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

In conventional holography, it is difficult to gain access on the three-dimensional surfaces of objects under test [1]. In contrast, there are well established techniques like the stereophotogrammetry that reconstruct surfaces of objects in dense three-dimensional point-clouds by using at least two cameras and structured illumination [2]. Recent works demonstrated, that reconstructed digital holograms can be used as images in the stereophotogrammetrical determination of object surfaces resulting in a three-dimensional point-cloud of the object [3]. Additionally, it is possible to combine the point-cloud with holographic techniques, such as holographic interferometry or phase-shifting techniques [4]. Due to such a combination, full three-dimensional deformation vectors can be calculated, providing the user with a method of high precision deformation measurements [5]. To accomplish these calculations, four cameras are used to connect three point-clouds with each other, so that the deformations measured, depending on the sensitivity-vector of each camera, can be combined to a full three-dimensional vector.

Stereophotogrammetric Image Field Holography

Ring self-pumped phase-conjugate mirror with a photorefractive Ba0.77Ca0.23TiO3 crystal as an active medium

View-synthesized 're-calibration' of an array projector for 3D measurement from an arbitrary monocular view

New experimental results about a reflection-type double-pass confocal microscope with a phase-conjugate mirror are presented. Exploiting the change of polarization of the light, when it is reflected at the object, gives the possibility to enhance the axial resolution of the microscope compared to that of the conventional confocal microscope by a factor of 65%. Furthermore, investigations on the time behavior of the PCM are presented.

Investigation of the reflection-type double-pass confocal microscope with a phase-conjugate mirror

The invention of Holography by Dennis Gabor goes back to the year 1948. Gabor could show that the complete original wavefront of the object both with its amplitude and phase can be reconstructed by adding a coherent background to a wave coming from an object while recording in a storage medium. However, because no adequate source of coherent light was available at this time, the small coherence length of the mercury lamp forced him to arrange everything along one axis. This setup, known as in-line holography, has the disadvantage that the desired reconstruction is in line with disturbing diffraction orders, and can be separated from these disturbances only by focusing. Gabor finally gave up his investigations and mentioned in 1955 to his collaborator Michael W. Haine, “ It was a very ill wind which I let out now almost eight years ago which blew nobody any good, least of all to myself.” The holographic principle, on which Gabor had set his high hopes, especially with respect to the enhancement of the resolution of the electronic microscope, seemed to retain only the status of a scientific curiosity. But the breakthrough would be achieved a couple of years later. In the early 1960s, Emmett Leith and Juris Upatnieks, two physicists at the University of Michigan, implemented their off-axis scheme as a smart combination of Gabor’s holographic principle with the carrier frequency technique known from side-looking radar. Their famous publication came out 61 years ago in 1961. In this way the twin image problem could be effectively eliminated. However, it resulted in the requirement of increased temporal coherence which could be satisfied using a powerful coherent light source only. Such a source was soon available. In 1958, Charles Townes at the Columbia University, and a Russian research group led by Nicolay Basov and Aleksandr Prokhorov at the Lebedev Institute for Physics, Moscow, simultaneously and independently analyzed the possibilities of applying the principle of the Microwave Amplification by Stimulated Emission of Radiation (MASER) for the optical region of the electromagnetic spectrum. The first operating optical MASER, now known as the laser, was constructed by Maiman and was demonstrated at the Hughes Research Laboratories in Malibu, California, in 1960. The symbiosis of the holographic and laser principle opened the door for a large variety of new technologies and applications in optical imaging, information processing, and metrology. The 1960s were filled with a variety of successful practical applications and innovations such as display or volume holography, computer generated holography, holographic interferometry, holographic nondestructive testing, holographic pattern recognition, and holographic storage technology. Gabor took an active part in this development. 50 years ago, on the occasion of his Nobel lecture at the Imperial Colleges in 1971, he also tried to take a look into the future of holography. Gabor mentioned “However, there are important areas in which we can do much more, and where an improvement is badly needed. This is the area of microholograms for storing and for display. An even more ambitious scheme, probably even farther in the future, is three-dimensional cinematography, without viewing aids such as Polaroids”. The basics for

Richard Kowarschik

Papers

Stereophotogrammetric Image Field Holography

Ring self-pumped phase-conjugate mirror with a photorefractive Ba0.77Ca0.23TiO3 crystal as an active medium

View-synthesized 're-calibration' of an array projector for 3D measurement from an arbitrary monocular view

Investigation of the reflection-type double-pass confocal microscope with a phase-conjugate mirror

Celebrating Holography after 60 years of successful application