J. Soler Penades

Suspended SOI waveguide with sub-wavelength grating cladding for mid-infrared

We present a new type of mid-infrared silicon-on-insulator (SOI) waveguide. The waveguide comprises a sub-wavelength lattice of holes acting as lateral cladding while at the same time allowing for the bottom oxide (BOX) removal by etching. The waveguide loss is determined at the wavelength of 3.8 μm for structures before and after being underetched using both vapor phase and liquid hydrofluoric acid (HF). A propagation loss of 3.4 dB/cm was measured for a design with a 300 nm grating period and 150 nm holes after partial removal (560 nm) of BOX by vapor phase HF etching. We also demonstrate an alternative design with 550 nm period and 450 nm holes, which allows a faster and complete removal of the BOX by liquid phase HF etching, yielding the waveguide propagation loss of 3.6 dB/cm.

Grating coupled low loss Ge-on-Si waveguides and multimode interferometers for the mid-infrared

Germanium-on-silicon is a promising platform for planar photonics over the entire mid-infrared range. We report here grating coupled Ge-on-Si waveguides with record low losses of 0.6dB/cm, multimode interferometers and Mach-Zehnder interferometers.

Group IV mid-­infrared photonics

In this paper we present SOI, suspended Si, and Ge-on-Si photonic platforms and devices for the mid-infrared. We demonstrate low loss strip and slot waveguides in SOI and show efficient strip-slot couplers. A Vernier configuration based on racetrack resonators in SOI has been also investigated. Mid-infrared detection using defect engineered silicon waveguides is reported at the wavelength of 2-2.5 μm. In order to extend transparency of Si waveguides, the bottom oxide cladding needs to be removed. We report a novel suspended Si design based on subwavelength structures that is more robust than previously reported suspended designs. We have fabricated record low loss Ge-on-Si waveguides, as well as several other passive devices in this platform. All optical modulation in Ge is also analyzed.

Group IV mid-infrared devices and circuits

In this paper we present silicon and germanium-based material platforms for the mid-infrared wavelength region and we report several active and passive devices realised in these materials. We particularly focus on devices and circuits for wavelengths longer than 7 micrometers.

Germanium and silicon photonic integrated circuits for the mid-infrared

Silicon and germanium are transparent up to approximately 8 µm and 15 µm, respectively, thus offering a range of applications in biochemical and environmental sensing, medicine, astronomy and communications. In this paper we present our recent results on germanium and silicon modulators and detectors suitable for mid-infrared communications.

Subwavelength engineered structures for integrated photonics

We report our advances in development of subwavelength engineered structures for integrated photonics. This unique technology allows synthesis of an effective photonic medium with an unprecedented control of material properties, constituting a powerful tool for a designer of photonic integrated circuits. By locally engineering the refractive index of silicon by forming a pattern of holes at the subwavelength scale it is possible to manipulate the flow of light in silicon photonic waveguides. We have demonstrated a number of subwavelength engineered devices operating at telecom wavelengths, including fiber-chip couplers, waveguide crossings, WDM multiplexers, ultra-fast optical switches, athermal waveguides, evanescent field sensors, polarization rotators, transceiver hybrids and colorless interference couplers. The subwavelength metamaterial concept has been adopted by industry (IBM) for fiber-chip coupling and subwavelength engineered structures are likely to become key building blocks for the next generation of integrated photonic circuits. We present an overview of different implementations of these structures in silicon photonic integrated circuits, such as high-efficiency fiber-chip couplers, wavelength multiplexers, microspectrometers, waveguide crossovers, ultra-broadband splitters and mid-infrared waveguide components, to name a few.

Silicon and germanium mid-infrared photonics

We present three main material platforms: SOI, suspended Si and Ge on Si. We report low loss SOI waveguides (rib, strip, slot) with losses of ~1dB/cm. We also show efficient modulators and detectors realized in SOI, as well as filters and multiplexers. To extend transparency of SOI waveguides, bottom oxide cladding can be removed. We have fabricated low loss passive devices in a suspended platform that employ subwavelength gratings. Ge on Si material can have larger transparency range than suspended Si. We have designed passive devices in this platform, demonstrated all optical modulation and carried out two photon absorption measurements. We have also investigated theoretically free carrier optical modulation in Ge.

