High-efficiency dual single layer graphene modulator integrated on slot waveguides.

This paper presents an experimental and theoretical investigation of a graphene-integrated electro-absorption modulator (EAM) based on a slot waveguide. Due to the enhanced light-matter interaction of graphene, the device exhibits an impressive modulation efficiency (0.038 dBµm-1V-1) and bandwidth (≈ 16 GHz). Starting from these results, we carried out an extensive design study, focusing on three crucial design parameters and exploring the associated trade-offs in insertion loss, extinction ratio and bandwidth. The simulation results offer valuable insights into the influence of each design parameter, reaffirming that our slot waveguide platform holds great promise for realizing a high-performance EAM balancing optical and electrical performance. It is important to note that the slot waveguide was defined through standard deep ultraviolet (DUV) lithography, allowing seamless integration into high-density systems.

Unique design approach to realize an O-band laser monolithically integrated on 300†mm Si substrate by nano-ridge engineering.

We introduce a new design space for optimizing III-V devices monolithically grown on Silicon substrates by extending the concept of nano-ridge engineering from binary semiconductors such as GaAs, InAs and GaSb to the ternary alloy InGaAs. This allows controlling the fundamental lattice constant of the fully relaxed ternary nano-ridge which thereby serves as a tunable base for the integration of diverse device hetero-layers. To demonstrate the flexibility of this approach, we realized an O-band nano-ridge laser containing three In0.45Ga0.55As quantum wells, which are pseudomorphically strained to an In0.25Ga0.75As nano-ridge base. The demonstration of an optically pumped nano-ridge laser operating around 1300 nm underlines the potential of this cost-efficient and highly scalable integration approach for silicon photonics.

Loss-coupled DFB nano-ridge laser monolithically grown on a standard 300-mm Si wafer.

We present a loss-coupled distributed feedback microlaser, monolithically grown on a standard 300-mm Si wafer using nano-ridge engineering. The cavity is formed by integrating a metallic grating on top of the nano-ridge. This allows forming a laser cavity without etching the III-V material, avoiding damaged interfaces and the associated carrier loss. Simulations, supported by experimental characterisation of the modal gain of the nano-ridge devices, predict an optimal duty cycle for the grating of ~0.4, providing a good trade-off between coupling strength and cavity loss for the lasing mode. The model was experimentally verified by characterising the lasing threshold and external efficiency of devices exhibiting gratings with varying duty cycle. The high modal gain and low threshold obtained prove the excellent quality of the epitaxial material. Furthermore, the low loss metal grating might provide a future route to electrical injection and efficient heat dissipation of these nanoscale devices.

5 × 25 Gbit/s WDM transmitters based on passivated graphene-silicon electro-absorption modulators.

Today, one of the key challenges of graphene devices is establishing fabrication processes that can ensure performance stability and repeatability and that can eventually enable production in high volumes. In this paper, we use up-scalable fabrication processes to demonstrate three five-channel wavelength-division multiplexing (WDM) transmitters, each based on five graphene-silicon electro-absorption modulators. A passivation-first approach is used to encapsulate graphene, which results in hysteresis-free and uniform performance across the five channels of each WDM transmitter, for a total of 15 modulators. Open-eye diagrams are obtained at 25 Gb/s using $ 2.5\;{{\rm V}_{{\rm pp}}} $2.5Vpp, thus demonstrating potential for multi-channel data transmission at ${5}\times {25}\;{\rm Gb/s}$5×25Gb/s on each of the three WDM transmitters.

Novel adiabatic coupler for III-V nano-ridge laser grown on a Si photonics platform.

