laser surface thermal lensing (stl): an optical method for
TRANSCRIPT
Eastern Michigan UniversityDigitalCommons@EMU
Master's Theses and Doctoral Dissertations Master's Theses, and Doctoral Dissertations, andGraduate Capstone Projects
2008
Laser surface thermal lensing (STL): An opticalmethod for surface analysis of glass transitioneffects in transparent polymersNandita Prasad
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Recommended CitationPrasad, Nandita, "Laser surface thermal lensing (STL): An optical method for surface analysis of glass transition effects in transparentpolymers" (2008). Master's Theses and Doctoral Dissertations. 193.http://commons.emich.edu/theses/193
LASER SURFACE THERMAL LENSING (STL): AN OPTICAL METHOD FOR
SURFACE ANALYSIS OF GLASS TRANSITION EFFECTS IN TRANSPARENT
POLYMERS
By
Nandita Prasad
Submitted to the Department of Chemistry
Eastern Michigan University
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE
In
Chemistry
Thesis Committee:
Dr. Donald Snyder, Chair
Dr. Marshall Thomsen
Dr. Timothy Brewer
June 3rd, 2008
Ypsilanti, Michigan
ii
APPROVAL
LASER SURFACE THERMAL LENSING (STL): AN OPTICAL METHOD FOR
SURFACE ANALYSIS OF GLASS TRANSITION EFFECTS IN TRANSPARENT
POLYMERS
By
Nandita Prasad
Dr. Donald M. Snyder ________________________________ ______________Thesis Advisor Date
Dr. J. Marshall Thomsen ________________________________ ______________Committee Member Date
Dr. Timothy Brewer ________________________________ ______________Committee Member Date
Dr. Ross Nord ________________________________ ______________Department Head Date
Dr. Deborah deLaski-Smith ________________________________ ______________Interim Dean of the Graduate School Date
iii
DEDICATION
This thesis is dedicated to my entire family, who has offered me unconditional support and
love throughout the course of this thesis.
iv
ACKNOWLEDGEMENTS
I would like to thank my thesis advisor, Dr. Donald Snyder for his support,
encouragement, and guidance throughout the length of this project. I would also like to
thank Dr. Marshall Thomsen, who collaborated with Dr. Donald Snyder, for his support and
guidance whenever I needed it.
I also thank the Department of Chemistry for giving me the opportunity to pursue my
graduate studies. My heartfelt gratitude to Dr. Krish Rengan for his timely guidance and
support throughout the admission process and during my entire stay at Eastern Michigan
University. I also thank Dr. Kevin Kuehn (Biology Department) for the use of the
microscope in his laboratory and the National Science Foundation for its partial funding to
support the project. I am also thankful to all the committee members for taking the time to
read and edit my thesis. I’m very grateful to my family, especially my husband, Steve M.
Fernandes, for their guidance and moral support.
Last but not least I would like to thank my co-worker, Bushra Hussain, for her
assistance and cooperation in the initial set-up of the experiments.
v
ABSTRACT
Absorption of energy from a modulated laser beam by a transparent substrate leads to cyclic
thermal expansion and contraction of the surface at the pump beam focus. This minute
distortion of the surface forms a transient optical lens structure, which can be monitored by
the changes it induces in a second laser beam reflected at an angle off the same focal point.
The Surface Thermal Lensing (STL) technique is based on detecting the modulated
interference pattern that results from interaction of the probe beam wavefront as it reflects
from both the flat surface surrounding the transient lens and the curved lens surface itself.
Since a variety of physical properties for a polymer change sharply as the solid is heated
through the glass transition temperature, changes in the STL signal can be used to detect Tg
in spatially resolved surface regions as small as the focal point of the pump laser. This report
will examine the relationship between the magnitude of the STL signal and such instrument
parameters as pump beam intensity and chopping frequency, and the thermo-mechanical
properties of several transparent polymers.
