From electronics to photonics; Modulating light with electricity

May 19, 2005
From electronics to photonics; modulating light with electricity

Much of our electronics could soon be replaced by photonics, in which beams of light flitting through microscopic channels on a silicon chip replace electrons in wires. Photonic chips would carry more data, use less power and work smoothly with fiber-optic communications systems. The trick is to get electronics and photonics to talk to each other.
Now Cornell University researchers have taken a major step forward in bridging this communication gap by developing a silicon device that allows an electrical signal to modulate a beam of light on a micrometer scale.

Image: Scanning electron microscope image of the ring coupled to the waveguide with a zoom-in picture of the coupling region. Copyright © Cornell University

Other electro-optical modulators have been built on silicon, but their size is on the order of millimeters, too large for practical use in integrated circuit chips. (a micrometer, or micron, is one millionth of a meter, or one thousandth of a millimeter.) Smaller modulators have been made using compound semiconductors such as gallium arsenide, but silicon is preferable for its ability to be integrated with current microelectronics.

The work is described in a paper published in the May 19, 2005, issue of Nature by Michal Lipson, Cornell assistant professor of electrical and computer engineering, and her research group.

Their modulator uses a ring resonator -- a circular waveguide coupled to a straight waveguide carrying the beam of light to be modulated. Light traveling along the straight waveguide loops many times around the circle before proceeding. The diameter of the circle, an exact multiple of a particular wavelength, determines the wavelength of light permitted to pass. For the experiments reported in Nature, the ring used was 12 microns in diameter to resonate with laser light at a wavelength of 1,576 nanometers, in the near infrared.

Schematic layout of the ring resonator based modulator. The inset shows a cross-section of the ring
Schematic layout of the ring resonator based modulator. The inset shows a cross-section of the ring. Copyright © Cornell University

The ring is surrounded by an outer ring of negatively doped silicon, and the region inside the ring is positively doped, making the waveguide itself the intrinsic region of a positive-intrinsic-negative (PIN) diode. When a voltage is applied across the junction, electrons and holes are injected into the waveguide, changing its refractive index and its resonant frequency so that it no longer passes light at the same wavelength. As a result, turning the voltage on switches the light beam off.

The PIN structure has been used previously to modulate light in silicon using straight waveguides. But because the change in refractive index that can be caused in silicon is quite small, a very long straight waveguide is needed. Since light travels many times around the ring resonator, the small change has a large effect, making it possible to build a very small device.

In tests, the researchers found that the device could completely interrupt the propagation of light with an applied voltage of less than 0.3 volts. The researchers note in their paper that devices using a PIN configuration have been relatively slow in switching but that the ring resonator configuration also eliminates this problem. Tests using a pulse-modulated electrical signal produced an output with a very similar waveform to the input at up to 1.5 gigabits per second.

The Nature paper is titled "Micrometer-scale Silicon Electro-Optic Modulator." Co-authors are Cornell graduate students Qianfan Xu and Bradley Schmidt and postdoctoral researcher Sameer Pradhan, now at Intel Corp.

Source: Cornell University

Explore further: Researchers create practical and versatile microscopic optomechanical device

Related Stories

Graphene balloons show their colors

November 7, 2016

Researchers from the Graphene Flagship have found a new potential application for graphene: mechanical pixels. By applying a pressure difference across graphene membranes, the perceived color of the graphene can be shifted ...

A diamond ring sparks a paradigm shift

June 6, 2011

The sweet smell of benzene gave birth to the term ‘aromatic’ molecules, but it is the chemical bonds within these compounds that have fascinated researchers for almost 200 years. Encasing alternating double- and ...

Topological light: Living on the edge

October 21, 2013

( —Topology—the understanding of how things are connected—remains abstract, even with the popular example of doughnuts and coffee cups. This concept, esoteric as it appears, is also neat because it is the basis ...

Recommended for you

Melting solid below the freezing point

January 23, 2017

Phase transitions surround us—for instance, liquid water changes to ice when frozen and to steam when boiled. Now, researchers at the Carnegie Institution for Science have discovered a new phenomenon of so-called metastability ...

Probe for nanofibers has atom-scale sensitivity

January 20, 2017

Optical fibers are the backbone of modern communications, shuttling information from A to B through thin glass filaments as pulses of light. They are used extensively in telecommunications, allowing information to travel ...

Magnetic recording with light and no heat on garnet

January 19, 2017

A strong, short light pulse can record data on a magnetic layer of yttrium iron garnet doped with Co-ions. This was discovered by researchers from Radboud University in the Netherlands and Bialystok University in Poland. ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.