Search

Modeling the optical response of grating profiled PtSi/Si structures is examined to demonstrate the potential of microstructuring in optimizing the absorption of infrared detectors. Coupling to angularly broad surface plasmon polariton resonances near normal incidence is, in fact, achieved at both Si/PtSi and SiO2/PtSi interfaces for the same grating parameters in the wavelength ranges 3.0 - 4.4 micrometer and 1.3 - 1.9 micrometer respectively. These ranges correspond to two infrared, atmospheric transmission windows, and demonstrate the potential for a single device geometry to operate optimally in two different spectral bands. It is also shown that, throughout these spectral bands, it is possible to attain reflectance significantly lower than that of the planar structure counterparts in the angle range 0 degrees to plus or minus 20 degrees (corresponding to the use of F1.4 optics), along with containment of low reflectance to that angle range. Absorption mediated by the PtSi/Si surface plasmon polariton mode may be of particular interest in these Schottky barrier structures, since there would be considerable enhancement in the generation of hot carriers in the near barrier region where they have a better chance of direct or indirect (via elastic scattering) promotion over the barrier to give rise to a detectable charge.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have excited surface plasmons in a YBCO thin film at different temperatures using attenuated total reflection of light. The 300 nm thick c-axis film was fabricated using pulsed laser deposition onto an MgO (100) substrate with 248 nm KrF excimer radiation. Critical temperature of the film was 89.6 K and its roughness, as shown by atomic force microscopy, 20 nm rms, without droplets over areas of 10 micrometer by 10 micrometer. The sample was mounted in Otto geometry on a cooled stage which allowed the temperature to be varied between 300 K and 70 K. An infrared HeNe laser at 3392 nm was used to excite the surface plasmons. The dielectric function of the film was determined between room temperature and 80 K. The imaginary part of the dielectric function decreased substantially with reduction in temperature. Results obtained were: (epsilon) r equals -24.1 plus 0.0013T and (epsilon) i equals 7.7 plus 0.067T where T is the temperature in Kelvin. The ratio (epsilon) i300/(epsilon) i80 at 2.13 is less than the resistance ratio R300/R80 at 2.81. An explanation is offered in terms of two temperature independent mechanisms operative at optical frequencies: enhanced Rayleigh scattering of surface plasmons at grain boundaries and intraband/interband transitions. The real part of the dielectric function, (epsilon) r, was found to be only slightly temperature dependent. It was, however, highly sample dependent when comparison was made with the results of other films, a feature attributed to surface and grain boundary contamination.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The real, e r , and imaginary, e i , components of the ab -plane dielectric function at 3392 nm of doped thin films of YBa 2 Cu 3 O 7− δ has been determined from measured attenuated total reflectance data. While e i , is nearly temperature independent, | e r | decreases by ∼25% between 85 and 295 K. A strong dependence on doping is observed: the magnitudes of e r and e i are both strongly suppressed as delta increases. A generalised Drude analysis shows that the scattering rate increases in the underdoped regime.

Photoresponse of n-type indium-doped ZnO and a p-type polymer (PEDOT:PSS) heterojunction devices are studied, juxtaposed with the photoluminescence of the In-ZnO samples. In addition to the expected photoresponse in the ultraviolet, the heterojunctions exhibit significant photoresponse to the visible (532 nm). However, neither the doped ZnO nor PEDOT:PSS individually show any photoresponse to visible light. The sub-bandgap photoresponse of the heterojunction originates from visible photon mediated e-h generation between the In-ZnO valence band and localized states lying within the band gap. Though increased doping of In-ZnO has limited effect on the photoluminescence, it significantly diminishes the photoresponse. The study indicates that optimally doped devices are promising for the detection of wavelengths in selected windows in the visible.

