Accurate in situ measurements of dielectric constants obtained in THz range
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Accurate in situ measurements of dielectric constants obtained in THz range
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ielectric constants give significant information on how materials behave and relax under the influence of electromagnetic fields. N. Matsumoto and T. Hosokura from the Murata Manufacturing Co. Ltd. and T. Nagashima and M. Hangyo from Osaka University have developed reflection-type time-domain terahertz spectrosopic ellipsometry (THz-TDSE). Reflection-type spectroscopy is a contactless technique used for high-absorption materials and thin films deposited on opaque substrates, and time-domain spectroscopy provides transmittance/reflectance as well as phase shift information. The accuracy of time-domain spectroscopy is limited by phase shift phenomena. The research-
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esearchers H.J. Cho of the University of Central Florida, J.H.Ahn of Sungkyunkwan University, and their colleagues have developed a method to fabricate graphene-on-organic film. This method stems out of the significant challenges in developing a compatible fabrication method of actuator materials for a large displacement and a rapid response at low voltages. Most work on actuators has focused on shape memory alloys, piezoelectric ceramics, and polymer-based materials; however, these materials require postprocessing steps that are not compatible with conventional batch microfabrication steps. The graphene-based organic film is compatible with microfabrication processes and used for electromechanically driven micro-actuators. As reported in the January 31st online edition of Nano Letters (DOI: 10.1021/ nl103618e), the researchers used a hybrid material of graphene/epoxy to
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MRS BULLETIN
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VOLUME 36 • APRIL 2011
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photoconductive antenna irradiated by the delayed probe laser beam. The photocurrent of the antenna is measured through a lock-in amplifier. The time domain waveforms of p- and s-polarized reflected waves are measured independently, and Fourier-transformed into complex reflection coefficients. Since the optical path lengths of the s- and p-polarized waves are strictly equal, the phase shift information between the p- and s-polarization can be obtained precisely. Reflection coefficients are used to compute real and imaginary parts of the dielectric constant. Interferences arising from the different interfaces were taken into account and this technique permitted measurement of the dielectric constant for GaAs thin film, as well as soft-mode phonon dispersion spectra in SrTiO3 thin films. Elsa Couderc
ers implemented THz-TDSE, eliminating previous reference and phase shift problems, thus allowing for accurate, contactless, and nondestructive in situ measurements of complex dielectric constants in the THz range. In the January 15th issue of Optics Letters (DOI: 10.1364/OL.36.000265; p. 265), the researchers report development of a setup that allows accurate measurements of the dielectric constants in the THz range of different materials, namely GaAs thin films and SrTiO3 on Pt substrate. In
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