Mid-infrared silicon-on-insulator waveguides with single-mode propagation over an octave of frequency.

Increasing the working optical bandwidth of a photonic circuit is important for many applications, in particular chemical sensing at mid-infrared wavelengths. This useful bandwidth is not only limited by the transparency range of waveguide materials, but also the range over which a waveguide is single or multimoded for predictable circuit behaviour. In this work, we show the first experimental demonstration of "endlessly single-mode" waveguiding in silicon photonics. Silicon-on-insulator waveguides were designed, fabricated and characterised at 1.95 µm and 3.80 µm. The waveguides were shown to support low-loss propagation (1.46 ± 0.13 dB/cm loss at 1.95 µm and 1.55 ± 0.35 dB/cm at 3.80 µm) and single-mode propagation was confirmed at 1.95 µm, meaning that only the fundamental mode was present over the wavelength range 1.95 - 3.80 µm. We also present the prospects for the use of these waveguides in sensing applications.

Near-IR & Mid-IR Silicon Photonics Modulators.

As the silicon photonics field matures and a data-hungry future looms ahead, new technologies are required to keep up pace with the increase in capacity demand. In this paper, we review current developments in the near-IR and mid-IR group IV photonic modulators that show promising performance. We analyse recent trends in optical and electrical co-integration of modulators and drivers enabling modulation data rates of 112 GBaud in the near infrared. We then describe new developments in short wave infrared spectrum modulators such as employing more spectrally efficient PAM-4 coding schemes for modulations up to 40 GBaud. Finally, we review recent results at the mid infrared spectrum and application of the thermo-optic effect for modulation as well as the emergence of new platforms based on germanium to tackle the challenges of modulating light in the long wave infrared spectrum up to 10.7 µm with data rates of 225 MBaud.

Suspended germanium waveguides with subwavelength-grating metamaterial cladding for the mid-infrared band.

In recent years, sensing and communication applications have fueled important developments of group-IV photonics in the mid-infrared band. In the long-wave range, most platforms are based on germanium, which is transparent up to ∼15-µm wavelength. However, those platforms are limited by the intrinsic losses of complementary materials or require complex fabrication processes. To overcome these limitations, we propose suspended germanium waveguides with a subwavelength metamaterial lateral cladding that simultaneously provides optical confinement and allows structural suspension. These all-germanium waveguides can be fabricated in one dry and one wet etch step. A propagation loss of 5.3 dB/cm is measured at a wavelength of 7.7 µm. These results open the door for the development of integrated devices that can be fabricated in a simple manner and can potentially cover the mid-infrared band up to ∼15 µm.

High-speed silicon Michelson interferometer modulator and streamlined IMDD PAM-4 transmission of Mach-Zehnder modulators for the 2 µm wavelength band.

We demonstrate high-speed silicon modulators optimized for operating at the wavelength of 2 µm. The Mach-Zehnder interferometer (MZI) carrier-depletion modulator with 2 mm phase shifter has a single-arm modulation efficiency (Vπ ·Lπ) of 2.89 V·cm at 4 V reverse bias. Using a push-pull configuration it operates at a data rate of 25 Gbit/s OOK with an extinction ratio of 6.25 dB. We also proposed a mathematically-analysed streamlined IMDD PAM-4 scheme and successfully demonstrated a 25 Gbit/s datarate PAM-4 with the same 2 mm modulator. A Michelson interferometer carrier-depletion modulator with 0.5 mm phase shift length has also been shown with modulation efficiency (Vπ ·Lπ) of 1.36 V·cm at 4 V reverse bias and data rate of 20 Gbit/s OOK. The Michelson interferometer modulator performs similarly to a Mach-Zehnder modulator with twice the phase shifter length.

Nanometallic antenna-assisted amorphous silicon waveguide integrated bolometer for mid-infrared.

Bolometers are thermal detectors widely applied in the mid-infrared (MIR) wavelength range. In an integrated sensing system on chip, a broadband scalable bolometer absorbing the light over the whole MIR wavelength range could play an important role. In this work, we have developed a waveguide-based bolometer operating in the wavelength range of 3.72-3.88 µm on the amorphous silicon (a-Si) platform. Significant improvements in the bolometer design result in a 20× improved responsivity compared to earlier work on silicon-on-insulator (SOI). The bolometer offers 24.62% change in resistance per milliwatt of input power at 3.8 µm wavelength. The thermal conductance of the bolometer is 3.86×10-5W/K, and an improvement as large as 3 orders magnitude may be possible in the future through redesign of the device geometry.

Broadband 2 × 2 multimode interference coupler for mid-infrared wavelengths.

Beam splitters are core components of photonic integrated circuits and are often implemented with multimode interference couplers. While these devices offer high performance, their operational bandwidth is still restrictive for sensing applications in the mid-infrared wavelength range. Here we experimentally demonstrate a subwavelength-structured 2×2 multimode interference coupler with high performance in the 3.1-3.7µm range, doubling the bandwidth of a conventional device.

