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A silicon-compatible light source is the final missing piece for completing high-speed, low-power on-chip optical interconnects. In this paper, we present a germanium nanowire light emitter that encompasses all the aspects of potential low-threshold lasers: highly strained germanium gain medium, strain-induced pseudoheterostructure, and high-Q nanophotonic cavity. Our nanowire structure presents greatly enhanced photoluminescence into cavity modes with measured quality factors of up to 2000. By varying the dimensions of the germanium nanowire, we tune the emission wavelength over more than 400 nm with a single lithography step. We find reduced optical loss in optical cavities formed with germanium under high (>2.3%) tensile strain. Our compact, high-strain cavities open up new possibilities for low-threshold germanium-based lasers for on-chip optical interconnects.

We present a new way to create a thermally stable, highly strained germanium (Ge) optical resonator using a novel Ge-on-dual-insulators substrate. Instead of using a conventional way to undercut the oxide layer of a Ge-on-single-insulator substrate for inducing tensile strain in germanium, we use thin aluminum oxide as a sacrificial layer. By eliminating the air gap underneath the active germanium layer, we achieve an optically insulating, thermally conductive, and highly strained Ge resonator structure that is critical for a practical germanium laser. Using Raman spectroscopy and photoluminescence experiments, we prove that the novel geometry of our Ge resonator structure provides a significant improvement in thermal stability while maintaining good optical confinement.

We have implemented an automated nanophotonic design algorithm with fabrication constraints. This was used to demonstrate a compact 3-channel wavelength demultiplexer with a channel spacing of 40 nm, insertion loss < 2.29 dB, and under 10.7 dB crosstalk.

In wavelength division multiplexing (WDM) schemes, splitters must be used to combine and separate different wavelengths. Conventional splitters are fairly large with footprints in hundreds to thousands of square microns, and experimentally-demonstrated MMI-based and inverse-designed ultra-compact splitters operate with only two channels and large channel spacing ($>$100 nm). Here we inverse design and experimentally demonstrate a three-channel wavelength demultiplexer with 40 nm spacing (1500 nm, 1540 nm, and 1580 nm) with a footprint of 24.75 $��\mathrm{m}^2$. The splitter has a simulated peak insertion loss of -1.55 dB with under -15 dB crosstalk and a measured peak insertion loss of -2.29 dB with under -10.7 dB crosstalk.

A dramatic and previously overlooked interaction of parasitic absorption with strain in germanium (Ge) is demonstrated through extensive simulations and experiments. Uniaxial strain of 4–5% and biaxial strain greater than 1% are the best candidates for a room temperature Ge laser.

We present a novel structure that simultaneously achieves high tensile strain, pseudo-heterostructure, and high-Q optical cavity in a pure Ge layer. Employing our structure in a GeSn layer will enable a truly practical Si-compatible laser.

We report uniaxial tensile strains up to 5.0% in lithographically patterned germanium nanowires, which is enough strain to make germanium a direct bandgap semiconductor. Theoretically, this strain can reduce a germanium laser's threshold by >16,000x.

We report recent advances in electrically controlled photonic crystal nanocavity light sources and modulators, including an electrically pumped laser with 181nA threshold, an ultrafast single-mode LED with sub-fJ/bit operation energy, and modulator with 0.2 fJ/bit modulation energy. Additionally, we model the electrical and thermal performance of the lateral p-i-n geometry utilized in these devices, determining design criteria for efficient electrical injection and finding minimal self-heating.

We model conduction and free-carrier injection in laterally doped GaAs p-i-n diodes formed in one and twodimensional photonic crystal (PC) nanocavities. Finite element simulations show that the lateral geometry exhibits high electrical conductivity for a wide range of PC parameters and allows for precise control over current flow, enabling efficient carrier injection despite fast surface recombination. Thermal simulations indicate that the temperature increase during steady-state operation is only 3.3K in nanobeams and 0.29K in L3 defect nanocavities. The results affirm the suitability of lateral doping in PC devices and indicate criteria for further design optimization.

We have demonstrated electrically driven photonic crystal nanocavity lasers, LEDs and modulators with record low operation powers (e.g., lasing threshold of 180nA and sub-fJ/bit modulator operation), and with the modulation speeds exceeding 10GHz.

A fabrication procedure for electrically pumping photonic crystal membrane devices using a lateral p-i-n junction has been developed and is described in this work. The lateral junction is optimized to efficiently inject current into a photonic crystal nanocavity. We have demonstrated electrically pumped lasing by using the lateral junction to pump a quantum dot photonic crystal nanocavity laser. Continuous wave lasing is observed at temperatures up to 150K, and a threshold of 181nA at 50K is demonstrated - the lowest threshold ever demonstrated in an electrically pumped laser. At room temperature we find that our devices do not lase, but behave as single-mode light-emitting diodes (LEDs). When directly modulated, we find that our LEDs have an ultrafast electrical response up to 10 GHz corresponding to less than 1 fJ/bit energy operation. In addition, we have demonstrated electrical pumping of photonic crystal nanobeam LEDs, and have built fiber taper coupled electro-optic modulators in the same lateral junction platform.

