Double-modulation reflection-type terahertz ellipsometer for measuring the thickness of a thin paint coating.

We constructed a double-modulation, reflection-type terahertz (THz) ellipsometer for precise measurement of the thickness of a paint film which is coated on a metal surface and which is not transparent to visible or mid-infrared light. The double-modulation technique enabled us to directly obtain two ellipsometric parameters, Δ(ω) and Ψ(ω), as a function of angular frequency, ω, with a single measurement while reducing flicker noise due to a pump laser. The bias voltage of a photoconductive antenna (PCA) used as a THz pulse emitter was modulated at 100 kHz, and a first lock-in amplifier (LA1) was connected to the output of an electro-optic (EO) signal-sampling unit. In addition, a wire-grid polarizer (WGP) was rotated at 100 Hz to conduct polarization modulation with a frequency of 200 Hz. The output signal from LA1 was fed into a second lock-in amplifier (LA2) that worked in synchronization with the rotating WGP (RWGP). By operating LA2 in a quadrature phase-detection mode, we were able to obtain in-phase and out-of-phase signals simultaneously, from which the two ellipsometric parameters for an isotropic sample could be derived at the same time while cancelling common-mode noise. The lower detection limit of the thickness measurement and the relative standard deviation (RSD) of a black paint film coated on an aluminum substrate were 4.3 µm and 1.4%, respectively. The possibility of determining all elements of the Jones matrix for an anisotropic material is also discussed.

[1]  C. Jagadish,et al.  Polarisation-sensitive terahertz detection by multicontact photoconductive receivers , 2005, cond-mat/0510076.

[2]  C. Morris,et al.  Polarization modulation time-domain terahertz polarimetry. , 2012, Optics express.

[3]  K. Vedam.  Spectroscopic ellipsometry: a historical overview , 1998 .

[4]  Masahiko Tani,et al.  Polarization state measurement of terahertz electromagnetic radiation by three-contact photoconductive antenna. , 2007, Optics express.

[5]  R. J. Bell,et al.  Optical properties of the metals Al, Co, Cu, Au, Fe, Pb, Ni, Pd, Pt, Ag, Ti, and W in the infrared and far infrared. , 1983, Applied optics.

[6]  Shinichi Watanabe,et al.  Precise real-time polarization measurement of terahertz electromagnetic waves by a spinning electro-optic sensor. , 2012, The Review of scientific instruments.

[7]  J. Munch,et al.  Double modulated differential THz-TDS for thin film dielectric characterization , 2002 .

[8]  Ryo Hattori,et al.  Polarization modulation of terahertz electromagnetic radiation by four-contact photoconductive antenna. , 2006, Optics express.

[9]  Keith S. Champlin,et al.  Microwave Conductivity of Silicon and Germanium , 1968 .

[10]  X. Zhang,et al.  Free‐space electro‐optic sampling of terahertz beams , 1995 .

[11]  T. Yasui,et al.  Terahertz paintmeter for noncontact monitoring of thickness and drying progress in paint film. , 2005, Applied optics.

[12]  M. Schubert,et al.  Hole diffusion profile in a p-p+ silicon homojunction determined by terahertz and midinfrared spectroscopic ellipsometry , 2009 .

[13]  Tino Hofmann,et al.  Terahertz magneto-optic generalized ellipsometry using synchrotron and blackbody radiation , 2006 .

[14]  D. S. Dunn,et al.  A method for measuring infrared reflection—Absorption spectra of molecules adsorbed on low-area surfaces at monolayer and submonolayer concentrations , 1981 .

[15]  Xiang Zhang,et al.  Two‐dimensional electro‐optic imaging of THz beams , 1996 .

[16]  T. Yasui,et al.  Absolute distance measurement of optically rough objects using asynchronous-optical-sampling terahertz impulse ranging. , 2010, Applied optics.

[17]  C. T. Ellis,et al.  Terahertz magneto-optical polarization modulation spectroscopy , 2012 .

[18]  T. Iwata,et al.  Prediction of the Thickness of a Thin Paint Film by Applying a Modified Partial-Least-Squares-1 Method to Data Obtained in Terahertz Reflectometry , 2013 .

[19]  Takeshi Yasui,et al.  Improvement of minimum paint film thickness for THz paint meters by multiple-regression analysis. , 2007, Applied optics.

[20]  Daniel F. Santavicca,et al.  Efficient measurement of broadband terahertz optical activity , 2012 .

[21]  Masanori Hangyo,et al.  Measurement of complex optical constants of a highly doped Si wafer using terahertz ellipsometry , 2001 .