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This contribution addresses both the general mid-infrared ͑MIR͒ properties of Cu 2 O and induced absorption due to intraexcitonic 1s → 2p transitions under simultaneous cw laser excitation. After an overview of the current state of knowledge and remaining open questions concerning excitons in Cu 2 O we will first discuss several absorption bands in the spectral range between approximately 100 and 150 meV in terms of multiphonon and biphonon features. The second, more extensive part will concern a pump-probe experiment in the limit of low densities of 1s excitons where we investigated intraexcitonic transitions appearing in the MIR spectral range as well. We find a signal which can be clearly assigned to a 1s para→ 2p para transition and present various data depending on excitation conditions and sample temperature. Applying the results of detailed theoretical considerations we can estimate the density and lifetime of the 1s para-exciton and at least speculate about its dispersion.

Dedicated to Professor Dr. Roland Zimmermann on the occasion of his 60th birthdayThe possibility of Bose-Einstein condensation of excitons in semiconductors and/or their superfluid transport are long-standing, still controversially discussed problems. In a recent series of experiments by E. Fortin and coworkers [1-3] [phys. stat. sol. (b) 191, 345 (1995); Phys. Rev. Lett. 77, 896 (1996); and Proc. Internat. Conf. Exciton Processes in Condensed Matter, World Scientific, Singapore 2000, respectively], (para-)excitons in Cu 2 O have been detected electrically by their field ionization in a Schottky barrier. We analyze the electric field in the Schottky barrier and its screening by the holes remaining after field ionization and find serious screening and saturation effects of this Schottky-barrier exciton detector under the excitation conditions of the experiment. These findings make a reinterpretation of the data very likely. IntroductionExcitons are the quanta of the collective excitation of the electron system in semiconductors and insulators. This excitation can be described to a very good approximation as a two-particle state, consisting of an electron in the conduction band and of the hole left behind in the valence band plus Coulomb and exchange interaction between these two particles. In so far the problem is closely related to the hydrogen or the positronium atom (see, e.g., [4,5], and references therein). Obviously the exciton is an electrically neutral quasiparticle with integer spin, but it is composed of two fermions, namely electron and hole. As a consequence of this fact, the commutation relations between exciton creation and annihilation operators are those of bosons at low densities, but with a deviation from ideal boson character via a term which increases with the density of excitons [4].The interplay of bosonic character and fermionic constituents triggered a lot of research over several decades about what happens with excitons at the highest densities. In the following we summarize shortly the present situation for bulk semiconductors. It became clear that at the highest densities a transition to a metallic electron-hole plasma (EHP) occurs [4][5][6]. Depending on the material parameters and excitation conditions such as carrier lifetime, excitation density, and carrier temperature, the plasma may even undergo a first-order phase transition to a liquid-like state, the analogon to molten metallic hydrogen. This is even true in CuCl [7] in spite of its large exciton binding energy and small Bohr radius, shifting the density for the occurrence of an EHP (also called Mott density) from values below 10 17 cm À3 , e.g., for Ge to very high values around 10 20 cm À3 . Experimental investigations and the theoretical modelling of the electron-hole plasma have been pushed by many research groups world-wide as can be seen, e.g., from the references in reviews and textbooks like [4,5]. These refer-

Progress at Rolls-Royce Fuel Cell Systems (RRFCS) continues towards demonstration of a 250 kW generator module for launch of a 1 MW pressurized SOFC hybrid system for stationary power generation. This paper describes recent progress on demonstration of the SOFC system, both as individual components - core cell and stack technology, the turbogenerator, packaging, and, crucially, the progress on building these components together into a functional hybrid Generator Module and system. The establishment of a truly global engineering team with skills and experience across the range of technologies required has been crucial to the development of the overall system with a view to cost effective manufacture, system simplicity and realistic fuel availability. Efficiency and key system components are discussed, together with design evolution as the technology moves towards a commercial product.

