Zi-li Wang

Heterostructure-based optical absorbers

We have fabricated optical absorbers based on heterostructures composed of thick metallic films and truncated all-dielectric photonic crystals. Under the tunneling mechanism, the light can enter the heterostructure without reflection and is greatly absorbed due to the strong local-field enhancement in the metallic film. With the increase in the thickness of the metal, the absorbance will tend to unity. Experiments, in good agreement with the simulations, demonstrate a maximum of absorbance close to 98%. Possible methods to realize a wide-angle or/and wideband absorption are also given.

Broad angular multilayer analyzer for soft X-rays

Using numerical optimization algorithm, non-periodic Mo/Si, Mo/Be, and Ni/C broad angular multilayer analyzers have been designed. At the wavelength of 13 nm and the angular range of 45~49 degrees , the Mo/Si and Mo/Be multilayer can provide the plateau s-reflectivity of 65% and 45%, respectively. At 5.7 nm, the s-reflectivity of Ni/C multilayer is 16% in the 44~46 degrees range. The non-periodic Mo/Si broad angular multilayer was also fabricated using DC magnetron sputtering, and characterized using the soft X-ray polarimeter at BESSY. The s-reflectivity is higher than 45.6% over the angular range of 45~49 degrees at 13 nm, where, the degree of polarization is more than 99.98%.

Highly efficient blazed grating with multilayer coating for tender X-ray energies

For photon energies of 1 - 5 keV, blazed gratings with multilayer coating are ideally suited for the suppression of stray and higher orders light in grating monochromators. We developed and characterized a blazed 2000 lines/mm grating coated with a 20 period Cr/C- multilayer. The multilayer d-spacing of 7.3 nm has been adapted to the line distance of 500 nm and the blaze angle of 0.84° in order to provide highest efficiency in the photon energy range between 1.5 keV and 3 keV. Efficiency of the multilayer grating as well as the reflectance of a witness multilayer which were coated simultaneously have been measured. An efficiency of 35% was measured at 2 keV while a maximum efficiency of 55% was achieved at 4 keV. In addition, a strong suppression of higher orders was observed which makes blazed multilayer gratings a favorable dispersing element also for the low X-ray energy range.

Enhancement of (nearly) homogeneous fields in a (effective) zero-index cavity

Enhanced homogeneous fields are realized in a zero-index medium embedded in Bragg reflectors. When the zero-index medium has Kerr-type nonlinearity, the threshold for optical bistability can be reduced noticeably due to the enhanced uniform fields. Based on a transmission line, we fabricate an effective zero-index medium sandwiched in electric and magnetic walls and experimentally demonstrate the enhanced nearly uniform fields.

Enhancement of self-collimated fields in photonic crystals consisting of two kinds of single-negative materials.

Self-collimations are found in one-dimensional (1D) photonic crystals consisting of two kinds of single-negative materials that effectively cancel each other out. Compared to the self-collimations in all-dielectric photonic crystals or 1D photonic crystals with negative-index materials, this kind of structure can amplify both far and near fields greatly during collimation.

Deposition and characterization of B4C/CeO2 multilayers at 6.x nm extreme ultraviolet wavelengths

New multilayers of boron carbide/cerium dioxide (B4C/CeO2) combination on silicon (Si) substrate are manufactured to represent reflective-optics candidates for future lithography at 6.x nm wavelength. This is one of only a few attempts to make multilayers of this kind. Combination of several innovative experiments enables detailed study of optical properties, structural properties, and interface profiles of the multilayers in order to open up a room for further optimization of the manufacturing process. The interface profile is visualized by high-angle annular dark-field imaging which provides highly sensitive contrast to atomic number. Synchrotron based at-wavelength extreme ultraviolet (EUV) reflectance measurements near the boron (B) absorption edge allow derivation of optical parameters with high sensitivity to local atom interactions. X-ray reflectivity measurements at Cu-Kalpha (8 keV) determine the period of multilayers with high in-depth resolution. By combining these measurements and choosing rob...

