Dipartimento SBAI, Università di ROMA “La Sapienza”, Via A. Scarpa 16, 00161 ROMA - ITALY PHOTOTHERMAL DEFLECTION TECHNIQUE Theory and applications: the experience at “La Sapienza” in Rome Roberto Li Voti • PHOTOTHERMAL TECHNIQUES • PRINCIPLE OF PHOTOTHERMAL DEFLECTION •THE HEAT DIFFUSION • MEASUREMENT OF THERMAL DIFFUSIVITY • OTHER APPLICATIONS Thanks to Photothermal at Roma La Sapienza Grigore L. LEAHU, Stefano PAOLONI, Concita SIBILIA, Mario BERTOLOTTI
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PHOTOTHERMAL DEFLECTION TECHNIQUEsabotin.ung.si/~isschool/2018Erice/RLV.pdfGas Vact sg Photoacoustic signal Effective cavity length …you can hear the light (Bell,1880) PHOTOTHERMAL
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Dipartimento SBAI, Università di ROMA “ La Sapienza”, Via A. Scarpa 16, 00161 ROMA - ITALY
PHOTOTHERMAL DEFLECTION TECHNIQUE
Theory and applications: the experience at “La Sapienza” in Rome
Roberto Li Voti
• PHOTOTHERMAL TECHNIQUES
• PRINCIPLE OF PHOTOTHERMAL DEFLECTION
•THE HEAT DIFFUSION
• MEASUREMENT OF THERMAL DIFFUSIVITY
• OTHER APPLICATIONS
Thanks to Photothermal at Roma La Sapienza
Grigore L. LEAHU, Stefano PAOLONI, Concita SIBILIA, Mario BERTOLOTTI
Tr transmission of the optics R detector sensitivity
S, PTR signal
PHOTOTHERMAL TECHNIQUESPhotoacoustic technique
( )dTVlRT
sdP
rg2
g2
π+
γ=
Pr ( )fDls ggg π= ,min
P acoustic pressureT temperatureγ specific heat ratioR radius of the celllg length of the cellVr residual volumesg effective lengthD gas thermal diffusivityf modulation frequency
THERMAL WAVE REFLECTION AND REFRACTIONfor plane waves
( ) ( ) ( )[ ] ( ) ( )[ ]
( ) ( ) ( )[ ]
=
+=
θ+θβ−
θ−θβ−θ+θβ−
zxsin2
zxsinzxsin1
222
111111
tAezxT
rAeAezxT
cos
'cos'cos
,~
,~
Medium 2Medium 1
z
x
∂∂=
∂∂
=
z
Tk
z
Tk
TT
22
11
21
~~
~~Temperature must be conserved at z=0
Normal heat flux must be conserved at z=0
( ) ( )
θ=θ
θ=θ
22
11
11
sinD
1sin
D
1
'
( ) ( )( ) ( )
( )( ) ( )2211
11
2211
2211
ee
e2t
ee
eer
θ+θθ=
θ+θθ−θ=
coscos
cos
coscos
coscos
( )211 DDarcsin=θ≤θ lim
ρ
2 2l
ζ
A
air
invar
− l /224 2l
heating line-2 2 l
“Thermal Snell law”
“Thermal Fresnel coefficients”
Numerical simulation of the temperature field at the Invar-Air interface. The diffusivities are DInvar=0.05 cm2/s,Dair=0.2 cm2/s. The incidence angle isθ1=20°<θlim =30°and the refracted angle is consequentlyθ2=43°.
Temperature field at the Invar- Air interface. The incidence angle isθ1=70°>θlim .
ρl212
6 2l
THERMAL WAVE REFRACTIONexperimental evidence
( )[ ]112 R θ+θ=θ arctg( )12z
x tgR θ−θ=ΦΦ= ~
~
0 0.2 0.4 0.6 0.8
1
0.5
0
10°
Incidence angle, sin ( )θ
20° 30° 40° 50° 60°
1R
efra
ctio
n a
ngle
, si
n (θ 2)
Linear scale in θ Linear scale in sin(θ )
airInP DD
0 20 40
50
40
30
20
10
0
-1010 30 50
Incidence angle, deg
1
1.7
2.2
5
Re
fra
ctio
n a
ngle
, de
g
DInP=0.44 cm2/s
Dinvar=0.04 cm2/s
( ) ( )22
11
sinD
1sin
D
1 θ=θ
Snell law
THERMAL WAVE INTERFEROMETRY
BASIC PRINCIPLE
To generateplane thermal wavesof a given frequency at the front surface of the sample byheating it periodically with a pump laser beam.
