M. A. Paesler

Reversible photodarkening of amorphous arsenic chalcogens

Abstract Illumination with bandgap light induces changes in physical properties of many chalcogenide semiconductors. Fundamental aspects of strike reversible photodarkening (the light-induced red-shift in the optical absorption edge) in arsenic-chalcogen glasses are critically reviewed. For understanding photodarkening at the microscopic level, details of the changes in the atomic structure that accompany the shift in the absorption edge are of particular importance. Study of the structural changes by a variety of techniques has revealed a phenomenon rich in basic physics but has not led a coherent picture of the underlying microscopic mechanism. Application of advanced experimental probes providing more detailed structural information has clarified some of the fundamental changes in the atomic structure and their relation to changes in the electronic and mechanical properties. Modifications in short-range and intermediate-range order accompany photodarkening. The changes in short-range order in the form of increased AsAs bonding are very small and probably do not play a predominant role in the changes in the electronic structure. Evidence suggests that the primary effect of the light-induced changes is the modification of intermediate-range correlations.

Raman imaging with near‐field scanning optical microscopy

Raman spectroscopy in conjunction with near‐field scanning optical microscopy is used to image Rb‐doped KTiOPO4 within a spectral feature with high spatial resolution. We present Raman spectra as well as the first Raman images obtained in the near field. Differences between near‐field and far‐field Raman measurements are discovered and discussed.

The temperature dependence of photoluminescence in a-Si: H alloys

Abstract Photoluminescence intensity observed near 1.3 eV in sputtered a-Si : H has been measured as a function of temperature for several samples prepared under differing conditions. The data are shown to obey an expression derived from a law of the form p nr p r ∝ e T T 0 where pnr and pnr are the probabilities for non-radiative and radiative recombination. We find T0≈23 K independent of sample preparation conditions.

Importance of argon pressure in the preparation of rf‐sputtered amorphous silicon–hydrogen alloys

We report the influence of argon partial pressure pAr on the photoconductivity, four‐probe conductivity and photoluminescence of rf‐sputtered a‐Si–H alloys. As pAr is increased from 5 to 30 mTorr, for fixed hydrogen partial pressure pH, photoconductivity increases by as much as three orders of magnitude and then saturates. Over this range the dark conductivity activation energy decreases, a plot of log conductivity versus inverse temperature becomes increasingly less linear and the photoluminescence intensity increases slightly and then drops back. We note that high pAr films become slowly contaminated upon exposure to air. These results are interpreted in terms of a reduction in energetic silicon atom bombardment of the growing film with increased pAr. For our sputtering arrangement, we show that silicon atoms ejected from the target become significantly more thermalized at pAr=10 mTorr than at 5 mTorr. Based primarily on the fact that the conductivity activation energy is more strongly dependent on pH f...

Resolution in collection‐mode scanning optical microscopy

The use of small apertures or sharpened tips as sensing elements in scanned‐probe optical sensing devices has led to the development of a number of instruments that provide lateral spatial resolution much finer than that available in conventional optical imaging instruments. Such a device might generally be classified as a scanning optical microscope, or SOM. One particular mode of SOM operation involves the use of a sharpened optical fiber to collect light emanating from a surface. The lateral spatial resolution of such a collection‐mode SOM is discussed in terms of the electromagnetic mode solutions of the probe tip. Numerical results indicate that, though bound modes solutions exist for increasingly fine unclad tips, classical diffraction effects limit resolution to a finite fraction (approximately 1/3) of the source wavelength λ. A second mechanism for signal transduction is shown to involve molecular scattering at the probe tip. An analysis of signal collection efficiency demonstrates that at tip rad...