While III-V lasers epitaxially grown on silicon have been demonstrated, an efficient approach for coupling them with a silicon photonics platform is still missing. In this paper, we present a novel design of an adiabatic coupler for interfacing nanometer-scale III-V lasers grown on SOI with other silicon photonics components. The starting point is a directional coupler, which achieves 100% coupling efficiency from the III-V lasing mode to the Si waveguide TE-like ground mode. To improve the robustness and manufacturability of the coupler, a linear-tapered adiabatic coupler is designed, which is less sensitive to variations and still reaches a coupling efficiency of around 98%. Nevertheless, it has a relatively large footprint and exhibits some undesired residual coupling to TM-like modes. To improve this, a more advanced adiabatic coupler whose geometry is varied along its propagation length is designed and manages to reach ∼100% coupling and decoupling within a length of 200 µm. The proposed couplers are designed for the particular case of III-V nano-ridge lasers monolithically grown using aspect-ratio-trapping (ART) together with nano-ridge engineering (NRE) but are believed to be compatible with other epitaxial III-V/Si integration platforms recently proposed. In this way, the presented coupler is expected to pave the way to integrating III-V lasers monolithically grown on SOI wafers with other photonics components, one step closer towards a fully functional silicon photonics platform.

Room Temperature O-band DFB Laser Array Directly Grown on (001) Silicon.

Several approaches for growing III-V lasers on silicon were recently demonstrated. Most are not compatible with further integration, however, and rely on thick buffer layers and require special substrates. Recently, we demonstrated a novel approach for growing high quality InP without buffer on standard 001-silicon substrates using a selective growth process compatible with integration. Here we show high quality InGaAs layers can be grown on these InP-templates. High-resolution TEM analysis shows these layers are free of optically active defects. Contrary to InP, the InGaAs material exhibits strong photoluminescence for wavelengths relevant for integration with silicon photonics integrated circuits. Distributed feedback lasers were defined by etching a first order grating in the top surface of the device. Clear laser operation at a single wavelength with strong suppression of side modes was demonstrated. Compared to the previously demonstrated InP lasers 65% threshold reduction is observed. Demonstration of laser arrays with linearly increasing wavelength prove the control of the process and the high quality of the material. This is an important result toward realizing fully integrated photonic ICs on silicon substrates.

Silicon photonics integrated circuits: a manufacturing platform for high density, low power optical I/O's.

Silicon photonics integrated circuits are considered to enable future computing systems with optical input-outputs co-packaged with CMOS chips to circumvent the limitations of electrical interfaces. In this paper we present the recent progress made to enable dense multiplexing by exploiting the integration advantage of silicon photonics integrated circuits. We also discuss the manufacturability of such circuits, a key factor for a wide adoption of this technology.

Trade-off between optical modulation amplitude and modulation bandwidth of silicon micro-ring modulators.

An analytic model is developed to study the dynamic response of carrier-depletion silicon ring modulators. Its validity is confirmed by a detailed comparison between the modeled and the measured small signal frequency response of a practical device. The model is used to investigate how to maximize the optical modulation amplitude (OMA) and how the OMA could be traded for the bandwidth by tuning the coupling strength and the operation wavelength. Our calculation shows that for a ring modulator with equal RC time constant and photon lifetime, if its operation wavelength shifts from the position of the maximum OMA towards the direction that is away from the resonance, the 3dB modulation bandwidth increases ~2.1 times with a penalty of 3 dB to the OMA.

Silicon dual-ring modulator driven by differential signal.

A silicon dual-ring modulator consisting of two serially cascaded rings with embedded PN junctions is driven by a differential signal pair. We show by simulation and experiment that the device has advantages over the single-ring modulator in terms of optical bandwidth, 3-dB modulation bandwidth and bit rate, at the expense of a 1.7-dB increase in the transmission penalty and a twofold increase of the RF power consumption. Driven by differential pseudo random binary sequence (PRBS) signals of 0.5-V peak-to-peak voltage (Vpp), the dual-ring modulator exhibits optical bandwidths of 66 pm and 40 pm at 12.5 Gb/s and 20 Gb/s, respectively. In contrast, the single-ring modulator has an optical bandwidth of 26 pm under a single-end PRBS signal of 0.5 Vpp at 12.5 Gb/s, and its eye diagram closes if the bit rate rises to 20 Gb/s.