Surface plasmons polaritons (SPPs) may be supported on the surface of PtSi electrodes in PtSi/Si Schottky barrier diodes. Using a prism-air gap-PtSi/p-Si or Otto coupling configuration we have excited SPPs at temperatures below 120 K on these devices. Changes in the dip in reflectance associated with SP excitation indicates changes in the PtSi optical data as a function of temperature. However it is important to understand what the optical data describe. In general they incorporate a surface roughness layer and are not truly characteristic of the material (PtSi) itself. In this context AFM analysis of the surface topography is presented. In addition the compositional structure of the PtSi as determined from Rutherford Backscattering Spectroscopy is discussed.

Hydrogen is detected using a Pd/n-InP Schottky diode in which the elongated, very thin Pd electrode is of greater resistance than the underlying semiconductor substrate. Four-probe measurements of the device resistance, as a function of hydrogen concentration, are made by contacting only the Pd electrode, with a sensitivity of 1 ppm being achieved. On hydrogen exposure the device resistance drops from an initial high value, characteristic of the Pd electrode alone, to a lower value due to a hydrogen-induced lowering of the Schottky barrier that opens up the InP substrate as a parallel current carrying channel.

Visible light is emitted from roughened metal-oxide-metal tunnel junctions subjected to a small (2 - 4 V) dc bias. The color of the emitted light is voltage tunable but device quantum efficiency is generally very low (10-5 - 10-7). The optical emission is due to the roughness induced scattering of electronically excited surface plasmon polaritons. The characteristics of the surface roughness are clearly important in determining the overall device efficiency and spectral output. With a view to better understanding and improving the efficiency we have examined the surface topography of variously roughened devices and relate this to the optical output. For example, we have roughened devices by means of holographic crossed diffraction grating substrates which possess surface topography of greater rms roughness height and larger transverse correlation length than that of devices roughened by a pre-deposited CaF2 layer. Devices roughened by means of a particulate aluminum substrate are also discussed.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Absorption of p-polarised light by the metal electrode of metal-semiconductor Schottky barrier photodetectors is considerably enhanced through the excitation of surface plasmon polaritons (SPPs) at the metal-air interface. Using a prism-air gap-sample arrangement SPPs have been excited at the Al-air interface of Al-GaAs diodes, giving rise to acorresponding enhancement in device photoresponse. By adjusting the air gap dimension the coupling efficiency between incoming light and SPPs may be varied and, for large air gaps (> 1 gm), it is shown that a number of reflectance dips occur (as a function of internal angle of incidence in the prism) due to excitation of leaky guided wave modes; there are accompanying peaks in the photosignal. The potential use of SPP enhanced Schottky diodes as sensitive chemical sensors is discussed in the context of results, both experimental and computer modelled, relating to surface contamination of silver films.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The ab-plane dielectric function at 3392nm of thin films of YBa2Cu3O7−δ with a wide range of doping has been measured using attenuated total reflectance. While e2, and therefore σ1, are temperature independent, |e1| decreases by ∼25% with increasing temperature. A stronger dependence on doping is observed: the magnitudes of e1 and e2 are both strongly suppressed as δ increases. A generalised Drude analysis shows that the plasma frequency is temperature-independent, but scales with doping. The scattering rate increases with temperature, and also increases with decreasing doping, consistent with stronger coupling in the underdoped regime.

The light emission spectrum from a scanning tunnelling microscope (LESTM) is investigated as a function of relative humidity and shown to provide a novel and sensitive means for probing the growth and properties of a water meniscus on the nanometre scale. An empirical model of the light emission process is formulated and applied successfully to replicate the decay in light intensity and spectral changes observed with increasing relative humidity. The modelling indicates a progressive water filling of the tip-sample junction with increasing humidity or, more pertinently, of the volume of the localized surface plasmons responsible for light emission; it also accounts for the effect of asymmetry in structuring of the water molecules with respect to the polarity of the applied bias. This is juxtaposed with the case of a non-polar liquid in the tip-sample nanocavity where no polarity dependence of the light emission is observed. In contrast to the discrete detection of the presence/absence of a water bridge in other scanning probe experiments through measurement of the feedback parameter for instrument control, LESTM offers a means of continuously monitoring the development of the water bridge with sub-nanometre sensitivity. The results are relevant to applications such as dip-pen nanolithography and electrochemical scanning probe microscopy.