vi
TABLE OF CONTENTS
DEDICATION ………………………………………………………………………… iii
ACKNOWLEDGEMENTS ……………………………………………………………. iv
ABSTRACT ……………………………………………………………………………. v
TABLE OF CONTENTS..……………………………………………………………. vi
LIST OF FIGURES ……………………………………………………………………. vii
CHAPTER 1: INTRODUCTION ………………………………………………………. 1
Detection techniques based on thermodynamic properties…..………………….….7
Photothermal Interferometry ……………………………………………………. 7
Photothermal Reflectance ………………………………………………………. 8
Photothermal Diffraction ……………………………………………………….. 9
Photothermal Deformation …………………………………………………….. 10
Transverse Photothermal Deflection ………………………………………….. 11
Collinear Thermal Lensing …………………………………………………….. 12
CHAPTER 2: SURFACE THERMAL LENSING…………………………………… 14
Glass Transition Temperature ……………………………………………………. 17
CHAPTER 3: EQUIPMENT SET-UP ………………………………………………… 19
CHAPTER 4: EXPERIMENTAL PROCEDURE ……………………………………... 22
CHAPTER 5: RESULTS AND DISCUSSION ………………………………………... 25
CHAPTER 6: CONCLUSION ………………………………………………………….. 37
REFERENCES ………………………………………………………………………… 39
vii
LIST OF FIGURES
Figure Page
1.1 Basic processes responsible for photothermal spectroscopy signal generation ………… 3
1.2 Thermal waves for characterizing thin films…………………………………………….. 4
1.3 Experimental set-up of photothermal interferometry……………………………………. 8
1.4 Schematic of photothermal reflectance………………………………………………….. 9
1.5 Experimental set-up of photothermal diffraction……………………………………… 10
1.6 Schematic of photothermal deformation…………………………………………………10
1.7 Schematic of the transverse photothermal technique…………………………………….11
1.8 Schematic of the collinear thermal lensing………………………………………………12
2.1 Schematic of STL signal…………………………………………………………………15
2.2 Schematic of STL signal generation……………………………………………………..16
2.3 Expected STL signal behavior at Tg with gradual increase in temperature…………… 18
3.1 Equipment set up for STL………………………………………………………………..19
4.1 Structure of polycarbonate……………………………………………………………… 24
4.2 Structure of Cyclic Olefin Polymer…………………………………………………… 24
5.1 Probe beam power calibration………………………………………………………… 25
5.2 STL signal as a function of time…………………………………………………………26
5.3 STL signal as a function of time…………………………………………………………27
5.4 Pictures of the damaged surface…………………………………………………………28
5.5 STL signal as a function of pump power………………………………………………...29
5.6 STL signal as a function of probe power………………………………………………...30
5.7 STL signal as a function of time…………………………………………………………31
viii
5.8 STL signal as a function of pump power for zeonex…………………………………….32
5.9 STL signal as a function of probe power………………………………………………...33
5.10 STL signal as a function of pump frequency…………………………………………...34
5.11 STL signal as a function of temperature………………………………………………..35
5.12 Phase as a function of temperature……………………………………………………..36
CHAPTER 1
Introduction
Photothermal effects in solid materials have been studied for a number of applications
since the 1880s, but the initial work in this field was limited by the weak responses of the
solid structures to the low intensity light sources then available. Tremendous advances have
been achieved in these studies since the development of lasers, which have better beam
quality and greater power densities. The applications of these studies are varied.
Photothermal spectroscopic methods have been applied in many scientific and technical
fields such as life sciences, material sciences, manufacturing, medical research,
environmental sciences, instrumentation for analysis, and non-destructive evaluation (1).
Photothermal spectroscopy is a set of highly sensitive analytical methods used to
monitor changes caused by absorption of radiation. Absorption of radiation causes heating in
the sample, which induces changes in temperature, pressure, and refractive index of the
sample. Local optical and thermal properties can be probed using this technique. The merit
of this technique is that it is a non-contact and non-destructive method. Photothermal
methods can be applied to a broad range of materials (gas, liquid, liquid crystal, and solid),
transparent or opaque. Samples of arbitrary shapes can be analyzed using this method in air
or vacuum (1-8).
Photothermal spectroscopy is classified as an indirect method for optical absorption
analysis where the effect of optical absorption on the sample is measured rather than directly
measuring the transmission of light used to excite the sample. Optical absorption leads to
sample heating, hence the photothermal spectroscopy signals are light absorption dependent.
Photothermal signals are not produced by scattering and reflection losses; rather
2
photothermal spectroscopy is used to measure optical absorption in scattering solutions, in
solids, and at interfaces, making this technique particularly attractive for application to
surface and solid absorption studies and studies in scattering media (9).
Photothermal spectroscopy usually utilizes laser light sources due to high spectral
purity, power, and spatial coherence. In the linear regime, the temperature change is
proportional to the optical power for an excitation of a sample with a given absorption
coefficient, and the photothermal spectroscopy signal is in turn proportional to the
temperature change. The magnitude of the signal depends on the power or energy of the
incident beam. High powers or pulse energies over very narrow optical bandwidths can be
delivered by lasers, thus enhancing the photothermal signals. The temperature change is also
inversely proportional to the volume over which the light is absorbed since heat capacity
scales with the amount of substance. The laser source can be focused to small diffraction
limited volumes, which enhance signal magnitudes. Thus photothermal spectroscopy can be
used in small volume sample analysis and allows for microscopic analysis of heterogeneous
materials (9).
Fig. 1.1 depicts the basic processes responsible for photothermal spectroscopy signal
generation. Excitation of a sample is accomplished by using optical radiation, usually from a
laser. The internal energy of the sample increases due to absorption of some of this radiation.
Two different modes of hydrodynamic relaxation lead to dispersion of the internal energy.
The increased internal energy results in a temperature change in the sample, which results in
a change in sample density and pressure. The density change results in a change of the
refractive index, which can be probed by a variety of methods (10).
3
Pressure change occurs if the photothermal induced temperature change occurs faster
than the time required for the material to expand or in a few cases contract, and this pressure
perturbation will disperse in an acoustic wave. A density change proportional to the
temperature will remain once the pressure has relaxed to its equilibrium value. The
absorption of optical energy induces a change in temperature in either case, which will in
turn result in a density change in the sample. Other properties of the sample are affected by
the combined temperature and density changes, and photothermal spectroscopy is based on a
measurement of these properties. In particular, the basis of sensitive photothermal methods is
the measurement of the refractive index change that occurs with changes in temperature and
density of the sample (10).
