In the last decade, silicon photonics has undergone an impressive development driven by an increasing number of technological applications. Plasmonics has not yet made its way to the microelectronic industry, mostly because of the lack of compatibility of typical plasmonic materials with foundry processes. In this framework, we have developed a plasmonic platform based on heavily n-doped Ge grown on silicon substrates. We developed growth protocols to reach n-doping levels exceeding 1020 cm-3, allowing us to tune the plasma wavelength of Ge in the 3-15 μm range. The plasmonic resonances of Ge-on-Si nanoantennas have been predicted by simulations, confirmed by experimental spectra and exploited for molecular sensing. Our work represents a benchmark for group-IV mid-IR plasmonics.
Mid-infrared (mid-IR) spectroscopy is a nearly universal way to identify chemical and biological substances, as most of the molecules have their vibrational and rotational resonances in the mid-IR wavelength range. The development of silicon-based mid-IR photonic circuits has recently gained a lot of attention. Among the different materials available in silicon photonics, germanium (Ge) and silicon-germanium (SiGe) alloys with a high Ge concentration are particularly interesting because of the wide transparency window of Ge extending up to 15 µm.
In this work we will review recent results in the development of photonics circuit based on Ge-rich SiGe waveguides.
Photonics integration in the mid-Infrared (mid-IR) spectral range, and more specifically the fingerprint region between 5 and 20 μm wavelength has garnered a great interest as it provides an immense potential for applications in spectroscopy and sensing. The unique vibrational and rotational resonances of the molecules at these wavelengths can be exploited for non-intrusive, unambiguous detection of the molecular composition of a broad variety of gases, liquids or solids, with a great interest for many high-impact applications. Fourier-transform spectrometers (FTS) are a particularly interesting solution for the on-chip integration due to their superior robustness against fabrication imperfections. However, the performance of current on-chip FTS implementations is limited by tradeoffs between bandwidth and resolution, for a given footprint. In this work we propose and experimentally demonstrate a new FTS approach that gathers the advantages of spatial heterodyning and optical path tuning by thermo-optic effect. The high resolution is provided by spatial multiplexing among different interferometers with increasing imbalance length, while the broadband operation is enabled by fine sampling interval of the optical path delay in each interferometer harnessing the thermo-optic effect. This novel approach overcomes the bandwidth-resolution tradeoff in conventional counterparts. The fabricated device enables a bandwidth as wide as 603 cm-1 (instead of 74 cm-1 with no-thermal tuning) near 7.7 μm wavelength, keeping a resolution better than 15 cm-1 with the same footprint. This device is fabricated in a Ge-rich graded-index SiGe platform with experimentally proven low loss operation up to 8.5 μm wavelength.
Due to their unique vibrational/rotational frequencies in the mid infrared (MIR) fingerprint region, which scans from 500 to 1500 cm-1, molecules can be assuredly identified and quantified. Thus, integrated on-chip mid infrared spectroscopic systems, with low power consumption and high performance, would show great value for numerous applications, such as medical diagnosis, astronomy, chemical and biological sensing or security. Different solutions can be envisioned, such as Fourier-Transform spectrometers, echelle gratings, or arrayed waveguide gratings (AWG). The integrated spatial heterodyne Fourier-Transform spectrometer (SHFTS) shows relaxed fabrication tolerances while applying a phase and amplitude correction algorithm. Meanwhile, it provides high optical throughput and high spectral resolution compared with AWG or echelle gratings. However, up to now in the literature, most of the reported integrated Fourier-Transform based spectrometer is based on silicon-on-insulator operating in the near infrared typically at 1.55 μm wavelength. Thereby the development of integrated Fourier-Transform spectrometers operating in the MIR covering the wide fingerprint region is highly desirable. In this work, we experimentally demonstrate the first duel polarization SHFTS operating in the mid infrared beyond 5 μm wavelength. The fabricated FTS, which is based on the graded-index Ge-rich SiGe platform, contains 19 Mach-Zehnder interferometers with a linearly increasing path difference. A spectral resolution better than 15 cm-1 has been demonstrated within an unprecedented spectral range of 800 cm-1 (from 5 to 8.5 μm wavelength).