Electrical transport and structural properties of platinum nanowires, deposited using the focused ion beam method, have been investigated. Energy dispersive x-ray spectroscopy reveals metal-rich grains (atomic composition 31% Pt and 50% Ga) in a largely non-metallic matrix of C, O and Si. Resistivity measurements (15–300 K) reveal a negative temperature coefficient with the room-temperature resistivity 80–300 times higher than that of bulk Pt. Temperature-dependent current–voltage characteristics exhibit non-linear behaviour in the entire range investigated. The conductance spectra indicate increasing non-linearity with decreasing temperature, reaching 4% at 15 K. The observed electrical behaviour is explained in terms of a model for inter-grain tunnelling in disordered media, a mechanism that is consistent with the strongly disordered nature of the nanowires observed in the structure and composition analysis.

Background: Low Secure Mental Health Services provide care for highly disturbed psychiatric inpatients. Little is known about the provision of such services in the UK.Aims: To perform the first UK wide survey developing a national data set for Low Secure Services.Method: A cross-sectional census day postal survey design was employed.Results: A total of 307 units were identified in the UK, this was comprised of 170 Psychiatric Intensive Care Units (PICUs) housing 1,242 patients and 137 Low Secure Units (LSUs) treating 1,583 patients. The survey achieved a 98% response rate. PICUs on average had more qualified staff and were more medically oriented than LSUs. PICUs offer a time limited, medically oriented treatment strategy with a higher number of qualified nursing staff then LSUs. Whereas, LSUs placed more of an emphasis on long-term therapeutic treatment and rehabilitation.Conclusions: Although there were a variety of names, Low Secure Services were organised into two main categories of unit: the PICU and the LSU. These names should be adopted nation-wide as standard. There is need for a national strategy relating to the way PICUs and LSUs are developed.

It has become evident that Psychiatric Intensive Care Units (PICU) accept aggressive, violent patients often with criminal histories and personality disorders. This paper aims to document the effects that this patient group can have on a PICU and the significant role that psychotherapy can offer this complex patient group. The conclusions have been drawn from psychotherapeutic work on a PICU (Pathways on Tagore, NELMHT). Such patients present unique therapeutic and emotional challenges to staff. As a result staff need to recognise and address these issues. This paper identifies and discusses a number of central themes, challenges and practical skills that may be important in enabling the successful treatment of such patients within a PICU setting.

The dielectric function of a YBCO film was determined at 3392nm at temperatures down to 80K. Results obtained were ʵr = -24.09 − 0.0013T and ʵi = 7.66 + 0.067T. The results for ʵi are compared with the dc resistance of the film. Intrinsic intragrain scattering, elastic and inelastic grain boundary scattering and optical interband absorption are estimated as 82%, 5%, 13% and 10% respectively at OK.

Surface plasmons have been observed directly in YBCO films in an Otto-geometry attenuated total reflection measurement at a wavelength of 3.392 μm. The laser deposited films are c -axis oriented on an MgO substrate. This observation confirms theoretical deductions from complex dielectric function data. Measured data have been fitted to a theoretical model and are compared with the optical constants determined by Bozovic [1]. The investigations have been extended to films with other orientations to investigate whether material anisotropy is reflected in the results and non-metallic behaviour is found.

It is shown that surface plasmons (SPs) are supported on thin PtSi films. Using a prism-air gap-sample configuration, p-polarised infra-red light (3.39 μm) has been coupled with 95% efficiency to SPs on the silicide electrode of PtSi-Si Schottky barrier structures. Stimulating SPs offers both a means of optically characterising silicide films and of enhancing optical absorption with a view to significantly increasing the Schottky barrier photoresponse.