Time-dependent temperature fields in a sample and its surrounding media due to heat
conduction are obtained when a time-dependent heat source is active within a solid state
material. The changing temperature fields are referred to as thermal waves. Thermal
Optical Excitation
Absorption
Excited State Relaxation
Increased Thermal Energy
Pressure Change(Acoustic wave)
Temperature Change(Thermal Diffusion)
Temperature Change Photothermal SignalOptical probe
Fig. 1.1 Basic processes responsible for photothermal spectroscopy signal generation
4
diffusivity can be measured by using the thermal waves. The thermal diffusion equation
shown below describes the heat conduction process in a homogenous and uniform solid state
material (10).
2T = (1/ α) x (δT / δt) (1)
where α = k/ ρ c is the thermal diffusivity and
k = thermal conductivity
ρ = density
c = specific heat of the material
T = temperature
t = time
A special class of solutions to the above equation are one dimensional thermal waves
which occur when the heat source is periodic in time.
Fig. 1.2 Thermal waves for characterizing thin films
Air
Film
Substrate
Laser Beam Laser Beam
Low ω High ω
5
In Figure 1.2, a periodic heat source having an angular frequency, ω is focused at the sample
surface, and the thermal wave is expressed as:
T(x, t) = T0 e-x/μ cos ((2πx/ λ) – ωt) (2)
where μ = (2α/ ω) 1/2 is the thermal diffusion length and
λ = 2π μ is the thermal wavelength
x = thickness of film
T = temperature
T0 = initial temperature
t = time
From Equation 2 it is observed that the thermal wave is heavily damped and heavily
dispersed. The thermal diffusion length μ determines the decay rate of the thermal wave as it
penetrates into the sample, and according to Equation 2 it is seen that the amplitude of the
thermal wave decays exponentially as it propagates. The diffusion length is inversely
proportional to the square root of the frequency, and the penetration depth of the thermal
waves decreases as frequency increases. Short diffusion length is obtained at high
modulation frequency, thus enabling the thermal wave method to be used as a powerful tool
for study of localized properties of thin films at microscopic levels.
Thermal waves being highly dispersive, the propagation velocity υ depends strongly
on frequency as shown by the Equation 3.
6
υ = ω (λ /2π) = (2ω) ½ (3)
Accordingly, increasing the frequency will result in an increase in thermal wave velocity.
Thus the two features of thermal waves (heavily damped and heavily dispersed) make
them suitable for characterizing thin film coatings. Fig. 1.2 shows a sample with a single
layer and the thermal waves generated. The waves generated are a combination of
contributions from the films, the substrate, and the film substrate interface (10).
Different diffusion length and thermal wave velocities are obtained at different
frequencies. Depth profiling of the thin film system and separation of the contributions of
the different parts can be done as each part plays a significantly different role in the detected
signal.
There are several methods used to monitor the thermal state of the analytical sample
(12-17). Older methods include photoacoustic spectroscopy, photothermal calorimetry and
photothermal radiometry, which did not make use of lasers (9, 18, 19). Table 1 lists major
techniques with their applications.
7
Table 1
Detection techniques based on thermodynamic properties
The following presents a brief review of some of the above techniques based on refractive
index changes.
Photothermal Interferometry
Sample heating occurs due to optical absorption, which subsequently changes the
refractive index and thus induces a phase shift in light passing through the heated region.
This optical phase shift can be detected with an interferometer. A change in power at a
photoelectric detector is observed when the phase of monochromatic light passes through the
heated sample, relative to the phase passing through the reference arm. There are various
interferometry methods used to detect changes in the optical path length induced by the
photothermal effect, and these methods are classified as photothermal interferometry (9, 20).
Thermodynamic property Measured Property Detection Technique
Temperature Infrared emission Photothermal radiometry
Pressure Acoustic wave velocity Photoacoustic spectroscopy
Refractive index Photothermal Lens
Photothermal Interferometry
Photothermal Deflection
Photothermal Reflectance
Density
Surface Deformation Photothermal Diffraction
Photothermal Deformation
8
Measuring refractive index changes using optical interferometry while using an
excitation laser to heat the sample is a fairly new concept. Most photothermal interferometry
apparatuses are based on laser excitation sources, and both coherent and wide-band
incoherent sources could be used. Photothermal interferometry is a highly sensitive
technique.
Fig. 1.3 Experimental set-up of photothermal interferometry
Photothermal Reflectance
This technique is based upon generating a photothermal signal due to induced
modulation of the reflectivity of the illuminated material (10, 21). The modulated reflectance
of the probe beam is based on the temperature dependence of the sample’s optical
reflectivity. A fractional change in the reflectivity (∆R) of the probe beam occurs due to a
small local temperature excursion (∆T), as defined by Equation 4:
∆ R / R0 = (1/R0) x (δR/ δT) x (∆T) (4)
where R0 = the reflectivity at a reference temperature T0
Optical Excitation
Detector
Sample
Mirror
Probe Laser
Mirror
9
(1/R0) x (δR/ δT) = the temperature coefficient of reflectivity, which is a function of the
material properties of the sample, sample temperature, and wavelength of the probe beam.
As shown in Fig. 1.4, a reflected signal without any angular deflection is obtained
when a probe beam is focused to a spot that is directly in the center of the pump beam area.
Due to variation in reflectance with temperature, the intensity of the reflected probe beam
shows a periodic variation in phase with the periodic variation in the pump beam intensity.
Fig. 1.4 Schematic of photothermal reflectance
Photothermal Diffraction
Photothermal diffraction is based upon measuring signals due to volume phase
diffraction gratings formed by the photothermal effects. A periodic spatial refractive index
modulation results in a volume phase diffraction grating, which will diffract light at an angle
that meets the requirements from Bragg's Law. The amount of light diffracted is proportional
to the refractive index change, and the diffracted light is measured using a photoelectric
detector (9).
