Photonic Integrated Circuit Based Temperature Sensor forOut-of-Autoclave Composite Parts Production Monitoring.

The use of composite materials has seen widespread adoption in modern aerospace industry. This has been facilitated due to their favourable mechanical characteristics, namely, low weight and high stiffness and strength. For broader implementation of those materials though, the out-of-autoclave production processes have to be optimized, to allow for higher reliability of the parts produced as well as cost reduction and improved production speed. This optimization can be achieved by monitoring and controlling resin filling and curing cycles. Photonic Integrated Circuits (PICs), and, in particular, Silicon Photonics, owing to their fast response, small size, ability to operate at higher temperatures, immunity to electromagnetic interference, and compatibility with CMOS fabrication techniques, can offer sensing solutions fulfilling the requirements for composite material production using carbon fibres. In this paper, we demonstrate a passive optical temperature sensor, based on a 220 nm height Silicon-on-Insulator platform, embedded in a composite tool used for producing RTM-6 composite parts of high quality (for use in the aerospace industry). The design methodology of the photonic circuit as well as the experimental results and comparison with the industry standard thermocouples during a thermal cycling of the tool are presented. The optical sensor exhibits high sensitivity (85 pm/°C), high linearity (R2 = 0.944), and is compatible with the RTM-6 production process, operating up to 180 °C.

Monolithic integration of microlenses on the backside of a silicon photonics chip for expanded beam coupling.

To increase the manufacturing throughput and lower the cost of silicon photonics packaging, an alignment tolerant approach is required to simplify the process of fiber-to-chip coupling. Here, we demonstrate an alignment-tolerant expanded beam backside coupling interface (in the O-band) for silicon photonics by monolithically integrating microlenses on the backside of the chip. After expanding the diffracted optical beam from a TE-mode grating through the bulk silicon substrate, the beam is collimated with the aid of microlenses resulting in an increased coupling tolerance to lateral and longitudinal misalignment. With an expanded beam diameter of 32 µm, a ±7 µm lateral and a ±0.6° angular fiber-to-microlens 1-dB alignment tolerance is demonstrated at the wavelength of 1310 nm. Also, a large 300 µm longitudinal alignment tolerance with a 0.2 dB drop in coupling efficiency is obtained when the collimated beam from the microlens is coupled into a thermally expanded core single-mode fiber.

Design and fabrication of blazed gratings for a waveguide-type head mounted display.

In a waveguide-type display for augmented reality, the image is injected in the waveguide and extracted in front of the eye appearing superimposed on the real-world scene. An elegant and compact way of coupling these images in and out is by using blazed gratings, which can achieve high diffraction efficiencies. We report the design of blazed gratings for green light (λ = 543 nm) and a diffraction angle of 43°. The blazed gratings with a pitch of 508 nm and a fill factor of 0.66 are fabricated using grayscale electron beam lithography. We outline the subsequent replication in a polymer waveguide material with ultraviolet nanoimprint lithography and confirm a throughput efficiency of 17.4%. We finally show the in- and outcoupling of an image through two blazed gratings appearing sharp and non-distorted in the environment.

Imprinted Polymer-Based Guided Mode Resonance Grating Strain Sensors.

Optical sensors based on guided mode resonance (GMR) realized in polymers are promising candidates for sensitive and cost effective strain sensors. The benefit of GMR grating sensors is the non-contact, easy optical read-out with large working distance, avoiding costly alignment and packaging procedures. The GMR gratings with resonance around 850-900 nm are fabricated using electron beam lithography and replicated using a soft stamp based imprinting technique on 175 µ m-thick foils to make them suitable for optical strain sensing. For the strain measurements, foils are realized with both GMR gratings and waveguides with Bragg gratings. The latter are used as reference sensors and allow extracting the absolute strain sensitivity of the GMR sensor foils. Following this method, it is shown that GMR gratings have an absolute strain sensitivity of 1.02 ± 0.05 p m / µ ϵ at 870 nm.

All-organic switching polarizer based on polymer waveguides and liquid crystals.

This paper reports on the design, fabrication and characterization of an all-organic photonic integrated circuit working as a switching polarizer for visible light (630nm), combining organic waveguides and liquid crystals that can be electrically driven. The device was made in commercially available epoxy by laser direct writing lithography. A device with a 2dB loss and a 20dB extinction ratio for both polarizations, was simulated; the manufactured devices proved the working principle of the design. The results have led to the design of a switching polarization splitter, in which a careful choice of waveguide material and liquid crystal can lead to devices working on a wide range of wavelengths.

