Ion implantation is a superb method for modifying surface properties of materials since it offers accurate control of dopant composition and structural modification at any selected temperature. In the field of semiconductor technology there was a time lag of some 20 years from the initial development of ion implantation to its becoming a cornerstone of production technology. A similar delay in the acceptance time occurred for metal surface treatments. For insulating crystals and glasses, the use of ion beams to modify such crucial optical parameters as refractive index, reflectivity, colour centre content, and luminescence has now passed this 20-year apprenticeship, and the subject is expanding to include the valuable application phase. Appreciation of possible uses of ion implantation is gaining momentum, in part as a result of the ease with which one can fabricate optical waveguides and waveguide lasers and tailor electro-optic and non-linear properties of the key materials of modern optics. Our own experience with these ion implanted property changes, and potential applications, encompasses a diversity of examples, from lasers to studies of fundamental imperfections in insulators, to fabrication of car rearview mirrors.

Since Sussex has been among the pioneers in the study of work with optical materials, we have written a text which has perhaps presented a disproportionate number of examples using our own data. They do, however, typify many aspects of the subject. The topics cover the basic ion beam interactions in solids, followed by the optical effects of absorption and luminescence. We have then included a chapter on waveguide theory and analysis in order to lead into the very exciting examples of ion implanted lasers, second harmonic generation and nonlinear waveguide optics.

Luminescence processes

Luminescence transitions may occur within localised defect sites, for example to give the characteristic line emissions of rare earth ions, narrow emission bands of chromium in ruby, or they may produce broad bands from charge transfer between defects. Overall, emission bands may vary greatly in width, but nevertheless the luminescence spectra provide a measure of specific defect types, and even offer some quantitative measure of the changes in defect concentrations. Since the excitation energy for luminescence may be provided by many routes, ion implantation is no exception and it frequently produces strong luminescence from insulating targets. This feature is often used as a means of aligning the ion beam and it is common practice to have defining apertures with silica plates to check the ion beam position visually. Such intense luminescence can reveal a number of features relating to the changing defect state of the target. For example, in many of the materials used to form optical waveguides by ion implantation there is a decrease in luminescence intensity which approximately follows the amorphisation in the crystal. Hence, one has a visual estimate of the progress of the amorphisation. Quantitative recording of the wavelength dependence of the signal, in terms of luminescence efficiency and spectral changes, should provide details on not only the defects pre-existing in the material but also the ion beam induced changes. Consequently, a number of research groups have used the luminescence produced during implantation to follow such modifications.

Lattices modified by the ion beam will show changes in their subsequent luminescence performance and the effects of implantation have been recorded in photoluminescence, laser emission, cathodoluminescence and thermoluminescence.

Analysis methods using absorption, ESR and RBS

Optical methods of studying defects have the advantage that if each defect has characteristic energy levels which lie within the forbidden energy gap, then they show separable optical absorption and luminescence bands. Higher photon energy absorption generally monitors electronic transitions, whereas infra-red absorption records vibrational spectra. Many of the optical transitions which result from the presence of impurities have energies in the visible part of the spectrum and consequently the defects are referred to as colour centres. Examples of colour centres are widespread and include the impurities which give colouration to ruby and sapphire or stained glass. They are the basis for photographic and photochromic materials, and frequently involve a mixture of impurity and intrinsic defects. Whilst analysis of absorption bands may determine defect symmetry and inter-relationships of different colour centres, it is unusual to be able to confirm precise models of defect sites solely from the absorption data. In this respect the processes which involve hyperfine interactions such as Electron Spin Resonance (ESR), Electron Nuclear Double Resonance (ENDOR), Mossbauer spectroscopy or spin precession techniques provide more specific answers if they can be applied. In the ion implantation literature there are frequent presentations of data from Rutherford Back-scattering Spectrometry (RBS) to give the depth distributions of impurities or damage in the target material. In part, RBS appears to be popular for implantation analysis because it requires a high energy ion accelerator, which is normally a feature of an implantation laboratory. The information is useful but, like electron microscopy, it rarely gives precise details of individual defect arrangements.

Over the past decade ion implantation has been demonstrated as a suitable technique for the fabrication of waveguide structures in an ever increasing number of optical materials. The stage has now been reached where actual device-oriented structures are being designed and implemented using this technology. This chapter will deal with the progress which is being made in the development of useful devices, and how ion implantation is being used to achieve these ends.