Nano-Raman Spectroscopy and Imaging with a Near-Field Scanning Optical Microscope

Raman spectroscopy was performed using a near-field scanning optical microscope. This avoids the diffraction limit inherent in conventional optical microscopy techniques involving far-field optical components, and allows volumes significantly smaller than the cube of the wavelength to be investigated. The small sample volume coupled with the light-starved nature of the Raman effect itself makes such nano-Raman studies difficult. A near-field Raman microscope is described and results showing near-field effects in an investigation of Rb-doped KTP are presented. An image taken within a Raman feature demonstrates that nano-Raman imaging is indeed possible if the near-field instrument has considerable long-term stability, and that several unique aspects of the near-field data recommend this approach.

Kinetic limits for sensing tip morphology in near‐field scanning optical microscopes

The lateral spatial resolution provided by near‐field scanning optical microscopes depends critically on the shape and size of the sharpened tip of the optical fiber used as a source of light or sensing element. The fabrication of a tip by the commonly used heating and pulling method is examined. It consists of an initially pulling and rupture followed by a cooling and relaxation. The presented analysis leads to a deeper physical understanding of these processes and suggests avenues for technical improvements.

Photoluminescence in sputtered amorphous silicon-hydrogen alloys

Abstract Results of photoluminescence (PL) experiments performed on amorphous silicon-hydrogen alloys prepared by sputtering in a plasma containing hydrogen are presented. A single, featureless peak centred near 1.3 eV and of 0.3 eV f.w.h.m. is observed upon excitation with 2.4 eV light in alloys of hydrogen concentration cH≥5 at.% held at temperatures below 200 K. The PL quantum efficiency and peak energy are increasing functions of ch. A high-temperature decrease in PL is observed, and electric fields of 105 V/cm are found to be sufficient to quench the PL. It is contended that the original argument in the literature concerning the necessity for Stokes shifts of the order of 0.5 eV is wrongly based. From the present data we conclude that radiative recombination in our films may best be viewed in the framework of a rigid-band model with only small perturbations caused by distortional effects of the host matrix. The high-temperature thermal quenching of the PL is analysed in terms of two quenching mechani...

Effects of partial evolution of H from a-Si:H on the infrared vibrational spectra and the photoluminescence

Abstract H-evolution vs. T in a-Si:H often displays two features near 300°C and 600°C. Evolution of H at 390°C and subsequent redetermination of the evolution vs. T characteristic removes the 300°C feature without affecting other features. IR measurements show that the peaks at 2090, 2000, 890 and 850 cm −1 are reduced uniformly. Thus, the assignments of the 2090 cm −1 peak and the 300°C feature to SiH 2 complexes cannot both be correct. IR spectra of one sample annealed at 265 and 310°C show no reduction in peak intensity, implying that the evolved H was IR inactive. The same anneals do affect the energy of the PL peak, indicating that this H does influence the EDOS.

Environments of Mid-Cretaceous Saharan dinosaurs

Abstract Recent studies of the oceanic record suggest that the Earth was a global greenhouse during middle Cretaceous time. A review of topographic, sedimentary and biologic data pertaining to terrestrial mid-Cretaceous equatorial environments broadly supports the climatic inferences of marine studies. In particular, analyses of widely-occurring low latitude Saharan sediments support the Cretaceous greenhouse hypothesis. In comparison to marine ecosystems, terrestrial ecosystems respond more sensitively to atmospherically transported heat and nutrients because of the more tenuous presence of the hydrosphere on land. Indeed, the morphologies of terrestrial biota suggest that: (1) equatorial mid-Cretaceous climates were episodic rather than seasonal; (2) convective storms although infrequent were violent; and (3) a moist intertropical convergence zone was absent. Cretaceous atmospheric dynamics apparently differed importantly from those of the present. Circumstantial evidence suggests that (1) higher atmospheric carbon dioxide levels probably stimulated the emission of other greenhouse gases; and (2) higher humidity levels reduced diurnal temperature variations but impaired evaporative thermoregulatory mechanisms. That terrestrial ecosystems withstood greenhouse conditions in low latitudes as well as they did underscores the adaptability of terrestrial life. Analogies for future environmental stresses and responses, whether anthropogenic or resulting from other causes, might be found in the terrestrial record of the Cretaceous equatorial zone.