Fabrication and characterization of polymer optical waveguide Bragg grating for pulse signal sensing.

Polymer materials have the advantages of a low Young's modulus and low-cost preparation process. In this paper, a polymer-based optical waveguide pressure sensor based on a Bragg structure is proposed. The change in the Bragg wavelength in the output spectrum of the waveguide Bragg grating (WBG) is used to linearly characterize the change in pressure acting on the device. The polymer-based WBG was developed through a polymer film preparation process, and the experimental results show that the output signal of the device has a sensitivity of 1.275 nm/kPa with a measurement range of 0-12 kPa and an accuracy of 1 kPa. The experimental results indicate that the device already perfectly responds to a pulse signal. It has significant potential application value in medical diagnostics and health testing, such as blood pressure monitoring, sleep quality monitoring, and tactile sensing.

Microring structure for flexible polymer waveguide-based optical pressure sensing.

Flexible pressure sensors provide a promising platform for artificial smart skins, and photonic devices provide a new technique to fabricate pressure sensors. Here, we present a flexible waveguide-based optical pressure sensor based on a microring structure. The waveguide-based optical pressure sensor is based on a five-cascade microring array structure with a size of 1500 µm × 500 µm and uses the change in output power to linearly characterize the change in pressure acting on the device. The results show that the device has a sensing range of 0-60 kPa with a sensitivity of 23.14 µW/kPa, as well as the ability to detect pulse signals, swallowing, hand gestures, etc. The waveguide-based pressure sensors offer the advantages of good output linearity, high integration density and easy-to-build arrays.

A dataset of color QR codes generated using back-compatible and random colorization algorithms exposed to different illumination-capture channel conditions.

Color QR Codes are often generated to encode digital information, but one also could use colors or to allocate colors in a QR Code to act as a color calibration chart. In this dataset, we present several thousand QR Codes images generated with two different colorization algorithms (random and back-compatible) and several tuning variables in these color encoding. The QR Codes were also exposed to three different channel conditions (empty, augmentation and real-life). Also, we derive the SNR and BER computations for these QR Code in comparison with their black and white versions. Finally, we also show if ZBar, a commercial QR Code scanner, is able to read them.

Silicon-photonics-based waveguide Bragg grating sensor for blood glucose monitoring.

We demonstrated the design of two different structures, a two-sided structure and a top-surface structure, of glucose waveguide Bragg grating (WBG) sensors in a single-mode silicon-on-insulator (SOI) chip. A two-sided WBG structure was fabricated, and chip preparation was realized by lithography and other processes. A photonic platform for testing the two-sided WBG using glucose was built and completed. When the blood glucose concentration changed by 1 mg/mL, the two-sided WBG had a wavelength offset of 78 pm. The experimental results show that the two structures can achieve the sensing of different blood glucose concentrations. The two-sided WBG had better sensing performance and thus has a wide range of application prospects.

Pursuing the Diffraction Limit with Nano-LED Scanning Transmission Optical Microscopy.

Recent research into miniaturized illumination sources has prompted the development of alternative microscopy techniques. Although they are still being explored, emerging nano-light-emitting-diode (nano-LED) technologies show promise in approaching the optical resolution limit in a more feasible manner. This work presents the exploration of their capabilities with two different prototypes. In the first version, a resolution of less than 1 µm was shown thanks to a prototype based on an optically downscaled LED using an LED scanning transmission optical microscopy (STOM) technique. This research demonstrates how this technique can be used to improve STOM images by oversampling the acquisition. The second STOM-based microscope was fabricated with a 200 nm GaN LED. This demonstrates the possibilities for the miniaturization of on-chip-based microscopes.

The Structural, Electronic, and Optical Properties of Ge/Si Quantum Wells: Lasing at a Wavelength of 1550 nm.

The realization of a fully integrated group IV electrically driven laser at room temperature is an essential issue to be solved. We introduced a novel group IV side-emitting laser at a wavelength of 1550 nm based on a 3-layer Ge/Si quantum well (QW). By designing this scheme, we showed that the structural, electronic, and optical properties are excited for lasing at 1550 nm. The preliminary results show that the device can produce a good light spot shape convenient for direct coupling with the waveguide and single-mode light emission. The laser luminous power can reach up to 2.32 mW at a wavelength of 1550 nm with a 300-mA current. Moreover, at room temperature (300 K), the laser can maintain maximum light power and an ideal wavelength (1550 nm). Thus, this study provides a novel approach to reliable, efficient electrically pumped silicon-based lasers.

Directly addressable GaN-based nano-LED arrays: fabrication and electro-optical characterization.

The rapid development of display technologies has raised interest in arrays of self-emitting, individually controlled light sources atthe microscale. Gallium nitride (GaN) micro-light-emitting diode (LED) technology meets this demand. However, the current technology is not suitable for the fabrication of arrays of submicron light sources that can be controlled individually. Our approach is based on nanoLED arrays that can directly address each array element and a self-pitch with dimensions below the wavelength of light. The design and fabrication processes are explained in detail and possess two geometries: a 6 × 6 array with 400 nm LEDs and a 2 × 32 line array with 200 nm LEDs. These nanoLEDs are developed as core elements of a novel on-chip super-resolution microscope. GaN technology, based on its physical properties, is an ideal platform for such nanoLEDs.

