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Realization of on-board and inter-chip optical interconnects requires a photonic data link with power consumption well below their electrical counterparts (i.e., 50 Gb/s requires 2–4 pJ/bit/channel. External reverse-biased modulators could drastically reduce this power consumption. Here we design ultralow power GaAs/AlGaAs multi quantum well electro-reflective modulators operating at 1 V for facile integration with polymer “optical bridges”, utilizing coupled quantum confined Stark effect between adjacent quantum wells and optical coupling to hybrid surface plasmon-slab modes for significantly enhanced extinction ratio and spectral bandwidth. Distinctive from conventional electro-optical or electro-absorption modulators, this new design synergistically leverages ultra-large changes in both refractive index (|Δn|∼0.05) and absorption coefficient (Δα∼104 cm−1), achieving 35-50 dB extinction ratio at 1 V reverse bias with a low insertion loss of 1–3 dB, an incident angle tolerance of ∼5°, and a spectral bandwidth of 7–10 nm. The modulator power consumption is ∼1.9 fJ/bit without the need of thermal tuning, and the RC-limited bandwidth well exceeds 100 GHz. This new modulator enables high bandwidth and ultralow power optical interconnect networks at >100 Gb/s/channel and

An electroreflective modulator based on multiple-pass electroabsorption (EA) of coupled multiple quantum wells (CMQWs) is demonstrated for integration with low-loss polymer optical waveguides towards inter-chip and on-board optical interconnects at 850 nm. Taking advantage of coupling between quantum states in MQWs, the top polymer cladding and the bottom Au mirror form a Fabry–Perot cavity to further enhance the EA of CMQWs, thereby greatly improving the extinction ratio (ER) at a low voltage. The device achieves an ER of ∼6 dB and an insertion loss (IL)

We propose and experimentally demonstrate a new photonics interconnect approach based on on-chip quadratic optical reflectors for interfacing a wide range of photonic devices. Here, we present its embodiments on compact waveguide bends and high performance fiber-to-chip optical couplers.

Coupling of light between different photonic devices, for example on-chip waveguides, fibers, and free-space optical elements, is an essential function enabling integrated optical systems. Efficient optical coupling demands matching the optical mode profiles and effective indices between two devices, and often changing propagation direction of the light. To date, such coupling is pre-dominantly accomplished via direct butt coupling of two devices, or meticulously optimized diffraction gratings. In this article, we present a new coupling scheme based on microfabricated free-form optical reflectors. The free-form reflector simultaneously achieves the functions of light beam re-directing and shaping (for mode matching), and can be versatilely adapted for coupling between photonic chips, fibers, and free-space surface-incident devices. We show that this technology uniquely fulfills all key performance requirements for optical interfaces with exceptionally low coupling loss (0.2–0.3 dB per coupler), large bandwidth (over half an octave), high density (large 2-D coupler arrays), polarization diversity, and superior alignment tolerance commensurate with passive alignment techniques. Preliminary experimental validation demonstrates waveguide-to-fiber coupling with a low insertion loss (IL) of 0.9 dB. We foresee that the technology will become a promising solution to the chip-level photonic interconnection and packaging challenges plaguing integrated photonics.

Here we present next generation polymer-based singlemode waveguide devices and architectures for chip-scale photonic interconnections. A suite of optical devices are developed showing record low propagation loss, high-performance signal/power routing, and high-density inter-layer coupling. Flexible waveguide ribbons are further demonstrated that are widely applicable across chip- and board-domains.

We describe an optical communication technology scalable across chip-, board-, and rack-interconnect levels for data centers. The approach claims high energy efficiency, large bandwidth density, and is compatible with standard area-bonding and pick-and-place chip assembly.

We analyze a chip-to-chip optical interconnect platform based on our recently developed flexible substrate integration technology. We show that the architecture achieves high bandwidth density (100 Tbs/cm2), and does not require optical alignment during packaging. These advantages make the flexible photonics platform a promising solution for chip-to-chip optical interconnects. We further report initial experimental characterizations of the flexible photonics platform fabricated using thermal nanoimprint patterning of glass waveguides and III-V die bonding.