The ternary oxide K2NiO2 has been prepared by the solid state reaction of potassium oxide and nickel oxide in an equimolar ratio. Reactions using an excess of potassium oxide give the ternary oxide K2NiO3 as an additional product. The compound K2NiO3 cannot be prepared as a single phase by the reaction of potassium oxide and nickel oxide in argon or under vacuum, but may be prepared by reaction in oxygen or by the oxidation of the ternary oxide, K2NiO2. Nickel oxide is reduced to nickel metal at 185 °C by liquid potassium, with the formation of the ternary oxides K2NiO2 and K2NiO3. Thermomagnetic analysis indicates that the reaction occurs in a single-stage.

Light emission from tunnel junctions occurs when the tunnelling electrons excite surface plasmon polaritons (SPPS) and these SPPS decay by photon emission. Laks and Mills (1979, 1980) predicted an additional direct process in which optical frequency fluctuations in the tunnel current couple directly to the radiation field. The authors have observed an anomalous component in the light output from a nominally smooth aluminium-aluminium oxide-gold tunnel junction which has many of the features expected in direct emission.

A detailed description of the experimentally observed light output from statistically rough ${\mathrm{A}\mathrm{l}\ensuremath{-}\mathrm{A}\mathrm{l}}_{2}$${\mathrm{O}}_{3}$-$M$ ($M=\mathrm{A}\mathrm{g},\mathrm{A}\mathrm{u},\mathrm{C}\mathrm{u}$) tunnel junctions is presented. These data include a comprehensive description of the polarization and angular distribution of the light emitted from ${\mathrm{A}\mathrm{l}\ensuremath{-}\mathrm{A}\mathrm{l}}_{2}$${\mathrm{O}}_{3}$-Au junctions as well as spectra from reverse-biased ${\mathrm{A}\mathrm{l}\ensuremath{-}\mathrm{A}\mathrm{l}}_{2}$${\mathrm{O}}_{3}$-Ag junctions. It is argued, principally on the grounds of an examination of surface-plasmon---polariton (SPP) damping, that the bulk of the output from statistically rough tunnel junctions is due to the fast-SPP mode. The idea of fast-SPP mediation is found, in many respects, to be much more consistent with currently available experimental results than that of slow- (or junction) SPP mediation. Extant theoretical models hold slow-SPP mediation to be the dominant means of visible-regime emission. The view of the emission mechanism presented in this paper suggests that the statistically rough tunnel junction could emit light more efficiently (if the scale of the surface roughness were altered) and that it has potential as a spectroscopic tool.

The light output from Al-I-Au tunnel junctions is observed to depend on the morphology of the Au film electrode. Films deposited quickly (2 nm s-1) give out less light, especially towards the blue end of the spectrum, than those deposited slowly (0.03 nm s-1). An explanations is offered in terms of elastic scattering of surface plasmon polaritons in the Au film. The scattering increases the chance of plasmon decay by internal electromagnetic absorption at the expanse of photon emission. A model, of more general applicability, has been developed which can explain the results. SEM has been used to determine the grain size in the Au films and STM to measure the surface roughness.

The ternary oxides KCuO and K 4 CuO 3 have been observed as products in the solid state reactions of potassium oxide with the oxides Cu 2 O and CuO under argon and in vacuum. The compound K 4 CuO 3 has been isolated as a single phase and its X-ray powder diffraction pattern has been obtained. Both Cu 2 O and CuO are reduced by liquid potassium to copper metal, with the formation of the ternary oxides KCuO and K 4 CuO 3 . Thermal analysis showed that Cu 2 O reacts with liquid potassium at 185 °C and that CuO undergoes the same reaction at 70 °C.

