Surface studies in ultra high vacuum

An ultra high vacuum system capable of attaining pressures of 10-12 mm Hg was used for thermal desorption experiments. The metal chosen for these experiments was tantalum because of its suitability for thermal desorption experiments and because relatively little work has been done using this metal. The gases investigated were carbon monoxide, hydrogen and ethylene. The kinetic and thermodynamic parameters relating to the desorption reaction were calculated and the values obtained related to the reaction on the surface. The thermal desorption reaction was not capable of supplying all the information necessary to form a complete picture of the desorption reaction. Further information was obtained by using a quadrupole mass spectrometer to analyse the desorbed species. The identification of the desorbed species combined with the value of the desorption parameters meant that possible adatom structures could be postulated. A combination of these two techniques proved to be a very powerful tool when investigating gas-metal surface reactions and gave realistic values for the measured parameters such as the surface coverage, order of reaction, the activation energy and pre-exponential function for desorption. Electron microscopy and X-ray diffraction were also used to investigate the effect of the gases on the metal surface.

Tuneable magneto-optical metamaterials based on photonic resonances in nickel nanorod arrays

We investigate the magneto-optical properties of a nanostructured metamaterial comprised of arrays of nickel nanorods embedded in an anodized aluminum oxide template. The rods are grown using a self-assembly bottom-up technique that provides a uniform, quasi-hexagonal array over a large area, quickly and at low cost. The tuneability of the magneto-optic response of the material is investigated by varying the nanorod dimensions: diameter, length and inter-rod spacing as well as the overall thickness of the template. It is demonstrated that the system acts as a sub-wavelength light trap with enhanced magneto-optical properties occurring at reflectivity minima corresponding to photonic resonances of the metamaterial. Changes in dimensions of the nickel rods on the order of tens of nanometers cause a spectral blue-shift in the peak magneto-optical response of 270 nm in the visible range. A plasmonic enhancement is also observed at lower wavelengths, which becomes increasingly damped with larger diameters and increased volume fraction of nickel inclusions. This type of structure has potential applications in high density magneto-optical data storage (up to 1011–12 rods per square inch), ultrafast magneto-plasmonic switching and optical components for telecommunications.

Magneto-optical trapping and background-free imaging for atoms near nanostructured surfaces

We demonstrate a combined magneto-optical trap and imaging system that is suitable for the investigation of cold atoms near surfaces. In particular, we are able to trap atoms close to optically scattering surfaces and to image them with an excellent signal-to-noise ratio. We also demonstrate a simple magneto-optical atom cloud launching method. We anticipate that this system will be useful for a range of experimental studies of novel atom-surface interactions and atom trap miniaturization.

Enhanced shape anisotropy and magneto-optical birefringence by high energy ball milling in NixFe1−xFe2O4 ferrofluids

Ferrofluids belonging to the series NixFe1 xFe2O4 were synthesised by two different procedures—one by standard co-precipitation techniques, the other by co-precipitation for synthesis of particles and dispersion aided by high-energy ball milling with a view to understand the effect of strain and size anisotropy on the magneto-optical properties of ferrofluids. The birefringence measurements were carried out using a standard ellipsometer. The birefringence signal obtained for chemically synthesised samples was satisfactorily fitted to the standard second Langevin function. The ball-milled ferrofluids showed a deviation and their birefringence was enhanced by an order. This large enhancement in the birefringence value cannot be attributed to the increase in grain size of the samples, considering that the grain sizes of sample synthesised by both modes are comparable; instead, it can be attributed to the lattice strain-induced shape anisotropy(oblation) arising from the high-energy ball-milling process. Thus magnetic-optical (MO) signals can be tuned by ball-milling process, which can find potential applications.

