Study on UV Vacuum Polarization Characteristics of Lithium Fluoride Polarizer

Vacuum ultraviolet optics has a wide range of applications in high temperature plasma diagnostics, vacuum ultraviolet spectroscopy radiation standards and other fields. With the development of vacuum ultraviolet optics, it is necessary to have a deep understanding of the polarization characteristics of the vacuum ultraviolet component.

There are mainly two types of vacuum ultraviolet polarizing elements: transmissive and reflective. Vacuum ultraviolet transmission materials mainly include magnesium fluoride and lithium fluoride. Using the good transmission characteristics and dual dispersion characteristics of magnesium fluoride crystals in the vacuum-UV band, Steinmetz et al. and Ohnson designed dual-Rochon and Wallston prisms, respectively, to generate linearly polarized radiation in the vacuum ultraviolet band, ie, with the incident plane. Parallel and perpendicular polarized light. Lithium fluoride crystals have a high transmittance in the vacuum ultraviolet band and are also suitable for use as a vacuum ultraviolet transmission polarizing element. Several pieces of lithium fluoride crystals are placed in parallel. When the incident angle is Brewster's angle, the transmitted light passing through a plurality of lithium fluoride pieces can generate a partially polarized light with a higher degree of polarization, thereby making it possible to use multiple pieces of lithium fluoride. Crystals make polarizers. Walked studied the polarization characteristics of a lithium fluoride polarizer using a normal-incidence vacuum-UV monochromator. Due to the small degree of polarization of outgoing light from a normal incidence monochromator, Walker determined the degree of polarization of transmitted light from a lithium fluoride polarizer. The incident light is non-polarized light, and the degree of polarization of the transmitted light of the lithium fluoride polarizer is determined directly according to the definition of the degree of polarization by measuring the intensities of the transmitted light in both directions parallel and perpendicular to the incident plane. Reflective polarizers are mainly reflective coatings with stable properties in the vacuum ultraviolet band, such as gold, silver, aluminum, and magnesium fluoride, etc. Reflective characteristics are used to determine lithium fluoride using a single-mirror method, depending on the reflection characteristics. The incident angle of the crystal is 60. In order to ensure that the direction of the outgoing light passing through the lithium fluoride polarizer is completely consistent with the direction of the emitted light of the monochromator, the lithium fluoride crystals of the lithium fluoride polarizers of each group need to be placed symmetrically. The lithium polarizer can continuously rotate within a range of 180 around the direction in which the monochromator emits light.

Also given are the measurement results of the extinction ratio of the two-group, four-group, and eight-group lithium fluoride polarizers.

3.1.1 The extinction ratio of the two-piece and four-piece lithium fluoride polarizers is basically the same. At the same time, the extinction ratio of the 1mSeya-Namioka monochromator was also obtained.

31.2 Extinction ratio of eight-piece lithium fluoride polarizer Using the extinction ratio of the obtained monochromator grating, two four-piece lithium fluoride polarizers are considered as an eight-piece lithium fluoride polarizer, which is measured by In principle (6), the extinction ratio of an eight-group lithium fluoride polarizer can be introduced, as shown.

So far, it can be seen that the resulting two-, four-, and eight-group lithium fluoride polarizers extinction ratio. Vacuum ultraviolet radiation.

If a depolarizer is added after the lithium fluoride polarizer, and then the analyzer is rotated around the incident light in the range of 0 to 180, the degree of polarization of the emitted light is measured to be less than 1, that is, the emitted light is non-polarized light.

3.2 Polarization characteristics of the vacuum ultraviolet component The polarization characteristics of the reflectance of the aluminum + magnesium fluoride film and the polarization characteristics of the absolute efficiency of the sinusoidal holographic grating were primarily studied using the vacuum ultraviolet polarized radiation produced by the lithium fluoride polarizer, and several were selected in the vacuum ultraviolet band. For specific wavelengths, the reflectivity R, and R of the aluminum and magnesium fluoride films are measured in parallel and in the vertical direction to the plane of incidence, and the absolute efficiencies n and n of the sinusoidal holographic grating are also measured in the presence of unpolarized light. The reflectance R of the film and the measurement result of the grating absolute efficiency n wavelength 174.2 nm are as shown in FIG. The measurement result satisfies the formula (8a, b).

