TE- and TM-polarization-resolved spectroscopy on quantum wells under normal incidence

We present TE- and TM-polarization-resolved photocurrent measurements on quantum well pin diodes under normal incidence. Usually, optical experiments performed in such a geometry yield information only about transitions involving in-plane (p_x and p_y) components of the hole wave functions because of the in-plane (TE) polarization of the light. Information on transitions sensitive to the p_z components can be obtained by focussing a radially polarized laser beam through a microscope objective with high numerical aperture (NA=0.9). With our setup, the electrical field vector at the focal tail has a significant component along the optical axis (TM-polarization!) which enables excitation of transitions sensitive to p_z components also. Additionally, the existence of a degenerate (azimuthally polarized) optical mode enables switching these p_z components on and off easily.Experimental evidence of these features has been achieved by exploiting the selection rules for e–hh and e–lh transitions in a quantum well structure. We present a comparison of our recorded spectra with theoretical predictions obtained from simple geometric optics assumptions. For our quantum wells the polarization effects are small because our measurement averages the intensity distribution of the whole focal plane. We plan to extend our measurements to polarization resolved single quantum dot spectroscopy. By restricting the detection region to the spatial extent of a single dot, one can exploit the almost pure TM-polarization on the optical axis for obtaining high contrast between heavy- and light-hole exciton absorption.     

Oxygen quenching of toluene fluorescence at elevated temperatures

Gas-phase oxygen quenching of toluene laser-induced fluorescence (LIF) is studied between 300 and 650 K in a nitrogen/oxygen bath gas of 1-bar total pressure with oxygen partial pressures up to 400 mbar. With increasing vibrational excitation of the laser-excited toluene, intramolecular decay becomes faster, resulting in a decreasing relative strength of collisional quenching by oxygen. Additionally, Stern–Volmer plots are found to be non-linear for temperatures above 500 K in the case of 266-nm excitation and at all temperatures for 248-nm excitation. This is attributed to the onset of internal conversion from specific vibrational levels. A photophysical model is developed that describes the experimental data and predicts toluene LIF signal strengths for higher oxygen partial pressures. One important result for practical application is that oxygen quenching is not the dominant de-excitation process for engine-related temperature and pressure conditions, and thus application of the popular fuel–air ratio LIF (FARLIF) concept leads to erroneous signal interpretation.This revised version was published online in August 2005 with a corrected cover date.     

Magnetic interaction between ultrafine amorphous Fe1-xCx alloy particles in ferrofluids

Ferrofluids with ultrafine magnetic particles of amorphous Fe_1-xC_x were studied by Mössbauer spectroscopy, transmission electron microscopy and magnetic measurements. Magnetic measurements showed that the initial susceptibility follows a Curie-Weiss law, indicating magnetic interaction between the particles. The influence of the magnetic interaction on the estimation of the particle size, made from the field dependence of the induced hyperfine splitting, is discussed.     

Hyperfine-mediated gate-driven electron spin resonance

An all-electrical spin resonance effect in a GaAs few-electron double quantum dot is investigated experimentally and theoretically. The magnetic field dependence and absence of associated Rabi oscillations are consistent with a novel hyperfine mechanism. The resonant frequency is sensitive to the instantaneous hyperfine effective field, and the effect can be used to detect and create sizable nuclear polarizations. A device incorporating a micromagnet exhibits a magnetic field difference between dots, allowing electrons in either dot to be addressed selectively.     

Laser-induced fluorescence modulation techniques for velocity measurements in gas flows

Broadband fluorescence of iodine, excited at 514.5 nm by a single-mode argon-ion laser tuned to the quasi-linear part of an absorption line, was used to detect the Doppler shift and hence the velocity of iodine molecules seeded in a nitrogen jet flow. The slope of the absorption line profile was measured directly using a frequency shift introduced by acoustooptic modulators (AOMs). A velocity of 36 m/s was measured in a jet of N₂ at 60 Torr in 2 ms with an accuracy of 11%. To reduce experimental noise, the laser beams were switched at 125 KHz and signal-tuned amplification was used.     

