Amorphous silicon (a-Si) is considered as one of the potential materials for multi-layer photonics due its high refractive index, linear and non-linear optical properties. This makes a-Si integration compatible with the Silicon-on-Insulator (SOI) photonics by increasing circuit density at each optical device layer. However, the high absorption loss of a-Si would require hydrogenation to passivate the dangling bonds for low loss optical waveguide interconnects and coupling of light between optical layers. Without an efficient passivation process, optical loss per layer would be too high for a viable multi-layer photonic platform. Therefore, we have developed a low temperature process hydrogenated a-Si (a- Si:H) with Hot-Wire Chemical Vapour Deposition (HWCVD) method that is compatible with back-end-of-the-line (BEOL) integration with active photonic or electronic layers. This work describes the experimental control of deposition temperature to achieve low loss a-Si:H waveguiding layer and the inter-layer waveguide coupling structures. Our latest results show a-Si:H deposited at 230 oC has the lowest propagation loss of 0.7 dB/cm for a sub-micron ridge waveguide at 1550 nm wavelength and 45 dB cross-talk isolation between two waveguides separated by 1 μm of SiO2 layer.
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.
Mid-Infrared (Mid-IR) techniques have gained considerable attention because of their inherent molecular selectivity and their potential for rapid label-free detection in applications such as water quality and environmental monitoring, security, food safety, and point-of-care diagnostics. Waveguide evanescent-field-based Mid-IR spectroscopy can detect analytes at very low concentrations using molecular absorption fingerprints, potentially offering high sensitivity and selectivity over a wide range of compounds. Moreover, significant footprint reduction compared to ATR-based FTIR measurements can be achieved with optical waveguide-based Mid-IR sensing through integration of various optoelectronic and microfluidic components realizing fully packaged lab-on-a-chip systems.
Recently we have developed low-loss chalcogenide optical waveguides and demonstrated waveguiding in the mid-wave and long-wave infrared spectral bands. High contrast GeTe4 and ZnSe channel waveguides were fabricated on bulk substrates and on silicon wafers (with suitable optical isolation layers) using lift-off and dry etching techniques after photolithographically patterning the thin films. These waveguides were exhibiting optical losses as low as 0.6 dB/cm in the mid-wave IR band and were validated for the Mid-IR evanescent wave spectroscopy with water and IPA. We have also demonstrated the effectiveness of simple paper-based fluidics with our waveguides.
In addition, we investigate a new family of free-standing Ta2O5 rib waveguides for trace gas detection with evanescent field overlap with the surrounding medium (air) up to about 70%. The waveguides are being fabricated and the fabrication and characterization results will be presented.
This work describes the integration of mid-infrared (MIR) silicon photonics with PDMS microfluidics to perform absorption spectroscopy of IPA-water solutions. The MIR spectral region contains strong absorption bands for many molecules, and photonic devices operating in the MIR can be used in many sensing applications. In this work a preliminary demonstration of a silicon-on-insulator (SOI) device is carried out in which the transmission spectra of different concentrations of water-IPA solutions are measured at wavelengths between 3.725 μm and 3.888 μm. A PDMS microfluidic channel is integrated with the waveguides in order to improve the repeatability of sample handing, reduce reagent volumes and prevent evaporation of the analyte. A microfluidic channel with 3000 x 100 μm cross-section and 30 mm length is bonded to a SOI chip comprising 500 nm thick rib waveguides and a 2 μm thick SiO2top cladding isolating the waveguide mode from the analyte. Trenches were patterned into the SiO2 cladding to create sensing windows of varying lengths (10 μm to 3mm) along different waveguides. The devices were used to detect an expected IPA absorption peak at 3.77 μm, and concentration as low as 3% IPA in water (by volume) was detected. Further work will focus on increasing the sensitivity of the measurement by using increased interaction lengths, reduction of noise and instability, and on the detection of drugs using transmission measurements over a broader wavelength range.
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.
The Lightwave Roadshow is an outreach program run by research students at the University of Southampton, UK, that seeks to educate and inspire young students with optics, through conducting workshops in local schools and exhibiting at local and regional educational fairs. Adopting a hands-on philosophy enabled by an extensive collection of experimental optical demonstrations, Lightwave aims to promote scientific interest and indirectly address the global STEM skills shortage. While Lightwave has become a well-established program in local schools since its inception in 1998, 2015 included an unprecedented number of overseas activities. Inspired by the In- ternational Year of Light and Light-based Technologies (IYL 2015), Lightwave organized a school workshop in a foreign country (Singapore) as well as exhibited at major events, including the IYL 2015 opening ceremony in France, which marked the first time that the roadshow used UK school students to deliver outreach activities beyond the UK. These recent successful overseas projects have encouraged the outreach team to continue expand- ing the reach of the roadshow internationally. Of particular note is the involvement of Lightwave at academic conferences, where experiences and best practices can be shared among outreach ambassadors from different programs, student chapters, universities, and organizations. This paper provides a review of these activities, and identifies the administrative and practical challenges of bringing a local outreach program abroad and some strategies to overcome them. We also outline our travel suite of experimental demonstration kit, a portable selection from our main equipment inventory. This won the recent OSA ‘IYL-To-Go’ student competition.
The accepted industrial skills shortage in the subjects of science, technology, engineering and mathematics (STEM) in the United Kingdom has led to an increasing drive for universities to work with a wider pool of potential students. One contributor to this drive is the Lightwave Roadshow, a photonics-focused outreach program run by postgraduate students from the University of Southampton. The program has benefitted from the unique platform of the International Year of Light (IYL) 2015 for the development and support of hands-on and interactive outreach activities. In this report we review Lightwave activities facilitated by IYL that focused on widening participation for students aged 6 to 18 years from a multitude of societal categories; the roadshow has directly benefitted from the significance and investment into the IYL in conjunction with university recruitment strategies, local schools and the support of international organizations such as SPIE and OSA. Lightwave has used the foundation of the IYL to provide a wide range of activities for over 1,200 UK students in 53 different schools; the assessment tools used to measure learning outcomes, reach and impact are also discussed. The program’s activities have been developed to make younger age groups the center of the outreach activity and create an environment which encourages youth pursuit of optics and science from a grassroots level upwards; to illustrate this we will outline a Lightwave project endorsed by the IYL steering committee to permit two 6th form students to attend the IYL opening ceremony in Paris.
GeTe4 waveguides were designed and fabricated on silicon substrates with a ZnSe isolation layer. GeTe4 has a refractive index of 3.25 at a wavelength of 9 μm and a lower refractive index isolation layer is needed to realise waveguides on silicon. Numerical modelling was carried out to calculate the thickness of the isolation layer (ZnSe, refractive index ~2.4) required to achieve low loss waveguides. For a loss between 0.1 and 1.0 dB/cm it was found that a ~ 4 μm thick ZnSe film is required at a wavelength of 9 μm. ZnSe thin films were deposited on silicon, GeTe4 waveguides were fabricated by lift-off technique and were characterised for mid-infrared waveguiding.
Realization of single-mode waveguides is essential for ultra-sensitive biosensing in the mid-infrared molecular “fingerprint” region for biomedical lab-on-chip applications. High contrast (Δn ≈ 1) germanium telluride (GeTe4) single mode rib waveguides were fabricated on zinc selenide (ZnSe) substrates for evanescent field based sensing to detect analytes at low concentration. Amorphous GeTe4 thin films were deposited by RF-sputtering and were found to transmit over the spectral range from 2μm - 20μm. Photolithography followed by reactive ion etching was carried out to etch the film, forming rib waveguide structures with minimum surface roughness and vertical sidewalls. It was found that films deposited at room temperature have average roughness of about 5nm. Optical constants were determined by IR-VASE ellipsometry.