For a single layer of aluminum film, the extinction ratio of 1 corresponds to two incident angles: and 90 pairs of aluminum + magnesium fluoride film, the extinction ratio is 1 when the corresponding incident angle is also two, but due to the magnesium fluoride film The interference effect is different from that of a single-layer aluminum film. One is 0 and the other is related to the thickness of the magnesium fluoride film, and changes with the thickness of the magnesium fluoride film in the range of 0 to 90°. That is, by appropriately adjusting the thickness of the magnesium fluoride film, the extinction ratio of the aluminum+magnesium fluoride film can be set to 1 at an arbitrary incident angle to generate non-polarized radiation. Conversely, the extinction ratio of the aluminum+magnesium fluoride film can also be improved. The reflectivity component is significantly added in a direction that is parallel or perpendicular to the plane of incidence to produce polarized radiation. It can be seen that the extinction ratio of the measured aluminum+magnesium fluoride film is close to 1 at an incident angle of 0 to 50. As the incident angle is large, Rs gradually increases and R gradually decreases. When the incident angle is greater than 60, the difference between R, and Rs is obviously increased, and the reflected light has a certain degree of polarization.

The diffraction efficiency of a holographic grating is affected by many factors such as the conditions of incidence, reception conditions, grating period, and trench depth. In this paper, only the first-order diffraction efficiency of a particular grating was experimentally studied. The sinusoidal holographic grating used had a period of 2400 gmmd and was obtained by holographic replication. The variation of absolute efficiency of grating first order diffraction with incident angle is studied. When the incident angle is in the range of 10~20° and 0, the first-order diffraction absolute efficiency n and 11rightsreserved, htfp degree inhomogeneity of the sinusoidal holographic grating make the emitted light with the lithium fluoride film transmit to the detector. Changes in the position, the unevenness of the detector area's response will also cause errors.

n close. When the incident angle is 30, n is significantly larger than the n-grating has a higher extinction ratio, and the degree of polarization of the first-order diffracted light is higher. When the incident angle is greater than 60, the first-order diffraction efficiency is less than 10%. 3.3 Error Analysis In the above experiment for measuring the polarization characteristics of the lithium fluoride polarizer, the causes of the error mainly include the following factors: (1) Theoretical calculation Assuming the light source is non-polarized, the actual polarization properties of the light source are unknown.

(2) The theoretical calculation assumes that the intensity of the light source during the measurement is constant, the random fluctuation of the intensity of the empty cathode light source, and the random drift of the optoelectronic signal of the detector and the amplifier is approximately %1i1. This error can be obtained through multiple sampling and statistical averaging. And eliminate it. The systematic drift of the amplifiers of the light source and the detector shows that they almost linearly change with time. Therefore, the strength at each position can be measured by the sequence and the reverse order, ie according to /p/2, /3, /3, I2. The order of A and A is averaged again as the measured value of each position and is eliminated to some extent.

(3) In the experiment, the axis of the monochromator was inconsistent with the axis of rotation of the lithium fluoride polarizer. Asymmetry in the position of each lithium fluoride in the lithium fluoride polarizer and the thickness of the lithium fluoride film were examined. A method for measuring the extinction ratio of a polarizing element using a monochromator. With this method, the specular grating and the polarizing element can be determined to be extinct without knowing the specific reflection system or transmission coefficient of each reflective polarizing element or transmission polarizing element. ratio. Using this method, the polarization characteristics of a lithium fluoride polarizer in the vacuum ultraviolet range of Zibu was studied on a 1m Seta-Namioka monochromator. At 149.2, 1742, 2142, 235.0, 300, and 340 nm, the extinction ratios of the two-, four-, and eight-pack lithium fluoride polarizers and the extinction of the lithium fluoride polarizers calculated using the Fresnel formula were measured. It is consistent within the error range. Lithium-fluoride polarizers were used as polarizers and analyzers, respectively, and their angle extinction characteristics were studied to satisfy Marius's law and it was confirmed that polarized radiation was actually generated. Using a lithium fluoride polarizer, a preliminary study was conducted on the reflectivity of the aluminum+magnesium fluoride film and the absolute efficiency of the holographic grating in parallel and perpendicular directions to the incident plane. The arithmetic average of the two-component arithmetic average and the non-polarized light incident time corresponded to each other. Basically the same.

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