We apply a fast terahertz time-domain spectroscopy (TDS) system based on electronically controlled optical sampling (ECOPS) to contact-free thickness gauging. Our setup achieves a measurement speed of 1600 terahertz pulse traces per second, which—to our knowledge—represents the fastest thickness measurement performed with any terahertz system to-date. Using a silicon wafer as a test sample, we compare data of the ECOPS experiment to results obtained with a conventional terahertz TDS system and a mechanical micrometer gauge. We show that all systems provide consistent results within the measurement accuracy. Moreover, we perform thickness measurements of a rapidly moving sample and characterize the ECOPS setup with respect to time-domain dynamic range, signal-to-noise ratio, and spectral properties.
A theoretical model for the responsivity and noise-equivalent power (NEP) of photoconductive antennas (PCAs) as coherent, homodyne THz detectors is presented. The model is validated by comparison to experimental values obtained for two ErAs:InGaAs PCAs. The responsivity and NEP were obtained from the measured rectified current, the current noise floor in the PCAs, and the incoming THz power for the same conditions. Since the THz power measurements are performed with a pyroelectric detector calibrated by the National Metrology Institute of Germany (PTB), the experimentally obtained values are directly traceable to the International System of Units (SI) for the described conditions. The agreement between the presented model and the experimental results is excellent using only one fitting parameter. A very low NEP of 1.8 fW/Hz at 188.8 GHz is obtained at room temperature.
The identification of various substances by multivariate data analysis of terahertz transmittance spectra is demonstrated. Transmittance spectra were obtained by the use of a Fourier transform infrared spectrometer. By means of principal component analysis and partial least squares regression, the spectral data were analyzed in order to identify substances and mixtures of several substances. With only three principal components, detection and identification of substances are possible with high accuracy. Using these methods, concentration ratios of substances in mixtures of two substances can be determined with an accuracy of 10 %. It is shown that the method is robust against disturbances in the spectra such as standing waves. This is particularly important for practical applications.
Superlattice structures of In(Al)GaAs with localized ErAs trap centers feature excellent material properties for terahertz (THz) generation and detection. The carrier lifetime of these materials as emitter and receiver has been measured as 1.76 ps and 0.39 ps, respectively. Packaged photoconductors driven by a 1550 nm, 90 fs commercial Toptica “TeraFlash pro” system feature a 4.5 THz single shot bandwidth with more than 60 dB dynamic range. The emitted THz power of the ErAs:In(Al)GaAs emitter versus laser power has been recorded with a pyroelectric detector calibrated by the Physikalisch Technische Bundesanstalt (PTB). The maximum power was 164 µW at a laser power of 42 mW and a bias of 200 V.
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