e) Time-Resolved Spectroscopy of THz Coherent Transients
In
recent
years
experiments
with
ultrashort
pulses
were
almost
exclusively
restricted
to
the
visi
-
ble
and
infrared
spectral
range.
However,
the
newly
developed
terahertz
beam
sources,
produ
-
cing
sub
picosecond
pulses
of
terahertz
radiation,
have
made
accessible
a
new
frequency
range
for time dom
ain spectroscopy and the studies of fast transients.
These
pulses
essentially
consist
of
a
single
cycle
over
the
pulse
duration
of
typically
300
fs,
and
are
characterized
by
a
transform-limited
white
spectrum
extending
from
low
frequencies
up
to
5
THz.
This
frequency
range,
midway
between
the
microwave
and
infrared
frequencies,
is
im
-
portant
owing
both
to
the
samples
that
can
be
investigated
and
to
the
unusual
experimental
conditions encoun
tered with these THz pulses.
They
allow
to
study
gases,
liquids
and
solids
in
a
frequency
range,
which
in
recent
years
was
not
or
only
hardly
accessible.
The
time
response
of
TeraHertz-matter
interactions
can
be
studied
by
obser
ving
the
pulse
reshaping
of
femtosecond
THz-pulses
propagating
through
resonant
and
nonreso
nant
media.
By
precisely
modeling
the
pulse
interaction
with
these
samples
the
mea
-
sure
ments
can
be
analyzed
directly
in
the
time
domain
or
parallel
in
the
frequency
domain
by
Fourier
transforming
the
pulses
and
comparing
them
with
theoretical
spectra.
This
gives
de
-
tailed insight into the material properties in the THz frequency range.
Generation and Detection of THz-Pulses
Meanwhile
different
techniques
are
known
to
generate
and
detect
THz
radiation,
e.g.,
by
mixing
op
tical
frequencies
on
an
opto
electronic
chip
or
using
nonli
n
ear
crystals
as
sour
-
ces and de
tectors.
A
very
successful
technique
for
generating
and
detec
-
ting
these
pulses
with
ex
-
cellent
signal-to-noise
ratios
is
to
use
photo-conducting
antenna
chips
irra
diated
with ultra
short laser pul
ses.
When
the
transmitting
chip
is
additionally
supplied
with
a
voltage,
the
incoming
light
pulses
instan
taneously
create
photo
carriers
which
are
accelerated
in
the
sup
plied
electric
field.
The
rapid
chan
ges
in
conductivity
result
in
short
pulses
of
the
electric
current,
and
it
is
well
known
from
classical
electrodynamics
that
a
changing
current
causes
a
radiation
field
which
is
pro
-
portional to the time derivative of the current.
The
so-called
Hertzian
dipole
antennas
are
fabricated
on
an
ion-implanted
silicon-on-sapphire
wa
fer
and
are
located
in
the
middle
of
two
coplanar
transmission
lines
separated
from
each
other
by
10
µm.
The
antenna
gap
is
5
µm
wide
and
photoconductively
shortened
with
60
-
70
fs
laser
pulses
coming
at
a
rate
of
100
MHz
from
a
colliding
pulse
mode-locked
(CPM)
dye
laser
or
a Ti:Sapphire la
ser.
The
emitted
TeraHz
radi
ation
is
collimated
by
a
silicon
lens
and
parabolo
idal
mirror
into
a
highly
directional beam with a 25 mrad divergence. After a 50 cm propaga
tion distance this beam is fo
-
cused
by
an
identical
combination
of
mirror
and
lens
onto
the
receiving
antenna
chip
which
in
this case is of the same design as the transmitter.
The
electric
field
of
the
THz
radiation
induces
a
voltage
across
the
gap
and
thus
replaces
the
voltage
source
of
the
transmitting
antenna.
The
actual
electric
field
of
a
THz
pulse
is
now
samp
-
led
by
photo
conductively
shorting
the
antenna
gap
with
laser
pulses
from
a
detection
beam
and
monitoring
the
respective
photocurrent
in
the
antenna
as
a
function
of
the
time
delay
between the optical excita
tion and probe pulse.
Special Properties and Advantages of Time Resolved THz Spectroscopy
•
The
measured
average
antenna
current
directly
represents
the
electric
field
amplitude
of
the
pulses
in
contrast
to
the
optical
region,
where
only
intensities
are
registered
and
the
phase in
formation of the field is lost.
•
The pulses are measured with a time resolution of 65 fs.
