"
Submitted to Applied Physics Letters
LBl-8749
Q
~"L
Preprint
POLARIZATION-SENSITIVE
COHERENT
ANTI-STOKES
RAMAN
SPECTROSCOPY
MAR
231979
LlBRARY
AND
OOCUME;NTS
SE'CTlON
Jean-Louis
Oudar,
Robert
W.
Smith,
and
Y.
R~
Shen
February
1979
Prepared for the
U.
S.
Department
of
Energy
under Contract
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~.
*
Submitted
to
Applied
Physics·Letters
LBL-8749
POLARIZATION-SENSITIVE
-COHERENT
~ANTI-:"STOKES-RAMAN-:
SPECTROSCOPY
,
*
Jean-Louis
Oudar,
Robert
W.
Smith,
and
Y. R.
Shen
Department
of
Physics,
University
of
California
Berkeley,
California
94720
and
Mat~rials
and
Molecular
Research
Division
Lawrence
Berkeley
Laboratory
Berkeley,
California
94720
FEBRUARY
1979
ABSTRACT
We
show
that
by
using
polarizat-ion
coherent
antiS
tokes
Raman
spectroscopy,
th.e
detection
sensitivity.
of
weak Raman
modes
is
greatly
enhanced.
The
spectra
of
the
real
part,
the
imaginary
part,
and
the
absolute
magnitude
of
the
resonant
nonlinear
susceptibility
can
be
separately
measured.
Raman
-4
modes
with
cross-sections
as
low
as
2 x
10
.times
that
of
-1
.
the
992
cm
mode
of
benzene
are
detectable
with
less
than
10
kW
peak_power
lasers.
On
leave
from
Centre
National
D'Etudes
des
Telecommunications,
Depart-
ment
RPM,
196,
rue
de
Paris,
92220
Bagneux,
France.
-2-
LBL-8749
A number
of
coherent
nonlinear
optical
spectroscopic
techniques
have
been
develo'ped
in
recent
years,1,2,
which
have
~lear
advantages
over
spon-
taneous
scattering
techniqu'es
when
high
resolution
is
required,
and/or
when
fluorescence
or
thermal
radiative
emission
is
strong.
However"
these
coherent
techniques
have
a
rather
limited
sensitivity
in
detecting
weak
resonan'ces,
due
to
the
strong
background
~fromthe
nonresonant
nonlinear
contribution.
The
major
source
of
noise
then
comes
from
fluctuations
in
the
power
or
the
mode
structure
of
the
input
lasers.
In
the
case
of
co-
herent'
antiStokes
Raman
spectroscopy
(CARS),
variations
of
the
basic
, 3 4
scheme
have
been
proposed
to
suppress
this
background,
,
but
they
make
use
of
a
third
input
beam
so
that
three
lasers
are
needed.
Recently,
Akhamanov
et
~1.5
have
shown
that
ellipsometric
measure-
ments
of
the
antiSt~kes
beam,in
a
CARS
experiment
can
yield
information
on
the
dispersion
of
the
nonlinear.
susceptibility
with
a
high
accuracy.
Such a
scheme
is
attractive
since
polarization
characteristics
are'inde-
pendent
of
the
laser
intensity
fluctuations.
However
itisa
tedious
pro-
cess
if
the
ellipsometric
measurement
is
to
be
performed
over
the
entire
spectrum,
as
was
done
in
Ref.
5.
In
this
letter,
we
show
that
with
simple
polarization
arrangement
in
CARS,
we
can
obtain
a
direct,
continuously
scanned
spectrum
which
elim-
inates
the
nonresonant
background
and
improves
cons~derably
the
detection
limit
of
weak
Raman
resonances.
In
addition,
by
admixing
the
resonant
contribution
with
a
controlled
(and
properly
phase-,shifted)
proportion
of
the
nonresonant
part,
we
can
obtain
spectra
which
are
linear
in
either
the
real
or
the
imaginary
part
of
the
resonant
susceptibility.
This,
of
6
course,
can
be
viewed
as
an
optical
heterodyne
detection
of
the
resonance"
