Ryoichi Ishihara

Associate Professor, Group leader

Qutech, Dep. Quantum and Computer Engineering, Faculty of Electrical Engineering, Mathematics and Computer Science, Delft University of Technology

Ishihara-lab focuses on the integration technologies for unconventional electronic systems; quantum computers, quantum sensors, neuromorphic computers, and biodegradable sensors. Our work involves new materials, scalable fabrication of electronic and photonic devices, and 3D heterogeneous integration, aiming to realize unconventional electronic systems.

Graphene on Diamond: A Scalable Approach to Photonic Cavity Tuning

Introduction & Motivation

Diamond is an outstanding material for photonics, thanks to its wide bandgap, low loss in visible and near-infrared wavelengths, high thermal conductivity, and ability to host optically active color centers—such as NV, SiV, GeV, and SnV. Nanophotonic devices made from diamond enable applications in quantum information processing, sensing, and integrated optics.

However, selective tuning of diamond cavities is particularly challenging, especially in low-temperature quantum systems. Diamond’s thermo-optic and electro-optic coefficients are relatively small, and thermal tuning is less effective at cryogenic temperatures. Yet, precise spectral alignment is crucial for coupling color centers (including SnVcenters) to cavity modes, enhancing their emission and enabling high-fidelity quantum operations.

Graphene, a two-dimensional material with exceptional electronic and optical properties, offers a promising route for active tuning. By placing graphene on or near a diamond nanophotonic cavity, one can exploit electrical gating to modify graphene’s refractive index or absorption, thereby shifting or modulating the cavity resonance. This enables on-chip, selective tuning of individual cavities, a key requirement for scalable quantum photonic architectures. Importantly, graphene gating can also facilitate spectral tuning of color centers, allowing for precise alignment of their zero-phonon lines to the cavity resonance—essential for optimized photon extraction and coherent quantum control at low temperatures.

Project Objectives:

Literature Review

• Examine state-of-the-art diamond nanophotonic cavity designs.
• Investigate existing demonstrations of graphene integration on photonic platforms

Theoretical Model & Simulation

• Develop a model for electro-optic tuning using graphene’s conductivity changes.
• Perform 2D/3D electromagnetic simulations to optimize resonance shifts, Q-factors, and coupling to color center transitions.

Design & Fabrication Flow

• Propose cavity geometries (e.g., photonic crystal or ring resonators) that accommodate graphene gating in a scalable manner.
• Outline a process flow for integrating graphene on diamond and defining gate electrodes.

Characterization & Experiment Planning

• Devise optical and electrical testing strategies to measure resonance shifts as a function of gate voltage at low temperatures.
• Propose methods to evaluate spectral tuning of color centers (NV, SnV) under gated graphene.

Scalability & Outlook

• Assess the feasibility of wafer-scale integration and the potential for individually addressable, cryogenic tuning of multiple cavities on a single chip.
• Identify next steps for fully integrated, tunable diamond photonic circuits in quantum information and sensing.

Expected Outcomes:

• Validated Simulation Model
  Demonstrate feasible resonance shifts (tuning range) while maintaining acceptable Q-factors and strong coupling to color center emission lines.
• Scalable Design
  Provide a well-documented cavity layout and integration approach for graphene gating on diamond, supporting selective tuning in large arrays.
• Experimental Strategy
  Develop clear guidelines for measuring optical responses and color center emission under electrical gating, particularly at cryogenic temperatures.
• Roadmap
  Offer a detailed plan for future fabrication, device testing, and the integration of multiple color centers in a single, scalable diamond photonic platform.

References:

1. Gan, X. et al. Electro-optical Modulation in Graphene Integrated Photonic Crystal Nanocavities. CLEO: 2013CTu1F.4 (2013) doi:10.1364/cleo_si.2013.ctu1f.4.
2. Gan, X. et al. High-Contrast Electrooptic Modulation of a Photonic Crystal Nanocavity by Electrical Gating of Graphene. Nano Lett. 13, 691–696 (2013).
3. Safaei, A., Chandra, S., Leuenberger, M. N. & Chanda, D. Wide Angle Dynamically Tunable Enhanced Infrared Absorption on Large-Area Nanopatterned Graphene. ACS Nano 13, 421–428 (2019).
4. Shiramin, L. A. et al. Electrically Tunable Absorption in Graphene-Integrated Silicon Photonic Crystal Cavity. 2017 IEEE 14th Int. Conf. Group IV Photonics (GFP) 181–182 (2017) doi:10.1109/group4.2017.8082256.
5. Chiba, H. & Notomi, M. Reconfigurable nanocavity formation in graphene-loaded Si photonic crystal structures. Opt. Express 27, 37952 (2019).
6. Brenneis, A. et al. Ultrafast electronic readout of diamond nitrogen-vacancy centres coupled to graphene. Nature Nanotechnology 10, 135–139 (2015).

Interested? Please contact Ryoichi Ishihara r.ishihara@tudelft.nl or Salahuddin Nur <S.Nur@tudelft.nl>