Continuous Live-Cell Culture Imaging and Single-Cell Tracking by Computational Lensfree LED Microscopy.

Continuous cell culture monitoring as a way of investigating growth, proliferation, and kinetics of biological experiments is in high demand. However, commercially available solutions are typically expensive and large in size. Digital inline-holographic microscopes (DIHM) can provide a cost-effective alternative to conventional microscopes, bridging the gap towards live-cell culture imaging. In this work, a DIHM is built from inexpensive components and applied to different cell cultures. The images are reconstructed by computational methods and the data are analyzed with particle detection and tracking methods. Counting of cells as well as movement tracking of living cells is demonstrated, showing the feasibility of using a field-portable DIHM for basic cell culture investigation and bringing about the potential to deeply understand cell motility.

A Parts Per Billion (ppb) Sensor for NO2 with Microwatt (µW) Power Requirements Based on Micro Light Plates.

A film of gas sensitive ZnO nanoparticles has been coupled with a low-power micro light plate (µLP) to achieve a NO2-parts-per-billion conductometric gas sensor operating at room temperature. In this µLP configuration, an InGaN-based LED (emitting at 455 nm) is integrated at a few hundred nanometers distance from the sensor material, leading to sensor photoactivation with well controlled, uniform, and high irradiance conditions, and very low electrical power needs. The response curves to different NO2 concentrations as a function of the irradiance displayed a bell-like shape. Responses of 20% to 25 ppb of NO2 were already observed at irradiances of 5 mWatts·cm-2 (applying an electrical power as low as 30 µW). In the optimum illumination conditions (around 60 mWatts·cm-2, or 200 µW of electric power), responses of 94% to 25 ppb were achieved, corresponding to a lower detection limit of 1 ppb of NO2. Higher irradiance values worsened the sensor response in the parts-per-billion range of NO2 concentrations. The responses to other gases such as NH3, CO, and CH4 were much smaller, showing a certain selectivity toward NO2. The effects of humidity on the sensor response are also discussed.

Highly Specific and Wide Range NO2 Sensor with Color Readout.

We present a simple and inexpensive method to implement a Griess-Saltzman-type reaction that combines the advantages of the liquid phase method (high specificity and fast response time) with the benefits of a solid implementation (easy to handle). We demonstrate that the measurements can be carried out using conventional RGB sensors; circumventing all the limitations around the measurement of the samples with spectrometers. We also present a method to optimize the measurement protocol and target a specific range of NO2 concentrations. We demonstrate that it is possible to measure the concentration of NO2 from 50 ppb to 300 ppm with high specificity and without modifying the Griess-Saltzman reagent.

DNA-Origami-Driven Lithography for Patterning on Gold Surfaces with Sub-10 nm Resolution.

Sub-10 nm lithography of DNA patterns is achieved using the DNA-origami stamping method. This new strategy utilizes DNA origami to bind a preprogrammed DNA ink pattern composed of thiol-modified oligonucleotides on gold surfaces. Upon denaturation of the DNA origami, the DNA ink pattern is exposed. The pattern can then be developed by hybridization with complementary strands carrying gold nanoparticles.

Energetics and carrier transport in doped Si/SiO2 quantum dots.

In the present theoretical work we have considered impurities, either boron or phosphorous, located at different substitutional sites in silicon quantum dots (Si-QDs) with diameters around 1.5 nm, embedded in a SiO2 matrix. Formation energy calculations reveal that the most energetically-favored doping sites are inside the QD and at the Si/SiO2 interface for P and B impurities, respectively. Furthermore, electron and hole transport calculations show in all the cases a strong reduction of the minimum voltage threshold, and a corresponding increase of the total current in the low-voltage regime. At higher voltages, our findings indicate a significant increase of transport only for P-doped Si-QDs, while the electrical response of B-doped ones does not stray from the undoped case. These findings are of support for the employment of doped Si-QDs in a wide range of applications, such as Si-based photonics or photovoltaic solar cells.

Polarity-driven polytypic branching in cu-based quaternary chalcogenide nanostructures.

An appropriate way of realizing property nanoengineering in complex quaternary chalcogenide nanocrystals is presented for Cu2CdxSnSey(CCTSe) polypods. The pivotal role of the polarity in determining morphology, growth, and the polytypic branching mechanism is demonstrated. Polarity is considered to be responsible for the formation of an initial seed that takes the form of a tetrahedron with four cation-polar facets. Size and shape confinement of the intermediate pentatetrahedral seed is also attributed to polarity, as their external facets are anion-polar. The final polypod extensions also branch out as a result of a cation-polarity-driven mechanism. Aberration-corrected scanning transmission electron microscopy is used to identify stannite cation ordering, while ab initio studies are used to show the influence of cation ordering/distortion, stoichiometry, and polytypic structural change on the electronic band structure.

Band engineered epitaxial 3D GaN-InGaN core-shell rod arrays as an advanced photoanode for visible-light-driven water splitting.

