The availability of silicon photonic integrated circuits (ICs) in the 2–4 μm wavelength range enables miniature optical sensors for trace gas and bio-molecule detection. In this paper, we review our recent work on III–V-on-silicon waveguide circuits for spectroscopic sensing in this wavelength range. We first present results on the heterogeneous integration of 2.3 μm wavelength III–V laser sources and photodetectors on silicon photonic ICs for fully integrated optical sensors. Then a compact 2 μm wavelength widely tunable external cavity laser using a silicon photonic IC for the wavelength selective feedback is shown. High-performance silicon arrayed waveguide grating spectrometers are also presented. Further we show an on-chip photothermal transducer using a suspended silicon-on-insulator microring resonator used for mid-infrared photothermal spectroscopy.

Mid-infrared spectroscopic techniques rely on the specific ”fingerprint” absorption lines of molecules in the mid-infrared band to detect the presence and concentration of these molecules. Despite being very sensitive and selective, bulky and expensive equipment such as cooled mid-infrared detectors is required for conventional systems. In this paper, we demonstrate a miniature CMOS-compatible Silicon-on-Insulator (SOI) photothermal transducer for mid-infrared spectroscopy which can potentially be made in high volumes and at a low cost. The optical absorption of an analyte in the mid-infrared wavelength range (3.25–3.6 μm) is thermally transduced to an optical transmission change of a microring resonator through the thermo-optic effect in silicon. The photothermal signal is further enhanced by locally removing the silicon substrate beneath the transducer, hereby increasing the effective thermal isolation by a factor of 40. As a proof-of-concept, the absorption spectrum of a polymer that has been locally patterned in the annular region of the resonator was recovered using photothermal spectroscopy. The spectrum is in good agreement with a benchmark Fourier-transform infrared spectroscopy (FTIR) measurement. A normalized noise equivalent absorption coefficient (NNEA) of 7.6 × 10–6 cm–1 W/Hz1/2 is estimated.

Abstract:A compact (1.2 mm 2 ) fully integrated mid-IR spectrometer operating in the 3 μm wavelength range is presented. To our knowledge this is the longest wavelength integrated spectrometer operating in the important wavelength window for spectroscopy of organic compounds. The spectrometer is based on a silicon-on-insulator arrayed waveguide grating filter. An array of InAs 0.91 Sb 0.09 p-i-n photodiodes is heterogeneously integrated on the spectrometers output grating couplers using adhesive bonding. The spectrometer insertion loss is less than 3 dB and the waveguide-referred responsivity of the integrated photodiodes at room temperature is 0.3 A/W.

Heterogeneously integrating III-V materials on silicon photonic integrated circuits has emerged as a promising approach to make advanced laser sources for optical communication and sensing applications. Tunable semiconductor lasers operating in the 2-2.5 µm are of great interest for industrial and medical applications since many gases (e.g., CO2, CO, CH4) and bio-molecules (such as blood glucose) have strong absorption features in this wavelength region. The development of integrated tunable laser sources in this wavelength range enables low-cost and miniature spectroscopic sensors. Here we report heterogeneously integrated widely tunable III-V-on-silicon Vernier lasers using two silicon micro-ring resonators as the wavelength tuning components. The laser has a wavelength tuning range of more than 40 nm near 2.35 µm. By combing two lasers with different distributed Bragg reflectors, a tuning range of more than 70 nm is achieved. Over the whole tuning range, the side mode suppression ratio (SMSR) is higher than 35 dB. As a proof-of-principle, this III-Von-silicon Vernier laser is used to measure the absorption lines of CO. The measurement results match very well with the HITRAN database and indicate that this laser is suitable for broadband spectroscopy.

We report on 2.3x lm wavelength InP-based type-II distributed feedback (DFB) lasers heterogeneously integrated on a silicon photonics integrated circuit. In the devices, a III-V epitaxial layer stack with a "W"-shaped InGaAs/GaAsSb multi-quantum-well active region is adhesively bonded to the first-order silicon DFB gratings. Single mode laser emission coupled to a single mode silicon waveguide with a side mode suppression ratio of 40 dB is obtained. In continuous-wave regime, the 2.32 lm laser operates close to room temperature (above 15 C) and emits more than 1 mW output power with a threshold current density of 1.8 kA/cm 2 at 5 C. A tunable diode laser absorption measurement of CO is demonstrated using this source. Published by AIP Publishing.[http://dx.doi.org/10.1063/1.4971350] Silicon photonics beyond the telecommunication wavelength range attracts interest as it is a promising platform to build compact photonics system for the integrated spectroscopic sensors.1,2 In the 2-3 lm wavelength range, many important gases exhibit narrow and dense absorption lines. Therefore, a number of silicon photonics components have been developed in this wavelength range for spectroscopic gas sensing applications, such as low-loss waveguides, high performance arrayed waveguide grating (AWG) spectrometers and photodetectors. [4][5][6] In order to realize compact silicon photonics gas sensor systems, the on-chip single mode lasers are required. Such single mode silicon photonics laser sources operating at telecommunication wavelengths have been demonstrated over the last years 7-9 but are to be demonstrated in the 2-3 lm wavelength range. Recently, a heterogeneously integrated Fabry-Perot laser on silicon at 2 lm wavelength was demonstrated based on molecular bonding technology and strained InGaAs type-I heterostructures. 10But the emission wavelength of the strained InGaAs type-I material system is limited to around 2.3 lm.11 GaSb-based type-I heterostructures can be used to realize light sources operated in 2-3 lm wavelength range.12-14 However, the heterogeneous processes of GaSb-based material are not as well-established as InP-based material, resulting in low process yield and poor device performance.1 In the recent years, InP-based type-II quantum well lasers with emission wavelengths up to 2.7 lm were reported. 15,16 The lasing wavelength in this material system can possibly be extended to longer wavelengths as photoluminescence up to 3.9 lm wavelength has been demonstrated. 17 These results indicate that compact III-V/silicon photonics sensor systems can be realized by integrating InP-based type-II active structures with silicon. Recently, we demonstrated heterogeneously integrated InP-based type-II Fabry-Perot lasers on silicon photonics integrated circuits (PICs) based on adhesive bonding technology. 18 However, for many spectroscopic sensing applications, a single mode laser is essential.In this paper, we report heterogeneously integrated single mode lasers on a silicon PIC at wavelengths beyond 2 lm, based on an In...