Pump Laser
Photodetector
Sample
Probe Laser
10
Fig. 1.5 Experimental set-up of photothermal diffraction
Photothermal Deformation
Photothermal deformation is a sensitive method in which the thermal waves are
generated by a modulated pump beam. As the slope of the transient thermal lens curvature
changes, the angle of the reflected beam also changes as the probe beam is reflected from the
surface of the lens at a point on the lens profile away from the pump beam. A quad cell
detector monitors the change in the deflection of the beam and records both the surface
normal and parallel components of the reflected beam. Lock in amplifiers collect these
signals and selectively amplify the ac component to a measurable strength (10, 22).
Fig. 1.6 Schematic of photothermal deformation
Probe Laser Sample Detector
Undiffracted probeIndex grating
Excitation Lasers
Pump LaserQuad-Cell Detector
Sample Surface
Probe Laser
11
Transverse Photothermal Deflection
Transverse photothermal deflection spectroscopy is based on the mirage effect. This
method is used to examine optical absorption at or near the surface of solid samples. When a
sample absorbs optical radiation, it results in the heating of the gas or liquid above the
surface. The heated fluid behaves like a prism and deflects the probe laser incident tangent
to the surface. Spatial gradients in refractive index change the direction in the propagation of
a ray of light, and the light exits a medium with a refractive index gradient at an angle
relative to the incident ray. This bending of light path is commonly called photothermal
deflection. A position-sensing detector monitors the deflection of the probe beam. This
technique is very easy to set up and produces highly sensitive measurements of surface
absorption. It is very versatile and can be used for solid, surface, liquid, and gas phase
analysis. Excitation sources can be either pulsed or chopped continuous (10, 23, 24).
Fig. 1.7 Schematic of the transverse photothermal technique
Probe Laser
Pump Laser Modulator
Detector
Lens
Lens
Mirror
Sample
12
Collinear Thermal Lensing
Fig. 1.8 shows the schematic of a collinear photothermal technique, which is an
alternative to the transverse photothermal deflection method. This method is employed to
detect the change of refractive index inside the sample due to the pump beam by its focusing
or defocusing of the collinear probe beam. Earlier work done using collinear thermal lensing
technique reported maximum sensitivity values as several parts per billion (ppb), and
standard photothermal deformation techniques have reported sensitivities of 0.1 ppm. The
advantages of this technique are that it is a straightforward method to model the signal and
offers high spatial and time resolution (10).
Fig. 1.8 Schematic of the collinear thermal lensing (3)
A beam deflection signal is not obtained if the pump and probe beams are aligned
collinearly in Fig. 1.8. The pump beam causes localized heating, and the refractive index
profile generated behaves as a lens. Depending on whether the derivative of the refractive
index with respect to temperature is positive or negative, the pump and the probe beams will
Probe Laser
Pump Laser Modulator
Detector
Lens
Lens
Sample
Filter
13
be focused or defocused. Small variations in the refractive index are sufficient to generate a
measurable thermal lens effect. Though the sensitivity of the thermal lens method is less
than the photothermal deflection method, this is a widely used technique for absorption
measurement in various materials.
14
CHAPTER 2
Surface Thermal Lensing (STL)
Highly localized heating of transparent materials is observed due to absorption of
even a very small fraction of an incident laser beam. This causes numerous physical changes
in the sample and surrounding media, which is subjected to secondary heating. Thermal
waves, which are modulated temperature fields, are created when the laser beam is
modulated and the heating effect is time dependent. Thermal conductivity, density, and
specific heat capacity of the sample control the magnitude, velocity of propagation, and
damping of the thermal waves (25).
Photothermal techniques have been useful for characterization of weak absorption,
measurement of thermal properties of solid samples, and analysis of local defects. One of the
primary disadvantages of using photothermal techniques for characterizing weak absorptions
is signal calibration requirements.
Characterization of the thermal properties of solids is important in studying their
structure over a wide range of scales. Measurement of thermal conductivity gives significant
information about defects and impurities, structural anisotropies, finite size effects and
effects of interfaces and grain boundaries. This is particularly important for optical materials
since local defects in solids may lead to laser-induced damage.
Surface thermal lensing is a technique that combines the sensitivity of photothermal
deformation methods and the simplicity of the thermal lensing technique. It is a powerful
tool for photothermal characterization of weakly absorbing optical thin films and has been
15
extensively used to study thin films only and bulk polymers have not been investigated until
recently. This method uses a probe beam that has a larger beam size than the pump beam.
Fig. 2.1 demonstrates the principle of STL in which the modulated pump beam is the heat
source (26-33).
Fig. 2.1 Schematic of STL signal
A Gaussian distribution of temperature is generated in the interaction volume, due to
absorption of the pump beam. A thermal lens is formed due to the competition between the
heat injected by absorption and thermal diffusion in the medium. The thermal lens alters the
propagation of the probe beam along the sample, and the resulting distortions in the spatial
profile of the transmitted beam are observed in the far field. Due to the periodic formation of
the surface lens, the reflected probe beam has an embedded diffraction pattern, which is
modulated in phase with the chopped beam.
