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This article analysed and optimised horizontal Silicon insulator Silicon capacitor (H-SISCAP) phase shifter structures with insulator thickness tox up to 40 nm. The phase shifter has an effective capacitance ( C eff) around 0.5 fF/μm and a phase change efficiency 1.8 V ·cm, of which the balance between capacitance and phase efficiency is comparable with silicon rib waveguide based depletion type phase shifters. Silicon Mach–Zehnder interferometer (MZI) modulators with 200 μm long H-SISCAP phase shifters optimised for TM polarised light have been fabricated and demonstrated with a 3 dB EO bandwidth above 35 GHz and intrinsic H-SISCAP RC bandwidth around 150 GHz. The modulator displays open eye-diagrams for data rates up to 60 Gbit/s without detection equalization and 72 Gbit/s with 2 taps equalization in NRZ-OOK operation. The demonstrated H-SISCAP phase shifter paves the way to build high bandwidth segmented, travelling wave and compact ring resonators modulators for applications of optical transmitters, microwave generations, and LIDAR applications, etc.

In this work, the authors present their recent progress in the development of a platform for the mid-infrared wavelength range, based on suspended silicon waveguides with subwavelength metamaterial cladding. The platform has some intrinsic advantages, which make it a very promising candidate for sensing applications in the molecular fingerprint region. Specifically, it can cover the full transparency window of silicon (up to a wavelength of 8 μm), only requires one lithographic etch-step and can be designed for strong light-matter interaction. Design rules, practical aspects of the fabrication process and experimental results of a complete set of elemental building blocks operating at two very different wavelengths, 3.8 and 7.7 μm, are discussed. Propagation losses as low as 0.82 dB/cm at λ0 = 3.8 μm and 3.1 dB/cm at λ0 = 7.7 μm are attained for the interconnecting waveguides.

In this work different silicon photonic devices, including straight waveguides, multi-mode interference devices and Mach-Zehnder interferometers, were fabricated and characterized on hot-wire chemical vapor deposition (HWCVD) silicon nitride (SiN) layers deposited at temperatures below 350 °C. These layers presented a hydrogen concentration of 13.1%, which is lower than that achieved with plasma enhanced chemical vapor deposition at these deposition temperatures. The lowest reported optical propagation losses of 6.1 dB/cm and 5.7 dB/cm, 1550 nm and 1310 respectively, for straight SiN waveguides prepared by HWCVD was measured. We demonstrated that silicon nitride SiN, prepared using HWCVD, is a viable material for silicon photonics fabrication.

A method of achieving hybrid integration of laser bars on a silicon platform, using chip-flip bonding, is presented. A support structure etched into the substrate is used to provide vertical alignment to the plane utilizing a gimballed placement laser tool. In-plane accuracy is provided by bonder tolerances, with facet-to-facet alignment enabled by high magnification optics and the dimensions of the laser bar. Chip design features have increased allowed tolerances during fabrication of the support structure giving a route to higher yield in manufacture.

A novel technique for realization of configurable/one-time programmable (OTP) silicon photonic circuits is presented. Once the proposed photonic circuit is programmed, its signal routing is retained without the need for additional power consumption. This technology can potentially enable a multi-purpose design of photonic chips for a range of different applications and performance requirements, as it can be programmed for each specific application after chip fabrication. Therefore, the production costs per chip can be reduced because of the increase in production volume, and rapid prototyping of new photonic circuits is enabled. Essential building blocks for the configurable circuits in the form of erasable directional couplers (DCs) were designed and fabricated, using ion implanted waveguides. We demonstrate permanent switching of optical signals between the drop port and through the port of the DCs using a localized post-fabrication laser annealing process. Proof-of-principle demonstrators in the form of generic 1×4 and 2×2 programmable switching circuits were fabricated and subsequently programmed.

High performance silicon optical modulators underpin a number of current and emerging applications based upon silicon photonic technology. Here we present our recent breakthroughs in this field.

