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Surface studies and microstructure fabrication using femtosecond
laser pulses
Rebecca J. Younkin
Ph.D. Thesis, Harvard University, 2001, 118 pages export citation
In the first of the two experiments described in this
thesis, we constructed and
characterized an apparatus for performing ultrafast surface
spectroscopy on Pt (111).
A combination of an optical parametric amplifier and a
difference frequency generation stage allow us to create
optical pulses with wavelengths in the range 550 nm-7.3
µm and pulsewidths of approximately 100 fs. We also
developed methods for using
the infrared output of this apparatus in infrared-visible
sum-frequency generation at
a Pt-vacuum interface. Our attempts to time-resolve a
chemical reaction taking place
at this interface were unsuccessful because of power
limitations, but the techniques
that we developed should be applicable in related
experiments.
In the second series of experiments, we created arrays of
conical microstructures
on the surface of silicon by irradiating a silicon wafer
with trains of ultrashort pulses in
the presence of an ambient gas. The microstructures are
typically tens of micrometers
high and separated by several micrometers. We showed that
the morphology of the
structures is highly dependent on the species of ambient
gas, the gas pressure, the
laser fluence, and the number of laser pulses used.The
sharpest structures are made
in halogen-containing gases,such as SF6 or Cl
2 .The use of
N2 or air results in blunt,
rounded structures.
In this thesis, I discuss the application of these silicon
microstructures in optoelectronics and vacuum
microelectronics. I note that surfaces covered with these
microstructures have striking optical properties:structures
made in SF6 absorb approximately 90% of incident
light with
wavelengths between 250 nm and 2.5 µm.The
sharp conical structures also show high field-emission
current. The remarkable and
potentially useful optical and field-emitting properties of
the structures are the result
of the conical morphology of the laser-induced
microstructures and the impurities
introduced into the silicon.
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