A change in light intensity can be measured by placing a detector behind a pinhole so
that some portion of the circular beam profile is sampled. In regions where the modulated
Probe beam
Pump beam
Detector
16
diffraction pattern forms, the modulated change in light intensity will produce an AC signal.
Areas of the beam profile outside the diffraction pattern will simply give a steady DC signal
with no AC component. The profile of the diffraction pattern can be plotted by scanning a
detector in one or two dimensions, or an entire diffraction pattern in principle might be
obtained by a CCD camera or an array detector.
Generation of the STL signal is demonstrated as shown in Fig. 2.2.
Fig. 2.2 Schematic of STL signal generation
17
The reflected beam sampling is performed using a photodiode detector equipped with
a pinhole. With the probe beam OFF, there is no current in the photodiode detector, but with
it being ON and the pump beam OFF, steady DC current flows in the photodiode detector.
With only the pump beam being chopped and both the pump and probe beams ON, a
corresponding modulation occurs in the reflected probe beam, which has the form of a
diffraction pattern embedded in the Gaussian profile of the probe beam (34).
The modulation frequency of the pump beam chopper is fed into a lock-in amplifier
as a reference signal. Thus, only the modulated AC component of the photodiode signal that
matches the reference signal is amplified by the detector, ignoring the constant background
of the Gaussian profile. An STL signal is generated in the form of peaks by a line scan
across the reflected probe beam, and this signal corresponds to the areas of the beam that are
being modulated by the diffraction pattern.
Glass Transition Temperature
Glass transition temperature (Tg) is that temperature when a changeover in behavior from
that typical of a disordered solid to a liquid occurs with increase in temperature (and vice
versa with decrease in temperature). This change occurs in a narrow temperature range, and
the plexus of changes observed at Tg can be divided into two families; one set involves
measurements made in equilibrium, which include changes in macroscopic properties such as
thermal expansion and heat capacity, and in microscopic quantities such as temporal
variation of the mean squared displacement of individual atoms. The other set involves
response to externally imposed stimuli that causes a shift from a system’s equilibrium state,
which also include changes in macroscopic category such as viscosity and acoustic
18
attenuation, and microscopic quantities such as drag forces on an atom constrained to move
with a specified velocity (35).
As mentioned in the previous section, highly localized heating results in formation of
a thermal lens, which alters propagation of the probe beam (basis of the STL technique). The
utilization of this technique to determine Tg of solid polymers has been studied and reported
in this thesis. With gradual increase in temperature of the solid polymer, a sudden change in
STL signal intensity is expected at Tg.
Fig. 2.3: Expected STL signal behavior at Tg with gradual increase in temperature
0
5
10
0 25 50 75 100 125 150
Temperature (˚C)
ST
L a
mp
litu
de
(nA
)
Tg
19
CHAPTER 3
Equipment Set-up
This section provides an in-depth view of the experimental setup and the various
configurations of the instrumentation used. The schematic is as shown in Fig. 3.1.
Fig. 3.1 Equipment set up for STL
The pump laser is a Spectra Physics Millennia II Diode pumped solid state CW Nd-
YAG laser that provides greater than 2W of green 532 nm from a standard 110 or 220V
single phase outlet. It is focused at right angles to the sample surface and behaves as the
excitation laser. The probe laser is a low power 20mW CW He-Ne laser operating at 632 nm
in the visible range and is used to monitor the changes caused by the pump beam. The probe
He-Ne Laser
Sample holder
532 nm DPSS Laser
Shutter
CRT Monitor
Video Camera
ChopperLock in amplifier
Detector
STL signal
Thermocouple reader
+Z
+Y
+X
Motion Controller
Lens
Heater Control Attenuator
20
beam is incident at 16º to the sample. The attenuator is used to control the power of the
probe beam. It is placed in the path of the probe beam to control its power using a motorized
actuator attached to it. A linear actuator (not shown in the schematic) is connected to the
attenuator and controls the power of the beam.
The sample holder is capable of moving in the X, Y, and Z directions. It has heating
coils wrapped around the exterior with a thermocouple attached to the sample stage, which is
connected to the temperature controller for feedback control. Another thermocouple is
attached to the surface of the sample to measure the temperature at the irradiated spot. A
mechanical chopper (Stanford Research Systems SR540) modulates the pump beam intensity
with a chopping frequency ranging from 1 Hz to 4000 Hz and a modulation ratio of 50:50.
The pump beam is focused on 100 µm of the sample surface, and the probe beam on 200 µm
of the sample surface. The position of the probe beam spot can be adjusted by movable
mirrors and by using lenses to collimate the probe laser. The motion controller helps in
making a 1-D or 2-D scan of the sample surface. It is also used to scan the detector across
the reflected probe beam. A CCD camera (Model # 250677 [COHU] 6400 Series) is used
mainly to check if the probe and pump beam are centered on the same spot. An
epifluorescence microscope (Leica DMRB) coupled with a DEI-470 Video Camera System
was used to capture images of the sample. The reflected probe beam that diverges from the
sample passes through a lens to focus into a small spot at a pinhole photodetector that can be
scanned across the reflected probe beam. The detector acquires the STL signal and sends it
to a Stanford Research System SR830 Dual-Phase DSP lock-in amplifier, which measures
the modulated changes in the probe beam intensity as a function of chopping frequency. The
lock-in amplifier is connected to a computer that uses software developed by Andrew Zhao
21
in Visual Basic to acquire and analyze real time data. The software package has the
following basic functions:
1. It controls the shutter to determine if the pump laser passes through the shutter.
2. It sets and maintains the chopping frequency by controlling the modulator SR540.
3. It controls the motion controller to make one or two dimensional scanning for the
sample stage and detector.
