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K Buse

Publications and source records attributed to K Buse.

At least 19 recordsLinked to original sources

High-temperature-recorded index gratings in periodically poled lithium niobate.

Holographic index gratings based on a zero-electric-field photorefractive effect are recorded at high temperatures in copper-doped periodically poled lithium niobate crystals. The interplay between the domain structure and the index grating is studied: the fundamental grating with spatial frequency K is strongly suppressed. Pronounced sideband gratings with K(s)=K+sG appear, where G is the domain grating vector and s is an integer number. After development, an additional grating based on the electro-optic effect shows up. In contrast with the previously mentioned gratings, this grating allows anisotropic diffraction.

Journal Article↗

Bragg selectivity of space-charge gratings in multidomain lithium niobate crystals.

It is shown experimentally and theoretically that a partial domain reversal in ferroelectric lithium niobate crystals containing a previously imprinted photorefractive space-charge grating results in a highly unusual change of the angular Bragg selectivity. This carries important in situ information about domain structures, including the quality of periodically poled samples.

Journal Article↗

Frequency mixing of photorefractive and ferroelectric gratings in lithium niobate crystals.

Holographically recorded photorefractive gratings in periodically poled lithium niobate crystals (PPLNs) are investigated. The principal spatial frequency K of the grating is strongly suppressed. Sideband gratings with grating vectors K +/- G appear. From the measurements the domain grating vector G and the duty cycle of the domain structure can be obtained. These findings allow for fast nondestructive quality inspection of PPLN and are of importance for any optical application combining holography and PPLN.

Journal Article↗

Investigation of chromophore-chromophore interaction by electro-optic measurements, linear dichroism, x-ray scattering, and density-functional calculations.

Free-beam interferometry and angle-resolved absorption spectra are used to investigate the linear electro-optic coefficients and the linear dichroism in photoaddressable bis-azo copolymer thin films. From the first- and second order parameters deduced, the chromophore orientation distribution is calculated and displayed for several poling temperatures and chromophore concentrations. The influence of dipole-dipole interaction on the overall polymer dynamics is discussed. The first order parameter, and therefore the Pockels effect, peaks for a poling temperature of around 10 degrees C above the glass transition. The decrease of the Pockels effect above this temperature region is triggered by a head-to-tail chromophore orientation, i.e., a transition to a microcrystalline phase, increasing the second order parameter. Comparison of the experimentally observed absorption spectra and those obtained by density-functional calculations support the picture of differently aligned bis-azo dye molecules in a trans,trans configuration. Complementary wide-angle x-ray scattering is recorded to confirm the various kinds of ordering in samples poled at different temperatures.

Algorithms↗

Femtosecond time-resolved absorption processes in lithium niobate crystals.

Femtosecond pump pulses are strongly attenuated in lithium niobate owing to two-photon absorption; the relevant nonlinear coefficient beta(p) ranges from approximately 3.5 cm/GW for lambda(p) = 388 nm to approximately 0.1 cm/GW for 514 nm. In collinear pump-probe experiments the probe transmission at the double pump wavelength 2lambda(p) = 776 nm is controlled by two different processes: A direct absorption process involving pump and probe photons (beta (r) = 0.9 cm/GW) leads to a pronounced short-duration transmission dip, whereas the probe absorption by pump-excited charge carriers results in a long-duration plateau. Coherent pump-probe interactions are of no importance. Hot-carrier relaxation occurs on the time scale of < or approximately equal to 0.1 ps.

Journal Article↗

Investigation of nonlinear absorption processes with femtosecond light pulses in lithium niobate crystals.

The propagation of high-power femtosecond light pulses in lithium niobate crystals (LiNb O3 ) is investigated experimentally and theoretically in collinear pump-probe transmission experiments. It is found within a wide intensity range that a strong decrease of the pump transmission coefficient at wavelength 388 nm fully complies with the model of two-photon absorption; the corresponding nonlinear absorption coefficient is beta(p) approximately = 3.5 cm/GW. Furthermore, strong pump pulses induce a considerable absorption for the probe at 776 nm. The dependence of the probe transmission coefficient on the time delay Delta t between probe and pump pulses is characterized by a narrow dip (at Delta t approximately = 0) and a long (on the picosecond time scale) lasting plateau. The dip is due to direct two-photon transitions involving pump and probe photons; the corresponding nonlinear absorption coefficient is beta(r) approximately = 0.9 cm/GW. The plateau absorption is caused by the presence of pump-excited charge carriers; the effective absorption cross section at 776 nm is sigma(r) approximately = 8 x 10(-18) cm(2). The above nonlinear absorption parameters are not strongly polarization sensitive. No specific manifestations of the relaxation of hot carriers are found for a pulse duration of approximately = 0.24 ps.

Journal Article↗

Atom lithography with a holographic light mask.

In atom lithography with optical masks, deposition of an atomic beam on a given substrate is controlled by a standing light-wave field. The lateral intensity distribution of the light field is transferred to the substrate with nanometer scale. We have tailored a complex pattern of this intensity distribution through diffraction of a laser beam from a hologram that is stored in a photorefractive crystal. This method can be extended to superpose 1000 or more laser beams. The method is furthermore applicable during growth processes and thus allows full 3D structuring of suitable materials with periodic and non-periodic patterns at nanometer scales.

Journal Article↗

Public-private health partnerships: a strategy for WHO.

