ABSORPTION AND LATTICE HEATING

The initial stage in the conversion of laser radiation to heat during irradiation involves the excitation of electrons to states of higher energy. For this process to occur, vacant states have to be available to accept excited electrons. When the photon energy hν is small, as for example when 10.6 μm laser radiation is absorbed, only electrons with energies within a narrow range hν near the Fermi energy, ∈F, can participate in absorption. At 0 K, the highest energy reached upon absorption is ∈F + hν.

At higher temperatures, electrons occupy a range of states given by the Fermi–Dirac distribution (Omar 1975). This reduces to a Boltzmann function for electron energies ∈ such that ∈ – ∈F ≫ kT, where T is the metal temperature. Absorption of photons then populates those states with energy ∈ + hν. Since ∈F is usually several electronvolts, whereas hν = 0.117 eV for CO2 laser photons, absorption of IR laser radiation then acts to redistribute electrons among states close to those on the Fermi surface.

This situation is different at excimer laser wavelengths, since hν is then comparable to or larger than the work function, φ, of many metals. When hν > φ, electrons may be directly excited from states near the Fermi surface to continuum states associated with the ejection of an electron from the metal. These electrons will originate from levels within the skin depth, δ. Those electrons that are not ejected will dissipate their excess energy as heat within the skin depth.

INTRODUCTION

Shortly after the announcement of high temperature superconductivity in the La–Ba–Cu–O (Bednorz and Müller 1986) and Y–Ba–Cu–O (Wu et al. 1987a) systems the first reports were published describing in situ preparation of superconducting thin films using laser ablation (Dijkkamp et al. 1987, Wu et al. 1987b, Narayan et al. 1987). The laser ablation method, which is a well known technique for the preparation of thin films of a variety of materials (Duley 1983, Bäuerle 1986, Braren et al. 1993, Chrisey and Hubler 1994), was found to be well suited to the deposition of superconducting films since it permits flexible control over deposition conditions and yields films with good stoichiometry.

Materials such as Y–Ba–Cu–O are, however, complex from both a chemical and a structural point of view (Burns 1992) and therefore vaporization and redeposition of these materials using laser radiation is anticipated to be a complicated process. A full understanding of the physical and chemical mechanisms that accompany laser ablation and in situ deposition has yet to be obtained. Nevertheless, useful progress has been made in the preparation of superconducting films with high zero resistance temperatures (about 90 K) and critical current densities exceeding 106 A cm–2 using the laser ablation method.

DEPOSITION AND PROPERTIES

The use of excimer laser radiation to prepare thin films of superconducting material by laser vaporization of the parent compound was first reported in 1987 (Dijkkamp et al. 1987, Wu et al. 1987b, Narayan et al. 1987).

INTRODUCTION

A wide variety of materials can be deposited from gaseous, solid and liquid precursors using laser techniques. Photothermal as well as photochemical routes are often available and range from the straightforward use of laser radiation as a vaporization source to photochemical decomposition of adsorbed layers. Laser deposition can be used for the creation of extended thin films or for selective deposition of specific features in localized regions with dimensions extending to less than 1 μm. The choice of deposition technique will depend on the required composition of the deposit together with the properties of the substrate. Laser wavelength may be of primary importance for photochemical deposition of sub-micrometer features. Some significant factors in the laser deposition of materials are:

1. chemical routes to the required deposit,

2. laser intensity and wavelength,

3. sensitivity of substrate to thermal/photochemical effects,

4. sensitivity of substrate to ambient atmosphere/chemical environment,

5. scale of features to be deposited, i.e. micro/macrofeatures,

6. required deposition rate,

7. sensitivity of deposited layer to contamination by secondary products and/or particulates, and

8. optical configuration required for deposition.

The fact that a multicolour image can be produced by a hologram recorded with three suitably chosen wavelengths was first pointed out by Leith and Upatnieks [1964].

The resulting recording can be considered as made up of three incoherently superposed holograms. When it is illuminated once again with the three wavelengths used to make it, each of these wavelengths is diffracted by the hologram recorded with it to give a reconstructed image in the corresponding colour. The superposition of these three images yields a multicolour reconstruction.

However, while multicolour holography was demonstrated at quite an early stage, its further development was held up initially by several practical problems. These problems, as well as later advances that have made multicolour holography practical, are described in this chapter (see also the review by Hariharan [1983]).

Light sources for colour holography

The most commonly used lasers for colour holography are the He-Ne laser (λ = 633 nm) and the Ar+ laser, which has two strong output lines (λ, = 514 nm and 488 nm; see Table 5.1). The range of colours that can be reconstructed with these three wavelengths as primaries can be determined by means of the C.I.E. chromaticity diagram [Optical Society of America, 1953]. In this diagram, as shown in fig. 9.1, points representing monochromatic light of different wavelengths constitute the horseshoe-shaped curve known as the spectrum locus; all other colours lie within this boundary.

Introduction

In the previous chapter we introduced the theory of geometrical optics, a very simplistic analysis of the propagation of radiation describing only the lines that trace the radiation trajectories. In that analysis, the lines, or rays, were not subjected to the effects of diffraction or interference; with the exception of dispersion, color too had no influence on these trajectories. The absolute value of the speed of light had no bearing on the propagation; only its magnitude relative to the speed in free space had to be known, and even that parameter could not be derived directly and had to be retrieved from other sources. Similarly, parameters of the important effect of dispersion could not be derived directly. Attenuation by absorption was outside the scope of geometrical optics, as were other effects related to the nature of radiation such as polarization, coherence, and wavelength. These shortcomings of geometrical optics were to be expected. After all, such fundamental questions as how radiation is created or how it interacts with a particular medium were not asked. Without consideration of these questions, the nature of radiation and the details of its propagation cannot be fully understood.

Historically, the first studies attempting to understand the nature of light, and not merely its patterns of propagation, were made in the seventeenth century. At that time, visible light was the only known mode of radiation.

Explanation of the various effects of light is a very elusive task. Although light has captured the imagination of human beings since the dawn of civilization, science has yet to deliver a single, comprehensive explanation of all its effects. The advanced theories that now exist create many new questions along with new answers. Part of the confusion can be blamed on our tendency to explain physical phenomena using the perception of our senses. Unfortunately, our senses do not tell the full story. Although we can see light, and even distinguish among some of its colors, we cannot see most of the radiation emitted by the sun. Even our ability to visually determine the brightness of light sources is limited by the rapid saturation of the eye retina. Our senses tell us that light propagates in straight lines, yet careful experiments have demonstrated that the trajectories of light can be bent by gravitation. We cannot even capture and store light.

Holographic interferometry is an extension of interferometric measurement techniques in which at least one of the waves that interfere is reconstructed by a hologram.

The unique capabilities of holographic interferometry are due to the fact that holography permits storing a wavefront for reconstruction at a later time. Wavefronts which were originally separated in time or space or even wavefronts of different wavelengths can be compared by holographic interferometry. As a result, changes in the shape of objects with rough surfaces can be studied with interferometric precision.

One of the most important applications of holographic interferometry is in nondestructive testing (see Erf [1974], Vest [1981], and Rastogi [1994]). It can be used wherever the presence of a structural weakness results in a localized deformation of the surface when the specimen is stressed, either by the application of a load or by a change in pressure or temperature. Crack detection and the location of areas of poor bonding in composite structures are fields where holographic interferometry has been found very useful. An allied area of applications has been in medical and dental research, where it has been used to study the deformations of anatomical structures under stress, as well as for nondestructive tests on prostheses [Greguss, 1975, 1976; von Bally, 1979]; see also, Podbielska [1991, 1992].

Holographic interferometry has also proved its utility in aerodynamics, heat transfer, and plasma diagnostics.

The past ten years have seen an upsurge of interest in optical holography because of several major advances in its technology. Holography is now firmly established as a display medium as well as a tool for scientific and engineering studies, and it has found a remarkably wide range of applications for which it is uniquely suited.

My aim in writing this book is to present a self-contained treatment of the principles, techniques, and applications of optical holography, with particular emphasis on recent developments. After a brief historical introduction, three chapters outline the theory of holographic imaging, the characteristics of the reconstructed image, and the different types of holograms. Five chapters then deal with the practical aspects of holography – optical systems, light sources and recording media – as well as the production of holograms for displays and colour holography. The next two chapters discuss computer-generated holograms and some specialized techniques such as polarization recording, holography with incoherent light, and hologram copying. These are followed by four chapters describing the more important applications of holography. Particle-size analysis, high-resolution imaging, multiple imaging, holographic optical elements, and information storage and processing are covered in two of these, and the other two are devoted to holographic interferometry and its use in stress analysis, vibration studies, and contouring.

To make the best use of the available space, the scope of the book has been limited to optical holography.

Introduction

Previous discussions (see Section 10.4) suggested that stimulated emission can be used to generate optical gain, that is, to amplify radiation. The reader certainly has experience in the amplification of electronic signal. For example, radio receivers capture faint radio waves and turn them into a signal that is powerful enough to drive large speakers. This electronic amplification increases the amplitude of the signal while faithfully preserving its acoustic frequencies and modulation characteristics. Similarly, optical amplification is expected to increase the amplitude of an optical signal while preserving its frequency, its modulation characteristics, and its coherence. The latter requirement is of particular significance for optical radiation, where the coherence of naturally occurring radiation rarely exceeds one micrometer. Lasers are the primary source for coherent radiation. They depend on stimulated emission for amplification and for the generation of coherent radiation (the word laser is the acronym of Light Amplification by Stimulated Emission of Radiation). For amplification, an atomic system that is part of the laser medium must be prepared with a sufficiently large number of particles in the excited state. (The term atomic system is used here to describe all microscopic systems including molecules and free electrons.) Radiation passing through that excited medium encounters multiple events of stimulated emission, each event contributing one photon that is added coherently to the propagating beam. When the number of events of stimulated emission exceed all losses by absorption or scattering, the incident radiation is amplified.

Introduction

In the previous chapter we saw that, when electric dipoles are forced to oscillate, they induce an electric field that oscillates at the same frequency. In addition, owing to the motion of the oscillating charges, a magnetic field oscillating at the same frequency is also induced. These simultaneous oscillating fields are the basis for all known modes of electromagnetic radiation. Thus, Xrays, UV radiation, visible light, and infrared and microwave radiation are all part of the same physical phenomenon. Although each radiating mode is significantly different from the others, all modes of electromagnetic radiation can be described by the same equations because they all obey the same basic laws.

Oscillation alone is insufficient to account for electromagnetic radiation. The other important observation is that radiation propagates. It is broadcast by a source and, if uninterrupted, can propagate indefinitely in both time and space. An example of the boundless propagation of electromagnetic waves – whether X-ray, visible, or microwave – is the radiation emitted by remote galaxies. Some of this radiation, generated at primordial times and at remote reaches of the universe, can be detected on earth billions of years later. Evidently, radiation is not limited to the immediate vicinity of the source. Although we know that certain media can block radiation, we find it more astonishing that electromagnetic waves can propagate through free space; unlike electrical currents or sound, conductors are not necessary for the transmission of radiation.