4. It acquires real time data from the lock-in amplifier and analyzes the signal.
22
CHAPTER 4
Experimental Procedure
The experimental set-up is as shown in Fig. 3.1. In order to keep dust out, to reduce
noise levels, and to maintain a controlled environment, the entire set-up was enclosed in
plexiglass. A thick dark cloth was used to cover the enclosure in order to protect the user’s
eyes and to avoid stray radiation from entering.
The pump beam laser source was set in the range 0.2W-1.0W according to the
experimental need, and the beam was projected perpendicular to the surface of the sample at
a specified spot. The probe beam laser source was set to give approximately 4.5mW-16.5
mW by using an attenuator, and this beam was projected on the sample at an off-normal
angle. Using mirrors, the pump and the probe beams were aligned to coincide on the same
sample spot. The alignment was checked using a CCD camera that was connected to a
monitor outside the enclosure to avoid affecting the stability of the output signal due to
inaccurate alignment.
The position of the detector was optimized by chopping the probe beam with the
pump beam blocked. The motion controller was used to adjust the detector position to obtain
an optimized signal. The reference frequency was supplied by the probe beam chopper, and
the signal for the lock-in-amplifier came from the detector. Whenever the lock-in-amplifier
showed overload, it was set for higher input current values until steady value was reached.
Further, if overload was observed, the average of values on either side of the overload for
both x and y positions on the motion controller were taken.
The probe beam chopper was turned off and with the pump beam chopper turned on,
the pump beam chopper serves as the reference frequency for the lock-in-amplifier. The
23
reference frequency was set at 67 Hz for strong and stable signal values. The lock-in-
amplifier’s time constant was set to one second, and the computer acquired one data point per
second. The modulated pump beam was unblocked, and an actual irradiation time of 5
minutes was used. A localized deformation, i.e. a thermal bump, was produced, which flexed
with the same frequency as that of the chopped pump beam, and the incident probe beam was
reflected from the fluctuating thermal bump, which produced an oscillating diffraction
pattern with the same frequency as that of the thermal bump. Its maximum intensity signal
entered the pinhole detector, sending the signal to the lock-in-amplifier, which amplified the
AC part of the signal whose frequency matched with that of the reference frequency, and the
rest was filtered out. The phase shift between the input (chopped pump beam) and the output
(reflected probe beam with diffraction pattern embedded in it) and the intensity of the signal
were measured by the lock-in-amplifier. The lock-in-amplifier then sent the data to the
computer, which used software (PTM YC using Visual Basic 6.0) for data acquisition. The
data were displayed in the form of plots (signal amplitude versus time and phase versus
time).
Various experiments were carried out using solid samples of transparent polymers.
Two types of materials were used, polycarbonate and zeonex discs, which were obtained
from G-S Plastic Optics. Polycarbonates are a group of thermoplastic polyesters that are
easily worked, molded, and thermoformed. They have excellent temperature resistance,
impact resistance, and optical properties that enable their use in modern manufacturing,
between commodity plastics and engineering plastics. Polycarbonate (M.W of repeat unit =
254.3), is synthesized from bisphenol A and phosgene (carbonyl dichloride, COCl2) and has
the following structure:
24
Fig. 4.1: Structure of polycarbonate
Zeonex (Fig. 4.2) is an ultra high purity optical grade Cyclic Olefin Polymer (COP)
with excellent chemical resistance, thermal stability, and optical properties and is used in
optical applications where refractive index stability over high heat and humidity is crucial. It
is also widely used in medical and laser devices.
Fig. 4.2: Structure of Cyclic Olefin Polymer
Time dependence data were obtained for a total of 4 hrs in periods of 1.5, 1.5, and 1
hours to avoid overflow error in the computer. STL signal as a function of pump power,
probe power, and pump frequency were obtained. The pump power dependent runs were
carried out by varying the pump power from 0.2 W – 1.0 W in steps of 0.2 W and the
amplitude noted down immediately.
Finally the effect of temperature over STL signal was observed for zeonex. The solid
zeonex sample was placed in a sample holder, and the temperature was raised gradually from
room temperature to 140˚C. At the end of each experiment, before shutting down, care was
taken to reset the motion controller to 00.00 setting to avoid shifting of its origin to the value
at which it was turned off.
25
CHAPTER 5
Results and Discussion
Experiment 1: Probe beam power calibration
Since the 633 nm He-Ne CW-laser used as the probe beam was a fixed power unit, adjusting
the intensity of the probe beam was accomplished with a Newport Model 935 variable
attenuator. The sensing head of a laser power meter was placed in the path of the probe
beam after it passed through the adjustable beam attenuator in order to calibrate the probe
beam intensity as a function of the actuator position for the attenuator. After warm-up, the
actuator was set at specific values and the probe beam power at the sensor head recorded to
show the expected linear decrease in the power meter reading as attenuation was increased,
as shown in Fig. 5.1.
