Experimental technique

1
High Resolution Thermal Imaging Technique Ø.
Haugen, T. H. Johansen and S. Niratisairak AMCS
group, Dept. of Physics, University of Oslo,
Norway
Introduction A visual map of the temperature
at the sample's surface can be obtained using the
thermal imaging based on fluorescent properties
of a thin polymer film 1. When the film is
illuminated with ultraviolet (UV) light (l 250
and 340 nm), it emits visible light (l 612 nm)
2 with intensity which is strongly
temperature-dependent 3. The technique can
work at low temperatures, down to 4K, where the
standard thermal imaging using infra-red
detectors is not applicable. The spatial
resolution of our thermo-optical imaging system
is 1 mm, while the temperature resolution is 0.1
K. The imaging has been applied to visualize hot
spots in superconducting circuits and faults of
the metal grid in solar cells.
Experimental technique The fluorescent thin
polymer film is deposited on the surface of
sample by the spin-coating technique. The film is
a solution of a mixture between rare-earth
compounds (e.g. EuTTA or EuTFC) and polymer
matrix (i.e. PMMA) in Acetone. The choice of
rare-earth compound depends on the temperature
range of examination. The UV-light illuminates
the film. This excites the rare-earth chelate
compound and leads to their narrow-band
fluorescence. However, a non-radiant relaxation
can happen too when the rare-earth compound is
thermally excited. Thus, the warmer area in the
excited film exhibits lower intensity of
fluorescence than the colder. The fluorescent
light passes through the objective lens in
microscope, which has a narrow-pass-filter, to
the CCD camera, and then to a computer. The
image contains both the optical contrast and
thermal information. By acquiring the images both
before and after heating the sample, we can get
rid of the optical contrast and focus only on the
thermal information.
Experimental setup The studied sample is
coated with the polymer film and mounted on the
cold finger in Janis ST-500 continuous flow
cryostat. It is cooled by liquid Helium. The
UV-source is 200 W mercury-xenon in a Hamamatsu
LC6 lamp-house. The UV-beam excites the polymer
film and the film emits a bright and a
narrow-band fluorescent light. This fluorescent
light passes through to the microscope Leica DMR,
which has a narrow-pass-filter at l 61010
nm., in its column. The CCD camera (Qimage Retiga
EXI) is used to acquire the image from the
microscope. The camera has 1360x1036 pixels and
12-bit digital output. Then, the digital image is
sent to PC for the post-processing using Matlab
program. Figure 2. Experimental
setup. The UV-light is conducted by the
fibre-optics from the housing lamp.

• Spatial resolution 1 mm
• Thermal resolution 0.1 K
• Working temperature down to 4 K

Figure 1. The excited film emits brighter
fluorescent light in the colder region
Results Three samples having different
structure were examined. Two of them are
bridge-structures made from superconductor YBCO,
which are shown in figures 3 and 4, and another
sample is a copper bridge, see figure 5. We
biased the electric current to those bridges and
the temperature distribution produced by Joule
heat has been visualized.
Figure 3 Optical and thermal
images of a fragmentary superconduucting YBCO
bridge. The transport current is 100 mA. The
background temperature is 84 K and the warmest
area is 13 K above the background.
Figure 4 The darker area is
YBCO and brighter is SrTiO3 (not superconductor).
The smallest width of YBCO is 9 mm. The biasing
current is 10 mA. and background temperature is
80K.

References 1 P. Kolodner et al, Microscopic
fluorescent imaging of surface temperature
profiles with 0.01 ºC resolution, Appl. Phys.
Lett. 40, 782, 1982 2 G. A. Crosby et al,
Intramolecular energy transfer in rare earth
chelates role of triplet states, J. Chem. Phys.
34, 743, 1961 3 M. L. Bhaumik, Quenching and
temperature dependence of fluorescence in
rare-earth chelates, J. Chem. Phys. 40, 3711,
1964 4 J. Pantoflícek et al, The luminescence
properties of SmD and EuTTA, Czech. J. Phys. B.
18, 1610, 1968 5 V. Y. Venchikov et al,
Thin-film luminescent UV radiation converters
based on Europium chelates, J. Appl. Spec. 68,
1036, 2001