Mid-infrared Silicon Photonics Research Articles

Sub-wavelength gratings in silicon photonic devices for mid-infrared spectroscopy and sensing

Mid-infrared spectroscopy enabled by silicon photonics has received great interest in recent years as a pathway for a scalable sensing technology. The development of such devices would realise inexpensive and accessible instrumentation for a wide variety of uses over numerous fields. However, not every sensing application is the same; to produce sensors for real-world scenarios, engineers need flexibility in device design but also need to maintain compatibility with scalable fabrication processes. Sub-wavelength gratings can offer a solution to this problem, as they enable the engineering of optical properties using standard fabrication techniques and without requiring new materials. By using sub-wavelength gratings, specific design approaches can be tailored to different applications, such as increasing the interaction of a sensor with an analyte or broadening the bandwidth of an integrated photonic device. Here, we review the development of sub-wavelength grating-based devices for mid-infrared silicon photonics and discuss how they can be exploited for spectroscopic and sensing devices.

Blazed subwavelength grating coupler

Short-wavelength mid-infrared (2–2.5 μm wave band) silicon photonics has been a growing area to boost the applications of integrated optoelectronics in free-space optical communications, laser ranging, and biochemical sensing. In this spectral region, multi-project wafer foundry services developed for the telecommunication band are easily adaptable with the low intrinsic optical absorption from silicon and silicon dioxide materials. However, light coupling techniques at 2–2.5 μm wavelengths, namely, grating couplers, still suffer from low efficiencies, mainly due to the moderated directionality and poor diffraction-field tailoring capability. Here, we demonstrate a foundry-processed blazed subwavelength coupler for high-efficiency, wide-bandwidth, and large-tolerance light coupling. We subtly design multi-step-etched hybrid subwavelength grating structures to significantly improve directionality, as well as an apodized structure to tailor the coupling strength for improving the optical mode overlap and backreflection. Experimental results show that the grating coupler has a recorded coupling efficiency of − 4.53 dB at a wavelength of 2336 nm with a 3-dB bandwidth of ∼ 107 nm . The study opens an avenue to developing state-of-the-art light coupling techniques for short-wavelength mid-infrared silicon photonics.

Advances in short-wavelength mid-infrared silicon photonics (Invited)

Advances in short-wavelength mid-infrared silicon photonics (Invited)

Mid-Infrared Silicon Photonics for Communications

The mid-infrared wavelength region is important for a number of application areas, two of which are optical fibre and free space communications. Silicon photonics can provide inexpensive photonic chips for such applications due to excellent electronic and photonic properties. In this paper, the realisation of active silicon and germanium photonic devices for the mid-infrared spectral region are given. High speed Si depletion type modulators, Si and Ge injection modulators operating at wavelengths up to 8 micrometers, and high speed Si detectors are presented. These devices are integrated with drivers and amplifiers and show very good performance, e.g. data rate in excess of 20 Gb/s.

Next Generation Mid-Infrared Optical Sensor

CEA-Leti has developed a prototype next-generation optical chemical sensor using mid-infrared silicon photonics

On-chip Bragg grating waveguides and Fabry-Perot resonators for long-wave infrared operation up to 8.4 µm.

Taking advantage of unique molecular absorption lines in the mid-infrared fingerprint region and of the atmosphere transparency window (3-5 µm and 8-14 µm), mid-infrared silicon photonics has attracted more research activities with a great potential for applications in different areas, including spectroscopy, remote sensing, free-space communication and many others. However, the demonstration of resonant structures operating at long-wave infrared wavelengths still remains challenging. Here, we demonstrate Bragg grating-based Fabry-Perot resonators based on Ge-rich SiGe waveguides with broadband operation in the mid-infrared. Bragg grating waveguides are investigated first at different wavelengths from 5.4 µm up to 8.4 µm, showing a rejection band up to 21 dB. Integrated Fabry-Perot resonators are then demonstrated for the first time in the 8 µm-wavelength range, showing Q-factors as high as 2200. This first demonstration of integrated mid-infrared Fabry-Perot resonators paves the way towards resonance-enhanced sensing circuits and non-linear based devices at these wavelengths.

Efficient and broadband subwavelength grating coupler for 3.7 μm mid-infrared silicon photonics integration.

