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

Based on the fanning effect self-pumped and mutually pumped phase conjugation is demonstrated in photorefractive BTO-crystals with applied electric ac-fields. We used fiber-like crystals that enable a long interaction region and high applied electric fields. We realized three kinds of phase conjugating mirrors, self-pumped, externally self- pumped, and mutually pumped. Experimental results are given characterizing the spatial and temporal behavior of the phase conjugated wave, the transmission (imaging) of intensity and phase patterns, and the quality of the phase conjugation.

Phase conjugation in fiber-like BTO crystals with applied electric field

The interplay of reflection and transmission gratings is observed and discussed for a ring self-pumped phase-conjugate mirror (RSPPCM) with a photorefractive Ba0.77Ca0.23TiO3 crystal (BCT). It is shown that the reflection grating decreases the threshold of the RSPPCM.

Ring self-pumped phase-conjugate mirror with a photorefractive Ba 0.77 Ca 0.23 TiO 3 crystal: interplay of reflection and transmission gratings

Results of experimental and theoretical investigations of a coherent 2D optical soliton interaction in a SBN crystal is presented. The influence of the distance between solitons with equal intensities along the optical axis is studied. The relative position of beams along the optical axis is taken into account. The interaction of four solitons with equal intensities is also investigated. Experimental and theoretical results show that the distance between the centers of input beams placed in the corners of a rectangle should be more than 3.2 FWHM along both directions to avoid the interaction of solitons.

The interaction of photorefractive solitons in a SBN crystal

We present observations of wave focusing and defocusing in a photorefractive Ba0.77Ca0.23TiO3 (BCT) crystal without external electric field and without background illumination. The effects depend on the cross-section of the input beam. We show that by decreasing of the diameter of the input beam the fanning effect disappears at 150 µm. The self-focusing is observed for input diameters from 150 µm down to 40 µm, the self-defocusing is observed for input diameters from 40 µm down to 20 µm. For diameters smaller than 20 µm no non-linear effects are observed. The 1D self-trapping is detected at 65 µm in BCT. The experimental results are used to estimate the effective density of ionized donors N eff ? 1.3 •1020 m -3 as well as to find the thermal excitation cross-section ? T ? 1.8 •10-14 W of the crystal. Light intensity and wavelength are correspondingly 3 mW and 633 nm.

Experimental observations of wave self-focusing and self-defocusing in a photorefractive Ba 0.77 Ca 0.23 TiO 3 (BCT) crystal

In this paper we present a technique to reconstruct object information from free space digital holograms with the application of the Moire-effect leading to an increase in the resolvable angle by a factor of 2 compared to phase shifting methods. To do so the spatial frequencies causing the Moire-structures are repositioned in their proper position and this way increase the image quality.

Richard Kowarschik

Papers

Phase conjugation in fiber-like BTO crystals with applied electric field

Ring self-pumped phase-conjugate mirror with a photorefractive Ba 0.77 Ca 0.23 TiO 3 crystal: interplay of reflection and transmission gratings

The interaction of photorefractive solitons in a SBN crystal

Experimental observations of wave self-focusing and self-defocusing in a photorefractive Ba 0.77 Ca 0.23 TiO 3 (BCT) crystal

Application of the Moiré-effect in digital holography