Laser Research
General
The
vast
majority
of
these
activities
goes
back
to
common
studies
with
graduate
and
doctoral
students
at
Helmut-Schmidt-University
and
also
to
sabbaticals
at
IBM-Watson
Research
Center,
Yorktown
Heights,
and
the
Oklahoma
State
University.
These
activities
were
mainly
focusing
on
laser sources, laser spectroscopy and laser applications.
Coherent
light
sources
allow
completely
new
approaches
to
study
the
structure
and
properties
of
materials.
Over
recent
years
this
has
contributed
to
a
much
deeper
insight
and
more
complete understanding of solids, fluids and gases.
Lasers
can
be
used
as
extremely
narrow-bandwidth
light
sources
for
high
resolution
laser
spec
-
troscopy
in
the
frequency
domain,
and
on
the
other
hand
-
under
special
operation
conditions
-
they
can
generate
extremely
short
and
bandwidth
limited
light
pulses
in
the
picosecond
and
femtosecond
range,
which
allows
time
resolved
studies
on
these
time
scales.
In
combination
with
coherent
excitation
phenomena
in
materials
this
also
makes
possible
ultrahigh
resolution
measurements in the frequency domain.
In
recent
years
the
Laser-Physics-Group
(LPG)
was
mainly
focusing
on
pulsed
laser
operation
and
respective
applications.
But
also
special
continuous-wave
laser
set-ups
were
developed
for
gas sensor applications and experiments with trapped ions.
a) Ultrashort Laser Pulses in the Pico- and Femtosecond Range
Nonlinear
interaction
of
laser
radiation
in
optical
crystals
and
glasses
are
successfully
used
for generating and detecting ultrashort light pulses.
Particularly
in
micro-structures
and
glass
fibers
with
peak
intensities
up
to
several
GW/cm
2
nonlinear
effects
can
favorably
be
used
for
reshaping
or
generating
pulses
on
new
wavelengths
and
to
realize
compact
fiber
lasers,
which
are
wavelength
tunable
and
can
generate extremely short laser pulses.
On
the
other
hand
these
effects
can
seriously
limit
the
bandwidth
and
transmission
distance
of
optical
fiber
communication
systems.
Therefore,
a
deeper
understanding
of
these
effects
and
their
mutual
interrelations
is
a
necessary
prerequisite
for
these
appli
-
cations.
b)
Time Resolved Laser Spectroscopy of Optical Transients
Optically
excited
atomic
and
molecular
states
spontaneously
decay
to
lower
states
or
the
ground
state,
typically
on
a
nanosecond
time
scale.
But
a
sufficiently
short
excitation
can
also
prepare
coherent
superpositions
of
sub-levels
in
the
excited
and
lower
state,
which
can
be
observed
in
the
time
domain
as
coherent
transients
on
the
free
induction
decay.
These
transients
are
oscillating
with
the
respective
sub-level
splitting
frequencies
and
are
known
as quantum beats. They allow Doppler-free measurements of splitting frequencies.
c)
Coherence Spectroscopy with Pulse Trains
A
train
of
short
light
pulses
can
create
an
enhanced
coherent
superposition
of
nearly
degenerate
atomic
states
or
sub-states,
when
the
excitation
rate
or
a
higher
harmonic
coincides
with
the
level
splitting
frequency.
By
slightly
tuning
the
laser
repetition
rate
ground
state
hyperfine
splittings
with
extremely
sharp
resonances
of
only
30
Hz
can
be
measured directly in the frequency domain.
d)
Spectroscopy with Trapped Ions
Ion-storage
techniques
have
disclosed
new
dimensions
of
fundamental
and
high-accuracy
experiments.
So,
with
single
ions
the
phenomenon
of
quantum
jumps
was
demonstrated,
and optically cooled ions were observed to form a crystal structure in the trap.
Ion
traps
are
also
promising
tools
for
ultrahigh
resolution
laser
spectroscopy
because
ions
can
be
observed
over
long
periods
without
perturbations.
Therefore,
transit
time
broadening and collisional effects can largely be eliminated.
e)
Time-Resolved Spectroscopy of THz Coherent Transients
The
newly
developed
technique
of
THz
time-domain
spectroscopy
allows
to
study
gases,
liquids
and
solids
in
a
frequency
range
which
in
recent
years
was
only
hardly
accessible.
The
time
response
of
TeraHertz-matter
interactions
can
be
studied
by
observing
the
pulse
reshaping
of
femtosecond
THz-pulses
propagating
through
resonant
and
non-resonant
media.
By
precisely
modeling
the
pulse
interaction
with
these
samples
one
gets
detailed
insight into the material properties in the THz frequency range.
f)
Optical Sensors for Environmental and Medical Applications
Photo-acoustic
gas
sensors
can
successfully
be
applied
in
medical
diagnostics
as
breath
test
analyzer
or
for
environmental
measurements
as
sensitive
detector
of
trace
gases
and
pollutants in the atmosphere.
In
the
meantime
photo-acoustic
sensors
allow
detection
of
special
gas
impurities
with
concentrations less than 1 ppb (part per billion).
g)
Surface Cleaning and Micro-Structuring with Lasers
For
cleaning
sensitive
surfaces
like
pigment
lacquer,
finishing
varnish
or
art
paintings
and
also
for
manufacturing
all-polymer
electronic
components
a
prototype
set-up
based
on
a
KrF-excimer laser with adapted beam homogenizer and scanning unit was developed.
h)
Opto-Electronic Components for Metrological Applications
Laser
based
systems
were
developed
to
measure
velocities
of
gases
and
liquids
with
a
laser
anemometer
,
shifts
and
vibrations
with
interferometers
or
longer
distances
with
laser range finders.
i)
Optical Communication through the Atmosphere
Optical
communication
systems
have
the
potential
of
a
higher
transmission
bandwidth
and
a
higher
directionality
than
radiowave
and
microwave
systems.
But
they
suffer
from
stronger
atmospheric
perturbation
effects.
The
propagation
and
transmission
of
optical
and
infrared
radiation
through
the
atmosphere
was
studied
under
the
influence
of
molecular absorption, turbulence and scattering processes.
j)
Spectral Calculation
A
program
platform,
called
MolExplorer,
has
been
developed
for
fast
computation
and
display
of
molecular
spectra
on
a
PC
Microsoft
Windows
system.
It
takes
advantage
of
a
database,
in
which
the
relevant
parameters
for
the
calculation
of
a
spectrum
are
stored.
It
provides
survey
spectra
from
mm-waves
up
to
the
ultra
violet
as
well
as
selected
parts
with
highest spectral resolution for relevant gases and pollutants in the atmosphere.
Physics & Climate