•
It's
particularly
noteworthy
that
these
pulses
are
detected
with
a
signal-to-noise
ratio
of
better
than
5.000:1,
which
for
the
power
corresponds
to
a
ratio
of
25
Mio:1.
The
average
power
of
the
THz
radiation
is
approximately
10
nW
and
therefore
the
NEP
of
the
system
only 10
-15
Watt/Hz
1/2
.
•
Compared
with
a
helium
cooled
bolometer
this
setup
is
more
than
1.000x
more
sensitive,
al
though it is operated at room temperature.
•
The
reason
for
this
high
sensitivity
is
mainly
due
to
the
subpicosecond
gating
at
the
detector
measuring
only
events
within
this
short
time
interval,
and
on
the
other
hand
the
integration
over
more
than
10
Mio
pulses
at
a
detection
bandwidth
of
1
Hz
at
the
current
amplifier.
•
This
coherent
detection
almost
completely
suppresses
thermal
or
shot
noise,
processes
limi
ting the conventional techniques.
•
The THz pulses are unique in terms of their pulse duration and corresponding bandwidth.
•
They
essentially
consist
of
a
single
cycle
and
are
characterized
by
a
transform-limited
white
spectrum, in this case extending up to 5 THz.
•
In
this
frequency
range
up
to
now
no
proper
radiation
for
time
resolved
measurements
of
fast transients is available, and
•
no other system with this S/N ratio, high sensitivity and broad spectral range is known.
Measurement of Gases
Almost
all
polar
molecules
have
their
rotational
absorption
lines
in
this
spectral
range
and
therefore
can
be
investigated
with
these
exceptional
pulses.
As
an
example
of
a
measurement
with symmetric top molecules is shown a plot with 10 hPa methyl fluoride in a gas cell.
For
the
analysis
of
such
measurement
two
scans
are
necessary,
one
without
the
vapor
to
determine
the
input
pulse,
and
a
second
one
with
the
vapor
in
the
gas
cell,
to
regist
er
the
pulse
being
re
-
shaped
and
attenu
ated
due
to
the
pro
-
pagation though the sample.
At
this
low
pressure
the
input
pulse
is
only
slightly
affected,
but
some
signi
-
ficant
changes
in
the
pulse
structure
are
already
observed.
The
main
pulse
is
followed
by
a
series
of
coherent
transients
reradiated
by
the
vapor
every
20
ps
after
the
excitation
pulse.
This
phenomenon
is
best
under
stood
when
switching
to
the
frequency
do
-
main.
The
spectrum
of
the
input
pulse
is
shown
in
the
lower
plot
as
the
green
line
in
direct
comparison
with
the
ab
-
sorption
spectrum
of
MF.
The
THz
pul
-
se
which
co
vers
this
full
spectrum,
ex
-
cites
the
mole
cu
les
simultaneously
on
a
multitude
of
pure
rotational
lines
and
the
molecules
respond
to
this
excita
-
tion
as
a
phased
array
of
oscil
la
tors
by
re-radi
ating
a
free-induction
decay
sig
-
nal consisting of coherent tran
si
ents.
Because
the
MF
molecule
is
distin
guished
by
absorp
tion
lines
with
an
almost
constant
frequen
-
cy
separation,
a
periodic
rephasing
and
depha
sing
of
the
entire
ensemble
of
more
than
50
exci
-
ted
transitions
occurs
during
the
free
induc
tion
decay,
mani
fest
as
a
train
of
subpsec
THz
pulses
with
a
repetition
rate
equal
to
the
frequency
separation
between
adjacent
lines.
This
situation
is
quite similar to mode-locking of lasers.
Because
of
the
rephasing
of
the
dipoles
and
the
commensurate
frequencies
involved,
we
term
these periodic pulses "
THz commensurate echoes
".
What New Informations are Derived from THz-Spectroscopy?
•
In
the
THz
frequency
range
we
observe
a
transition
from
the
classical
collision
theory
with
a
Lorentzian lineshape to a Van Vleck-Weisskopf line shape of collisionally broadened lines.
•
From
this
lineshape
we
get
informations
on
the
temporal
response
of
the
polar
molecules
which
can
align
to
the
electric
field
of
the
THz
pulses
in
the
presence
of
molecular
colli
-
sions.
•
The
alignment
characterizes
the
thermalization
to
the
THz-field
in
the
presence
of
colli
-
sions.
•
From this time response we derive the molecular collision time.
•
Also
can
be
measured
the
absorption
and
collisional
broadening
at
zero
frequency
origi
-
nating from molecular tunneling.
