Mid-infrared photonic crystal waveguides in silicon

Efficient 4.95 µm-8.5 µm dual-band grating coupler with crosstalk suppression capability

In many integrated optics systems, grating couplers are a key component of interfacing the external light source with in-plane photonic devices. Grating couplers with dual-band capability are often desired for expanding the operation spectrum of photonic systems. Here, we propose and theoretically investigate, for the first time, a 4.95 µm-8.5 µm dual-band grating coupler on a Ge-on-SOI platform. In addition to conventional structures, Bragg gratings are introduced to two wavelength division directions for crosstalk suppression. With this design, the simulated coupling efficiencies have respectively reached 59.93% and 46.38% for the 4.95 µm and 8.5 µm bands. This mid-infrared dual-band grating coupler may be useful for defense and environmental monitoring applications.

Integrated Nanophotonic Waveguide-Based Devices for IR and Raman Gas Spectroscopy

On-chip devices for absorption spectroscopy and Raman spectroscopy have been developing rapidly in the last few years, triggered by the growing availability of compact and affordable tunable lasers, detectors, and on-chip spectrometers. Material processing that is compatible with mass production has been proven to be capable of long low-loss waveguides of sophisticated designs, which are indispensable for high-light-analyte interactions. Sensitivity and selectivity have been further improved by the development of sorbent cladding. In this review, we discuss the latest advances and challenges in the field of waveguide-enhanced Raman spectroscopy (WERS) and waveguide infrared absorption spectroscopy (WIRAS). The development of integrated light sources and detectors toward miniaturization will be presented, together with the recent advances on waveguides and cladding to improve sensitivity. The latest reports on gas-sensing applications and main configurations for WERS and WIRAS will be described, and the most relevant figures of merit and limitations of different sensor realizations summarized.

CORNERSTONE’s Silicon Photonics Rapid Prototyping Platforms: Current Status and Future Outlook

The field of silicon photonics has experienced widespread adoption in the datacoms industry over the past decade, with a plethora of other applications emerging more recently such as light detection and ranging (LIDAR), sensing, quantum photonics, programmable photonics and artificial intelligence. As a result of this, many commercial complementary metal oxide semiconductor (CMOS) foundries have developed open access silicon photonics process lines, enabling the mass production of silicon photonics systems. On the other side of the spectrum, several research labs, typically within universities, have opened up their facilities for small scale prototyping, commonly exploiting e-beam lithography for wafer patterning. Within this ecosystem, there remains a challenge for early stage researchers to progress their novel and innovate designs from the research lab to the commercial foundries because of the lack of compatibility of the processing technologies (e-beam lithography is not an industry tool). The CORNERSTONE rapid-prototyping capability bridges this gap between research and industry by providing a rapid prototyping fabrication line based on deep-UV lithography to enable seamless scaling up of production volumes, whilst also retaining the ability for device level innovation, crucial for researchers, by offering flexibility in its process flows. This review article presents a summary of the current CORNERSTONE capabilities and an outlook for the future.

Demonstration of mid-infrared slow light one-dimensional photonic crystal ring resonator with high-order photonic bandgap

Integrated mid-infrared sensing offers opportunities for the compact, selective, label-free and non-invasive detection of the absorption fingerprints of many chemical compounds, which is of great scientific and technological importance. To achieve high sensitivity, the key is to boost the interaction between light and analytes. So far, approaches like leveraging the slow light effect, increasing optical path length and enhancing the electric field confinement (f) in the analyte are envisaged. Here, we experimentally investigate a slow light one-dimensional photonic crystal ring resonator operating at high-order photonic bandgap (PBG) in mid-infrared range, which features both strong field confinement in analyte and slow light effect. And the optical path length can also be improved by the resoantor compared with waveguide structure. The characteristics of the first- and second-order bandgap edges are studied by changing the number of patterned periodical holes while keeping other parameters unchanged to confine the bands in the measurement range of our setup between 3.64 and 4.0 µm. Temperature sensitivity of different modes is also experimentally studied, which helps to understand the field confinement. Compared to the fundamental PBG edge modes, the second PBG edge modes show a higher field confinement in the analyte and a comparable group index, leading to larger light-matter interaction. Our work could be used for the design of ultra-sensitive integrated mid-infrared sensors, which have widespread applications including environment monitoring, biosensing and chemical analysis.

