L.E. Helseth

Roles of polarization, phase and amplitude in solid immersion lens systems

By altering the polarization, phase and amplitude at the exit pupil, the intensity distribution near the focal plane of a solid immersion lens (SIL) system can be changed. We have studied how the resolution and focal depth changes for a few particular cases. It was seen that by impinging radial polarization on a SIL-system, we may obtain a rotational symmetric z-component of the focused wavefront with spot size similar to that predicted by scalar theory. We also observed that it was possible to manipulate the contributions from the homogeneous and inhomogeneous waves behind the SIL by changing the amplitude and phase distribution at the aperture. In this way it may be possible to improve both the resolution and focal depth of the system.

Faraday rotation spectra of bismuth-substituted ferrite garnet films with in-plane magnetization

Single crystal films of bismuth-substituted ferrite garnets have been synthesized by the liquid phase epitaxy method where gadolinium gallium garnet substrates are dipped into the flux. The growth parameters are controlled to obtain films with in-plane magnetization and virtually no domain activity, which makes them excellently suited for magnetooptic imaging. The Faraday rotation spectra were measured across the visible range of wavelengths. To interpret the spectra we present a simple model based on the existence of two optical transitions of diamagnetic character, one tetrahedral and one octahedral. We find excellent agreement between the model and our experimental results for photon energies between 1.77 and 2.53 eV, corresponding to wavelengths between 700 and 490 nm. It is shown that the Faraday rotation changes significantly with the amount of substituted gallium and bismuth. Furthermore, the experimental results confirm that the magnetooptic response changes linearly with the bismuth substitution.

Faraday rotation and sensitivity of (100) bismuth-substituted ferrite garnet films

We have investigated the Faraday rotation of in-plane magnetized bismuth-substituted ferrite garnet films grown by liquid phase epitaxy on (100) oriented gadolinium gallium garnet substrates. The Faraday spectra were measured for photon energies between 1.7 and 2.6 eV. To interpret the spectra, we use a model based on two electric dipole transitions: one tetrahedral and one octahedral. Furthermore, the Faraday rotation sensitivity was measured at 2.3 eV, and found to be in good agreement with the theoretical predictions. In particular, we find that the sensitivity increases linearly with the bismuth content and nonlinearly with the gallium content.

Focusing of atoms with strongly confined light potentials

Focusing of atoms with light potentials is studied. In particular, we consider strongly confined, cylindrical symmetric potential, and demonstrate their applications in both red and blue-detuned focusing of atoms. We also study the influence of aberrations, and find that a resolution of 1 nm should in principle be possible.

Interaction between a magnetic domain wall and a superconductor

memory device based on active control of generation and annihilation of vortices by means of one or more domain walls. In recent years superconducting circuits based on single-flux-quantum pulses have been shown to provide a family of digital electronics with ultrahigh speed and very low-power dissipitation. At clock rates exceeding 10 GHz and an operation speed of many hundred GHz, these devices can in the future outrun any semiconductor device. 7 Using domain walls as active ‘‘vortex gates,’’ we may add an additional degree of freedom in these devices. It is known that bismuth-substituted ferrite garnet films with in-plane magnetization have domain walls with very low coercivity that can be moved without ambiguity at frequencies up to several GHz. 6 Furthermore, in such materials Bloch walls are easily formed by external magnetic fields or stress patterns, and these could be manipulated in numerous ways suitable for a memory device.

Contact electrification and energy harvesting using periodically contacted and squeezed water droplets.

We investigate the contact electrification occurring when a small water droplet resting on a metal electrode is brought periodically in contact with a hydrophobic film of fluorinated ethylene propylene. It is found that the maximum current increases with the drop volume according to a power law. The time scale for the contact current to develop is consistent with that required for a droplet to spread and is, therefore, longer than the time required to form the electric double layer. Adding salt into the water does reduce the contact current but not entirely, which suggests that any remaining water layer cannot entirely neutralize the charges developed upon contact. With an average power of 0.7 μW and a peak power near 5 μW at a frequency of 5 Hz, a 200 μL droplet of pure water can be used to light up a light-emitting diode.

Adsorption of bovine serum albumin on polyelectrolyte-coated glass substrates: applications to colloidal lithography.

The adsorption of bovine serum albumin (BSA) labeled with fluorescein isothiocyanate (FITC) on polyelectrolyte-coated glass substrates was investigated using fluorescence microscopy. Glass substrates may inhibit adsorption of proteins due to electrostatic repulsion. However, when the substrate is modified with a thin film of positively charged polyelectrolytes, proteins can be adsorbed due to the attractive electrostatic interactions. In this study, poly(allylamine-hydrochloride) (PAH) molecules, which have positively charged amino groups at pH 7, were used to generate a positively charged layer on the glass substrate. A surfactant, sodium dodecyl sulphate (SDS), was used to alter the glass-protein interaction and subsequently modulate the coverage of adsorbed protein. We applied this technique to control the heterogeneously charged microscopic patterns of biomolecules created when the adsorption of protein is done in conjunction with colloidal lithography.

Paramagnetic particles as sensitive force detectors in liquids

We study how paramagnetic colloids in external magnetic fields can be used as sensitive force detectors in liquid systems. An experimental setup has been constructed which uses optical tweezers to calibrate the force between paramagnetic dipoles. In the operative mode the optical tweezers are switched off and the unknown force is measured by tuning a magnetic field and therefore the dipolar interaction between the two paramagnetic dipoles. The setup can be used to measure the force versus distance as well as for probing critical bond forces in the range between 5 fN and 1 pN. We apply the system to measure capillary forces at a water–air interface and for probing polyelectrolyte bridges between colloids.

Hydrophobic polymer covered by a grating electrode for converting the mechanical energy of water droplets into electrical energy

Water contact electric harvesting has a great potential as a new energy technology for powering small-scale electronics, but a better understanding of the dynamics governing the conversion from mechanical to electrical energy on the polymer surfaces is needed. Important questions are how current correlates with droplet kinetic energy and what happens to the charge dynamics when a large number of droplets are incident on the polymer simultaneously. Here we address these questions by studying the current that is generated in an external electrical circuit when water droplets impinge on hydrophobic fluorinated ethylene propylene film containing a grating electrode on the back side. Droplets moving down an inclined polymer plane exhibit a characteristic periodic current time trace, and it is found that the peak current scales with sine of the inclination angle. For single droplets in free fall impinging onto the polymer, it is found that the initial peak current scales with the height of the free fall. The transition from individual droplets to a nearly continuous stream was investigated using the spectral density of the current signal. In both regimes, the high frequency content of the spectral density scales as f −2. For low frequencies, the low frequency content at low volume rates was noisy but nearly constant, whereas for high volume rates an increase with frequency is observed. It is demonstrated that the output signal from the system exposed to water droplets from a garden hose can be rectified and harvested by a 33 μF capacitor, where the stored energy increases at a rate of about 20 μJ in 100 s.

Microscopic magnetic squeezer

A microscopic magnetic squeezer based on two magnetic domain walls moving along a one-dimensional potential well generated by a stress line in ferrite garnet films is demonstrated. The squeezer can operate on magnetic objects of size 1–200μm and exert compressive forces up to 10pN. The squeezer operation, i.e., the relative motion of the two domain walls, is well controlled by a small external magnetic field modulation. The squeezer has potential applications in microfluidics and also as sensitive pressure gauges for microbiological systems.