Callum G. Littlejohns

Surface-Grating-Coupled Low-Loss Ge-on-Si Rib Waveguides and Multimode Interferometers

Germanium-on-silicon is a highly promising platform for planar photonics for the midinfrared, due to germaniums wide transparency range. In this letter, we report Ge-on-Si waveguides with record low losses of only 0.6 dB/cm, which is achieved using a 2.9-μm thick germanium layer, thus minimizing mode interaction with dislocations at the germanium/silicon interface. Using these waveguides, multimode interferometers with insertion losses of only 0.21 ± 0.02 dB are also demonstrated.

Silicon Photonic Waveguides and Devices for Near- and Mid-IR Applications

Silicon photonics has been a very buoyant research field in the last several years mainly because of its potential for telecom and datacom applications. However, prospects of using silicon photonics for sensing in the mid-IR have also attracted interest lately. In this paper, we present our recent results on waveguide-based devices for near- and mid-infrared applications. The silicon-on-insulator platform can be used for wavelengths up to 4 μm; therefore, different solutions are needed for longer wavelengths. We show results on passive Si devices such as couplers, filters, and multiplexers, particularly for extended wavelength regions and finally present integration of photonics and electronics integrated circuits for high-speed applications.

Mid-Infrared Thermo-Optic Modulators in SoI

We report experimental results for thermo-optic modulators in silicon-on-insulator (SoI) material operating at the wavelength of 3.8 μ m. These devices are based on asymmetric Mach-Zehnder interferometers (MZIs) with aluminum heaters placed above one MZI arm. The SoI rib waveguides with 400-nm Si device layer thickness are used. Devices with conventional straight MZI arm and spiral MZI arm geometries are investigated. Straight-arm MZIs exhibited higher modulation depths, of up to 30.5 dB, whereas spiral-arm MZIs required smaller switching powers, as low as 47 mW. Measured -3 dB bandwidths were up to 23.8 kHz and did not vary significantly with device configuration.

Next Generation Device Grade Silicon-Germanium on Insulator

High quality single crystal silicon-germanium-on-insulator has the potential to facilitate the next generation of photonic and electronic devices. Using a rapid melt growth technique we engineer tailored single crystal silicon-germanium-on-insulator structures with near constant composition over large areas. The proposed structures avoid the problem of laterally graded SiGe compositions, caused by preferential Si rich solid formation, encountered in straight SiGe wires by providing radiating elements distributed along the structures. This method enables the fabrication of multiple single crystal silicon-germanium-on-insulator layers of different compositions, on the same Si wafer, using only a single deposition process and a single anneal process, simply by modifying the structural design and/or the anneal temperature. This facilitates a host of device designs, within a relatively simple growth environment, as compared to the complexities of other methods, and also offers flexibility in device designs within that growth environment.

50 Gb/s Silicon Photonics Receiver With Low Insertion Loss

Optical (de)multiplexers that can provide low insertion loss, low crosstalk, high thermal stability, and insensitivity to fabrication tolerances are essential components for silicon photonics integration. In this letter, we demonstrate the integration of an angled multimode interferometer coarse wavelength division multiplexer with germanium p-i-n photodetectors to form a 50 Gb/s receiver with a low insertion loss of <;-0.5 dB and crosstalk of <;-15 dB. The angled multimode interferometer has the flexibility to be designed on a platform with a wide range of waveguide thicknesses with little variation in performance, which allows for the optimization of other optical components in the photonics integrated circuit.

Germanium-on-silicon mid-infrared grating couplers with low-reflectivity inverse taper excitation

A broad transparency range of its constituent materials and compatibility with standard fabrication processes make germanium-on-silicon (Ge-on-Si) an excellent platform for the realization of mid-infrared photonic circuits. However, the comparatively large Ge waveguide thickness and its moderate refractive index contrast with the Si substrate hinder the implementation of efficient fiber-chip grating couplers. We report for the first time, to the best of our knowledge, a single-etch Ge-on-Si grating coupler with an inversely tapered access stage, operating at a 3.8 μm wavelength. Optimized grating excitation yields a coupling efficiency of -11  dB (7.9%), the highest value reported for a mid-infrared Ge-on-Si grating coupler, with reflectivity below -15  dB (3.2%). The large periodicity of our higher-order grating design substantially relaxes the fabrication constraints. We also demonstrate that a focusing geometry allows a 10-fold reduction in inverse taper length, from 500 to 50 μm.

Low Loss SOI Waveguides and MMIs at the MIR Wavelength of

In this letter, we demonstrate high-performance waveguides and MMIs on an SOI platform at the fixed wavelength of 2 μm. The propagation loss demonstrated by the waveguides and the insertion loss of the MMIs are as low as 1 dB/cm and 0.29 dB, respectively, with TE polarization. To the best of our knowledge, this is the lowest loss for Si rib waveguides reported at a wavelength of 2 μm.

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The development of low-thermal-budget Ge-on-Si epitaxial growth for the fabrication of low-cost Ge-on-Si devices is highly desirable for the field of silicon photonics. At present, most Ge-on-Si growth techniques require high growth temperatures, followed by cyclic annealing at temperatures > 800°C, often for a period of several hours. Here, we present a low-temperature (400°C) low-cost plasma-enhanced chemical vapor deposition (PECVD) Ge-on-Si growth study and, subsequently, fabricate a high-speed zero-bias 12.5-Gb/s waveguide integrated photodetector with a responsivity of 0.1 A/W at a wavelength of 1550 nm. This low-energy device demonstrates the feasibility of the PECVD method for the fabrication of low-cost low-thermal-budget Ge-on-Si devices.

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Silicon-germanium (Si1-xGex) has become a material of great interest to the photonics and electronics industries due to its numerous interesting properties including higher carrier mobilities than Si, a tuneable lattice constant, and a tuneable bandgap. In previous work, we have demonstrated the ability to form localised areas of single crystal, uniform composition SiGe-on-insulator. Here we present a method of simultaneously growing several areas of SiGe-on-insulator on a single wafer, with the ability to tune the composition of each localised SiGe area, whilst retaining a uniform composition in that area. We use a rapid melt growth technique that comprises of only a single Ge growth step and a single anneal step. This innovative method is key in working towards a fully integrated photonic-electronic platform, enabling the simultaneous growth of multiple compositions of device grade SiGe for electro-absorption optical modulators operating at a range of wavelengths, photodetectors, and bipolar transistors, on the same wafer. This is achieved by modifying the structural design of the SiGe strips, without the need to modify the growth conditions, and by using low cost, low thermal-budget methods.

Ge-on-Si Plasma-Enhanced Chemical Vapor Deposition for Low-Cost Photodetectors

A fabrication-tolerant mid-infrared silicon polarization splitter and rotator (PSR) based on a partially etched grating-assisted coupler is proposed. The design of the partially etched structure allows to use different cladding layers, such as SiO2, to make the device compatible with the metal back-end of line process. Moreover, by using the grating-assisted coupler, the device is no longer limited by the precise requirement of the coupling length and strength as those in its counterparts based on directional couplers. The simulation results show that the PSR can work over a wide spectral range of 50 nm around the mid-infrared wavelength of 2.5 μm with the typical transverse electric (TE) to transverse magnetic (TM) polarization conversion efficiency of 96.83%, the conversion loss of -0.97 dB, and the polarization crosstalk of -21.48 dB. The TM-to-TM through insertion loss is around -0.76 dB. The effects of the fabrication errors are analyzed. The numerical simulation results demonstrate that the device has a good fabrication tolerance larger than 45 nm.