Following early success with a number of high-profile partnerships, WHO is increasingly working with the private for-profit sector. In so doing, the organization finds itself in the maelstrom of a vibrant debate on the roles of public, civic, and commercial entities in society and on the appropriate modes of interaction among them. This paper examines WHO's involvement with the commercial sector, particularly in partnerships. WHO's approach to this sector is outlined and the criticisms levelled at public-private partnerships are reviewed. An indication is given of the steps recently taken by WHO to confront the concerns that have been expressed. The paper argues that partnership between WHO and the commercial sector is inevitable and that it presents considerable opportunities, but also significant risks, for the organization and for public health. A strategy is proposed for directing the debate on issues critical to WHO and its role in the promotion and protection of public health.

Communicable Diseases↗

3D imaging: wave front sensing utilizing a birefringent crystal

Utilizing a rotatable thin birefringent crystal and two polarizers, the three-dimensional (3D) wave front of any light wave can be measured conveniently. In first experiments phase-front distortions as small as 15 &mgr;m are detected with a dynamic range of 3 mm and a spatial resolution of 50 &mgr;m. Such a dynamic range and spatial resolution exceed the performance of conventional wave front sensors of, e.g., the Shack-Hartmann type, significantly. Furthermore, the new system is rather insensitive against mechanical instabilities in opposition to interferometric and holographic techniques.

Journal Article↗

Global public-private partnerships: Part II--What are the health issues for global governance?

This is the second of a two-part review of global public-private partnerships (GPPPs) for health development. Part I was published in the April issue of the Bulletin (Vol. 78, No. 4). The recent emergence of GPPPs is rapidly reconfiguring the international health landscape. While most multilateral and bilateral agencies are currently grappling with how to proceed, there is little information in the public domain concerning how individual partnerships work and to date very little consideration of the many implications of this trend. This paper differentiates between product-based, product development-based and issues/systems-based GPPPs and describes a number of examples of each type in the health sector. The benefits of these initiatives, not least the major resources which they harness for specific health problems, are identified. The final section of the paper explores the implications and dilemmas posed by GPPPs. It discusses whether or not shared goals can transcend conflicting values and mandates and how governance of partnership arrangements may transform and undermine certain attributes of multilateral organizations. The paper concludes that the current climate of goodwill between public and private sectors offers an opportunity that should not be missed: it can be used not only to foster new partnership but to ensure that partnership is truly in the interests of international public health.

Delivery of Health Care↗

Global public-private partnerships: Part I--A new development in health?

The proliferation of public-private partnerships is rapidly reconfiguring the international health landscape. This article (part I of two on the subject) traces the changing nature of partnership, and discusses the definitional and conceptual ambiguities surrounding the term. After defining global public-private partnerships (GPPPs) for health development, we analyse the factors which have led to the convergence of public and private actors and discuss the consequences of the trend toward partnership between UN agencies (including the World Bank) and commercial entities in the health sector. Generic factors such as globalization and disillusionment with the UN, and factors specific to the health sector, such as market failure in product development for orphan diseases, are examined. Reviewed are the interests, policies, practices and concerns of the UN, the private-for-profit sector, bilateral organizations, and governments of low-income countries with respect to public-private partnership. While GPPPs bring much needed resources to problems of international health, we highlight concerns regarding this new organizational format. Part II, which will be published in the May issue of the Bulletin, presents a conceptual framework for analysing health GPPPs and explores the issues raised.

Global Health↗

Health sector development: from aid coordination to resource management.

Aid coordination has assumed a prominent place on health policy agendas. This paper synthesizes the findings of research undertaken to explore the changing practices of aid coordination across a number of countries. It begins by reviewing the key issues giving rise to increased attention to aid coordination in the health sector. The second section describes, assesses and compares the strengths and weaknesses of the dominant mechanisms or instruments which were found to be employed to coordinate health sector aid in the case studies. From this analysis, four factors become clear. First, in many countries, coordination mechanisms have been introduced as a part of an incremental process of trying out different approaches--there is no one model that stands out at any one time. Secondly, some instruments function largely for consultation, predominantly coordinating inputs, while others are more directive and operational, and are used to manage inputs, processes and outputs. Third, many of the mechanisms have not excelled, although, fourth, it is difficult to judge the effectiveness or impact of aid coordination. It is therefore argued that concern with the effectiveness of aid coordination arrangements must give way to a broader analysis of the processes, outputs and outcomes governing the use of both external and domestic resources, focusing on institutional characteristics, the distribution and nature of influence among the actors, and the interests which they pursue through the aid regime. These factors varied considerably across the countries indicating that aid management is context dependent and subject to continuing changes. Finally, the paper looks at the findings in the light of the introduction of sector-wide approaches.

Budgets↗

Keeping a tight grip on the reins: donor control over aid coordination and management in Bangladesh.

A long-standing consensus that aid coordination should be owned by recipient authorities has been eclipsed by accord on the desirability of recipient management of aid along-side domestic resources. Nonetheless, in many low and lower-middle income countries, donors remain remarkably uncoordinated; where attempts at coordination are made, they are often donor-driven, and only a small proportion of aid is directly managed by recipients. This paper draws on evidence from an in-depth review of aid to the health sector in Bangladesh to analyze the systems by which external resources are managed. Based on interviews with key stakeholders, a questionnaire survey and analysis of documentary sources, the factors constraining the government from assuming a more active role in aid management are explored. The results suggest that donor perceptions of weak government capacity, inadequate accountability and compromised integrity only partially account for the propensity for donor leadership. Equally important is the consideration that aid coordination has a markedly political dimension. Stakeholders are well aware of the power, influence and leverage which aid coordination confers, an awareness which colours the desire of some stakeholders to lead aid coordination processes, and conditions the extent and manner by which others wish to be involved. It is argued that recipient management of external aid is dependent on major changes in the attitudes and behaviours of recipients and donors alike.

Bangladesh↗