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

We investigate experimentally and theoretically the self-trapping and the self-bending of a laser beam with input diameters between 18 and 40 μm in a photorefractive strontium barium niobate crystal dependent on the applied electric field, the input beam diameter, and the input beam intensity. We find out the parameters for the optimal focusing, i.e., the beam is self-trapped to a determined diameter independently on the input diameter.

Beam self-trapping and self-bending dynamics in a strontium barium niobate crystal

A transparent optical sensor for detecting water on a windscreen surface of vehicles, a so-called rain sensor (RS), is presented. The RS is based on a light source, coupling holographic diffraction elements (HDEs), a waveguide (WG), and a detector. The advantage of the novel RS is its transparency, which allows it to be located in the driver's view area. The HDEs that couple the light signal into and out of the WG are manufactured as a laminate of a Plexiglas polymethylmethacrylate-Phenanthrenequinone photopolymer composite integrated into a windscreen. In the detecting area the light signal is coupled out of the WG onto the windscreen surface under total internal reflection angles [between 42 $^{\circ}$ for glass and air border (for the transition from glass to air), and 62 $^{\circ}$ for glass and water]. Depending on the moisture deposited on the windscreen surface, the light signal will be reflected back into the WG and will either propagate to the detector or will be coupled out of the glass.

A Transparent Optical Sensor for Moisture Detection Integrated in a PQ-PMMA Medium

The measurement of three-dimensional object shapes plays an important role in machine vision, solid modeling and other industrial applications. Well known optical methods for 3D- shape inspection are moire-techniques and contouring. Unfortunately, as a rule they are lacking because they measure only one coordinate, i.e., the height above a reference surface. The other coordinates in the reference plane have to be determined using the pixel distance of the observing CCD-camera or by other means. Here, we demonstrate a real 3D-coordinate measuring system using fringe projection techniques where the scale of coordinates is given by the illumination-structures. This method has the advantage that the aberration of the observing system and the depth-dependent imaging scale have no influence on the measuring accuracy. Moreover, the measurements are independent of the position of the camera with respect to the object under test.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Three-coordinate measuring system with structured light

The paper is aimed at the description of an optical shape measuring technique based on a consistent principle using fringe projection technique. We demonstrate a real 3D- coordinate measuring system where the sale of coordinates is given only by the illumination-structures. This method has the advantages that the aberration of the observing system and the depth-dependent imaging scale have no influence on the measuring accuracy and, moreover, the measurements are independent of the position of the camera with respect to the object under test. Furthermore, it is shown that the influence of specular effects of the surface on the measuring result can be eliminated. Moreover, we developed a very simple algorithm to calibrate the measuring system. The measuring examples show that a measuring accuracy of 10-4 (i.e. 10 micrometers ) within an object volume of 100 X 100 X 70 mm3 is achievable. Furthermore, it is demonstrated that the set of coordinate values can be processed in CNC- and CAD-systems.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optical 3D-coordinate measuring system using structured light

Geometric calibration of digital light processing projectors in single-camera, fringe-projecting 3D measurement systems have been studied assuming the projector is inverse pinhole modeled Conversely, a high-speed multi-aperture array projector (MAAP) projecting aperiodic fringes is not dependent on a digital mirror device and cannot be pinhole modeled With MAAP projection, a stereo camera setup is required This paper presents a model-less method to calibrate a MAAP by direct measurement of its illumination field and re-enables 3D measurements with a single camera even with surface discontinuities present Experimental proof of principle and preliminary measurement performance are shown

Richard Kowarschik

Papers

Beam self-trapping and self-bending dynamics in a strontium barium niobate crystal

A Transparent Optical Sensor for Moisture Detection Integrated in a PQ-PMMA Medium

Three-coordinate measuring system with structured light

Optical 3D-coordinate measuring system using structured light

Calibration of an array projector used for high-speed three-dimensional shape measurements using a single camera.