- 19 May 2005 -

Silicon device modulates light

Cornell University researchers have taken a major step forward by developing a silicon device that allows an electrical signal to modulate a beam of light on a micrometer scale.

Other electro-optical modulators have been built on silicon, but their size is on the order of millimeters, too large for practical use in IC chips.

Smaller modulators have been made using compound semiconductors such as GaAs, but silicon is preferable for its ability to be integrated with current microelectronics.

The Cornell 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 1,576nm, in the near-IR.

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.

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 that devices using a PIN configuration have been relatively slow in switching, but that the ring resonator configuration also eliminates this. Tests using a pulse-modulated electrical signal produced an output with a very similar waveform to the input at up to 1.5Gb/s.

The work is described in a paper published in Nature by Michal Lipson, Cornell assistant professor of electrical and computer engineering, and her research group.Co-authors are Cornell graduate students Qianfan Xu, Bradley Schmidt and postdoctoral researcher Sameer Pradhan, now at Intel Corp.