Group delay in THz spectroscopy with ultra-wideband log-spiral antennae
| Autor: | Langenbach, M., Roggenbuck, A., Mayorga, I. Cámara, Deninger, A., Thirunavukkuarasu, K., Hemberger, J., Grüninger, M. |
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| Rok vydání: | 2014 |
| Předmět: | |
| Zdroj: | J. Infrared Milli. Terahz Waves 35, 918 (2014) |
| Druh dokumentu: | Working Paper |
| DOI: | 10.1007/s10762-014-0098-4 |
| Popis: | We report on the group delay observed in continuous-wave terahertz spectroscopy based on photomixing with phase-sensitive homodyne detection. We discuss the different contributions of the experimental setup to the phase difference \Delta\phi(\nu) between transmitter arm and receiver arm. A simple model based on three contributions yields a quantitative description of the overall behavior of \Delta\phi(\nu). Firstly, the optical path-length difference gives rise to a term linear in frequency. Secondly, the ultra-wideband log-spiral antennae effectively radiate and receive in a frequency-dependent active region, which in the most simple model is an annular area with a circumference equal to the wavelength. The corresponding term changes by roughly 6 pi between 100 GHz and 1 THz. The third contribution stems from the photomixer impedance. In contrast, the derivative (d\Delta\phi / d\nu) is dominated by the contribution of periodic modulations of \Delta\phi(\nu) caused by standing waves, e.g., in the photomixers' Si lenses. Furthermore, we discuss the Fourier-transformed spectra, which are equivalent to the waveform in a time-domain experiment. In the time domain, the group delay introduced by the log-spiral antennae gives rise to strongly chirped signals, in which low frequencies are delayed. Correcting for the contributions of antennae and photomixers yields sharp peaks or "pulses" and thus facilitates a time-domain-like analysis of our continuous-wave data. Comment: 7 pages, 7 figures |
| Databáze: | arXiv |
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