0
2
4
6
8
10
12
14
0 500 1000 1500 2000
actuator position
pow
er m
eter
rea
ding
(m
W)
Fig. 5.1 Probe beam power calibration
26
Experiment 2: STL signal as a function of time (Sample: Polycarbonate)
A solid polycarbonate sample of 4 mm thickness and 1” diameter was used. The pump
power was 1.0 W and the probe power set at 10 mW. The pump frequency was set at 67 Hz
and the experiment was carried out at a room temperature of 27ºC. The STL signal of the
sample was monitored as a function of time. Time dependence data were obtained for a total
of 4 hours to eliminate initial transient behavior, and the data were collected in periods of
1.5, 1.5, and 1.0 hours to avoid overflow error in the computer. Data were collected from
seven sample positions where the signal was fairly strong as the sample was not uniform
(Fig. 5.2).
0
10
20
30
40
50
0 5000 10000 15000
time (s)
STL
am
plit
ude
(nA
)
Position 1
Position 2
Position 3
Position 4
Position 5
Position 6
Position 7
Fig. 5.2 STL signal as a function of time
Similar signal curves were obtained for all of the seven spots as seen. At the end of
the four-hour run, each signal was leveling out to an approximately constant value.
The phase throughout all the runs was relatively constant. The initial and final
27
amplitudes acquired by the lock-in amplifier at each spot were different, and the
differences in the initial and final values for each spot could be due to non-uniformity
in the sample. At other spots, dissimilar curves were obtained as shown in Fig. 5.3.
0
5
10
15
20
25
0 1000 2000 3000 4000 5000 6000
time (s)
STL
am
plit
ude
(nA
)
Position 1
Position 2
Position 3
Fig. 5.3 STL signal as a function of time
During these runs, the phase changed monotonically and these runs were carried out
for just 1.5 hours. Careful inspection of the sample revealed noticeable damage on
the surface of the sample at the three spots where the anomalous curves were
obtained. Epifluorescence microscope studies revealed damage on the surface of the
sample as shown in Fig. 5.4.
28
Fig. 5.4 Pictures of the damaged surface
Crater-like pits and sharp-edged holes were observed on the surface with cracks
radiating out from the damaged area, which indicates non-uniformity in the samples. Further
experiments were carried out using a polycarbonate solid sample of thickness 2 mm and 2”
diameter as the above sample was rendered unusable.
29
Experiment 3: STL signal as a function of pump power (Sample: Polycarbonate)
A solid polycarbonate sample of thickness 2 mm and 2” diameter was used. A short test run
at 1.0 W pump power damaged the surface; hence a pump power dependence run was carried
out at a pump frequency of 67 Hz. The STL signal varied linearly with the pump power as
shown in Fig. 5.5. A fairly good signal was obtained at the smallest pump power of 0.2W.
0
5
10
15
20
25
30
35
0 0.2 0.4 0.6 0.8 1
pump power (W)
STL
am
plit
ude
(nA
)
Fig. 5.5 STL signal as a function of pump power
30
Experiment 4: STL signal as a function of probe power (Sample: Polycarbonate)
The effect of probe power on the STL signal was studied with the solid polycarbonate sample
of thickness 2 mm and 2” diameter. The pump power was set to 0.2W and the pump
frequency was 70 Hz. Although the correlation coefficient for the data series was somewhat
low, the relationship was approximately linear over the probe power measured. Fig. 5.6
shows the STL signal as a function of probe beam power.
0.60.650.7
0.750.8
0.850.9
0.951
1.051.1
7 8 9 10 11 12 13
probe power (mW)
ST
L a
mp
litu
de
(nA
)
Fig. 5.6 STL signal as a function of probe power
Under ideal conditions, we expect the linear fit to pass through the origin, but extrapolation
to zero probe power gave a Y-intercept at 0.3237 nA. As a fairly good signal was obtained
at the smallest pump power of 0.2W, the next set of time dependence runs were carried out at
pump power of 0.2W and probe power of 12.3 mW, as shown in Fig. 5.7. The pump
frequency was set to 70 Hz, and the experiments were carried out at room temperature.
31
0
1
2
3
4
5
6
7
0 5000 10000 15000
time (s)
STL
am
plit
ude
(nA
)
Position 1
Position 2
Position 3
Fig. 5.7 STL signal as a function of time
An exponential decay with time, as expected, was observed over a period of 4 hours. As
with the previous sample, the initial and final amplitude varied from spot to spot. The initial
amplitude observed was considerably less than that obtained for runs with 1.0 W pump
power. The experiment was repeated at a new spot with a pump power of 0.4 W. Damages
were noticed after 10 minutes of the run, and due to unavailability of sample surface for
further experiments, it was replaced with a Zeonex sample.
32
Experiment 5: STL signal as a function of Pump power (Sample: Zeonex)
The following preliminary experiments were carried out using the Zeonex sample to establish
working conditions with that sample.
The probe power was maintained at 16.5 mW and the pump chopping frequency at
200Hz. The pump power was increased from 0.2 W to 1.0 W in steps of 0.2 W, and the
corresponding maximum STL signal recorded. Then, the pump power was decreased from
1.0 W to 0.2 W in steps of 0.2 W and the amplitude recorded. The STL signal increased
(blue diamond)/ decreased (pink square) linearly with change in pump power as shown in the
Fig. 5.8.
