Characterization of solar cells by photocurrent spectroscopy and current-voltage characteristics with high spatial resolution.

Spatially resolved photocurrent-spectroscopy and spatially resolved current-voltage characteristics are introduced as new methods to characterize solar cells. A combination of these two methods is shown to localize and characterize deficiencies and structural damages in processed solar cells with high spatial resolution. The local external and internal quantum efficiencies as well as the local characteristic parameters of the p-n junction like the short circuit current, the saturation current, the ideality factor, and the optically induced shunt resistance can be determined quantitatively. Both, a slab of a damaged and an undamaged (GaIn)(NAs) concentrator solar cell, are used as test structures. Upon these test structures domains with a high concentration of impurities in the crystal structure and structural imperfections in the upper contact region are identified and analyzed. Additional numerical simulations prove the reliability and show limits of the methods.

Direct observation of optically injected spin-polarized currents in semiconductors.

Quantum interference of one- and two-photon excitation of unbiased semiconductors yields ballistic currents of carriers. The magnitudes and directions of the currents and the spin orientations of the carriers are controlled by the polarization and relative phase of the exciting femtosecond laser fields. We provide direct experimental evidence for the spin polarization of the optically injected spin currents by detecting a phase-dependent spatial shift of the circularly polarized photoluminescence in cubic ZnSe.

Chlorophyll b is involved in long-wavelength spectral properties of light-harvesting complexes LHC I and LHC II.

Chlorophyll (Chl) molecules attached to plant light-harvesting complexes (LHC) differ in their spectral behavior. While most Chl a and Chl b molecules give rise to absorption bands between 645 nm and 670 nm, some special Chls absorb at wavelengths longer than 700 nm. Among the Chl a/b-antennae of higher plants these are found exclusively in LHC I. In order to assign this special spectral property to one chlorophyll species we reconstituted LHC of both photosystem I (Lhca4) and photosystem II (Lhcb1) with carotenoids and only Chl a or Chl b and analyzed the effect on pigment binding, absorption and fluorescence properties. In both LHCs the Chl-binding sites of the omitted Chl species were occupied by the other species resulting in a constant total number of Chls in these complexes. 77-K spectroscopic measurements demonstrated that omission of Chl b in refolded Lhca4 resulted in a loss of long-wavelength absorption and 730-nm fluorescence emission. In Lhcb1 with only Chl b long-wavelength emission was preserved. These results clearly demonstrate the involvement of Chl b in establishing long-wavelength properties.

Efficient intracavity generation of visible pulses in a femtosecond near-infrared optical parametric oscillator.

We have achieved efficient simultaneous intracavity doubling of the signal frequency and sum-frequency generation between the signal and the pump in a periodically poled LiNbO(3) femtosecond optical parametric oscillator pumped by a Ti:sapphire laser. The responsible mechanisms are second-, third-, and fourth-order quasi-phase matching (QPM). An ~56% poling duty cycle permits efficient even-order QPM. Femtosecond pulses tunable in the visible were generated with a total efficiency of more than 8% for only 480-mW pump power at 80 MHz. Optimization of the poling duty cycle can improve the efficiency further.

Ultrafast nonlinear sub-wavelength solid immersion spectroscopy at T = 8 K: an alternative to nonlinear scanning near-field optical microscopy.

Pump-probe measurements were performed with a subwavelength spatial resolution of 355 nm and a temporal resolution of 130 fs in a multiple quantum well sample at T= 8 K. A solid immersion lens was used to increase the spatial resolution to 0.41 lambda, demonstrating that the limit of conventional microscopy was surpassed.

Pump geometry for resonant and quasi-resonant optical excitation of microcavity lasers.

A simple new pump geometry for optical excitation of microcavities and vertical-cavity surface-emitting lasers is presented. The technique circumvents the high reflectivity of the cavity stop band by excitation through the substrate at a large angle of incidence. Under these conditions, the reflectivity of the bottom Bragg reflector is small, and optical pumping at any desired photon energy becomes possible. Experimental results for optical excitation with this new geometry are compared with resonant optical pumping through the cavity mode.

Ultrabroadband chirped mirrors for femtosecond lasers.

We report on the performance of widely tunable femtosecond and continuous-wave Ti:sapphire lasers that use a newly developed ultrabroadband mirror set. The mirrors exhibit high reflectivity (R > 99%) and smooth variation of group delay versus frequency over a wavelength range from 660 to 1060 nm. Mode-locked operation with pulse durations of 85 fs was achieved from 693 to 978 nm with only one set of ultrabroadband mirrors.

[Correlation of cotinine level in amniotic fluid, umbilical artery blood and maternal blood]. / Korrelation des Cotininspiegels in Fruchtwasser, Nabelarterienblut und mütterlichem Blut.

