Birefringent Thin Films and Polarizing Elements

The following sections are included:

• Dedication
• Preface to the Second Edition
• Contents
• List of Figures
• List of Tables
• Glossary

Light and other electromagnetic waves are our most important carriers of information. In everyday life we make direct use of the properties of light in receiving and analysing visual data. The focusing action of the lens of our eye depends on the speeds of light in air and in the biological materials of the eye, and the colour that we see for an object is determined by the wavelengths of the light waves that enter our eye to form an image on our retina…

The interaction of light with matter has been modelled successfully at both microscopic and macroscopic levels. In the former case individual actions involving an atom and a photon can be considered and the quantum model, as it is called, is particularly useful for explaining phenomena such as the emission of characteristic line spectra by atoms and the photoelectric effect in which light releases charge carriers in an optical medium…

In practice, polarizing components are used with many different types of optical source. The result obtained in a particular case depends on the characteristics of the light that is incident on the polarizing component, including the spread of wavelengths and the polarization. Thus the light from the sun is broadband and unpolarized, the light from a spectral lamp may be described as quasimonochromatic and unpolarized, and the radiation emitted by a cw laser may be coherent and linearly polarized…

Many applications in optics and, indeed, other branches of physics require computation of an output such as transmittance that results from a given input to a component. A general method of approach for a linear system is to represent the input and output parameters by column vectors and determine a transfer matrix for transforming from the input column vector to the output vector, or vice-versa. A transfer matrix may apply to an entire component, to a distinct part of a component, or to any pair of reference planes within a part…

In this chapter we consider methods for computing the optical reflection and transmission coefficients from bulk and layered biaxial media. Our objectives are to enhance understanding of this problem and to provide simple, yet general algorithms for computation…

A biaxial thin film deposited onto a glass substrate, and of sufficient thickness, can act as an optical planar waveguide. Light that enters the film may be trapped by total internal reflection at both the film/cover interface and the film/substrate interface. One useful property of waveguides is that very large power densities can be achieved. For example, a laser beam can be coupled into a film of thickness about 1 μm…

Biaxial thin films are deposited in much the same way as isotropic films. The main difference is that the substrate is positioned to receive the impinging vapour at an oblique angle during the deposition of a biaxial film, and at normal incidence for an isotropic (or uniaxial) film…

Form birefringence is refractive anisotropy due to shape. As one example, a system of parallel layers is asymmetric in a three-dimensional sense and exhibits uniaxial form birefringence. As a second example, the nanostructural columns in a film deposited at an oblique angle are asymmetric in shape and the film is biaxial…

A natural progression in nanoengineering practice is to twist the columns of a growing birefringent film, to develop inorganic materials with optical properties similar to liquid crystals. In turn a new field of chiral optics may follow, in which thin film structures control both the intensity and state of polarization of light beams. As in the case of inorganic birefringent structures the technology is planar and suitable for integrated optical devices…

In this chapter we introduce two methods, one exact and one approximate, that treat the helical medium of the previous chapter as a continuum. Rather than forming the medium by incremental rotation of successive ultra-thin films, we will here assume that the medium is formed by a continuous rotation of the substrate during deposition…

For a given wavelength and p- or s-polarization the symmetric stack of isotropic films (A/2)B(A/2) is known to be equivalent to a single layer. This result is evident from the form of the characteristic matrix that represents the group of layers. The refractive index of the equivalent layer is called the Herpin index [132] and, in practice, the method allows an index that is otherwise unavailable to be generated…

There is a rule-of-thumb in thin film physics that states “the diameter of nanostructural columns is about th the thickness of the film”. Hence it is not surprising that very thick vacuum-deposited films can have a milky appearance due to scatter from the nanostructure. Further, when the films are deposited obliquely, the scatter is found to be spatially anisotropic…

The tilted-columnar nanostructure that endows an evaporated film with unique polarizing properties for light at normal incidence also has the potential to be its Achilles heel. Moisture from the atmosphere is able to enter into the voids between loosely packed columns, and this has the effect of decreasing the difference in refractive index between the crystallites and the voids. Form birefringence depends on this difference in index, and decreases as moisture penetrates into the coating…

It is well known that in situ optical measurements can reveal morphological changes that occur during the deposition of thin metal films. For example, the knee feature that is observed in the transmittance versus mass thickness profile during the deposition of silver at normal incidence is due to a refractive index resonance that occurs as the film structure changes from a planar array of globules to a more or less continuous layer. Further, nanostructural differences in deposited and sputter-etched gold films, deduced originally from observations of the knee feature and later confirmed by TEM studies, have been used to explain normal incidence optical anisotropies in these films. Thus, the basic morphology of a thin film (particularly a noble metal) may be probed by monitoring the reflectance or transmittance with light at normal incidence as the film is being deposited, and nanostructural effects lateral to the deposition plane can be probed by measuring normal incidence optical anisotropies.

An ideal linear polarizer can be regarded as a device that produces linearly polarized light from any input light. Thus a mixture of unpolarized light and elliptically polarized light, for example, should lead to a transmitted beam of linearly polarized light. Several different types of linear polarizer are available [162], and it is possible to classify them according to the physical principles that are utilized. Already we have seen that any polarization state can be resolved into two superposed linearly polarized beams vibrating, say, along the y- and z-axes, and the action required of a linear polarizer can be considered to be isolation of one of the component beams…

A polarized beam of light can be represented as the superposition of two linearly polarized beams. As in other sections of this book, we assume that the beam is travelling along the x-axis, that one of the components vibrates in the y-direction and the other vibrates along z. If the electric fields along y and z are in phase then the polarization of the beam is linear. If the field strengths are unequal and E_y lags E_z by π/2, then the polarization state of the beam is right elliptical…

Birefringent filters are used in a variety of applications and scientific disciplines. As examples, the Lyot–Ohman filter is used in astronomy and another series-of-plates filter is used as an intracavity tuning device for dye lasers. Birefringent filters have specific properties that cannot be matched by conventional high-resolution devices such as the Fabry–Perot interferometer. Thus, the Lyot–Ohman filter constructed with wide-field elements provides both a wide field of view and a narrow band of wavelengths…

Thin film design methods and production techniques have been developed over a long period for isotropic layers. Examples of coatings that are readily available include antireflection coatings, multilayered high-reflectance stacks, narrowband filters, broadband filters, lowpass and highpass filters. These may be fabricated to a predetermined plan by using a quartz crystal monitor to achieve the required film thicknesses, but in many cases the coating is designed as a stack of low-index and high-index materials and all the films are deposited to have an optical thickness of one quarter-wave. It is relatively easy to monitor the thickness of a quarter-wave film during deposition, by waiting for a turning point in the reflectance or transmittance. One disadvantage of both methods is that only a few materials are suitable for optical coatings, and hence only a few refractive indices are available to the designer…

The following sections are included:

• Appendix A: PS Sampler
  • Anisotropic Antireflection Coating
  • Anisotropic Reflector
  • Anisotropic-Phase Reflector
  • Achromatic Antireflection Coating
  • Achromatic Fifty Percent Reflector
  • Single-Cavity Narrowband Filter
  • Multi-Cavity Narrowband Filter
  • Edge Filter
  • Common-Index Thin Film Polarizer
  • Multi-Cavity Linear Polarizer
• Appendix B: Birefringent Thin Films Toolbox
  • Quick Reference
  • Commands and Functions
• Bibliography
• Index