Photochemical Etching of Carbonyl Groups from a Carbon Matrix: The (001) Diamond Surface.

The surface of diamond is reported to undergo nonablative photochemical etching when exposed to ultraviolet (UV) radiation which allows controlled single and partial layer removal of lattice layers. Oxygen termination of surface dangling bonds is known to be crucial for the etching process; however, the exact mechanism of carbon ejection remains unclear. We investigate the interaction of UV laser pulses with oxygen-terminated diamond surfaces using atomic-scale surface characterization combined with first-principles time-dependent density functional theory calculations. We present evidence for laser-induced desorption (LID) from carbonyl functional groups at the diamond {001} surface. The doubly bonded carbonyl group is photoexcited into a triply bonded CO-like state, including scission of the underlying C─C bonds. The carbon removal process in LID is atom by atom; therefore, this mechanism provides a novel "top-down" approach for creating nanostructures on the surface of diamond and other carbon-containing semiconductors.

All-solid-state parametric Raman anti-Stokes laser at 508 nm.

We report a parametric anti-Stokes Raman laser using potassium gadolinium tungstate, generating output chiefly at the first anti-Stokes at 508 nm. The compact 4.5 cm long device is pumped by a Q-switched 532 nm laser and uses an off-axis Stokes resonator to provide non-collinear phase matching between the pump and the generated Stokes and anti-Stokes fields. Anti-Stokes output energies up 0.27 mJ were obtained at a conversion efficiency from the pump of 0.46%. Second- and third-order anti-Stokes lines at 486 nm and 465 nm were also observed.

Highly efficient diamond Raman laser.

We report an efficient 532 nm pumped external cavity diamond Raman laser generating output chiefly at the 573 nm first Stokes. At a pulse repetition rate of 5 kHz, the Raman laser generated 1.2 W output with a conversion efficiency of 63.5%, a slope efficiency of 75%, a pulse peak instantaneous conversion efficiency of 85%, and a peak photon conversion efficiency of 91%. The laser generated a maximum output energy of 0.67 mJ by increasing the pump beam size and the pulse energy. The efficiency is commensurate with the highest previously reported for other Raman materials pumped by Q-switched lasers.

Increased wavelength options in the visible and ultraviolet for Raman lasers operating on dual Raman modes.

We report increased wavelength options from Raman lasers for Raman media having two Raman modes of similar gain coefficient. For an external-cavity potassium gadolinium tungstate Raman laser pumped at 532 nm, we show that two sets of Stokes orders are generated simultaneously by appropriate orientation of the Raman crystal, and also wavelengths that correspond to sums of the two Raman modes. Up to 14 visible Stokes lines were observed in the wavelength range 555-675 nm. The increase in Stokes wavelengths also enables a much greater selection of wavelengths to be accessed via intracavity nonlinear sum frequency and difference frequency mixing. For example, we demonstrate 30 output wavelength options for a wavelength-selectable 271-321 nm Raman laser with intracavity sum frequency mixing in BBO. We also present a theoretical analysis that enables prediction of wavelength options for dual Raman mode systems.

Two-photon polarization-selective etching of emergent nano-structures on diamond surfaces.

Optical techniques have advanced considerably in recent years to enable processing of surfaces with a resolution less than the wavelength of light. Despite the highly selective nature of light-matter interactions, however, efforts to increase resolution to the scale of single atoms are hampered by rapid and efficient dissipation of the absorbed energy to the surrounding matrix. Here we show that two-photon surface excitation using ultraviolet light provides a method for selectively removing carbon from diamond surfaces. Polished surfaces etched by this method develop ultra-deep subwavelength structures with morphologies dependent on the polarization of the incident laser with respect to the crystal axes. As well as revealing a practical and versatile method for nano-patterning of diamond surfaces, we show that the results comprise mesoscopic evidence for bond scission via a highly localized optical interaction that may lead to the development of new optical approaches for ultra-nanoscale (<10 nm) surface structuring.

Hook method: recovery of density information from interferograms distorted by large spatial gradients.

The hook method is a well-established technique for measuring the spatial distribution of species' densities in the gas phase, particularly in optically thick plasmas. However, in the presence of large density gradients (such as those occurring in a metal vapor laser plasma), the hook interferogram suffers severe distortion and the standard hook equation is invalid. By the use of a computer simulation of fringe formation, it is shown that this effect arises as a result of the strong wavelength-dependent lensing of probe rays in the test medium. On the basis of this lensing mechanism, a criterion has been derived for the maximum permissible density gradient, above which the standard hook analysis cannot be accurately applied. Finally, a new technique is presented that permits density data to be recovered from interferograms that are too distorted to analyze by the use of standard techniques. This technique is based on extracting density gradient values from distortion-free features of the fringe pattern. The new technique also permits density data to be obtained with an increased spatial resolution over that of the standard hook analysis.