Hao-Yu Greg Lin

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.

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.

Real-Time and Label-Free Chemical Sensor-on-a-chip using Monolithic Si-on-BaTiO 3 Mid-Infrared waveguides

Chip-scale chemical detection is demonstrated by using mid-Infrared (mid-IR) photonic circuits consisting of amorphous silicon (a-Si) waveguides on an epitaxial barium titanate (BaTiO3, BTO) thin film. The highly c-axis oriented BTO film was grown by the pulsed laser deposition (PLD) method and it exhibits a broad transparent window from λ = 2.5 μm up to 7 μm. The waveguide structure was fabricated by the complementary metal–oxide–semiconductor (CMOS) process and a sharp fundamental waveguide mode has been observed. By scanning the spectrum within the characteristic absorption regime, our mid-IR waveguide successfully perform label-free monitoring of various organic solvents. The real-time heptane detection is accomplished by measuring the intensity attenuation at λ = 3.0–3.2 μm, which is associated with -CH absorption. While for methanol detection, we track the -OH absorption at λ = 2.8–2.9 μm. Our monolithic Si-on-BTO waveguides establish a new sensor platform that enables integrated photonic device for label-free chemical detection.

Monolithically Integrated Si-on-AlN Mid-Infrared Photonic Chips for Real-Time and Label-Free Chemical Sensing

Chip-scale chemical sensors were demonstrated using optical waveguides consisting of amorphous silicon (a-Si) and aluminum nitride (AlN). A mid-infrared (mid-IR) transparent AlN thin film was prepared by room-temperature sputtering, which exhibited high Al/N elemental homogeneity. The Si-on-AlN waveguides were fabricated by a complementary metal-oxide-semiconductor process. A sharp fundamental mode and low optical loss of 2.21 dB/cm were obtained. Label-free chemical identification and real-time monitoring were performed by scanning the mode spectrum while the waveguide was exposed to various chemicals. Continuous tracing of heptane and methanol was accomplished by measuring the waveguide intensity attenuation at λ = 2.5-3.0 μm, which included the characteristic -CH and -OH absorptions. The monolithically integrated Si-on-AlN waveguides established a new sensor platform that can operate over a broad mid-IR regime, thus enabling photonic chips for label-free chemical detection.

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

A mid-infrared sensor for label-free on-chip chemical detection was developed using an engineered nanofluidic channel consisting of a Si-liquid-Si slot-structure. A sensitivity with 75 times improvement was achieved compared to conventional evanescent-wave sensing. Mid-infrared spectroscopy is a detection technique commonly used for identifying biochemicals and tracing of toxic molecules, and is free of target labels and sensor surface functionalization. The use of mid-IR spectrum circumvents the need for labeling the sample, because the characteristic wavelength of absorption by many functional groups present in chemical or biological molecules falls within this region of the spectrum. Herein, we present a new chip-scale optofluidic device that utilizes mid-IR techniques for label-free and surface functionalization-free chemical sensing. The optofluidic platform is built using CMOS processes, and is capable of accomplishing broad mid-IR spectral sensing. Fig. 1 schematically illustrates the structure of the mid-IR opto-nanofluidic device. The sensing element is a nanofluidic-channel slot-waveguide with its two ends connected to Si-SiO2-Si slot-waveguides. We embed the entire nanofluidic channel and part of the silicon-oxide slot-waveguides in the PDMS chamber. Upon filling the interior of the chamber with liquid analyte, the solution inside the nanofluidic channel converts the fluid-filled channel into a fluidic slot-waveguide. The mid-IR probe light, after passing through the nanofluidic channel, propagates into the second SiO2 slot-waveguide at the other end. The transmitted light is encoded with the absorption spectrum of the analyte in the fluid because the absorption of probe light by the analyte that fills the nanofluidic channel heavily modulates the intensity of the guided light at the characteristic absorption wavelengths. The enhancement of chemical sensitivity of our fluidic slot-waveguide is evaluated. Fig. 2 (a) compares the predicted optical-field profiles for propagating mid-IR (? = 3.3 μm) radiation within a rectangular-strip waveguide, to that of a nanofluidic slot waveguide. In the case of the rectangular strip-waveguide, the optical field is mainly retained inside the Si core and its penetration as an evanescent wave into the surrounding fluid is small. In the slot-waveguide the optical field is highly concentrated at the center of the fluidic channel and interacts strongly with the liquid inside the channel. Thus, even a slight change in the concentration of analyte will result in a significant modulation of intensity to the guided mid-IR that consequently boosts the sensitivity when sensing chemicals. From the plot in Fig. 2 (b), the enhancement-factor Sslot/Sstrip rises to 75 times as the slot-width narrows to d = 80 nm.

Low-stress silicon nitride platform for broadband mid-infrared microphotonics

We experimentally demonstrate a sophisticated mid-IR microphotonics platform adopting engineered Si-rich and low-stress silicon nitride (SiNx) thin films where an extensive infrared transparency up to λ = 8.5 µm is achieved. Furthermore, because of the designed low-stress property, the SiNx deposition is able to reach a thickness > 2 µm that significantly reduces mid-IR waveguide loss to less than 0.2 dB/cm. We show directional couplers functioning over a broad infrared spectrum, thus enabling monolithic mid-IR multiplexing schemes for integrated linear and nonlinear photonics leading to sophisticated label-free sensing technologies.

Low-Stress Silicon Nitride for Mid-Infrared Microphotonics

We experimentally demonstrate a sophisticated mid-IR microphotonics platform adopting engineered Si-rich and low-stress silicon nitride thin films. An extensive infrared transparency up to λ = 8.5 µm and efficient directional couplers are achieved.