Polytypic InP nanolaser monolithically integrated on (001) silicon.

On-chip optical interconnects still miss a high-performance laser monolithically integrated on silicon. Here, we demonstrate a silicon-integrated InP nanolaser that operates at room temperature with a low threshold of 1.69 pJ and a large spontaneous emission factor of 0.04. An epitaxial scheme to grow relatively thick InP nanowires on (001) silicon is developed. The zincblende/wurtzite crystal phase polytypism and the formed type II heterostructures are found to promote lasing over a wide wavelength range.

Graphene Phase Modulators Operating in the Transparency Regime.

Next-generation data networks need to support Tb/s rates. In-phase and quadrature (IQ) modulation combine phase and intensity information to increase the density of encoded data, reduce overall power consumption by minimizing the number of channels, and increase noise tolerance. To reduce errors when decoding the received signal, intersymbol interference must be minimized. This is achieved with pure phase modulation, where the phase of the optical signal is controlled without changing its intensity. Phase modulators are characterized by the voltage required to achieve a π phase shift, Vπ, the device length, L, and their product, VπL. To reduce power consumption, IQ modulators are needed with <1 V drive voltages and compact (sub-cm) dimensions, which translate in VπL < 1Vcm. Si and LiNbO3 (LN) IQ modulators do not currently meet these requirements because VπL > 1Vcm. Here, we report a double single-layer graphene (SLG) Mach-Zehnder modulator (MZM) with pure phase modulation in the transparency regime, where optical losses are minimized and remain constant with increasing voltage. Our device has VπL ∼ 0.3Vcm, matching state-of-the-art SLG-based MZMs and plasmonic LN MZMs, but with pure phase modulation and low insertion loss (∼5 dB), essential for IQ modulation. Our VπL is ∼5 times lower than the lowest thin-film LN MZMs and ∼3 times lower than the lowest Si MZMs. This enables devices with complementary metal-oxide semiconductor compatible VπL (<1Vcm) and smaller footprint than LN or Si MZMs, improving circuit density and reducing power consumption by 1 order of magnitude.

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.

Using carrier-depletion silicon modulators for optical power monitoring.

Defect-mediated subbandgap absorption is observed in ion-implanted silicon-on-oxide waveguides that experience a rapid thermal annealing at 1075°C. With this effect, general carrier-depletion silicon modulators exhibit the capability of optical power monitoring. Responsivity is measured to be 22 mA/W for a 3 mm long Mach-Zehnder modulator of 2×10(18) cm(-3) doping concentration at -7.1 V bias voltage and 5.9 mA/W for a ring modulator of 1×10(18) cm(-3) doping concentration at -10 V bias voltage. The former is used to demonstrate data detection of up to 35 Gbits/s.

2D-3D integration of hexagonal boron nitride and a high-κ dielectric for ultrafast graphene-based electro-absorption modulators.

Electro-absorption (EA) waveguide-coupled modulators are essential building blocks for on-chip optical communications. Compared to state-of-the-art silicon (Si) devices, graphene-based EA modulators promise smaller footprints, larger temperature stability, cost-effective integration and high speeds. However, combining high speed and large modulation efficiencies in a single graphene-based device has remained elusive so far. In this work, we overcome this fundamental trade-off by demonstrating the 2D-3D dielectric integration in a high-quality encapsulated graphene device. We integrated hafnium oxide (HfO2) and two-dimensional hexagonal boron nitride (hBN) within the insulating section of a double-layer (DL) graphene EA modulator. This combination of materials allows for a high-quality modulator device with high performances: a ~39 GHz bandwidth (BW) with a three-fold increase in modulation efficiency compared to previously reported high-speed modulators. This 2D-3D dielectric integration paves the way to a plethora of electronic and opto-electronic devices with enhanced performance and stability, while expanding the freedom for new device designs.