![]() Radially polarized beams frequently exhibit a kind of donut profile.Ī radially polarized laser beam may be generated from a linearly polarized beam with some optical element, but it is also possible to obtain radially polarized emission directly from a laser. Note that radial or azimuthal polarization state requires a zero electric field strength and thus also a vanishing optical intensity on the beam axis it is not compatible with a Gaussian beam, for example. For example, there are beams with radial polarization, where the polarization at any point on the beam profile is oriented in the radial direction, i.e., away from the beam axis. However, there are light beams where that is not the case. In the previous cases, the direction of the electric field vector was assumed to be constant over the full beam profile. ![]() That is called the Faraday effect, and is exploited in Faraday rotators and Faraday isolators. While optical activity usually results from the presence of chiral molecules, with a concentration difference between the two possible enantiometers, it can also be induced by a magnetic field in a substance which is not naturally optically active. Optical activity can be accurately measured with polarimeters. Some optically active substances such as ordinary sugar (saccharose) can produce substantial rotation angles already within e.g. True polarization rotation, where a linear polarization state is always maintained (just with variable direction), can occur in the form of optical activity. With a combination of one half waveplate and two quarter waveplates, one can realize a polarization controller, with which one can do arbitrary polarization conversions by properly rotating the three plates.Īs explained above, a waveplate or other birefringent optical element may rotate the direction of linear polarization, but more generally one will obtain an elliptical polarization state after such an element.With a quarter waveplate (/4 plate), having its axis oriented at 45° to the polarization direction, one may convert a linear polarization state to a circular one (and vice versa).With a half waveplate (/2 plate), one may rotate a linear polarization state into any other direction.The polarization state of light is often manipulated using different kinds of optical waveplates. If the oscillations of the horizontal and vertical electric field vector do not have the same strengths, one has the case of an elliptical polarization, where the electric field vector, projected to a plane perpendicular to the propagation direction, moves along an ellipse. For an observer looking against the beam, the rotation of course has the opposite direction. For example, left circular polarization means that the electric (and magnetic) field vector rotates in the left direction, seen in the direction of propagation. One distinguishes left and right circular polarization (see Figure 2). Effectively, this leads to a rapid rotation of the electric field vector – once per optical cycle – which maintains a constant magnitude. Circular and Elliptical Polarization Figure 2: Different polarization states of laser emission, illustrated for a few-cycle pulse propagating from left to right.Ī circular polarization state can mathematically be obtained as a superposition of electric field oscillations in the vertical and horizontal direction, both with equal strength but a relative phase change of 90°. Note that a rotation of the polarization by 180° does not lead to a physically distinct state. Of course, the polarization can have any other direction perpendicular to the beam axis. The electric field (red) oscillates in the vertical direction and the magnetic field (blue) in a direction perpendicular to the drawing plane. Figure 1: An electromagnetic wave traveling from left to right. In a different perspective, this is also shown in the second part of Figure 2. For example, a laser beam propagating in direction may have the electric field oscillations in the vertical () direction and the magnetic field oscillations in the horizontal () direction (see Figure 1) it can be called vertically polarized or -polarized. ![]() The direction of polarization is taken to be the direction of the electric field oscillations (i.e., not the magnetic ones). In the simplest case, a light beam is linearly polarized, which means that the electric field oscillates in a certain linear direction perpendicular to the beam axis, and the magnetic field oscillates in a direction which is perpendicular both to the propagation axis and the electric field direction. Linear Polarization The direction of polarization is associated with the electrical oscillations, not the magnetic ones. ![]() More specifically, light waves are recognized as electromagnetic transverse waves, i.e., with transverse oscillations of the electric and magnetic field. In many respects, light can be described as a wave phenomenon (→ wave optics).
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