Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

We first provide an overview of 3-D shape measurement us- ing various optical methods. Then we focus on structured light tech- niques where various optical configurations, image acquisition tech- niques, data postprocessing and analysis methods and advantages and limitations are presented. Several industrial application examples are presented. Important areas requiring further R&D are discussed. Finally, a comprehensive bibliography on 3-D shape measurement is included, although it is not intended to be exhaustive. © 2000 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(00)00101-X)

Overview of three-dimensional shape measurement using optical methods

Fluorescence is widely used in optical devices, microscopy imaging, biology, medical research and diagnosis. Improving fluorescence sensitivity, all the way to the limit of single-molecular detection needed in many applications, remains a great challenge. The technique of surface enhanced fluorescence (SEF) is based upon the design of surfaces in the vicinity of the emitter. SEF yields an overall improvement in the fluorescence detection efficiency through modification and control of the local electromagnetic environment of the emitter. Near-field coupling between the emitter and surface modes plays a crucial role in SEF. In particular, plasmonic surfaces with localized and propagating surface plasmons are efficient SEF substrates. Recent progress in tailoring surfaces at the nanometre scale extends greatly the realm of SEF applications. This review focuses on the recent advances in the different mechanisms involved in SEF, in each case highlighting the most relevant applications.

http://turroserver.chem.columbia.edu/surfaceplasmons/pdf/53_jpd_41_1_2008_sefsrev.pdf

Surface enhanced fluorescence

This work presents the fabrication of biologically inspired artificial compound eyes The artificial ommatidium, like that of an insect's compound eyes, consists of a refractive polymer microlens, a light-guiding polymer cone, and a self-aligned waveguide to collect light with a small angular acceptance The ommatidia are omnidirectionally arranged along a hemispherical polymer dome such that they provide a wide field of view similar to that of a natural compound eye The spherical configuration of the microlenses is accomplished by reconfigurable microtemplating, that is, polymer replication using the deformed elastomer membrane with microlens patterns The formation of polymer waveguides self-aligned with microlenses is also realized by a self-writing process in a photosensitive polymer resin The angular acceptance is directly measured by three-dimensional optical sectioning with a confocal microscope, and the detailed optical characteristics are studied in comparison with a natural compound eye

http://science.sciencemag.org/content/sci/312/5773/557.full.pdf

Biologically inspired artificial compound eyes.

Modal decomposition of light has been known for a long time, applied mostly to pattern recognition. With the commercialization of liquid-crystal devices, digital holography as an enabling tool has become accessible to all, and with it all-digital tools for the decomposition of light have finally come of age. We review recent advances in unravelling the properties of light, from the modal structure of laser beams to decoding the information stored in orbital angular momentum (OAM)-carrying fields. We show application of these tools to fiber lasers, solid-state lasers, and structured light created in the laboratory by holographic laser beam shaping. We show by experimental implementation how digital holograms may be used to infer the intensity, phase, wavefront, Poynting vector, polarization, and OAM density of some unknown optical field. In particular, we outline how virtually all the previous ISO-standard beam diagnostic techniques may be readily replaced with all-digital equivalents, thus paving the way for unravelling of light in real time. Such tools are highly relevant to the in situ analysis of laser systems, to mode division multiplexing as an emerging tool in optical communication, and for quantum information processing with entangled photons.

Creation and detection of optical modes with spatial light modulators

To solve a special task of laser-beam material treatment, a new diffractive element was designed, fabricated and investigated, which converts a given single-mode CO2-laser beam into a ring-shaped intensity distribution in the working plane. This was accomplished by application of refined ray-tracing methods and an enhanced micro technology using e-beam generated masks. Relevant steps of calculation and manufacturing of these masks are outlined. First results of measured intensity distribution demonstrate a good conformity with results of computer simulations.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Investigation of computer-generated diffractive beam shapers for the task of Gauss-to-ring laser beam transformation