The recent advances in the development of quantum cascade laser with room temperature operation in the mid infrared paved the way for the realization of wideband communication systems. Particularly, two mid-infrared atmosphere transparency windows lying between 3-5 μm and between 8-14 μm exhibit great potential for further implementation of wideband free space communications. Additionally this wide unregulated spectral region shows reduced background noise and low Mie and Rayleigh scattering. Despite the development of a plethora of photonic components in mid infrared such as sources, detectors, passive structures, less efforts have been dedicated to investigate polarization management for information transport. In this work, the potential of Ge-rich SiGe waveguides is exploited to build a polarization insensitive platform in the mid-infrared. The gradual index evolution in SiGe alloys and geometric parameter optimization are used to obtain waveguides with birefringence below 2×10-4 and an unprecedented bandwidth in both atmosphere transparency windows i.e. near 3.5 μm and 9 μm. Following waveguide birefringence optimization an ultra-wideband and polarization insensitive multimode interference coupler was designed. The optimized structure shows a 4.5 μm wide bandwidth in transverse electric and transverse magnetic polarization at 9 μm wavelength. The developed ultra-wideband polarization insensitive photonic building blocks presented in this work pave the way for further implementation of free space communication systems in the mid infrared spectral region.
Mid-infrared racetrack resonators are demonstrated working at 8μm wavelength. The devices are based on a graded SiGe platform providing low propagation loss on a large wavelength range in the mid-IR. Different resonators designs have been fabricated, with varying gap distances in the directional coupler. Q factors of more than 3000 have been experimentally demonstrated. These results pave the way towards compact mid-IR sensors or efficient active devices.
The third-order nonlinear parameter of Ge-rich SiGe waveguides are experimentally retrieved using a bi-directional top hat D-scan at λ = 1.58 μm. The obtained values are then used to fit the theoretical equation, providing promising values in the mid-IR, where the nonlinear effects are no longer limited by two-photon absorption. New Ge-rich SiGe waveguide designs are provided to exploit the nonlinear properties in the mid-IR, showing a flat anomalous dispersion over one octave spanning from λ = 3 µm to λ = 8 μm and a γ parameter that decreases from γ = 10 W-1m-1 .
The mid infrared (MIR) region, which ranges from 2 μm to 20 μm, has attracted a lot of interest, particularly for novel applications in medical diagnosis, astronomy, chemical and biological sensing or security, to name a few. Most recently, Germanium-rich Silicon Germanium (Ge-rich SiGe) has emerged as a promising waveguide platform to realize complex mid-IR photonic integrated circuits. The Ge-rich SiGe graded buffer benefits from a wide transparency window, strong 3rd order nonlinearity, and the compatibility with mature large-scale fabrication processes, which in turn, paves the way for the development of mid-IR photonic devices that afford improved on-chip functionalities, altogether with compact footprints and cost-effective production. Albeit, low-loss waveguides and wideband Mach-Zehnder interferometers (MZIs) have been recently successfully demonstrated at mid-IR wavelengths, the coupling of light between external access ports, typically optical fibers, and integrated circuits remains challenging. Surface grating couplers provide technologically attractive scenario for light coupling, since they allow flexible placement on the chip, thereby enabling automatic testing of fabricated devices on a wafer-scale, preferred for large-volume developments. In this work, we report two designs for surface grating couplers implemented on the Ge-rich SiGe graded buffer. The grating couplers are designed for transverse electric (TE) and transverse magnetic (TM) polarizations, respectively, both operating at 7.5 μm wavelength. In particular, the TE-designed grating coupler with an inverse taper excitation arrangement yields a coupling efficiency of 6.3% (-12 dB), a 1-dB bandwidth of 300 nm, and reduced back-reflection less than 1%. Furthermore, the TM-designed grating coupler with a conventional taper injection stage predicts an improved coupling performance up to 11% (-9.6 dB), with a 1-dB bandwidth of 310 nm, and only 1% back-reflection. These results open up the way for the realization of complex and multifunctional photonics integrated circuits on Ge-rich SiGe platform with operation at midIR wavelengths.
Mid-infrared (mid-IR) silicon photonics is becoming a prominent research with remarkable potential in several applications such as in early medical diagnosis, safe communications, imaging, food safety and many more. In the quest for the best material platform to develop new photonic systems, Si and Ge depart with a notable advantage over other materials due to the high processing maturity accomplished during the last part of the 20th century through the deployment of the CMOS technology. From an optical viewpoint, combining Si with Ge to obtain SiGe alloys with controlled stoichiometry is also of interest for the photonic community since permits to increase the effective refractive index and the nonlinear parameter, providing a fascinating playground to exploit nonlinear effects. Furthermore, using Ge-rich SiGe gives access to a range of deep mid-IR wavelengths otherwise inaccessible (λ ~2-20 μm). In this paper, we explore for the first time the limits of this approach by measuring the spectral loss characteristic over a broadband wavelength range spanning from λ = 5.5 μm to 8.5 μm. Three different SiGe waveguide platforms are compared, each one showing higher compactness than the preceding through the engineering of the vertical Ge profile, giving rise to different confinement characteristics to the propagating modes. A flat propagation loss characteristic of 2-3 dB/cm over the entire wavelength span is demonstrated in Ge-rich graded-index SiGe waveguides of only 6 μm thick. Also, the role of the overlap fraction of the confined optical mode with the Si-rich area at the bottom side of the epitaxial SiGe waveguide is put in perspective, revealing a lossy characteristic compared to the other designs were the optical mode is located in the Ge-rich area at the top of the waveguide uniquely. These Ge-rich graded-index SiGe waveguides may pave the way towards a new generation of photonic integrated circuits operating at deep mid-IR wavelengths.
