Aperiodic Multilayers

Aperiodic multilayers are typically used to achieve broad spectral response at a fixed incidence angle, or broad angular response at a fixed photon energy. In contrast to periodic and depth-graded multilayers, the individual layer thicknesses in an aperiodic multilayer coating are specified numerically, rather than analytically. These coatings can be designed using various numerical methods, including those available in our IMD software.

To illustrate the concept, shown in Figure 1 is the performance of an aperiodic Al/Zr multilayer designed for high reflectance at normal incidence from 171 Å to 211 Å. Also shown in Figure 1 are the reflectance curves for the periodic Si/Mo multilayers used for the Hinode/EIS instrument (green) and the TXI sounding rocket instrument (blue); by using a reduced number of periods, the EIS and TXI multilayer coatings sacrificed peak reflectance in order to achieve a wider spectral response. The new aperiodic Al/Zr coating design achieves relatively high reflectance over the entire target wavelength band, particularly at the edges that include important solar emission lines. The layer thickness profile of this aperiodic Al/Zr multilayer is shown in Figure 2.

Figure 1. Theoretical EUV reflectance vs. wavelength of an aperiodic Al/Zr multilayer coating designed to provide high reflectance from 171 Å to 211 Å at normal incidence. The aperiodic coating achieves much flatter spectral response relative to the periodic Si/Mo coatings used on the Hinode/EIS satellite and the TXI sound-rocket instruments.

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Figure 2. Layer thickness vs. layer index (left) and structure diagram (right) for the aperiodic Al/Zr multilayer shown in Figure 1.

Aperiodic multilayer coatings can be optimized for performance at wavelengths in the EUV, soft X-ray, or hard X-ray bands, using a variety of multilayer material combinations.

Laterally-Graded Multilayers

Certain applications require multilayer coatings that are intentionally designed to be non-uniform over the substrate surface. As just one example, a new X-ray polarimeter concept currently being developed at MIT [Marshall et al, SPIE 8861 (2013)] will utilize laterally-graded multilayer (LGML) coatings operating near the Brewster angle at 45° in the soft X-ray band from 20 Å to 80 Å, approximately. For that project, the gradation in multilayer period must be ~linear over the full length of the substrate.

The measured uniformity of a prototype W/B₄C LGML coating for the MIT polarimetry project is shown in Figure 3, and its measured performance at 45° incidence is shown in Figure 4; the reflectance-vs-wavelength measurements (filled circles) were made at the ALS (courtesy E. Gullikson) using a partially-polarized beam, and calculations of reflectance for pure s-polarization (solid lines) shown in Figure 4 are based on fits (dotted lines) to those measurements.

Figure 3. Measured uniformity of a W/B₄C LGML coating for soft X-ray polarimetry.

Figure 4. Measured reflectance vs. wavelength vs. position of a ~50-mm-long W/B₄C LGML mirror for soft X-ray polarimetry. The measurements are shown as filled circles, and were made at the ALS (E. Gullikson) using a partially-polarized beam. Fits (dotted lines) to the measurements were used to compute the reflectance curves for pure s-polarization, which are shown as solid lines.

A variety of multilayer material combinations can be used to produce laterally-graded multilayer coatings optimized for performance in the EUV, soft X-ray, or hard X-ray bands.

Introduction Periodic Multilayers Normal-Incidence Multilayer Optics Depth-Graded Multilayers Grazing-Incidence Multilayer Optics Aperiodic and Laterally-Graded Multilayers