Stereophotogrammetric 3D shape measurement using structured illumination is an established class of methods for industrial inspection. One essential step in the measurement process for all stereophotogrammetric techniques is the assignment of corresponding points between the stereo views. As the purpose of the used structured illumination is to ease and improve the correspondence assignment, the choice of said sequence is of utmost importance. The precision of the correspondence assignment directly affects the noise of the final point cloud and therefore this assignment should be conducted with the highest precision possible. Depending on the chosen structured illumination sequence, different degrees of freedom for the pattern design exist and may affect the precision of the correspondence assignment and thus the noise of the 3d point cloud. In our contribution we want to discuss the influence of the frequency content of the structured illumination for a scheme employing bandlimited statistical patterns, which have been fruitfully used for highspeed applications in the past years. To evaluate the limits of the correspondence assignment accuracy we created a simple numerical signal-detector model. Using this model the correspondence assignment in dependence of the chosen structured illumination can be compared to ground truth-data. Furthermore, the noise of point clouds in real measurements is investigated to validate the results of the used simulation. Therefore, illumination sequences using different spatial frequency bands are created and projected onto a reference object. Afterwards, the noise of the resulting pointcloud is evaluated. The results indicate that it is advisable to optimize the pattern design depending on the used sensor and object properties.

Influence of the structured illumination frequency content on the correspondence assignment precision instereophotogrammetry

Laser speckle projection is a reliable method to generate statistical illumination patterns for 3D reconstruction purposes as in stereo photogrammetry. This type of pattern has several advantages compared to incoherent methods. However, the biggest disadvantage is given by the coherent noise, the so-called "subjective speckles," developing when a coherently illuminated surface is imaged by a lens system. Some experimental techniques have been published already, being costly in measurement time or leading to loss in light intensity and/or depth of field. In this work we want to present numerical one-dimensional filtering techniques that reduce this kind of noise and increase the performance of 3D reconstruction, while no experimental changes to the classical speckle projection technique have to be made. Therefore, a model describing the expectable contrast reduction is derived, the dependency between filter orientation and setup geometry is investigated, and results from simulations and real experiments are shown. It is found that for small filter sizes the results can be improved independent of the filter, but that in the general case a vertical orientation of the filter towards the setup geometry is most useful.

Subjective speckle suppression for 3D measurement using one-dimensional numerical filtering.

Since many years laser triangulation is a key technology for metrological measurements. Manifold sensors exist that measure single spot distances or distance profiles via triangulation between the illumination source (e.g. laser diode) and an imaging system incorporating typically a digital detector. The physical limits have been investigated and derived by several researchers [1]. It has been stated that the main limitation for laser triangulation is subjective laser speckle noise that occurs when the laser spot or line is imaged onto the detector.

Coherent Pattern Projection for Highspeed 3D Shape Measurements

Light-polarizing films are a basis of sheet polarizers, which are used in liquid crystal systems. A light-polarizing film is generally produced by the formation of a uniaxial stretched film made of polyvinyl alcohol (PVA) or its derivatives containing a light-polarizing element like iodine or dichroic dye. Films with iodine are excellent in their initial polarizing performance but poor in resistance to moisture and heat. In order to improve the durability of polarizers more stable dichroic element are used instead of iodine. Films with dichroic organic dyes first of all azodyes are superior in the resistance to moisture and heat as compared with iodine systems. Searching for thermally stable dyes, which do not change their colour under of the influence of high temperature and humidity and are suitable for the manufacturing of effective polarizers is an actual problem. In this paper we show the results of optical investigations of uniaxial PVA-films with iodine (with and without transparent layer) and dichroic azodye.

Richard Kowarschik

Papers

Investigation of computer-generated diffractive beam shapers for the task of Gauss-to-ring laser beam transformation

Influence of the structured illumination frequency content on the correspondence assignment precision instereophotogrammetry

Subjective speckle suppression for 3D measurement using one-dimensional numerical filtering.

Coherent Pattern Projection for Highspeed 3D Shape Measurements

Chemical and optical investigations of film polarizers with azodyes