Interest in photonic crystal nanocavities is fueled by advances in device performance, particularly in the development of low-threshold laser sources. Effective electrical control of high performance photonic crystal lasers has thus far remained elusive due to the complexities associated with current injection into cavities. A fabrication procedure for electrically pumping photonic crystal membrane devices using a lateral p-i-n junction has been developed and is described in this work. We have demonstrated electrically pumped lasing in our junctions with a threshold of 181 nA at 50K - the lowest threshold ever demonstrated in an electrically pumped laser. At room temperature we find that our devices behave as single-mode light-emitting diodes (LEDs), which when directly modulated, have an ultrafast electrical response up to 10 GHz corresponding to less than 1 fJ/bit energy operation - the lowest for any optical transmitter. In addition, we have demonstrated electrical pumping of photonic crystal nanobeam LEDs, and have built fiber taper coupled electro-optic modulators. Fiber-coupled photodetectors based on two-photon absorption are also demonstrated as well as multiply integrated components that can be independently electrically controlled. The presented electrical injection platform is a major step forward in providing practical low power and integrable devices for on-chip photonics., Comment: 10 pages, 13 figures

A fabrication procedure for electrically pumping photonic crystal membrane devices using a lateral p-i-n junction has been developed and is described in this work. The lateral junction is optimized to efficiently inject current into a photonic crystal nanocavity. We have demonstrated electrically pumped lasing by using the lateral junction to pump a quantum dot photonic crystal nanocavity laser. Continuous wave lasing is observed at temperatures up to 150K, and a threshold of 181nA at 50K is demonstrated - the lowest threshold ever demonstrated in an electrically pumped laser. We have demonstrated electrical pumping of photonic crystal nanobeam light emitting diodes, and observe linewidth narrowing at room temperature.

The use of silicon nanostructures in solar cells offers a number of benefits, such as the fact they can be used on flexible substrates. A silicon wire-array structure, containing reflecting nanoparticles for enhanced absorption, is now shown to achieve 96% peak absorption efficiency, capturing 85% of light with only 1% of the silicon used in comparable commercial cells. Si wire arrays are a promising architecture for solar-energy-harvesting applications, and may offer a mechanically flexible alternative to Si wafers for photovoltaics1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17. To achieve competitive conversion efficiencies, the wires must absorb sunlight over a broad range of wavelengths and incidence angles, despite occupying only a modest fraction of the array’s volume. Here, we show that arrays having less than 5% areal fraction of wires can achieve up to 96% peak absorption, and that they can absorb up to 85% of day-integrated, above-bandgap direct sunlight. In fact, these arrays show enhanced near-infrared absorption, which allows their overall sunlight absorption to exceed the ray-optics light-trapping absorption limit18 for an equivalent volume of randomly textured planar Si, over a broad range of incidence angles. We furthermore demonstrate that the light absorbed by Si wire arrays can be collected with a peak external quantum efficiency of 0.89, and that they show broadband, near-unity internal quantum efficiency for carrier collection through a radial semiconductor/liquid junction at the surface of each wire. The observed absorption enhancement and collection efficiency enable a cell geometry that not only uses 1/100th the material of traditional wafer-based devices, but also may offer increased photovoltaic efficiency owing to an effective optical concentration of up to 20 times.

We present results on electrically driven nanobeam photonic crystal cavities formed out of a lateral p-i-n junction in gallium arsenide. Despite their small conducting dimensions, nanobeams have robust electrical properties with high current densities possible at low drive powers. Much like their two-dimensional counterparts, the nanobeam cavities exhibit bright electroluminescence at room temperature from embedded 1,250 nm InAs quantum dots. A small room temperature differential gain is observed in the cavities with minor beam self-heating suggesting that lasing is possible. These results open the door for efficient electrical control of active nanobeam cavities for diverse nanophotonic applications.

We have demonstrated an electrically driven photonic crystal nanocavity laser with record low threshold (180nA) and a single mode photonic crystal nanocavity LED directly modulated at 10GHz speed with 0.25 fJ/bit energy consumption.

Extensive modeling and experiments demonstrate a dramatic reduction in parasitic absorption with strain due to a previously overlooked mechanism. A special resonance at 4–5% uniaxial strain indicates that this is the optimal strain level for a room temperature Ge laser.

We propose a novel low threshold, CMOS-compatible laser structure with a strained germanium gain medium and a photonic crystal cavity. We demonstrate 1.70% uniaxial tensile strain through experiments and design a high quality factor (>11,000) optical cavity around the gain medium.

Low-power and electrically controlled optical sources are vital for next generation optical interconnect systems to meet strict energy demands. Current optical transmitters consisting of high-threshold lasers plus external modulators consume far too much power to be competitive with future electrical interconnects. Here we demonstrate a directly modulated photonic crystal nanocavity light-emitting diode (LED) with 10 GHz modulation speed and less than 1 fJ per bit energy of operation, which is orders of magnitude lower than previous solutions. The device is electrically controlled and operates at room temperature, while the high modulation speed results from the fast relaxation of the quantum dots used as the active material. By virtue of possessing a small mode volume, our LED is intrinsically single mode and, therefore, useful for communicating information over a single narrowband channel. The demonstrated device is a major step forward in providing practical low-power and integrable sources for on-chip photonics.