3D single-crystalline, well-aligned GaN-InGaN rod arrays are fabricated by selective area growth (SAG) metal-organic vapor phase epitaxy (MOVPE) for visible-light water splitting. Epitaxial InGaN layer grows successfully on 3D GaN rods to minimize defects within the GaN-InGaN heterojunctions. The indium concentration (In ∼ 0.30 ± 0.04) is rather homogeneous in InGaN shells along the radial and longitudinal directions. The growing strategy allows us to tune the band gap of the InGaN layer in order to match the visible absorption with the solar spectrum as well as to align the semiconductor bands close to the water redox potentials to achieve high efficiency. The relation between structure, surface, and photoelectrochemical property of GaN-InGaN is explored by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), current-voltage, and open circuit potential (OCP) measurements. The epitaxial GaN-InGaN interface, pseudomorphic InGaN thin films, homogeneous and suitable indium concentration and defined surface orientation are properties demanded for systematic study and efficient photoanodes based on III-nitride heterojunctions.

Localized growth and in situ integration of nanowires for device applications.

Simultaneous localized growth and device integration of inorganic nanostructures on heated micromembranes is demonstrated for single crystalline germanium and tin oxide nanowires. Fully operating CO gas sensors prove the potential of the presented approach. With this simple CMOS compatible technique, issues of assembly, transfer and contact formation are addressed.

Characterization of individual barium titanate nanorods and their assessment as building blocks of new circuit architectures.

In this work, we report on the integration of individual BaTiO(3) nanorods into simple circuit architectures. Polycrystalline BaTiO(3) nanorods were synthesized by electrophoretic deposition (EPD) of barium titanate sol into aluminium oxide (AAO) templates and subsequent annealing. Transmission electron microscopy (TEM) observations revealed the presence of slabs of hexagonal polymorphs intergrown within cubic grains, resulting from the local reducing atmosphere during the thermal treatment. Electrical measurements performed on individual BaTiO(3) nanorods revealed resistivity values between 10 and 100 Ω cm, which is in good agreement with typical values reported in the past for oxygen-deficient barium titanate films. Consequently the presence of oxygen vacancies in their structure was indirectly validated. Some of these nanorods were tested as proof-of-concept humidity sensors. They showed reproducible responses towards different moisture concentrations, demonstrating that individual BaTiO(3) nanorods may be integrated in complex circuit architectures with functional capacities.

Effectiveness of nitrogen incorporation to enhance the photoelectrochemical activity of nanostructured TiO2:NH3 versus H2-N2 annealing.

Highly ordered TiO(2) nanohole layers were synthesized by anodic oxidation of titanium foils using ethylene glycol and ammonium fluoride as the electrolyte. The effectiveness of different methods, namely annealing at 500 °C in NH(3) and in H(2) diluted in N(2), to incorporate nitrogen into TiO(2) and thus extend its photoelectrochemical (PEC) activity to the visible range was studied. The intra-gap levels introduced by both processes were identified by means of XPS and PL measurements. Water splitting experiments demonstrated that annealing in H(2) improved the photocatalytic activity of pure TiO(2), while annealing in ammonia led to a decrease in the PEC performance.

On the photoconduction properties of low resistivity TiO2 nanotubes.

TiO(2) nanotubes were synthesized by anodic oxidation of titanium foils using dimethyl sulfoxide and hydrofluoric acid as the electrolyte. The electrical properties of individual nanotube-based devices were evaluated and modeled after exposing some of them to different gas and illumination conditions. Resistivity values fully comparable to those of TiO(2) single crystal anatase (ρ(SA) = 1.09 ± 0.01Ω cm) were found, and their photoconductive characteristics, explained in terms of the Shockley-Read-Hall model for non-radiative recombination in semiconductors, were found to be strongly influenced by the applied experimental conditions such as the surrounding atmosphere. These devices may have potential applications in photocatalytic processes, such as CO(2) reduction or H(2)O splitting, avoiding the interfering effects typical of nanotube arrays.

A model for the response towards oxidizing gases of photoactivated sensors based on individual SnO2 nanowires.

The paper presents a quantitative model to elucidate the role of impinging photons on the final response towards oxidizing gases of light-activated metal oxide gas sensors. The model is based on the competition between oxygen molecules in air and oxidizing target gases (such as NO(2)) for the same adsorption sites: the surface oxygen vacancies (OV). The model fairly reproduces the experimental measurements of both the steady-state and the dynamic response of individual SnO(2) nanowires towards oxidizing gases. Quantitative results indicate that: (1) at room temperature NO(2) adsorbs onto OV more avidly than oxygen; (2) the flux of photons and the NO(2) concentration determine the partition of the two gas populations at the surface; and (3) the band-to-band generation of electron-hole pairs plays a significant role in the photodesorption process of gas molecules. The model also offers a methodology to estimate some fundamental parameters, such as the adsorption rates and the photodesorption cross sections of oxidizing molecules interacting with the nanowires' surface. All these results, enabled by the use of individual nanowires, provide deep insight about how to control the response of metal oxide nanowires towards oxidizing gases, paving the way to the development and consolidation of this family of low consumption conductometric sensors operable at room temperature.