1999

Quick as a Flash: Observing Ultrafast Laser-Induced Dynamics in Semiconductors, at Physics Colloquium, University of Massachusetts at Lowell (Lowell, MA), Wednesday, April 21, 1999:
We track laser-induced phase changes in materials on ultrafast time scales by measuring the material's dielectric function (from near-UV to near-IR) with 100-fs time resolution. Experiments on GaAs excited with intense femtosecond pulses reveal details of the ultrafast electron and lattice dynamics at large excited carrier densities. Future experiments will use our technique to measure the plasma dynamics during femtosecond laser-induced breakdown of transparent materials, a process which has been shown to have several potential applications in micro-machining and data storage.
Laser-induced microexplosions: creating stellar conditions on an optical bench, at Condensed Matter Seminar, Old Dominion University (Norfolk, VA), Friday, April 9, 1999:
Using femtosecond laser pulses we study the effects of intense laser radiation on transparent materials. By tightly focusing these laser pulses below the surface of transparent materials, we initiate highly nonlinear absorption processes which produce a dense, highly-excited plasma inside the sample. The high density, tightly-confined plasma leads to a micron-sized explosion within the material, with temperatures and pressures approaching stellar conditions. We have recently shown that it is possible to create internal submicron-sized structures by optically initiating microexplosions inside... Read more about Laser-induced microexplosions: creating stellar conditions on an optical bench
How I Got to Know My 200 Students (Almost) Overnight, at Seminar, National Science Foundation (Arlington, VA), Thursday, April 8, 1999:
Direct interaction between students and their teacher is an essential component of effective teaching. Unfortunately, this component is often absent in large classes. On one hand, lecture periods and office hours offer insufficient opportunities to interact with large numbers of students. On the other hand, electronic communication, such as e-mail or web-based forums quickly become an intractable burden on the teacher. This past year, with the support from the NSF, we developed an effective and efficient system allowing asynchronous communication between students and teacher. The system helps... Read more about How I Got to Know My 200 Students (Almost) Overnight
Photodisruption in turbid tissue with 100-fs and 200-ps laser pulses, at APS Centennial Meeting 1999 (Atlanta, GA), Friday, March 26, 1999:
We compare the potential of 100-fs and 200-ps laser pulses for photodisruptive surgery on the surface and in the bulk of turbid tissue. Water, human epidermis cultures, and pig skin were used as tissue models. In our technique, tightly-focused femtosecond and picosecond laser pulses are nonlinearly absorbed, vaporizing tissue in the focal volume. We find that there are several advantages in using femtosecond pulses for photodisruption. The breakdown threshold is lower and the energy deposition is more deterministic for 100-fs pulses compared to 200-ps pulses. In human skin culture we observe... Read more about Photodisruption in turbid tissue with 100-fs and 200-ps laser pulses
The role of multiphoton excitation in ultrafast white-light continuum generation, at APS Centennial Meeting 1999 (Atlanta, GA), Thursday, March 25, 1999:
When a powerful ultrashort laser pulse is focused into a transparent medium, it can undergo severe spectral broadening and be transformed into a white-light spectral continuum. Despite the continuum's widespread use as a tunable ultrafast light source, the mechanisms of continuum generation are still poorly understood. We report an experimental investigation of white-light continuum generation in transparent media using 100-fs pulses from a Ti:sapphire laser system. We measured the continuum's spectral width for a variety of materials and pump wavelengths. The materials were selected to cover... Read more about The role of multiphoton excitation in ultrafast white-light continuum generation
Electron-beam-induced oxidation of benzene to phenol in C6H6/O2/Pt(111), at APS Centennial Meeting 1999 (Atlanta, GA), Tuesday, March 23, 1999:
We have observed the formation of phenol when C6H6/O2/Pt(111) is exposed to a beam of electrons with energy on the order of 10-100 eV. With this sample preparation, the oxygen is bound directly to the platinum; the benzene overlayer does not displace the oxygen and chemisorb to the platinum. In contrast, no electron-beam-induced phenol production is observed when the benzene and oxygen are applied in the reverse order, for O2/C6H6/Pt(111). Following exposure to the electron beam, the yield of phenol is measured during temperature-programmed desorption. In the absence of the electron beam, no... Read more about Electron-beam-induced oxidation of benzene to phenol in C6H6/O2/Pt(111)
Microstructuring of bulk transparent solids using nanojoule, femtosecond laser pulses, at APS Centennial Meeting 1999 (Atlanta, GA), Tuesday, March 23, 1999:
We produce sub-micron sized permanent damage in the bulk of dielectric materials using 110-fs laser pulses with only 40 nJ of energy. Tight external focusing (0.65 NA) of the ultrashort laser pulses enables us to achieve a high intensity at the focus with low laser energy. The high intensity leads to nonlinear absorption of the laser pulse by the material, resulting in permanent damage. Achieving high intensity with low energy reduces the effects of self-focusing, and eliminates the need for an amplified laser system. We report thresholds for damage and critical self-focusing in fused silica... Read more about Microstructuring of bulk transparent solids using nanojoule, femtosecond laser pulses
Factors affecting gender disparity in introductory physics, at APS Centennial Meeting 1999 (Atlanta, GA), Tuesday, March 23, 1999:
In America, the disparity between the performance of women and men in physics is a matter of common concern; however, it is poorly understood. Is this disparity observed among physics majors at elite institutions as well as non-majors in one-year introductory courses? Does this disparity depend on the pedagogical approach used to teach the class? The high school physics or mathematics background of the students? The gender of the instructor?
Teaching and Research: Inseparable responsibilities of the modern physicist, at APS Centennial Meeting 1999 (Atlanta, GA), Tuesday, March 23, 1999:
Mention the word ""physics"" to the average high-school student and you are not likely to see many happy faces. Public opinion of science in general--and physics in particular--is not high. More importantly, misunderstandings about the goals and procedures of physics are rampant. In part, these problems arise because physics education has focused nearly exclusively on generating a steady supply of future physicists. The need to educate non-majors, let alone the public at large, has generally not been perceived as an important mission of physics departments. Now that the need for physics is no... Read more about Teaching and Research: Inseparable responsibilities of the modern physicist

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