Luca Alloatti

Single-chip microprocessor that communicates directly using light

Data transport across short electrical wires is limited by both bandwidth and power density, which creates a performance bottleneck for semiconductor microchips in modern computer systems—from mobile phones to large-scale data centres. These limitations can be overcome by using optical communications based on chip-scale electronic–photonic systems enabled by silicon-based nanophotonic devices8. However, combining electronics and photonics on the same chip has proved challenging, owing to microchip manufacturing conflicts between electronics and photonics. Consequently, current electronic–photonic chips are limited to niche manufacturing processes and include only a few optical devices alongside simple circuits. Here we report an electronic–photonic system on a single chip integrating over 70 million transistors and 850 photonic components that work together to provide logic, memory, and interconnect functions. This system is a realization of a microprocessor that uses on-chip photonic devices to directly communicate with other chips using light. To integrate electronics and photonics at the scale of a microprocessor chip, we adopt a ‘zero-change’ approach to the integration of photonics. Instead of developing a custom process to enable the fabrication of photonics, which would complicate or eliminate the possibility of integration with state-of-the-art transistors at large scale and at high yield, we design optical devices using a standard microelectronics foundry process that is used for modern microprocessors. This demonstration could represent the beginning of an era of chip-scale electronic–photonic systems with the potential to transform computing system architectures, enabling more powerful computers, from network infrastructure to data centres and supercomputers.

Optical properties of highly nonlinear silicon-organic hybrid (SOH) waveguide geometries

Geometry, nonlinearity, dispersion and two-photon absorption figure of merit of three basic silicon-organic hybrid waveguide designs are compared. Four-wave mixing and heterodyne pump-probe measurements show that all designs achieve high nonlinearities. The fundamental limitation of two-photon absorption in silicon is overcome using silicon-organic hybrid integration, with a five-fold improvement for the figure of merit (FOM). The value of FOM = 2.19 measured for silicon-compatible nonlinear slot waveguides is the highest value published.

Silicon-Organic Hybrid Electro-Optical Devices

Organic materials combined with strongly guiding silicon waveguides open the route to highly efficient electro-optical devices. Modulators based on the so-called silicon-organic hybrid (SOH) platform have only recently shown frequency responses up to 100 GHz, high-speed operation beyond 112 Gbit/s with fJ/bit power consumption. In this paper, we review the SOH platform and discuss important devices such as Mach-Zehnder and IQ-modulators based on the linear electro-optic effect. We further show liquid-crystal phase-shifters with a voltage-length product as low as VπL = 0.06 V·mm and sub-μW power consumption as required for slow optical switching or tuning optical filters and devices.

Reduced propagation loss in silicon strip and slot waveguides coated by atomic layer deposition

When silicon strip and slot waveguides are coated with a 50 nm amorphous titanium dioxide (TiO2) film, measured losses at a wavelength of 1.55 μm can be as low as (2 ± 1)dB/cm and (7 ± 2)dB/cm, respectively. We use atomic layer deposition (ALD), estimate the effect of ALD growth on the surface roughness, and discuss the effect on the scattering losses. Because the gap between the rails of a slot waveguide narrows by the TiO2 deposition, the effective slot width can be back-end controlled. This is useful for precise adjustment if the slot is to be filled with, e. g., a nonlinear organic material or with a sensitizer for sensors applications.

Silicon-organic hybrid (SOH) IQ modulator using the linear electro-optic effect for transmitting 16QAM at 112 Gbit/s

Advanced modulation formats call for suitable IQ modulators. Using the silicon-on-insulator (SOI) platform we exploit the linear electro-optic effect by functionalizing a photonic integrated circuit with an organic χ(2)-nonlinear cladding. We demonstrate that this silicon-organic hybrid (SOH) technology allows the fabrication of IQ modulators for generating 16QAM signals with data rates up to 112 Gbit/s. To the best of our knowledge, this is the highest single-polarization data rate achieved so far with a silicon-integrated modulator. We found an energy consumption of 640 fJ/bit.

Performance tradeoff between lateral and interdigitated doping patterns for high speed carrier-depletion based silicon modulators

Carrier-depletion based silicon modulators with lateral and interdigitated PN junctions are compared systematically on the same fabrication platform. The interdigitated diode is shown to outperform the lateral diode in achieving a low VπLπ of 0.62 V∙cm with comparable propagation loss at the expense of a higher depletion capacitance. The low VπLπ of the interdigitated PN junction is employed to demonstrate 10 Gbit/s modulation with 7.5 dB extinction ration from a 500 µm long device whose static insertion loss is 2.8 dB. In addition, up to 40 Gbit/s modulation is demonstrated for a 3 mm long device comprising a lateral diode and a co-designed traveling wave electrode.

Low-Loss Silicon Strip-to-Slot Mode Converters

We demonstrate compact highly efficient broadband strip-to-slot mode converters in silicon with record-low losses of 0.02 (±0.02) dB and negligible reflections between 1480 nm and 1580 nm. The new strip-to-slot transition is logarithmically tapered, which enables a compact design. The new logarithmic tapers are compared with more conventional linearly tapered converters.

Silicon-organic hybrid (SOH) frequency comb sources for terabit/s data transmission

We demonstrate frequency comb sources based on silicon-organic hybrid (SOH) electro-optic modulators. Frequency combs with line spacings of 25 GHz and 40 GHz are generated, featuring flat-top spectra with less than 2 dB power variations over up to 7 lines. The combs are used for WDM data transmission at terabit/s data rates and distances of up to 300 km.

Low Power Mach–Zehnder Modulator in Silicon-Organic Hybrid Technology

We report on a silicon-organic hybrid modulator based on a Mach-Zehnder interferometer (MZI) operating at 10 Gbit/s with an energy consumption of 320 fJ/bit. The device consists of a striploaded slot waveguide covered with an electro-optic polymer cladding. The MZI modulator is poled to be driven in push-pull operation by a single coplanar RF line. Our nonlinear coefficient r33 = 15 pm/V in combination with an 80 nm narrow slot enables RF peak-to-peak drive voltages as low as 800 mVpp to suffice for an extinction ration of 4.4 dB for a 1.5 mm long modulator.

Silicon-organic hybrid phase shifter based on a slot waveguide with a liquid-crystal cladding.

A highly efficient phase shifter based on the silicon-organic hybrid (SOH) platform is theoretically investigated and experimentally tested. The device consists of a silicon slot waveguide covered with an organic liquid-crystal (LC) cladding. A record-low voltage-length product of U(π)L = 0.085 Vmm can be achieved for high-purity materials where an optimum operation point can be set by a DC bias. With standard materials and without a DC bias, we measure a phase shift of 35π with a drive voltage of only 5 V for a 1.7 mm long device corresponding to a voltage-length product of U(π)L = 0.24 Vmm. The power dissipation is about six orders of magnitude smaller than that of state-of-the-art thermo-optic devices, thereby enabling dense integration of LC phase shifters in advanced photonic integrated circuits.