In our group we are using intense, short-pulse lasers to modify the optical properties of materials with micron-size precision. Because the lasers that we use have pulse durations on the order of 100 femtosecond (fs) this technique is referred to as fs-laser micro-machining or fs-laser processing.
The fs-laser processing technique relies on the fact that ≈100 fs laser pulses, when tightly focused inside a material, can give rise to very high intensities. For example, a laser pulse with a pulse energy of 1 μJ focused to a spot with a spot radius of 1 μm has a peak intensity on the order of 1014 W/cm2 . Under these conditions the laser-materials interaction becomes highly nonlinear, resulting in permanent modification of the material. By precisely controlling sample position and movement it is possible to optically “write” 3-dimensional patterns inside a material. These modified patterns can differ from the unmodified material in a wide variety of properties including refractive index, absorption coefficient, nonlinear optical susceptibility, crystal structure, morphology etc. and they can be used to create waveguides (see figure below), photonic crystals and other nano-structured optical elements. The structures that can be fabricated with this so-called fs-laser writing technique have applications in optical data storage, telecommunications and bio-sensing and –imaging.
While fs-laser processing is studied by many research groups world-wide we still understand very little about the mechanism responsible for the laser-induced modification of the material. Therefore, in addition to modifying materials with high intensity lasers, we also use tightly focused low-intensity lasers to characterize materials with micron-size resolution. Using a confocal laser microscope set-up we can measure fluorescence and Raman scattering tohelp us understand how the structure of the material is changed by the fs laser. Our work has shown that structural characterization with confocal spectroscopy is a key tool for understanding the structural changes associated with fs-laser fabrication of waveguides in glass. We have used this technique to investigate the effects of glass composition as well as laser processing parameters, such as pulse energy, pulse repetition rate, scan speed.
Recently we have embarked on several new directions, funded by the National Science Foundation (grants DMR-0801786 and CMMI-0825572). In one of these projects the objective is to experimentally monitor the dynamics of fs laser modification in glass to gain a better understanding of the process. While the structural changes that result from modification have been characterized, little is known about the processes occurring between laser absorption and resulting modification. These intermediate processes are difficult to study because they occur in very small volumes and on time scales that can range from femtoseconds to microseconds. We have built a fs pump-probe set-up and have started to measure the dynamics of the fs-laser induced plasma on picosecond time scales.
Other projects involve fs laser fabrication of waveguide lasers and amplifiers in rare-earth doped phosphate glass as well as using fs-laser processing to tailor-make nano-and microstructures in glass with a degree of precision and resolution unattainable by, for example, conventional patterning and lithography techniques.
More information about these projects can be found on the individual project web pages: