The mid-infrared (mid-IR) is an important wavelength range for vibrational absorption spectroscopy (e.g. for gas sensing, medical diagnostics, environmental monitoring), and thus there is a strong need for small and stable on-chip spectrometers. It is also desirable for it to be inexpensive to fabricate and for it to be able to perform high-resolution measurements over a wide bandwidth. To this end, we demonstrate two kinds of mid-IR thermo-optic type Fourier Transform spectrometers (FTS). Both variations of the device are designed to target a central wavelength of 3.8 μm and are based on the silicon-on-insulator platform. These two devices are verified by using them to retrieve the spectrum of a quantum cascade laser when it is tuned to different wavelengths. They have the potential to achieve higher resolution and bandwidth through subsequent design optimization, and could in future be integrated with mid-infrared photodetectors.
We review our recent results on modulators and detectors for the 2μm range, which may become very relevant for future communications, and on the development of mid-IR broadband devices for sensing applications. We show Mach-Zehnder and Michelson based modulators operating at data rates up to 25 Gb/s and Ge based detectors operating up to 12.5 Gb/s. For longer wavelengths relevant for sensing applications, we present broadband waveguides and splitters, waveguide integrated bolometers, and the realisation of a mid-infrared sensor.
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.
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.
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.
Group IV platforms can operate at longer wavelengths due to their low material losses. By combining graphene and Si and Ge platforms, photodetection can be achieved by using graphene’s optical properties and coplanar integration methods. Here, we presented a waveguide coupled graphene photodetector operating at a wavelength of 3.8 μm.
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