Abstract
A Ti seed film is investigated towards improving the far UV reflectance of Al/MgF2 mirrors. Samples were initially coated with a Ti film in half of the area and they were later coated in the full area with an Al film and protected with MgF2. All materials were deposited by evaporation. Samples were prepared with the MgF2 layer deposited either at room temperature (RT) or at 225°C. A 3-nm thick Ti seed film was seen to significantly increase the reflectance of Al/MgF2 mirrors at the well-known reflectance dip centered at ∼160 nm; this was attributed to a reduction of short-range surface roughness at the Al/MgF2 interface, which is responsible for radiation absorption through surface-plasmon (SP) coupling. SP absorption was more efficiently reduced with a Ti seed film on samples fully deposited at RT. A Ti seed film as thin as 1 nm provided the largest SP absorption reduction, and the SP dip was almost completely removed.
Highlights
Space observations in the far ultraviolet (FUV, wavelength in the ∼100-200-nm range) for astrophysics, as well as for solar physics and atmosphere physics, require effective optics for optimum detection of the typically scarce photons
A 3-nm thick Ti seed film was seen to significantly increase the reflectance of Al/MgF2 mirrors at the well-known reflectance dip centered at ∼160 nm; this was attributed to a reduction of short-range surface roughness at the Al/MgF2 interface, which is responsible for radiation absorption through surface-plasmon (SP) coupling
A common fact is observed on films deposited at the two temperatures: the area seeded with Ti presents a remarkable reflectance increase over the non-seeded coating at the SP range, which is interpreted as a reduction of SP absorption
Summary
Space observations in the far ultraviolet (FUV, wavelength in the ∼100-200-nm range) for astrophysics, as well as for solar physics and atmosphere physics, require effective optics for optimum detection of the typically scarce photons. Over so many years since these mirrors were first developed and until recently, their reflectance had been only slightly enhanced, based on improvements in vacuum technology, such as deposition chamber cleanliness It has not been until the last years that Quijada et al [6] reported an important advancement for these coatings. The procedure results in a significant FUV reflectance increase over the standard room-temperature (RT) deposition procedure, and such increase is remarkable in the range close to the MgF2 cutoff This reflectance increase can be attributed to the higher FUV transparency and lower porosity of the hot-deposited fluoride film. There is still room for improvement at longer FUV wavelengths, where Al/MgF2 mirrors display a reflectance dip peaked at ∼160 nm This is attributed to absorption through excitation of a resonant surface-plasmon (SP) mode, and it is remarkable in free-electron metals. The FUV reflectance enhancement enables to estimate the reduction of the SP absorption
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