Broadband (0.3~11THz) reflection spectroscopy using terahertz air photonics
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1163-K02-01
Broadband (0.3~11THz) reflection spectroscopy using terahertz air photonics
Xiaoyu Guo, I-Chen Ho and X.-C. Zhang Center for Terahertz Research, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, U.S.A
ABSTRACT Pulsed THz wave spectroscopy using air as the THz wave emitter with the excitation of femtosecond laser provides intense (>100 kV/cm) and broadband THz waves (usable bandwidth from 0.3 to 11 THz). Using the air-biased-coherent-detection (ABCD) method, air can also coherently detect pulsed THz waves over a broadband spectrum. By utilizing these two technologies, we developed a prototype THz air photonics time-domain reflection spectrometer, and applied it on many materials in normal reflection geometry. Optical properties of CaCO3 crystals and several other samples in the THz range were studied. The system provided a signal-to-noise ratio (SNR) over 1000:1, with 0.1 cm-1 frequency resolution. The results acquired from both transmission and reflection measurements were then compared.
INTRODUCTION In the last decade, terahertz time-domain spectroscopy (THz-TDS) has become a powerful technique to study the properties of various materials from dielectrics to semiconductors [1] and has also been applied to precise gas sensing [2, 3]. THz-TDS operates with subpicosecond pulses of electromagnetic radiation spanning from tens of gigahertz to several terahertz in the frequency domain. Many groups have applied THz-TDS to explosive and other chemical characterization [4-7]. For real-world applications, reflection measurements are preferred since most bulky targets are impossible to measure in transmission mode, in which the targets would completely attenuate the incident THz waves. Additionally, various prospective THz imaging applications, especially THz reflection tomography [8], are based on the measurement of the complex reflectivity. To date, there have been many works published reporting the realization of THz-TDS in reflection mode [9-11]. However, traditional THz-TDS employing a photo-conductive antenna or electro-optic crystal technique provides a useful bandwidth of less than 5 THz, which is not sufficient to fill the entire
THz gap (0.3~10THz) [12, 13]. The limitations arise mainly from the carrier lifetime in the photoconductors and/or phonon absorption in the electro-optic crystals. A THz-ABCD spectrometer uses dry air (or selected gases), which works in a completely different way from the solid emitters and sensors. The only limitation of bandwidth in this air photonic system is the laser pulse duration. In this paper, we demonstrate that a prototype THz air photonics spectrometer operating with 100 fs amplified laser pulses provides 11 THz of usable THz bandwidth. The SNR is over 1000:1 with a spectral resolution of 0.1 cm-1.
EXPERIMENTAL DETAILS
Figure 1. Experimental setup. The THz wave is generated by mixing the fundamental pump beam and second harmonic (SH) (after a type-I beta BBO crystal) at the first air plasma point in front of parabolic mirror P1. A high resistivity silicon
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