The wavespropagate inside the structure and, if they approach a buried layer withdifferent thermal properties, they are partiallyreflectedgiving rise, together with theincident waves, to an interference effect at the front surface.
DETECTION
APPLICATIONS
Photoacoustic
Radiometry
Photothermal Deflection techniques
Nondestructive evaluation of the thermophysical properties and
the thickness of layered samples
INTERNAL TEMPERATURE RISE
MATERIAL/BULK INTERFACE
BOUNDARY CONDITIONS AT THE SURFACE (z=0)
I; Power intensity
Temperature
heat flux
Thermal Resonator
Forward thermal wave
Backward thermal wave
Pump laser beam1 D illumination
L (Cavity Length)
Buried layer
at Front surfaceTemperature rise
1° Layer
THERMAL WAVE INTERFEROMETRY
zz BeAezT ββ− +=)(
bulkm
bulkm
ee
eeR
+−=
L
L
Ae
Be
waveincidence
wavereflectedR β−
β==
( )( )BAk
dz
dTkI
BA0T
−β=−=
+=
LeARB ⋅−⋅⋅= β2
( )LeAkdz
dTkI ⋅−−=−= ββ 21
( )Lk
IA ⋅−−
= ββ 2Re1
1
-0.9
+0.9
TEMPERATURE RISEAT THE SURFACE
PHASE SIGNAL ( ) ( )
−
π−=ϕ
π−
π−
DfL42
DfL2
eR1
eDfL2Rsin2arcf tan
( )( )
( )
−
+=
+−
+−
DfLj
DfLj
air
eR
k
IoT
/12
/12
Re1
1π
π
β
THERMAL WAVE INTERFEROMETRY
( )2/ DfL π
bulkm
bulkm
ee
eeR
+−=
-0.9
+0.9
phase
-0.9
+0.9
( )2/ DfL π
amplitude
0 0.4 0.8 1.2 1.6 2 2.4
40
30
20
10
0
-10
-20
-30
-40
Depth normalized to the thermal diffusion length
R=0.9
R= -0.9
R=0.6
R= -0.6
R= -0.3
R=0.3
R=0
Phase shift (degree)
TEMPERATURE RISEAT THE SURFACE
PHASE SIGNAL
Note that the interference effect(oscillation) is seen when the cavitylength is of the same order of thethermal diffusion length of theresonator and when the coefficient Ris large enough.
The maximum oscillation is of 45°degrees and is obtained in case ofperfect reflection from the buriedlayer with R=±1
( ) ( )
−
π−=ϕ
π−
π−
DfL42
DfL2
eR1
eDfL2Rsin2arcf tan
( )( )
( )
−
+=
+−
+−
DfLj
DfLj
air
eR
k
IoT
/12
/12
Re1
1π
π
β
DfL /π
THERMAL WAVE INTERFEROMETRY
Opaque coating
Bulk
Air
Vertical Deflection
Illuminated area
Probe beam
Pump beam
z
L
THERMAL WAVE INTERFEROMETRYDiffusivity measurement by Mirage
( )( )[ ]( )[ ]
+−−+−+⋅
ω+=
c
c
aircsurf LjRR
LjR
jee
IT
l
l
12exp1
12exp1ˆ21
2 11 ≈+−=
airc
airc
ee
eeR
bc
bc
ee
eeR
+−=2
Inox L=200µm
D=0.04, 0.046 or 0.06 cm2/s
( )
⋅−⋅⋅−=ϕ−ϕ=ϕ∆ −
−
c
c
L
Lc
refeR
eLsinRarc
l
ll
422
22
1
22tan
Opaque coating
Bulk
Air
Vertical Deflection
Illuminated area
Probe beam
Pump beam
z
L
THERMAL WAVE INTERFEROMETRYDiffusivity measurement by Mirage
( )( )[ ]( )[ ]
+−−+−+⋅
ω+=
c
c
aircsurf LjRR
LjR
jee
IT
l
l
12exp1
12exp1ˆ21
2 11 ≈+−=
airc
airc
ee
eeR
bc
bc
ee
eeR
+−=2
Inox L=200µm
D=0.046 cm2/s
( )
⋅−⋅⋅−=ϕ−ϕ=ϕ∆ −
−
c
c
L
Lc
refeR
eLsinRarc
l
ll
422
22
1
22tan
Ph
ase
con
tra
st,
deg.