•
New
analyses
for
the
molecular
re
spon
-
se
time
allow
a
generali
zed
unification
of
the
basic
collision
and
line-shape
theories
of
Lorentz,
van
Vleck-Weiss
-
kopf
and
Debye
which
can
be
described
by
our
molecular
response
theory
(MRT).
•
This
shows
that
the
applied
THz
experi
-
mental
set-up
allows
the
direct
obser
-
vation
of
the
ulti
mate
time
response
of
mole
cules to an external applied electric field in the presence of molecular collisions.
•
This
response
is
limited
by
the
uncertainty
principle
and
is
determined
by
the
inverse
of
the
spitting frequency between adjacent levels.
Doctoral Theses
J. Zhao
Zeitaufgelöste Terahertz-Spektroskopie an Polymeren, Silizium und Ammoniak
School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 2003,
Fortschr.-Ber. VDI Reihe 8 Nr. 1010, Düsseldorf, VDI-Verlag 2003, ISBN: 3-18-501008-6
B. Heinemann
Weiterentwicklung und Einsatz eines Mess-Systems mit asynchroner Abtastung zur zeitaufgelösten Terahertz-
Spektroskopie an Gasen
School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 2014
Refereed Publications in Journals and Conference Digests
H. Harde, S. Keiding, D. Grischkowsky
THz Commensurate Echoes: Rephasing of Molecular Transitions in Free-Induction Decay
Physical Review Letters 66, p. 1834 (1991)
H. Harde, S. Keiding, D. Grischkowsky
THz Coherent Transients from N
2
O Vapor
in ''Quantum Electronics Laser Science'', 1991 Technical Digest Series (Optical Society of America, Washington,
D.C. 1991), p. 248 (1991)
H. Harde, D. Grischkowsky
Coherent Transients Excited by Subpicosecond Pulses of Terahertz Radiation
Journal Optical Society America B 8, p. 1642 (1991)
H. Harde, D. Grischkowsky
Time-Domain Spectroscopy of Molecular Vapors with Subpicosecond Pulses of THz Radiation
in:''Ultrafast Processes in Spectroscopy'', Institute of Physics Conference Series No. 126, ed. by A. Laubereau
and A. Seilmeier (IOP Publishing Ltd, Bristol, 1992), pp. 217-222 (1992)
D. Grischkowsky, N. Katzenellenbogen, H. Harde
Time-Domain Spectroscopy with Femtosecond Pulses of THz Radiation
in: ''Nonlinear Optics: Materials, Fundamentals and Applications Technical Digest, 1992'' (Optical Society of
America, Washington, D.C., 1992), Vol. 18, pp. 187-188 (1992)
H. Harde, N. Katzenellenbogen, D. Grischkowsky
THz Time-Domain Spectroscopy of Methyl Chloride Vapor
in: ''Quantum Electronics and Laser Science Conference'', 1993 OSA Technical Digest Series, Vol. 3
(Optical Society of America, Washington DC), p. 89 (1993)
H. Harde, N. Katzenellenbogen, D. Grischkowsky
TeraHz Coherent Transients from Methyl Chloride Vapor
Journal Optical Society America B 11, 1018 (1994)
H. Harde, N. Katzenellenbogen, D. Grischkowsky
Time-Domain Studies in Dense Molecular Vapors with Femtosecond Pulses of TeraHz Radiation
in: ''5th European Quantum Electronics Conference'', Technical Digest, IEEE Catalog No. 94TH0615-5,
ISBN: 0-7803-1791-2, Piscataway, p. 253 (1994)
H. Harde, N. Katzenellenbogen, D. Grischkowsky
Line-Shape Transition of Collision Broadened Lines
Physical Review Letters 74, 1307 (1995)
H. Harde, R. A. Cheville, D. Grischkowsky
Collision Induced Orientation of Polar Molecules in the Field of Femtosecond Terahertz Pulses
Proceedings of the 1996 European Quantum Electronics Conference, IEEE Catalog No. 96TH8162,
ISBN: 0-7803-3171-0, p. 24 (1996)
D. Grischkowsky, R. A. Cheville, H. Harde
THz-Studies of Rotational Line-Shapes
Quantum Electronics and Laser Science Conference, Vol. 12,1997 OSA Technical Digest Series (Optical Society
of America), Washington DC, p. 82 (1997)
H. Harde, R. A. Cheville, D. Grischkowsky