Critical coupling control is an important concept used in integrated photonics to obtain functionalities such as single and coupled resonator optical filters and wavelength multiplexers. Realization of critical coupling depends strongly on device fabrication, and reproducibility is therefore an ongoing challenge.Post-fabrication trimming offers a solution for achieving optimal performance for individual devices. Ion implantation into silicon causes crystalline lattice damage which results in an increase of the material’s refractive index and therefore creates a platform for realization of various optical devices. In recent years, we have presented results on the development of erasable gratings, optical filters and Mach-Zehnder interferometers using ion implantation of germanium into silicon. Here, we report the design, fabrication and testing of silicon-on-insulator racetrack resonators, trimmed by localised annealing of germanium ion implanted silicon using continuous and pulsed wave laser sources. The results demonstrate the ability to permanently tune the critical coupling condition of racetrack resonators. Compared to the pulsed lasers used for annealing, continuous wave lasers revealed much higher extinction ratio due to improved material quality after silicon recrystallization.

In this work, potential ways of accessing a greater spectral range at mid-infrared (MIR) wavelength ranges in silicon photonics are explored, in particular for sensing applications that use on-chip spectroscopy. To utilise the full low-loss transmission of silicon, silicon membranes are transfer printed onto MIR-transparent substrates for waveguides with no absorption from the substrate. A Y-junction splitter with low loss across a 1 µm optical bandwidth is also designed and experimentally demonstrated.

This paper presents recent results on the use of silicon-rich silicon nitride waveguides for intermodal four wave mixing. Through appropriate design of the mode propagation characteristics, broadband wavelength conversion of signals is achieved.

A CMOS compatible three-dimensional (3D) integrated photonics circuit for multilayer silicon photonics is reported. Slopes with angles between 10o and 15° were created in the oxide layer using single step wet etching to connect the two Si waveguide layers. Amorphous Si (a-Si) deposited using hot wire chemical vapor deposition (HWCVD) at a temperature of 230°C was used to fabricate the device. Losses of 0.5 dB/slope were measured in the slope waveguides at 1310 nm wavelength. As a demonstration, we propose a 4x4 network switch using a-Si based vertical directional coupler.

The mid-infrared (MIR) wavelength region is attracting more and more research for applications such as medical diagnostics, environmental monitoring, and free space communications. In the MIR, thermal detectors play an important role because they can operate over a large wavelength range, can be fabricated using CMOS compatible processes, and do not require cooling. Today no other MIR detector technology is able to fill this gap. We demonstrate the first uncooled silicon waveguide-based bolometers, in the Silicon-on-Insulator (SOI) and suspended silicon waveguide platforms. The bolometers comprise gold plasmonic antennas on the waveguide surface that heat up when they absorb light, and amorphous silicon thermometers (formed by ion implantation), whose electrical resistance changes by 0.90 ± 0.26 % K-1 when they are heated. We show that suspending the bolometers improves their performance, and achieve sensitivities of up to 1.13 ± 0.04 % change in resistance per milliwatt of input power, with a noise equivalent power of 66 μW/√Hz. Calculations suggest the NEP could in future, be further reduced by 4 orders of magnitude.

Advanced photonic probing techniques are of great importance for the development of non-contact wafer-scale testing of photonic chips. Ultrafast photomodulation has been identified as a powerful new tool capable of remotely mapping photonic devices through a scanning perturbation. Here, we develop photomodulation maps into a quantitative technique through a general and rigorous method based on Lorentz reciprocity that allows the prediction of transmittance perturbation maps for arbitrary linear photonic systems with great accuracy and minimal computational cost. Excellent agreement is obtained between predicted and experimental maps of various optical multimode-interference devices, thereby allowing direct comparison of a device under test with a physical model of an ideal design structure. In addition to constituting a promising route for optical testing in photonics manufacturing, ultrafast perturbation mapping may be used for design optimization of photonic structures with reconfigurable functionalities., Advanced photonic probes are important for the development of non-contact wafer-scale testing of photonic chips. Here, Vynck et al. develop a quantitative technique based on mapping of transmittance variations by ultrafast perturbations to analyze arbitrary linear multi-port photonic devices.