Enhanced shape anisotropy and magneto-optical birefringence by high energy ball milling in NixFe1 xFe2O4 ferrofluids

Ferrofluids belonging to the series NixFe1 xFe2O4 were synthesised by two different procedures—one by standard co-precipitation techniques, the other by co-precipitation for synthesis of particles and dispersion aided by high-energy ball milling with a view to understand the effect of strain and size anisotropy on the magneto-optical properties of ferrofluids. The birefringence measurements were carried out using a standard ellipsometer. The birefringence signal obtained for chemically synthesised samples was satisfactorily fitted to the standard second Langevin function. The ball-milled ferrofluids showed a deviation and their birefringence was enhanced by an order. This large enhancement in the birefringence value cannot be attributed to the increase in grain size of the samples, considering that the grain sizes of sample synthesised by both modes are comparable; instead, it can be attributed to the lattice strain-induced shape anisotropy(oblation) arising from the high-energy ball-milling process. Thus magnetic-optical (MO) signals can be tuned by ball-milling process, which can find potential applications

High Verdet constant Ga : S : La : O chalcogenide glasses for magneto-optical devices

The magneto-optical rotation at room temperature was measured for three Ga:S:La:O chalcogenide glasses at several laser lines in the visible. The first sample was a binary system constituted by 70 mol % Ga2S3 and 30 mol % La2O3, whereas in the second and third ones the lanthanum oxide was partially substituted by lanthanum sulfide, keeping the amount of gallium sulfide fixed. A pulsed magnetic field between 50 and 80 kG was used for the Faraday rotation measurements. The Verdet constant for one of the ternary samples was found to be as high as 0.205 min G(-1) cm(-1) at 543 nm, indicating that these chalcogenide glasses are very promising for magneto-optical applications. The data for each sample were fitted using the expected analytical expression for the magneto-optical dispersion. Measurements of the refractive index of the glasses at 632.8 nm are also reported. Data on the magneto-optical properties of two high Verdet constant, heavy-metal oxide diamagnetic glasses are also included for comparison. (C) 1999 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(99)00102-6].

Assessment of ultra-high resolution optical coherence tomography for monitoring tissue effects caused by laser photocoagulation of ex-vivo porcine retina

Retinal laser photocoagulation is an established and successful treatment for a variety of retinal diseases. While being a valuable treatment modality, laser photocoagulation shows the drawback of employing high energy lasers which are capable of physically destroying the neural retina. For reliable therapy, it is therefore crucial to closely monitor the therapy effects caused in the retinal tissue. A depth resolved representation of optical tissue properties as provided by optical coherence tomography may provide valuable information about the treatment effects in the retinal layers if recorded simultaneously to laser coagulation. Therefore, in this work, the use of ultra-high resolution optical coherence tomography to represent tissue changes caused by conventional and selective retinal photocoagulation is investigated. Laser lesions were placed on porcine retina ex-vivo using a 577 nm laser as well as a pulsed laser at 527 nm built for selective treatment of the retinal pigment epithelium. Applied energies were varied to generate lesions best representing the span from under- to overtreatment. The lesions were examined using a custom-designed optical coherence tomography system with an axial resolution of 1.78 μm and 70 kHz Ascan rate. Optical coherence tomography scans included volume scans before and after irradiation, as well as time lapse scans (Mscan) of the lesions. Results show OCT lesion visibility thresholds to be below the thresholds of ophthalmoscopic inspection. With the ultra-high resolution OCT, 42% - 44% of ophthalmoscopically invisible lesions could be detected and lesions that were under- or overexposed could be distinguished using the OCT data.

Real-time ultra-high optical coherence tomography monitoring of optical tissue effects caused by selective retina therapy

Purpose: Selective retina therapy (SRT) has shown great promise compared to conventional retinal laser photocoagulation as it avoids collateral damage and selectively targets the retinal pigment epithelium (RPE). Its use, however, is challenging in terms of therapy monitoring and dosage because an immediate tissue reaction is not biomicroscopically discernibel. To overcome these limitations, real-time optical coherence tomography (OCT) might be useful to monitor retinal tissue during laser application. We have thus evaluated a proprietary OCT system for its capability of mapping optical changes introduced by SRT in retinal tissue. Methods: Freshly enucleated porcine eyes, covered in DMEM upon collection were utilized and a total of 175 scans from ex-vivo porcine eyes were analyzed. The porcine eyes were used as an ex-vivo model and results compared to two time-resolved OCT scans, recorded from a patient undergoing SRT treatment (SRT Vario, Medical Laser Center Lübeck). In addition to OCT, fluorescin angiography and fundus photography were performed on the patient and OCT scans were subsequently investigated for optical tissue changes linked to laser application. Results: Biomicroscopically invisible SRT lesions were detectable in OCT by changes in the RPE / Bruch's complex both in vivo and the porcine ex-vivo model. Laser application produced clearly visible optical effects such as hyperreflectivity and tissue distortion in the treated retina. Tissue effects were even discernible in time-resolved OCT imaging when no hyper-reflectivity persisted after treatment. Data from ex-vivo porcine eyes showed similar to identical optical changes while effects visible in OCT appeared to correlate with applied pulse energy, leading to an additional reflective layer when lesions became visible in indirect ophthalmoscopy. Conclusions: Our results support the hypothesis that real-time high-resolution OCT may be a promising modality to obtain additional information about the extent of tissue damage caused by SRT treatment. Data shows that our exvivo porcine model adequately reproduces the effects occurring in-vivo, and thus can be used to further investigate this promising imaging technique.