Development of Multilayer Optics for EUV, Soft X-Ray and X-Ray Regions in IPOE

In EUV and X-ray regions, multilayer mirrors are the essential and necessary optics elements. The good prospects of the EUV and X-ray optics for next generation lithography system, microscopy in the “water windows”, astronomy telescope, spectroscopy, plasma diagnostics, and X-ray laser have impelled the development of multilayer. This report introduced the recent results of the multilayer optics elements in institute of precision optical engineering (IPOE), Tongji University, China, including beam splitters, broad band/angular polarizers, supermirrors, and high-reflectance mirrors. The product of reflectivity and transmittance is above 4% for the Mo/Si multilayer beam splitter at 13.9 nm. Over the 15-17 nm wavelength range, the s-reflectivity of the non-periodic Mo/Si broadband multilayer polarizers is reasonably constant, as high as 36.6%, and the degree of polarization is more than 97.8%. The experimental results of some X-ray supermirrors and high-reflectance mirrors in our lab were also presented.

Development of X‐ray Microscopy at IPOE

In order to meet the different requirements of applications in synchrotron radiation and plasma diagnosis in China, focusing and imaging optics based on Kirkpatrick‐Baez (KB) mirrors, compound refractive lenses (CRLs), and multilayer Laue lenses (MLLs) were studied in our lab. A one‐dimensional KB microscope using mirrors with a dual‐periodic multilayer coating was developed. The multilayer mirror can reflect both 4.75 keV (Ti K‐line) and 8.05 keV (Cu K‐line) simultaneously, which makes alignment easier. For hard x‐ray microscopy, CRL was studied. Using a SU‐8 resist planar parabolic CRL, a focal line of 28.8‐μm width was obtained. To focus hard x‐rays to nanometer levels efficiently, an MLL was fabricated using a WSi2/Si multilayer. The MLL consists of 324 alternating WSi2 and Si layers with a total thickness of 7.9 μm. (Recently, a much thicker multilayer has been deposited with a layer number of n = 1582 and a total thickness of 27 μm.) After deposition, the sample was sliced and polished into an appro...

Applications of wavelet analysis to study growth characteristics of x-ray multilayer mirrors

Transmission electron microscopy images and hard x-ray reflectivity curves are used to obtain information on the growth characteristics of x-ray multilayer mirrors. A multi-resolution approach based on wavelet analysis is used to decompose the interface profiles obtained from transmission electron microscopy images into a number of different spatial frequency ranges. The replication factor (vertical) and the characteristic length (lateral) of the interfacial roughness are determined in these spatial frequency ranges. By changing the upper or lower limits of the wavelet transform of the x-ray reflectivity curves, the structure uniformity (lateral) and evolution of the surface damage (vertical) are determined. These results are significant for the study of the growth characteristics of ultrathin x-ray multilayer mirrors and enable more effective methods to determine their imperfections.

Fabrication of Multilayer Reflective Mirrors for Soft X-Ray Laser Working at the Wavelength of 4.48 nm

Near normal incident high reflective and polarized multilayer mirrors were designed and fabricated for Ni-like Ta X-ray laser working at the wavelength of 4.48 nm. The influence of the imperfect interface on the reflectivity of the multilayer was simulated. Using the direct current magnetron sputtering technique, the Cr/C and Cr/Sc multilayers were deposited. Then, the layer thickness and structure of the multilayer were measured and fitted by an X-ray diffractometer. The reflectivities of these multilayers were measured on the beam line UE56/1-PGM at BESSY-II. At grazing incident angle of 85°, the measured peak reflectivities were 7.50% and 6.12% for Cr/C and Cr/Sc multilayer mirrors, respectively. At the grazing angle of 44°, the s-reflectivity is 13.61% for Cr/C multilayer polarizer. The measurement and simulation results suggest that the interface roughness strongly reduces the reflectivity of the multilayers. Thus, there is still a potential for improvement by introducing diffusion barriers and using more perfect substrate.