Silicon chips dominate electronics while optical fibres dominate long-distance information transfer. Recent work, in search of the best of both worlds, has led to silicon devices capable of modulating light; these show promise but still rely on weak physical mechanisms found in silicon itself. Now a team working at Stanford University and at Hewlett-Packard's Palo Alto labs has developed thin germanium ‘quantum well’ nanostructures grown on silicon that generate a strong quantum-mechanical effect capable of turning light beams on and off. Their performance rivals the best seen in any material. This development may allow silicon/germanium chips to handle both electronics and optics, uniting computing and communications at the integrated chip level. Silicon is the dominant semiconductor for electronics, but there is now a growing need to integrate such components with optoelectronics for telecommunications and computer interconnections1. Silicon-based optical modulators have recently been successfully demonstrated2,3; but because the light modulation mechanisms in silicon4 are relatively weak, long (for example, several millimetres) devices2 or sophisticated high-quality-factor resonators3 have been necessary. Thin quantum-well structures made from III-V semiconductors such as GaAs, InP and their alloys exhibit the much stronger quantum-confined Stark effect (QCSE) mechanism5, which allows modulator structures with only micrometres of optical path length6,7. Such III-V materials are unfortunately difficult to integrate with silicon electronic devices. Germanium is routinely integrated with silicon in electronics8, but previous silicon–germanium structures have also not shown strong modulation effects9,10,11,12,13. Here we report the discovery of the QCSE, at room temperature, in thin germanium quantum-well structures grown on silicon. The QCSE here has strengths comparable to that in III-V materials. Its clarity and strength are particularly surprising because germanium is an indirect gap semiconductor; such semiconductors often display much weaker optical effects than direct gap materials (such as the III-V materials typically used for optoelectronics). This discovery is very promising for small, high-speed14, low-power15,16,17 optical output devices fully compatible with silicon electronics manufacture.

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Strong quantum-confined Stark effect in germanium quantum-well structures on silicon

A phase modulator that is only 29 µm long and operates at 65 GHz is demonstrated using plasmonics and the Pockels effect in a nonlinear polymer. The device operates across a 120-nm-wide wavelength range centred on 1,550 nm and at temperatures up to 85 °C.

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High-speed plasmonic phase modulators

The authors demonstrate a 70 GHz modulator in a 10-μm-long two-dimensionally localized gap-plasmon waveguide system.

All-plasmonic Mach–Zehnder modulator enabling optical high-speed communication at the microscale

Silicon dominates the electronics industry, but its poor optical properties mean that III–V compound semiconductors are preferred for photonics applications. Photoluminescence at visible wavelengths was observed from porous Si at room temperature in 1990, but the origin of these photons (do they arise from highly localized defect states or quantum confinement effects?) has been the subject of intense debate ever since. Attention has subsequently shifted from porous Si to Si nanocrystals, but the same fundamental question about the origin of the photoluminescence has remained. Here we show, based on measurements in high magnetic fields, that defects are the dominant source of light from Si nanocrystals. Moreover, we show that it is possible to control the origin of the photoluminescence in a single sample: passivation with hydrogen removes the defects, resulting in photoluminescence from quantum-confined states, but subsequent ultraviolet illumination reintroduces the defects, making them the origin of the light again.

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Classification and control of the origin of photoluminescence from Si nanocrystals

Optical modulators on silicon promise to deliver ultralow power communication networks between or within computer chips. Here, the authors demonstrate a silicon modulator operating with less than one femtojoule energy and are able to compensate for thermal drift over a 7.5 °C temperature range.

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An ultralow power athermal silicon modulator

We demonstrate a high-speed silicon Mach-Zehnder modulator (MZM) with low insertion loss, based on the carrier depletion effect in a lateral PN junction. A 1.9 dB on-chip insertion loss and a VπLπ < 2 V·cm were achieved in an MZM with a 750 μm-long phase shifter by properly choosing the doping concentration and precisely locating the junction. High-speed modulations up to 45-60 Gbit/s have been demonstrated with an additional 1.6 dB optical loss, indicating a total insertion loss of 3.5 dB. A high extinction ratio of 7.5 dB was also realized at the bit rate of 50 Gbit/s with an acceptable insertion loss of 6.5 dB.

High-speed, low-loss silicon Mach–Zehnder modulators with doping optimization

A detailed analysis of the photoluminescence (PL) from Si nanocrystals (NCs) embedded in a silicon-rich ${\mathrm{SiO}}_{2}$ matrix is reported. The PL spectra consist of three Gaussian bands (peaks $A,B$, and $C$), originated from the quantum confinement effect of Si NCs, the interface state effect between a Si NC and a ${\mathrm{SiO}}_{2}$ matrix, and the localized state transitions of amorphous Si clusters, respectively. The size and the surface chemistry of Si NCs are two major factors affecting the transition of the dominant PL origin from the quantum confinement effect to the interface state recombination. The larger the size of Si NCs and the higher the interface state density (in particular, $\mathrm{Si}\mathrm{O}$ bonds), the more beneficial for the interface state recombination process to surpass the quantum confinement process, in good agreement with Qin's prediction in Qin and Li [Phys. Rev. B 68, 85309 (2003)]. The realistic model of Si NCs embedded in a ${\mathrm{SiO}}_{2}$ matrix provides a firm theoretical support to explain the transition trend.

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Origin and evolution of photoluminescence from Si nanocrystals embedded in a SiO 2 matrix

A two-mode (de)multiplexer based on adiabatic couplers is proposed and experimentally demonstrated. The experimental results are in good agreement with the simulations. An ultralow mode cross talk below -36 dB and a low insertion loss of about 0.3 dB over a broad bandwidth from 1500 to 1600 nm are measured. The design is also fabrication-tolerant, and the insertion loss can be further improved in the future.

Two-mode multiplexer and demultiplexer based on adiabatic couplers

A high-speed depletion-mode silicon-based microring modulator with interleaved PN junctions optimized for high modulation efficiency and large alignment tolerance is demonstrated. It is fabricated using standard 0.18 μm complementary metal–oxide–semiconductor processes and provides low VπLπs of 0.68 V·cm to 1.64 V·cm with a moderate doping concentration of 2 × 1017 cm−3. The measured modulation efficiency decreases by only 12.4% under ± 150 nm alignment errors. 25 Gbit/s non-return-zero modulation with a 4.5 dB extinction ratio is experimentally realized at a peak-to-peak driving voltage of 2 V, demonstrating the excellent performance of the novel doping profile.

25 Gbit/s silicon microring modulator based on misalignment-tolerant interleaved PN junctions

A high speed silicon Mach-Zehnder modulator is proposed based on interleaved PN junctions This doping profile enabled both high modulation efficiency of VπLπ = 15~20 V·cm and low doping-induced loss of ~10 dB/cm by applying a relatively low doping concentration of 2 × 1017 cm−3 High speed operation up to 40 Gbit/s with 701 dB extinction ratio was experimentally demonstrated with a short phase shifter of only 750 μm

Jinzhong Yu

Papers

High-speed, low-loss silicon Mach–Zehnder modulators with doping optimization

Origin and evolution of photoluminescence from Si nanocrystals embedded in a SiO 2 matrix

Two-mode multiplexer and demultiplexer based on adiabatic couplers

25 Gbit/s silicon microring modulator based on misalignment-tolerant interleaved PN junctions

High speed silicon Mach-Zehnder modulator based on interleaved PN junctions.