Detection of sub-8-nm movements of kinesin by high-resolution optical-trap microscopy.

Kinesin is a molecular motor that transports organelles along microtubules. This enzyme has two identical 7-nm-long motor domains, which it uses to move between consecutive tubulin binding sites spaced 8 nm apart along a microtubular protofilament. The molecular mechanism of this movement, which remains to be elucidated, may be common to all families of motor proteins. In this study, a high-resolution optical-trap microscope was used to measure directly the magnitude of abrupt displacements produced by a single kinesin molecule transporting a microscopic bead. The distribution of magnitudes reveals that kinesin not only undergoes discrete 8-nm movements, in agreement with previous work [Svoboda, K., Schmidt, C. F., Schnapp, B. J. & Block, S.M. (1993) Nature (London) 365, 721-727], but also frequently exhibits smaller movements of about 5 nm. A possible explanation for these unexpected smaller movements is that kinesin's movement from one dimer to the next along a protofilament involves at least two distinct events in the mechanical cycle.

Mechanisms for triggering high-voltage breakdown in vacuum

The electrical and optical characteristics of a cylindrical alumina insulator (94% Al203) have been measured under ultra-high vacuum (P < 10-8 mBar) conditions. A high-resolution CCD camera was used to make real-time optical recordings of DC prebreakdown luminescence from the ceramic, under conditions where DC current magnitudes were limited to less than 50μA. Two concentric metallized rings formed a pair of co-axial electrodes, on the end-face of the alumina tube; a third 'transparent' electrode was employed to study the effect of an orthogonal electric field upon the radial conduction processes within the metallized alumina specimen. The wavelength-spectra of the emitted light was quantified using a high-speed scanning monochromator and photo-multiplier tube detector. Concurrent electrical measurements were made alongside the recording of optical-emission images. An observed time-dependence of the photon-emission is correlated with a time-variation observed in the DC current-voltage characteristics of the alumina. Optical images were also recorded of pulsed-field surface-flashover events on the alumina ceramic. An intensified high-speed video technique provided 1ms frames of surface-flashover events, whilst 100ns frames were achieved using an ultra high-speed fast-framing camera. By coupling this fast-frame camera to a digital storage oscilloscope, it was possible to establish a temporal correlation between the application of a voltage-pulse to the ceramic and the evolution of photonic emissions from the subsequent surface-flashover event. The electro-optical DC prebreakdown characteristics of the alumina are discussed in terms of solid-state photon-emission processes, that are believed to arise from radiative electron-recombination at vacancy-defects and substitutional impurity centres within the surface-layers of the ceramic. The physical nature of vacancy-defects within an alumina dielectric is extensively explored, with a particular focus placed upon the trapped electron energy-levels that may be present at these defect centres. Finally, consideration is given to the practical application of alumina in the trigger-ceramic of a sealed triggered vacuum gap (TVG) switch. For this purpose, a physical model describing the initiation of electrical breakdown within the TVG regime is proposed, and is based upon the explosive destabilisation of trapped charge within the alumina ceramic, triggering the onset of surface-flashover along the insulator. In the main-gap prebreakdown phase, it is suggested that the electrical-breakdown of the TVG is initiated by the low-field 'stripping' of prebreakdown electrons from vacancy-defects in the ceramic under the influence of an orthogonal main-gap electric field.