Modern electronics based on semiconductors is meeting the fundamental speed limit caused by the interconnect delay and large heat generation when the sizes of components reach nanometer scale. Photons as a carrier of the information are superior to electrons in bandwidth, density, speed, and dissipation. More over, photons could carry intensity, polarization, phase, and frequency information which could break through the limitation of binary system as in electronic devices. But due to the diffraction limitation, the photonic components and devices can not be fabricated small enough to be integrated densely. Surface plasmon polariton is quanta of collective oscillations of free electrons excited by photons in metal nanostrucrures, which offers a promising way to manipulate light at the nanoscale and to realize the miniaturization of photonic devices. Hence, plasmonic circuits and devices have been proposed for some time as a potential strategy for advancing semiconductor-based computing beyond the fundamental performance limitations of electronic devices, as epitomized by Moore's law. A variety of individual plasmonic nanodevices have been intensively studied recently, but the crucial and necessary step to enable nanophotonic circuits for future information technology, namely cascade logics integrated on-chip, has not been achieved. Here we demonstrate that a nanophotonic binary logic NOR gate can be realized by cascading plasmonic OR and NOT gates in four-terminal nanowire networks. We explain the operating principle for the device based on quantum dot luminescence imaging, which reveal the interferences for different logic functions between propagating plasmon wave packets in the nanowire network in great detail. Since the NOR gate is logic complete, i.e. any Boolean logic gate can be constructed from it, our results could have a key role in defining a viable path for the development of novel subwavelength optical processor architectures.

Cascaded Logic Gates in Nanophotonic Plasmon Networks

The modulator is a key component in optical communications. Several graphene-based amplitude modulators have been reported based on electro-absorption. However, graphene phase modulators (GPMs) are necessary for functions such as applying complex modulation formats or making switches or phased arrays. Here, we present a 10 Gb s–1 GPM integrated in a Mach–Zehnder interferometer configuration. This is a compact device based on a graphene-insulator–silicon capacitor, with a phase-shifter length of 300 μm and extinction ratio of 35 dB. The GPM has a modulation efficiency of 0.28 V cm at 1,550 nm. It has 5 GHz electro-optical bandwidth and operates at 10 Gb s–1 with 2 V peak-to-peak driving voltage in a push–pull configuration for binary transmission of a non-return-to-zero data stream over 50 km of single-mode fibre. This device is the key building block for graphene-based integrated photonics, enabling compact and energy-efficient hybrid graphene–silicon modulators for telecom, datacom and other applications. A 10 Gb s–1 phase modulator based on a graphene-on-silicon Mach–Zehnder interferometer (MZI) is reported. The compact device has a phase-shifter length of only 300 μm and provides modulation of light at 1,550 nm with a 35 dB extinction ratio.

Graphene-silicon phase modulators with gigahertz bandwidth

Leveraging the spatial modes of multimode waveguides using mode-division multiplexing on an integrated photonic chip allows unprecedented scaling of bandwidth density for on-chip communication. Switching channels between waveguides is critical for future scalable optical networks, but its implementation in multimode waveguides must address how to simultaneously control modes with vastly different optical properties. Here we present a platform for switching signals between multimode waveguides based on individually processing the spatial mode channels using single-mode elements. Using this wavelength-division multiplexing-compatible platform, we demonstrate a 1×2 multimode switch for a silicon chip that routes four data channels with low (<−16.8  dB) crosstalk. We show bit-error rates below 10−9 and power penalties below 1.4 dB on all channels while routing 10 Gb/s data when each channel is input and routed separately. The switch exhibits an additional power penalty of less than 2.4 dB when all four channels are simultaneously routed. These results enable individual processing of multimode signals and high-bandwidth, flexible optical networks.

On-chip mode-division multiplexing switch

We demonstrate a high-speed silicon Mach-Zehnder modulator (MZM) with low insertion loss, based on the carrier depletion effect in a lateral PN junction. A 1.9 dB on-chip insertion loss and a VπLπ < 2 V·cm were achieved in an MZM with a 750 μm-long phase shifter by properly choosing the doping concentration and precisely locating the junction. High-speed modulations up to 45-60 Gbit/s have been demonstrated with an additional 1.6 dB optical loss, indicating a total insertion loss of 3.5 dB. A high extinction ratio of 7.5 dB was also realized at the bit rate of 50 Gbit/s with an acceptable insertion loss of 6.5 dB.

