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 propose a high-speed projection system that is able to project statistical speckle patterns at a rate of 500Hz. Its purpose is to generate structured light for a real-time photogrammetry stereo vision setup. As conventional digital light projector (DLP) projection setups are limited in their maximum projection rate to 250Hz for gray-value patterns, stripe projection systems are usually applied for real-time three-dimensional (3D) measurements. However, these techniques can only be used on steady surfaces as phase unwrapping has to be done. In contrast, the proposed setup is able to measure the shape of multiple spatially separated objects at once. We compare the speckle setup with a system using a DLP projector and with other fast 3D shape measurement setups, like the widely used stripe projection methods, qualitatively and quantitatively.

High-speed pattern projection for three-dimensional shape measurement using laser speckles

The described 3-D measurement system with fringe projec- tion applies the principle of uniform scale representation by the exclusive use of phase measurement values for the coordinates of each point. The test object is successively illuminated with a grating structure from at least three different directions with a telecentric system, where gray code is combined with five 90-deg phase shifts. A CCD camera records the intensity distribution of the fringes that appear as intersection lines on the surface of the object. This provides the linearly independent absolute phase values that are necessary for the calculation of the coordinates. Note that all coordinates (x,y,z) are determined with the same accuracy. To compensate the influence of specular reflections or shadowed areas up to 15 light projection directions can be used. Moreover, the object can be rotated around a second axis, yielding other views of the object. Thus we acquire different patches of the object that are transformed into a global coordinate system without any interactive user help. During this procedure, correlation methods or special points are not necessary. The calibration of the 3-D orientation of the second axis is realized with a special calibration body. © 2000 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(00)01701-3) Subject terms: adaptive optical three-dimensional measurement system; struc- tured light; gray code; uniform scale representation; phase measurement; calibra- tion method development.

Adaptive optical 3-D-measurement with structured light

This article presents a fast and accurate method to measure human faces for medical applications To encode an object point, several random patterns are projected A correlation technique, which takes only the area of one pixel into account, is used to locate the homologous points It could be shown that band limited random patterns are helpful for noise reduction The comparison of the point cloud of a measured plane with an ideal one showed a standard deviation less then 50μm Furthermore a depth difference of 20μm is detectable

Human face measurement by projecting bandlimited random patterns

Many three-dimensional (3D) shape measurement techniques in stereophotogrammetry with temporal coded structured illumination are limited to static scenes because the time for measurement is too long in comparison to the object speed. The measurement of moving objects result in erroneous reconstructions. This is apparent to reduce measurement time to overcome this limitation, which is often done by increasing the projection rate for illumination while shrinking the amount of images taken for reconstruction. The projection rate limits most applications in its speed because digital light processing (DLP) projectors, which are widely used, bring a limited projection rate along. Our approach, in contrast, does not take a DLP. Instead we use laser speckles as projected patterns which are switched using an acousto-optical deflector. The projection rate is 10× higher than what the fastest stripe projection systems to our knowledge achieve. Hence, we present this uncommon but potential approach for highspeed (≈250 3Dfps= [3D measurements per second]), dense, and accurate 3D measurements of spatially separated objects and show the media that emphasizes the ability of accurate measurements while the objects under testing move.

High-speed three-dimensional shape measurements of objects with laser speckles and acousto-optical deflection

The field of an electric dipole inside an arbitrary system of parallel slabs is evaluated with a Green’s function approach. Application of boundary conditions yields a matrix formalism that allows compact formulation of the problem. The method is extended to the general case of parallel stratified medialike cavities containing a dipole. Effects on spontaneous-emission rate of dipole emitters are evaluated and discussed for different types of planar microcavities.

Richard Kowarschik

Papers

High-speed pattern projection for three-dimensional shape measurement using laser speckles

Adaptive optical 3-D-measurement with structured light

Human face measurement by projecting bandlimited random patterns

High-speed three-dimensional shape measurements of objects with laser speckles and acousto-optical deflection

Dipole lifetime in stratified media