This Review covers the state-of-the-art technologies on photonics-based terahertz communications, which are compared with competing technologies based on electronics and free-space optical communications. Future prospects and challenges are also discussed. Almost 15 years have passed since the initial demonstrations of terahertz (THz) wireless communications were made using both pulsed and continuous waves. THz technologies are attracting great interest and are expected to meet the ever-increasing demand for high-capacity wireless communications. Here, we review the latest trends in THz communications research, focusing on how photonics technologies have played a key role in the development of first-age THz communication systems. We also provide a comparison with other competitive technologies, such as THz transceivers enabled by electronic devices as well as free-space lightwave communications.

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Advances in terahertz communications accelerated by photonics

All-inorganic colloidal cesium lead halide perovskite quantum dots (CsPbX3 , X = Cl, Br, I) are revealed to be a new class of favorable optical-gain materials, which show -combined merits of both colloidal quantum dots and halide perovskites. Low-threshold and -ultrastable stimulated emission is -demonstrated under atmospheric conditions with wavelength tunability across the whole -visible spectrum via either size or composition control.

All-Inorganic Colloidal Perovskite Quantum Dots: A New Class of Lasing Materials with Favorable Characteristics.

Lasing is experimentally demonstrated in a direct bandgap GeSn alloy, grown directly onto Si(001). The authors observe a clear lasing threshold as well as linewidth narrowing at low temperatures.

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Lasing in direct-bandgap GeSn alloy grown on Si

Reliable, efficient electrically pumped silicon-based lasers would enable full integration of photonic and electronic circuits, but have previously only been realized by wafer bonding. Here, we demonstrate continuous-wave InAs/GaAs quantum dot lasers directly grown on silicon substrates with a low threshold current density of 62.5 A cm–2, a room-temperature output power exceeding 105 mW and operation up to 120 °C. Over 3,100 h of continuous-wave operating data have been collected, giving an extrapolated mean time to failure of over 100,158 h. The realization of high-performance quantum dot lasers on silicon is due to the achievement of a low density of threading dislocations on the order of 105 cm−2 in the III–V epilayers by combining a nucleation layer and dislocation filter layers with in situ thermal annealing. These results are a major advance towards reliable and cost-effective silicon-based photonic–electronic integration.

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Electrically pumped continuous-wave III–V quantum dot lasers on silicon

Optics offers unique opportunities for reducing energy in information processing and communications while simultaneously resolving the problem of interconnect bandwidth density inside machines. Such energy dissipation overall is now at environmentally significant levels; the source of that dissipation is progressively shifting from logic operations to interconnect energies. Without the prospect of substantial reduction in energy per bit communicated, we cannot continue the exponential growth of our use of information. The physics of optics and optoelectronics fundamentally addresses both interconnect energy and bandwidth density, and optics may be the only scalable solution to such problems. Here we summarize the corresponding background, status, opportunities, and research directions for optoelectronic technology and novel optics, including subfemtojoule devices in waveguide and novel two-dimensional (2-D) array optical systems. We compare different approaches to low-energy optoelectronic output devices and their scaling, including lasers, modulators and LEDs, optical confinement approaches (such as resonators) to enhance effects, and the benefits of different material choices, including 2-D materials and other quantum-confined structures. With such optoelectronic energy reductions, and the elimination of line charging dissipation by the use optical connections, the next major interconnect dissipations are in the electronic circuits for receiver amplifiers, timing recovery, and multiplexing. We show we can address these through the integration of photodetectors to reduce or eliminate receiver circuit energies, free-space optics to eliminate the need for timing and multiplexing circuits (while also solving bandwidth density problems), and using optics generally to save power by running large synchronous systems. One target concept is interconnects from ∼1 cm to ∼10 m that have the same energy (∼10 fJ/bit) and simplicity as local electrical wires on chip.

Attojoule Optoelectronics for Low-Energy Information Processing and Communications

Growing a group III–V quantum dot laser directly on a group IV substrate could provide silicon photonics with a convenient new form of laser source for use in optoelectronic circuitry.

Long-wavelength InAs/GaAs quantum-dot laser diode monolithically grown on Ge substrate

We report the first room-temperature continuous-wave operation of III-V quantum-dot laser diodes monolithically grown on a Si substrate. Long-wavelength InAs/GaAs quantum-dot structures were fabricated on Ge-on-Si substrates. Room-temperature lasing at a wavelength of 1.28 μm has been achieved with threshold current densities of 163 A/cm2 and 64.3 A/cm2 under continuous-wave and pulsed conditions for ridge-waveguide lasers with as cleaved facets, respectively. The value of 64.3 A/cm2 represents the lowest room-temperature threshold current density for any kind of laser on Si to date.

Continuous-wave InAs/GaAs quantum-dot laser diodes monolithically grown on Si substrate with low threshold current densities.

We compare InAlAs/GaAs and InGaAs/GaAs strained-layer superlattices (SLSs) as dislocation filter layers for 1.3-μm InAs/GaAs quantum-dot laser structures directly grown on Si substrates. InAlAs/GaAs SLSs are found to be more effective than InGaAs/GaAs SLSs in blocking the propagation of threading dislocations generated at the interface between the GaAs buffer layer and the Si substrate. Room-temperature lasing at ~1.27 μm with a threshold current density of 194 A/cm(2) and output power of ~77 mW has been demonstrated for broad-area lasers grown on Si substrates using InAlAs/GaAs dislocation filter layers.

1.3-μm InAs/GaAs quantum-dot lasers monolithically grown on Si substrates using InAlAs/GaAs dislocation filter layers

The realization of semiconductor lasers on Si substrates will enable the fabrication of complex optoelectronic circuits. This will permit the creation of the long-dreamed chip-to-chip and system-to-system optical interconnects. This paper reports recent developments in our work on InAs/GaAs quantum-dot (QD) lasers monolithically grown on Si, Ge, and Ge-on-Si (Ge/Si) substrates. A thin AlAs nucleation layer (NL) was first investigated for the growth of InAs/GaAs QDs on Si substrates. The AlAs NL enables more defects to be confined in the interface between the GaAs epitaxial layer and Si substrate, and hence leads to higher photoluminescence intensity for InAs/GaAs QDs. Room-temperature lasing at 1.29 μm with a threshold current density of 650 A/cm2 was demonstrated with the use of an AlAs NL. The growth of InAs/GaAs QDs on Ge and Ge/Si substrates was further studied. A low threshold current density of ~200 A/cm2 for 1-mm long QD lasers has been demonstrated for QD lasers grown on Ge substrates by using Ga prelayer technique. This growth technique has also been explored for Ge/Si substrates. Room-temperature lasing at 1.28 μm with threshold current density of ~164 A/cm2 and lasing operation up to 84°C has been demonstrated for a 3-mm long device.

Qi Jiang

Papers

Electrically pumped continuous-wave III–V quantum dot lasers on silicon

Long-wavelength InAs/GaAs quantum-dot laser diode monolithically grown on Ge substrate

Continuous-wave InAs/GaAs quantum-dot laser diodes monolithically grown on Si substrate with low threshold current densities.

1.3-μm InAs/GaAs quantum-dot lasers monolithically grown on Si substrates using InAlAs/GaAs dislocation filter layers

InAs/GaAs Quantum-Dot Lasers Monolithically Grown on Si, Ge, and Ge-on-Si Substrates