Nonlinear combining of laser beams

We propose to combine multiple laser beams into a single diffraction-limited beam by the beam self-focusing (collapse) in the Kerr medium. The beams with the total power above critical are first combined in the near field and then propagated in the optical fiber/waveguide with the Kerr nonlinearity. Random fluctuations during propagation eventually trigger strong self-focusing event and produce diffraction-limited beam carrying the critical power.Comment: 5 pages, 5 figure

Tardos fingerprinting is better than we thought

We review the fingerprinting scheme by Tardos and show that it has a much better performance than suggested by the proofs in Tardos' original paper. In particular, the length of the codewords can be significantly reduced. First we generalize the proofs of the false positive and false negative error probabilities with the following modifications: (1) we replace Tardos' hard-coded numbers by variables and (2) we allow for independently chosen false positive and false negative error rates. It turns out that all the collusion-resistance properties can still be proven when the code length is reduced by a factor of more than 2. Second, we study the statistical properties of the fingerprinting scheme, in particular the average and variance of the accusations. We identify which colluder strategy forces the content owner to employ the longest code. Using a gaussian approximation for the probability density functions of the accusations, we show that the required false negative and false positive error rate can be achieved with codes that are a factor 2 shorter than required for rigid proofs. Combining the results of these two approaches, we show that the Tardos scheme can be used with a code length approximately 5 times shorter than in the original construction.Comment: Modified presentation of result

Spin waves in magnetic quantum wells with Coulomb interaction and sdsd exchange coupling

We theoretically describe the spin excitation spectrum of a two dimensional electron gas embedded in a quantum well with localized magnetic impurities. Compared to the previous work, we introduce equations that allow to consider the interplay between the Coulomb interaction of delocalized electrons and the $sd$ exchange coupling between electrons and magnetic impurities. Strong qualitative changes are found : mixed waves propagate below the single particle continuum, an anticrossing gap is open at a specific wavevector and the kinetic damping due to the electron motion strongly influences the coupling strength between electrons and impurities spins

Nuclear spin warm-up in bulk n-GaAs

We show that the spin-lattice relaxation in n-type insulating GaAs is dramatically accelerated at low magnetic fields. The origin of this effect, that cannot be explained in terms of well-known diffusion-limited hyperfine relaxation, is found in the quadrupole relaxation, induced by fluctuating donor charges. Therefore, quadrupole relaxation, that governs low field nuclear spin relaxation in semiconductor quantum dots, but was so far supposed to be harmless to bulk nuclei spins in the absence of optical pumping can be studied and harnessed in much simpler model environment of n-GaAs bulk crystal.Comment: 5 pages, 4 figure

An Internal Perspective of Business Model Innovation in Manufacturing Companies

Business model innovation is increasingly being used as a concept in different academic fields, although it is still missing a ground theoretical conceptualization. This work aims at advancing the understanding of business model innovation from an internal perspective in manufacturing companies. It builds on a literature review regarding two main concepts – business model and business architecture – and proposes a set of key areas for internally-driven business model innovation. Six main areas have been identified: i) strategy & business goals; ii) organizational culture; iii) product and service management; iv) technology management; v) operations management; vi) performance management systems. Further research will analyze the innovation processes within each area, based on existing theoretical foundations and empirical studies. Moreover, new empirical studies will be performed to further investigate different potential pathways for business model innovation.This work builds on research initially undertaken on SustainValue project which received funding from the European Community´s Seventh Framework Programme (FP7/2007-2013) under grant agreement n°262931. Further review and analysis leading to these results was supported by the EPSRC Centre for Innovative Manufacturing in Industrial Sustainability (grant n° EP/I033351/1).This is the author accepted manuscript. The final version is available from IEEE via http://dx.doi.org/10.1109/CBI.2015.4

Creating and Capturing Value Through Sustainability: The Sustainable Value Analysis Tool. A new tool helps companies discover opportunities to create and capture value through sustainability.

Recent research and practice have shown that business model innovation can be one way to create and capture new value and drive production and consumption toward sustainability. However, business model tools typically do not create a space to consider how sustainability concerns may be integrated into the innovation process. To address this gap, this article describes a tool that can help companies identify new opportunities to create and capture value through sustainability by analyzing value captured and uncaptured for key stakeholders across the product life cycle. The Sustainable Value Analysis Tool is shown to help companies recognize value uncaptured and turn it into opportunities; it facilitates sustainability-focused business model innovation by identifying value uncaptured—and hence, opportunities for innovation—associated with environmental and social sustainability in production, use, and disposal.This study was supported by the EPSRC Centre for Innovative Manufacturing in Industrial Sustainability (grant EP/I033351/1) and the EPSRC project Business Models for Sustainable Industrial Systems (grant EP/L019914/1)

Ballistic spin transport in exciton gases

Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. This brings several complications: the Pauli principle prevents the carriers from moving with the same speed; Coulomb repulsion leads to rapid dephasing of electron flows. Spin-optronics is a valuable alternative to traditional spintronics. In spin-optronic devices the spin currents are carried by electrically neutral bosonic quasi-particles: excitons or exciton-polaritons. They can form highly coherent quantum liquids and carry spins over macroscopic distances. The price to pay is a finite life-time of the bosonic spin carriers. We present the theory of exciton ballistic spin transport which may be applied to a range of systems where bosonic spin transport has been reported, in particular, to indirect excitons in coupled GaAs/AlGaAs quantum wells. We describe the effect of spin-orbit interaction of electrons and holes on the exciton spin, account for the Zeeman effect induced by external magnetic fields, long range and short range exchange splittings of the exciton resonances. We also consider exciton transport in the non-linear regime and discuss the definitions of exciton spin current, polarization current and spin conductivity.Comment: 16 pages, 10 figures to be published in Phys. Rev.

Ultimate photo-induced Kerr rotation achieved in semiconductor microcavities

Photoinduced Kerr rotation by more than $\pi /2$ radians is demonstrated in planar quantum well microcavity in the strong coupling regime. This result is close to the predicted theoretical maximum of $\pi$. It is achieved by engineering microcavity parameters such that the optical impedance matching condition is reached at the smallest negative detuning between exciton resonance and the cavity mode. This ensures the optimum combination of the exciton induced optical non-linearity and the enhancement of the Kerr angle by the cavity. Comprehensive analysis of the polarization state of the light in this regime shows that both renormalization of the exciton energy and the saturation of the excitonic resonance contribute to the observed optical nonlinearities.Comment: Shortened version prepared to submit in Phys. Rev. Letter