Its is found that the spectrum of the light emitted by statistically rough tunnel junctions in the range 400–700 nm correlates with the calculated damping of the fast surface plasmon polariton mode of the tunnel junction structure. It is deduced that the radiative decay of this mode is responsible for the bulk of the light emission. It is shown that the view of existing theory that emission from statistically rough junctions is mediated by the slow surface plasmon polariton mode is untenable. Our deductions firmly consolidate a suggestion due to Kroo, Szentirmay and Felszerfalvi.

We have observed light emission from the rear of CaF2-roughened Al-I-Au tunnel sandwiches. Like the forward emitted light, its spectral intensity shows a sharp drop at energies > 2.5 eV. We interpret the results in terms of both emissions being mediated by a common surface or interface plasmon; the plasmon is damped above 2.5 eV by excitation of an interband transition in Au.

We have excited mid-infrared surface plasmons in two YBCO thin films of contrasting properties using attenuated total reflection of light and found that the imaginary part of the dielectric function decreases linearly with reduction in temperature. This result is in contrast with the commonly reported conclusion of infrared normal reflectance studies. If sustained it may clarify the problem of understanding the normal state properties of YBCO and the other cuprates. The dielectric function of the films, e = e 1 + i e 2 , was determined between room temperature and 80K: e 1 was found to be only slightly temperature dependent but somewhat sample dependent, probably as a result of surface and grain boundary contamination. The imaginary part, e 2 , (and the real part of the conductivity, σ 1 ,) decreased linearly with reduction in temperature in both films. Results obtained were: for film 1: e 1 = − 14.05 − 0.0024 T and e 2 = 4.11 + 0.086 T and for film 2: e 1 = −24.09 + 0.0013 T and e 2 = 7.66 + 0.067 T where T is the temperature in Kelvin. An understanding of the results is offered in terms of temperature-dependent intrinsic intragrain inelastic scattering and temperature-independent contributions: elastic and inelastic grain boundary scattering and optical interband (or localised charge) absorption. The relative contribution of each is estimated. A key conclusion is that the interband (or localised charge) absorption is only ∼ 10%. Most importantly, the intrinsic scattering rate, 1/τ, decreases linearly with fall in temperature, T , in a regime where current theory predicts dependence on frequency, ω, to dominate. The coupling constant, λ, between the charge carriers and the thermal excitations has a value of 1.7, some fivefold greater than the far infrared value. These results imply a need to restate the phenomenology of the normal state of high temperature superconductors and seek a corresponding theoretical understanding.

Excitation of surface plasmons by attenuated total reflection is used to optically characterize platinum silicide films and to produce enhanced photosignals from PtSi-Si Schottky barrier devices. In this work the Otto configuration (prism-air gap-sample) for attenuated total reflection is used to excite the mode at the air-PtSi interface on PtSi-Si structures; surface plasmon excitation is evidenced as a prominent dip in measurement of reflectance as a function of internal angle of incidence in the prism. Coupling efficiencies of 90 - 95% between incoming p-polarized radiation and surface plasmons on PtSi have been achieved at wavelengths ranging from the visible to the infrared (0.633 micrometers to 3.39 micrometers ). The surface plasmon energy is dissipated by absorption through the creation of electron-hole pairs in the lossy silicide layer. Subsequent hot carrier emission over the Schottky barrier can thus yield an enhanced photosignal associated with the enhanced absorption due to surface plasmon excitation--initial results demonstrating this phenomenon are reported. here. Finally, reflectivity calculations show also the possibility of infrared surface plasmon excitation in PtSi grating structures with periodicity of the same order as the wavelength.