A SiGe-on-SOI carrier-depletion Mach-Zehnder modulator is presented. Compared with Si modulators, it eliminates advanced lithography by enabling a dual-waveguide design using small SiGe waveguides for modulators and large Si waveguides for edge coupling. DC and high speed test results are included.

A III-V on Si hybrid integration is demonstrated by using Si slab waveguides with two parabolic reflective facets to increase the alignment tolerance between flip-chip bonded lasers and modulators. DC and high speed test results of the integrated transmitter are included.

A SiGe-on-SOI carrier-depletion Mach-Zehnder modulator is presented. Compared with Si modulators, it enables a dual-waveguide design using small SiGe waveguides for modulators and large Si waveguides for edge coupling thus eliminates advanced lithography for inverse tapers. DC and high speed test results are included.

Electronic–photonic synergy has become an increasingly clear solution to enhance the bandwidth and improve the energy efficiency of information systems. Monolithic integration of optoelectronic devices is the ideal solution for large-scale electronic–photonic synergy. Due to its pseudo-direct gap behavior in optoelectronic properties and compatibility with Si electronics, epitaxial Ge-on-Si has become an attractive solution for monolithic optoelectronics. In this paper we will review recent progress in Ge-on-Si optoelectronics, including photodetectors, electroabsorption modulators, and lasers. The performance of these devices has been enhanced by band-engineering such as tensile strain and n-type doping, which transforms Ge towards a direct gap material. Selective growth reduces defect density and facilitates monolithic integration at the same time. Ge-on-Si photodetectors have approached or exceeded the performance of their III–V counterparts, with bandwidth-efficiency product > 30 GHz for p-i-n photodiodes and bandwidth-gain product > 340 GHz for avalanche photodiodes. Enhanced Franz–Keldysh effect in tensile-strained Ge offers ultralow energy photonic modulation with 100 GHz intrinsic bandwidth. Room temperature optically-pumped lasing as well as electroluminescence has also been achieved from the direct gap transition of band-engineered Ge-on-Si waveguides. These results indicate that band-engineered Ge-on-Si is promising to achieve monolithic active optoelectronic devices on a Si platform.

Electronic-photonic synergy has become an increasingly clear solution to enhance the bandwidth and improve the energy efficiency of information systems. Monolithic lasers on silicon are ideal for large scale electronic-photonic integration. Germanium is a particularly interesting candidate for this application due to its pseudo-direct gap behavior in the near infrared regime for optical communications and compatibility with advanced silicon complementary metal oxide semiconductor (CMOS) technology. In this paper, we review theoretical modeling and experimental studies on optical gain and lasing from monolithic epitaxial Ge-on-Si devices band-engineered by tensile strain and n-type doping to compensate the energy difference between the direct and indirect conduction valleys. Demonstrations of optical gain and lasing at room temperature indicate that the Ge-on-Si laser is a promising candidate for monolithic electronic-photonic integrated circuits.

It has been demonstrated theoretically and experimentally that germanium, with proper strain engineering and n-type doping, can be an efficient light emitter and a gain medium at its direct bandgap within the third optical communication window ( ~1520-1620 nm). In this paper, we systematically discuss the effect of strain, doping, and temperature on the direct-gap optical gain in germanium. For electrically pumped devices, properties and design guidelines of Ge/Si heterojunction are also analyzed and compared with the results from fabricated Ge/Si heterojunction LEDs.

Thin film Ge on Si is a potential active material candidate for electrically pumped monolithically integrated Si-based light emitters. Theoretical analysis has shown that a combination of strain and n-type doping can modify the band structure of Ge so that it exhibits direct bandgap properties. Direct band-to- band optical transition is observed from the room temperature photoluminescence at around 1550nm. An optical bleaching ef- fect occurring in lock-in pump-probe measurements indicates a precursor to optical net gain.

