Absorption and extinction spectra of fractal and nonfractal small-particle composites are studied. General solutions of the coupled-dipole equations with the exact operator for the dipole interaction ͑including the near-, intermediate-, and far-zone terms͒ are found and compared with those in the quasistatic approximation. Broadscale numerical simulations of optical spectra for clusters containing a large number of particles ͑up to 10 000͒ are performed. A significant fraction of dipolar eigenmodes in fractal aggregates is shown to be strongly localized. The eigenmodes cover a wide spectral region providing resonant enhancement in the visible and infrared parts of the spectrum. In contrast to previous predictions, the absorption spectrum is shown to be significantly different from the spectral distribution of the density of dipole eigenmodes. It clearly indicates the importance of symmetry properties of the modes and corresponding selection rules for the absorption by different modes in random fractal composites. Our experimental data obtained for extinction spectra of silver colloid fractal aggregates are in good agreement with the results of numerical simulations.

Localization of optical excitations and selective photomodification are studied experimentally in fractal aggregates of silver colloidal nanoparticles. The localized optical excitations of the fractal colloids cover a broad spectral range, from the visible to the infrared. We show that the absorbed laser energy is localized in an increasingly smaller number of particles with increasing the laser wavelength from 355 to 1900 nm. The size of the photomodified regions can be as small as 20 nm. The observed modification is explained by optically induced sintering (coalescence) of colloidal nanoparticles. [S0031-9007(97)05209-5]

Cathepsin K is a cysteine protease of the papain family, which is predominantly expressed in osteoclasts, and is regarded as a key protease in bone remodeling. To facilitate structural studies of the protein, the wild-type sequence of the protease has been mutated so as to replace a potential N-glycosylation site. We have expressed the mutant human cathepsin K to 190 mg/5 L using the Pichia pastoris expression system. Cathepsin K was inactivated with the mechanism-based inhibitor, APC3328, and crystallized from magnesium formate. A 2.2 8, X-ray data set has been collected on crystals belonging to space group P 2 1 2~2~. with a = 41.66 A, b = 5 1.41 A, and c = 107.72 A. There is most likely one molecule per asymmetric unit.

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A direct manifestation of electron energy quantization in metal nanoparticles is observed in two-photon excited luminescence. Experiments reveal the discrete spectra in broadband anti-Stokes photoluminescence from aggregates of silver colloid particles. A theory based on a spherical quantum-well model for metal nanoparticles is in good agreement with experimental observations.

Nano-engineering III-nitride semiconductors offers a route to further control the optoelectronic properties, enabling novel functionalities and applications. Although a variety of lithography techniques are currently employed to nano-engineer these materials, the scalability and cost of the fabrication process can be an obstacle for large-scale manufacturing. In this paper, we report on the use of a fast, robust and flexible emerging patterning technique called Displacement Talbot lithography (DTL), to successfully nano-engineer III-nitride materials. DTL, along with its novel and unique combination with a lateral planar displacement (D2TL), allow the fabrication of a variety of periodic nanopatterns with a broad range of filling factors such as nanoholes, nanodots, nanorings and nanolines; all these features being achievable from one single mask. To illustrate the enormous possibilities opened by DTL/D2TL, dielectric and metal masks with a number of nanopatterns have been generated, allowing for the selective area growth of InGaN/GaN core-shell nanorods, the top-down plasma etching of III-nitride nanostructures, the top-down sublimation of GaN nanostructures, the hybrid top-down/bottom-up growth of AlN nanorods and GaN nanotubes, and the fabrication of nanopatterned sapphire substrates for AlN growth. Compared with their planar counterparts, these 3D nanostructures enable the reduction or filtering of structural defects and/or the enhancement of the light extraction, therefore improving the efficiency of the final device. These results, achieved on a wafer scale via DTL and upscalable to larger surfaces, have the potential to unlock the manufacturing of nano-engineered III-nitride materials.

Large local optical activity in fractal aggregates of silver nanoparticles has been observed by means of photon scanning tunneling microscopy. The effect occurs because resonant plasmon modes in random fractals can have handedness in spatial distribution of their amplitudes. In agreement with experimental observations, numerical simulations show dramatic difference in dipole-moment distributions for right-and left-circularly polarized incident light when the cluster size is comparable with or larger than the wavelength. Variations in the local parameter describing the circular intensity difference of scattered light show that fractal aggregates are characterized by broad and random distributions of chiral plasmon modes.