Waveguide-Integrated Compact Plasmonic Resonators for On-Chip Mid-Infrared Laser Spectroscopy

The integration of nanoplasmonic devices with a silicon photonic platform affords a new approach for efficient light delivery by combining the high field enhancement of plasmonics and the ultralow propagation loss of dielectric waveguides. Such a hybrid integration obviates the need for a bulky free-space optics setup and can lead to fully integrated, on-chip optical sensing systems. Here, we demonstrate ultracompact plasmonic resonators directly patterned atop a silicon waveguide for mid-infrared spectroscopic chemical sensing. The footprint of the plasmonic nanorod resonators is as small as 2 μm², yet they can couple with the mid-infrared waveguide mode efficiently. The plasmonic resonance is directly measured through the transmission spectrum of the waveguide with a coupling efficiency greater than 70% and a field intensity enhancement factor of over 3600 relative to the evanescent waveguide field intensity. Using this hybrid device and a tunable mid-infrared laser source, surface-enhanced infrared absorption spectroscopy of both a thin poly(methyl methacrylate) film and an octadecanethiol monolayer is successfully demonstrated.

Dispersion engineering and thermo-optic tuning in mid-infrared photonic crystal slow light waveguides on silicon-on-insulator

In this Letter, the design, fabrication, and characterization of slow light devices using photonic crystal waveguides (PhCWs) in the mid-infrared wavelength range of 3.9-3.98 μm are demonstrated. The PhCWs are built on the silicon-on-insulator platform without undercut to leverage its well-developed fabrication process and strong mechanical robustness. Lattice shifting and thermo-optic tuning methods are utilized to manipulate the slow light region for potential spectroscopy sensing applications. Up to 20 nm wavelength shift of the slow light band edge is demonstrated. Normalized delay-bandwidth products of 0.084-0.112 are obtained as a result of dispersion engineering. From the thermo-optic characterization results, the slow light enhancement effect of thermo-optic tuning efficiency is verified by the proportional relationship between the phase shift and the group index. This work serves as a proof of concept that the slow light effect can strengthen light-matter interaction and thereby improve device performance in sensing and nonlinearity applications.

Graded SiGe waveguides with broadband low-loss propagation in the mid infrared

Mid-infrared (mid-IR) silicon photonics is expected to lead key advances in different areas including spectroscopy, remote sensing, nonlinear optics or free-space communications, among others. Still, the inherent limitations of the silicon-on-insulator (SOI) technology, namely the early mid-IR absorption of silicon oxide and silicon at λ~3.6 µm and at λ ~8.5 µm respectively, remain the main stumbling blocks that prevent this platform to fully exploit the mid-IR spectrum (λ ~2-20 µm). Here, we propose using a compact Ge-rich graded-index Si_1-xGe_x platform to overcome this constraint. A flat propagation loss characteristic as low as 2-3 dB/cm over a wavelength span from λ = 5.5 µm to 8.5 µm is demonstrated in Ge-rich Si_1-xGe_x waveguides of only 6 µm thick. The comparison of three different waveguides design with different vertical index profiles demonstrates the benefit of reducing the fraction of the guided mode that overlaps with the Si substrate to obtain such flat low loss behavior. Such Ge-rich Si_1-xGe_x platforms may open the route towards the implementation of mid-IR photonic integrated circuits with low-loss beyond the Si multi-phonon absorption band onset, hence truly exploiting the full Ge transparency window up to λ ~15 µm.

Enhanced spectral sensitivity of a chip-scale photonic-crystal slow-light interferometer

We experimentally demonstrate that the spectral sensitivity of a Mach-Zehnder (MZ) interferometer can be enhanced through structural slow light. We observe a 20-fold resolution enhancement by placing a dispersion-engineered, slow-light, photonic-crystal waveguide in one arm of a fiber-based MZ interferometer. The spectral sensitivity of the interferometer increases roughly linearly with the group index, and we have quantified the resolution in terms of the spectral density of interference fringes. These results show promise for the use of slow-light methods for developing novel tools for optical metrology and, specifically, for compact high-resolution spectrometers.

M-line spectroscopy on mid-infrared Si photonic crystals for fluid sensing and chemical imaging

The presented work demonstrates the design and characterization of Si-based photonic crystal waveguides operating as an evanescent wave absorption sensor in the mid-IR range λ = 5-6 µm. The photonic crystal structure is fabricated in a Si slab upon a thin Si(3)N(4)/TEOS/Si(3)N(4) membrane. M-line spectroscopy is used to verify the presence of guided waves. Different fillings of the photonic crystal holes have been realized to avoid sample residuals in the holes and, at the same time, to obtain spectral tuning of the structures by modification of the refractive index contrast with the photonic background. The chip displays sensitivity to fluid droplets in two-prism experiments. The output signal is quantitatively related to the fluid's absorption coefficient thereby validating the experimental method.

