Submitted to Applied Optics
N
PHOTOTHERMAL
DEFLECTION
SPECTROSCOPY
AND
DETECTION
W.
Jackson,
Nabi1
M.
Amer,
A.C.
Boccara,
and
D.
Fournier
September
1980
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Photothermal
Deflection
Spectroscopy
and
Detection
w.
Jackson
and
Nabil
M.
Amer
Applied
Physics
&
Laser
Spectroscopy
Group
Lawrence
Berkeley
Laboratory
University
of
California
Berkeley,
California
94 720,
USA
A.
c.
Boccara
and
D.
Fournier
Laboratoire
d'Optique
Physique
- E.
R.
5 du
CNRS
Ecole
Superieure
de
Physique
et
de Chimie
Industrielles
10,
rue
Vauquelin
75231
Paris
Cedex
05,
France
I.
Introduction
It
is
well
known
that
upon
the
absorption
of
electromagnetic
radiation
by
a
given
medium. a
fraction
of
or
all
of
the
excitation
energy
will
be
converted
to
thermal
energy.
In
recent
years,
this
de=exci
tat
ion
mechanism
has
provided
the
physical
basis
for
a new
class
of
sensitive
photothermally
based
spectroscopies.
Among
the
better
known
examples
of
these
spectroscopies
are
interferometric
tech-
niques,
1
thermal
lensing
2 3 4
(TL}, •
photoacoustic
spec
troscopy(PAS),
and •
most
5-10
recently
•
photothermal
deflection
spectroscopy
(PDS). While
the
theoretical
foundation
of
interferometry
1
•
TL
2
•
3
and
PAs
11
-
16
are
fairly
well
understood,
this
is
not
the
case
for
PDS.
Even
though
the
concept
of
beam
deflection
by
thermally
induced
changes
in
the
index
of
refraction
has
been
known
for
a
long
time
•
17
to
Photothermal
Deflection
Spectroscopy
and
Detection
w.
Jackson
and
Nabil
M.
Amer
Applied
Physics
&
Laser
Spectroscopy
Group
Lawrence
Berkeley
Laboratory
University
of
California
Berkeley,
California
94720,
USA
A.
c.
Boccara
and
D.
Fournier
Laboratoire
d'Optique
Physique
- E.
R.
5 du
CNRS
Ecole
Superieure
de
Physique
et
de
Chimie
Industrielles
10,
rue
Vauquelin
75231
Paris
Cedex
OS.
France
ABSTRACT
The
theory
for
a
sensitive
spectroscopy
based
on
the
photother~
mal
deflection
of
a
laser
beam
is
developed.
We
consider
cw
and
pulsed
cases
of
both
transverse
and
collinear
photothermal
deflection
spectroscopy
for
solids,
liquids,
gases,
and
thin
films.
The
predic~
tions
of
the
theory
are
experimentally
verified.
its
implications
for
imaging
and
microscopy
are
given.
and
the
sources
of
noise
are
analyzed.
The
sensitivity
and
versatility
of
photothermal
deflection
spectroscopy
are
compared
with
thermal
lensing
and
photoacoustic
spectroscopy.
the
best
of
our
knowledge,
no
one
has
published
a
complete
systematic
theoretical
or
experimental
investigation
of
the
applicability
of
this
phenomenon
to
spectros·~
copy.
In
this
paper.
we
develop
and
experimentally
verify
a
general
theoretical
treatment
of
PDS.
Before
proceeding
with
the
theoretical
treatment
of
PDS,
a
brief
ical
description
of
PDS
is
in
order.
The
absorption
of
the
optically
exciting
beam
(purap
beam)
causes
a
corresponding
change
in
the
index
of
refraction
of
the
opti~
cally
heated
region.
The
absorption
also
causes
an
index-of~refraction
gradient
in
a
thin
layer
adjacent
to
the
sample
surface.
By
probing
the
gradient
of
the
varying
index
of
refraction
with
a
second
beam
(probe
beam)
•
one
can
relate
its
deflection
to
the
optical
absorption
of
the
sample.
This
is
in
contrast
with
probing
thermally
induced
changes
in
optical
path
lengths,
as
in
interferorJetr
ic
techniques
or
probing
the
curvature
of
the
index
of
refraction
as
in
TL.
As
can
be
seen
from
the
above
description,
one
has
two
choices
in
performing
PDS:
l)
col~
. 6 7 10
linear
photothermal
deflect1on
• '
where
the
gradient
of
the
index
of
refraction
is
both
created
and
probed
within
the
sample
(Fig.
l)
or
2)
transverse
photother~
5 8 9
mal
deflection
• •
where
the
probing
of
the
gradient
of
the
index
of
refraction
is
accomplished
in
the
thin
layer
adjacent
to
the
sample--an
approach
most
suited
for
opaque
samples
and
for
materials
of
poor
optical
quality
(Fig.
1).
We
have
already
demonstrated
the
high
sensitivity
of
PDS
for
measuring
in
small
5-8
absorptions
in
thin
films,
solids,
liquids,
and
gases
•
Its
potential
for
ing
and
scanning
microscopy
has
been
demonstrated
18
•
In
Sec.
II,
we
present
the
theory
of
PDS
for
the
cw
collinear
and
transverse
cases
•
and
the
pulsed
collinear
case.
In
Sec.
Ill
we
deal
with
experimental
con~
siderations.
The
experimental
results
are
compared
with
theoretical
predictions
in
Sec.
IV.
lJoise
and
background
analysis
are
described
in
Sec.
v.
and
in
Sec.
VI,
we
discuss
our
findings
and
compare
them
with
related
techniques.
Finally,
the
implications
of
our
calculations
for
imaging
and
microscopy
are
presented
in
Sec.