Nav: Home

Light and sound in silicon chips: The slower the better

September 16, 2019

Integrated circuits in silicon enable our digital era. The capabilities of electronic circuits have been extended even further with the introduction of photonics: components for the generation, guiding and detection of light. Together, electronics and photonics support entire systems for data communication and processing, all on a chip. However, there are certain things that even electrical and optical signals can't do simply because they move too fast.

Sometimes moving slowly is actually better, according to Prof. Avi Zadok of Bar-Ilan University's Faculty of Engineering and Institute of Nanotechnology and Advanced Materials. "Important signal processing tasks, such as the precise selection of frequency channels, require that data is delayed over time scales of tens of nano-seconds. Given the fast speed of light, optical waves propagate over many meters within these timeframes. One cannot accommodate such path lengths in a silicon chip. It is unrealistic. In this race, fast doesn't necessarily win."

The problem, in fact, is a rather old one. Analog electronic circuits have been facing similar challenges in signal processing for sixty years. An excellent solution was found in the form of acoustics: A signal of interest is converted from the electrical domain to the form of an acoustic wave. The speed of sound, of course, is slower than that of light by a factor of 100,000. Acoustic waves acquire the necessary delays over tens of micro-meters instead of meters. Such path lengths are easily accommodated on-chip. Following propagation, the delayed signal can be converted back to electronics.

In a new work published today (September 16, 2019) in the journal Nature Communications, Zadok and co-workers carry over this principle to silicon-photonic circuits.

"There are several difficulties with introducing acoustic waves to silicon chips," says doctoral student Dvir Munk, of Bar-Ilan University, who participated in the study. "The standard layer structure used for silicon photonics is called silicon on insulator. While this structure guides light very effectively, it cannot confine and guide sound waves. Instead, acoustic waves just leak away." Due to this difficulty, previous works that combine light and sound waves in silicon do not involve the standard layer structure. Alternatively, hybrid integration of additional, nonstandard materials was necessary.

"That first challenge can be overcome by using acoustic waves that propagate at the upper surface of the silicon chip," continues Munk. "These surface acoustic waves do not leak down as quickly. Here, however, there is another issue: Generation of acoustic waves usually relies on piezo-electric crystals. These crystals expand when a voltage is applied to them. Unfortunately, this physical effect does not exist in silicon, and we much prefer to avoid introducing additional materials to the device."

As an alternative, students Munk, Moshe Katzman and coworkers relied on the illumination of metals. "Incoming light carries the signal of interest," explains Katzman. "It irradiates a metal pattern on the chip. The metals expand and contract, and strain the silicon surface below. With proper design, that initial strain can drive surface acoustic waves. In turn, the acoustic waves pass across standard optical waveguides in the same chip. Light in those waveguides is affected by the surface waves. In this way, the signal of interest is converted from one optical wave to another via acoustics. In the meantime, significant delay is accumulated within very short reach."

The concept combines light and sound in standard silicon with no suspension of membranes or use of piezo-electric crystals. Acoustic frequencies up to 8 GHz are reached, however the concept is scalable to 100 GHz. The working principle is applicable to any substrate, not only silicon. Applications are presented as well: the concept is used in narrowband filters of input radio-frequency signals. The highly selective filters make use of 40 nano-second long delays. "Rather than use five meters of waveguide, we achieve this delay within 150 microns," says Munk.

Prof. Zadok summarizes: "Acoustics is a missing dimension in silicon chips because acoustics can complete specific tasks that are difficult to do with electronics and optics alone. For the first time we have added this dimension to the standard silicon photonics platform. The concept combines the communication and bandwidth offered by light with the selective processing of sound waves."

One potential application of such devices is in future cellular networks, widely known as 5G. Digital electronics alone might not be enough to support the signal processing requirements in such networks. Light and sound devices might do the trick.

Bar-Ilan University

Related Silicon Articles:

A leap in using silicon for battery anodes
Scientists have come up with a novel way to use silicon as an energy storage ingredient.
Flexible thinking on silicon solar cells
Combining silicon with a highly elastic polymer backing produces solar cells that have record-breaking stretchability and high efficiency.
No storm in a teacup -- it's a cyclone on a silicon chip
University of Queensland researchers have combined quantum liquids and silicon-chip technology to study turbulence for the first time, opening the door to new navigation technologies and improved understanding of the turbulent dynamics of cyclones and other extreme weather.
Black silicon can help detect explosives
Scientists from Far Eastern Federal University (FEFU), Far Eastern Branch of the Russian Academy of Sciences, Swinburne University of Technology, and Melbourne Center for Nanofabrication developed an ultrasensitive detector based on black silicon.
2D antimony holds promise for post-silicon electronics
Researchers in the Cockrell School of Engineering are searching for alternative materials to silicon with semiconducting properties that could form the basis for an alternative chip.
Silicon technology boost with graphene and 2D materials
In a review published in Nature, ICFO researchers and collaborators report on the current state, challenges, opportunities of graphene and 2D material integration in Silicon technology.
Light and sound in silicon chips: The slower the better
Acoustics is a missing dimension in silicon chips because acoustics can complete specific tasks that are difficult to do with electronics and optics alone.
Silicon as a semiconductor: Silicon carbide would be much more efficient
In power electronics, semiconductors are based on the element silicon -- but the energy efficiency of silicon carbide would be much higher.
New insight into glaciers regulating global silicon cycling
A new review of silicon cycling in glacial environments, led by scientists from the University of Bristol, highlights the potential importance of glaciers in exporting silicon to downstream ecosystems.
Understanding the (ultra-small) structure of silicon nanocrystals
New research provides insight into the structure of silicon nanocrystals, a substance that promises to provide efficient lithium ion batteries that power your phone to medical imaging on the nanoscale.
More Silicon News and Silicon Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Making Amends
What makes a true apology? What does it mean to make amends for past mistakes? This hour, TED speakers explore how repairing the wrongs of the past is the first step toward healing for the future. Guests include historian and preservationist Brent Leggs, law professor Martha Minow, librarian Dawn Wacek, and playwright V (formerly Eve Ensler).
Now Playing: Science for the People

#566 Is Your Gut Leaking?
This week we're busting the human gut wide open with Dr. Alessio Fasano from the Center for Celiac Research and Treatment at Massachusetts General Hospital. Join host Anika Hazra for our discussion separating fact from fiction on the controversial topic of leaky gut syndrome. We cover everything from what causes a leaky gut to interpreting the results of a gut microbiome test! Related links: Center for Celiac Research and Treatment website and their YouTube channel
Now Playing: Radiolab

The Flag and the Fury
How do you actually make change in the world? For 126 years, Mississippi has had the Confederate battle flag on their state flag, and they were the last state in the nation where that emblem remained "officially" flying.  A few days ago, that flag came down. A few days before that, it coming down would have seemed impossible. We dive into the story behind this de-flagging: a journey involving a clash of histories, designs, families, and even cheerleading. This show is a collaboration with OSM Audio. Kiese Laymon's memoir Heavy is here. And the Hospitality Flag webpage is here.