Experimental demonstration of propagation characteristics of mid-infrared photonic crystal waveguides in silicon-on-sapphire

We provide the first experimental demonstration of optical transmission characteristics of a W1 photonic crystal waveguide in silicon on sapphire at mid infrared wavelength of 3.43 μm. Devices are studied as a function of lattice constant to tune the photonic stop band across the single wavelength of the source laser. The shift in the transmission profile as a function of temperature and refractive index is experimentally demonstrated and compared with simulations. In addition to zero transmission in the stop gap, propagation losses less than 20 dB/cm are observed for group indices greater than 20 below the light line while more than 300 dB/cm propagation losses are observed above the light line, characteristic of the waveguiding behavior of photonic crystal line defect modes.

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.

Contra-directional coupling into slotted photonic crystals for spectrometric applications

We propose and demonstrate the concept of a contra-directional coupler between a W1 and a slotted photonic crystal waveguide. The bandwidth and operating wavelength of such a coupler can be controlled via its geometrical parameters, and power transfer is not periodic unlike in the more familiar codirectional case. Light of specific wavelengths can be extracted from the W1 mode into air slot modes using this design, with W1/slot coupling efficiencies of up to 99±1%, and waveguide extracted coupling efficiencies of up to 51±12% demonstrated experimentally. Combining several of these couplers in series, we demonstrate the spectral filtering functionality on-chip. The device therefore combines the well-known sensing function of the slotted waveguide geometry with the spectrometer function, thus uniting two essential biosensor functions in a monolithic device.

Experimental demonstration of polarization-insensitive air-cladding grating couplers for silicon-on-insulator waveguides

We present an air-cladding apodized focusing subwavelength grating that can effectively couple two polarizations into a single waveguide. For the transverse magnetic mode, -3.2  dB maximum coupling efficiency with ∼28  nm 1 dB bandwidth is achieved. With the same grating, -4.3  dB maximum coupling efficiency with ∼58  nm 1 dB bandwidth is achieved for the transverse electric mode. The minimum difference between two polarizations' coupling peaks is demonstrated to be ∼32  nm. At the 1525 nm wavelength range, the polarization-insensitive grating is demonstrated with -6.5  dB coupling efficiency. The polarization-insensitive coupling wavelength can be controlled experimentally.

Mid-infrared materials and devices on a Si platform for optical sensing

In this article, we review our recent work on mid-infrared (mid-IR) photonic materials and devices fabricated on silicon for on-chip sensing applications. Pedestal waveguides based on silicon are demonstrated as broadband mid-IR sensors. Our low-loss mid-IR directional couplers demonstrated in SiN x waveguides are useful in differential sensing applications. Photonic crystal cavities and microdisk resonators based on chalcogenide glasses for high sensitivity are also demonstrated as effective mid-IR sensors. Polymer-based functionalization layers, to enhance the sensitivity and selectivity of our sensor devices, are also presented. We discuss the design of mid-IR chalcogenide waveguides integrated with polycrystalline PbTe detectors on a monolithic silicon platform for optical sensing, wherein the use of a low-index spacer layer enables the evanescent coupling of mid-IR light from the waveguides to the detector. Finally, we show the successful fabrication processing of our first prototype mid-IR waveguide-integrated detectors.

Demonstration of mid-infrared waveguide photonic crystal cavities

We have demonstrated what we believe to be the first waveguide photonic crystal cavity operating in the mid-infrared. The devices were fabricated from Ge23Sb7S70 chalcogenide glass (ChG) on CaF2 substrates by combing photolithographic patterning and focused ion beam milling. The waveguide-coupled cavities were characterized using a fiber end fire coupling method at 5.2 μm wavelength, and a loaded quality factor of ~2000 was measured near the critical coupling regime.

Demonstration of Silicon-on-insulator mid-infrared spectrometers operating at 3.8 μm

The design and characterization of silicon-on-insulator mid-infrared spectrometers operating at 3.8 μm is reported. The devices are fabricated on 200 mm SOI wafers in a CMOS pilot line. Both arrayed waveguide grating structures and planar concave grating structures were designed and tested. Low insertion loss (1.5-2.5 dB) and good crosstalk characteristics (15-20 dB) are demonstrated, together with waveguide propagation losses in the range of 3 to 6 dB/cm.