The aim of the present study was to determine the correlation of cotinine levels in amniotic fluid and in fetal and corresponding maternal blood samples. Amniotic fluid samples (N = 130) were taken during second trimester amniocentesis, umbilical artery blood samples (N = 75) at birth, both together with corresponding maternal blood samples. Self-reported smokers showed maternal serum cotinine levels > 15 ng/ml in 93%, self-reported nonsmokers levels < 15 ng/ml in 89%. Correlation of corresponding values for cotinine was 0.81-0.92. Cotinine values were increased in fetal blood and amniotic fluid in comparison to maternal serum levels. Despite the fact that pathophysiology is not fully understood, an accumulation of nicotine and its metabolites both in the fetus and in the amniotic fluid appears to be evident.

All-optical spatial light modulator with megahertz modulation rates.

We demonstrate a novel all-optical spatial light modulator capable of megahertz modulation rates. It is based on the quantum-confined Stark effect, but the modulating electric field is entirely photogenerated by strongly asymmetric photocarrier transfer in GaAs/AlAs layers. In a nonoptimized sample, cw optical excitation of approximately 50 W/cm(2) created a 30-kV/cm electric field, inducing a 9-meV exciton red shift at room temperature. Under pulsed excitation the photogenerated electric field can be switched on in a few tens of picoseconds and relaxes in a few hundred nanoseconds, permitting megahertz modulation rates.

[Antepartum monitoring and peripartal findings in mono- and dichorial twin pregnancy]. / Antepartale Uberwachung und peripartale Befunde bei mono- und dichorialer Zwillingsschwangerschaft.

In a group of 65 twin pregnancies the difference of perinatal findings and antepartal test results was evaluated in relation to amnionicity and chorionicity. Monochorionic placentation was found in 33% of the pregnancies. The rate of foetal malformation (11%), neuromuscular dysfunction (6%), perinatal mortality (11%) and duration of neonatal intensive care was increased in those cases. The most useful diagnostic tool was B-Mode-ultrasound (first detection and surveillance of multiple pregnancy, especially diagnosis of inter-twin growth discordancy). Non stress test and Doppler sonography were found to be of value as additional tests for detection of functional differences between both twins. There were no differences between findings in first and second twin as well as between findings in pregnancies with mono- or dichorionic placentation.

[Premature separation of the placenta--clinical findings and fetal monitoring]. / Vorzeitige Plazentalösung--klinische Befunde und fetale Uberwachungsmethoden.

In a study group of 41 pregnant women of postpartally confirmed placental abruption, the prognostic value of clinical and diagnostic findings was investigated. The incidence of placental abruption was 1.4% of all deliveries within a three-year interval. 51% of patients showed vaginal bleeding before delivery. Retroplacental haematoma was found in 49% of cases ultrasonographically. A total of 75% had a pathological CTG test. More than 95% of these findings occurred within 3 days before delivery. Abnormal Doppler flow findings in foetal vessels more than 3 days before delivery were seen in 62% of cases. In the last three days before delivery, 86% were abnormal. Preterm delivery before 37 weeks of gestation was registered in 82% of patients. Perinatal mortality amounted to 12%. The rate of severely dystrophic newborn was 30%. Even in cases with lack of the clinical and/or sonographical findings, the possibility of placental abruption should be considered, if an acute deterioration of cardiotocographic or Doppler-sonographic findings.

[Evaluation of the antepartum CTG course in cases of highly pathologic Doppler flow findings]. / Auswertung des antepartalen CTG-Verlaufs in Fällen mit hochpathologischen Doppler-Flow-Befunden.

In a group of 79 pregnancies with highly abnormal Doppler-flow findings in foetal vessels the value of antepartal CTG criteria was evaluated in detail. In cases with absent enddiastolic flow (AEDF) the prognostical value was not further increased by antepartal pathological CTG findings. In cases with permanently normal antepartal CTG's, however, foetal outcome was fairly good (median duration of pregnancy 38 + 6 weeks, median foetal birth weight 2,545 grams, arterial pH < 7.20 in 9%). This was true also in the presence of pathological Doppler-flow findings except AEDF. In cases with pathological antepartal CTG findings in the antepartal course of pregnancy the loss of acceleration was most frequent (82%), followed by decreased frequency (53%) or narrowed amplitude (38%) of the foetal heart rate variation. Combination of both methods (Doppler sonography and CTG) is recommended clinically because the rate of uncertain findings can be reduced. Especially in cases with AEDF an active management would be justified before CTG's become pathological.

[Fetal heart rate and Doppler flow parameters in fetal blood vessels--evaluation of 2,517 individual signals]. / Fetale Herzfrequenz und Doppler-Flow-Parameter in fetalen Gefässen--Auswertung von 2517 Einzelsignalen.

Analysis of more than 2500 doppler flow signals in three patients was performed to evaluate the influence of fetal heart rate (fhr) variation on S/D-ratio in fetal vessels. Little differences of S/D-ratio in fetal vessels (descending aorta, umbilical arteries) were detected within the physiological variations of fhr (120-160 bpm) (envelope of regression line -0.007, -0.006). Analysing inter- as well as intraindividual variation only minor effects of fhr alteration can be observed in clinical application of doppler flow velocimetry. Dopplersonographic measurements, however, should be performed for at least a three minute period (for each vessel) to avoid false-positive or false-negative interpretation of short term changes of doppler signals. This is especially important in distressed fetuses.