Light-induced manipulation techniques have been utilized to transport, trap, or levitate microscopic objects for a wide range of applications in biology, electronics, and photonics. Without making direct physical contact, they can provide simple yet powerful means for high-precision assembly of microscale functional blocks and components within the integrated circuit platforms, thereby offering a viable alternative to the conventional heterogeneous integration techniques, such as wafer/die bonding and transfer printing. Using a microbubble-based optofluidic pick-and-place assembly process, heterogeneous integration of compact III-V semiconductor photodetectors on a silicon-based photonic integrated circuit chip, enabling direct high-speed vertical electrical contacts for significantly improved photogenerated carrier transit distance/time, is experimentally demonstrated. The microdisk-shaped InGaAs p-i-n photodetector integrated on the silicon waveguide has a 3 dB bandwidth exceeding 50 GHz under the applied bias voltage of -1 V for near-infrared wavelengths around 1.55 mu m. The light-induced optofluidic assembly will provide a promising route for seamless heterogeneous integration of various optoelectronic components with high-speed and low-noise electrical interconnection on the fully processed silicon photonic/electronic integrated circuit platforms.

We propose and experimentally demonstrate an apodized bidirectional grating coupler for high-efficiency, perfectly vertical coupling. Through grating apodization, the coupling efficiency (CE) can be notably improved, and the parasitic reflections can be minimized. For ease of fabrication, subwavelength gratings are introduced, which are also beneficial for the coupling performance. Simulation shows a record CE of 72%. We found that the coupler is quite robust to the variation of incidence mode field diameter and fiber misalignment. A CE of −1.8 dB is experimentally measured with a 1-dB bandwidth of 37 nm.

A 500μm long capacitive silicon electro-optical MZI modulator for TM polarization light has been proposed and experimentally demonstrated with data rate 30 Gbit/s and 2 V peak-to-peak driving voltage in a push-pull configuration.

We demonstrate the first waveguide-based bolometers on Silicon-on-Insulator (SOI) platform for mid-infrared (MIR) wavelengths, which use plasmonic antennas as absorbing elements. The bolometer achieves a sensitivity of 0.80 % change in resistance per milliwatt of input power at the 3.8 μm wavelength.

Protein sensing in biological fluids provides important information to diagnose many clinically relevant diseases. Mid-infrared (MIR) absorption spectroscopy of Bovine Serum Albumin (BSA) is experimentally demonstrated on a germanium on silicon (GOS) waveguide in the 1900-1000 cm-1 (5.3-10.0 µm) region of the MIR. GOS waveguides were shown to guide light up to a wavelength of 12.9 µm. The waveguide absorption spectrum of water, showing molecular bending vibrations, was obtained experimentally and compared with a theoretical model showing good agreement. Measurement of a concentration series of BSA protein in phosphate buffered saline (PBS) from 0.1 mg/ml to 100mg/ml was performed on the waveguide using filter paper as a flow strip and the amide I, II and III peaks were observed and quantified.

Silicon-on-insulator (SOI) is the dominant platform in near-infrared silicon photonics. However, the high absorption of SiO2 at wavelengths above 4 μm limits the range at which this material can be used in the midinfrared wavelengths (2 – 20 μm). In this paper we review our recent investigation on suspended devices, building on our previous work based on suspended silicon devices with sub-wavelength grating (SWG) lateral cladding at 3.8 μm, followed by our more recent results at 7.67 μm. This platform which uses the well-known fabrication techniques of SOI, only requires a single lithographic etch step and it allows the use of the whole transparency range of the waveguide core material. Due to this, it could be a good candidate for sensing applications since the design covers a very large wavelength range. As the bottom cladding of the waveguide is air, the waveguide can be designed so that the mode is squeezed to increase the evanescent field proportion and therefore the interaction with a surrounding analyte., 2019 21st International Conference on Transparent Optical Networks (ICTON), July 9-13, 2019, Angers, France

High-speed and low-power silicon optical modulators are key in realising low cost photonic circuits for data transmission purposes. Depending on the application, different wavelength bands are preferred and therefore designs of optical modulator, which operate in these bands, are required. Here we present our recent work on silicon optical modulators at 1310, 1550, 2000 and 3800 nm wavelengths.

