S. Haacke

Improving the signal-to-noise ratio of femtosecond luminescence upconversion by multichannel detection

We have improved the sensitivity and signal-to-noise ratio of a luminescence upconversion experiment, using a charge-coupled device (CCD) as the detector. We show experimentally and numerically that the bandwidth of a 1-mm-thick β-barium borate crystal is large enough to take full advantage of the multichannel capabilities of the CCD. The improvement is significant in a standard experiment with a single laser as well as in experiments with resonant excitation that use two synchronized femtosecond pulse sources at different wavelengths. The characteristics of the two-color scheme are discussed in detail.

Spectral and kinetic fluorescence properties of native and nonisomerizing retinal in bacteriorhodopsin

Steady-state and picosecond (ps) fluorescence studies of wild-type bacteriorhodopsin (wt-bR) and of a nonisomerizing analog locked in the all-trans configuration have been performed. Extending earlier work done by femtosecond absorption spectroscopy, we observe a strong similarity between both proteins in both fluorescence spectra and Stokes shift thus confirming the previous result that the fluorescent state I460 of the native bR proteins is in the all-trans configuration. Comparison of the spectra of fluorescence and stimulated emission of the locked pigments indicates the presence of an excited-state absorption situated around 750 nm. Upon increase of the excitation energy, the time-integrated fluorescence shows an interesting weak blue shift, which is identical for both pigments. Finally, we discuss the primary structural processes in retinal and in the protein that lead to the sub-100 fs formation of I460 and in particular to the considerable Stokes shift.

DYNAMICS OF SINGLET EXCITONS IN 1D CONJUGATED POLYDIACETYLENE CHAINS : A FEMTOSECOND FLUORESCENCE STUDY

Abstract The 1 B u exciton lifetime has been measured in isolated polydiacetylene chains, using fluorescence up-conversion. It is found to be 130–135 fs at 20 K, and corresponds to the non-radiative internal conversion to a lower-lying excited Ag state, as in polyenes. At low temperature, the absorption linewidth is almost lifetime-limited: inhomogeneous broadening is small (≤30 cm−1). The lineshape remains Lorentzian up to 150 K, but with a three-fold increase in width, whereas the lifetime is merely constant. The broadening is most probably due to a reduction of the coherence time.

Resonant Rayleigh scattering by Wannier excitons in a two-dimensional disordered potential

Resonant Rayleigh scattering (RRS) in semiconductors is due to imperfections breaking the symmetry of translational invariance of the crystal: impurities, defects or interfaces. The RRS observed from semiconductor quantum wells (QWs) is mainly due to fluctuations in the lateral two-dimensional potential energy confining charge carriers and excitons. Time resolving the RRS allows us to extract the statistical properties of the disorder potential, including a certain degree of spatial correlation. Furthermore, the temporal coherence of RRS has been demonstrated by interferometric experiments with a single speckle, allowing us to discriminate RRS very clearly from incoherent fluorescence. This lively ongoing field of research has brought about generic fundamental notions readily applicable to spectroscopy of other semiconductor nanostructures or possibly inorganic crystals, which are worth compiling. This paper aims at reviewing a large fraction of the experiments carried out on GaAs QWs, which have contributed significantly to the understanding of the static and dynamic properties of RRS.

Ultrafast energy relaxation in bacteriorhodopsin studied by time-integrated fluorescence

Time-integrated fluorescence experiments on native bacteriorhodopsin and on its non-isomerizing form bR5.12 are reported. The experimental set-up was designed such as to observe emission exclusively from the excited state intermediate I-460. We obtain the first systematic investigation of the fluorescence spectra as a function of the excitation wavelength tuned throughout the entire absorption band of bR. An important finding is that the position of the fluorescence maximum does not show a systematic shift when the excitation wavelength is shortened. For excitation with high excess energy, we observe a broadening of the blue wing of the bR fluorescence, indicating incomplete vibrational energy relaxation on the time scale of the lifetime of I-460. Due to a much longer excited state lifetime, vibrational energy relaxation is more effective in bR5.12 and the fluorescence spectra are much less dependent on excitation wavelength. The results are placed in the general framework of thermalization between the retinal chromophore and the protein environment, and are compared with information obtained by femtosecond experiments.

