Dichroism is an optical property of certain materials to selectively absorb light of different polarizations. This means that the material will appear to have different colors when viewed through polarizing filters oriented in different directions. The materials that exhibit dichroism are called dichroic materials. Mirrors, filters, and prisms with optical coatings are usually used dichroic materials to reflect and transmit light over certain wavelengths. This material absorbs one plane-polarized component of transmitted light more strongly than the other, resulting in differing coloration of the transmitted or reflected light.

Figure 1: Dichroism in a dichroic crystal

A dichroic prism, commonly employed in certain camcorders utilizes multiple coatings to divide light into its constituent red, green, and blue components, which are subsequently recorded on separate CCD arrays. Dichroic materials are used for a variety of applications, from splitting visible light into distinct wavelengths to absorbing light rays of differing polarizations. Figure 1 shows a dichroic crystal that shows the dichroic property.

Dichroism and Polarization  

Dichroism is closely related to polarization, which refers to the orientation of the electric field vector of light. By studying the polarization-dependent absorption of light in a material, dichroism can provide valuable information about the electronic and structural properties of the material, making it an important tool in various fields such as materials science, chemistry, and biophysics.

Figure 2: Linearly and circularly polarized light

The two types of polarization in electromagnetic (EM) light are linear polarization and circular polarization. Linear polarization occurs when the electric field vector of the EM wave oscillates in a single plane, while the magnetic field oscillates perpendicular to the electric field in the same plane. Circular polarization occurs when the electric field vector of the EM wave rotates in a circle, rather than oscillating in a plane. Figure 2 represents a linearly and circularly polarized light.

Linearly polarized light exhibits oscillations that are restricted to a single plane. Any polarized light state can be expressed as a combination of two linearly polarized states that are perpendicular to one another, typically referred to as vertical and horizontal polarization. If two light waves with horizontal and vertical polarization respectively are in phase, they will generate a resultant light wave that is linearly polarized at an angle of 45º. The properties of the resulting electromagnetic wave are determined by the relative intensities and phase difference of the constituent waves.

When one of the polarized states is out of phase by a quarter of a wavelength relative to the other, the resulting wave takes on a helical shape and is called circularly polarized light (CPL). A device that converts linearly polarized light to circularly polarized light is called a quarter-wave plate. By delaying one of the linear components of the light beam relative to the other by a quarter of a wavelength, a quarter-wave plate can convert linearly polarized light to circularly polarized light. This results in the production of either left- or right-circularly polarized light. If left and right circularly polarized waves with equal amplitudes and wavelengths are superimposed, they create a plane-polarized wave.

Types of Dichroism

There are two types of dichroism: linear dichroism and circular dichroism. Linear dichroism refers to the selective absorption of light polarized in one direction over the other. Circular dichroism is the phenomenon in which there is a difference in the absorption of right- and left-circularly polarized light. Both types of dichroism are commonly observed in a variety of materials, including crystals, solutions, and biological molecules.

Circular dichroism (CD) is the most commonly observed form of dichroism. It involves the selective absorption of two types of circularly polarized light: left-handed circularly polarized light (L-CPL) and right-handed circularly polarized light (R-CPL). Circular dichroism arises when a molecule contains chiral chromophores, such as those found in DNA molecules that possess a chiral sugar within their structure, resulting in intrinsic asymmetry. This leads to a strong interaction between the chromophore bases and generates an intense circular dichroism spectrum.

When a plane-polarized light wave (blue) traverses a medium that does not absorb the left circularly polarized component (red) of the wave but highly absorbs the right circularly polarized component (green), the intensity of the green component decreases in comparison to the red one. 

Applications

Dichroic materials are widely used in the field of optics due to their unique optical properties. One of the most common applications of dichroic materials is in color filters used for photography and video. Dichroic filters are able to transmit only specific wavelengths of light, resulting in vivid, true-to-life colors in photographs and videos.

Along with filters, dichroic materials are also used in polarizing filters. These filters are used to remove unwanted reflections from surfaces, such as water or glass, and can also be used to reduce glare from electronic displays. Polarizing filters work by selectively absorbing light waves that are polarized in one direction while allowing those polarized in the other direction to pass through.

Dichroism is also observed in certain biological molecules, such as DNA and proteins. Circular dichroism spectroscopy is a powerful analytical tool used to study the structure and function of these molecules. Scientists can gain valuable insights into the three-dimensional structure of biological molecules by measuring the differential absorption of left- and right-circularly polarized light.