Purpose Outline Properties of Multilayer Optics



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Purpose Outline Properties of Multilayer Optics

Delta State University, US has reference to this Academic Journal, Properties of Multilayer Optics An Investigation of Methods of Polarization Analysis in consideration of the ICS Experiment at UCLA 8/4/04 Oliver Williams Outline Purpose Review of Polarization/Analysis in Visible EUV/Soft x-ray Behavior in Materials Basics of Multilayer Optics ML Optics as Polarizers/Phase Retarders Summary Purpose Beam electrons scattering off CO2 laser photons (~0.12 eV) expected so that produce ~110 eV photons Scattered photons assume polarization state of laser photons = circular in consideration of ICS experiment Require analyzer capable of determining degree of polarization of soft x-rays

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Review of Polarization A preferential orientation of the electric field along a single axis perpendicular so that the direction of propagation= Linearly polarized When the horizontal in addition to vertical components of electric field are 90 degrees out of phase= Circularly polarized At a boundary of different index of refraction (e.g. air-glass): Horizontal = s-component (E-field perpendicular so that plane of incidence) Vertical = p-component (E-field parallel so that plane of incidence) Brewster?s Angle: Angle at which the s-component of the E-field is completely reflected while the p-component is completely absorbed or transmitted; dependent on the index of refraction of material. Some Methods of Polarization/Analysis Dichroic/Wire grid polarizers ? Linear Brewster?s angle based ? Linear Birefringence (crystals) ? Linear & Circular 1/4 wave plates ? Circular Of particular interest so that the ICS experiment are analyzers based on quarter wave plates due so that their ability so that retard (or advance) the relative phases of the s in addition to p components of the electric field by 90 degrees (ideally). The well-known method of using a ¬ wave plate followed by a linear polarizer as an analyzer can be used so that determine the degree of circular polarization of the scattered photons. However? EUV/Soft X-ray Behavior in Materials The real part of the refractive index (nr) of materials seen by light in the EUV/SXR (>30 eV approx.) approaches unity as the energy increases. The complex refractive index is, where ? is the decrement of the real part of the refractive index in addition to ? is the absorption index alongside both typically very small (

Basics of Multilayer Optics If the reflections of 30-100 surfaces can be arranged such that their phases interfere constructively at the top surface, a total reflectivity close so that one can be achieved. Bragg condition: Light reflecting off many surfaces will be strongly dependent on the distance between reflection layers (d) in addition to the angle of incidence (?) as the total distance traveled must be an integer number (m) of wavelengths (?) so that constructively interfere in addition to is given by the equation, The dependence of the reflectivity on the wavelength of light makes ML optics inherently good filters ML Optics as Linear Polarizers Because the index of refraction in consideration of EUV/SXR light is close so that one, Brewster?s angle is near 45 degrees. Adjusting the spacing between layers (d) in the Bragg equation one can place this Bragg reflection peak also near 45 degrees This causes the ML optic so that be highly reflective (due so that the multilayer reflections) in addition to especially so that the s-component since the angle of maximum reflection is at Brewster?s angle This makes ML optics almost perfect linear polarizers as the relative reflection (transmission) between the s in addition to p components is very large (small). www-cxro.lbl.gov/optical_constants/multi2.html ML Optics as Phase Retarders There already exist modest phase shifts between the s in addition to p components at dielectric/vacuum interfaces at certain angles of incidence, however because the reflectivity is so low it is inefficient so that be used as a retarder. Graphs so that the right show the reflectance of the s in addition to p components of 90 eV photons on a Molybdenum mirror as a function of incidence angle as well as the (normalized) relative phase shift, ??. Adding multilayers changes the optical properties around Bragg peak, especially.

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Reflection Retarders As seen from the plots, phase retardation based on reflection can be quite effective, producing ???45 degrees. Because of the necessity of operating near 45 degrees in consideration of decent phase shifts, the reflectivity of the p-component is quite low (?0.1), which is extended so that ?0.01(two reflections) if it is so that function as a ¬ wave plate. Besides the inefficiency, the introduction of one more optical element so that align, especially under vacuum in addition to only receiving a signal every few minutes (for us), makes reflection retarders difficult so that use. Transmission Retarders Essentially same fabrication process as reflection type except necessity of removing substrate by etching gives higher surface roughness. However, the transmission retarder is able so that achieve ???90 degrees between s in addition to p on only one pass alongside about the same efficiency as one reflection. This is made possible by the modulation of the electric field between the low in addition to high absorption regions of the ML. Transmission vs. Reflection Upon strong reflection off a layer a standing wave is formed which effectively changes the refractive index felt by the total field. If the Bragg peak is placed near Brewster?s angle, only the s-component feels this electric field modulation in addition to change in refractive index which causes a phase shift alongside respect so that the p-component. The effect of this modulation in the transmission case (top graph) is compared so that reflection in addition to it can be seen in the plots that the normalized phases of the s in addition to p components in transmission more strongly ?slip? past each other at certain angles than in consideration of reflection. Because this change in phase is so heavily dependent on strong E-field modulation, in addition to hence strong reflection, poorly fabricated transmission ML?s are much less forgiving in phase shifts than reflection ML?s.

Summary Multilayer optics are excellent small bandwidth filters Reflective linear polarizers based on ML?s are very effective in the EUV/SXR region ¬ wave plates based on transmissive multilayers have shown so that be more efficient in addition to practical than the reflection type Sensitivity so that imperfections make the fabrication of transmission ML?s more difficult in addition to (most likely) more expensive than reflection ML?s An x-ray detector in combination alongside a transmission retarder in addition to reflection analyzer (polarizer) mounted on vacuum compatible motorized rotation stages could be used so that verify the circular polarization of the ICS photons at Neptune

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