Time-resolved relaxation oscillations in gain-clamped semiconductor optical amplifiers by pump and probe measurements

Pump and probe measurements with femtosecond resolution are performed on a gain-clamped semiconductor optical amplifier, a structure that contains a lasing mode. The corresponding relaxation oscillations are observed in the temporal gain recovery of the amplifier. The current dependence of the oscillations is well reproduced by a small-signal analysis of the laser rate equations. The gain recovery can be very fast, we measure damping rates of up to 40 GHz.

Compositional heterogeneity reflects partial dehydration in three-dimensional crystals of bacteriorhodopsin.

Absorption, fluorescence and excitation spectra of three-dimensional bacteriorhodopsin crystals harvested from a lipidic cubic phase are presented. The combination of the spectroscopic experiments performed at room temperature, controlled pH and full external hydration reveals the presence of three distinct protein species. Besides the well-known form observed in purple membrane, we find two other species with a relative contribution of up to 30%. As the spectra are similar to those of dehydrated or deionized membranes containing bacteriorhodopsin, we suggest that amino acid residues, located in the vicinity of the retinal chromophore, have changed their protonation state. We propose partial dehydration during crystallization and/or room temperature conditions as the main source of this heterogeneity. This assignment is supported by an experiment showing interconversion of the species upon intentional dehydration and by crystallographic data, which have indicated an in-plane unit cell in 3D crystals comparable to that of dehydrated bacteriorhodopsin membranes. Full hydration of the proteins after the water-withdrawing crystallization process is hampered. We suggest that this hindered water diffusion originates mainly from a closure of hydrophobic crystal surfaces by lipid bilayers. The present spectroscopic work complements the crystallographic data, due to its ability to determine quantitatively compositional heterogeneity resulting from proteins in different protonation states.

Relevance of Dephasing Processes for the Ultrafast Rise of Emission from Resonantly Created Excitons in Quantum Wells

We present a comparative study of time-integrated four-wave-mixing and femtosecond emission under resonant excitation on excitons in weakly disordered GaAs quantum wells. At highest exciton densities when dephasing dominates the spectral width (homogeneous broadening), we find that the rise time of the incoherent luminescence signal is given by T-2/2. At lowest densities, optical coherence times approach the exciton radiative lifetime (15 to 20 ps). This confirms our previous result that coherent resonant Rayleigh scattering is responsible for the short rise time of the excitonic emission. We also show clear evidence for dephasing due to exciton-phonon interaction, as the rise time of the emission decreases dramatically when the sample temperature is increased.

Negatively charged excitons in CdTe-based quantum wells: A time-resolved study

Using time-resolved photoluminescence, we have investigated the radiative behavior of neutral and negatively charged excitons in CdTe-based quantum wells. We find that the photoluminescence of negatively charged excitons can be well reproduced by a model of delocalized and thermalized three-particle complexes. A large radiative zone in k-space results from the transfer of the charged exciton momentum to the remaining electron (recoil effect). On the other hand, the photoluminescence decay of excitons is not well described in the framework of a model of delocalized and thermalized excitons. We infer that the main difference between the behavior of neutral and charged excitons in the time domain stems from the large radiative zone of charged excitons.

Radiative lifetime of negative trions in GaAs and CdTe quantum wells

We have studied experimentally the radiative lifetimes of negatively charged excitons (X-) and neutral excitons (X) in GaAs and CdTe quantum wells. Despite the different optical properties (e.g. trion binding energies) and the different quantum well structures of the studied samples, we find quite similar X- lifetimes in the range of 45 to 60 ps. On the other side, the X lifetimes are very different and influenced by the material. For relatively low electron concentrations, the X- lifetime is found to be constant with increasing temperature in a given range. This time behaviour of X- suggests that X- is localised for low electron concentrations in the CdTe QW.