A compact parallel optical engine platform using Si optical bench technology to realize highly manufacturable vertical coaxial alignment approach is demonstrated and an in-production 40G optical engine using this platform is presented.

ZnO nanorod arrays with peculiar morphologies were synthesized on (111)-oriented Si substrate and glass via a vapor phase growth. The morphology of the individual nanorod can be flat-headed bottle-like, and needle-like, which depends on the deposition positions relative to the source materials in the presence of a controlling element Se. In addition, the arrays of all the three morphologies exhibit good alignment and high coverage. This fabrication technique can be also used to direct the controllable growth of other nanomaterials with similar morphologies.

A high bandwidth density chip-to-chip optical interconnect architecture is analyzed. The interconnect design leverages our recently developed flexible substrate integration technology to circumvent the optical alignment requirement during packaging. Initial experimental results on fabrication and characterization of the flexible photonic platform are also presented.

To meet the unprecedented demands for data transmission speed and bandwidth silicon integrated photonics that can generate, modulate, process and detect light signals is being developed. Integrated silicon photonics that can be built using existing CMOS fabrication facilities offers the tantalizing prospect of a scalable and cost-efficient solution to replace electrical interconnects. Silicon, together with commonly used dielectric materials in CMOS processes such as silicon dioxide, is a great material system for optical confinement and wave transmission in near infrared range. However, silicon is not a good choice for active photonic devices due to its transparency in such wavelength range. Germanium and GeSi alloy, the materials that have long been adopted to improve the performance of silicon transistors in many ways, have been showing their potential as the building blocks of such active integrated optical devices. This chapter discusses the research of using germanium and GeSi for silicon-integrated photodetection and light source in the contexts of material physics and growth, device design and fabrication. This introduction section briefly introduces the background of integrated silicon photonics and some germanium properties which are important for photonic applications. Next section focuses on waveguide-integrated germanium photodetectors which can readily be integrated with silicon waveguide on mature silicon or silicon-on-insulator (SOI) platform. The physics and design considerations of these devices are presented with details. The fabrication processes of these devices are also discussed with some extent. Next section includes a newly developed field that using germanium for light sources in silicon photonics applications such as light-emitting-diodes (LEDs) and lasers. There has been a few breakthroughs in this topic including the author’s work of epitaxial germanium LEDs and optically pumped lasers operating at room temperature. The physics of this unusual concept of using indirect band gap material for light emission is discussed in details and some important results are presented.

We report optically-pumped Ge-on-Si lasers with direct gap emission near 1600 nm at room temperature. The Ge-on-Si material was band-engineered by tensile strain and n-type doping to compensate the energy difference between direct and indirect band gaps for efficient light emission. Modeling of Ge/Si double heterojunction LEDs shows that it is possible to achieve >10% quantum efficiency even assuming an Auger coefficient 10 times larger than reports in literature. Edge-emitting Ge-on-Si waveguide LEDs have also been demonstrated at room temperature. These simulation and experimental results strongly indicate the feasibility of electrically-pumped Ge-on-Si lasers.

We report room temperature Ge-on-Si lasers with direct gap emission at 1590–1610 nm. Modeling of Ge/Si double heterojunction structures, which is supported by experimental results of Ge/Si LEDs, indicates the feasibility of electrically pumped lasers.

We propose and theoretically analyze what we believe to be a novel design of cavity-enhanced photodetectors capable of sensing multiple wavelengths simultaneously in a single pixel. The design is based on phase-tuned propagation of resonant modes in cascaded planar resonant cavities. We show that this concept can be generalized to detect multiple wavelength combinations covering the entire near to far infrared spectrum. Besides its multispectral detection capability, the design also features minimal spectral cross talk and significantly suppressed noise. The intrinsic design versatility and scalability, as well as process compatibility with planar microfabrication, suggest the design's wide application potential for telecommunications, infrared imaging, and biochemical sensing.

