An electro-optic modulator plays the key role in fiber optic networks. Just as a transistor is a switch for electronic signals, an electro-optic modulator is a switch for optical signals. Optical communication uses light, so the modulator turns on and off the light that sends a stream of binary signals over optical fibers.
The new modulator is 10 times smaller and can potentially be 100 times more energy efficient than the best previous devices. It is roughly the size of a bacterium, measuring 0.6 by 8 microns.
“This is by far the most exciting research I have ever done because of the impact the device will bring and because of the challenge it was for design and fabrication,” said Alan Wang, associate professor of electrical engineering in the OSU College of Engineering.
For their invention, Wang and his doctoral student, Erwen Li, leveraged technology also developed at Oregon State: transparent conductive oxide materials. The structure they invented uses a transparent conductive oxide gate instead of a typical metal gate to combine a metal-oxide semiconductor capacitor with an ultra-compact photonic crystal nanocavity.
The design, combining innovations in materials and devices, enhanced the interaction between electronics and photonics, which enabled the researchers to create a smaller electro-optic modulator.
Wang had consulted his colleagues in industry about whether he was on the right track for developing something they could use.
“They told me reducing the size and reducing the energy consumption is going to be the trend in the next five to 10 years in industry. So this is exactly the kind of device they’re looking for,” Wang said.
Silicon photonic modulators rely on the plasma dispersion effect by free-carrier injection or depletion, which can only induce moderate refractive index perturbation. Therefore, the size and energy efficiency of silicon photonic modulators are ultimately limited as they are also subject to the diffraction limit. Here we report an ultracompact electro-optic modulator with total device footprint of 0.6 × 8 μm2 by integrating voltage-switched transparent conductive oxide with one-dimensional silicon photonic crystal nanocavity. The active modulation volume is only 0.06 um3, which is less than 2% of the lambda-cubic volume. The device operates in the dual mode of cavity resonance and optical absorption by exploiting the refractive index modulation from both the conductive oxide and the silicon waveguide induced by the applied gate voltage. Such a metal-free, hybrid silicon-conductive oxide nanocavity modulator also demonstrates only 0.5 dB extra optical loss, moderate Q-factor above 1000, and high energy efficiency of 46 fJ/bit. The combined results achieved through the holistic design opened a new route for the development of next generation electro-optic modulators that can be used for future on-chip optical interconnects.