Pao Tai Lin

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

Abstract 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 SiNx 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.

Chip-scale Mid-Infrared chemical sensors using air-clad pedestal silicon waveguides

Towards a future lab-on-a-chip spectrometer, we demonstrate a compact chip-scale air-clad silicon pedestal waveguide as a Mid-Infrared (Mid-IR) sensor capable of in situ monitoring of organic solvents. The sensor is a planar crystalline silicon waveguide, which is highly transparent, between λ = 1.3 and 6.5 μm, so that its operational spectral range covers most characteristic chemical absorption bands due to bonds such as C-H, N-H, O-H, C-C, N-O, C=O, and C≡N, as opposed to conventional UV, Vis, Near-IR sensors, which use weaker overtones of these fundamental bands. To extend light transmission beyond λ = 3.7 μm, a spectral region where a typical silicon dioxide under-clad is absorbing, we fabricate a unique air-clad silicon pedestal waveguide. The sensing mechanism of our Mid-IR waveguide sensor is based on evanescent wave absorption by functional groups of the surrounding chemical molecules, which selectively absorb specific wavelengths in the mid-IR, depending on the nature of their chemical bonds. From a measurement of the waveguide mode intensities, we demonstrate in situ identification of chemical compositions and concentrations of organic solvents. For instance, we show that when testing at λ = 3.55 μm, the Mid-IR sensor can distinguish hexane from the rest of the tested analytes (methanol, toluene, carbon tetrachloride, ethanol and acetone), since hexane has a strong absorption from the aliphatic C-H stretch at λ = 3.55 μm. Analogously, applying the same technique at λ = 3.3 μm, the Mid-IR sensor is able to determine the concentration of toluene dissolved in carbon tetrachloride, because toluene has a strong absorption at λ = 3.3 μm from the aromatic C-H stretch. With our demonstration of an air-clad silicon pedestal waveguide sensor, we move closer towards the ultimate goal of an ultra-compact portable spectrometer-on-a-chip.

Mid-Infrared Spectrometer Using Opto-Nanofluidic Slot-Waveguide for Label-Free On-Chip Chemical Sensing

A mid-infrared (mid-IR) spectrometer for label-free on-chip chemical sensing was developed using an engineered nanofluidic channel consisting of a Si-liquid-Si slot-structure. Utilizing the large refractive index contrast (Δn ∼ 2) between the liquid core of the waveguide and the Si cladding, a broadband mid-IR lightwave can be efficiently guided and confined within a nanofluidic capillary (≤100 nm wide). The optical-field enhancement, together with the direct interaction between the probe light and the analyte, increased the sensitivity for chemical detection by 50 times when compared to evanescent-wave sensing. This spectrometer distinguished several common organic liquids (e.g., n-bromohexane, toluene, isopropanol) accurately and could determine the ratio of chemical species (e.g., acetonitrile and ethanol) at low concentration (<5 μL/mL) in a mixture through spectral scanning over their characteristic absorption peaks in the mid-IR regime. The combination of CMOS-compatible planar mid-IR microphotonics, and a high-throughput nanofluidic sensor system, provides a unique platform for chemical detection.

Air-clad silicon pedestal structures for broadband mid-infrared microphotonics

Toward mid-infrared (mid-IR) silicon microphotonic circuits, we demonstrate broadband on-chip silicon structures, such as: (i) straight and bent waveguides and (ii) beam splitters, utilizing an air-clad pedestal configuration which eliminates the need for typical mid-IR-lossy oxide cladding. We illustrate a sophisticated fabrication process that can create high-quality pedestal structures in crystalline silicon, while preserving its mid-IR transparency. A fundamental waveguide mode is observed between λ=2.5 μm and λ=3.7 μm, and an optical loss of 2.7 dB/cm is obtained at λ=3.7 μm. Our pedestal silicon structures show 50:50 mid-IR power splitting enabling the further development of mid-IR silicon microphotonics.

Planar silicon nitride mid-infrared devices

Integrated mid-infrared devices including (i) straight/bent waveguides and (ii) directional couplers are demonstrated on silicon nitride (SiN) thin films prepared by optimized low-pressure chemical vapor deposition. The deposited SiN film has a broad spectral transparency from visible up to a wavelength of λ = 8.5 μm (as seen from Fourier transform infrared spectroscopy). Our SiN waveguide shows a dominant fundamental mode with an optical loss of 2.1 dB/cm at λ = 3.7 μm. In addition, we demonstrate an efficient SiN directional coupler between λ = 3.55 μm to λ = 3.75 μm where an 8 dB extinction ratio is achieved within each channel upon wavelength scanning. With the inherent advantage of complementary metal–oxide–semiconductor compatibility, our SiN platform paves the way to create sophisticated photonic circuits that are desired for mid-infrared nonlinear light generation and chip-scale biochemical sensors.

