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Introduction to Nanosensors
Published in Vinod Kumar Khanna, Nanosensors, 2021
Reflection is the process in which light bounces back or is deflected at the boundary between two media. Refraction is the process of change of direction in which light bends in crossing from one medium to another of different density. The ratio of the speed of light in a vacuum to the speed of light in a medium is called the refractive index of the medium. Dispersion is the splitting of light of mixed wavelengths into its component colors. Diffraction is the spreading of light when it passes through an aperture or around the edge of a barrier. Polarization is the process of restricting the vibrations of the electric vector of light waves to one direction; in unpolarized light, the electric field vibrates in all directions perpendicular to the direction of propagation.
Polarization
Published in Myeongkyu Lee, Optics for Materials Scientists, 2019
Understanding and manipulating the polarization of light is crucial for many optical applications. Light is an electromagnetic wave. In free space, both the electric field and magnetic field are oscillating perpendicular to the direction of propagation. By convention, the polarization of light refers to the direction of the electric field. If the direction of the electric field of light is well defined, it is called polarized. The most common source of polarized light is a laser. Light is said to be unpolarized when the electric field has randomly fluctuating directions. Many light sources such as sun, lamps, and incandescent bulbs produce unpolarized light. Such ordinary light is produced by a number of independent atomic sources whose radiation is not synchronized. The resultant electric field does not maintain a constant direction of oscillation. As discussed in the previous chapter, an isotropic medium preserves the polarization of a wave because it does not differentiate between polarization states. However, an anisotropic medium (such as a birefringent crystal) can modify the polarization state of a wave. In addition, some crystals preferentially absorb light polarized in particular directions. Polarization is an important property of light for many optical systems. The polarization of light is classified into three types, depending on how the electric field is oriented: linear polarization, circular polarization, and elliptical polarization.
Polarization
Published in Vasudevan Lakshminarayanan, Hassen Ghalila, Ahmed Ammar, L. Srinivasa Varadharajan, Understanding Optics with Python, 2018
Vasudevan Lakshminarayanan, Hassen Ghalila, Ahmed Ammar, L. Srinivasa Varadharajan
In Chapter 3, we learned that electromagnetic waves are transverse with the magnetic field B→ and the electric field E→ lying in a plane perpendicular to the direction of propagation of the wave. In addition, they are also perpendicular to each other. If we were to observe how the E→ changes with time at any given point in space, in general, we would find that it changes in a random manner; such a wave is said to be unpolarized. In unpolarized light, E→ has no favored orientation and the planes of vibration are symmetrically distributed around the direction of propagation. On the other hand, if the variation takes a certain predictable form, then the wave is said to be polarized. There are many of these polarization states. The most common ones are linear (or plane) polarization, circular polarization, and elliptical polarization. Such light waves are referred to as linearly (or plane) polarized light, circularly polarized light, and elliptically polarized light, respectively.
Equal-efficiency diffraction of unpolarized wideband light with acousto-optic filters
Published in Journal of Modern Optics, 2022
Samuel Dupont, Justine Champagne, J.C. Kastelik
The diffraction of unpolarized light using AOTFs, which can be used to analyse linear polarization, is possible over a limited wavelength range as the diffraction efficiency is wavelength-sensitive at a given incident angle [7]. Indeed, in birefringent acousto-optic crystals, an unpolarized input beam is divided into two orthogonally polarized beams. The simultaneous interaction of these two beams with the same efficiency is possible at a given wavelength for one incident angle and one operating frequency. These two parameters constitute the ‘Double Diffraction’ (DD) operating point. When the wavelength is changed, a new incident angle-interaction frequency pairing must be determined but when a polychromatic incident beam is analysed, maintaining a fixed incident angle can be necessary for practical reasons. Consequently, as the wavelength of interest moves away from the operating point wavelength corresponding to the incident angle chosen, the more the interaction efficiency of the two polarization states differs. Hence with polychromatic light, knowledge of the polarization state of the incident beam is normally only possible over a very limited wavelength band.
Current Profile Reconstruction Using Motional Stark Effect Polarimeter Data on HL-2A Tokamak
Published in Fusion Science and Technology, 2020
W. J. Chen, D. L. Yu, L. W. Yan, B. S. Yuan, X. X. He, L. Liu, Y. L. Wei, N. Zhang, X. F. He, H. Wu, Z. B. Shi, Y. Liu, Q. W. Yang
Before measuring the pitch angle of the magnetic field on the HL-2A tokamak, the MSE system is calibrated in the laboratory to determine the response to all linear polarization angles. An integrated sphere is used to produce unpolarized light in front of a polarizer. This polarizer sheet mounted on a rotator (Zolix: RSA200) is applied to produce linearly polarized light with a polarization fraction of nearly 100%. The rotator can ensure its position resolution within 0.01 deg. It is used as a reference to determine the deviation between the measured angle and the actual polarizated angle. As shown in Fig. 6a, the slope of the line is 1.09 not 1, mainly due to misalignment of the linear polarizer behind the dual PEMs. And, the offset angle of 22.02 deg is determined by the mechanical installation of the dual PEMs and the linear polarizer. The systematic error based on the root-mean-square error with 200 samples is less than 0.14 deg. In Fig. 6b, the input angle of the system response ranges from −90 to 100 deg. For specific input angles 5, 0, and −3 deg, the deviation, defined as ±|max(γmeasure−γRMS(γinput))|, is within ±0.2 deg, as shown in Figs. 6c, 6d, and 6e.
Design of an achromatic wide-view circular polarizer using normal dispersion films
Published in Journal of Information Display, 2019
Seung-Won Oh, Sang-Hyeok Kim, Jong-Min Baek, Tae-Hoon Yoon
A uniaxial film with normal dispersion commonly increases the light reflection [16–20]. In the proposed structure, however, the strong dispersion of uniaxial films makes the polarization states accumulate at S3 on the Poincaré sphere so that it can be used in the opposite way to remove the light reflection. Figure 2(a) shows the proposed CP with a +A/−A/+C configuration using two A plates and a C plate. When unpolarized light traverses through the polarizer, it becomes linearly polarized (located at point 1), as shown in Figure 2(b). As the optic axes of the two A plates are the same but the sign of the phase retardations is opposite, the rotations of the polarization state caused by the A plates (on the Poincaré sphere) proceed in opposite directions. Thus, the wavelength dispersion is canceled by the two A plates. The dispersion can be effectively removed by setting the degree of dispersion of the plates so that the first A plate (which has a high retardation value) will have a weak dispersion while the second A plate (which has a low retardation value) will have a strong dispersion. Finally, the polarization states at all the visible wavelengths are effectively accumulated at the S3 axis (point 3).