Research - Laser Interactions with Materials

Fundamentals of laser interactions with materials

Laser ablation and desorption- Understanding how lasers interact with materials to remove them is crucial not only for laser drilling and machining, but for analytical chemistry (where one wishes to quantitatively sample a solid material by mass spectrometry, for example), and for pulsed laser deposition (PLD) of thin films, where the ablated material is deposited to grow a thin film.

In most cases the
Gas dynamics of laser-vaporized materials are very important, since they can lead to redeposition of some of the material back onto the target, slowing of the ejected material, and condensation of the material into nanoparticles.

Our research utilizes fast intensified CCD-array photography (ICCD) in conjunction with pulsed lasers to locate, and then spectroscopically characterize the plume of ablated material as it travels from the target surface toward a substrate, where it deposits some material as a smooth film or particulate layer.

Amorphous Diamond Films  

Shown at right is an ICCD-photograph (false-colors) of a plume of carbon ions, atoms, and clusters traveling at high velocity (center of the ball is moving about 4 centimeters each microsecond) through vacuum. Ejected from laser-vaporized graphite, this high-velocity carbon forms a coating of 'amorphous diamond', an amorphous (glassy) material with many of the properties of crystalline diamond, including extreme hardness. Touch here to read more...  

Photoluminescent Si Nanoparticles - Reveal how Nanoparticles form by Laser Ablation  - When a Si wafer is ablated into a background gas, ablated atoms and molecules are confined as a hot plasma. After cooling by the gas, however, they condense into clusters of atoms and nanoparticles (a 25-atom Si cluster is 1-nm in size). Luminescent silicon has been the subject of much research. Here, we create photoluminescent SiOx clusters in the gas phase by condensation, and use this luminescence to track the nanoparticles as they form, and propagate. The gas-phase photoluminescence spectrum (induced by a time-delayed second laser) reveals interesting vibronic structure.

MALDI - First Imaging of Heavy Biomolecules and Matrix Molecules Desorbed for Mass Spectrometry - The MALDI (Matrix Assisted Laser Desorption and Ionization) process is used routinely to analyze the mass of heavy biomolecules by time-of-flight mass spectrometry. During the ablation process, some of the molecules become ionized. In this paper, the plume of ejected material is imaged for the first time, and a plume sharpening effect is noted. "Imaging of Vapor Plumes Produced by Matrix Assisted Laser Desorption: A Plume Sharpening Effect" A. A. Puretzky, D. B. Geohegan, G. B. Hurst, M. V. Buchanan and B. S. Luk'yanchuk Phys. Rev. Lett. 83, 444, (1999). Download PDF file (553k)

"Plume Splitting" - How the flux of ions becomes split into two distributions when penetrating a background gas - When a plume of ablated material encounters a background gas at high pressures, it can be described hydrodynamically. At very low pressures, a scattering model is most appropriate. At intermediate pressures, a new model must be introduced to describe the experimentally-observed effect where the flux of ions arriving at an ion probe appears to arrive in two distributions – a fast pulse nearly identical to the velocity distribution of the ejected material as measured in vacuum, and a distribution of material which is significantly delayed, and coincides with that of a bright shock front. Please see...

"Dynamics of laser ablation plume penetration through low pressure background gases" David B. Geohegan and Alexander A. Puretzky Appl.Phys. Lett.67, 197 (1995) . Download PDF file (312k)

"Laser ablation plume thermalization dynamics in background gases: Combined imaging, optical absorption and emission spectroscopy, and ion probe measurements"  Geohegan, D.B.; Puretzky, A.A.Applied Surface Science, Vol: 96-98, pp. 131-138, (1996). Download PDF file (585k)

"Dynamics of Plume Propagation and Splitting during Pulsed-Laser Ablation" R. F. Wood, K. R. Chen, J. N. Leboeuf, A. A. Puretzky, and D. B. Geohegan Phys. Rev. Lett. 79, 1571, (1997). Download PDF file (130k)

"Nanoparticles Formed during PLD of YBCO" - Do nanoparticles form during pulsed laser deposition of high-temperature superconductor thin films? It turns out that they'd do at low temperaturs, but thermophoresis prevents them from being incorporated into the growing film at high temperatures. See...

"Gas-phase nanoparticle formation and transport during pulsed laser deposition of Y1Ba2Cu3O7 – d" D. B. Geohegan, A. A. Puretzky, and D. J. Rader Appl.Phys. Lett. 74, 3788 (1999) . Download PDF file (287k)