Page 366 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 6 EVALUATION METHODS FOR PROPERTIES OF NANOSTRUCTURED BODY
AIST/NMIJ of JAPAN has developed ultra fast face heating/surface or front-face heating to detect a
laser flash methods which are evolution of the con- temperature change of the thin film surface where
ventional laser flash method and the picosecond laser metallic thin film deposited on transparent substrate
flash method [3,4]. As shown in Fig. 6.3.7, the ultra was heated from the transparent substrate side facing
fast laser flash method corresponds to the picosecond the heated region. Fig. 6.3.8 shows the block diagram
thermoreflectance measurement in geometry of rear of a measurement system. This geometry is essen-
tially equivalent to the laser flash method which is the
standard measurement method to measure thermal
diffusivity of bulk material. Thermal diffusivity of
κ f the film can be calculated with small uncertainty
from the thickness of a thin film and the heat diffu-
Probe pulse Pump pulse sion time across the thin film.
The temperature probe beam is focused to a spot
diameter of about 50 m at the specimen front face just
opposite to the heating light focus position. Reflected
light intensity of the temperature probe light is in pro-
portion to a change of specimen front-face tempera-
ture, and change of the reflected light intensity is
detected by photodiode. Temperature coefficient of
reflectivity of normal metal such as aluminum is as
5
Thin film Transparent substrate small as 10 /K. Since temperature rise of the speci-
x men front face after picosecond pulse heating is
0 d smaller than a few degrees, sinusoidal component cor-
responding to a temperature change is small in com-
Figure 6.3.7 parison with offset level. Such a small signal can be
Principle of rear face heating/front-face detection measured by lock-in detection at modulation frequency
picosecond thermoreflectance method. of heating light by an acoustic–optic modulator.

Picosecond
CW Argon-ion laser
Thermal Ti-Sapphire laser
diffusivity

Auto correlator

Lock-in Amp.
Optical delay line

PC

Polarizer
Reference signal
Specimen
Photo diode
Probe pulse

λ/2 plate
AO modulator
Pump pulse
Polarizer

Figure 6.3.8
Block diagram of the rear face heating/front-face detection picosecond thermoreflectance system.

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