Significantly High Modulation Efficiency of Compact Graphene Modulator Based on Silicon Waveguide

Design of a silicon Mach-Zehnder modulator via deep learning and evolutionary algorithms

As an essential block in optical communication systems, silicon (Si) Mach-Zehnder modulators (MZMs) are approaching the limits of possible performance for high-speed applications. However, due to a large number of design parameters and the complex simulation of these devices, achieving high-performance configuration employing conventional optimization methods result in prohibitively long times and use of resources. Here, we propose a design methodology based on artificial neural networks and heuristic optimization that significantly reduces the complexity of the optimization process. First, we implemented a deep neural network model to substitute the 3D electromagnetic simulation of a Si-based MZM, whereas subsequently, this model is used to estimate the figure of merit within the heuristic optimizer, which, in our case, is the differential evolution algorithm. By applying this method to CMOS-compatible MZMs, we find new optimized configurations in terms of electro-optical bandwidth, insertion loss, and half-wave voltage. In particular, we achieve configurations of MZMs with a [Formula: see text] bandwidth and a driving voltage of [Formula: see text], or, alternatively, [Formula: see text] with a driving voltage of [Formula: see text]. Furthermore, the faster simulation allowed optimizing MZM subject to different constraints, which permits us to explore the possible performance boundary of this type of MZMs.

On-Chip Reconfigurable and Ultracompact Silicon Waveguide Mode Converters Based on Nonvolatile Optical Phase Change Materials

Reconfigurable mode converters are essential components in efficient higher-order mode sources for on-chip multimode applications. We propose an on-chip reconfigurable silicon waveguide mode conversion scheme based on the nonvolatile and low-loss optical phase change material antimony triselenide (Sb₂Se₃). The key mode conversion region is formed by embedding a tapered Sb₂Se₃ layer into the silicon waveguide along the propagation direction and further cladding with graphene and aluminum oxide layers as the microheater. The proposed device can achieve the TE₀-to-TE₁ mode conversion and reconfigurable conversion (no mode conversion) depending on the phase state of embedded Sb₂Se₃ layer, whereas such function could not be realized according to previous reports. The proposed device length is only 2.3 μm with conversion efficiency (CE) = 97.5%, insertion loss (IL) = 0.2 dB, and mode crosstalk (CT) = -20.5 dB. Furthermore, the proposed device scheme can be extended to achieve other reconfigurable higher-order mode conversions. We believe the proposed reconfigurable mode conversion scheme and related devices could serve as the fundamental building blocks to provide higher-order mode sources for on-chip multimode photonics.

High-performance Mach-Zehnder modulator using tailored plasma dispersion effects in an ITO/graphene-based waveguide

A high-performance electro-optic Mach-Zehnder modulator (MZM) with outstanding characteristics is proposed. The MZM is in a push-pull configuration that is constructed using an ITO/graphene-based silicon waveguide. A novel idea for engineering of the plasma dispersion effect in an ITO/graphene-based waveguide is proposed so that the modulation characteristics of the MZM are highly improved. Plasma dispersion effects of ITO and graphene layers are tailored in such a way that a large difference between real parts of guided mode effective index of the two arms is achieved while their corresponding imaginary parts are equal. As a result, a very low [Formula: see text] of [Formula: see text] is achieved. To the best of our knowledge, this is one of the lowest [Formula: see text] reported for an electro-optic modulator. In addition, the proposed modulator exhibits a very high extinction ratio of more than 30 dB, low insertion loss of 2.8 dB and energy consumption of as low as 10 fJ/bit, which are all promising for optical communication and processing systems.

Silicon-Based Graphene Electro-Optical Modulators

With the increasing demand for capacity in communications networks, the use of integrated photonics to transmit, process and manipulate digital and analog signals has been extensively explored. Silicon photonics, exploiting the complementary-metal-oxide-semiconductor (CMOS)-compatible fabrication technology to realize low-cost, robust, compact, and power-efficient integrated photonic circuits, is regarded as one of the most promising candidates for next-generation chip-scale information and communication technology (ICT). However, the electro-optic modulators, a key component of Silicon photonics, face challenges in addressing the complex requirements and limitations of various applications under state-of-the-art technologies. In recent years, the graphene EO modulators, promising small footprints, high temperature stability, cost-effective, scalable integration and a high speed, have attracted enormous interest regarding their hybrid integration with SiPh on silicon-on-insulator (SOI) chips. In this paper, we summarize the developments in the study of silicon-based graphene EO modulators, which covers the basic principle of a graphene EO modulator, the performance of graphene electro-absorption (EA) and electro-refractive (ER) modulators, as well as the recent advances in optical communications and microwave photonics (MWP). Finally, we discuss the emerging challenges and potential applications for the future practical use of silicon-based graphene EO modulators.