Silicon photonics: some remaining challenges

This paper discusses some of the remaining challenges for silicon photonics, and how we at Southampton University have approached some of them. Despite phenomenal advances in the field of Silicon Photonics, there are a number of areas that still require development. For short to medium reach applications, there is a need to improve the power consumption of photonic circuits such that inter-chip, and perhaps intra-chip applications are viable. This means that yet smaller devices are required as well as thermally stable devices, and multiple wavelength channels. In turn this demands smaller, more efficient modulators, athermal circuits, and improved wavelength division multiplexers. The debate continues as to whether on-chip lasers are necessary for all applications, but an efficient low cost laser would benefit many applications. Multi-layer photonics offers the possibility of increasing the complexity and effectiveness of a given area of chip real estate, but it is a demanding challenge. Low cost packaging (in particular, passive alignment of fibre to waveguide), and effective wafer scale testing strategies, are also essential for mass market applications. Whilst solutions to these challenges would enhance most applications, a derivative technology is emerging, that of Mid Infra-Red (MIR) silicon photonics. This field will build on existing developments, but will require key enhancements to facilitate functionality at longer wavelengths. In common with mainstream silicon photonics, significant developments have been made, but there is still much left to do. Here we summarise some of our recent work towards wafer scale testing, passive alignment, multiplexing, and MIR silicon photonics technology.

Group IV mid-IR photonics

Silicon and germanium are transparent up to approximately 8 μm and 15 μm, respectively, thus offering a range of applications in biochemical and environmental sensing, medicine, astronomy and communications [1]. Silicon-on-insulator (SOI), can be used only up to 4 μm due to the high absorption loss of silicon dioxide, and therefore alternative material platforms have to be utilized for longer wavelengths. Also, to fully exploit the transparency range of SOI, 400 or 500 nm thick overlayers need to be used rather than the most popular 220 nm platform [2]. In this paper we report record low loss MIR SOI strip and slot waveguides, as well as Vernier racetrack configurations. If the buried oxide can be removed and replaced with air, such a platform would be transparent up to 8 μm. We report a robust design based on single etch suspended Si waveguides. Ge-on-Si waveguides have already been demonstrated with losses of 2.5-3.0 dB/cm at λ=5.8 μm by Chang et. al [3] and Shen et. al [4]. We report a record low loss in Ge-on-Si and a demonstration of all optical modulation in such waveguides. Although Si is transparent beyond 1.1 μm, it has been demonstrated that it can be used as a photodetector if mid-bandgap states are created by ion implantation. In this paper we show that detection in Si can be extended to up to 2.5 μm by implantation of SOI waveguides with boron. Finally, we also report theoretical analysis of electroabsorption and electrorefraction in Ge.

Subwavelength waveguide structures for optical interconnects

We report our advances in development of subwavelength engineered waveguide structures. This unique NRC patented technology [1,2] allows synthesis of a metamaterial with an unprecedented control of material properties, constituting a powerful tool for a designer of photonic integrated circuits. We have demonstrated a number of subwavelength engineered devices operating at telecom wavelengths [3-7], for example fibre-chip couplers, waveguide crossings, WDM multiplexers, ultra-fast optical switches, athermal waveguides, evanescent field sensors, polarization rotators, transceiver hybrids and ultra-broadband interference couplers. The subwavelength metamaterial concept has been adopted by industry (IBM) for fibre-chip coupling and subwavelength structures are likely to become key building blocks for the next generation of integrated photonic circuits. Here we present an overview of recent examples of our subwavelength engineered structures, with an emphasis on couplers for optical interconnects and evanescent field sensors. We demonstrate an unprecedented control over the light coupling between optical fibers and silicon chips by constructing metamaterial couplers operating at telecom (1.55 μm) and datacom (1.3 μm) wavelengths. We also show that by subwavelength patterning of silicon-wire waveguides the field delocalization can be engineered to increase the sensitivity of evanescent field waveguide sensors [8]. Finally, we discuss some emerging applications of subwavelength engineered structures in mid-infrared photonics.