b) Time Resolved Laser Spectroscopy of Optical Transients
Ultrashort
excitation
of
atoms
and
molecules
allows
time
resolved
observation
of
excited
states
relaxation and measurement of coherent transients.
For
observations
on
a
picosecond
time
scale
typical
electronic
detectors
are
not
fast
enough.
Therefore,
methods
of
higher
time
resolution
are
applied,
which
take
advantage
of
transforming
the time scale t to a length l via the velocity v with t = l/v.
Fast Beam Spectroscopy
Ions
can
be
accelerated
up
to
velocities
of
several
%
of
the
speed
of
light
c.
When
these
ions
pass
a
gas
cell
or
a
thin
carbon
foil,
they
are
further
ionized
or
also
neutralized
and
at
the
same
time excited to higher atomic states.
Subsequent
selective
excitation
with
a
conti
-
nuously
operating
laser
allows
a
well
controlled
population change of the involved atomic states.
By
measuring
the
fluorescence
of
the
atoms
or
ions
with
and
without
laser
excitation
as
a
func
-
tion
of
the
time
of
flight
(distance
after
excita
-
tion)
makes
possible
cascade
free
lifetime
mea
-
surements
of
highly
excited
atomic
or
ionic
states on a picosecond time scale.
A
sufficiently
short
excitation
can
also
prepare
coherent
superpositions
of
sub-levels
in
the
ex
-
cited
or
lower
state,
when
the
inverse
of
the
excitation
time
covers
the
splitting
of
adjacent
sub-
levels.
Then
coherent
transients
are
observed
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.
Quantum Beats in Forward Scattering
An
excitation
and
detection
scheme
using
a
transformation
of
time
to
length
by
the
speed
of
light
c
is
known
as
pump-probe
experiment.
The
time
evolution
of
coherently
excited
states
after
an
excitation
by
an
ultrashort
pump
pulse
can
sensitively
be
detected
by
a
time
delayed
probe
pulse,
which
passes
an
optical
delay
line
and
crossed
optical
polarizers.
Recording
the
average
transmitted
intensity
of
the
probe
pul
-
ses
behind
the
polarizer
as
a
function
of
the
delay
then
allows
to
sample
the
time
evolution
of
the
coherently
prepared
sub-states
in
for
-
ward scattering.
Since
mode-locked
lasers
are
typically
generating
pulses
with
repetition
rates
in
the
MHz
up
to
the
GHz
range,
integration
over
millions
of
pulses
for
each
delay
time
significantly
improves
the
signal-to-noise ratio.
A
measurement
with
85
Rb
(72%
abundance)
and
87
Rb
(28%)
demonstrates
the
high
sensiti
-
vity
and
time
resolution
of
the
set-up
for
the
observation
of
coherent
transients
in
the
time
domain,
reflecting
directly
the
respective
level
splittings of the excited and ground states.
Quantum Beats at “Negative” Delay Times
The
accuracy
of
measuring
frequency
splittings
in
the
time
domain
is
generally
limited
by
the
finite delay time, and thus the observation time of typically several ten nanoseconds.
With
a
synchronously
pumped
mode-locked
and
cavity
dumped
dye-laser,
however,
long
lived
coherent
transients
as
they
are
expected
in
the
ground
state,
can
also
be
measured
after
delay
times of several µs, and this with ps time resolution.
Taking
advantage
of
the
well-known
and
extremely
stable
laser
pulse
separation
time
an
atomic
coherence
created
by
a
pump
pulse,
can
be
sampled
by
a
probe
pulse
derived
from
the
next
pulse
of
the
pulse
train.
At
“negative”
delay
times
then
the
time
evolution
of
atoms
is
measured
after one pulse period reduced by the respective delay time.
This
figure
shows
the
beat
pattern
of
the
85
Rb
ground
state
splitting
with
3.04
GHz,
on
the
left
side
for
negative
delays
displaying
beats
after
almost
2.4
µs,
on
the
right
side
directly
after
zero
delay.
From
such
measurement
the
beat
period
can
be
interpolated
over
the
full
time
interval,
and
according
to
Fourier’s
theorem
the
ground
state
splitting
can
be
determined
within
100 kHz.
Observation of 500 GHz Quantum Beats
A
coherent
superposition
of
the
3p
sodium
fine-structure
states
with
a
wavelength
splitting
of
0.6
nm
or
a
frequency
difference
of
517
GHz
requires
simultaneous
excitation
of
the
D
1
and
D
2
resonance lines at 589 nm with subpicosecond pulses.
A
synchronously
pumped
mode-locked
dye-
laser
with
saturable
absorber
in
the
dye
gene
-
rates
pulses
of
400
femtoseconds
duration
at
a
repetition
rate
of
84
MHz.
