Room-Temperature Phosphorescence and Low-Energy Induced Direct Triplet Excitation of Alq3 Engineered Crystals.

Crystal engineering is a practical approach for tailoring material properties. This approach has been widely studied for modulating optical and electrical properties of semiconductors. However, the properties of organic molecular crystals are difficult to control following a similar engineering route. In this Letter, we demonstrate that engineered crystals of Alq3 and Ir(ppy)3 complexes, which are commonly used in organic light-emitting technologies, possess intriguing functional properties. Specifically, these structures not only process efficient low-energy induced triplet excitation directly from the ground state of Alq3 but also can show strong emission at the Alq3 triplet energy level at room temperatures. We associate these phenomena with local deformations of the host matrix around the guest molecules, which in turn lead to a stronger host-guest triplet-triplet coupling and spin-orbital mixing.

Advanced One- and Two-Dimensional Mesh Designs for Injectable Electronics.

The unique structure and mechanical properties of syringe-injectable mesh electronics have enabled seamless tissue integration and stable chronic recording of the activities of the same neurons on a year scale. Here, we report studies of a series of structural and mechanical mesh electronics design variations that allow injection using needles at least 4-fold smaller than those previously reported to minimize the footprint during injection of the electronics in soft matter and tissue. Characterization of new ultraflexible two-dimensional (2D) and one-dimensional (1D) probes has demonstrated reproducible injection of the newly developed mesh electronics designs via needles as small as 100 µm in inner diameter (ID) with reduced injection volumes. In vitro hydrogel and in vivo mouse brain studies have shown that ultraflexible 2D and 1D probes maintain their structural integrity and conformation post-injection after being transferred through the reduced diameter needles. In addition, analysis of the variation of the post-injection mesh cross sections suggests a smaller degree of tissue deformation and relaxation with decreasing needle diameters. The capability to implement rational design for mesh electronic probes that can be delivered via much smaller diameter needles should open up new opportunities for integration of electronics with tissue and soft matter in fundamental and translational studies.

Flexible Mid-infrared Photonic Circuits for Real-time and Label-Free Hydroxyl Compound Detection.

Chip-scale chemical detections were demonstrated by mid-Infrared (mid-IR) integrated optics made by aluminum nitride (AlN) waveguides on flexible borosilicate templates. The AlN film was deposited using sputtering at room temperature, and it exhibited a broad infrared transmittance up to λ = 9 µm. The AlN waveguide profile was created by microelectronic fabrication processes. The sensor is bendable because it has a thickness less than 30 µm that significantly decreases the strain. A bright fundamental mode was obtained at λ = 2.50-2.65 µm without mode distortion or scattering observed. By spectrum scanning at the -OH absorption band, the waveguide sensor was able to identify different hydroxyl compounds, such as water, methanol, and ethanol, and the concentrations of their mixtures. Real-time methanol monitoring was achieved by reading the intensity change of the waveguide mode at λ = 2.65 µm, which overlap with the stretch absorption of the hydroxyl bond. Due to the advantages of mechanical flexibility and broad mid-IR transparency, the AlN chemical sensor will enable microphotonic devices for wearables and remote biomedical and environmental detection.

Mid-Infrared Chalcogenide Waveguides for Real-Time and Nondestructive Volatile Organic Compound Detection.

A mid-infrared (mid-IR) sensor chip was demonstrated for volatile organic compound (VOC) detection. The sensor consisted of As2Se3 optical waveguides built by microelectronic fabrication processes. The VOC sensing performance was characterized by measuring acetone and ethanol vapors at their characteristic C-H absorption from λ = 3.40 to 3.50 µm. Continuous VOC detection with <5 s response time was achieved by measuring the intensity attenuation of the waveguide mode. The miniaturized noninvasive VOC sensor can be applied to breath analysis and environmental toxin monitoring.

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.

Real-Time and Label-Free Chemical Sensor-on-a-chip using Monolithic Si-on-BaTiO3 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.

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.

Peptide-Decorated Tunable-Fluorescence Graphene Quantum Dots.

We report here the synthesis of graphene quantum dots with tunable size, surface chemistry, and fluorescence properties. In the size regime 15-35 nm, these quantum dots maintain strong visible light fluorescence (mean quantum yield of 0.64) and a high two-photon absorption (TPA) cross section (6500 Göppert-Mayer units). Furthermore, through noncovalent tailoring of the chemistry of these quantum dots, we obtain water-stable quantum dots. For example, quantum dots with lysine groups bind strongly to DNA in solution and inhibit polymerase-based DNA strand synthesis. Finally, by virtue of their mesoscopic size, the quantum dots exhibit good cell permeability into living epithelial cells, but they do not enter the cell nucleus.

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.