Graphene on Silicon Photonics: Light Modulation and Detection for Cutting-Edge Communication Technologies

Graphene—a two-dimensional allotrope of carbon in a single-layer honeycomb lattice nanostructure—has several distinctive optoelectronic properties that are highly desirable in advanced optical communication systems. Meanwhile, silicon photonics is a promising solution for the next-generation integrated photonics, owing to its low cost, low propagation loss and compatibility with CMOS fabrication processes. Unfortunately, silicon’s photodetection responsivity and operation bandwidth are intrinsically limited by its material characteristics. Graphene, with its extraordinary optoelectronic properties has been widely applied in silicon photonics to break this performance bottleneck, with significant progress reported. In this review, we focus on the application of graphene in high-performance silicon photonic devices, including modulators and photodetectors. Moreover, we explore the trend of development and discuss the future challenges of silicon-graphene hybrid photonic devices.

Integrated non-volatile plasmonic switches based on phase-change-materials and their application to plasmonic logic circuits

Integrated photonic devices or circuits that can execute both optical computation and optical data storage are considered as the building blocks for photonic computations beyond the von Neumann architecture. Here, we present non-volatile hybrid electro-optic plasmonic switches as well as novel architectures of non-volatile combinational and sequential logic circuits. The electro-optic switches consist of a plasmonic waveguide having a thin layer of a phase-change-material (PCM). The optical losses in the waveguide are controlled by changing the phase of the PCM from amorphous to crystalline and vice versa. The phase transition process in the PCM can be realized by electrical threshold switching or thermal conduction heating via external electrical heaters or the plasmonic waveguide metal itself as an integrated heater. We have demonstrated that all logic gates, a half adder circuit, as well as sequential circuits can be implemented using the plasmonic switches as the active elements. Moreover, the designs of the plasmonic switches and the logic operations show minimum extinction ratios greater than 20 dB, compact designs, low operating power, and high-speed operations. We combine photonics, plasmonics and electronics on the same platform to design an effective architecture for logic operations.

Tuning of Graphene-Based Optical Devices Operating in the Near-Infrared

Graphene is a material with exceptional optical, electrical and physicochemical properties that can be combined with dielectric waveguides. To date, several optical devices based on graphene have been modeled and fabricated operating in the near-infrared range and showing excellent performance and broad application prospects. This paper covers the main aspects of the optical behaviour of graphene and its exploitation as electrodes in several device configurations. The work compares the reported optical devices focusing on the wavelength tuning, showing how it can vary from a few hundred up to a few thousand picometers in the wavelength range of interest. This work could help and lead the design of tunable optical devices with integrated graphene layers that operate in the NIR.

Tunable multilayer-graphene-based broadband metamaterial selective absorber

We propose a tunable multilayer-graphene-based broadband metamaterial selective absorber using the finite-difference time domain. The simulation results reveal that the absorption spectra of the proposed metamaterial with the nano-cylinder and 30-layer graphene show high absorption (88.3%) in the range of 250-2300 nm, which covers the entire solar spectrum. Moreover, the graphene-based metamaterial has a low thermal emittance of 3.3% in the mid-infrared range (4-13 µm), which can greatly reduce the heat loss. The proposed metamaterial has a tunable cutoff wavelength, which can be tuned by controlling the Fermi level of graphene. In addition, our structure is an angle-insensitive absorber, and the device has the potential to be widely used in solar cell and thermal detectors.

Device architectures for low voltage and ultrafast graphene integrated phase modulators

The atomic layer thin geometry and semi-metallic band diagram of graphene can be utilized for significantly improving the performance matrix of integrated photonic devices. Its semiconductor-like behavior of Fermi-level tunability allows graphene to serve as an active layer for electro-optic modulation. As a low loss metal layer, graphene can be placed much closer to active layer for low voltage operation. In this work, we investigate hybrid device architectures utilizing semiconductor and metallic properties of the graphene for ultrafast and energy efficient electro-optic phase modulators on semiconductor and dielectric platforms. (1) Directly contacted graphene-silicon heterojunctions. Without oxide layer, the carrier density of graphene can be modulated by the directly contact to silicon layer, while silicon intrinsic region stays mostly depleted. With doped silicon as electrodes, carrier can be quickly injected and depleted from the active region in graphene. The ultrafast carrier transit time and small RC constant promise ultrafast modulation speed (3dB bandwidth of 67 GHz) with an estimated V_π·L of 1.19 V·mm. (2) Graphene integrated lithium niobite modulator. As a transparent electrode, graphene can be placed close to integrated lithium niobate waveguide for improving coupling coefficient between optical mode profile and electric field with minimal additional loss (4.6 dB/cm). Numerical simulation indicates 2.5× improvement of electro-optic field overlap coefficient, with estimated V _π of 0.2 V.

