silicon enables the high confinement of light in the waveguides and leads to the reduction of the footprint of the silicon-based photonic devices. Silicon-on-insulator processes, which are now widely available for CMOS, can easily create a low-loss waveguide by patterning the same semiconductor layer used for transistors and then covering it with insulator.
However, the high thermo-optic (TO) coefficient of silicon which is around 1.86 × 10−4/K and the high confinement of light in the waveguide core leads to high temperature sensitivity of the silicon photonic devices. Researchers from University of California, Santa Barbara (UCSB) and the California Institute of Technology (CalTech) came up with new microchip-scale, integrated waveguides for photonic delay employing silicon processing. Such photonic delays are useful in military applications ranging from small navigation sensors to wideband phased array radar and communication antennas, according to DARPA
Rajesh Menon and other Engineers at the University of Utah (Salt Lake City, UT) have developed an ultracompact beamsplitter—measuring only 2.4 by 2.4 microns or one-fiftieth the width of a human hair –for dividing light waves into two separate channels of information. In 2015, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) developed the first on-chip metamaterial with a refractive index of zero, meaning that the phase of light could be stretched infinitely long. The metamaterial represented a new method to manipulate light and was an important step forward for integrated photonic circuits, which use light rather than electrons to perform a wide variety of functions.
Now, SEAS researchers have pushed that technology further – developing a zero-index waveguide compatible with current silicon photonic technologies. In doing so, the team observed a physical phenomenon that is usually unobservable — a standing wave of light. When the refractive index is reduced to zero the light no longer behaves as a moving wave, traveling through space in a series of crests and troughs, otherwise known as phases. Instead, the wave is stretched infinitely long, creating a constant phase. The phase oscillates only as a variable of time, not space.
This is exciting for integrated photonics because most optical devices use interactions between two or more waves, which need to propagate in sync as they move through the circuit. If the wavelength is infinitely long, matching the phase of the wavelengths of light isn’t an issue, since the optical fields are the same everywhere.
“This adds an important tool to the silicon photonics toolbox,” said Camayd-Muñoz. “There’s exotic physics in the zero-index regime, and now we’re bringing that to integrated photonics. That’s an important step, because it means we can plug directly into conventional optical devices, and find real uses for zero-index phenomena. In the future, quantum computers may be based on networks of excited atoms that communicate via photons. The interaction range of the atoms is roughly equal to the wavelength of light. By making the wavelength large, we can enable long-range interactions to scale up quantum devices.”
Scientists at the Naval Information Warfare Center Pacific (NIWC-Pacific) have recently invented a new type of optical waveguide deposited on a silicon wafer that results in greater confinement of light, as reported in Sep 2020. The waveguide has applications for low-loss, high-bandwidth data processing in modern computing systems.
Silicon photonics involves patterning a silicon wafer with optical components using existing semiconductor manufacturing techniques. These miniaturized photonics systems show significant promise for next-generation optical communications technology. A key component of these devices is optical waveguides, which ideally guide light with minimal loss of energy. Ideal light confinement requires cladding material with a very low refractive index. While silicon dioxide works well as a cladding material, a convenient method of thin-film deposition — called plasma-enhanced chemical vapor deposition (PECVD) — tends to produce suboptimal silicon dioxide films.
To this end, Navy researchers have developed a way to manufacture an optical waveguide with silicon dioxide cladding using PECVD. First, a waveguide is patterned onto a silicon wafer using electron-beam resist lithography or photoresist etching. The waveguide and wafer are then covered in a thick silicon dioxide cladding through a particular PECVD process. The cladding has a greatly reduced refractive index and thus achieves better light confinement than traditional PECVD waveguides.
Silicon waveguides move us closer to faster computers that use light, reported in feb 2021
With results published in Light Science & Applications, our Zurich-based team of researchers has just managed to efficiently guide visible light through a silicon wire — an important milestone towards faster, more efficient integrated circuits. Our low-loss silicon waveguide could enable new photonic chip designs for applications that rely on visible light, and could lead to more efficient lasers and modulators used in telecoms.
In optical circuits, information is encoded in light rather than electronics. In 2019, together with partners from Skolkovo Institute of Science and Technology and University of Southampton, we built the world’s first ultrafast all-optical transistor that can operate at room temperature. Our latest work is a build-up on that: a silicon waveguide that can be used to connect such transistors, carrying light between them with minimal losses.
Wiring up transistors of an optical circuit with silicon waveguides is crucial for compact, highly integrated chips. And it’s easier to integrate other components such as electrodes if the waveguide is made of silicon — a cheap and abundant material that happens to be an excellent semiconductor. One challenge, though, has been the ability of silicon to absorb visible light — for which it’s called a ‘dark’ material. While important for capturing sunlight in solar panels, this is not great for a waveguide where light absorption means signal loss.
