In many spectroscopic and photonics applications, it is desirable to work with light that has been polarized in a particular direction. In this article we explore what polarization is, and how it may be obtained from an unpolarized light source.
Electro-magnetic radiation (light)
What we generally refer to as “light” is more correctly known as 'electromagnetic radiation' (often abbreviated as EM radiation). The the main characteristics of electromagnetic radiation are its frequency and wavelength (λ). We broadly classify frequency into types of EM radiation, such as radio waves, microwaves, terahertz, infrared, visible light, ultraviolet, X-rays and gamma rays. In this sequence, radio waves are radiation with the lowest frequency (and largest wavelength) and gamma rays have the highest frequency (and shortest wavelength).
EM radiation has an electric and magnetic field component which oscillates in phase perpendicular to each other and to the direction in which the radiation propagates. These two oscillating fields are often visualised as in the diagram above and are continually self-propagating.
Unpolarized vs. Partially-polarized vs. fully-polarized
The orientation of the electric field plane is known as the “polarization direction” (E). This can be broken down into three basic categories:
Unpolarized polarized – Most sources of light are described as `unpolarized` light (i.e. natural light), meaning that they consist of many photons with random orientations of their individual polarization directions.
Partially polarized – However, in some circumstances certain polarization directions can be preferentially selected over others and the light becomes `partially` polarized. A common example is specular reflection from surfaces (e.g. a window or the surface of a lake) and many people are familiar with the effect that polarizing sunglasses can have in controlling these reflections.
Fully polarized – At the other extreme, some sources can be `fully` polarized(or plane-polarized), meaning that all of the photons have their electric fields oriented in the same direction. Many types of laser have this property.
Light that has been polarized is useful because it enables the user to be selective over what part of the electromagnetic spectrum is used (whether for photography, night-vision, coloured lighting or analytical measurement). Of course, this filtering of light can offer the user a finer level of detail for their application, similarly to a sharp knife or a small paint brush.
How polarization affects the reflection of light
Many interactions of light with matter depend on its polarization. For example, at a reflective interface, components of light whose polarizations are oriented perpendicular to the plane of incidence are reflected more strongly than those oriented parallel to it. At one angle of incidence in particular – Brewster’s Angle – the reflected ray is completely polarized perpendicular to the plane of incidence.
Common terminology when discussing light reflected from a surface is to refer to `p-polarized` and `s-polarized` rays.
Perpendicular Polarization (Transverse Electric) – This occurs when the magnetic field is parallel to the plane of incidence, but the electric field is perpendicular to the plane of incidence. This is also known as `S-polarized` light, the `s` coming from the German word for perpendicular, senkrecht.
Parallel Polarization (Transverse Magnetic) – Meanwhile, parallel polarization is the opposite. This occurs when the electric fiels is parallel to the plane of incidence and the magnetic field is perpendicular. This is also known as `P-polarized` light.
Controlling reflections is an important application of polarizers. If the specific interactions with the surface are of interest, then the polarizer can be oriented parallel to select only the reflected rays; if the reflections are an unwanted source of measurement noise, the polarizer can be oriented to reduce them.
How a Wire Grid Polarizer works
Specac provide a range of infrared wire grid polarizers for use in analytical testing across the mid-and-far-infrared spectrum of light. Some of these are compatible with our spectrometer accessories, allowing the user to mount a polarizing filter directly into an accessory or sample cell, further refining their analysis.
A Wire Grid Polarizer are included in the Specac polarizer product range. It consists of an array of fine parallel conductive wires placed perpendicular to the incident beam, with the spacing of the wires being smaller than the wavelength of the light being filtered.
Some light waves will be parallel to these wires and those electrons will move along the wires instead of passing through to the other side of the filter. Any electrons that are not angled at the same plane as the wires (perpendicular) do not collide and are therefore free to move to the other side.
For waves with their electric fields perpendicular to the wires, the electrons cannot move far across the wires (remember, the diameter of the wires are much smaller than the wavelength of light). So the perpendicular light passes through un-blocked (save for very small amounts).
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XRF spectroscopy and FTIR spectroscopy is covered, along with other uses for sample preparation equipment. Tips on sample preparation and then a Q/A session also features.
Equipment/supplies mentioned includes:
Manual Hydraulic Press
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XRF Sample Cups
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