Convergence of Terahertz Sciences in Biomedical Systems
Recent technological breakthrough in the field of Terahertz radiation has triggered new applications in biology and biomedicine. Particularly, biological applications are based on the specific spectroscopic fingerprints of biological matter in this spectr
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Abstract We review the recent progress in the terahertz (THz) apertureless near-field microscopes. We demonstrate quantitative analysis and measurements of the THz near-fields interactions in the probe-sample system. We also present a self-consistent line dipole image method for the quantitative analysis of the nearfield interaction. The measurements of approach curves and relative contrasts on gold and silicon substrates were in excellent agreement with calculations based on the self-consistent line dipole image method.
1 Introduction Nanoscale near-field imaging in the terahertz (THz) spectral range provides a powerful means for studying intriguing phenomena such as intermolecular vibrational spectroscopy and dynamic charge transport in a variety of quantumconfined nanostructures. Conventional THz time-domain spectroscopy (TDS) can provide macroscopic imaging averaged over an ensemble of such nanostructures, which inevitably suffers from inhomogeneous spectral broadening. Moreover, its spatial resolution is limited to *k/2 by diffraction. Therefore, several types of THz pulse scanning near-field optical microscopes (SNOMs) have been developed to achieve sub-wavelength resolutions [1–9]. In contrast to visible or IR SNOMs based on frequency-domain spectroscopy systems, most THz SNOMs have been based on THz pulse TDS systems [1–8], K. Moon M. Lim Y. Do H. Han (&) Department of Electrical and Computer Engineering, National Laboratory for Nano-THz Photonics, POSTECH, Pohang, Gyeongbuk 790-784, Korea e-mail: [email protected]
G.-S. Park et al. (eds.), Convergence of Terahertz Sciences in Biomedical Systems, DOI: 10.1007/978-94-007-3965-9_13, Ó Springer Science+Business Media Dordrecht 2012
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making it possible to perform broadband coherent THz spectroscopy. Among the THz SNOM systems, the scattering-type SNOM (s-SNOM) has been the most successful technique so far in terms of spatial resolution and image quality [6–13]. In the THz s-SNOM, the scattered field from the tip apex is measured in the farfield region. Sub-micrometer resolutions are enabled by the strongly localized near-field around the probe tip [14]. Thus, it is essential to understand the nearfield interaction in the tip-substrate system, and there have been several analytic models [15–18] and also numerical simulations [19, 20] to solve the problems. The most popular approach has been the point dipole image method (PDIM) [8–12] where the probe tip is replaced by a polarizable point dipole [10]. The PDIM has been widely used to analyze experimental data [8–12], and has provided qualitative understanding on s-SNOMs, including resolution [10] and optical phase contrast [11]. However, because the electromagnetic boundary conditions are not fully matched on the surface of the probe sphere, the PDIM becomes incorrect as the probe sphere approaches the substrate [15–18]. In this part, we analyze the THz near-field scattering signals of a THz pulse s-SNOM by using a self-consistent line dipole image method (LDIM), closely follow
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