We study the patterns formed on Ar+ ion-sputtered Si surfaces at room temperature as a function of the control parameters ion energy and incidence angle. We observe the sensitivity of pattern formation to artifacts such as surface contamination and report the procedures we developed to control them. We identify regions in control parameter space where holes, parallel mode ripples and perpendicular mode ripples form, and identify a region where the flat surface is stable. In the vicinity of the boundaries between the stable and pattern-forming regions, called bifurcations, we follow the time dependence from exponential amplification to saturation and examine the amplification rate and the wavelength in the exponential amplification regime. The resulting power laws are consistent with the theory of nonequilibrium pattern formation for a type I (constant wavelength) bifurcation at low angles and for a type II (diverging wavelength) bifurcation at high angles. We discuss the failure of all sputter rippling models to adequately describe these aspects of the simplest experimental system studied, consisting of an elemental, isotropic amorphous surface in the simplest evolution regime of linear stability.

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Methods for the fabrication of large areas of nanoscale features with controlled period and intraperiod organization are of interest because of the potential for high-throughput mass production of nanoscale devices. Due to their potential in this regard, much recent attention has been devoted to self-organization processes, [1][2][3][4][5] in which processing causes the spontaneous emergence of a nanoscale pattern. The short-range order can be quite high [2][3][4] but some envisaged applications require long-range order, which is destroyed by uncontrolled topological defects arising spontaneously from the self-organization process. A potentially successful hierarchical fabrication strategy is the fabrication of controlled features at a small, but lithographically tractable, length scale by methods such as conventional mask or optical-standing-wave lithography, in order to guide a self-organization process at the finest length scale. [6][7][8] Topographic patterning has been used for templating the local disorder in two-dimensional (2D) self-assembled monolayers [9] and for templating defect organization or elimination in three-dimensional (3D) colloidal crystallization.[10] Topography has also been used to manipulate semiconductor quantum-dot placement, composition, and strain, through its effect on stress, [11] surface energy, and mobility.[12]3D short-range ordering of grown-in quantum-dot short-period superlattices can result from multilayer growth; [1,4] nanoscale topographic templating of the first layer could dramatically accelerate the development of order and lead to true long-range order. Lithographically and focused ion beam (FIB)-patterned topographies have recently been used to template quantum-dot growth in linear chains, [8] periodic 2D lattices, [7] and in more complex configurations that are promising for novel nanoelectronic architectures, such as quantum cellular automata.[13] The finest features have been templated by serial writing with a FIB, a prohibitively expensive process for mass production that might be circumvented by using a hierarchical fabrication strategy. Here we report the influence of patterned boundaries on the primary material of complementary metal oxide semiconductor (CMOS) technology, i.e., a Si(001) substrate, in guiding self-organized topographic ripples spontaneously appearing during uniform irradiation with low energy Ar + ions. We show that the long-range order of the features can be greatly enhanced by this lateral-templating approach. The emerging pattern can be manipulated by changing the boundary spacing and misorientation with respect to the projected ion-beam direction. We develop a scalar figure of merit, a dimensionless topological defect density, to characterize the degree of order of the pattern. At small boundary separation, greatest order is observed when the separation is near an integer multiple of the spontaneously arising feature size. The defect density is exceedingly low up to a critical misorientation angle, beyond which topological defects develop i...

Seemingly symmetric nanoscale cylinders have hidden asymmetry of charge distribution.

Nanopores fabricated in free-standing amorphous silicon thin films were observed to close under 3 keV argon ion irradiation. The closing rate, measured in situ, exhibited a memory effect: at the same instantaneous radius, pores that started larger close more slowly. An ion-stimulated viscous flow model is developed and solved in both a simple analytical approximation for the small-deformation limit and in a finite element solution for large deformations. The finite-element solution exhibits surprising changes in cross-section morphology, which may be extremely valuable for single biomolecule detection, and are untested experimentally. The finite-element solution reproduces the shape of the measured nanopore radius versus fluence behavior and the sign and magnitude of the measured memory effect. We discuss aspects of the experimental data not reproduced by the model, and successes and failures of the competing adatom diffusion model.

Coherent ultrashort X-ray pulses provide new ways to probe matter and its ultrafast dynamics 1-3 . One of the promising paths to generate these pulses consists of using a nonlinear interaction with a system to strongly and periodically distort the waveform of intense laser fields, and thus produce high-order harmonics. Such distortions have so far been induced by using the nonlinear polarizability of atoms, leading to the production of attosecond light bursts 4 , short enough to study the dynamics of electrons in matter 3 . Shorter and more intense attosecond pulses, together with higher harmonic orders, are expected 5,6 by reflecting ultraintense laser pulses on a plasma mirror-a dense (≈10 23 electrons cm −3 ) plasma with a steep interface. However, short-wavelength-light sources produced by such plasmas are known to generally be incoherent 7 .In contrast, we demonstrate that like in usual low-intensity reflection, the coherence of the light wave is preserved during harmonic generation on plasma mirrors. We then exploit this coherence for interferometric measurements and thus carry out a first study of the laser-driven coherent dynamics of the plasma electrons.One of the challenges of high-order harmonic generation (HHG), beyond the production of very high orders with good efficiencies, is to preserve the initial properties of the laser beam in this frequency conversion process. In particular, keeping a high degree of coherence is essential for many applications, such as coherent imaging 8 . This has already been shown to be possible for HHG in gases 9 , but remains an open question for dense plasmas. In this case, an intense laser pulse interacts with an initially solid target, and creates a dense reflective plasma at the surface. In an early study using picosecond laser pulses 10 , interference measurements in the far-field using Young slits suggested that the coherence of the light field in the source plane was lost in interaction with such an extremely dense and hot (a few 10 6 K) plasma, for instance through stochastic processes or plasma instabilities. This conclusion was consistent with the observation of an uncollimated harmonic emission. Here, we demonstrate that such deleterious effects can be avoided using well-controlled interaction conditions, that is, ultrashort (<100 fs) laser pulses with a high temporal contrast. In these conditions, the plasma hardly has time to expand during the interaction (density gradient scale length L l/10, with l being the laser wavelength), and thus behaves as a high-flatness mirror-a plasma mirror 11 . We then exploit the coherence of the produced harmonics to study the dynamics of plasma electrons on the attosecond timescale.Our experiment uses 60 fs pulses with a high temporal contrast (10 10 on the nanosecond timescale), to produce high-order harmonics on plasma mirrors through coherent wake emission 12 (CWE), at intensities from a few 10 16 W cm −2 to a few 10 17 W cm −2 .Groups of harmonics in the beam diverging from the plasma mirror are selected with different ...

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