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We study the interaction of intense, femtosecond laser pulses with bulk
transparent materials. The intensity of a femtosecond laser pulse can be high
enough to cause nonlinear interactions between a transparent medium and the
laser field. The material can strongly absorb energy from the laser field,
producing free electrons in the material. The laser pulse, in turn, can be
transformed by the material's nonlinear response. The nature of the interaction
between the laser pulse and the material depends on how the laser pulse is
focused into the sample. Roughly speaking, with tight focusing the laser pulse
modifies the material, while with slow focusing the material transforms the
laser pulse.
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Snapshot of a microexplosion in water 10 ns after excitation. |
White-light continuum
generation in water |
When a powerful femtosecond laser pulse is tightly focused into a transparent
sample, nonlinear absorption occurs only in the very small focal volume. This
absorption results in a hot, micrometer-sized plasma which expands into the
surrounding volume, creating a microexplosion and leaving behind a permanently
damaged region. We study the nonlinear mechanisms responsible for these
microexplosions and explore their applications. For example, we have observed
damage structures as small as 200-nm in diameter, offering exciting
possibilities for high-precision microstructuring of transparent solids and for
minimally disruptive laser surgery.
When a powerful femtosecond laser pulse is focused slowly into a transparent
material, it is strongly affected by the material's nonlinear response. In
particular, the frequency spectrum of the pulse can be dramatically broadened to
produce a white-light continuum that covers and extends beyond the visible
spectrum. Despite its extensive use in the laser community, the mechanism for
producing this continum is still not well understood. We investigate continuum
generation in different materials with various pulse characteristics, with the
goal of understanding and optimizing the continuum.
| | PLAINLY SPEAKING |
Usually when light goes through a piece of glass, nothing happens to either the
light nor the glass, i.e. the glass is transparent. With a powerfull femtosecond
laser pulse, both the laser light and the glass can be changed. When we
concentrate the laser light using a microscope lens, we produce a microscopic
explosion inside the glass which leaves behind a minuscule ball-shaped hole. In
other conditions, the single color laser pulse is transformed into a short pulse
which contains all colors -- a white-light pulse.
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Array of microscopic holes in glass produced using a femtosecond laser
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We use microexplosions as a miniature "punch" to make patterns inside glass for
such applications as high-density data storage. Microexplosions can also be used
as a high-precision laser scalpel -- we have been able to eliminate a single cell
in a skin sample, without affecting the neighboring cells!
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Recent progress