0
1
2
3
4
5
6
0 0.2 0.4 0.6 0.8 1 1.2
pump power (W)
STL
am
plit
ude
(nA
)
Fig. 5.8 STL signal as a function of pump power for Zeonex
33
Experiment 6: STL signal as a function of probe power(Sample: Zeonex)
The pump power was set at 1.0 W and the pump chopping frequency at 200Hz. The probe
power was increased from 4.5 mW to 16.5 mW in steps of 2 mW, and the corresponding
maximum STL signal recorded. Then, the probe power was decreased from 16.5 mW to 4.5
mW in steps of 2 mW and the amplitude recorded. The STL signal increased (blue
diamonds)/ decreased (pink square) linearly with change in probe power as shown in
Fig. 5.9.
0
0.5
1
1.5
2
2.5
3
0 5 10 15 20
probe power (mW)
STL
am
plit
ude
(nA
)
Fig. 5.9 STL signal as a function of probe power
34
Experiment 7: STL signal as a function of pump frequency (Sample: Zeonex)
The pump modulation frequency is used as a reference for the lock-in amplifier. The pump
power was set to 1.0 W and the probe power at 16.5 mW. The pump chopping frequency
was varied from 67Hz to 400 Hz and the STL signal recorded (Fig. 5.10). The probe beam
was not chopped.
44.14.24.34.44.54.64.74.84.9
0 100 200 300 400 500
frequency (Hz)
STL
am
plit
ude
(nA
)
Fig. 5.10 STL signal as a function of pump frequency
The STL signal was stronger at low chopping frequency and decreases with increases in the
chopping frequency. Experimental working frequency is usually set at around 200-250 Hz
due to the presence of environmental noise at low frequency.
35
Experiment 8: Effect of temperature over STL signal (Sample: Zeonex)
The pump power was set to 1.0 W and the probe power at 16.5mW. The pump frequency
was set to 200Hz and the temperature was increased gradually. The solid Zeonex sample
was placed in a sample holder, and the temperature of the sample was gradually increased
from room temperature to 140 ˚C, and the corresponding STL signal recorded as shown in
Fig. 5.11
0123456789
10
0 20 40 60 80 100 120 140
temperature (˚C)
STL
am
plit
ude
(nA
)
Fig. 5.11 STL signal as a function of temperature
The STL signal was fairly constant with temperature over the range 20 ˚C to 115 ˚C. At
about 115 ˚C, the STL signal rose considerably and leveled out at higher temperatures as
shown in Fig. 5.11. The range over which the sharp change in the STL signal occurred
corresponds to the Tg of Zeonex, 118 ˚C.
36
A similar change in the STL phase was recorded. The phase was almost constant over the
temperature range of 20 ˚C to 115 ˚C as shown in Fig. 5.12. At about 115 ˚C, there was a
sudden decrease in phase, which later stabilized at higher temperatures as shown in Fig. 5.12.
0
20
40
60
80
100
0 50 100 150
temperature (˚C)
ST
L p
has
e (˚
)
Fig. 5.12 Phase as a function of temperature
37
CHAPTER 6
Conclusions
The STL technique, which is based on detecting the modulated interference pattern
resulting from the reflected probe beam off the curved surface thermal lens, has been utilized
to investigate the relationship between the STL signal amplitude and the properties of bulk
polycarbonate and Zeonex discs. The properties of the polymer samples, such as thermal
diffusivity, thermal expansion coefficient, and glass transition temperature, determine the
size of the thermal bump, but as yet there is no definitive mathematical model that can
consistently account for the changes that occur during the process of STL signal generation.
The amplitude decay over time seems to follow a general pattern regardless of the
type of polymer. Constant phase over time was observed for both the types of polymers
used. It was assumed that the amplitude remained constant after the time decay runs were
carried out, which suggests complete relaxation. This was verified through experiments such
as that shown in Fig. 5.8, demonstrating reproducibility after relaxation has occurred. As
expected the STL signal increased with pump and probe power. The maximum amplitude
obtained for each spot (at the same pump and probe power) at the start of any run was
different, which suggests the non-uniformity of the samples (Fig. 5.1, 5.2, and 5.7). Also, the
damage caused at a few spots, which limited the availability of the surface for further studies,
does suggest non-uniformity of the sample in terms of damage resistance.
Following the time decay runs, a number of frequency dependence runs were carried
out by B. Hussain (30) for the polycarbonate samples. It was observed that the STL signal
increased gradually with frequency, reached a peak and then decayed with increasing
frequency. Frequency dependence of the amplitude was studied at a single spot for the
38
Zeonex sample for comparison with the polycarbonate sample. For the Zeonex sample, the
amplitude decayed with increasing frequency from the start of the experiment. There was no
peak in the amplitude observed as in the polycarbonate sample. Although both polymers
were transparent to visible light and thus should have been non-absorbent at either of the
laser frequencies, the polycarbonate samples were distinctly more sensitive to the pump
beam, sustaining significant damage at relatively low powers. Whether this was an inherent
property of the polycarbonate or due to some manufacturing-related problem, such as surface
contamination by absorptive impurities or physical flaws, is not known. Further
investigation is necessary to determine the differential behavior of the two polymer samples.
Finally, a temperature dependent run was carried out with Zeonex, and a sudden
change in the STL signal (as expected in Fig. 2.3) was observed at the glass transition
temperature of the polymer, indicating that thermomechanical properties can be studied using
the STL technique. The number of runs carried out was limited due to the damage caused on
the sample surface and the number of samples available. Further work using different
polymer samples needs to be carried out, and intense experimentation is required to
determine the actual cause of the observed phenomenon.
39
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