Together
with
a
pola
-
rization
selective
detection
scheme
well
resolv
-
ed
fine-structure
quantum
beats
in
forward
scattering
with
an
oscillation
period
of
1.9
ps
can
be
measured,
demonstrating
the
ultrahigh
time resolution of this experimental set-up.
Doctoral Theses
H. Harde
Cascade-Free Lifetime Measurements with Fast Ion Beams after Combined Gas-Laser-Excitation
- Development of a Set-Up and First Measurements with Ba+, Na, Ne und Li+
Physics Department, University of Kaiserslautern 1974
H. Burggraf
Hochauflösende Spektroskopie mit ultrakurzen Laserpulsen am Natrium
School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 1985
H. Lehmitz
Kohärenzspektroskopie mit ultrakurzen Lichtimpulsen – Zeit- und frequenzaufgelöste Messungen am Beispiel von
Cäsium sowie Voruntersuchungen zu Messungen an gespeicherten Ytterbiumionen
School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 1989
Refereed Publications in Journals and Conference Digests
H. Harde, G. Guthöhrlein
New Method for Cascade-Free Lifetime Measurements
Physical Review A 10, 1488 (1974)
H. Harde
Intracavity Dye-Laser Excitation of Fast Ions and Atoms
Summaries of the Sixth Conference of the ''European Group of Atomic Spectroscopy'', European Physical Society,
Berlin, p. 65 (1974)
H. Harde
Cascade-Free Lifetime Measurements by Laser Excitation of Foil- or Gas-Excited Beams
in ''Beam-Foil Spectroscopy 2'', ed. by J. A. Sellin and D. J. Pegg (Plenum Press, New York 1976), p. 859 (1976)
D. Schulze-Hagenest, H. Harde, W. Brand, W. Demtröder
Fast-Beam Spectroscopy by Combined Gas-Cell-Laser Excitation for Cascade-Free Lifetime Measurements of
Highly Excited States
Zeitschrift für Physik A 282, 149 (1977)
H. Harde, H. Burggraf, J. Mlynek, W. Lange
Time-Resolved Polarization Spectroscopy of Coherent Transients: Picosecond Studies with a Mode-Locked Dye
Laser
Technical Digest of Topical Meeting of Picosecond Phenomena (Optical Society of America, Washington DC,
1980), post-deadline paper, p. THB16 (1980)
H. Harde, H. Burggraf, J. Mlynek, W. Lange
Hyperfine Quantum Beats in Transmission: Subnanosecond Studies by Means of a Mode-Locked Dye Laser
Europhysics Conference Abstracts of 12th EGAS-Conference 4 E (European Physical Society, Pisa, 1980),
p. 71 (1980)
H. Harde, H. Burggraf, J. Mlynek, W. Lange
Quantum Beats in Forward Scattering: Subnanosecond Studies with a Mode-Locked Dye Laser
Optics Letters 6, 290 (1981)
H. Harde
Time-Resolved Coherence Spectroscopy in Rb Vapor
Conference Abstracts of Ninth International Conference on Atomic Physics, ed. by R. S. Van Dyck Jr. and
E. N. Fortson (University of Washington, Seattle, 1984), p. B 48 (1984)
H. Lehmitz, H. Harde
Polarization Selective Detection of Hyperfine Quantum Beats in Cs
in ''Methods of Laser Spectroscopy'', ed. by Y. Prior et al., Plenum Press, New York, p. 101 (1986)
H. Burggraf, M. Kuckartz, H. Harde
Observation of 517 GHz Fine Structure Quantum Beats in Na
in ''Methods of Laser Spectroscopy'', ed. by Y. Prior et al., Plenum Press, New York, p. 105 (1986)
H.Lehmitz, H. Harde
Measurement of First-Order Free-Induction-Decay in Cs
in ''Methods of Laser Spectroscopy'', ed. by Y. Prior et al., Plenum Press, New York, p. 109 (1986)
H. Burggraf, M. Kuckartz, H. Harde
Measurement of 1.9 ps Fine Structure Quantum Beats in Na
Technical Digest of International Quantum Electronics Conference (Optical Society of America, Washington, D.C.,
1986), p. 200 (1986)
Contributions on National Conferences and Meetings
H. Burggraf, H. Harde
Zeitaufgelöste Polarisationsspektroskopie mit Pikosekunden Laserimpulsen
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik, Münster, 12. März 1981,
Verhandl. DPG (VI) 16, 418 (1981)
H. Lehmitz, H. Hoidis, H. Harde
Quantunm Beats in Vorwärtsstreuung am Cs mit ps-Zeitauflösung
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik, Bayreuth, 27. März 1985,
Verhandl. DPG (VI) 20, 1054 (1985)
M. Kuckartz, H. Harde
Beobachtung von 517 GHz Quantum Beats in Na
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik, Heidelberg, 17. März 1986,
Verhandl. DPG (VI) 21, 728 (1986)
Physics & Climate