Claudia Blöser

Time-resolved photocurrent spectroscopy of the evolution of the electric field in optically excited superlattices and the prospects for Bloch gain

We report on photocurrent spectroscopy on undoped GaAs∕AlGaAs semiconductor superlattices subjected to femtosecond optical excitation. The evolution of the carrier-drift-induced inhomogeneity of the electric field is studied by tracing the shifting and broadening of Wannier–Stark transitions as a function of delay time and bias field. Based on experimental data and results of numerical simulations, we find that the superlattice rapidly splits into two moving field regions, one with strong field gradient and low electron density, the other with partially screened field at low gradient and high electron density. Concerning future Bloch-gain measurements, we find that gain is expected in spite of the inhomogeneous field if the electron-rich region is not heavily screened. The time window during which Bloch gain exists is determined by the sweep out of the electrons (10–30ps).

Degrees of Responsibility in Kant’s Practical Philosophy

It has been argued that Kant’s practical philosophy cannot allow for degrees of responsibility for one’s actions. However, it would be uncompromising to allow for only two possibilities: either full responsibility or none. Moreover, in the Metaphysics of Morals Kant himself claims that there can be degrees of responsibility, depending on the magnitude of the obstacles that have to be overcome when acting. I will show that this claim is consistent with Kant’s theory as a whole and thereby make transparent how degrees of responsibility are possible for Kant. The solution is based on the distinction between two senses of responsibility: taking oneself to be an accountable person is an all-or-nothing affair, whereas praise- or blameworthiness for a particular action can still be a matter of degree.

Time-resolved photocurrent spectroscopy of optically excited superlattices and the prospects for Bloch gain

We report on experiment and theory of the evolution of the electric field in undoped GaAs/AlGaAs semiconductor superlattices subjected to femtosecond optical excitation. We performed time-resolved pump-probe experiments and measured the photocurrent generated by spectrally narrowed and wavelength-tuned probe pulses as a function of delay time, pump power and bias field. The drift of the charge carriers, subsequent to the optical excitation, leads to the buildup of an inhomogeneity of the electric field which was traced via the temporal changes of the Wannier-Stark spectra. Although the photocurrent spectra by themselves only yield information on the absorption integrated spatially over the superlattice, we extract information on the local electric fields and the charge-carrier densities by a comparison of the measured data with the results of Monte-Carlo simulations. We find that at moderate excitation densities (1016-cm-3 range) the superlattice within a few picoseconds splits into two moving field regions, one with strong field gradient and low electron density, the other with partially screened field at low gradient and high electron density. The largest field differences are found just when the last electrons are swept out after 10-30 ps, the exact time depending on the superlattice parameters and excitation conditions. The initial homogeneous field is restored on a much longer time scale of hundreds of picoseconds which is defined basically by the drift of the heavy holes. Our calculations show that Bloch gain in optically excited semiconductor superlattice is expected in spite of the inhomogeneous field if the field in the electron-rich region is not heavily screened. The time window during which Bloch gain exists is determined by the sweep-out of the electrons.

Fundamental Hope and Practical Identity

Abstract This article considers the question ‘What makes hope rational?’ We take Adrienne Martins recent incorporation analysis of hope as representative of a tradition that views the rationality of hope as a matter of instrumental reasons. Against this tradition, we argue that an important subset of hope, ‘fundamental hope’, is not governed by instrumental rationality. Rather, people have reason to endorse or reject such hope in virtue of the contribution of the relevant attitudes to the integrity of their practical identity, which makes the relevant hope not instrumentally but intrinsically valuable. This argument also allows for a new analysis of the reasons people have to abandon hope and for a better understanding of non-fundamental, ‘prosaic’ hopes.

Werte und Normen

Die Metaphysik von Werten und Normen behandelt vor allem Fragen nach der Ontologie, d. h. der Existenzform von Werten und Normen. Gegenstand der Werttheorie sind sowohl ›dunne‹ Werte (›gut‹ und ›schlecht‹) als auch ›dichte‹ evaluative Eigenschaften (›gerecht‹, ›mutig‹, ›grausam‹).