In this review we describe fundamentals of scanning near-field optical microscopy with aperture probes. After the discussion of instrumentation and probe fabrication, aspects of light propagation in metal-coated, tapered optical fibers are considered. This includes transmission properties and field distributions in the vicinity of subwavelength apertures. Furthermore, the near-field optical image formation mechanism is analyzed with special emphasis on potential sources of artifacts. To underline the prospects of the technique, selected applications including amplitude and phase contrast imaging, fluorescence imaging, and Raman spectroscopy, as well as near-field optical desorption, are presented. These examples demonstrate that scanning near-field optical microscopy is no longer an exotic method but has matured into a valuable tool.

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Scanning near-field optical microscopy with aperture probes: Fundamentals and applications

The phase-shifting mask consists of a normal transmission mask that has been coated with a transparent layer patterned to ensure that the optical phases of nearest apertures are opposite. Destructive interference between waves from adjacent apertures cancels some diffraction effects and increases the spatial resolution with which such patterns can be projected. A simple theory predicts a near doubling of resolution for illumination with partial incoherence σ < 0.3, and substantial improvements in resolution for σ < 0.7. Initial results obtained with a phase-shifting mask patterned with typical device structures by electron-beam lithography and exposed using a Mann 4800 10× tool reveals a 40-percent increase in usuable resolution with some structures printed at a resolution of 1000 lines/mm. Phase-shifting mask structures can be used to facilitate proximity printing with larger gaps between mask and wafer. Theory indicates that the increase in resolution is accompanied by a minimal decrease in depth of focus. Thus the phase-shifting mask may be the most desirable device for enhancing optical lithography resolution in the VLSI/VHSIC era.

Improving resolution in photolithography with a phase-shifting mask

Several performance criteria are described to enable a fair comparison among the various correlation filter designs: signal-to-noise ratio, peak sharpness, peak location, light efficiency, discriminability, and distortion invariance. The trade-offs resulting between some of these criteria are illustrated with the help of a new family of filters called fractional power filters (FPFs). The classical matched filter, phase-only filter (POF), and inverse filter are special cases of FPFs. Using examples, we show that the POF appears to provide a good compromise between noise tolerance and peak sharpness.

Performance measures for correlation filters.

We report a new, to our knowledge, technique for encoding amplitude information onto a phase-only filter with a single liquid-crystal spatial light modulator. In our approach we spatially modulate the phase that is encoded onto the filter and, consequently, spatially modify the diffraction efficiency of the filter. Light that is not diffracted into the first order is sent into the zero order, effectively allowing for amplitude modulation of either the first-order or the zero-order diffracted light. This technique has several applications in both optical pattern recognition and image processing, including amplitude modulation and inverse filters. Experimental results are included for the new technique.

Encoding amplitude information onto phase-only filters

The present invention is a nonlinear joint transform image correlator which employs a spatial modulator operating in a binary mode at the Fourier plane. The reference and input images are illuminated by a coherent light at the object plane of a Fourier transform lens system. A image detection device, such as a charge coupled device, is disposed at the Fourier plane of this Fourier transform lens system. A thresholding network detects the median intensity level of the imaging cells of the charge coupled device at the Fourier plane and binarizes the Fourier transform interference intensity. The correlation output is formed by an inverse Fourier transform of this binarized Fourier transform interference intensity. In the preferred embodiment this is achieved via a second Fourier transform lens system. This binary data is then applied to spatial light modulator device operating in a binary mode located at the object plane of a second Fourier transform lens system. This binary mode spatial light modulator device is illuminated by coherent light producing the correlation output at the Fourier plane of the second Fourier transform lens system. The inverse Fourier transform may also be formed via a computer. In an alternative embodiment, the Fourier transform interference intensity is thresholded into one of three ranges. An inverse Fourier transform of this trinary Fourier transform interference intensity produces the correlation output.

Joint transform image correlation using a nonlinear spatial light modulator at the fourier plane

Recently, many applications of the liquid crystal television (LCTV) to real-time signal processing have been reported. A basic description of the application of the LCTV to realtime pattern recognition was first reported by Liu et al. Gregory later also showed a successful space-invariant correlation using a LCTV as a spatial light modulator. Both methods were based on the use of a complex matched (VanderLugt) spatial filter. The joint transform architecture is an alternative approach to optical pattern recognition. The joint transform method has proved to be suitable for adaptive programmable correlation because no matched spatial filter is required. The reference pattern may simply be generated by a computer and input to a low space-bandwidth product (SBP), electronically addressed, spatial light modulator. Conversely, it is very difficult to generate a dynamic matched spatial filter in the Fourier plane. A microcomputer-based programmable optical correlator using a magnetooptical device (MOD) and a liquid crystal light valve (LCLV) was proposed recently by Yu and Ludman. In this Letter, a new implementation of the real-time joint transform correlator architecture using inexpensive LCTVs will be discussed. Preliminary experimental results are presented to verify the usefulness of the technique. There are three major objections to using commercially available liquid crystal TVs for optical processing applications: (1) low contrast ratio, (2) phase nonuniformity, and (3) low resolution and low SBP. These problems must be minimized if a LCTV is to be used as a spatial light modulator in a coherent optical system. The contrast uniformity

Adaptive real-time pattern recognition using a liquid crystal TV based joint transform correlator

From the Publisher:
In recent years, optical computing and optical neural networks research has enriched the field originally known as optical signal processing Optical Signal Processing, Computing, and Neural Networks is a self-contained textbook that offers an introductory survey which examines photonics, linear and nonlinear signal processing, and numerical, symbolic, and neural computing This comprehensive sourcebook is a basic text for students who lack an intensive background in optic, electromagnetic, computer, and neural network theories It will also serve as a working reference for optical physicists and engineers involved in current research and development of modern optical signal processing that includes optical computing and neural networks The first chapter of this book contains the basic coherent theory and concepts of optical transformation The second chapter introduces the fundamental concept of optical signal processing and its architectures The third chapter presents selected applications in coherent optics while the fourth chapter discusses white-light processing and its applications The advances of spatial-light modulators are discussed as well as hybrid-optical architectures using spatial-light modulators in later chapters Applications of photorefractive crystals in optical signal processing are presented in chapter 7 Digital-optical computing is described in chapter 8 while optical neural networks and their architectures, designs, and models are thoroughly covered in chapter 9 Examples and experimental results are included throughout the book to emphasize the concepts Chapters include problem sets, 330 throughout, that reinforce key elements in the text

Optical Signal Processing, Computing, and Neural Networks

1. Entropy Information and Optics 2. Signal Processing with Optics 3. Communication with Optics 4. Switching with Optics 5. Transformation with Optics 6. Interconnection with Optics 7. Pattern Recognition with Optics 8. Information Storage with Optics 9. Computing with Optics 10. Sensing with Optics 11. Information Display with Optics 12. Networking with Optics Index

Introduction to information optics

Novel electron trapping materials capable of performing optical parallel Boolean logic operations are described. An application to binary full addition based on a parallel algorithm is discussed, and experimental results are presented.

Use of electron trapping materials in optical signal processing. 1: Parallel Boolean logic.

A novel polarization-encoded two-channel joint transform correlator is described that can produce unity interference modulation, independent of the illumination of the target and the reference.

Suganda Jutamulia

Papers

Adaptive real-time pattern recognition using a liquid crystal TV based joint transform correlator

Optical Signal Processing, Computing, and Neural Networks

Introduction to information optics

Use of electron trapping materials in optical signal processing. 1: Parallel Boolean logic.

Illumination-independent high-efficiency joint transform correlator