The physical and chemical properties of low-dimensional structures depend on their size and shape, and can be very different from those of bulk matter. If such structures have at least one dimension small enough that quantum-mechanical effects prevail, their behaviour can be particularly interesting. In this way, for example, magnetic nanostructures can be made from materials that are non-magnetic in bulk 1 , catalytic activity can emerge from traditionally inert elements such as gold 2 , and electronic behaviour useful for device technology can be developed 3,4 . The controlled fabrication of ordered metal and semiconductor nanostructures at surfaces remains, however, a difficult challenge. Here we describe the fabrication of highly ordered, two-dimensional nanostructure arrays through nucleation of deposited metal atoms on substrates with periodic patterns defined by dislocations that form to relieve strain. The strain-relief patterns are created spontaneously when a monolayer or two of one material is deposited on a substrate with a different lattice constant. Dislocations often repel adsorbed atoms diffusing over the surface, and so they can serve as templates for the confined nucleation of nanostructures from adatoms. We use this technique to prepare ordered arrays of silver and iron nanostructures on metal substrates.Arbitrary atomic-scale structures can be assembled with the tip of a scanning tunnelling microscope (STM), either through direct displacement of adsorbed atoms 5 , or through tip-assisted decomposition of chemical species 6 . The principal drawback of methods based on scanning probes is their serial character. Approaches where a large number of structures can be created in parallel are selforganized growth-either in the kinetic 7 or in the thermodynamic regime 8-10 -and the controlled deposition of size-selected clusters from the gas phase 11 . Whereas the latter technique produces nanostructures of nearly uniform sizes on surfaces, the self-organized growth suffers from broad size distributions. In addition, both methods yield largely uncorrelated spatial distributions caused by the statistics of deposition and diffusion.There have been several attempts to improve the spatial order in nanostructure growth. The vertical correlation of island nucleation in sequences of quantum dot and spacer layers yielded improved lateral order 12. Also, misfit dislocations have been used for nanoscale structuring. Preferred nucleation of Ni at dislocations of the Au(111) reconstruction resulted in ordered islands 13 ; for this system, the mechanism was identified as site-specific exchange, a finding that strongly reduces the number of elements suitable for this type of ordering 14 . Also in semiconductors, island accumulation at dislocations has been reported 15 . However, there the bulk-like dislocations were not mobile enough to order into periodic patterns. Due to the attraction towards dislocations, islands were lined up in one dimension but were not periodic in two dimensions.In heteroepitaxial systems t...

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We present biomedical imaging using a single frequency terahertz imaging system based on a low threshold quantum cascade laser emitting at 3.7THz (λ=81μm). With a peak output power of 4mW, coherent terahertz radiation and detection provide a relatively large dynamic range and high spatial resolution. We study image contrast based on water/fat content ratios in different tissues. Terahertz transmission imaging demonstrates a distinct anatomy in a rat brain slice. We also demonstrate malignant tissue contrast in an image of a mouse liver with developed tumors, indicating potential use of terahertz imaging for probing cancerous tissues.

Quantum cascade lasers are semiconductor devices based on the interplay of perpendicular transport through the heterostructure and the intracavity lasing field. We employ femtosecond time-resolved pump-probe measurements to investigate the nature of the transport through the laser structure via the dynamics of the gain. The gain recovery is determined by the time-dependent transport of electrons through both the active regions and the superlattice regions connecting them. As the laser approaches and exceeds threshold, the component of the gain recovery due to the nonzero lifetime of the upper lasing state in the active region shows a dramatic reduction due to the onset of quantum stimulated emission; the drift of the electrons is thus driven by the cavity photon density. The gain recovery is qualitatively different from that in conventional lasers due to the superlattice transport in the cascade.

We demonstrate bio-medical imaging using a Terahertz quantum cascade laser. This new optoelectronic source of coherent Terahertz radiation allows building a compact imaging system with a large dynamic range and high spatial resolution. We obtain images of a rat brain section at 3.4 THz. Distinct regions of brain tissue rich in fat, proteins, and fluid-filled cavities are resolved showing the high contrast of Terahertz radiation for biological tissue. These results suggest that continuous-wave Terahertz imaging with a carefully chosen wavelength can provide valuable data on samples of biological origin; these data appear complementary to those obtained from white-light images.