The Cancer Genetics Network: recruitment results and pilot studies

OBJECTIVE: The National Cancer Institute established the Cancer Genetics Network (CGN) to support collaborative investigations into the genetic basis of cancer susceptibility, explore mechanisms to integrate this new knowledge into medical practice, and identify ways of addressing the associated psychosocial, ethical, legal, and public health issues. SUBJECTS AND METHODS: The CGN has developed the complex infrastructure required to support the projects, including the establishment of guidelines and policies, uniform methods, standard questionnaires to be used by all of the centers, and a standard format for submission of data to the Informatics Center. Cancer patients and their family members have been invited to enroll and be included in a pool of potential study participants. The Information Technology Group is responsible for support of the design, implementation, and maintenance of the multicenter Network-wide research protocols. RESULTS: As of January 2004, the CGN contained data on 23,995 probands (participants) and 425,798 family members. As a resource for cancer genetic studies, the CGN has a large number of probands and first-degree relatives with and without cancer and with multiple ethnicities. Different study designs can be used including case-control, case-case, and family studies. CONCLUSIONS: The unique resources of the CGN are available for studies on cancer genetic susceptibility, translational research, and behavioral research. The CGN is now at a point where approved collaborators may have access to enrolled patients and their families for special studies, as well as to the clinical, environmental and family cancer history data banked in the Informatics Center.     

Genetics in primary care: a USA faculty development initiative

The Genetics in Primary Care (GPC) project is a USA national faculty development initiative with the goal of enhancing the training of medical students and primary care residents by developing primary care faculty expertise in genetics. Educational strategies were developed for the project by an executive committee with input from an advisory committee, comprising individuals with primary care, medical education and genetics expertise. These committees identified the key issues in genetics education for primary care as (1) considering inherited disease in the differential diagnosis of common disorders; (2) using appropriate counseling strategies for genetic testing and diagnosis, and (3) understanding the implications of a genetic diagnosis for family members. The group emphasized the importance of a primary care perspective, which suggests that the clinical utility of genetic information is greatest when it has the potential to improve health outcomes. The group also noted that clinical practice already incorporates the use of family history information, providing a basis for discussing the application of genetic concepts in primary care. Genetics and primary care experts agreed that educational efforts will be most successful if they are integrated into existing primary care teaching programs, and use a case-based teaching format that incorporates both clinical and social dimensions of genetic disorders. Three core clinical skills were identified: (1) interpreting family history; (2) recognizing the variable clinical utility of genetic information, and (3) acquiring cultural competency. Three areas of potential controversy were identified as well: (1) the role of nondirective counseling versus shared decision-making in discussions of genetic testing; (2) the intrinsic value of genetic information when it does not influence health outcomes, and (3) indications for a genetics referral. The project provides an opportunity for ongoing discussion about these important issues.     

The distributions of chain lengths in a crosslinked polyisoprene network

A fundament of classical rubber elasticity theory is the Gaussian chain approximation formula, P(n,r) for the probability distribution of end-to-end distances of a polymer chain composed of n beads. It is considered to provide a realistic distribution of end-to-end distances, r, provided that the length of the polymer chain is much greater than its average end-to-end distance. By considering the number of beads (n) to be the independent variable, we can use P(n,r) to construct the probability distributions of network chain lengths, for fixed r. Since the network crosslinks reduce the probability for the occurrence of longer chains, the formula must be modified by a correction factor that takes this effect into account. We find that, both the shape of the n-probability distribution, its height, and the position of the peak vary significantly with r. We provide a numerical procedure for constructing networks that respect these distributions. The algorithm was implemented in a three-dimensional, random polymer-and-node network model to construct polyisoprene networks at two common crosslink densities. Although the procedure does not constrain the density, we find that the networks constructed have densities very close to the measured bulk density.