Many authors have recognised the advantages of waveguide structures for signal processing, for coupling to optical fibres or for frequency conversion. Optical circuitry has been proposed which, at least initially, was purely hypothetical, but has advanced to include ideas of entirely solid state waveguide structures (sometimes termed holosteric systems). Forseeable objectives include multi-frequency, or tunable, compact lasers in which the pump power is provided by a semiconductor diode. This inherently waveguide power source could then be matched into other waveguides for a combination of frequency conversion, SHG, pumping a tunable laser waveguide such as alexandrite or Ti:sapphire, or driving an optical parametric oscillator (OPO). The conversion efficiencies in the various stages are often as high as 30%, hence even in a several-stage process a 100 W semiconductor array could result in a few watts of tunable laser power. Such systems could of course supersede the highly inefficient Ar or Kr gas laser sources for many applications. The concepts are clear, and this final chapter will indicate how close the components are to realisation when using ion implantation fabrication routes.

Characteristics of ion implanted waveguides

A waveguide is characterised by a region of high refractive index bounded by regions of lower index. The confinement of the light, as well as the spatial distribution of optical energy inside the guiding layer depends on the refractive index profile. There are several conventional techniques for fabricating optical waveguides. These techniques, including epitaxial growth, metal diffusion and ion exchange, increase the refractive index of the surface layer for a few microns (Figure 5.1), and this high index layer is surrounded by the low index of air and substrate to form an optical waveguide. Ion implantation, as a surface modification technique, can modify the optical properties of an insulator surface. However, when light ions are used, particularly when dealing with crystals, instead of changing the refractive index of the surface layer, a low index optical barrier is built up at the end of the ions' track due to elastic energy deposition from ions to the lattice. Therefore, the surface layer, ideally the same as the substrate, is surrounded by the low index of air and this optical barrier (Figure 5.1(c)). During the ion implantation, some point defects may be produced in the surface layer due to ionisation and excitation when the ions are travelling fast. These simple defects will change the properties of the material, and induce absorption and scattering loss. In practice, it has been found that post-implant annealing at a moderate temperature can either reduce or aggregate these defects depending on the material in question. In many materials, a low loss optical waveguide (∼0.5dB/cm) can be produced by ion implantation and subsequent annealing.

Development of ion implantation

The control of surface properties is of paramount importance for a wide range of materials applications, and craftsmen and technologists of all scientific disciplines have battled with problems of corrosion, surface hardness, friction and electrical and optical behaviour for many hundreds of years. Even for the simplest of articles, whether they be knives, bottles or non-stick frying pans, the manufacture of materials which have the desired surface properties is often incompatible with bulk performance, and so there is an emphasis on finding ways to modify surface layers. Processes such as thermal quenching prove effective for hardening steel and glass bottles but lack the finesse which is required for more sophisticated technology. Instead, these use more controllable treatments, including the deposition of surface coatings or diffusion of impurities into the surface layer and, of course, ion implantation.

Historically, ion implantation has generally been the last of the treatments to receive widespread acceptance. The reason for this is that, compared with coating and diffusion treatments, it appeared to require more complex and expensive equipment which was not readily available. Figure 1.1 indicates that implantation systems may come in several levels of complexity. There are those similar to sophisticated laboratory research machines, which have ion sources, pre-acceleration, mass analysis followed by additional acceleration and then the target region. Commercial applications with requirements of uniformity and a large sample throughput may result in sample handling and beam sweeping equipment as complex and expensive as the accelerator.

Predictions of range distributions

An essential first step in the consideration of ion implantation effects is to understand how energy is coupled into the target material. We will first present examples of energy transfer and ion range, and then indicate how these features have been calculated. In practice there has been a continuous interaction between the theoretical and experimental assessments of ion ranges. This has resulted in modifications to the theories so that there are now tabulations and computer codes which predict ion ranges in virtually any ion/target combination. These computations are accurate to within 5–15%. Consequently, although it is useful to know the underlying assumptions of the range theories, and hence their limitations, the majority of the profiles for the distributions of implanted ions are calculated from standard computer simulations. Since knowledge of the ion range, damage distribution or surface sputtering involves many factors in addition to the initial ion range, the existing level of accuracy is perfectly acceptable. Indeed, divergence between measured and computed ranges is frequently not a result of a failure of the computation, but, rather, it results from the fact that such computer codes do not allow for subsequent migration and secondary processes. As has already been mentioned briefly in Chapter 1, there are two main processes which slow down the incoming ion. These are electronic excitations and nuclear collisions. The rate of energy transfer for each process is a function of the nuclear charge and mass of the incoming ion (Z1, M1), and the target (Z2, M2), as well as the energy.