A grating coupler is an essential building block for compact and flexible photonics integration. In order to meet the increasing demand of mid-infrared (MIR) integrated photonics for sensitive chemical/gas sensing, we report a silicon-on-insulator (SOI) based MIR subwavelength grating coupler (SWGC) operating in the 3.7 μm wavelength range. We provide the design guidelines of a uniform and apodized SWGC, followed by numerical simulations for design verification. We experimentally demonstrate both types of SWGC. The apodized SWGC enables high coupling efficiency of -6.477 dB/facet with 3 dB bandwidth of 199 nm, whereas the uniform SWGC shows larger 3dB bandwidth of 263.5 nm but slightly lower coupling efficiency of -7.371 dB/facet.

Midwave Infrared Quantum Dot Quantum Cascade Photodetector Monolithically Grown on Silicon Substrate

Mid-infrared photodetector based on submonolayer (SML) quantum dot quantum cascade structure monolithically grown on silicon substrate has been demonstrated in this paper. Both the optical and electrical characteristics of the SML quantum dot quantum cascade photodetectors (QD-QCD) were analyzed quantitatively. The performances of these devices were compared with that on native GaAs substrate. A large resistance-area (R0A ) product of 1.13 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> Ω.cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is achieved at 77 K for the silicon-based devices, which is only roughly one order less than that on GaAs substrate. The device shows a normal-incident peak responsivity of 0.59 mA/W under zero bias at the wavelength of 6.2 μm at 77 K, indicating a photovoltaic operation mode. Johnson noise limited specific detectivity is 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> cm·Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> /W at 77 K, with photoresponse up to 100 K. These results suggest that the silicon-based QD-QCD in this paper is a very promising candidate for large format mid-infrared focal plane array and mid-infrared silicon photonics applications.

Demonstration of InAs/InGaAs/GaAs Quantum Dots-in-a-Well Mid-Wave Infrared Photodetectors Grown on Silicon Substrate

In this paper, we have demonstrated the first InAs/InGaAs/GaAs quantum dots-in-a-well (DWELL) photodetector monolithically grown on silicon substrate. We studied both the optical and electrical characteristics of the DWELL photodetectors. Time-resolved photoluminescence spectra measured from the DWELL photodetector revealed a long carrier lifetime of 1.52 ns. A low dark current density of ${\text{2.03}}\times {\text{10}}^{-3}\;{\text{mA/cm}}^{2}$ was achieved under 1 V bias at 77 K. The device showed a peak responsivity of 10.9 mA/W under 2 V bias at the wavelength of 6.4 μ m at 77 K, and the corresponding detectivity was ${\text{5.78}}\times {\text{10}}^{8}\,{{\text{cm}\cdot \text{Hz}}}^{1/2}{\text{/W}}$ . These results demonstrated that these silicon-based DWELL photodetectors are very promising for future mid-infrared applications, which can enjoy the potential benefit from mid-infrared silicon photonics technology.

Monolithic Integration of InSb Photodetector on Silicon for Mid-Infrared Silicon Photonics

The InSb photodetector on a Si substrate acts as a signal receiver for the mid-infrared silicon photonics application to overcome the limitation of group IV semiconductors. In this paper, we demonstrated an InSb p–i–n photodetector with an InAlSb barrier layer grown on (100) silicon substrates via a GaAs/Ge buffer by molecular beam epitaxy. The lattice mismatch between InSb and GaAs was accommodated by an interfacial misfit array. The 50% cutoff detectable wavelength of this detector increased from 5.7 μm at 80 K to 6.3 μm at 200 K. An 80 K detectivity of 8.8 × 109 cmHz1/2 W–1 at 5.3 μm was achieved with a quantum efficiency of 16.3%. The dark current generating mechanism of this detector is both generation–recombination and surface leakage above 140 K, while it is only surface leakage from 120 to 40 K.

Heterogeneous Integration for Mid-infrared Silicon Photonics

Heterogeneous integration enables the construction of silicon (Si) photonic systems, which are fully integrated with a range of passive and active elements including lasers and detectors. Numerous advancements in recent years have shown that heterogeneous Si platforms can be extended beyond near-infrared telecommunication wavelengths to the mid-infrared (MIR) (2-20 μm) regime. These wavelengths hold potential for an extensive range of sensing applications and the necessary components for fully integrated heterogeneous MIR Si photonic technologies have now been demonstrated. However, due to the broad wavelength range and the diverse assortment of MIR technologies, the optimal platform for each specific application is unclear. Here, we overview Si photonic waveguide platforms and lasers at the MIR, including quantum cascade lasers on Si. We also discuss progress toward building an integrated multispectral source, which can be constructed by wavelength beam combining the outputs from multiple lasers with arrayed waveguide gratings and duplexing adiabatic couplers.

