Tunable semiconductor lasers are often listed in critical technology road maps for future dense-wavelength-division-multiplexing (DWDM) systems and high-performance computing systems, and they are increasingly demanded in long-haul, metropolitan, and access networks. The capability to produce such lasers directly on silicon (Si) could boost the use of Si photonics and facilitate the adoption of optical data transmission even at the chip scale. Moreover, just the use of Si as a cheap and large-diameter substrate for device production is very advantageous, as the fabrication can take advantage of the highly optimized processing techniques and economy of scale enabled by decades of development in Si microelectronics. Here, we report a tunable single-wavelength quantum dot (QD) laser directly grown on Si. The high carrier confinement and a real dot density of QDs provide reduced sensitivity to crystalline defects, which allows for exceptional lasing performance even in lattice-mismatched material systems. The discrete density of states of dots yields unique gain properties that show promise for improved device performance and new functionalities relative to the quantum well counterparts, including high temperature stability, low threshold operation, reduced sidewall recombination, and isolator-free stability. We implement a simple, integrable architecture to achieve over 45 dB side-mode-suppression-ratio without involving regrowth steps or subwavelength grating lithography. Under continuous-wave electrical injection at room temperature, we achieved a 16 nm tuning range with output powers of over 2.7 mW per tuning wavelength. This work represents a step towards using III–V/Si epitaxy to form efficient, easily manufacturable on-chip Si light sources for not only DWDM networks, but also spectroscopy, biosensors, and many other emerging applications.

Tunable quantum dot lasers grown directly on silicon

Distributed feedback (DFB) lasers represent a central focus for wavelength-division-multiplexing-based transceivers in metropolitan networks. Here, the first 1.3 μm quantum dot (QD) DFB lasers grown on a complementary metal-oxide-semiconductor (CMOS)-compatible (001) Si substrate are reported. Temperature-stable, single-longitudinal-mode operation is achieved with a side-mode suppression ratio of more than 50 dB and a threshold current density of 440 A cm−2. A single-lane rate of 128 Gbit s−1 with a net spectral efficiency of 1.67 bits−1 Hz−1 is demonstrated, with an aggregate total transmission capacity of 640 Gbit s−1 using five channels in the O-band. Apart from the QD active region growth, the overall fabrication is essentially identical to the commercial process for quantum well (QW) DFB lasers. This provides a process-compatible path for QD technology into commercial applications previously filled by QW devices. In addition, the capability to grow laser epi across entire CMOS-compatible (001) Si wafers adds extra benefits of reduced cost, improved heat dissipation, and manufacturing scalability. Through direct epitaxial integration of III–Vs and Si, one can envision a revolution of the photonics industry in the same way that CMOS design and processing revolutionize the microelectronics industry. This is discussed from a system perspective for on-chip optical interconnects.

1.3 µm Quantum Dot-Distributed Feedback Lasers Directly Grown on (001) Si

Electrically-pumped lasers directly grown on silicon are key devices interfacing silicon microelectronics and photonics. We report here, for the first time, an electrically-pumped, room-temperature, continuous-wave (CW) and single-mode distributed feedback (DFB) laser array fabricated in InAs/GaAs quantum-dot (QD) gain material epitaxially grown on silicon. CW threshold currents as low as 12 mA and single-mode side mode suppression ratios (SMSRs) as high as 50 dB have been achieved from individual devices in the array. The laser array, compatible with state-of-the-art coarse wavelength division multiplexing (CWDM) systems, has a well-aligned channel spacing of 20 0.2 nm and exhibits a record wavelength coverage range of 100 nm, the full span of the O-band. These results indicate that, for the first time, the performance of lasers epitaxially grown on silicon is elevated to a point approaching real-world CWDM applications, demonstrating the great potential of this technology.

Monolithic quantum-dot distributed feedback laser array on silicon

In the past few decades, numerous high-performance silicon (Si) photonic devices have been demonstrated. Si, as a photonic platform, has received renewed interest in recent years. Efficient Si-based III–V quantum-dot (QDs) lasers have long been a goal for semiconductor scientists because of the incomparable optical properties of III–V compounds. Although the material dissimilarity between III–V material and Si hindered the development of monolithic integrations for over 30 years, considerable breakthroughs happened in the 2000s. In this paper, we review recent progress in the epitaxial growth of various III–V QD lasers on both offcut Si substrate and on-axis Si (001) substrate. In addition, the fundamental challenges in monolithic growth will be explained together with the superior characteristics of QDs.

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Recent progress in epitaxial growth of III–V quantum-dot lasers on silicon substrate

Research into self-assembled semiconductor quantum dots (QDs) has helped advance numerous optoelectronic applications, ranging from solid-state lighting to photodetectors. By carefully controlling molecular beam epitaxy (MBE) growth parameters, we can readily tune QD light absorption and emission properties to access a broad portion of the electromagnetic spectrum. Although this field is now sufficiently mature that QDs are found in consumer electronics, research efforts continue to expand into new areas. By manipulating MBE growth conditions and exploring new combinations of materials, substrate orientations, and the sign of strain, a wealth of opportunities exist for synthesizing novel QD nanostructures with hitherto unavailable properties. As such, QDs are uniquely well positioned to make critical contributions to the development of future quantum technologies. In this tutorial, we summarize the history of self-assembled QDs, outline some examples of quantum optics applications based on QDs, discuss the science that explains the spontaneous formation of QDs, and provide recipes for successful QD growth by MBE for some of the most commonly used semiconductor materials systems. We hope that compiling this information in one place will be useful both for those new to QD self-assembly and for experienced researchers, ideally supporting the community’s efforts to continue pushing the boundaries of knowledge in this important field.

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Strain-driven quantum dot self-assembly by molecular beam epitaxy

Monolithic integration of efficient III–V light-emitting sources on planar on-axis Si (001) has been recognized as an enabling technology for realizing Si-based photonic integrated circuits (PICs). The field of microdisk lasers employing quantum dot (QD) materials is gaining significant momentum because it allows massive-scalable, streamlined fabrication of Si-based PICs to be made cost effectively. Here, we present InAs/GaAs QD microdisk lasers monolithically grown on on-axis Si (001) substrate with an ultra-low lasing threshold at room temperature under continuous-wave optical pumping. The lasing characteristics of microdisk lasers with small diameter (D) around 2 μm and sub-wavelength scale (D∼1.1 μm) are demonstrated, with a lasing threshold as low as ∼3 μW. The promising lasing characteristics of the microdisk lasers with ultra-low power consumption and small footprint represent a major advance towards large-scale, low-cost integration of laser sources on the Si platform.

Marie-Leonor Touraton

Papers

Ultra-low threshold InAs/GaAs quantum dot microdisk lasers on planar on-axis Si (001) substrates