High-speed, low-loss silicon Mach–Zehnder modulators with doping optimization

Graphene is an ideal material for optoelectronic applications. Its photonic properties give several advantages and complementarities over Si photonics. For example, graphene enables both electro-absorption and electro-refraction modulation with an electro-optical index change exceeding 10$^{-3}$. It can be used for optical add-drop multiplexing with voltage control, eliminating the current dissipation used for the thermal detuning of microresonators, and for thermoelectric-based ultrafast optical detectors that generate a voltage without transimpedance amplifiers. Here, we present our vision for grapheme-based integrated photonics. We review graphene-based transceivers and compare them with existing technologies. Strategies for improving power consumption, manufacturability and wafer-scale integration are addressed. We outline a roadmap of the technological requirements to meet the demands of the datacom and telecom markets. We show that graphene based integrated photonics could enable ultrahigh spatial bandwidth density , low power consumption for board connectivity and connectivity between data centres, access networks and metropolitan, core, regional and long-haul optical communications.

Graphene-Based Integrated Photonics For Next-Generation Datacom And Telecom

We experimentally demonstrate a spatially non-blocking five-port optical router, which is based on microring resonators tuned through the thermo-optic effect. The characteristics of the microring-resonator-based switching element are investigated to achieve balanced performances in its two output ports. The optical router is fabricated on the SOI platform using standard CMOS processing. The effective footprint of the device is about 440×660 μm2. The microring resonators have 3-dB bandwidths of larger than 0.31 nm (38 GHz), and extinction ratios of better than 21 dB for through ports and 16 dB for drop ports. Finally, 12.5 Gbps high-speed signal transmission experiments verify the routing functionality of the optical router.

Five-port optical router for photonic networks-on-chip

We have designed and fabricated a directed logic architecture consisting of two silicon microring resonators that can perform XOR and XNOR operations. The microring resonators are modulated through thermo-optic effect. Two electrical modulating signals applied to the microring resonators represent the two operands of the logical operation. The logical function is evaluated through the directed propagation of light in the device, and the result is represented by the output optical signal. Both XOR and XNOR operations at 20 kbits are demonstrated.

Demonstration of directed XOR/XNOR logic gates using two cascaded microring resonators

We propose and demonstrate a directed OR/NOR and AND/NAND logic circuit consisting of two parallel microring resonators (MRRs). We use two electrical signals representing the two operands of the logical operation to modulate the two MRRs through the thermo-optic effect, respectively. The final operation results are represented by the output optical signals. Both OR/NOR and AND/NAND operations at 10 kbps are demonstrated.

Proof of concept of directed OR/NOR and AND/NAND logic circuit consisting of two parallel microring resonators

We design and fabricate a four-port optical router, which is composed of eight microring-resonator-based switching elements, four optical waveguides and six waveguide crossings. The extinction ratio is about 13 dB for the through port and larger than 30 dB for the drop port. The crosstalk of the measured optical links is less than -13 dB. The average tuning power consumption is about 10.37 mW and the tuning efficiency is 5.398 mW/nm. The routing functionality and optical signal integrity are verified by transmitting a 12.5 Gb/s PRBS optical signal.

Microring-resonator-based four-port optical router for photonic networks-on-chip

We demonstrate a 26 Gbit/s Mach-Zehnder silicon optical modulator. The doping concentration and profile are optimized, and a modulation efficiency with the figure of merit (VπL) of 1.28 V·cm is achieved. We design an 80-nm-wide intrinsic silicon gap between the p-type and n-type doped regions to reduce the capacitance of the diode and prevent the diode from working in a slow diffusion mode. Therefore, the modulator can be driven with a small differential voltage of 0.5 V with no bias. Without the elimination of the dissipated power of the series resistors and the reflected power of the electrical signal, the maximum power consumption is 3.8 mW.

Ping Zhou

Papers

Five-port optical router for photonic networks-on-chip

Demonstration of directed XOR/XNOR logic gates using two cascaded microring resonators

Proof of concept of directed OR/NOR and AND/NAND logic circuit consisting of two parallel microring resonators

Microring-resonator-based four-port optical router for photonic networks-on-chip

Ultra-low-power carrier-depletion Mach-Zehnder silicon optical modulator.