[Doppler ultrasound of fetal blood vessels--optimizing the diagnostic value by a combined diagram]. / Doppler-Sonographie der fetalen Gefässe--Optimierung der Aussagekraft durch ein Kombinationsdiagramm.

The dopplersonographic data of 1926 pregnant women in the 3. trimester of pregnancy was used to evaluate a graphic system for the analysis of A/B-ratio in fetal vessels (pulsed wave duplex system; descending aorta and umbilical arteries). The basis of the newly developed combination diagram were dopplersonographic standard values (single cut off and pregnancy duration related percentiles). One of the results was the fact, that the combined measurement and analysis of both fetal vessels increased the positive predictive value compared to single measurements. The value of dopplersonographic examinations in the aorta was slightly better than those in the umbilical arteries. The combined diagram showed a marked improvement in graphicness, especially in follow-up cases, and a partial improvement in statistical values.

[Pathologic Doppler flow findings and cardiotocography results]. / Pathologische Doppler-Flow-Befunde und kardiotokographische Ergebnisse.

Of 1950 pregnant patients (2870 Doppler ultrasound measurements) we observed, in a study group with highly abnormal Doppler-flow findings (n = 66, Feb. 1990), a correlation of Doppler flow and FHR-recordings. Among these 66 patients we retained 60 (91%) in the hospital. They had at least 2 FHR-recordings a day. The results of Doppler flow measurements in the fetal aorta and umbilical artery correlated well with diagnosis of IUGR. The comparison of the overall results of both fetal vessels did not indicate any significant difference. In 21% of all patients with highly abnormal Doppler flow findings, was no abnormal FHR record until delivery. 26% already showed an abnormal non-stress test before the first pathological Doppler assessment, in 44% abnormal FHR-recordings were observed later than the first abnormal Doppler flow finding in the course of pregnancy. The median interval was 13.5 days in cases with increased Doppler flow parameters but with detectable end-diastolic blood flow and was reduced to 8 days in cases with absent end-diastolic blood flow. In 9% of all cases, abnormal results were found with both methods on the same day. In 32% we observed a reproducible notch in Doppler flow velocimetry of uteroplacental vessels. The rate of congenital malformations was 14%. Thus abnormal Doppler flow signals can be estimated as "early" prognostic criterias for a compromised fetus at risk.

[Doppler ultrasound in obstetrics--contribution to understanding reverse flow in the umbilical artery]. / Dopplersonographie in der Geburtshilfe--Beitrag zum Verständnis des "Reverse Flow" in der Arteria umbilicalis.

"Reverse Flow" as a Doppler sonographic finding indicates the appearance of retrograde blood flow in the diastolic part of the cardiac cycle. It has so far not been observed in uncomplicated pregnancies and seems to be an indicator of severe deterioration of the foetal condition. Several pathophysiological hypotheses are discussed in this paper on the basis of own results and the findings of other investigators.

[Doppler ultrasonography--perinatal data in cases with end-diastolic block and reverse flow]. / Doppler-Sonographie--Perinatologische Daten bei Fällen mit enddiastolischem Block bzw. Reverse Flow.

In 50 cases with an end-diastolic zero flow or reverse flow all antenatal and perinatal abnormalities have been recorded. The fetal outcome was registered. The percentage of highly dystrophic newborns (percentile less than 5) was 88%. The perinatal mortality counted up to 16% and the percentage of congenital malformations (including chromosomal anomalies) was 12%. A reverse flow was registered in 4 cases. The perinatal mortality of those cases with reverse flow was 100%. In approx. one-quarter of those pregnancies and in 50% of the perinatally deceased newborns, there were no pathological or suspicious changes in the antepartal and/or subpartal CTG-recordings. The Duplex sonographical diagnosis of an end-diastolic zero flow/reverse flow, has a highly positive predictive value, whereas its sensitivity is low. It can be regarded as a very helpful parameter in clinical diagnosis, particularly as it is independent of borderline values. The correct choice of the high-pass wall filter (50-100 Hz) is important.

[Doppler sonography--effect of fetal position, site of the placenta and measurement site of the fetal aorta]. / Doppler-Sonographie--Einfluss des fetalen Lagetyps, des Plazentasitzes sowie des Messortes an der fetalen Aorta.

The influence of fetal position, the site of placenta and the site of measurement in the fetal aorta on doppler-flow parameters was investigated in patients with uncomplicated pregnancies. In 20 cases the S/D-ratio was obtained in three different regions of the descending aorta. Measurements superior the renal vessels showed comparable values (4.1-4.3). No differences were found between signals obtained cranial or caudal of the diaphragma. Measurements at the aortal bifurcation showed significantly increased median values (6.6, p less than 0.05). No differences could be obtained concerning the influence of fetal position or site of placenta on S/D-ratio. All median values of a total of 197 patients were in the range of 4.8. Therefore it may be concluded that the above mentioned parameters do not have significant influence on doppler flow parameters in fetal vessels. However, a proximal site of measurement in the descending aorta fetalis should be reassured.