Light is an oscillating electromagnetic field that can be decomposed into its electric and magnetic field components. The electric field oscillates perpendicular to the direction of propagation of the light. The behavior of this field through time and its orientation determines a property called the polarization of the light.
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Optical polarization is an important property of light across all regions of the electromagnetic spectrum. This is because the polarization state of an electromagnetic beam affects its interactions with different kinds of matter. In spectroscopy, this can be exploited to look at completely different types of light-matter interaction in a molecule of interest. Varying the incident polarization of light can also be used to initiate different types of photoinduced processes in a sample.
Polarization control can also be desirable in many optical applications. This is because using specific polarizations can be an effective way of preventing unwanted back reflections from optical components. Varying polarization states can also be exploited for use in telecommunications and for the transfer of information through optical fibers.1 Changes in the polarization response of a material to given environmental conditions, such as stress as strain, can even be used as a way of creating sensors.2 The challenge is developing optics that can maintain and control the polarization states of the light as required.
Polarized Versus Unpolarized
Not all light sources inherently produce polarized light. The sun and most types of room lights and light-emitting diodes produce what is known as unpolarized light. The difference between polarized and unpolarized light is whether the electric field has a well-defined plane of incidence or fluctuates randomly in time.
While to the human eye unpolarized and polarized light may appear the same, the differences in the behavior of the oscillating electric fields can be seen when comparing how the two types of light interact with matter. Many applications require a well-defined polarization state. For this reason, optical components known as polarizers are often used to select a given polarization.
Polarizers work by only allowing light to pass through with a given orientation of the electric field – the remaining components are all absorbed by the material.
There are three main types of polarization: linear, circular, and elliptical. For linearly polarized light, the oscillation of the electric field is restricted to a single plane. This can be either vertical or horizontal polarization. For many optical applications, it can be helpful to consider the relative direction of the oscillating electric field to the plane of incidence of light reflected from a surface. This is known as either s or p polarization whether the electric field of the light is perpendicular or parallel to the plane of incidence.
Linear polarizers are commonly used to reduce glare in photography as scattered light is often unpolarized. This can be particularly beneficial in machine vision applications where automated data processing algorithms rely on high-quality, clear images, though some applications can make use of polarization information in addition to wavelength and intensity data. Multiple polarizers can be used together to modulate the intensity of light by changing the relative orientation of the filters to absorb more or less light.
Circularly polarized light consists of two perpendicular electric field vectors that have a fixed phase difference of π/2. This means that the direction of the polarization evolves in time. The electric field vectors can be thought of as tracing a circular path as the light propagates in time. Much like linearly polarized light can be considered as either vertical or horizontal, circularly polarized light can be described as left or right-handed depending on the orientation of the electric field vector relative to the direction of propagation.
Elliptically polarized light is similar to circularly polarized light but the phase difference between the two electric field components has different magnitudes. As they still have a phase difference of π/2, rather than generating a circular trace with the electric field vectors, it becomes elliptical in shape.
Applications of Polarized Light
One of the largest applications of circularly polarized light is for the investigation of chiral molecules. Chiral molecules are chemically identical species that form non-superimposable mirror images of each other, much like our own hands. There is no type of rotational motion that means chiral objects will become identical. Most chiral molecules have identical linear absorption spectra but can be differentiated using techniques that make use of circularly polarized light to record circular dichroism spectra.3
Elliptical polarization and other exotic polarization schemes can be used for scattering suppression and depth-resolved imaging schemes.4 Controlling the polarization can be one way to intentionally probe certain layer depths in biomedical imaging. One of the challenges in microscopy for obtaining high-quality images can be the suppression of unwanted signals from the scattering of nearby tissues.
Advances in optics are making it possible to extend the range of polarization control options to more regions of the electromagnetic spectrum, such as the extreme ultraviolet, opening the possibility of new optical methodologies.
References and Further Reading
- Damask, J. N. (2004). Polarization optics in telecommunications (Vol. 101). Springer Science & Business Media.
- Caucheteur, C., Guo, T., & Albert, J. (2017). Polarization-Assisted Fiber Bragg Grating Sensors : Tutorial and Review. Journal of Lightwave Technology, 35(16), 3311–3322. https://www.osapublishing.org/jlt/abstract.cfm?uri=jlt-35-16-3311
- Ranjbar, B., & Gill, P. (2009). Circular Dichroism Techniques Biomolecular and Nanostructural Analyses‐ A Review. Chemical Biology Drug Design, 74, 101–120. https://doi.org/10.1111/j.1747-0285.2009.00847.x
- Silva, A. Da, Deumié, C., & Vanzetta, I. (2012). Elliptically polarized light for depth resolved optical imaging. Biomedical Optics Express, 3(11), 2907–2915. https://www.osapublishing.org/boe/fulltext.cfm?uri=boe-3-11-2907&id=244657