![]() ![]() ![]() When weak sources are being detected, there can be interference from the laboratory surroundings due to blackbody radiation. ![]() ![]() However, the power delivered by an arc lamp falls off precipitously at long wavelengths. 3 The earliest work typically employed arc lamp sources and bolometric detection. The history of far-IR spectroscopy in general goes back much further, and there are strong ties to the microwave and mm- wave communities. In this respect, THz spectroscopy provides a convenient method for determining the complex permittivity, even for studies that are not time-resolved. While there exist methods for determining the index of refraction when using conventional far-IR sources and detectors, it is noted that the great majority of the far-IR results reported in the literature present the frequency-dependent absorption coefficient, but not the refractive index. The amplitude and phase are directly related to the absorption coefficient and index of refraction of the sample, and thus the complex-valued permittivity of the sample is obtained without having to carry out a Kramers−Kronig analysis. An additional advantage of THz spectroscopy is that the transient electric field itself is measured, not simply its intensity, and this determines the amplitude and phase of each of the spectral components that make up the pulse. The majority of the work presented in this article exploits this time-resolved aspect of THz spectroscopy. In contrast, other sources of far-IR radiation such as arc lamps or globars are continuous, and pulsed sources such as free electron lasers or synchrotrons produce far-IR pulses with ∼3 ps duration or greater. The fact that THz pulses are created and detected using short-pulsed visible lasers with pulse widths ranging from ∼100 fs down to ∼10 fs adds a new element that is not present in conventional far-IR studies: It is now possible to carry out time-resolved far-IR studies with subpicosecond temporal resolution. ![]()
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