Bragg-Grating-Based Photonic Strain and Temperature Sensor Foils Realized Using Imprinting and Operating at Very Near Infrared Wavelengths.

Thin and flexible sensor foils are very suitable for unobtrusive integration with mechanical structures and allow monitoring for example strain and temperature while minimally interfering with the operation of those structures. Electrical strain gages have long been used for this purpose, but optical strain sensors based on Bragg gratings are gaining importance because of their improved accuracy, insusceptibility to electromagnetic interference, and multiplexing capability, thereby drastically reducing the amount of interconnection cables required. This paper reports on thin polymer sensor foils that can be used as photonic strain gage or temperature sensors, using several Bragg grating sensors multiplexed in a single polymer waveguide. Compared to commercially available optical fibers with Bragg grating sensors, our planar approach allows fabricating multiple, closely spaced sensors in well-defined directions in the same plane realizing photonic strain gage rosettes. While most of the reported Bragg grating sensors operate around a wavelength of 1550 nm, the sensors in the current paper operate around a wavelength of 850 nm, where the material losses are the lowest. This was accomplished by imprinting gratings with pitches 280 nm, 285 nm, and 290 nm at the core-cladding interface of an imprinted single mode waveguide with cross-sectional dimensions 3 × 3 µm². We show that it is possible to realize high-quality imprinted single mode waveguides, with gratings, having only a very thin residual layer which is important to limit bend losses or cross-talk with neighboring waveguides. The strain and temperature sensitivity of the Bragg grating sensors was found to be 0.85 pm/µÎµ and -150 pm/°C, respectively. These values correspond well with those of previously reported sensors based on the same materials but operating around 1550 nm, taking into account that sensitivity scales with the wavelength.

Microfabricated devices for single objective single plane illumination microscopy (SoSPIM).

Light sheet microscopy is a relatively new form of fluorescence microscopy that has been receiving a lot of attention recently. The strong points of the technique, such as high signal to noise ratio and its reduced photodamage of fluorescently labelled samples, come from its unique feature to illuminate only a thin plane in the sample that coincides with the focal plane of the detection lens. Typically this requires two closely positioned perpendicular objective lenses, one for detection and one for illumination. Apart from the fact that this special configuration of objective lenses is incompatible with standard microscope bodies, it is particularly problematic for high-resolution lenses which typically have a short working distance. To address these issues we developed sample holders with an integrated micromirror to perform single lens light sheet microscopy, also known as single objective single plane illumination microscopy (SoSPIM). The first design is based on a wet-etched silicon substrate, the second on a microfabricated polished polymer plug. We achieved an on-chip light sheet thickness of 2.3 µm (FWHM) at 638 nm with the polymer micromirror and of 1.7 µm (FWHM) at 638 nm with the silicon micromirror, comparable to reported light sheet thicknesses obtained on dedicated light sheet microscopes. A marked contrast improvement was obtained with both sample holders as compared to classic epi-fluorescence microscopy. In order to evaluate whether this technology could be made available on a larger scale, in a next step we evaluated the optical quality of inexpensive replicas from both types of master molds. We found that replicas from the polished polymer based mold have an optical quality close to that of the master component, while replicas from the silicon based mold were of slightly lower but still acceptable quality. The suitability of the replicated polymer based sample holder for single-lens light sheet microscopy was finally demonstrated by imaging breast cancer spheroids.

Laser ablation- and plasma etching-based patterning of graphene on silicon-on-insulator waveguides.

We present a new approach to remove monolayer graphene transferred on top of a silicon-on-insulator (SOI) photonic integrated chip. Femtosecond laser ablation is used for the first time to remove graphene from SOI waveguides, whereas oxygen plasma etching through a metal mask is employed to peel off graphene from the grating couplers attached to the waveguides. We show by means of Raman spectroscopy and atomic force microscopy that the removal of graphene is successful with minimal damage to the underlying SOI waveguides. Finally, we employ both removal techniques to measure the contribution of graphene to the loss of grating-coupled graphene-covered SOI waveguides using the cut-back method.

Stretchable optical waveguides.