Experimental quantification of the free-carrier effect in silicon waveguides at extended wavelengths.

We examine the electro-optic effect at wavelengths ranging from 1.31 to 2.02 µm for: (1) an Electronic Variable Optical Attenuator (EVOA); and (2) a Micro-Ring Resonator (MRR). For the EVOA, simulations were performed to ascertain the relationship between free-carrier concentration and optical attenuation, and are in agreement with our observation of an increase in attenuation with increasing wavelength. MRRs were fabricated for use around wavelengths of 2 µm to explore the sensitivity of operation to bus-to-ring coupling gap and p-n junction offset. Trends observed in the experiment are replicated by simulation, calibrated using the observations of the EVOA operation. The previously proposed efficiency increase of operation around 2 µm compared to more traditional wavelengths is demonstrated. Future development of devices for these wavelengths, supported by amplification using Thulium Doped Fiber Amplifier (TDFA) technology, is a promising route to aid in the alleviation of increasing demands on communication networks.

Chalcogenide glass waveguides with paper-based fluidics for mid-infrared absorption spectroscopy.

We demonstrate the integration of paper fluidics with mid-infrared (MIR) chalcogenide waveguides to introduce liquid samples to the waveguide evanescent field for analysis. Spectroscopy of model analytes (water and isopropyl alcohol) having well-defined mid-IR absorptions, on a ZnSe rib waveguide fabricated on silicon, is demonstrated in the wavelength range of 2.6-3.7 µm, showing their O-H and C-H stretching absorptions. The results are compared with a theoretical waveguide model, achieving good agreement. It is concluded that the presence of paper in the evanescent field does not interfere with the waveguide measurements, opening up opportunities to combine low-cost paper-based fluidics and integrated photonic technologies.

Germanium-on-silicon waveguides operating at mid-infrared wavelengths up to 8.5 µm.

We report transmission measurements of germanium on silicon waveguides in the 7.5-8.5 µm wavelength range, with a minimum propagation loss of 2.5 dB/cm at 7.575 µm. However, we find an unexpected strongly increasing loss at higher wavelengths, potential causes of which we discuss in detail. We also demonstrate the first germanium on silicon multimode interferometers operating in this range, as well as grating couplers optimized for measurement using a long wavelength infrared camera. Finally, we use an implementation of the "cut-back" method for loss measurements that allows simultaneous transmission measurement through multiple waveguides of different lengths, and we use dicing in the ductile regime for fast and reproducible high quality optical waveguide end-facet preparation.

Ultra-compact MMI-based beam splitter demultiplexer for the NIR/MIR wavelengths of 1.55 µm and 2 µm.

Based on restricted interferences mechanism in a 1x2 MMI beam splitter, we theoretically investigate and experimentally demonstrate an ultra-compact MMI-based demultiplexer for the NIR/MIR wavelengths of 1.55 µm and 2 µm. The device is fabricated on 340 nm SOI platform, with a footprint of 293x6 µm2. It exhibits extremely low insertion losses of 0.14 dB and 1.2 dB at the wavelengths of 1.55 µm and 2 µm, respectively, with contrasts of approximately 20 dB for both wavelengths, and a cross-talk of 18.83 dB.

Silicon ring resonator-coupled Mach-Zehnder interferometers for the Fano resonance in the mid-IR.

We present ring resonator (RR)-coupled Mach-Zehnder interferometers (MZIs) based on silicon-on-insulator rib waveguides, operating around the mid-IR wavelength of 3.8 µm. A number of different photonic integrated devices have been designed and fabricated experimentally to obtain the asymmetric Fano resonances in the mid-IR. We have investigated the influence of the coupling efficiency between the RR and the MZI as well as the phase shift between the two arms of the MZI on the Fano-type resonance spectral features, in agreement with theoretical predictions. Finally, wavelength-dependent Fano transmittances have been successfully measured with insertion losses up to ∼1 dB and extinction ratios of ∼20 dB. A slope of sharp Fano resonances as high as -574.6/µm has been achieved and estimated to be 35.5% higher than the slope of single RR Lorentzian-type resonances.

Germanium-on-silicon Vernier-effect photonic microcavities for the mid-infrared.

We present Vernier-effect photonic microcavities based on a germanium-on-silicon technology platform, operating around the mid-infrared wavelength of 3.8 µm. Cascaded racetrack resonators have been designed to operate in the second regime of the Vernier effect, and typical Vernier comb-like spectra have been successfully demonstrated with insertion losses of ∼5 dB, maximum extinction ratios of ∼23 dB, and loaded quality factors higher than 5000. Furthermore, an add-drop racetrack resonator designed for a Vernier device has been characterized, exhibiting average insertion losses of 1 dB, extinction ratios of up to 18 dB, and a quality factor of ∼1700.