The intensity of surface enhanced Raman scattering (SERS) from benzoic acid and benzoic acid derivatives on mildly roughened, thermally evaporated Ag films shows a strong dependence on the rate at which the Ag film was deposited. Slowly deposited Ag films give a superior SERS response, by up to a factor of 10, to quickly deposited films, with films deposited at an intermediate rate yielding intermediate results. Careful examination using STM indicates that little distinction can be made between the differently prepared films in terms of surface roughness. By contrast TEM measurements reveal that the average metal grain dimension in slowly deposited Ag films is about 3-4 times greater than that in their quickly deposited counterparts. On the premise that the fundamental excitation of importance to the enhancement mechanism is the surface plasmon polariton (SPP) it is argued that the contrast in Raman scattering efficiency is due to differences in elastic grain boundary scattering of SPPs (leading to different degrees of internal SPP damping), rather than differences in the interaction of SPPs with surface inhomogeneities. Corollary data on elastic SPP-photon scattering obtained in a related experiment are also presented.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The field of surface polariton physics really took off with the prism coupling techniques developed by Kretschmann and Raether, and by Otto. This article reports on the construction and operation of a rotatable, in vacuo, variable temperature, Otto coupler with a coupling gap that can be varied by remote control. The specific design attributes of the system offer additional advantages to those of standard Otto systems of (i) temperature variation (ambient to 85 K), and (ii) the use of a valuable, additional reference point, namely the gap-independent reflectance at the Brewster angle at any given, fixed temperature. The instrument is placed firmly in a historical context of developments in the field. The efficacy of the coupler is demonstrated by sample attenuated total reflectance results on films of platinum, niobium, and yttrium barium copper oxide and on aluminum/gallium arsenide (Al/GaAs) Schottky diode structures.

The construction and operation of a prism/variable-gap/sample system (or variable-gap Otto coupler) for the excitation of surface electromagnetic modes is reported. This system has been used for the observation and characterization of surface plasmon polaritons on thin film structures. The initial alignment of prism and sample is performed under gravity and the subsequent gap variation is performed by means of a single actuator operating a flexure stage on which the prism is mounted. The flexure stage ensures the maintenance of good parallelism between sample and prism as the gap dimension is varied. The coupler has also served as a prototype, in terms of design principle, for the construction of a more sophisticated, variable-gap Otto coupler that can operate in vacuum at temperatures from ambient to 85 K.

To study some of the interfacial properties of PtSi/Si diodes, Schottky structures were fabricated on (100) crystalline silicon substrates by conventional thermal evaporation of Pt on Si followed by annealing at different temperatures (from 400°C to 700°C) to form PtSi. The PtSi/n-Si diodes, all yielded Schottky barrier (SB) heights that are remarkably temperature dependent. The temperature range (20–290 K) over which the I–V characteristics were measured in the present study is broader with a much lower limit (20 K), than what is usually reported in literature. These variations in the barrier height are adequately interpreted by introducing spatial inhomogeneity into the barrier potential with a Gaussian distribution having a mean barrier of 0.76 eV and a standard deviation of 30 meV. Multi-frequency capacitance–voltage measurements suggest that the barrier is primarily controlled by the properties of the silicide–silicon interface. The forward C–V characteristics, in particular, show small peaks at low frequencies that can be ascribed to interface states rather than to a series resistance effect.

Observation of both narrowband 3.8 eV emission and broadband, mostly visible emission from single, statistically rough CaF 2 AlAl 2 O 3 Ag tunnel junction structures under high applied bias (> 4 V) is reported. Resolution between the two components of the output is much superior in the tunnel junction arrangement than in experiments where high energy (keV) electron beams impinge on rough Ag films. Importantly, we find that the broadband emission cuts off at 3.5 eV even when the applied bias exceeds 4 V. This cut-off energy is higher than that reported elsewhere and confirms that for Ag junctions the bulk of the broadband output is indeed mediated by the fast surface plasmon polariton mode (which has a maximum energy of 3.5 eV) and not by the slow or junction mode. Both components of the output are found to be most intense in the direction normal to the junction surface and to be substantially unpolarised. For the broadband emission these features are consistent with results of theoretical calculation; for the narrowband 3.8 eV emission they contrast sharply with the observed characteristics of the 3.8 eV emission from smooth Ag junctions.

No abstract provided