Longhai Yu

Tunable pattern-free graphene nanoplasmonic waveguides on trenched silicon substrate

Graphene has emerged as a promising material for active plasmonic devices in the mid-infrared (MIR) region owing to its fast tunability, strong mode confinement, and long-lived collective excitation. In order to realize on-chip graphene plasmonics, several types of graphene plasmonic waveguides (GPWGs) have been investigated and most of them are with graphene ribbons suffering from the pattern-caused edge effect. Here we propose a novel nanoplasmonic waveguide with a pattern-free graphene monolayer on the top of a nano-trench. It shows that our GPWG with nanoscale light confinement, relatively low loss and slowed group velocity enables a significant modulation on the phase shift as well as the propagation loss over a broad band by simply applying a single low bias voltage, which is very attractive for realizing ultra-small optical modulators and optical switches for the future ultra-dense photonic integrated circuits. The strong light-matter interaction as well as tunable slow light is also of great interest for many applications such as optical nonlinearities.

Local and Nonlocal Optically Induced Transparency Effects in Graphene–Silicon Hybrid Nanophotonic Integrated Circuits

Graphene is well-known as a two-dimensional sheet of carbon atoms arrayed in a honeycomb structure. It has some unique and fascinating properties, which are useful for realizing many optoelectronic devices and applications, including transistors, photodetectors, solar cells, and modulators. To enhance light-graphene interactions and take advantage of its properties, a promising approach is to combine a graphene sheet with optical waveguides, such as silicon nanophotonic wires considered in this paper. Here we report local and nonlocal optically induced transparency (OIT) effects in graphene-silicon hybrid nanophotonic integrated circuits. A low-power, continuous-wave laser is used as the pump light, and the power required for producing the OIT effect is as low as ∼0.1 mW. The corresponding power density is several orders lower than that needed for the previously reported saturated absorption effect in graphene, which implies a mechanism involving light absorption by the silicon and photocarrier transport through the silicon-graphene junction. The present OIT effect enables low power, all-optical, broadband control and sensing, modulation and switching locally and nonlocally.

Thermally tunable silicon photonic microdisk resonator with transparent graphene nanoheaters

Efficient tunable photonic integrated devices are important for the realization of reconfigurable photonic systems. Thermal tuning is a convenient and effective approach, and silicon’s large heat conductivity, thermo-optical coefficient, and CMOS fabrication compatibility make it a good candidate material for tunable optical microcavities, which are versatile elements in low-cost, large-scale photonic integrated circuits. Metal heaters are traditionally used for tuning, and a thick SiO2 upper-cladding layer is usually needed to prevent light absorption by the metal since that could reduce response speed and heating efficiency. In this paper, we propose and experimentally demonstrate thermally tunable silicon photonic microdisk resonators by introducing transparent graphene nanoheaters, which contact the silicon core directly without any isolator layer. The theoretical and experimental results show that the transparent graphene nanoheaters improve the heating efficiency, the temporal response, and the achievable temperature in comparison with a traditional metal heater. Furthermore, the graphene nanoheater is convenient for use in ultrasmall nanophotonic integrated devices due to its single-atom thickness and excellent flexibility. Both experiments and simulations show that the transparent graphene nanoheater is a promising option for other thermally tunable photonic integrated devices such as optical filters and switches.

Graphene-based transparent flexible heat conductor for thermally tuning nanophotonic integrated devices

Graphene, a well-known two-dimensional sheet, has attracted strong interest for both fundamental studies and applications. Due to its high intrinsic thermal conductivity, graphene has many potential applications in thermal management, such as in heat spreaders and flexible heaters. In this paper, a graphene-based transparent flexible heat conductor for nanophotonic integrated devices is demonstrated. The graphene heat conductor is designed to deliver heat from a non-local traditional metal heater to nanophotonic integrated devices for realizing efficient thermal tuning. With the present graphene heat conductor, a thermally tuning silicon Mach-Zehnder interferometer and micro-disk have been realized with good performance in terms of heating efficiency and temporal response. This indicates that the present graphene-based transparent flexible heat conductor provides an efficient and beneficial heating method for thermally tuning nanophotonic integrated devices.

Silicon-graphene conductive photodetector with ultra-high responsivity

Graphene is attractive for realizing optoelectronic devices, including photodetectors because of the unique advantages. It can easily co-work with other semiconductors to form a Schottky junction, in which the photo-carrier generated by light absorption in the semiconductor might be transported to the graphene layer efficiently by the build-in field. It changes the graphene conduction greatly and provides the possibility of realizing a graphene-based conductive-mode photodetector. Here we design and demonstrate a silicon-graphene conductive photodetector with improved responsivity and response speed. An electrical-circuit model is established and the graphene-sheet pattern is designed optimally for maximizing the responsivity. The fabricated silicon-graphene conductive photodetector shows a responsivity of up to ~105 A/W at room temperature (27 °C) and the response time is as short as ~30 μs. The temperature dependence of the silicon-graphene conductive photodetector is studied for the first time. It is shown that the silicon-graphene conductive photodetector has ultra-high responsivity when operating at low temperature, which provides the possibility to detect extremely weak optical power. For example, the device can detect an input optical power as low as 6.2 pW with the responsivity as high as 2.4 × 107 A/W when operating at −25 °C in our experiment.

Observation of optically induced transparency effect in silicon nanophotonic wires with graphene

Graphene, a well-known two-dimensional sheet of carbon atoms in a honeycomb structure, has many unique and fascinating properties in optoelectronics and photonics. Integration of graphene on silicon nanophotonic wires is a promising approach to enhance light-graphene interactions. In this paper, we demonstrate on-chip silicon nanophotonic wires covered by graphene with CMOS-compatible fabrication processes. Under the illumination of pump light on the graphene sheet, a loss reduction of silicon nanophotonic wires, which is called optically induced transparency (OIT) effect, is observed over a broad wavelength range for the first time. The pump power required to generate the OIT effect is as low as ~0.1mW and the corresponding power density is about 2×103mW/cm2, which is significantly different from the saturated absorption effect of graphene reported previously. The extremely low power density implies a new mechanism for the present OIT effect, which will be beneficial to realize silicon on-chip all-optical controlling in the future. It also suggests a new and efficient approach to tune the carrier concentration (doping level) in graphene optically.

Tunable Silicon Micro-disk Resonator with Flexible Graphene-based Ultra-thin Heaters

A thermally-tuning silicon-on-insulator micro-disk resonator with a flexible graphene-based ultra-thin heater is demonstrated. The experimental results show graphene heaters have excellent performances on the heating efficiency and the temporal response.

Utilization of thermal effects for silicon photonics

Thermal effect plays a key role and has been utilized for various photonic devices. For silicon photonics, the thermal effect is usually important because of the large thermo-optical coefficient of silicon material. This paper gives a review for the utilization of thermal effects for silicon photonics. First, the thermal effect is very beneficial to realize energy-efficient silicon photonic devices with tunability/switchability (including switches, variable optical attenuators, etc). Traditionally metal micro-heater sitting on a buried silicon-on-insulator (SOI) nanowire is used to introduce a phase shift for thermal tunability by injecting a electrical current. An effective way to improve the energy-efficiency of thermal tuning is reducing the volume of the optical waveguide as well as the micro-heater. Our recent work on silicon nanophotonic waveguides with novel nano-heaters based on metal wires as well as graphene ribbons will be summarized. Second, the thermal resistance effect of the metal strip on a hybrid plasmonic waveguide structure can be utilized to realize an ultra-small on-chip photodetector available for an ultra-broad band of wavelength, which will also be discussed.