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.

Two-photon absorption and all-optical modulation in germanium-on-silicon waveguides for the mid-infrared.

The nonlinear absorption properties of a germanium-on-silicon waveguide have been characterized across the two-photon absorption (TPA) transmission window. The results show that the TPA parameters in germanium waveguides are much stronger than the peak values in silicon, in good agreement with selected measurements conducted in bulk materials. Exploiting the large nonlinear absorption near the bandedge, efficient all-optical modulation is achieved with a modulation depth of ∼8 dB and a response time <5 ps.

Mid-infrared all-optical modulation in low-loss germanium-on-silicon waveguides.

All-optical modulation has been demonstrated in a germanium-on-silicon rib waveguide over the mid-infrared wavelength range of 2-3 µm using a free-carrier absorption scheme. Transmission measurements have shown the waveguides to have low propagation losses that are relatively independent of wavelength out to 3.8 µm, indicating that the modulation could be extended further into the mid-infrared region for applications in sensing and spectroscopy. By monitoring the material recovery, the free-carrier lifetime of the micron-sized waveguides has been estimated to be ∼18 ns, allowing for modulation speeds within the megahertz regime.

Design Procedure and Fabrication of Reproducible Silicon Vernier Devices for High-Performance Refractive Index Sensing.

In this paper, we propose a generalized procedure for the design of integrated Vernier devices for high performance chemical and biochemical sensing. In particular, we demonstrate the accurate control of the most critical design and fabrication parameters of silicon-on-insulator cascade-coupled racetrack resonators operating in the second regime of the Vernier effect, around 1.55 µm. The experimental implementation of our design strategies has allowed a rigorous and reliable investigation of the influence of racetrack resonator and directional coupler dimensions as well as of waveguide process variability on the operation of Vernier devices. Figures of merit of our Vernier architectures have been measured experimentally, evidencing a high reproducibility and a very good agreement with the theoretical predictions, as also confirmed by relative errors even lower than 1%. Finally, a Vernier gain as high as 30.3, average insertion loss of 2.1 dB and extinction ratio up to 30 dB have been achieved.

Cascade-coupled racetrack resonators based on the Vernier effect in the mid-infrared.

In this paper we report the experimental demonstration of racetrack resonators in silicon-on-insulator technology platform operating in the mid-infrared wavelength range of 3.7-3.8 µm. Insertion loss lower than 1 dB and extinction ratio up to 30 dB were measured for single resonators. The experimental characterization of directional couplers and bending losses in silicon rib waveguides are also reported. Furthermore, we present the design and fabrication of cascade-coupled racetrack resonators based on the Vernier effect. Experimental spectra of Vernier architectures were demonstrated for the first time in the mid-infrared with insertion loss lower than 1 dB and maximum interstitial peak suppression of 10 dB.

Mid-infrared photonic crystal waveguides in silicon.

We demonstrate the design, fabrication and characterization of mid-infrared photonic crystal waveguides on a silicon-on-insulator platform, showing guided modes in the wavelength regime between 2.9 and 3.9 µm. The characterization is performed with a proprietary intra-cavity Optical Parametric Oscillator in a free space optical setup and with a fibre coupled setup using a commercial Quantum Cascade Laser. We discuss the use of an integrated Mach-Zehnder interferometer for dispersion measurements and report a measured group velocity of up to a value of n(g) = 12, and determine the propagation loss to be 20 dB/cm.

Low loss silicon waveguides for the mid-infrared.

Silicon-on-insulator (SOI) has been used as a platform for near-infrared photonic devices for more than twenty years. Longer wavelengths, however, may be problematic for SOI due to higher absorption loss in silicon dioxide. In this paper we report propagation loss measurements for the longest wavelength used so far on SOI platform. We show that propagation losses of 0.6-0.7 dB/cm can be achieved at a wavelength of 3.39 µm. We also report propagation loss measurements for silicon on porous silicon (SiPSi) waveguides at the same wavelength.

Ge on Si waveguide mid-infrared absorption spectroscopy of proteins and their aggregates.

Specific proteins and their aggregates form toxic amyloid plaques and neurofibrillary tangles in the brains of people suffering from neurodegenerative diseases such as Alzheimer's and Parkinson's. It is important to study these conformational changes to identify and differentiate these diseases at an early stage so that timely medication is provided to patients. Mid-infrared spectroscopy can be used to monitor these changes by studying the line-shapes and the relative absorbances of amide bands present in proteins. This work focusses on the spectroscopy of the protein, Bovine Serum Albumin as an exemplar, and its aggregates using germanium on silicon waveguides in the 1900-1000 cm-1 (5.3-10.0 µm) spectral region.