# Presentation: 252040

## Atomically smooth and rough surfaces by ultrafast laser ablation

- Atomic force microscopy image of the structure of a sapphire surface exposed to a single laser pulse of fluence 6.4 J/cm2 and 150 fs duration. The edge of the crater is atomically rough but the center, where atomic steps are visible, is smooth.

- Summary of the depth and roughness of the craters as a function of the energy density of the laser pulse. The threshold is 4.2 J/cm2.

- Intense laser pulses with a duration of less than a picosecond can be used to machine transparent materials with superb control but fundamental understanding of the process by which material is removed from the surface is incomplete. We have studied the morphology of sapphire exposed to individual laser pulses and have discovered that the morphology near the threshold intensity for ablation is intrinsically rough while the morphology just beyond the threshold is atomically smooth. Theoretical descriptions of femtosecond laser ablation do not explain these changes in morphology with laser intensity.

- 5 m

- Fluence

**Notes:**
Progress: Our instrumentation grant is for the development  of an ultrafast, high-energy, laser system for fundamental studies of laser-material interactions and ultrafast phenomena in materials synthesis and processing.  Our Spectra-Physics amplified Ti:sapphire laser was delivered at the beginning of January 2005.  Since that time, we have initiated experiments on i) laser ablation of transparent materials (described in the attached “nugget”); ii) the maximum cooling rate that can be achieved for thin layers, and iii) the competition between melting and ablation during laser processing of metallic multilayers.  The design of an ultrafast electron diffraction system for time-resolved studies of structural transformation has been completed and is now under construction.

Atomically smooth and rough surfaces by ultrafast laser ablation

Experimental details:  Low-miscut R-plane sapphire substrates are annealed at 1400 C in air to produce atomically flat surfaces;  terraces, approximately 1 micron wide, are separated by atomic-height steps.  The laser ablation experiments use microjoule pulses from an amplified Ti:sapphire laser focused to a spot diameter of approximately 10 microns.  The sapphire substrates are laser-processed in vacuum.  The morphologies of the ablated surfaces are characterized by atomic force microscopy with nanometer resolution.

Intellectual Merit: The mechanisms of ultrafast laser ablation of transparent materials are controversial. By starting with nearly perfect single crystal surfaces, and conducting the experiments in vacuum, we avoid the complexities that are introduced by microscopic surface defects and interactions of the laser pulse with air adjacent to the crystal surface.   In this way, we provide data for detailed comparisons with theoretical descriptions of the ablation process.

Broader Impacts: The interaction of intense laser pulses with materials are of concern for a wide variety of science and engineering disciplines ranging from inertial confinement fusion to laser eye surgery.  Advances in fundamental understanding of laser material interactions on ultrashort time-scales will impact these more applied topics by enabling advances in the methods and procedures used for laser processing of materials.