Generally and in one form of the invention this is a periodic surface filter comprising at least one element at a surface of the filter and electronic controls to change the optical characteristics of the element. Other methods and devices are disclosed.

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We perform an accuracy analysis of several possible reflectance-difference (RD) configurations that are compatible with standard molecular-beam epitaxy (MBE) growth chambers, and describe in detail an optical-bridge system that can determine relative changes in RD signals as small as 5 X 10-5 under standard growth conditions. Using this system, we determine the RD response ofGaAs for changes in surface conditions at different wavelengths and correlate these to simultaneously measured reflection high-energy electron diffraction (RHEED) intensities. Maximum anisotropies are found at 2.0-2.5 and 3.5 eV for Ga on GaAs and Al on AlAs, respectively, providing a way of spectrally distinguishing Ga-Ga and AI-AI dimers for surfacechemical investigations, and suggesting that these photon energies are also optimal for modifying growth by light. At photon energies well removed from these anisotropy maxima, RD signals follow changes in surface structure, as RHEED. Our RD-RHEED correlations provide insight concerning crystal growth by MBE and establish a common experimental link between MBE and non-UHV methods of crystal growth where RHEED cannot be used. Finally, our results illustrate various possibilities of using reflectance difference spectroscopy to investigate surface structure, surface chemistry, and surface dynamics.

Epitaxial liftoff is an alternative to lattice-mismatched heteroepitaxial growth. Multilayer AlxGa1−xAs epitaxial films are separated from their growth substrates by undercutting an AlAs release layer in HF acid (selectivity ≳108 for x≤0.4). The resulting AlxGa1−xAs films tend to bond by natural intermolecular surface forces to any smooth substrate (Van der Waals bonding). We have demonstrated GaAs thin-film bonding by surface tension forces onto Si, glass, sapphire, LiNbO3, InP, and diamond substrates, as well as self-bonding onto GaAs substrates. In transmission electron microscopy the substrate and thin-film atomic lattices can be simultaneously imaged, showing only a thin (20–100 Å) amorphous layer in between.

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Cu-Nb wire composites with 0.105, 0.148, and 0.182 volume fraction of Nb filaments were produced in situ and their mechanical properties measured as a function of filament size and interfilament spacing. The yield stress and the ultimate tensile strength increased with both niobium volume fraction and overall composite reduction. At room temperature, the ultimate tensile strength of the Cu–18.2 vol% Nb composite reduced by 99.999% in cross-sectional area (100–200 Å filament thickness) reached the value of 2230 MN/m2 (323 ksi) and further increased to 2850 MN/m2 (413 ksi) when measured at 77 °K. These values are higher by a factor of 4 than the values predicted by the rule of mixtures based on the highest reported strength of both niobium and copper. The composite strength is as high as that of the best copper whiskers and is shown to closely approach the theoretical strength of the material. The anomalous increase in strength despite the low volume fraction of reinforcing filaments suggests that the filaments act primarily as barriers to the motion of matrix dislocations and that the strength of the filamentary material is only of secondary importance. This hypothesis is supported by microstructural obsevations (transmission and scanning electron microscopy) which reveal the deformation modes during composite fabrication and mechanical testing. The excellent transport properties (in both the normal and superconducting state) make these composites attractive as conductors for high-stress applications.

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