Silicon Thermo-Optic Switches with Graphene Heaters Operating at Mid-Infrared Waveband

Abstract

The mid-infrared (MIR, 2–20 μm) waveband is of great interest for integrated photonics in many applications such as on-chip spectroscopic chemical sensing, and optical communication. Thermo-optic switches are essential to large-scale integrated photonic circuits at MIR wavebands. However, current technologies require a thick cladding layer, high driving voltages or may introduce high losses in MIR wavelengths, limiting the performance. This paper has demonstrated thermo-optic (TO) switches operating at 2 μm by integrating graphene onto silicon-on-insulator (SOI) structures. The remarkable thermal and optical properties of graphene make it an excellent heater material platform. The lower loss of graphene at MIR wavelength can reduce the required cladding thickness for the thermo-optics phase shifter from micrometers to tens of nanometers, resulting in a lower driving voltage and power consumption. The modulation efficiency of the microring resonator (MRR) switch was 0.11 nm/mW. The power consumption for 8-dB extinction ratio was 5.18 mW (0.8 V modulation voltage), and the rise/fall time was 3.72/3.96 μs. Furthermore, we demonstrated a 2 × 2 Mach-Zehnder interferometer (MZI) TO switch with a high extinction ratio of more than 27 dB and a switching rise/fall time of 4.92/4.97 μs. A comprehensive analysis of the device performance affected by the device structure and the graphene Fermi level was also performed. The theoretical figure of merit (2.644 mW−1μs−1) of graphene heaters is three orders of magnitude higher than that of metal heaters. Such results indicate graphene is an exceptional nanomaterial for future MIR optical interconnects.

Optically Transparent and Thermally Efficient 2D MoS2 Heaters Integrated with Silicon Microring Resonators

“…From the dynamic response (Figure , red line), we extracted t rise ∼ 26 μs and t fall ∼ 24 μs using a 10–90% criterion with the average response time of the heater in the order of τ = ( t rise + t fall )/2 ∼ 25 μs, which corresponds to the heating time constant according to the 1/ e criterion of τ 1/ e = τ/2.2 = 11.4 μs. The obtained τ is faster than the reported results on conventional tungsten heaters on top of the waveguide without thermal insulation; however, it is slower compared to TiN, silicon-doped, ,, and graphene-integrated heaters, − as seen in Table . The response time in our device is severely affected by the high resistance (>60 kΩ) of Au/1L-MoS 2 Schottky contacts, considering that the portion of the heat source (hot spot) is located ∼1 μm off the waveguide, instead of being directly distributed on top of the waveguide if the power is mostly dissipated in the MoS 2 channel and not at the contact area.…”

“…Eventually, to improve heating efficiency without involving waveguide doping one should target bringing a heater in direct contact with the waveguide without compromising α IL . Recently, there were several attempts in this direction using two-dimensional (2D) graphene heaters. − Owing to the 2D nature of single-layer graphene (SLG), being an atomically thin semi-metal, , the cross-sectional overlap between the waveguide mode and SLG is minimized resulting in a smaller α IL . The latter allows bringing the graphene heater closer to the waveguide, improving P π with optimized α IL yet not at the loss-less level.…”

“…In particular, placing SLG in close proximity (<200 nm) or direct contact with the waveguide would result in a considerable insertion loss (Figure c). Thermo-optic tuning of SiPh devices using SLG heaters was demonstrated in microdisks, microrings, − Mach–Zehnder interferometers (MZI), , photonic-crystal (PhC) waveguides, and nanobeam cavities . Specifically, ref shows that heating Si microdisks by a graphene heater in direct contact with the disk core can reduce the optical loss α IL ∼2·10 –4 dB/μm, however, without providing performance advantages over metal heater devices. − A sub-microsecond response time (τ ∼ 0.7 μs) was achieved using SLG as a heat conductor for heating a MRR, yet producing higher P π ∼ 100 mW and considerable α IL .…”

“…However, it implies additional fabrication complexity in realizing an external gating to the graphene heater and obtaining a high doping level (Fermi level, E F > 0.5 eV @1550 nm) in order to bring SLG to transparency. The doping level for graphene transparency can be reduced by operating the device at longer wavelengths (e.g., 2 μm) as is described in ref .…”

Optical Bistability in a Tunable Gourd-Shaped Silicon Ring Resonator

“…Among different nonlinear optics applications, optical bistability has attracted widespread concern in ultrafast signal processing and modulation [ 5 , 6 ]. In addition, a small-scale and steady tuning platform for generating the bistability can be offered by a microcavity, which would be further applied to all-optical modulation, memory, memristor, switch, and so forth [ 7 , 8 , 9 , 10 ].…”