Professor Mazur and his group study the dynamics of molecules, chemical reactions, and condensed matter on very short timescales—down to femtoseconds (millionths of billionths of a second). Physics in this ultrafast regime can only be studied using light, specifically using short laser pulses. The intensity of these short laser pulses is enormous and allows the creation of conditions that approach those found in stars and study a host of new phenomena.

In one of the group's projects, femtosecond laser pulses are tightly focused in bulk transparent dielectrics creating a hot, micrometer-sized plasma which expands into the surrounding volume. These "microexplosions" alter the structure of the material on the nanometer scale creating features with applications in data storage, optical communication, and medicine. The group is currently investigating exciting possibilities for high-precision microstructuring of transparent solids and for minimally disruptive laser surgery.

In a second project, Professor Mazur's group uses ultrafast optical techniques to study highly non-equilibrium electron and lattice dynamics in semiconductors. With an ultrafast broadband spectrometer developed by the group, the electronic and structural response of semiconductors to intense optical excitation can be observed with unprecedented detail. These studies allow the group to answer basic questions about the dynamics of the electrons and atoms in a semiconductor when the two systems are far off equilibrium. In most semiconductors the electronic excitation drives a rapid structural phase transition which in turn drastically alters the electronic bandstructure. The group's experiments provide an unusual opportunity to observe these structural phase transitions as they occur.

A third project, which is currently receiving much attention in the national press, deals with novel, nanoscale structures that are produced by femtosecond laser-induced chemical etching of silicon. Professor Mazur and his group recently discovered that irradiation of silicon surfaces with femtosecond laser pulses in the presence of a halogen containing gas transforms the flat, mirror-like surface of a silicon wafer into a forest of microscopic spikes. The spiked surface is strongly light-absorbing: the surface of silicon, normally gray and shiny, turns deep black. The optical and electronic properties of this "black silicon" turn out to be remarkable: the material's absorption is nearly one over the entire visible range and even in the infrared range where silicon is normally transparent. Even more surprisingly, the enhanced absorption results in photocarrier production at wavelengths near the band gap. Because silicon is the basic substrate of microelectronics and is also the material of choice for many optoelectronic devices such as solar cells and photodetectors, black silicon has many promising applications in remote sensing and telecommunications.