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.

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.

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.

Introduction

Of the three phenomena that result from the wavelike nature of light – polarization, interference, and diffraction – the third is the most puzzling. It does not render itself to intuitive explanation, since intuition suggests that light propagates in straight lines. Diffraction, however, allows for light under certain conditions to travel “around corners.” Because of this effect, light may be detected at points that could not be reached by straight rays. This effect also prevents indefinite propagation of collimated beams; invariably, after a certain distance, collimated beams appear to diverge. Similarly, when a focusing lens designed using considerations of geometrical optics is employed to focus radiation, the spot size at the focus cannot be reduced below a defined limit. In these examples, diffraction is seen to pose limitations on the application range of many optical devices. Thus, imaging resolution is reduced by the diffraction limits of lenses, power delivery by collimated laser beams is limited by their divergence, and the application of masks for processing semiconductor chips with photolithographic techniques is limited by diffraction induced by the minute pattern of the masks.

However, there exist numerous applications where diffraction presents an advantage. One example is the diffraction grating used for spectral separation of radiation (see Section 7.3). Another example is the advent of Fourier optics. This relatively new technology is based on the diffraction-limited imaging properties of lenses.

Introduction

The emission and absorption of radiation, as well as the conversion of radiation into other modes of energy such as heat or electricity, all involve interaction between electromagnetic waves and atoms, molecules, or free electrons. Such daily phenomena as the radiative emission by the sun, the shielding of earth from harmful UV radiation by the ozone layer, the blue color of the sky, and red sunsets are all – despite their celestial magnitude – generated by microscopic particles. Most lasers depend on emission by excited atoms (e.g. the He-Ne laser), ionized atoms (the Ar+ laser), molecules (CO or CO2 lasers), impurities trapped in crystal structures (Nd: YAG or Ti:sapphire lasers), or semiconductors (GaAs diode lasers). Similarly, many scattering processes of interest (e.g., Rayleigh or Mie scattering) result from the exchange of energy and momentum between incident radiation and atomic or molecular species. In the previous chapter we saw that the energy of microscopic particles is quantized: their energy can be acquired, stored, or released only in fixed lumps called quanta. The example of the “particle in the box” (eqn. 8.19) illustrated that these energy quanta are specific not only to the particle itself but to the system to which it belongs. Thus, in the box, the energy of the particle is specified by its own mass and by the dimension of the box; in a different box, the same particle will have an entirely different system of energy levels and the quanta will have different magnitudes.

Introduction

Until now, our discussion of the interaction between radiation and matter has concentrated only on the spectral aspects of radiation. The results could determine the wavelengths for absorption and emission or the selection rules for such transitions, but could not be used to determine the actual extent of emission or absorption. These too are important considerations which are needed to fully quantify radiative energy transfer. Unfortunately, none of the classical theories can predict the extent of emission from an excited medium, or even the extent of absorption. Although the discussion in Section 4.9 (on the propagation of electromagnetic waves through lossy media) touched briefly on the concept of attenuation by absorption, it failed to show the reasons for the spectral properties of the absorption or to accurately predict its extent. We will see later that the classical results are useful only as a benchmark against which the actual absorber is compared. The objective of this chapter is therefore to present an introduction to quantum mechanical processes that control the emission and absorption by microscopic systems consisting of atoms and molecules. The results will then be used to predict the extent of emission by media when excited by an external energy source and to evaluate the absorption of incident radiation.

It is now well recognized that all emission or absorption processes are the result of transitions between quantum mechanical energy levels.

Introduction

The discussion in Section 4.10 on the scattering by gas molecules and by submicron particles illustrated the rules of superposition of radiation from several sources. Without much detail, the analysis there showed that the irradiance resulting from such superposition depends on the coherence properties of the sources: when the radiation emanating from several sources is coherent, the fields are additive; if incoherent, only the energies are additive. The distinction between these two modes of addition is important in view of the quadratic dependence (eqn. 4.42) between the irradiance and the electric field. Thus, the analysis of the superposition of radiation emitted by incoherent sources requires only the summation of the irradiance from all sources at a point. No consideration of the frequencies or the phases of the interacting fields is needed. On the other hand, the irradiance that results from the superposition of radiation from a multitude of sources that are coherent with each other depends on the spatial and temporal distribution of the interacting fields, on their phases, and on their frequencies. Thus, before such irradiance can be determined, the distribution of the combined fields must be found. The spatial and temporal distribution of the irradiance is then obtained from the field distribution using (4.42). Here we discuss the details of the superposition of coherent electromagnetic fields. Such detailed analysis can be simplified when considering the superposition of only two beams obtained by splitting one beam emitted by a single source.

Introduction

The classical description of radiation and optics provides two alternative approaches. In the first and more rigorous approach, radiation is viewed as waves of electric and magnetic fields propagating in space. In the second approach, radiation is modeled by thin rays traveling from a source to a target while neglecting all aspects of its wave nature. Rigorous considerations show that the second approach, geometrical optics, is merely a class within the broader picture described by the first approach, which is called physical optics or electromagnetic theory. Electromagnetic theory is normally used to describe the propagation characteristics of electromagnetic waves. It is a very general theory that can depict most effects associated with the propagation of light. Many effects – such as the interference between several waves and diffraction – can be explained only by electromagnetic theory. Electromagnetic theory can also be used to design imaging and illuminating optical devices such as telescopes, microscopes, projectors, and mirrors. However, many of the wave characteristics of radiation are irrelevant for the successful design of these devices; only higher-order corrections require electromagnetic wave considerations. Therefore, in applications where the wave nature of radiation can be neglected, the alternative description of radiation and optics – geometrical optics – can be used. Although the information generated by geometrical optics is less detailed than results of electromagnetic theory, it is far less complex and yet provides a remarkable prediction of the performance of imaging and projecting optical devices.

Introduction

Many of the characteristics of laser beams are determined by properties of their gain medium and by the loss and gain characteristics of the laser cavity. The previous chapter discussed factors that determine the wavelength and spectral bandwidth of laser beams, the characteristics of their longitudinal modes, gain requirements for steady-state oscillation, the ultimate power (or energy) of laser beams, the duration of a laser pulse when Q-switched or mode-locked, and so on. However, this wealth of information is insufficient for design applications where the spatial pattern of the energy delivery must be well defined. To illustrate this, recall that when a laser is used for illumination (such as in PLIF), a relatively uniform distribution of the energy may be required; for material processing, the beam energy may need to be concentrated into a narrow well-defined spot; and for holography or interferometry, the shape of the incident wavefronts may need to be geometrically simple. Furthermore, in all applications, the distribution of the energy passing through any optical element must be carefully controlled to prevent laser-induced damage by localized high-energy concentration. Popular belief has it that laser beams are always collimated and that their wavefronts are planar. But this is true only in the limit, when the beam diameter approaches infinity. Because of diffraction, the beam cannot remain collimated indefinitely when the diameter is finite; with the exception of a narrow range where the beam may be considered as nearly collimated, it must either converge or diverge.