The atmospheric-transmission window and the fingerprint region of many substances overlaps with the long-wave infrared band. This has enabled the emergence of a new path for photonic integrated circuits, which could exploit the potential applications of this wavelength range, including chemical and bio sensing. In this work we review our latest advances in the suspended silicon platform with subwavelength grating lateral cladding at 7.7-μm wavelength. Suspended waveguides only require one lithographic etch step and can be specifically designed to maximize sensitivity when used as sensors. Waveguides with propagation loss of 3.1±0.3 dB/cm are demonstrated, as well as bends with less than 0.1 dB/bend. Suspended waveguides based on shifted Bragg grating lateral cladding are also reported, with propagation loss of 5.1±0.6 dB/cm. These results prepare the ground for the development of a platform capable of covering the entire mid-infrared band., 2019 21st International Conference on Transparent Optical Networks (ICTON), July 9-13, Angers, France

We present interlayer slope waveguides, designed to guide light from one level to another in a multi-layer silicon photonics platform. The waveguide is fabricated from hydrogenated amorphous silicon (a-Si:H) film, deposited using hot-wire chemical vapor deposition (HWCVD) at a temperature of 230°C. The interlayer slope waveguide is comprises of a lower level input waveguide and an upper level output waveguide, connected by a waveguide on a slope, with vertical separation to isolate other crossing waveguides. Measured loss of 0.17 dB/slope was obtained for waveguide dimensions of 600 nm waveguide width (w) and 400 nm core thickness (h) at a wavelength of 1550 nm and for transverse electric (TE) mode polarization.

In recent years, we have presented results on the development of a variety of silicon photonic devices such as erasable gratings and directional couplers, tunable resonators and Mach-Zehnder interferometers, and programmable photonic circuits using germanium ion implantation and localised laser annealing. In this paper we have carried out experiments to analyse a series of devices that can be fabricated using the same technology, particularly silicon-on-insulator racetrack resonators which are very sensitive to fabrication imperfections. Simulation and experimental results revealed the ability to permanently optimise the coupling efficiency of these structures by selective localised laser annealing.

Germanium has become a material of high interest for mid-infrared (MIR) integrated photonics due to its complementary metal-oxide-semiconductor (CMOS) compatibility and its wide transparency window covering the 2-15 μm spectral region exceeding the 4 μm and 8 μm limit of the Silicon-on-Insulator (SOI) platform and Si material respectively. Here, we present suspended germanium waveguides operating at wavelengths of 3.8 μm and 7.67 μm with propagation losses of 2.9 ± 0.2 dB/cm and 2.6 ± 0.3 dB/cm respectively.

Experimental demonstrations of silicon-on-insulator waveguide-based free-carrier effect modulators operating at 3.8 μm are presented. PIN diodes are used to inject carriers into the waveguides, and are configured to (a) use free-carrier electroabsorption to create a variable optical attenuator with 34 dB modulation depth and (b) use free-carrier electrorefraction with the PIN diodes acting as phase shifters in a Mach–Zehnder interferometer, achieving a V π L π of 0.052 V · mm and a DC modulation depth of 22 dB. Modulation is demonstrated at data rates up to 125 Mbit/s.

Integration of paper fluidics with Ge-on-Si waveguides for evanescent-field sensing of liquid analytes is demonstrated. Mid-infrared absorption spectroscopy of BSA protein in water and of toluene is shown in the fingerprint region of 1900-1000 cm−1.

We demonstrate low-loss hydrogenated amorphous silicon (a-Si:H) waveguides by hot-wire chemical vapor deposition (HWCVD). The effect of hydrogenation in a-Si at different deposition temperatures has been investigated and analyzed by Raman spectroscopy. We obtained an optical quality a-Si:H waveguide deposited at 230°C that has a strong Raman peak shift at 480 cm−1, peak width (full width at half-maximum) of 68.9 cm−1, and bond angle deviation of 8.98°. Optical transmission measurement shows a low propagation loss of 0.8 dB/cm at the 1550 nm wavelength, which is the first, to our knowledge, report for a HWCVD a-Si:H waveguide.