Monolithic lasers on Si are ideal for high-volume and large-scale electronic-photonic integration. Ge is an interesting candidate owing to its pseudodirect gap properties and compatibility with Si complementary metal oxide semiconductor technology. Recently we have demonstrated room-temperature photoluminescence, electroluminescence, and optical gain from the direct gap transition of band-engineered Ge-on-Si using tensile strain and n-type doping. Here we report what we believe to be the first experimental observation of lasing from the direct gap transition of Ge-on-Si at room temperature using an edge-emitting waveguide device. The emission exhibited a gain spectrum of 1590-1610 nm, line narrowing and polarization evolution from a mixed TE/TM to predominantly TE with increasing gain, and a clear threshold behavior.

We report the design and fabrication of a novel multispectral photodetector capable of detecting multiple wavebands in a single pixel. Our prototypical device demonstrates spectral selectivity with a peak responsivity of 65.4V/W at 3.9μm wavelength.

A multi-wavelength receiver was fabricated on SOI using a monolithically integrated arrayed waveguide grating (AWG) and 32 germanium waveguide photodetectors. The CMOS compatible high-speed detectors are capable for OC-192 or 10Gb/s data rate.

We demonstrate monolithic Ge-on-Si lasers, band-engineered by tensile strain and n-type doing, that exhibit direct gap emission around 1600 nm at room temperature. Direct gap electroluminescence from heterojunction devices verifies the feasibility of electrical pumping.

We report direct band gap optical gain of tensile strained n+ epitaxial Ge-on-Si at room temperature, which confirms that band-engineered Ge-on-Si is a promising gain medium for monolithic optical amplifiers and lasers on Si.

We report the design, fabrication, and characterization of spectral selective mid-infrared PbTe photodetector pixels monolithically integrated on a silicon platform. We demonstrate spectral selectivity with a peak responsivity of 65.4V/W.

Some of the crucial building blocks for Si photonics are high performance photodetectors, modulators, and efficient light sources. High quality Ge epitaxial films on Si have opened new possibilities for high performance CMOS compatible devices.

We demonstrate room temperature photoluminescence and optical gain from the direct band gap transition of tensile strained n-type Ge-on-Si around 1600 nm, which can be applied to a Si-based laser for optical interconnects and communications.

We report experimental observation of direct band gap photoluminescence (PL) and optical bleaching of band-engineered epitaxial Ge-on-Si at room temperature, confirming that this material is a promising candidate for efficient light emitting devices on Si.

Optical gain and net material gain via direct bandgap transition at around 1550nm of the tensile-strain band-engineered n-type Ge has been analyzed. Photoluminescence spectra measured at room temperature confirm the theoretical predictions.

We demonstrate and characterize both one-dimensional photonic crystal monolithically integrated on a Si platform, and thermally evaporated PbTe film with room temperature photoconductivity, which are key components for low cost resonant-cavity-enhanced mid-infrared photodetectors.

A novel photo sensor pixel using a one-dimensional (1D) photonic crystal structure incorporating photoconductive layers has been realized. The fabricated device exploits mode discrimination to provide simultaneous multispectral photo sensing capability. Resonant cavity enhancement (RCE) design allows the use of very thin photoconductive layer to achieve dramatically suppressed shot noise, as well as high quantum efficiency. Low cost amorphous silicon is used to be photoconductive material and the simply fabrication process is completely CMOS-compatible. Detectivities as high as 2.6×1010 cmHz1/2W−1 and 2.0×1010 cmHz1/2W−1 at the two pre-selected wavelengths, 632nm and 728nm, were achieved, respectively.