To deal with the absorption issue, we’ve opted for nanostructures called high contrast gratings. Such a grating consists of nanometer-sized ‘posts’ lined up to form a ‘fence’ that prevents light from escaping. The posts are 150 nanometers in diameter and are spaced so that light passing through them interferes destructively with light passing between them. Destructive interference is a phenomenon where waves – including electromagnetic waves such as visible light – that oscillate out of sync cancel each other out. This way, no light can “leak” through the grating and most of it gets reflected back inside the waveguide.
Our team has also shown that absorption of light inside the posts is minimal. Together, these two features lead to losses of only 13 percent along the light’s travel path of one millimeter inside the waveguide. For comparison, in a pure silicon waveguide without the grating, losses would amount to 99.7 percent in just 10 micrometers.
Simulations for precise grating design
The idea behind the high contrast gratings may look simple. Still, we were surprised to find that gratings could keep light from being absorbed by a ‘dark’ material like silicon. We first observed the grating effect in 2010 in a laser microcavity – which helped because the light amplification by the laser compensated for the losses. Also, then the light hit the gratings at almost 90 degrees – a sweet spot for the grating effect to kick in.
But keeping the losses low in a waveguide without the benefit of the laser gain, and at an almost-grazing angle of the incoming light, is much more challenging.
To ensure the grating design would work, we ran simulations to see how light propagation inside the waveguide would change with different grating dimensions. We found that the grating would provide efficient guiding over a broad band of wavelengths.
We just needed to determine the right spacing between the grating posts and make the posts the right thickness, within a precision margin of 15 nanometers. We did that — and our tests confirmed the simulations when we obtained low loss for visible light in the range between 550 and 650 nanometers.
In simulations, we also showed that the design could be used to guide the light around corners. In the future, we aim to confirm this experimentally. If it works, we’ll need to optimize the technology further to keep the additional losses low.
The next step is to engineer the efficient coupling of the light out of the waveguides into other components. That’s a crucial step in our research, with the ultimate goal of integrating the all-optical transistors into integrated circuits that would be able to perform simple logic operations.
Urbonas, D., Mahrt, R.F. & Stöferle, T. Low-loss optical waveguides made with a high-loss material. Light Sci Appl 10, 15 (2021). https://doi.org/10.1038/s41377-020-00454-w
Silicon waveguides with material absorption of 13,000 dB/cm transmit well in the visible reported in May 2021
Because silicon is transparent in the near-IR spectral region but quite opaque at visible wavelengths, silicon photonic circuits cannot ordinarily be made to operate in the visible region. However, Darius Urbonas, Rainer Mahrt, and Thilo Stöferle of IBM Research Europe–Zurich (Rüschlikon, Switzerland)—the same group of researchers who in 2019 created the world’s first ultrafast all-optical transistor capable of operating at room temperature—have experimentally achieved this feat, fabricating waveguides with losses down to 6 dB/cm in the 550–650 nm region in silicon, which has a material absorption of 13,000 dB/cm in this region. Their solution involves nanostructures, in the form of high-contrast gratings, with a striking behavior that some of the team members had already discovered over 10 years ago, albeit for another application.
The high-contrast grating consists of nanometer-sized posts lined up to form a fence that prevents light from escaping. The posts are 150 nm in diameter and are spaced so that light passing through the posts interferes destructively with light passing between posts. Instead, most of the light gets reflected back inside the waveguide. The IBM researchers also showed that absorption of light inside the posts themselves is minimal. All this together translates to losses of only 13% along a light travel path of 1 mm inside the waveguide (see figure: blue = 600 nm, red = 700 nm, black = 655 nm, and orange = simulation at 600 nm). For comparison, in a pure silicon waveguide without the gratings, the losses would amount to 99.7% in a distance of only 10 μm.
Simulations showed that the grating would provide efficient broadband guiding of light over a broad band of wavelengths. Using a standard silicon-photonics fabrication process, the grating waveguides were fabricated; experiments confirmed low loss for visible light in the range between 550 and 650 nm. Simulations also show that this design may be used to make not only straight waveguides, but also to guide light around corners; however, the researchers have not yet experimentally confirmed this. Even if it proves feasible, some further optimization will be needed to keep the additional losses low in that case. A next step will be to engineer the efficient coupling of the light out of the waveguides into other components. That will be a crucial step in the team’s multiyear exploratory research project with the goal of integrating the all-optical transistors they demonstrated in 2019 into integrated circuits capable of performing simple logic operations. Reference: D. Urbonas, R. Mahrt, and T. Stöferle, Light Sci. Appl. (2021); https://doi.org/10.1038/s41377-020-00454-w.
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