We employ ultrabroadband terahertz-midinfrared probe pulses to characterize the optical response of photoinduced charge-carrier plasmas in high-resistivity silicon in a reflection geometry, over a wide range of excitation densities (10 15 -10 19 cm −3 ) at room temperature. In contrast to conventional terahertz spectroscopy studies, this enables one to directly cover the frequency range encompassing the resultant plasma frequencies. The intensity reflection spectra of the thermalized plasma, measured using sum-frequency (up-conversion) detection of the probe pulses, can be modeled well by a standard Drude model with a density-dependent momentum scattering time of ∼200 fs at low densities, reaching ∼20 fs for densities of ∼10 19 cm −3 , where the increase of the scattering rate saturates. This behavior can be reproduced well with theoretical results based on the generalized Drude approach for the electron-hole scattering rate, where the saturation occurs due to phase-space restrictions as the plasma becomes degenerate. We also study the initial subpicosecond temporal development of the Drude response and discuss the observed rise in the scattering time in terms of initial charge-carrier relaxation, as well as the optical response of the photoexcited sample as predicted by finite-difference time-domain simulations.

In a differential absorption experiment the induced infrared transitions from the excitonic 1s to the 2p levels in Cu 2 O have been investigated. Intermediate densities of 1s excitons were created by cw-laser excitation while the interexcitonic 1s to 2p transitions were probed simultaneously, using Fourier spectroscopy. Our data give evidence for a surprisingly large splitting of the 2p level (%3.7 meV) the origin of which is a matter of speculation. An analysis of lineshape and width of the transitions results in a ratio of the effective masses m 1s =m 2p which deviates from the literature value.1 Introduction Compared to normal interband spectroscopy, the study of infrared transitions between different excitonic states can reveal valuable additional information. Due to the different selection rules, states which are invisible for interband spectroscopy, might show up in intersubband spectroscopy which therefore can serve to explore possible finestructures. Furthermore it gives access to the whole populated k-space and as a consequence, the lineshape provides information about dispersion and distribution of the initial and final states.In our contribution we apply the concept of excitonic intersubband spectroscopy to Cu 2 O, which is a naturally grown bulk semiconductor with a direct bandgap at the G point (E g % 2:17 eV at T ¼ 6 K [1]). Cu 2 O is well known for its beautiful series of np excitons (n ¼ main quantum number), the energies of which perfectly fulfill the 1=n 2 relation known from atomic hydrogen. These p-like excitons, showing up in one photon absorption spectroscopy, are only a subseries of all existing p states, namely those having symmetry G

In cubic CdS/ZnSe type-II heterostructures collective excitations have been studied using infrared spectroscopy. The CdS/ZnSe structures were grown by solid-source molecular-beam epitaxy on semi-insulating GaAs substrates. Highly n-type-doped multiple-quantum-well and superlattice samples show strong intersubband and interminiband absorption in the midinfrared. The validity of the polarization selection rule is verified experimentally. The CdS/ZnSe conduction band offset is determined using a combination of interband and intersubband spectroscopy. Measured transition energies agree well to model calculations if many-body effects and band nonparabolicity are included. Intensity-dependent pump and probe measurements on doped and undoped samples reveal a fast increase of the photoinduced absorption signal at low pump intensities. At high pump intensities the absorption signal saturates. This behavior is explained by the existence of a subgroup of long-lived photogenerated electron-hole pairs. An observed redshift of the photoinduced interminiband transitions is explained by filling of the lowest miniband. The effective electron mass of cubic CdS is determined from thick films using infrared reflection spectroscopy.

The photoluminescence quantum efficiency of the yellow series 1s orthoexciton in Cu 2 O, including its phonon sidebands, was measured in an Ulbricht sphere. The obtained efficiency values between 10 Ϫ4 and 10 Ϫ6 are remarkably low. The nonmonotonous temperature dependence is analyzed.