Silicon is evolving as a versatile photonic platform with multiple functionalities that can be seamlessly integrated. The tool box is rich starting from the ability to guide and amplify multiple wavelength sources at GHz bandwidths, to optomechanical MEMS. The strong confinement of light in ultra small structures also enables the generation of strong optical forces. We have recently shown that nonlinear optical forces can enable controllable manipulation of photonic structures. These advances should enable future micro-optomechanical systems (MOMS) with novel and distinct functionalities. A research area that recently has emerged is nonlinear optics using silicon photonics. Since the birth of nonlinear optics, researchers have continually focused on developing efficient nonlinear optical devices that require low optical powers. The strong light confinement in silicon waveguides results in a high effective nonlinearity ad enables fine tuning of waveguide dispersion which is essential for phase matching of parametric nonlinear optical processes such as four-wave-mixing (FWM) We demonstrated FWM-based frequency conversion in waveguides using as little as 1 mW of pump power in a ring-resonator geometry, and ∼100 mW of pump power over bandwidths exceeding 800 nm in a straight-waveguide device. In addition, by using the concept of time-space duality we have shown the temporally stretching and compressing of optical waveforms which allows for seamless transformation between the GHz and THz regimes.

Nonlinear silicon photonics

Optical phase change materials (O-PCMs) are being explored for a variety of photonic applications due to the extraordinarily large changes in optical properties that occur during electronic and/or structural phase transitions. Here, recent work integrating O-PCMs in integrated silicon photonic devices is presented. Conceptually proposed and experimentally realized thermo-optic, electro-optic, and all-optical Si/O-PCM devices are described and perspectives on the potential for Si/O-PCM electro-optic and all-optical modulators are outlined.

Optical phase change materials in integrated silicon photonic devices: review

Metal-insulator transition in vanadium dioxide Metal-insulator transition in vanadium dioxide

Metal–insulator transition in vanadium dioxide

Phase-change materials (PCMs) have emerged as promising active elements in silicon (Si) photonic systems. In this work, we design, fabricate, and characterize a hybrid Si-PCM optical switch. By integrating vanadium dioxide (a PCM) within a Si photonic waveguide, in a non-resonant geometry, we achieve ~10 dB broadband optical contrast with a PCM length of 500 nm using thermal actuation.

Silicon waveguide optical switch with embedded phase change material.

Tunable Optical Properties of 2D Materials and Their Applications

In this paper, a horizontal slot Si–VO2–Si optical waveguide is proposed and its optical properties are investigated. Numerical simulation results show that the effective index and the propagation loss of the proposed waveguide undergo substantial changes upon the VO2 transition from insulating to metallic phase. The effective index and the propagation loss variations of the proposed waveguide are then maximized by optimizing waveguide dimensions. It is shown that 0.226 change in the effective index (Δ n eff = 0.226) and 30 dB/ μ m change in the propagation loss (Δ l dB = 30 dB/μm) are achievable using the optimum dimensions. These extraordinary variations in waveguide properties recommend the proposed waveguide as an excellent candidate for optical active device realization. To investigate these applications, performance parameters of the proposed waveguide are further studied in terms of the transition speed and the power consumption. In these studies, the VO2 phase transition is assumed to be actuated by applying an electric field. Two examples of optical active devices based on the proposed waveguide are then presented: an electro-absorption modulator and a 1 × 2 directional coupler optical switch. Finite-difference time-domain simulation of the proposed devices shows very high extinction ratio of 21 dB along the ultrasmall propagation length of 1 μ m, for the proposed electro-absorption modulator, and high extinction ratios of ∼18.5 dB and ∼8.6 dB in off - and on -state of the proposed 1 × 2 switch, which has very small length of ∼6 μ m. Further simulations also show interesting properties of the proposed devices in terms of the power consumption, insertion loss, and bandwidth.

