<s>
Terahertz	B-Algorithm
spectroscopy	I-Algorithm
detects	O
and	O
controls	O
properties	O
of	O
matter	O
with	O
electromagnetic	O
fields	O
that	O
are	O
in	O
the	O
frequency	O
range	O
between	O
a	O
few	O
hundred	O
gigahertz	O
and	O
several	O
terahertz	O
(	O
abbreviated	O
as	O
THz	O
)	O
.	O
</s>
<s>
In	O
many-body	B-Algorithm
systems	I-Algorithm
,	O
several	O
of	O
the	O
relevant	O
states	O
have	O
an	O
energy	O
difference	O
that	O
matches	O
with	O
the	O
energy	O
of	O
a	O
THz	O
photon	B-Application
.	O
</s>
<s>
Therefore	O
,	O
THz	B-Architecture
spectroscopy	I-Architecture
provides	O
a	O
particularly	O
powerful	O
method	O
in	O
resolving	O
and	O
controlling	O
individual	O
transitions	O
between	O
different	O
many-body	B-Algorithm
states	O
.	O
</s>
<s>
By	O
doing	O
this	O
,	O
one	O
gains	O
new	O
insights	O
about	O
many-body	B-Algorithm
quantum	O
kinetics	O
and	O
how	O
that	O
can	O
be	O
utilized	O
in	O
developing	O
new	O
technologies	O
that	O
are	O
optimized	O
up	O
to	O
the	O
elementary	O
quantum	O
level	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
they	O
constitute	O
an	O
interesting	O
many-body	B-Algorithm
system	I-Algorithm
whose	O
quantum	O
properties	O
can	O
be	O
modified	O
,	O
e.g.	O
,	O
via	O
a	O
nanostructure	O
design	O
.	O
</s>
<s>
Consequently	O
,	O
THz	B-Architecture
spectroscopy	I-Architecture
on	O
semiconductors	O
is	O
relevant	O
in	O
revealing	O
both	O
new	O
technological	O
potentials	O
of	O
nanostructures	O
as	O
well	O
as	O
in	O
exploring	O
the	O
fundamental	O
properties	O
of	O
many-body	B-Algorithm
systems	I-Algorithm
in	O
a	O
controlled	O
fashion	O
.	O
</s>
<s>
There	O
are	O
a	O
great	O
variety	O
of	O
techniques	O
to	O
generate	O
THz	B-Architecture
radiation	I-Architecture
and	O
to	O
detect	O
THz	O
fields	O
.	O
</s>
<s>
One	O
can	O
,	O
e.g.	O
,	O
use	O
an	O
antenna	O
,	O
a	O
quantum-cascade	O
laser	O
,	O
a	O
free-electron	O
laser	O
,	O
or	O
optical	B-Algorithm
rectification	I-Algorithm
to	O
produce	O
well-defined	O
THz	O
sources	O
.	O
</s>
<s>
Besides	O
the	O
strength	O
and	O
duration	O
of	O
ETHz(t )	O
,	O
the	O
THz	O
field	O
's	O
photon	B-Application
energy	O
plays	O
a	O
vital	O
role	O
in	O
semiconductor	O
investigations	O
because	O
it	O
can	O
be	O
made	O
resonant	O
with	O
several	O
intriguing	O
many-body	B-Algorithm
transitions	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
the	O
exciton	O
binding	O
energy	O
is	O
few	O
to	O
hundreds	O
of	O
meV	O
that	O
can	O
be	O
matched	O
energetically	O
with	O
a	O
THz	O
photon	B-Application
.	O
</s>
<s>
The	O
frequency	O
of	O
transversal-optical	O
phonons	B-Language
,	O
to	O
which	O
THz	O
photons	B-Application
can	O
couple	O
,	O
lies	O
for	O
most	O
semiconductors	O
at	O
several	O
THz	O
.	O
</s>
<s>
Free	O
carriers	O
in	O
doped	B-Algorithm
semiconductors	I-Algorithm
or	O
optically	O
excited	O
semiconductors	O
lead	O
to	O
a	O
considerable	O
absorption	O
of	O
THz	O
photons	B-Application
.	O
</s>
<s>
In	O
particular	O
,	O
1s-to-2p	O
transition	O
is	O
dipole	O
allowed	O
and	O
can	O
be	O
directly	O
generated	O
by	O
ETHz(t )	O
if	O
the	O
photon	B-Application
energy	O
matches	O
the	O
transition	O
energy	O
.	O
</s>
<s>
In	O
gallium	O
arsenide-type	O
systems	O
,	O
this	O
transition	O
energy	O
is	O
roughly	O
4	O
meV	O
that	O
corresponds	O
to	O
1	O
THz	O
photons	B-Application
.	O
</s>
<s>
This	O
technique	O
is	O
called	O
transient	O
THz	B-Architecture
spectroscopy	I-Architecture
.	O
</s>
<s>
Since	O
a	O
THz	O
pulse	O
can	O
be	O
intense	O
and	O
short	O
,	O
e.g.	O
,	O
single-cycle	O
,	O
it	O
is	O
experimentally	O
possible	O
to	O
realize	O
situations	O
where	O
duration	O
of	O
the	O
pulse	O
,	O
time	O
scale	O
related	O
to	O
Rabi	O
-	O
as	O
well	O
as	O
the	O
THz	O
photon	B-Application
energy	O
ħω	O
are	O
degenerate	O
.	O
</s>
<s>
The	O
Rabi	O
splitting	O
has	O
also	O
been	O
measured	O
with	O
a	O
short	O
THz	O
pulse	O
and	O
also	O
the	O
onset	O
to	O
multi-THz-photon	O
ionization	O
has	O
been	O
detected	O
,	O
as	O
the	O
THz	O
fields	O
are	O
made	O
stronger	O
.	O
</s>
<s>
Terahertz	O
transitions	O
in	O
solids	O
can	O
be	O
systematically	O
approached	O
by	O
generalizing	O
the	O
semiconductor	O
Bloch	O
equations	O
and	O
the	O
related	O
many-body	B-Algorithm
correlation	O
dynamics	O
.	O
</s>
<s>
Therefore	O
,	O
a	O
systematic	O
THz	O
analysis	O
must	O
include	O
the	O
quantum	O
kinetics	O
of	O
many-body	B-Algorithm
correlations	O
,	O
that	O
can	O
be	O
treated	O
systematically	O
,	O
e.g.	O
,	O
with	O
the	O
cluster-expansion	O
approach	O
.	O
</s>
