image: Figure | Multimode Si-ITO electro-optical modulators for balanced signal routing. a, Schematic representation of the plasmonic multimode silicon modulator: the structure reshapes signal into complementary outputs within a single waveguide. b, Measured (points) and exponentially fitted (red curve) DC ER for the 1.6 µm-long modulator in the maximum modulation regime (ER = 20.6 dB). c, Output waveform of the analogue optical link with the 3.6 µm-long modulator in the maximum modulation regime at 10 MHz under +16 dBm RF input
Credit: Anastasia K. Zemtsova et al.
Modern integrated photonics demands compact, high-speed modulators capable of efficiently controlling optical signals. Silicon platforms offer high integration density, but achieving high-speed modulation on silicon faces limitations. Moreover, many applications (e.g., balanced detection schemes) require two complementary (phase-inverted) signals. Traditional solutions such as Mach–Zehnder interferometers are bulky and complex to manufacture.
In a new paper published in Light: Advanced Manufacturing, a team of scientists, led by Professor Vladimir P. Drachev from Plasmonics Laboratory, Center for Engineering Physics, Skolkovo Institute of Science and Technology, Russia and co-workers have developed a hybrid plasmonic modulator based on Si-ITO-SiO₂-Au. Its key feature is the use of a multimode silicon waveguide 7 µm wide and 220 nm thick. Unlike conventional single-mode devices, several transverse magnetic (TM) modes are excited simultaneously. Their interference at the output creates two spatially separated channels with a precise π phase shift. This yields two inverted signals within a single waveguide and a single grating coupler, eliminating the need for bulky interferometers.
Modulation is achieved by changing the electron concentration in a 2 nm-thick ITO accumulation layer upon voltage application. Depending on the position of the input optical fiber, the device can operate in either amplitude‑dominant or phase‑dominant modulation regime.
Key Features and Parameters
- Record extinction ratio per length: The DC extinction ratio (ER) reaches 20.6 dB with a modulator length of only 1.6 µm – a record 12.8 dB/µm.
- High-frequency operation: Under AC, the ER is 2.48 dB at 10 MHz and 1.25 dB at 1 GHz (for a 3.6 µm‑long device).
- Broadband performance: Devices with 30 nm ITO exhibit a –3 dB bandwidth of 13.4 GHz.
- Built‑in signal inversion: The two output spots on the grating coupler are separated by about 2.5 µm and maintain the π phase shift across the entire measurable frequency range (up to 43 GHz). This is ideal for balanced (differential) detection, suppressing common‑mode noise and improving the signal‑to‑noise ratio.
- Tunability: By varying the positions of input and output optical probes (or, in the future, switching modes on‑chip), the modulation depth and type (amplitude or phase) can be continuously tuned after fabrication.
- Compact and CMOS‑compatible: The structure is fabricated using electron‑beam lithography and plasma etching, compatible with large‑scale manufacturing.
These results represent a major step forward in integrated photonics, enabling compact, high‑speed optical links with balanced signal routing for data centers, telecommunications, and microwave photonics.
This work was supported by the Russian Science Foundation (Grant No. 25‑72‑31023).
Journal
Light: Advanced Manufacturing
Article Title
Advantages of multimode Si-ITO electro-optical modulators for balanced signal routing