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Time-Domain Thermoreflectance is a method by which the thermal properties of a material can be measured, most importantly thermal conductivity. This method can be applied most notably to thin film materials (up to hundreds of nanometers thick), which have properties that vary greatly when compared to the same materials in bulk. The idea behind this technique is that once a material is heated up, the change in the reflectance of the surface can be utilized to derive the thermal properties. The reflectivity is measured with respect to time, and the data received can be matched to a model which contain coefficients that correspond to thermal properties. ==Experiment setup== The technique of this method is based on the monitoring of acoustic waves that are generated with a pulsed laser. Localized heating of a material will create a localized temperature increase, which induces thermal stress. This stress build in a localized region causes an acoustic strain pulse. At an interface, the pulse will be subjected to a transmittance/reflectance state, and the characteristics of the interface may be monitored with the reflected waves. A probe laser will detect the effects of the reflecting acoustic waves by sensing the piezo-optic effect. The amount of strain is related to the optical laser pulse as follows. Take the localized temperature increase due to the laser, where R is the sample reflectivity, Q is the optical pulse energy, C is the specific heat per unit volume, A is the optical spot area, ζ is the optical absorption length, and z is the distance into the sample (Ref A). This temperature increase results in a strain that can be estimated by multiplying it with the linear coefficient of thermal expansion of the film. Usually, a typical magnitude value of the acoustic pulse will be small, and for long propagation nonlinear effects could become important. But propagation of such short duration pulses will suffer acoustic attenuation if the temperature is not very low (Ref B). Thus, this method is most efficient with the utilization of surface acoustic waves, and studies on investigation of this method toward lateral structures are being conducted. To sense the piezo-optic effect of the reflected waves, fast monitoring is required due to the travel time of the acoustic wave and heat flow. Acoustic waves travel a few nanometers in a picosecond, where heat flows about a hundred nanometers in a second.〔G. Andrew Antonelli, Bernard Perrin, Brian C. Daly, and David G. Cahill, "Characterization of mechanical and thermal properties using ultrafast optical metrology", ''MRS Bulletin'', August 2006〕〔Scott Huxtable, David G. Cahill, Vincent Fauconnier, Jeffrey O. White, and Ji-Cheng Zhao, "Thermal conductivity imaging at micrometre-scale resolution for combinatorial studies of materials", Nature Materials 3 298-301 (2004)〕 Thus, lasers such as titanium sapphire (Ti:Al2O3) laser, with pulse width of ~200 fs, are used to monitor the characteristics of the interface. Other type of lasers include Yb:fiber, Yb:tungstate, Er:fiber, Nd:glass. Second-harmonic generation may be utilized to achieve frequency of double or higher. The output of the laser is split into pump and probe beams by a half-wave plate followed by a polarizing beam splitter leading to a cross-polarized pump and probe. The pump beam is modulated on the order of a few megahertz by an acousto-optic or electro-optic modulator and focused onto the sample with a lens. The probe is directed into an optical delay line. The probe beam is then focused with a lens onto the same spot on the sample as the probe. Both pump and probe have a spot size on the order of 10–50 μm. The reflected probe light is input to a high bandwidth photodetector. The output is fed into a lock-in amplifier whose reference signal has the same frequency used to modulate the pump. The voltage output from the lock-in will be proportional to ΔR. Recording this signal as the optical delay line is changed provides a measurement of ΔR as a function of optical probe-pulse time delay.〔David G. Cahill, Wayne K. Ford, Kenneth E. Goodson, Gerald D. Mahan, Arun Majudar, Humphrey J. Maris, Roberto Merlin, and Simon R. Phillpot. "Nanoscale thermal transport", ''J. Appl. Phys.'' 93, 793 (2003), 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Time-domain thermoreflectance」の詳細全文を読む スポンサード リンク
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