Si-CMOS compatible materials and devices for mid-IR microphotonics

CMOS compatible mid-Infrared (mid-IR) microphotonics including (1) broadband SOUP (Silicon on Oxide Undercladding Pedestal) waveguides; and (2) mid-IR transparent chalcogenide glass (ChGs) waveguides monolithically integrated with a PbTe thin film photodetector; are demonstrated. Using a pedestal undercladding geometry we obtain an optical loss for our Si waveguide which is 10 dB/cm lower compared to other waveguides using planar SiO2 cladding at λ = 5 µm, and a fundamental mode is seen over a broad mid-IR spectral range. To realize a fully integrated mid-IR on-chip system, in parallel, we develop PbTe thin film detectors that can be deposited on various mid-IR platforms through a thermal evaporation technique, offering high photoresponsivity of 25 V/W from λ = 1 µm to 4 µm. The detector can be efficiently integrated, using a suitable spacer, to an underlying Chalcogenide glass (ChGs) waveguide. Our results of low loss waveguides and integrated thin film detectors enable Si-CMOS microphotonics for mid-IR applications.

Label-Free Water Sensors Using Hybrid Polymer–Dielectric Mid-Infrared Optical Waveguides

A chip-scale mid-IR water sensor was developed using silicon nitride (SiN) waveguides coated with poly(glycidyl methacrylate) (PGMA). The label-free detection was conducted at λ=2.6-2.7 μm because this spectral region overlaps with the characteristic O-H stretch absorption while being transparent to PGMA and SiN. Through the design of a hybrid waveguide structure, we were able to tailor the mid-IR evanescent wave into the PGMA layer and the surrounding water and, consequently, to enhance the light-analyte interaction. A 7.6 times enhancement of sensitivity is experimentally demonstrated and explained by material integration engineering as well as waveguide mode analysis. Our sensor platform made by polymer-dielectric hybrids can be applied to other regions of the mid-IR spectrum to probe other analytes and can ultimately achieve a multispectral sensor on-a-chip.

Monolithic Mid-Infrared Integrated Photonics Using Silicon-on-Epitaxial Barium Titanate Thin Films

Broadband mid-infrared (mid-IR) photonic circuits that integrate silicon waveguides and epitaxial barium titanate (BTO) thin films are demonstrated using the complementary metal-oxide-semiconductor process. The epitaxial BTO thin films are grown on lanthanum aluminate (LAO) substrates by the pulsed laser deposition technique, wherein a broad infrared transmittance between λ = 2.5 and 7 μm is observed. The optical waveguiding direction is defined by the high-refractive-index amorphous Si (a-Si) ridge structure developed on the BTO layer. Our waveguides show a sharp fundamental mode over the broad mid-IR spectrum, whereas its optical field distribution between the a-Si and BTO layers can be modified by varying the height of the a-Si ridge. With the advantages of broad mid-IR transparency and the intrinsic electro-optic properties, our monolithic Si on a ferroelectric BTO platform will enable tunable mid-IR microphotonics that are desired for high-speed optical logic gates and chip-scale biochemical sensors.

Direct Electrospray Printing of Gradient Refractive Index Chalcogenide Glass Films.

A spatially varying effective refractive index gradient using chalcogenide glass layers is printed on a silicon wafer using an optimized electrospray (ES) deposition process. Using solution-derived glass precursors, IR-transparent Ge23Sb7S70 and As40S60 glass films of programmed thickness are fabricated to yield a bilayer structure, resulting in an effective gradient refractive index (GRIN) film. Optical and compositional analysis tools confirm the optical and physical nature of the gradient in the resulting high-optical-quality films, demonstrating the power of direct printing of multimaterial structures compatible with planar photonic fabrication protocols. The potential application of such tailorable materials and structures as they relate to the enhancement of sensitivity in chalcogenide glass based planar chemical sensor device design is presented. This method, applicable to a broad cross section of glass compositions, shows promise in directly depositing GRIN films with tunable refractive index profiles for bulk and planar optical components and devices.

Ferroelectric barium titanate for mid-infrared integrated photonics

Mid-Infrared (mid-IR) photonic using barium titanate (BaTiO3) thin films are demonstrated through the complementary metal-oxide-semiconductor (CMOS) process. The BaTiO3 thin films have broad infrared transmittance between λ = 2.5 μm to λ = 7 um. Our waveguides show a fundamental mode over broad mid-IR regime. With the broad mid-IR transparency and electro-optic (E-O) property, our BaTiO3 platform will enable mid-IR microphotonics that is desired for high-speed optical circuits.