Ion implantation may be used to change the optical properties of insulators, either because of the chemical presence of the dopant ions, or more generally because of the radiation damage caused during their implantation. The latter effect produces a significant change in the refractive indices of most materials, and consequently He+ implantation has been used to define optical waveguides in a wide variety of substrates. These include electro-optic, non-linear and laser host materials, with key successes in quartz, LiNbO3, KNbO3, KTiOPO4 (KTP), Bi4Ge3O12 (BGO), garnets such as Y3Al5O12 (YAG), and amorphous glasses such as silica and lead germanate.

Although this technique has wide applicability, the refractive index profiles vary considerably between materials, and even between different indices of the same material. The index change may vary in degree, and even in sign, for both the nuclear collision and the electronic ionisation regions. These effects are discussed in this chapter, together with their applicability in the formation of optical waveguides, and more complex structures. Of particular interest are the three detailed examples of quartz, LiNbO3 and Bi4Ge3O12 since between them they embody most of the features so far observed in ion implanted waveguides in insulating materials. The performance of the implanted waveguides is considered in terms of their thermal stability and their attenuation due to absorption, scattering and tunnelling losses. The He+ guides are first compared with those produced by conventional chemical diffusion methods. At the end of the chapter, waveguides formed by implantation of chemically active components are discussed.

Spectroscopy is concerned with the interaction of light with matter. This monograph deals with collision-induced absorption of radiation in gases, especially in the infrared region of the spectrum. Contrary to the more familiar molecular spectroscopy which has been treated in a number of well-known volumes, this monograph focuses on the supermolecular spectra observable in dense gases; it is the first monograph on the subject.

For the present purpose, it is useful to distinguish molecular from supermolecular spectra. In ordinary spectroscopy, the dipole moments responsible for absorption and emission are those of individual atoms and molecules. Ordinary (or allowed) spectra are caused by intra-atomic and intra molecular dynamics. Collisions may shift and broaden the observable lines, but in ordinary spectroscopy collisional interactions are generally not thought of as a source of spectral intensity. In other words, the integrated intensities of ordinary spectral lines are basically given by the square of the dipole transition matrix elements of individual molecules, regardless of intermolecular interactions that might or might not take place. Supermolecular spectra, on the other hand, arise from interaction-induced dipole moments, that is dipole moments which do not exist in the individual (i.e., non-interacting) molecules. Interaction-induced dipole moments may arise, for example, by polarization of the collisional partner in the electric multipole field surrounding a molecule, or by intermolecular exchange and dispersion forces, which cause a temporary rearrangement of electronic charge for the duration of the interaction.

In Chapter 5 the absorption spectra of complexes of interacting atoms were considered. If some or all of the interacting members of a complex are molecular, additional degrees of freedom exist and may be excited in the presence of radiation. As a result, besides the translational profiles discussed in Chapter 5, new spectral bands appear at the rotovibrational transition frequencies of the molecules involved, and at sums and differences of such frequencies – even if the non-interacting molecules are infrared inactive. The theory of absorption by small complexes involving molecules is considered in the present Chapter.

We will be concerned with the spectral bands in the microwave and infrared regions. The translational and the purely rotational bands appear both at low frequencies and form in general one composite band, especially at the higher temperatures where individual lines tend to overlap (‘rototranslational band’). Moreover, various rotovibrational bands in the near infrared will be considered, such as the fundamental and the overtone bands. Even high overtone bands in the visible are of interest, e.g., of H2. We have seen in Chapter 3 that induced spectra of the kind are readily discernible in gases whose (non-interacting) molecules are infrared inactive, but evidence exists that suggests the presence of induced absorption in the allowed molecular bands as well. Induced absorption involving electronic transitions will be briefly considered in Chapter 7.