Spot size
-40
-30
-20
-10
0
0 2.5 5.0 7.5 10.0 12.5
(1)
(2)
(3)
Spot size = 1mm, 2.3mm, 5mm
Opaque coating
Bulk
Air
Vertical Deflection
Illuminated area
Probe beam
Pump beam
z
L
THERMAL WAVE INTERFEROMETRYDiffusivity measurement by Mirage
( )( )[ ]( )[ ]
+−−+−+⋅
ω+=
c
c
aircsurf LjRR
LjR
jee
IT
l
l
12exp1
12exp1ˆ21
2 11 ≈+−=
airc
airc
ee
eeR
bc
bc
ee
eeR
+−=2
Inox L=200µm
D=0.046 cm2/s
( )
⋅−⋅⋅−=ϕ−ϕ=ϕ∆ −
−
c
c
L
Lc
refeR
eLsinRarc
l
ll
422
22
1
22tan
Spot size = 2.3 mm
0
0.5
1.0
0.02 0.03 0.04 0.05 0.06 0.07
Sta
nda
rd d
evia
tion
, deg
.
Thermal diffusivity, cm2/s
Spot size, mm1 1.5 2 2.5 3 3.5
(1)
(2)
Opaque coating
Bulk
Air
Vertical Deflection
Illuminated area
Probe beam
Pump beam
z
L
THERMAL WAVE INTERFEROMETRYDiffusivity measurement by Mirage – Reflectivity
bc
bc
ee
eeR
+−=2
Inox L=200µm
D=0.046 cm2/sSpot size = 2.3 mm
-2.0
-1.5
-1.0
-0.5
0
1 2 3 4 5
Frequency square root, Hz1/2
Ref
lect
ivity
( )( ) ( )
( ) ( ) ( ) ⇒
π+−⋅=
⋅+⋅
⋅−⋅=Γ
∞→
∞→
csurfp
surf
surfp
surf
surfD
fLjR
pTpfTf
pTpfTf
f 12expˆlimˆ
ˆlimˆ
2
( ) [ ]( ) csurf
csurf
DfL
RDfL
π−=Γ
+π−=Γ
⇒2arg
ln2ln 2
•Thermal diffusivity and effusivity measurements
•Absorption spectroscopy
•Effusivity and optical absorption depth profiling
•Measurement of the attenuation in optical waveguides
•Evaluation of the thickness of thin layers
•Trace gas analysis
•Evaluation of the photoelastic constants
Main applications
Università degli Studi di Roma “La Sapienza” - Dipartimento di Energetica - Via A Scarpa 14 - 00161 - RomaPer informazioni Prof. M.Bertolotti Tel. 06.49916542 - Email: [email protected]
IR PDS device for trace gas analysis
Ge, Brewster angle
Gas chamber
He-Ne probe
CO2 pump
Position Sensor
Hot zone
Cold zone
Cold zone
Probe He-Ne
Undeflected beam
Deflected beam
Trace Gas Analysis – Infrared Photothermal Deflection Spectroscopy
( ) ( ) ( ) LAew
y
wc
eP
dT
dn
ny wy
L
απωρ
α
≅⋅
−
−=Φ −−
2
22
112
r
Collinear configuration
Photothermal deflection angle
yPump CO2 w
L
0
1
2
3
4
9.0 9.5 10.0 10.5
Lungezza d'onda, µm
Seg
nale
di d
efle
ssio
ne, a
.u.
Spettro di assorbimento CO2
Experimental results – Test on CO 2
9R
9P
10R 10P
0
0.005
0.010
0.015
0.020
0.025
9.5 10.0
exp.
Lunghezza d'onda, µm
Seg
nale
di d
efle
ssio
ne, V
/W
Spettro di assorbimento di una miscella 50 ppm C2H
4
Experimental Results – Test on C 2H4
10P(14) di C2H4
0
0.005
0.010
0.015
0.020
0.025
9.0 9.5 10.0 10.5
Etilenecarbossimetilcellulosa
Lunghezza d'onda, µm
Seg
nale
di d
efle
ssio
ne, V
/W
Experimental results on carbossimetilcellulosa
Carbossimetilcellulosa
Temperature of the treatment Concentration of the emitted ethylene
450°C 7.07 ppm
480°C 46.8 ppm
PLANE THERMAL WAVE RESONATOR
Forward wave
Backward wave
Pump laser beam
1 D illumination
L (Cavity Length)
2° thermal mirror
Absorbing layer1° thermal mirror (glass)
Probe beam
h
Experimental setup
0.1 0.3 0.5 0.7 0.9 1.1 1.3
Cavity length (mm)
1
0.5
0
-0.5
Photothermal deflection signal
Phase
Amplitude logarithm
frequency 36 Hz
THERMAL WAVE RESONATORExperimental evidence in air