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The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.

Electric-field-induced second-harmonic generation (EFISHG) allows the direct observation of charge trapping at near-interface oxide defects, which alters the reflected second-harmonic (SH) signal due to a change of the band-bending in the silicon space-charge region (SCR) at the interface.

Infrared transmission spectra of dentin reveal a broad absorption band between 6.0 and 7.0 microns composed of absorption peaks of water, collagen and carbonated hydroxyapatite. The nearly constant absorption and the existence of absorption peaks of different tissue components were used to investigate ablation as a function of the primary absorber.Laser ablation of dentin as a function of fluence was studied in the wavelength range between 6.0 and 7.5 microns using the Vanderbilt Free-Electron Laser (FEL). Depth and volume of the ablation crater were determined with a silicon replica method and subsequent confocal laser topometry. SEM investigations were performed on the irradiated surfaces. For the description of the experimental data an ablation model is developed.At all applied wavelengths we found a linear increase of ablation depth as a function of fluence above a threshold fluence. The lower absorption of dentin at 7.5 microns compared to the absorption at 6.0, 6.5 and 7.0 microns results in a greater ablation threshold. At 6.0, 6.5 and 7.0 microns wavelengths the ablation thresholds are comparable. The experimental data are in good agreement with an ablation model using a mean absorption coefficient of the target material. No thermal cracking is observed after ablation in dentin. The post ablative surface structure at 6.0 and 7.0 microns looks similar whereas at 7.5 microns the surface reveals a greater roughness.The ablation efficiency and threshold depend on the mean absorption but do not depend upon the chemical identity of the primary absorber in dentin. Calculations show that heat conduction during the laser pulse leads to a thermal equalization between the heated microstructures and surrounding tissue resulting in an ablation with little dependence on the primary absorber.

High resolution scans were made of Comet Kohoutek (1973f) using the McMath solar telescope at Kitt Peak National Observatory. The data were taken on January 1 and 4, 1974 UT, just after the comet perihelion. Hα emission (∼4.1 × 10 27 photon sec −1 ) was observed from the head of the Comet. An upper limit on the He I (5015) radiation was determined to be less than 2% of the observed Hα emission. The Na D 1/ D 2 line intensities on both nights were approximately 0.5, indicating an optically thin emission region.

Impact of energetic heavy particles on surfaces gives rise to emission of optical radiation from reflected particles, sputtered particles and also from excited states of the solid. We review the present status of research in this area with emphasis on understanding the basic mechanisms which give rise to formation of excited states. The spectral line shape from ejected atoms may be analyzed to provide information on the distribution of speeds and directions of the excited species; the line intensity provides a measure of the probability for creating the state. Formation of excited species is related both to the collision processes within the solid and also to the interaction of the recoiling ejected species with the target surface. Most ejected species are atomic but important examples of ejected molecules are also discussed. Luminescence induced in the solid itself is related to recombination of electron hole pairs and is related significantly to the presence of defects.

I will discuss recent measurements of (a) laser-induced fluroescence of neutral atoms electronically desorbed from surfaces, and (b) coherence in excited states of hydrogen formed near surfaces.

Measurements of the time history of ground state neutral lithium desorption from LiF during pulsed irradiation have resulted in a new model for lithium desorption.

Scans in the region of the sodium D lines were made of the first three Galilean satellites of Jupiter in late December 1973. Sodium emissions from Io (J I) were observed on three out of four nights. No evidence of emissions was observed from either Europa (J II) or Ganymede (J III).

Scanning near-field optical microscopy (SNOM) makes it routinely possible to overcome the fundamental diffraction limit of standard (far-field) microscopy. Recently, aperture-based infrared SNOM performed in the spectroscopic mode, using the Vanderbilt University free electron laser, started delivering spatially-resolved information on the distribution of chemical species and on other laterally-fluctuating properties. The practical examples presented here show the great potential of this new technique both in materials science and in life sciences. (C) 2004 Elsevier B.V. All rights reserved.