Recent works towards the development of Ge-rich SiGe photonic integrated circuits will be presented, such as the demonstration of low-loss waveguides and ultra-wideband Mach Zehnder interferometer from 5.5 to 8.6 μm wavelength, as well as the first steps towards the realization of efficient wideband optical sources.
We propose germanium-rich silicon germanium waveguides as a basic building block for polarization insensitive circuitry on silicon. In this work a detailed study of SiGe waveguides geometries is performed to find optimal parameters to simultaneously obtain low polarization sensitivity and single mode operation at λ=1.55μm. The polarization dependence of the effective index, group index and dispersion coefficient is investigated. Optimized geometries are tolerant to fabrication errors and can be realized with the current state of the art CMOS technology. As a next step polarization insensitive multimode interference structures have been designed.
The extension of silicon photonics towards the mid infrared (mid-IR) spectral range has recently attracted a lot of attention. The development of photonic devices operating at these wavelengths is crucial for many applications including environmental and chemical sensing, astronomy and medicine. Recent works regarding the development of Ge-rich SiGe waveguides on graded buffer layers will be presented. It will be shown that these waveguides demonstrate low loss and strong mode confinement for a large range of wavelengths and that they have a good potential for being a major building block of mid-infrared photonic integrated circuits.
We propose germanium-rich silicon-germanium (SiGe) as a new platform for optical interconnects. The platform viability is experimentally and theoretically investigated through the realization of main building blocks of passive circuitry. Germanium-rich Si1-xGex guiding layer on a graded SiGe layer is used to experimentally show 12μm radius bends by light confinement tuning at a wavelength of 1550nm. As a next step, Mach Zehnder interferometer with 10 dB extinction ratio is demonstrated. High Ge content of the proposed platform allows the coupling with Ge-based active devices, relying on a high quality epitaxial growth. Hence, the integration on Silicon of high speed and low power consumption Ge-rich active components is possible, despite the high lattice mismatch between silicon and germanium.
Electro-absorption and electro-refraction in Ge/SiGe coupled quantum wells (CQW) grown on Si have been investigated by means of optical transmission measurements. The separate confinement of electrons and holes in the heterostructure gives rise to an anomalous Quantum Confined Stark Effect (QCSE) that can be exploited to strongly enhance the electro refractive effect with respect to uncoupled quantum wells. A refractive index variation up to 2.3 x 10-3 has been measured at 1.5 V, with an VπLπ of 0.046 V cm. This result is very promising for the realization of an efficient and compact phase modulator based on the Ge/SiGe material system.
Ge quantum wells are emerging as a relevant material system for enabling fast and power-efficient optical modulation in the framework of Si-photonics. The need for reliable designs of QW structures, matching given operating wavelengths and bias voltages, calls for the implementation of modelling tools capturing the optical properties of SiGe heterostructures. Here we report on the calculation of the quantum confined Stark effect based on an eight-band k×p model. The obtained spectra are analysed and compared with experimental data showing a good agreement between calculation and measurements.
We experimentally and theoretically investigate the use of silicon germanium (SiGe) on silicon substrate as a new platform for optical interconnects. The system composed of Germanium (Ge) rich Si1-xGex guiding layer on a graded SiGe layer is showed to be suitable for the realization of all main building blocks of passive optical circuitry. We show experimentally at a wavelength of 1550nm that sharp 12μm radius bends can be obtained by light confinement tuning. Mach-Zehnder interferometer with more than 10 dB extinction ratio is also demonstrated. Moreover, Ge-rich Si1-xGex based passive components are very interesting for their native integration with Ge-rich active optical devices. Hence, by using this new platform for optical integrated circuits, lattice mismatch between silicon and germanium is no longer a major constraint for the integration of Ge-rich active photonic components on silicon.
