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We present a reference-free transmission spectroscopy of two kinds of aminophenol (Tyrosine and Phenylalanine) using
terahertz time domain spectroscopy (THz-TDS). The THz band, which refers to the spectral region between 0.1 to 10
THz, offers a plethora of fingerprints of many chemical and biological materials. Within the past few years, efforts have
been focused on exploiting the broadband nature of the THz time domain spectroscopy (THz-TDS) system for material
identification and characterization. The conventional spectroscopic sensing method involves measuring both the
terahertz signal carrying the sample information and a reference terahertz signal. In transmission geometry measurement,
the absorption peaks of the sample material are found by taking the logarithm of the power spectrum of the transmitted
signal beam divided by a reference power spectrum. In this work, we propose a reference-free approach to extract the
absorption feature in THz transmission spectroscopy. The samples are identified by their absorption peaks extracted from
the negative first-order derivative of the sample signal phase divided by the frequency. Unlike in conventional
transmission spectroscopy measurement, in this method, the amplitude spectrum of the terahertz signal is not considered
at all. Instead, the absorption features are extracted exclusively from the phase information by taking advantage of the
almost-linear phase spectrum of terahertz pulses and the correlation between dispersion and absorption. It is also noted
that the spectral phase of the terahertz pulse can be determined with far greater accuracy than the amplitude, which
makes this method even more favorable. We measured two kinds of aminophenol (Tyrosine and Phenylalanine), and
calculated the absorbance spectrum of each by both methods: taking the ratio between the power spectra of the sample
signal and the reference signal and the reference-free phase spectrum of each material. The agreement between the
positions of the absorption lines calculated from both methods indicates that the reference-free method is valid.
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