J. Jussi Toppari

Vacuum rabi splitting and strong-coupling dynamics for surface-plasmon polaritons and rhodamine 6G molecules

We report on strong coupling between surface-plasmon polaritons (SPP) and Rhodamine 6G (R6G) molecules, with double vacuum Rabi splitting energies up to 230 and 110 meV. In addition, we demonstrate the emission of all three energy branches of the strongly coupled SPP-exciton hybrid system, revealing features of system dynamics that are not visible in conventional reflectometry. Finally, in analogy to tunable-Q microcavities, we show that the Rabi splitting can be controlled by adjusting the interaction time between waveguided SPPs and R6G deposited on top of the waveguide. The interaction time can be controlled with sub-fs precision by adjusting the length of the R6G area with standard lithography methods.

Dielectrophoretic Trapping of DNA Origami

In this thesis three-dimensional tube-shaped DNA-origamis were dielectrophoretically trapped within lithographically fabricated nanoelectrodes. The origamis had been premade while the electrodes were fabricated specifically for these experiments with two different gapsizes, 150 nm and 400 nm. The aim of the work was to capture individual nanotubes in the gap between the electrodes by utilizing the dielectrophoretic forces present in the structure when a solution containing the origamis was put onto the electrodes and a voltage was applied. It was observed during the experiments that the success of the dielectrophoretic trapping depended strongly on the trapping conditions. This caused the trapping to be somewhat challenging and it was also noticed that the electrode structure with the 400 nm gap particularly required patience in order to produce good results, since the origamis to be trapped were of the same size as the gap between the electrodes making the successful trapping problematic. Despite this, a sufficient amount of trapped single nanotubes were produced.

Carbon nanotubes as electrodes for dielectrophoresis of DNA

Dielectrophoresis can potentially be used as an efficient trapping tool in the fabrication of molecular devices. For nanoscale objects, however, the Brownian motion poses a challenge. We show that the use of carbon nanotube electrodes makes it possible to apply relatively low trapping voltages and still achieve high enough field gradients for trapping nanoscale objects, e.g., single molecules. We compare the efficiency and other characteristics of dielectrophoresis between carbon nanotube electrodes and lithographically fabricated metallic electrodes, in the case of trapping nanoscale DNA molecules. The results are analyzed using finite element method simulations and reveal information about the frequency-dependent polarizability of DNA.

Dielectrophoresis of nanoscale double-stranded DNA and humidity effects on its electrical conductivity

The dielectrophoresis method for trapping and attaching nanoscale double-stranded DNA between nanoelectrodes was developed. The method gives a high yield of trapping single or a few molecules only which enables transport measurements at the single molecule level. Electrical conductivity of individual 140-nm-long DNA molecules was measured, showing insulating behavior in dry conditions. In contrast, clear enhancement of conductivity was observed in moist conditions, relating to the interplay between the conformation of DNA molecules and their conductivity.

Trapping of 27 bp-8 kbp DNA and immobilization of thiol-modified DNA using dielectrophoresis

Dielectrophoretic trapping of six different DNA fragments, sizes varying from 27 to 8416 bp, has been studied using confocal microscopy. The effect of the DNA length and the size of the constriction between nanoscale fingertip electrodes on the trapping efficiency have been investigated. Using finite element method simulations in conjunction with the analysis of the experimental data, the polarizabilities of the different size DNA fragments have been calculated for different frequencies. Also the immobilization of trapped hexanethiol- and DTPA-modified 140 nm long DNA to the end of gold nanoelectrodes was experimentally quantified and the observations were supported by density functional theory calculations.

One dimensional arrays and solitary tunnel junctions in the weak coulomb blockade regime: CBT thermometry

In this article we review the use of the tunnel junction arrays for primary thermometry. In addition to our basic experimental and theoretical results we stress the insensitivity of this method to the fluctuating background charges, to nonidealities in the array and to magnetic field. Important new results of this article are the low temperature corrections to the half width and depth of the measured conductance dip beyond the linear approximation. We also point ou that short arrays, single tunnel junctions in particular, show interesting deviations from the universal behaviour of the long arrays.

Characterization of the conductance mechanisms of DNA origami by AC impedance spectroscopy.

For some time now, ample interest has been directed to DNA as a functional or electrically operational molecule. This is due to the superior self-assembly properties that may give DNA a major role in future bottom-up fabrication processes. A striking example of DNA self-assembly techniques is DNA origami, which involves folding a long single-stranded DNA molecule with the help of short oligonucleotides, that is, staple strands. Furthermore, each of the staple strands can serve as a functionalization center (i.e., the template has individually addressable pixels with 6 6-nm spacing). This has further led to the idea that such origami could serve as a nanobreadboard for complex self-assembled nanoelectronic systems. Due to this, apart from the fundamental interest, the intriguing question of DNA electrical conductivity gains further importance: the electrical properties of such a nanobreadboard must be well understood before it can be exploited in nanotechnology. In this Communication, we measure the conductivity, and experimentally analyze the conductivity mechanisms, of single rectangular DNA origamis trapped and immobilized between nanoelectrodes by utilizing alternating-current impedance spectroscopy (AC-IS). The experiments show that the nature of the DNA origami conductivity is not purely Ohmic but that it is a combination of ionic diffusion and electronic conductivity, with a resistance

Fabrication of mesoscopic superconducting Nb wires using conventional electron-beam lithographic techniques

Conventional electron-beam lithography has been used to fabricate mesoscopic Nb wires. Nb was deposited in an ultrahigh vacuum evaporation chamber using electron gun heating. The typical linewidth and the thickness were 200 and 50 nm, respectively. The transition temperatures were above 7 K. They increased with thickness and linewidth. To demonstrate the feasibility of two angle evaporation techniques, we also fabricated Nb/(Al–)AlOx/Nb tunnel junctions showing superconducting single-electron transistor characteristics.

Custom-shaped metal nanostructures based on DNA origami silhouettes

The DNA origami technique provides an intriguing possibility to develop customized nanostructures for various bionanotechnological purposes. One target is to create tailored bottom-up-based plasmonic devices and metamaterials based on DNA metallization or controlled attachment of nanoparticles to the DNA designs. In this article, we demonstrate an alternative approach: DNA origami nanoshapes can be utilized in creating accurate, uniform and entirely metallic (e.g. gold, silver and copper) nanostructures on silicon substrates. The technique is based on developing silhouettes of the origamis in the grown silicon dioxide layer, and subsequently using this layer as a mask for further patterning. The proposed method has a high spatial resolution, and the fabrication yields can approach 90%. The approach allows a cost-effective, parallel, large-scale patterning on a chip with fully tailored metallic nanostructures; the DNA origami shape and the applied metal can be specifically chosen for each conceivable implementation.

ADIABATIC TRANSPORT OF COOPER PAIRS IN ARRAYS OF JOSEPHSON JUNCTIONS

We present a quantitative theory of Cooper pair pumping in gated one-dimensional arrays of Josephson junctions. The pumping accuracy is limited by quantum tunneling of Cooper pairs out of the propagating potential well and by direct supercurrent flow through the array. Both corrections decrease exponentially with the number N of junctions in the array, but give a serious limitation of accuracy for any practical array. We also consider the Cooper pair trap and how it can be used to test the results experimentally.