Silicon Optical Modulator Achieves >1-GHz Operation

In a development that points to the advent of silicon as a viable photonic material, a group of scientists from Intel Corp. in Santa Clara, Calif., and in Jerusalem has fabricated an all-silicon optical modulator with a modulation frequency greater than 1 GHz. A report of the work appears in the Feb. 12 issue of Nature and suggests further avenues of inquiry that may enable such a device to achieve higher performance, point-ing to potential advances in optical circuits.

Until now, silicon largely has resisted efforts to utilize it as an optical modulator because of its material properties. Specifically, unlike electro-optic materials such as lithium niobate and potassium dihydrogen phosphate, silicon does not exhibit the Pockels effect, in which the application of an electric field induces a linear change in the refractive index of the material. Silicon-based modulators have been demonstrated that rely on the introduction of free carriers to modify the refractive index, but this phenomenon is relatively slow and, until now, has limited such devices to modulation frequencies of only about 20 MHz.

The Intel optical modulator also employs the free-carrier plasma dispersion effect, but it crucially adopts a new means of doing so: the introduction of a 2.5-mm-long metal-oxide-semiconductor capacitor phase shifter into one arm of a silicon asymmetric Mach-Zehnder interferometer. The phase shifter consists of 900-nm-thick P-type polysilicon atop 1.4-µm-thick N-type crystalline silicon and separated by a 12-nm-thick insulating oxide layer.

The application of a positive voltage to the P-type polysilicon causes a 10-nm-thick charge layer to accumulate on either side of the oxide -- electrons in the N-type silicon and holes in the P-type polysilicon -- that changes the effective refractive index of the silicon waveguide. This induces a phase shift in the 1.55-µm radiation propagating through that arm, which leads to interference in the output region of the interferometer.

Using a pseudorandom electrical input to the phase shifter, the researchers verified that the modulator supports optical data transmission rates of 1 Gb/s. They expect that the modulation frequency can be scaled to 10 GHz and are investigating means of reducing on-chip loss in the device, such as by replacing the polysilicon with single-crystal silicon.