We introduce the concept of mechanically stretchable optical waveguides. The technology to fabricate these waveguides is based on a cost-efficient replication method, employing commercially available polydimethylsiloxane (PDMS) materials. Furthermore, VCSELs (λ = 850 nm) and photodiodes, embedded in a flexible package, were integrated with the waveguides to obtain a truly bendable, stretchable and mechanically deformable optical link. Since these sources and detectors were integrated, it was possible to determine the influence of bending and stretching on the waveguide performance.

Fabrication of a laser patterned flexible organic light-emitting diode on an optimized multilayered barrier.

The fast-growing market of organic electronics stimulates the development of versatile technologies for structuring thin-film materials. Ultraviolet lasers have proven their full potential for patterning organic thin films, but only a few studies report on interaction with thin-film barrier layers. In this paper, we present an approach in which the laser patterning process is optimized together with the barrier film, leading to a highly selective patterning technology without introducing barrier damage. This optimization is crucial, as the barrier damage would lead to moisture and oxygen ingress, with accelerated device degradation as a result. Following process optimization, a laser processed flexible organic LED has been fabricated and thin-film encapsulated and its operation is shown for the first time in atmospheric conditions.

Patterning of flexible organic light emitting diode (FOLED) stack using an ultrafast laser.

A femtosecond laser has been successfully utilized for patterning thin Flexible Organic Light Emitting Diode (FOLED) structures of individual layer thickness around 100nm. The authors report in this paper a step-like ablation behavior at the layer interfaces which accounts for a local removal of entire layers. Various surface analyzing techniques are used to investigate the morphologies and chemical compositions within and in the vicinity of the ablation areas. This study opens a new avenue in selectively ablating different layers from a multilayer stack on flexible substrates using fs lasers allowing post deposition structuring of large area flexible organic electronic devices.

Laser-induced forward transfer for flip-chip packaging of single dies.

Flip-chip (FC) packaging is a key technology for realizing high performance, ultra-miniaturized and high-density circuits in the micro-electronics industry. In this technique the chip and/or the substrate is bumped and the two are bonded via these conductive bumps. Many bumping techniques have been developed and intensively investigated since the introduction of the FC technology in 1960(1) such as stencil printing, stud bumping, evaporation and electroless/electroplating2. Despite the progress that these methods have made they all suffer from one or more than one drawbacks that need to be addressed such as cost, complex processing steps, high processing temperatures, manufacturing time and most importantly the lack of flexibility. In this paper, we demonstrate a simple and cost-effective laser-based bump forming technique known as Laser-induced Forward Transfer (LIFT)3. Using the LIFT technique a wide range of bump materials can be printed in a single-step with great flexibility, high speed and accuracy at RT. In addition, LIFT enables the bumping and bonding down to chip-scale, which is critical for fabricating ultra-miniature circuitry.

Enzyme-gelatin electrochemical biosensors: scaling down.

In this article we investigate the possibility of scaling down enzyme-gelatin modified electrodes by spin coating the enzyme-gelatin layer. Special attention is given to the electrochemical behavior of the selected enzymes inside the gelatin matrix. A glassy carbon electrode was used as a substrate to immobilize, in the first instance, horse heart cytochrome c (HHC) in a gelatin matrix. Both a drop dried and a spin coated layer was prepared. On scaling down, a transition from diffusion controlled reactions towards adsorption controlled reactions is observed. Compared to a drop dried electrode, a spin coated electrode showed a more stable electrochemical behavior. Next to HHC, we also incorporated catalase in a spin coated gelatin matrix immobilized on a glassy carbon electrode. By spincoating, highly uniform sub micrometer layers of biocompatible matrices can be constructed. A full electrochemical study and characterization of the modified surfaces has been carried out. It was clear that in the case of catalase, gluteraldehyde addition was needed to prevent leaking of the catalase from the gelatin matrix.

Electrochemical study of gelatin as a matrix for the immobilization of horse heart cytochrome c.

The aim of this paper is to emphasize the strength of gelatin as a stable matrix for redox enzymes. Cyclic voltammetry has been applied for a detailed electrochemical study of horse heart cytochrome c (HHC) entrapped in a gelatin matrix immobilized on a gold electrode. The influence of the HHC concentration, the mass percentage of the gelatin and the nature of the gelatin on the electrochemical behaviour of HHC have been described in detail. In addition, attenuated total reflection infrared (ATR-IR) spectroscopy was used to prove the immobilization on a qualitative and conformational level. The thickness of the gelatin film was determined using a non-contact optical profiler. These results open up new perspectives in the development of stable, biocompatible matrices for redox enzymes. The latter has its relevance in the field of biosensor development.