SiN layers are typically deposited by low-pressure chemical vapour deposition (LPCVD) and plasma-enhanced chemical vapour deposition (PECVD). The issue with these deposition techniques is that they require high temperature processing (>400ºC) in order to produce waveguides with low propagation losses. As a result, they are incompatible with multilayer platforms in which the integration of active devices is intended, because the fabrication of these devices requires processes that are sensitive to high temperatures such as metallisation and doping. To address this situation, we are proposing hot wire chemical vapour deposition (HWCVD) as a means to deposit SiN layers at temperatures below 400ºC. This technique has already been used in solar cell applications [1,2] to produce amorphous SiN layers with low hydrogen content and high mass density [3]. In this study, several deposition recipes were defined with a design of experiments methodology (DoE) in which different combinations of deposition parameters were investigated to understand how they affect the material structure of the SiN layers in order to optimise the quality and the losses of the deposited materials. The physical, chemical and optical properties of the obtained SiN layers were characterised using ellipsometry, scanning electron microscopy (SEM), Fourier transform infrared spectrometry (FTIR) and atomic force microscopy (AFM). Also, waveguide structures were fabricated on the deposited materials to measure their propagation losses at 1310 and 1550nm using the cutback method. SiN layers with good uniformity, roughness as low as 0.61nm, H concentration as low as 10.4x1021 atoms/ cm3 and losses of 7.1dB/cm at 1310nm and 12.3dB/cm at 1550nm were obtained. [1] H. D. Goldbach, C. H. M. van der Werf, J. Löffler, A. Scarfo, A. M. C. Kylner, B. Stannowski, W. M. Arnoldbik, A. W. Weeber, H. Rieffe, W. J. Soppe, J. K. Rath, and R. E. I. Schropp,“Hot-wire chemical vapor deposition of silicon nitride for multicrystalline silicon solar cells,” in Conference Record of the Thirsy-first IEEE Photovoltaic Specialists Conference, pp. 1249–1252, IEEE, January 2005. [2] J. K. Holt, D. G. Goodwin, A. M. Gabor, F. Jiang, M. Stavola, and H. A. Atwater, “Hot-wire chemical vapor deposition of high hydrogen content silicon nitride for solar cell passivation and anti-reflection coating applications,” Thin Solid Films 430, pp. 37–40, April 2003. [3] V. Verlaan, R. Bakker, C. H. M. van der Werf, Z. S. Houweling, Y. Mai, J. K. Rath, and R. E. I. Schropp, “High-density silicon nitride deposited at low substrate temperature with high deposition rate using hot wire chemical vapour deposition,” Surface and Coating Technologies 201, pp. 9285–9288, September 2007.

The high speed conversion of signals between the optical and electrical domains is crucial for many key applications of silicon photonics. Electro-optic modulators integrated with electronic drive amplifiers are typically used to convert an electrical signal to the optical domain. Design of these individual elements is important to achieve high performance, however a true optimisation requires careful co-design of the photonic and electronic components considering the properties of each other. Here we present our recent results in this area together with a MOSCAP type modulator with the potential for high speed, high efficiency and highly linear modulation.

A four-port grating coupler with etched square holes is proposed and demonstrated for surface-normal and polarization independent coupling. Benefiting from the perfectly vertical coupling and 1 × 4 power splitting/combing scheme, efficient polarization independent operation was achieved. The coupling efficiency was further improved by grating apodization. According to the simulations, the proposed 2D apodized grating coupler can achieve a coupling efficiency (CE) of 64.5% (-1.9 dB), a low upward back-reflection of 8.5% (-11 dB), and polarization-dependent loss (PDL) lower than 0.04 dB across the wavelength range of 1520-1620 nm. A CE of 56.3% (-2.5 dB) was experimentally measured, with PDL below 0.3 dB within the C-band and lower than 0.5 dB within the L-band. Waveguide delay lines were used to compensate the waveguide channel phase differences at the output port.

We reviewed our recent developments on the post-fabrication trimming techniques and programmable photonic circuits based on germanium ion implanted silicon waveguides. Annealing of ion implanted silicon can efficiently change the refractive index. This technology has been employed to fine-tune the optical phase, and therefore the operating point of photonic devices, enabling permanent correction of optical phase error induced by fabrication variations. High accuracy phase trimming was achieved with laser annealing and a real-time feedback control system. Erasable waveguides and directional couplers were also demonstrated, which can be used to implement programmable photonic circuits with low power consumption.