We demonstrate a novel photoconductor pixel, by using a photonic crystal structure incorporating photoconductive layers, which exploits resonant cavity enhancement to provide us with multispectral capability, high quantum efficiency, and dramatically suppressed shot noise

The Si/Si1-xGex quantum well system is attractive for high temperature thermoelectric applications and for demonstration of proof-of-principle for enhanced thermoelectric figure of merit Z, since the interfaces and carrier densities can be well controlled in this system. We report theoretical calculations for Z in this system, based on which Si/Si1-xGex quantum-well structures were grown by molecular-beam epitaxy. Thermoelectric and other transport measurements were made, indicating that an increase in Z over bulk values is possible through quantum confinement effects in the Si/Si1-xGex quantum-well structures.

Room temperature direct gap photoluminescence (PL) was observed from n-type tensile-strained epitaxial Ge-on-Si. The PL intensity increases with n-type doping due to a higher electron population in the direct Γ valley as a result of increased Fermi level. The direct gap emission also increases with temperature due to thermal excitation of electrons into the direct Γ valley, exhibiting robustness to heating effects. These unique properties of direct gap emission in an indirect gap material agree with our theoretical model and make Ge a promising light emitting material in 1550 nm communication band.

Lasers on Si are crucial components of monolithic electronic-photonic integration. Recently our theoretical analysis has shown that Ge, a pseudodirect bandgap material compatible with Si complementary metal oxide semiconductor technology, can be band engineered by tensile strain and n-type doping to achieve efficient light emission and optical gain from its direct gap transition. We report on what is to our knowledge the first experimental observation of optical gain in the wavelength range of 1,600-1,608 nm from the direct-gap transition of n(+) tensile-strained Ge on Si at room temperature under steady-state optical pumping. This experimental result confirms that the band-engineered Ge on Si is a promising gain medium for monolithic lasers on Si.

Nanocrystalline PbTe films are deposited on different substrates at room temperature through thermal evaporation. The films are stoichiometric single-phase polycrystalline with (200) texture. Electrical properties of the films are analyzed in the framework of a grain boundary channel conduction model. The index of refraction and extinction coefficient of PbTe films are extracted from infrared spectroscopic ellipsometry measurement in the wavelength range of 2–8 μm, yielding an optical band gap of 0.386 eV and evidence for the presence of an Urbach band tail. The optical band gap is larger than the typical value for bulk material due to quantum confinement effect.

A photodetector pixel using a photonic crystal structure incorporating photoconductive layers has been realized. The fabricated device exploits mode discrimination and resonant cavity enhancement to provide simultaneous multispectral detection capability, high quantum efficiency, and dramatically suppressed shot noise. Detectivities as high as 2.6×1010 and 2.0×1010cmHz1∕2W−1 at the two preselected wavelengths, 632 and 728nm, were achieved, respectively.

Strategically reducing the speed of waves, which greatly improves both the energy density and information capacity of carrier signals in space, is a key enabling factor for signal-processing devices. Among these devices, especially in the prosperous wireless communication industry, surface acoustic wave (SAW) devices based on interdigital transducers (IDTs) currently hold an essential status. However, velocity reduction in traditional IDT-based SAW devices can be achieved only by using specific substrate materials that are generally of lower hardness, which inevitably leads to an increase in device size and less-optimal electromechanical coupling coefficients. Here, we demonstrate a technological means of realizing slow on-chip SAWs that is relevant for practical rf signal processing, gyrometers, sensing, and transduction. This method takes advantage of the gradual flattening of a Rayleigh-type dispersion band due to the spatial lattice evolution of a surface phononic crystal. In our experiment, the speed of an ultraslow SAW is measured to be approximately 200 m/s, which is even slower than the speed of sound in air and equivalent to 1/17.4 of the speed of the original Rayleigh waves in ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_{3}$. Such ultraslow SAWs may have promising applications in time-dependent SAW modulation, high-sensitivity SAW sensors, and SAW nonlinear even quantum-dynamic systems. Additionally, our technique can be similarly applied to a broad range of other two-dimensional or quasi-two-dimensional wave structures, e.g., in electronic, optical, acoustic, and thermal systems.