Introduction to Polarizing Optics

Light travels as transverse electro-magnetic waves. The magnetic and electric fields are perpendicular to the direction of propagation and each other, as shown in Figure 1. Defining the direction of a ray and its electric field denotes the three vector directions: magnetic field, electric field, and propagation. Most incoherent light sources include a large number of molecular or atomic emitters. The rays generated from such sources have electric fields with no preferred orientation and, hence, these rays are unpolarized.

Electro-magnetic wave

Figure 1. Electro-magnetic wave

Linearly Polarized Light

Polarization symbols

Figure 2. Polarization symbols

The direction of the electric field vector is utilized to describe polarization. If a light beam includes rays where the electric field vectors are oriented in the same direction, the beam is said to be linearly polarized. If the E field vector is vertical, the light is said to be vertically polarized. Figure 2 displays some symbols utilized to designate linear polarization.

The terms “p” and “s” polarization are easy when linearly polarized light is incident on an optic. If the direction of polarization is perpendicular to the plane of incidence, the ray is said to be “s” polarized. If the direction of polarization is parallel to the plane of incidence, the ray is “p” polarized.

Vector Resolution Simplifies Analysis

Polarization Resolution of a polarized ray into two orthogonally polarized components

Figure 3. Polarization Resolution of a polarized ray into two orthogonally polarized components

In a plane polarized light beam, the polarization direction may not be parallel to the “X” or “Y” axis of an optical system. Analysis of what happens to a light beam, unpolarized or polarized, when going through an optical system is simplified if the beam is split into two components, one polarized along each axis (Figure 3).

The system alters the phase and amplitude of each component beam, and the emergent “beams” are again combined to provide the intensity and polarization state of the output beam.

Circularly and Elliptically Polarized Light

Many kinds of materials have different refractive indices for light polarized in orthogonal directions. Two equal rays travelling through such a medium travel at different speeds and become out of phase. If the phase difference when the rays exit the medium is an odd multiple of π/2, the two rays integrate to give circularly polarized light.

If the Ê vector rotates at the frequency of the radiation but differs in amplitude, then the light is elliptically polarized. Circular and linear polarizations are unique versions of elliptical polarization.

Production of Polarized Light

Polarization by Reflection

When an unpolarized beam of light is incident at an off-normal angle onto an optical surface, the transmitted and reflected beams become polarized to some extent. This is because the reflectance varies for p and s polarized light. Any unpolarized beam is equivalent to p and s linearly polarized components. This effect is vital for choosing beam splitters, and in polarization sensitive spectroscopy and radiometry.

Polarization by Wire Grids

Wire grid polarizer

Figure 4. Wire grid polarizer

A set of fine parallel metal wires can work as a polarizer (Figure 4). The component of the incident radiation, which has its Ê vector parallel to the wire grids, is absorbed and reflected and, hence, the transmitted component is largely polarized. The electric field along the grid drives the conduction electrons, leading to reradiation and Joule heating. For improved efficiency, the space between the wires must be small when compared to the wavelength and, hence, these polarizers can be easily constructed for infrared radiation.

Polarization by Scattering

Generally, light scattered at 90° by charged particles is polarized perpendicular to the plane of incidence. In the atmosphere, sun light is dispersed by charged molecules and particles. The intensity of scatter increases with increased frequency, which is responsible for the blue appearance of the sky. In space, there is no scattering atmosphere and, hence, the sky is not blue.

Polarization by Dichroism

Certain materials absorb light polarized in a single direction more strongly than light of the orthogonal polarization.

Sheet polarizers have this property called dichroism. Dichroic sheet polarizers for the near infrared, visible, and ultraviolet are inexpensive and moderately efficient. However, they cannot tolerate high power or ultraviolet beams and, hence, are not effective for use with intense sources. Dichroic sheet polarizers are relatively insensitive to angle of incidence.

Polarization by Double Refraction

Some transparent materials, such as mica, calcite crystal quartz, and sapphire, do not exhibit a single value for refractive index. This is because of structural anisotropy. The physical properties of these crystal materials differ with the orientation of crystal lattice. The refractive index for a ray travelling through these materials depends on the direction of the ray, with respect to the direction of the Ê vector and the crystalline structure. Materials characterized by two refractive indices are called birefringent.

Birefringent Polarizing Materials

Table 1. Refractive Indices at 589nm

Material n0 ne ne - n0
Calcite 1.658 1.486 -0.172
Crystal Quartz 1.544 1.553 0.009
Mica* 1.598 1.593 -0.005
Sapphire (Al2O3) 1.768 1.760 -0.008

Table 2. Variation of ne - no with wavelength

Wavelength (µm) Calcite Crystalline Quartz Mica* Sapphire
0.2 -0.326 0.0130   -0.0117
0.3 -0.206 0.0103   -0.0091
0.4 -0.184 0.0091   -0.0085
0.5 -0.176 0.0093 -0.0047 -0.0082
0.6 -0.172 0.0091 -0.0048 -0.0081
0.7 -0.169 0.0090 -0.0048 -0.0080
0.8 -0.167 0.0089   -0.0079
0.9 -0.165 0.0088   -0.0079
1.0 -0.164 0.0088    

* As used in Oriel waveplates

The polarization behavior of a crystal is ascertained by the electron bonding and crystal structure. Isotropic crystals exhibit a single index of refraction and, hence, are not birefringent. Sapphire, calcite, magnesium fluoride, and crystalline quartz are birefringent with a single optic axis. These crystals have two main refractive indices, n0 and ne. Table 1 shows the index of refraction values at 589nm for the ordinary (n0) and extraordinary (ne) rays of several materials.

Calcite is extensively utilized as a polarizing material because of its excellent transmission, and the huge difference in the two indices of refraction values. This large difference simplifies the isolation of the two different polarizations. Table 2 shows the difference of ne - n0 with wavelength.

Conclusion

Light from natural and incoherent artificial sources is often slightly polarized. Many lasers, on the other hand, produce polarized radiation. The above details demonstrate some of the ways to produce polarized light from unpolarized light. Some of these methods are employed deliberately, while others are unavoidable and can cause major errors in radiometric measurements.

About Oriel Instruments

Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.

Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.

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

For more information on this source, please visit Oriel Instruments.

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