Extrinsic photodiodes for integrated mid-infrared silicon photonics

Silicon photonics has recently been proposed for a diverse set of applications at mid-infrared wavelengths including spectroscopy, chemical and biological sensing, free-space communications, and nonlinear optics. While optical-to-electronic signal conversion is essential to these applications, on-chip photodetection remains an important and challenging task. In this Letter, we present room temperature operation of Zn+-implanted Si waveguide photodiodes from 2.2 to 2.4 μm, with measured responsivities of up to 87±29 mA/W and low dark currents of <10 μA. Photocurrent generation is achieved by transitions from dopant-induced sub-bandgap trap states located ≈0.58 eV above the valence band to the conduction band, resulting in a peak detection wavelength of ≈2.3 μm. The wavelength of operation can be increased by choosing a dopant with an appropriate trap level, opening the possibility for on-chip detection throughout the mid-infrared.

Mid-infrared silicon photonics for sensing applications

ABSTRACTThe mid-infrared wavelength region offers a plethora of possible applications ranging from sensing, medical diagnostics and free space communications, to thermal imaging and IR countermeasures. Hence group IV mid-infrared photonics is attracting more research interest lately. Sensing is an especially attractive area as fundamental vibrations of many important gases are found in the 3 to 14 μm spectral region. To realise group IV photonic mid-infrared sensors several serious challenges need to be overcome. The first challenge is to find suitable material platforms for the mid-infrared. In this paper we present experimental results for passive mid-infrared photonic devices realised in silicon-on-insulator (SOI), silicon-on-sapphire (SOS), and silicon on porous silicon (SiPSi). Although silicon dioxide is lossy in most parts of the mid-infrared, we have shown that it has potential to be used in the 3-4 μm region. We have characterized SOI waveguides with &lt; 1 dB/cm propagation loss. We have also designed and fabricated SOI passive devices such as MMIs and ring resonators. For longer wavelengths SOS or SiPSi structures could be used. An important active device for long wavelength group IV photonics will be an optical modulator. We present relationships for the free-carrier induced electro-refraction and electro-absorption in silicon in the mid-infrared wavelength range. Electro-absorption modulation is calculated from impurity-doping spectra taken from the literature, and a Kramers-Kronig analysis of these spectra is used to predict electro-refraction modulation. We have examined the wavelength dependence of electro-refraction and electro-absorption, and found that the predictions suggest longer-wave modulator designs will in many cases be different than those used in the telecom range.

Device engineering for silicon photonics

Silicon photonics has attracted increasing attention and research effort in recent years because of its potential for low-cost integration using existing complementary metal–oxide–semiconductor technology. One motivation for research on silicon photonics has been its potential application in energy-efficient high-speed optical interconnects for computing systems, particularly in circumstances where large power dissipation from electrical interconnects will limit performance. The high-density integration of photonic devices, the wide availability of fabrication facilities and the potential for monolithic integration with electronic circuits offer the prospect for future low-cost, reliable and energy-efficient silicon photonic devices suitable for applications in computing, communications and sensing. Herein we review recent progress in the engineering of new devices and functional elements in silicon photonics, including low-loss waveguides, passive integrated devices, integrated lasers, modulators, photodetectors and fiber–chip coupling techniques. We also identify some emerging opportunities with potential for future development, such as mid-infrared silicon photonics and optomechanical devices, and summarize some current trends, including the development of photonic devices based on amorphous silicon and the use of foundries for silicon-photonic device fabrication.

Rib waveguides for mid-infrared silicon photonics

Design rules for both single-mode and polarization-independent strained silicon-on-insulator rib waveguides at the wavelength of 3.39 μm are presented for the first time to our knowledge. Waveguide geometries with different parameters, such as waveguide height, rib width, etch depth, top oxide cover thickness and sidewall angle, have been studied in order to investigate and define design rules that will make devices suitable for mid-IR applications. Chebyshev bivariate interpolation with a standard deviation of less than 1% has been used to represent the zero-birefringence surface. Experimental results for the upper cladding stress level have been used to determine the influence of top oxide cover thickness and different levels of upper cladding stress on waveguide characteristics. Finally, the polarization-insensitive and single-mode locus is presented for different waveguide heights.