Design and Simulation of Compact Optical Modulators and Switches Based on Si–VO2 –Si Horizontal Slot Waveguides

Silicon photonic platforms are of significant interest for a variety of applications that operate in the mid-infrared regime. However, the realization of efficient mid-IR modulators, key components in any integrated optics platform, is still a challenging topic. Here, an ultra-compact high-speed hybrid Si/VO2 modulator operating at a mid-IR wavelength of 3.8 μm is presented. Electrical properties of graphene are employed to achieve a reversible insulating-metal phase transition in VO2 by electrical actuation. The thermal characteristics of graphene are employed to improve the response time of the VO2 phase transition through speed up heating and dissipation processes, thus enhancing the modulation speed. Optical and thermal simulations show an extinction ratio of 4.4 dB/μm, an insertion loss of 0.1 dB/μm, and high modulation speed of 23 ns. A larger modulation depth as high as 10 dB/μm can be achieved at the cost of lower modulation speed.

Mid-infrared hybrid Si/VO2 modulator electrically driven by graphene electrodes

In this paper, the distinctive dispersion characteristic of hybrid plasmonic waveguides is exploited for designing ultra-wideband directional couplers. It is shown that by using optimized geometrical dimensions for hybrid plasmonic waveguides, nearly wavelength-independent directional couplers can be achieved. These broadband directional couplers are then used to design Mach-Zehnder-interferometer-based switches. Our simulation results show the ultra-wide bandwidth of ∼260  nm for the proposed hybrid plasmonic-waveguide-based switch. Further investigation of the proposed Mach-Zehnder switch confirms that because of the strong light confinement in the hybrid plasmonic waveguide structure, the switching time, power consumption, and overall footprint of the device can be significantly improved compared to silicon-ridge-waveguide-based Mach-Zehnder switches. For the Mach-Zehnder switch designed by using the optimized directional coupler, the switching time is found to be less than one picosecond, while the power consumption, VπLπ figure of merit, and active length of the device are ∼61  fJ/bit, 85  V×μm, and 30 μm, respectively.

Ultra-wideband high-speed Mach-Zehnder switch based on hybrid plasmonic waveguides.

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A high-performance TE modulator/TM-pass polarizer using selective mode shaping in a VO2-based side-polished fiber

Si $_3$ N $_4$ has emerged as a prominent material for expanding the capability of silicon photonics to wavelengths below $ $\mu$ m. However, realizing an efficient optical modulator, a key building block for any integrated optics platform, remains a major challenge in Si $_3$ N $_4$ mainly because this material has a vanishing Pockels coefficient. Here, we propose a compact Si $_3$ N $_4$ based optical modulator by using a thin VO $_2$ layer on top of a Si $_3$ N $_4$ strip waveguide where amplitude modulation is achieved via phase transition of the VO $_2$ layer. To reduce the actuation time of the temperature-induced VO $_2$ phase transition, a mono-layer graphene microheater is designed for the active Si $_3$ N $_4$ VO $_2$ waveguide. Our simulations indicate a high extinction ratio of $\sim$ 8.28 dB/ $\mu$ m with an insertion loss of $\sim$ 2.8 dB/ $\mu$ m at the design wavelength of 850 nm for the proposed modulator and wideband operation in the wavelength range of 800–900 nm. It is shown that employing the electrical and thermal properties of graphene not only leads to a significant reduction of the power consumption of the device but also, decreases the actuation time compared to previous modulators based on the thermal phase transition of the VO $_2$ .

Babak Janjan

Papers

Design and Simulation of Compact Optical Modulators and Switches Based on Si–VO2 –Si Horizontal Slot Waveguides

Mid-infrared hybrid Si/VO2 modulator electrically driven by graphene electrodes

Ultra-wideband high-speed Mach-Zehnder switch based on hybrid plasmonic waveguides.

A high-performance TE modulator/TM-pass polarizer using selective mode shaping in a VO2-based side-polished fiber

Hybrid Si $_3$ N $_4$ /VO $_2$ Modulator Thermally Triggered by a Graphene Microheater