Theory of Brewster Angle Microscopy

By AZoNano

Table of Contents

Introduction

Light reflection from surfaces is part of our everyday experience. A wonderful optics property is the possibility of having zero reflection. It must be noted that no reflection takes place from a clean and perfect interface illuminated under a unique angle of incidence with p-polarized light. This phenomenon is explained by Brewster´s law providing the Brewster’s angle for the involved optical media. Any modification of the optical properties at the interface will lead to reflection. This fact forms the basis of Brewster angle microscopy, a recent technique for the study of nanofilms at the water/air interface or at the surface of other transparent non-adsorbing dielectric substrates.

By simply having a monolayer on an interface Brewster’s law is not satisfied and the covered part of the surface reflects light with a very low level of intensity. By increasing the intensity of the illumination, the contrast between the covered and the clean part of the surface can be considerably increased. By using a microscope optic and a CCD-detector, high contrast images of the lateral morphology of the layer can be taken. Typical fields of application are colloid, life sciences, surface chemistry and materials research.

Brewster’s Law

When a light beam passes through the boundary between two media having varying refractive indexes, normally a small amount of the light is reflected. The Brewster's angle (q_B) is a specific angle of incidence where light with a single particular polarization state cannot be reflected. The state that cannot be reflected is parallel to the plane of incidence. Light with this polarization is said to be p-polarized.

As all the p-polarized light is refracted and transmitted, any light reflected from the interface at this angle must be s -polarized. A stack of plates or a glass plate placed in a light beam under Brewster's angle can thus be used as a polarizer.

For glass as a medium (n₂ » 1.5) in an ambient air (n₁ » 1), Brewster's angle for visible light is around 56° to the normal while for an air-water interface (n₂ » 1.33) it reaches around 53°. Since the refractive index for a given medium is a function of the light wavelength Brewster's angle will also vary with wavelength.

Brewster Angle Microscopy

Brewster angle microscopy was introduced in 1991. Zero light is reflected from the air-water interface under Brewster’s angle incidence when p-polarized light is used. With a constant angle of incidence, a monolayer is formed on the water surface, which modifies the Brewster’s angle condition and observes light reflection. Figure 1 demonstrates the principle of obtaining the contrast for a surface covered with a nanofilm using Brewster angle microscopy.

Figure 1. Principle of Brewster angle microscopy: making use of Brewster`s Law for imaging ultra thin films with high contrast.

Brewster angle microscopy is an excellent visualization method for substructures having a long range orientation order. In order to obtain a contrast between subdomains of different molecule orientations, an analyzer is placed in the reflected beam path.

Instrumentation

The origin of commercial Brewster angle microscopes was the instrument developed by Dirk Hönig in his diploma-thesis shown in Figure 2.

Figure 2. Prototype, Diploma Thesis, Dirk Hönig.

Since the first presentation at the LB6 in Paris, Brewster angle microscopy has been established as a worldwide standard technique for the investigation of ultra thin films. Nanofilm´s BAM1 as shown in Figure 3 was the first commercial instrument available followed by several other versions of the BAM2 as shown in Figure 4.

Figure 3. BAM1 (1991) – first commercial Brewster Angle microscope.

Figure 4. BAM2 Brewster Angle Microscope (1995)

Presently Accurion offers two different Brewster angle microscopes. The nanofilm_ep3bam as shown in Figure 5 is based on the ellipsometric platform nanofilm_ep3 and can be upgraded to an imaging ellipsometer.

Figure 5. nanofilm_ep3bam (since 2003)

The novel nanofilm_ultrabam as shown in Figure 7 is based on a completely new optical pathway of light. The unique imaging optic provides totally focused images at a maximum of 35 fps.

Figure 6. nanofilm_ultrabam (since 2010)

Hence, for the first time in a commercial instrument high resolution and overall focused real time imaging of monolayers becomes possible. It enables the visualization of Langmuir monolayers or adsorbed films in real time.

Applications

Some of the applications of Brewster angle microscopes are listed below:

• To determine the morphological features of monolayers during compression/decompression
• To determine the inner structure of 2D condensed phase domains
• To enable formation dynamics of non equilibrium structures
• For Monolayers & chirality
• For monitoring 2D-structures
• For biofluids

About Accurion

Accurion is a high-tech company providing advanced instrumentation in the field of surface analysis and active vibration isolation.

This information has been sourced, reviewed and adapted from materials provided by Accurion.

For more information on this source, please visit Accurion.