Torben Rasmussen

Atomistic simulations of cross-slip of jogged screw dislocations in copper

We have performed atomic-scale simulations of cross-slip processes of screw dislocations in copper, simulating jog-free dislocations as well as different types of jogged screw dislocations. Minimum-energy paths and corresponding transition state energies are obtained using the nudged-elastic-band path technique. We find low barriers and effective masses for the conservative motion along the dislocations of elementary jogs on both ordinary {111}<110> and nonoctahedral {110}<110> slip systems. The jogs are found to be constricted and therefore effectively act as pre-existing constrictions; the cross-slip activation energy is thereby dramatically reduced, yielding values in agreement with experiment.

Single mode solid state distributed feedback dye laser fabricated by gray scale electron beam lithography on a dye doped SU-8 resist

We demonstrate gray scale electron beam lithography on a functionalized SU-8 resist for fabrication of single mode solid state dye laser devices. The resist is doped with Rhodamine 6G perchlorate and the lasers are based on a first-order Bragg grating distributed feedback resonator. The lasers are optically pumped at 532 nm, and exhibit low lasing threshold from 530 nJ mm?2 and single mode output at selectable wavelengths from 580 to 630 nm, determined by the grating pitch. The lasers are well suited for integration into polymer based lab-on-chip circuits for interference based sensing.

Simulation of structure and annihilation of screw dislocation dipoles

Abstract Large scale atomistic simulations are used to investigate the properties of screw dislocation dipoles in copper. Spontaneous annihilation is observed for dipole heights less than 1 nm. Equilibrated dipoles of heights larger than 1 nm adopt a skew configuration due to the elastic anisotropy of Cu. The equilibrium splitting width of the screw dislocations decreases with decreasing dipole height, as expected from elasticity theory. The energy barriers, and corresponding transition states for annihilation of stable dipoles are determined for straight and for flexible dislocations for dipole heights up to 5.2 nm. In both cases the annihilation is initiated by cross-slip of one of the dislocations. For straight dislocations the activation energy shows a linear dependence on the inverse dipole height, and for flexible dislocations the dependence is roughly linear for the dipoles investigated.

Real-time tunability of chip-based light source enabled by microfluidic mixing

We demonstrate real-time tunability of a chip-based liquid light source enabled by microfluidic mixing. The mixer and light source are fabricated in SU-8 which is suitable for integration in SU-8-based laboratory-on-a-chip microsystems. The tunability of the light source is achieved by changing the concentration of rhodamine 6G dye inside two integrated vertical resonators, since both the refractive index and the gain profile are influenced by the dye concentration. The effect on the refractive index and the gain profile of rhodamine 6G in ethanol is investigated and the continuous tuning of the laser output wavelength is demonstrated using an ethanolic rhodamine 6G solution of 2×10−2mol∕l mixed with pure ethanol. This yields rhodamine 6G concentrations from 5×10−3 to 1.5×10−2mol∕l inside the laser resonators and a wavelength change of 10 nm with a response time of 110 s.

Deep Reactive Ion Etching for High Aspect Ratio Microelectromechanical Components

A deep reactive ion etch (DRIE) process for fabrication of high aspect ratio trenches has been developed. Trenches with aspect ratios exceeding 20 and vertical sidewalls with low roughness have been demonstrated. The process has successfully been used in the fabrication of silicononinsulator (SOI) released comb drive based resonators and tunable capacitors for MEMS applications. Brief characterizations of the devices are presented.

Comment on ‘Atomistic simulation of cross-slip processes in model fee structures’

Abstract Recently Rao et al. (1999, Phil. Mag. A, 79, 1167) claimed reasonable agreement between their results for cross-slip activation energies (CSAEs) in Ni (from atomistic simulation) and in Cu (from scaling of the Ni result) and experimental results. Furthermore, the atomistic result for Cu by Rasmussen et al. (1997, Phys. Rev. B, 56, 2977) was described as deviating ‘significantly’ from theirs. This comment demonstrates that Rao et al.s scaling approach is too simple and that it is possible to obtain good agreement between atomistic estimates of CSAES with a more accurate, yet still approximate, scaling approach. It is noted that no experimentally derived value for the CSAE in Ni has been published.

Atomic structure and energetics of constricted screw dislocations in copper

Atomistic simulations of cross slip of a dissociated screw dislocation have been performed. Shapes and energetics of different dislocation configurations relevant to cross slip in an f.c.c. metal (Cu) are determined. The minimum stress-free activation energy and activation length in the Friedel-Escaig cross-slip mechanism are determined. The simulations reveal a new energetically favourable configuration of a dissociated screw dislocation not previously considered. The importance of this configuration to surface nucleated cross slip is discussed.

Genetic manipulation of structural color in bacterial colonies

Significance We demonstrate the genetic modification of structural color in a living system by using bacteria Iridescent 1 (IR1) as a model system. IR1 colonies consist of rod-shaped bacteria that pack in a dense hexagonal arrangement through gliding and growth, thus interfering with light to give a bright, green, and glittering appearance. By generating IR1 mutants and mapping their optical properties, we show that genetic alterations can change colony organization and thus their visual appearance. The findings provide insight into the genes controlling structural color, which is important for evolutionary studies and for understanding biological formation at the nanoscale. At the same time, it is an important step toward directed engineering of photonic systems from living organisms. Naturally occurring photonic structures are responsible for the bright and vivid coloration in a large variety of living organisms. Despite efforts to understand their biological functions, development, and complex optical response, little is known of the underlying genes involved in the development of these nanostructures in any domain of life. Here, we used Flavobacterium colonies as a model system to demonstrate that genes responsible for gliding motility, cell shape, the stringent response, and tRNA modification contribute to the optical appearance of the colony. By structural and optical analysis, we obtained a detailed correlation of how genetic modifications alter structural color in bacterial colonies. Understanding of genotype and phenotype relations in this system opens the way to genetic engineering of on-demand living optical materials, for use as paints and living sensors.

Single-mode and tunable microfluidic dye lasers

We present a technology for miniaturized, chip-based liquid dye lasers, which may be integrated with microfluidic networks and planar waveguides without addition of further process steps. The microfluidic dye lasers consist of a microfluidic channel with an embedded optical resonator. The lasers are operated with Rhodamine 6G laser dye dissolved in a suitable solvent, such as ethanol or ethylene glycol, and optically pumped at 532 nm with a pulsed, frequency doubled Nd:YAG laser. Both vertically and laterally emitting devices are realized. A vertically emitting Fabry-Perot microcavity laser is integrated with a microfluidic mixer, to demonstrate realtime wavelength tunability. Two major challenges of this technology are addressed: lasing threshold and fluidic handling. Low threshold, in-plane emission and integration with polymer waveguides and microfluidic networks is demonstrated with distributed feed-back lasers. The challenge of fluidic handling is addressed by hybridization with mini-dispensers, and by applying capillary filling of the laser devices.

Grey scale electron-beam lithography in functionalized SU-8 for active optical devices

Miniaturized, single mode polymer dye lasers are realized by means of grey scale electron beam lithography (EBL) in functionalized SU-8 2000 resist, doped with Rhodamine 6G laser dye. These devices offer the possibility of easy integration of single mode laser sources in polymer based lab-on-a-chip microsystems. The demonstrated laser device consists of a planar waveguide with a 1st-order distributed feedback grating (DFB) surface corrugation, which forms an optical resonator. When optically pumped at 532 nm, single mode lasing is obtained in the wavelength range 570 nm - 630 nm, determined by the grating period. Our results demonstrate the feasibility of fabricating advanced nano-structured active optical components in a rapid prototyping process.