Current modulation in graphene p-n junctions with external fields

In this work we describe a proposal for a graphene-based nanostructure that modulates electric current even in the absence of a gap in the band structure. The device consists of a graphene p-n junction that acts as a Veselago lens that focuses ballistic electrons on the output lead. Applying external (electric and magnetic) fields changes the position of the output focus, reducing the transmission. Such device can be applied to low power field effect transistors, which can benefit from graphene's high electronic mobility.

Nanoscale phase modulator and optical switch based on graphene-coated fiber

The new, to the best of our knowledge, phase modulator and optical switch based on graphene-coated fiber are proposed and studied. The fields of surface plasmon polaritons (SPP) modes are highly concentrated on the graphene so that very high effective indices (>400) can be achieved. By virtue of this feature, the dimensions of these devices can be as small as several nanometers in length. The minimum transmission losses for the phase modulator and optical switch are 0.54 dB and 0.18 dB, respectively. The maximum voltage adjustment range needed is only 0.3 V. The optical switch possesses a complete switch-off state and is adaptable to variation in phase difference and light intensity. The performance of these devices can be effectively tuned by the parameters, such as the graphene-coated lengths and the applied voltages. These devices may have important applications in fiber-based systems and other nanoscale optic and optoelectronic systems.

Slow light enabled high-modulation-depth graphene modulator with plasmonic metasurfaces

Graphene has attracted the interest of researchers seeking to develop compact optical modulators with the flexible tunability of graphene conductivity by tuning the Fermi level. Plasmonic structures have provided a robust way to enhance the modulation depth (MD) of graphene optical modulators, but the available schemes suffer from low MD, fabrication complexities, or both. Here, an ultra-thin plasmonic metasurface structure capable of guiding slow surface plasmons (SPs) is proposed to construct graphene-based optical modulators. The designs take advantage of the strong field enhancement of slow SP modes as well as the orientation match between the electric field and the graphene plane. A typical 0.96-μm-long metasurface-based graphene modulator presents a significantly improved MD of 4.66 dB/μm and an acceptable insertion loss of 1.4 dB/μm, while still having ease of fabrication.

Numerical investigation of the linearity of graphene-based silicon waveguide modulator

We investigated the linearity of graphene-based silicon waveguide modulators used in microwave photonics links. A theoretical model was developed to systematically analyze the linearity performance for the second-order harmonic distortions (SHD2) and the third-order intermodulation distortions (IMD3). For the graphene-based silicon waveguide electro-absorption (EA) modulator, the distortions were suppressed through bias optimization. As a result, the maximal spurious free dynamic range (SFDR) obtained for SHD2 and IMD3 were 105.9 dB ·Hz^1/2 and 117.8 dB·Hz^2/3, respectively. For the graphene-based silicon waveguide electro-refraction (ER) phase modulator, SHD2 was fully eliminated through the push-pull modulation and quadrature biasing, while the remaining IMD3 term was linearized by the proper adjustment of the bias voltage and phase shifter length to obtain an ultrahigh SFDR of 130 dB ·Hz^2/3. Moreover, the graphene-based silicon waveguide ER modulator is more compact and tolerant to bias errors than a pure silicon modulator. These results reveal that the graphene-based silicon waveguide EA and ER modulators can be potentially utilized in integrated microwave photonics.

Large modulation capacity in graphene-based slot modulators by enhanced hybrid plasmonic effects

We present an effective scheme to improve the modulation capacity in graphene-based silicon modulator by employing the double slots configuration with hybrid plasmonic effects. Two modulators, i.e., metal-insulator-metal and insulator-metal-insulator configurations have been demonstrated, showing that the double slots design can significantly improve the modulation efficiency. The obtained modulation efficiency is up to 0.525 dB/μm per graphene layer, far exceeding previous studies. It can be found that the light-graphene interaction plays a pivotal role in the modulation efficiency, whereas the height of metal has profound influence on the modulation. Our results may promote various future modulation devices based on graphene.