Samuli Siitonen

Coupling of light from an LED into a thin light guide by diffractive gratings.

Ring-shaped and radial diffractive gratings are designed with rigorous diffraction theory to couple light of a nearly monochromatic LED into a thin planar light guide on the bottom side. The theoretical coupling efficiencies for ring-shaped and radial gratings are 41% and 66%, respectively. Optimized diffractive elements are manufactured with direct electron-beam lithography and reactive-ion-etching into SiO2 substrates. Good agreement between experimental and theoretical results for selected radial gratings is reached. Furthermore, the mass production tests using injection molding are carried out with good replicability.

A double-sided grating coupler for thin light guides

A simple and efficient solution for coupling a collimated light beam into a thin light guide is presented. The approach is based on two gratings, with their grating lines perpendicular to each other, fabricated into the opposite surfaces of the light guide. The presented numerical simulation shows that an optimized double-sided solution for unpolarized light enables around 2-7 times higher incoupling efficiencies than what is possible with conventional solution based on only one grating. Experimental verification is made by using UV-replicated binary gratings on both sides of a PMMA foil.

Replication of sub-micrometre features using microsystems technology

Abstract Many fields of industry are looking for growth from miniaturisation. Because competition is increasing, costs are becoming more important in mass production. These are the driving forces for improving and developing micro injection moulding and replication technologies. This experimental study was carried out to replicate sub-micrometre features using the micro injection moulding technique. For the experiments, polycarbonate (PC) was selected because of its flow properties. The effects of processing parameters on the replication properties were observed. The best replication results were achieved when a small injection piston diameter and large shot size were used in micro injection moulding.

White LED light coupling into light guides with diffraction gratings

Radial diffractive gratings are used to couple light of a white LED into a light guide. Theoretical coupling efficiencies are evaluated with rigorous diffraction theory in a pure conical mounting. It is shown that when the refractive index of the grating increases from 1.46 to 2.05 the incoupling efficiency increases from 42% to 63%. Also, with the increasing refractive index the incoupling efficiency is shown to become more nearly uniform over the visible spectrum. Experimental results for the incoupled efficiencies and the color coordinates of the incoupled spectra are introduced for refractive indices n=1.46 and n=1.56.

Disposable gold coated pyramidal SERS sensor on the plastic platform.

In this paper we investigate suitability of arrays of gold coated pyramids for surface-enhanced Raman scattering (SERS) sensing applications. Pyramidarrays composed of 1000 nm pit size with 1250 nm pitch lengthwerereplicated on a plastic substrate by roll-to-roll (R2R) ultraviolet (UV) embossing. The level of SERS enhancement, and qualitative performance provided by the new substrate is investigated by comparing Raman spectrum of benzenethiol (BTh) test molecules to the benchmark Klarite SERS substrate which comprises inverted pyramid arrays(1500 nm pit size with 2000 nm pitch length) fabricated on a silicon substrate. The new substrate is found to provide upto 11 times increase in signal in comparison to the inverted pyramid (IV-pyramid) arrays fabricated on an identical plastic substrate. Numerical simulation and experimental evidence suggest that strongly confined electromagnetic fields close to the base of the pyramids, are mainly responsible for the Raman enhancement factor, instead of the fields localized around the tip. Unusually strong plasmon fields are projected upto 200nm from the sidewalls at the base of the pyramid increasing the cross sectional sensing volume.

Disposable photonic integrated circuits for evanescent wave sensors by ultra-high volume roll-to-roll method

Flexible photonic integrated circuit technology is an emerging field expanding the usage possibilities of photonics, particularly in sensor applications, by enabling the realization of conformable devices and introduction of new alternative production methods. Here, we demonstrate that disposable polymeric photonic integrated circuit devices can be produced in lengths of hundreds of meters by ultra-high volume roll-to-roll methods on a flexible carrier. Attenuation properties of hundreds of individual devices were measured confirming that waveguides with good and repeatable performance were fabricated. We also demonstrate the applicability of the devices for the evanescent wave sensing of ambient refractive index. The production of integrated photonic devices using ultra-high volume fabrication, in a similar manner as paper is produced, may inherently expand methods of manufacturing low-cost disposable photonic integrated circuits for a wide range of sensor applications.

Double-groove, two-depth grating coupler for light guides

A double-groove, two-depth dielectric grating structure for high-efficiency light coupling into a light guide is introduced. We show computationally that the optimized gratings can couple a monochromatic TE- or TM-polarized light beam with nearly 100% efficiency. For an unpolarized light beam 90% efficiency can be reached. In all cases the highest achieved coupling efficiencies require that the refractive index of the grating material be greater than 1.7. The illumination and fabrication tolerances of the couplers are also analyzed.

55.3: Diffractive Backlight Light Guide Plates in Mobile Electrowetting Display Applications

Power efficiency demands on mobile displays are increasing rapidly, as new multimedia services and applications are being adopted by users. Diffractive backlights and electrowetting displays have been proposed as some of the solutions to solve the poor efficiency of current mobile display systems. In this study, an electrowetting display was coupled with a pixelated, diffractive becklight light guide plate. This is to our knowledge the first time when a pixelated, diffractive backlight has been demonstrated in conjunction with an actual display. The results of the study show that the backlight and display panel need to be optimized as a system in order to obtain an applicable module for mobile use.

Semi-crystalline poly(4-methyl-1-pentene) polymers for replication of high aspect ratio diffractive features

Two transparent semi-crystalline poly(4-methyl-1-pentene) copolymers (P4MP1) of different molar mass distributions were used for injection molding of high aspect ratio (height/width) diffractive gratings. High quality sub-micron and micron features with aspect ratios of 1.6:1 were obtained with low mold temperatures. According to SEM and AFM measurements, replication of the features depended on substrate thickness and polymer. The copolymer with lower Mn and Mw penetrated deeper into the feature cavities in thin substrates, but showed poorer thermal stability and replication fidelity.

High efficiency half-wave retardation in diffracted light by coupled waves

We demonstrate high-efficiency half-wave retardation in diffracted light in the 2nd order Littrow mounting. The diffracting structure is a slanted crossed grating with subwavelength period in the direction of the second grating vector, which makes it possible to mix the polarization states of the input light inside the grating layer, and hence to create the half-wave retardation. We present an experimental result with 58.9 % diffraction efficiency and a near perfect half-wave retardation. We explain the effect qualitatively using the classical coupled-wave approach.