Researchers from Nanyang Technological University, University of Chicago, University of Wisconsin, Chungnam National University, National Institute for Materials Science, MIT and Singapore University of technology and design have developed hybrid tungsten oxyselenide/graphene electrodes that enable near-lossless optical phase modulation in two-dimensional semiconductor devices. The work targets a long-standing trade-off in integrated photonics, where improving modulation efficiency typically increases optical loss, especially for graphene-based designs at telecommunication wavelengths.
Schematic of the fabricated device structure, consisting of a capacitor stack (monolayer WS2/hBN/graphene/TOS) integrated onto a SiN microring resonator. Right top: Side-view schematic of the device, highlighting electrical contacts with the bottom WS2 layer and top graphene electrode, connected independently to gold electrodes for voltage biasing. Right bottom: Optical image of the actual device. Image from: Light: Science & Applications
In this approach, monolayer WSe₂ is converted by UV–ozone treatment into tungsten oxyselenide (TOS), which acts as a strong p‑type dopant for graphene. Heavy p‑doping shifts graphene’s Fermi level such that its absorption around 1550 nm is strongly suppressed, while its conductivity is enhanced, allowing graphene to function as a transparent, low-resistance top electrode in the near‑infrared.
The complete phase modulator stacks a hybrid TOS/graphene transparent electrode, a hexagonal boron nitride (hBN) dielectric spacer and a monolayer WS₂ electro‑optic layer on a silicon nitride (SiN) microring platform. This heterostructure maintains telecom-band transparency while still providing strong electro‑optic tuning via the WS₂, which supports efficient phase modulation under a vertical electric field without incurring parasitic absorption from the electrode.
Experimentally, the device reached a modulation efficiency of 0.202 V·cm with an extinction ratio variation of only 0.08 dB, significantly lower loss than comparable devices using pristine graphene or indium tin oxide (ITO) electrodes under similar bias conditions. The results confirm that the TOS/graphene electrode can induce substantial phase shifts in the microring resonance while preserving transmission, effectively decoupling phase modulation from intensity modulation.
By combining graphene’s high carrier mobility with the tailored band structure and doping effect of TOS, the architecture demonstrates a practical route to near-lossless, 2D-material-based phase modulators compatible with silicon nitride photonic platforms. This provides a basis for more energy-efficient optical interconnects, integrated photonic computing elements and quantum photonic circuits where low insertion loss and precise phase control are key requirements.
