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

Phase-shifting fringe projection is the primary structured illumination method for high-accuracy, three-dimensional (3D) shape measurements in the fields of profilometry and stereophotogrammetry. Many different schemes for the phase evaluation and the phase-shifted fringe pattern design exist. Here we focus on the role of the phase evaluation in the context of stereophotogrammetry, where the nominal phase value itself is merely used as an image feature that can be exploited to establish a correspondence between the two camera views. Starting from the classical phase evaluation function, we will discuss its essential properties for a highly accurate correspondence mapping. Based on the findings, we generalize the classical function to derive a generalized phase value for a sequence of stereo images. An experimental comparison between a correspondence assignment using the classical phase evaluation function and a specifically chosen general phase evaluation function is given.

Generalized phase evaluation for stereophotogrammetric correspondence assignment

Spatial solitons in photorefractive (PR) crystals have attracted great interest in the last two decades because they represent a model scheme for solitons in other materials which are more complicated to obtain experimentally. In our presentation, we show that we have succeeded in generating stable self-focused states in an optically active PR crystal (Bi/sub 12/TiO/sub 20/) at 633 nm with a lifetime of several hours.

https://ieeexplore.ieee.org/iel5/10486/33242/01567951.pdf

Generation of quasi-steady and steady state photorefractive solitons in an optically active crystal

We present a technique for the measurement of temporal phase differences. In this method, the holographic stor-age of a given wavefront in a photorefractive BaTiO3 crystal is combined with the interferometric phase-steptechnique. We obtain a novelty-filter phase-step method which allows for the measurement of phase changeswithin an unique process. The application and some features ofthe technique are demonstrated.2. INTRODUCTIONThe measurement of temporal phase changes may be carried out by means of two different ways. In conventionalinterferometry, the phase distributions of the original and of the altered states are estimated by means of a phase-step algorithm. Huntley and Saldner1 published a method for the measurement of phase differences that includes asolution ofthe phase unwrapping problem.A different way for the measurement of phase changes consists of the use of optical real-time storage media. Asin holographic interferometry, the original wavefront may be stored in the medium and superimposed with a givenchanged phase distribution. The phase difference is then estimated by means of interferometric techniques. Suchtechniques have to be applied once (and not twice as before) but the interferogram analysis will be still necessary.For more than 20 years photorefractive crystals have been used as optical storage materials for real-time hologra-phy and holographic interferometry. If the wavefront which has been stored in such a crystal is compared to achanged wavefront, the photorefractive medium not only acts as an optical memory but also as an optical noveltyfilter2. This principle can result in visualization techniques for transient phase changes3.If a novelty filter is to be applied for the measurement of transient phase changes, one has to find a way for thecalculation of the original phase values from the intensity at the novelty filter output.In this paper, we demonstrate such a technique on the basis of the coupled waves theory4, by means of which theinteraction of two coherent light beams in a photorefractive crystal can be described.3. THEORYThe principal scheme for the application of the procedure consists of a two-wave mixing5 (TWM) arrangement

Measurement of phase differences with a novelty-filter phase-step technique

We report a novel scheme for an incoherent-to-coherent converter by means of the selective erasure of the random dynamic gratings created by the beam-fanning process in BaTiO3. Furthermore, the spatial resolution properties of a two-wave mixing incoherent-to-coherent converter are discussed in the geometrical limit, and using the two-dimensional two-wave mixing theory.

Incoherent-to-coherent Conversion Via Photorefractive Fan-out and Two-wave Mixing

In this paper we investigate the steady-state partial self-trapping of a laser beam with input diameters from 12.6 μm up to 40.8 μm in a photorefractive SBN-crystal in dependence on applied electrical field, input beam diameter and input beam intensity, whereas cross-section of the output beam, saturation time and self-bending are investigated. Under partial self-trapping we understand the waveguiding effect that is not completely compensated by beam diffraction. We find that the output beam size does not depend on the input beam diameter used in the experiment. Beam partial self-trapping as well as self-bending are calculated using a Douglas scheme and compared with experiments. Numerical simulations and experiments are carried out for input beams with Gaussian profile.

Richard Kowarschik

Papers

Generalized phase evaluation for stereophotogrammetric correspondence assignment

Generation of quasi-steady and steady state photorefractive solitons in an optically active crystal

Measurement of phase differences with a novelty-filter phase-step technique

Incoherent-to-coherent Conversion Via Photorefractive Fan-out and Two-wave Mixing

Beam self-trapping in a SBN crystal