We address the behavior of mid-infrared localized plasmon resonances in elongated germanium antennas integrated on silicon substrates. Calculations based on Mie theory and on the experimentally retrieved dielectric constant allow us to study the tunability and the figures of merit of plasmon resonances in heavily-doped germanium and to preliminarily compare them with those of the most established plasmonic material, gold.
The use of heavily doped semiconductors to achieve plasma frequencies in the mid-IR has been recently proposed as a promising way to obtain high-quality and tunable plasmonic materials. We introduce a plasmonic platform based on epitaxial n-type Ge grown on standard Si wafers by means of low-energy plasma-enhanced chemical vapor deposition. Due to the large carrier concentration achieved with P dopants and to the compatibility with the existing CMOS technology, SiGe plasmonics hold promises for mid-IR applications in optoelectronics, IR detection, sensing, and light harvesting. As a representative example, we show simulations of mid-IR plasmonic waveguides based on the experimentally retrieved dielectric constants of the grown materials.
We experimentally and theoretically investigate GeSi-based photonics for future on-chip optical interconnect on bulk Silicon substrates with dense wavelength division multiplexing (WDM) system. We experimentally show that Ge-rich Si1-xGex can be used as both a passive low loss waveguide and a substrate to facilitate low-temperature epitaxial growth of Ge-based active devices working at low optical loss wavelength of Ge-rich Si1-xGex waveguides. We also theoretically discussed the possibilities to realize a compact passive component based on Ge-rich Si1-xGex material system on bulk Si wafer. From simulation the system based on Ge-rich Si1-xGex waveguide and the Si1-yGey (y < x) lower cladding layer is good enough to ensure compactness of important on-chip photonic components including passive waveguide and GeSi-based array waveguide grating (AWG). The small refractive index contrast between Ge-rich Si1-xGex waveguide and the Si1-yGey lower cladding layer potentially avoid the polarization dependent loss and detrimental fabrication tolerance of WDM system. Our studies show that GeSi-based photonics could uniquely provide both passive and active functionalities for dense WDM system.
We report on the developments of Ge/SiGe quantum well (QW) waveguide modulators operating at 1.3 μm. First we studied QW structures grown on a 13-μm SiGe buffer on bulk silicon. Light was directly coupled and propagated in the active region. Using a 3-μm wide and 50-μm long modulator, an extinction ratio larger than 4 dB was obtained for a drive voltage lower than 5 V in a 15 nm wavelength range. Then simulations were performed to evaluate the performances of an integrated modulator on silicon on insulator (SOI) platform. An eigenmode expension method was used to model the vertical optical coupling between SOI waveguide and Ge/SiGe devices. It is shown that a reduction of the thickness of the buffer leads to a significant improvement in the performances (extinction ratio, insertion loss) and footprint of the waveguide-integrated devices.
We report on the electro-refractive effect in Ge/SiGe multiple quantum wells grown by low energy plasma enhanced chemical vapor deposition (LEPECVD). The electro-refractive effect was experimentally characterized by the shift of Fabry-Perot fringes in the transmission spectra of a 64 μm long slab waveguide. A refractive index variation up to 1.3 × 10-3 was measured with an applied electric field of 88 kV/cm at 1475 nm, 50 meV below the excitonic resonance, with a VπLπ figure of merit of 0.46 V×cm. The device performances are promising for the realization of Mach Zehnder modulators in the Ge-Si material platform.
We report different experimental results showing the large potential of Ge/SiGe quantum well structures as a promising
solution forlow power consumption and large bandwidth optical modulators in silicon photonics technology. First, high
speed operation of such a Ge/SiGe multiple quantum well (MQW) electro-absorption modulator is reported, with 23
GHz bandwidth demonstrated from a 3 μm wide and 90 μm long Ge/SiGe MQW waveguide. Then the flexibility to shift
the absorption band edge from 1.42 to 1.3 μm is illustrated by strain engineering of the Ge wells. Finally electrorefraction by Quantum Confined Stark Effect (QCSE) is demonstrated, opening the route towards phase modulators
based on Ge/SiGe MQWs.
Room temperature direct gap electroluminescence (EL) from a Ge/Si0.15Ge0.85 MQW waveguide was experimentally
studied. The dependence of the EL intensity on the injection current and temperature was measured. The direct gap EL
from Ge/SiGe MQWs was shown to be transverse-electric (TE) polarized, confirming that the EL originates from
recombination with a HH state.
High speed Ge multiple quantum well (MQWs) electro-absorption (EA) modulator is reported. Device development
procedures from the epitaxial growth of high quality Ge MQWs by LEPECVD technique, fabrication, and
characterization of optoelectronic device are described.