Collision-Induced Tunneling in Methyl Halides
Quantum Electronics and Laser Science Conference, Vol. 12, 1997 OSA Technical Digest Series (Optical Society
of America), Washington DC, p. 115 (1997)
H. Harde, R. A. Cheville, D. Grischkowsky
Terahertz Studies of Collision-Broadened Rotational Lines
Feature Article in: Journal of Physical Chemistry A, 3646-3660 (1997)
H. Harde, R. A. Cheville, D. Grischkowsky
Collision Induced Tunneling in Methyl Halides
J. Opt. Soc. Am. B 14, 3282 (1997)
H. Harde, R.A. Cheville, M. Wolff, D. Grischkowsky
Line-Shape Studies of Zero-Frequency Transitions
Proceedings of the 1998 European Quantum Electronics Conference, IEEE
Catalog No. 98TH8326, ISBN 0-7803-4233, p. 177 (1998)
H. Harde, J.-X. Zhao, M. Wolff, A. Cheville, D. Grischkowsky
THz Time-Domain Spectroscopy on Ammonia
7. European Conference on Atomic and Molecular Physics (2001)
H. Harde, J. Zhao, M. Wolff, R. A. Cheville, D. Grischkowsky
THz Spectroscopy on Ammonia
Conference on Lasers and Electrooptics (CLEO), Technical Digest Series 15, pp. 324-325 (2001)
H. Harde, J. Zhao, M. Wolff, R.A. Cheville, D. Grischkowsky
THz-Time-Domain-Spectroscopy on Ammonia
J. Phys. Chem. A, 105 (25), pp. 6038-6047 (2001)
H. Harde, J. Zhao, J. Pfuhl
Ausbreitung von THz-Impulsen in resonanten und nichtresonanten Medien
Symposium „Simulation in Physik, Informatik und Informationstechnik“, 66. Physikertagung, Leipzig 2002,
Herausg. H. Hofman, ISSN: 0944-7121, 27-30 (2002)
B. Heinemann, C. J. Fox, H. Harde
Improved Fast Scanning TeraHz Pulse System
Proceedings of XVIth Intern. Conference on Ultrafast Phenomena, Stresa, Italy, 09-13 June, 2008
H. Harde, D. Grischkowsky
Molecular Response Theory in Terms of the Uncertainty Principle
J. Phys. Chem. A 119 (34), pp 9085–9090 (2015).
https://pubs.acs.org/doi/10.1021/acs.jpca.5b05909
Contributions on National Conferences and Meetings
H. Harde, D. Grischkowsky
Zeitaufgelöste THz-Spektroskopie an dichten Gasen
Poster Lasertag 1994, Hannover, 7. Dezember 1994
H. Harde, D. Grischkowsky, R. A. Cheville
Messungen und Modellbildung zur stoßinduzierten Thermalisierung von Molekülen
Poster Lasertag 1995, Hannover, 6. Dezember 1995
H. Harde
Erweiterte Theorie für molekulare Stöße
Lasertag 1996, Hannover, 11. Dezember 1996
H. Harde, D. Grischkowsky, R. A. Cheville
Stoß-induziertes Tunneln in Methylhalogeniden
Frühjahrstagung der DPG, Sektion Quantenoptik, Mainz, 6. März 1997,
Verhandl. DPG (VI) 32, Q 39.5, 349 (1997)
H. Harde, D. Grischkowsky, R. A. Cheville
Untersuchungen mit Femtosekunden THz-Impulsen zum stoßinduzierten Tunneln in Molekülen
Poster Norddeutsches Laserkolloquium 1999, Hannover, 9. Dezember 1999
H. Harde, J. Zhao, M. Wolff, A. Cheville, D. Grischkowsky
THz Time Domain Spectroscopy on Ammonia
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Fachverband Molekülphysik, Berlin, 2.-4. April
2001, Verhandl. DPG (VI) 36, M-Methoden (2001)
J. Zhao, H. Harde
Characterization of Plastic Materials in the Terahertz Frequency Range
Poster, Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Fachverband Quantenoptik, Berlin,
2.-4. April 2001, Verhandl. DPG (VI) 36, Q-Ultrakurze Pulse (2001)
J. Zhao, H. Harde
Untersuchungen von Polymeren im THz-Frequenzbereich
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik, Osnabrück, 4.-8. März 2002,
Verhandl. DPG (VI), Q 434.19 (2002)
H. Harde, J. Zhao, M. Wolff, R. A. Cheville, D. Grischkowsky
Stoßprozesse und (NH
3
)
2
–Dimerenbildung auf einer fs-Zeitskala
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik,
Verhandl. (DPG) (VI) 37, Q 434.20 (2002)