###### Abstract  
Over the last two decades, photomixer-based continuous wave systems developed into versatile and practical tools for terahertz (THz) spectroscopy. The high responsivity to the THz field amplitude of photomixer-based systems is predetermined by the homodyne detection principle that allows the system to have high sensitivity. Here, we show that the advantages of homodyne detection can be exploited with broadband power detectors combined with two photomixer sources. For this, we employ a THz detector based on a complementary metal-oxide-semiconductor field-effect transistor and a broadband bow-tie antenna (TeraFET). At 500 GHz and an effective noise bandwidth of 1 Hz, the response from one photomixer-based THz source resulted in an about 43 dB signal-to-noise ratio (SNR). We demonstrate that by employing a homodyne detection system by overlaying the radiation from two photomixers, the SNR can reach up to 70 dB at the same frequency with an integration time 100 ms. The improvement in SNR and the spectroscopic evidence for water vapor lines demonstrated up to 2.2 THz allow us to conclude that these detectors can be successfully used in practical continuous wave THz spectrometry systems.  
THz power detector; terahertz detector; broadband antenna; field-effect transistor; homodyne detection; terahertz spectroscopy +
Footnote †: journal: Medical  
Recently the development of photonic THz systems took a path that combines photoswitch-based generation and a fast power detector (zero-biased Schottky diode) [13]. This solution became attractive for applications that prefer fast response, and the spectral discrimination was not of primary interest. Whereas commercially offered systems employ zero-bias Schottky diode detectors, there were reports that demonstrated that TeraFETs could also be used in conjunction with time-domain photonic systems [14; 15]. Furthermore, CMOS TeraFETs have been reported to be effectively applied as detectors for photomixer-generated THz frequency combs [16].  
The advantages of homodyne detection can also be exploited by utilizing broadband power THz detectors and a photomixer-based THz generation scheme. In the beginning, Schottky diodes were demonstrated as candidates for mixing two THz waves [17]. Recently, a diode of an asymmetrically-shaped bow-tie geometrical form was used for homodyne-based imaging [18]. Reported state-of-the-art performance [19; 20; 21] values prove that field-effect transistors (FET) are becoming strong competitors to other established THz detection technologies. The technological handling, temperature stability, and large-area high-volume integration of FETs are, however, much easier to use, because unlike Schottky diodes, they are part of the mature standard processing technology of microelectronics, which is now projected for novel THz photonic use.  
Successful homodyne detection at a single frequency in the THz range with FET was demonstrated several times [22; 23; 24]. Recently, a homodyne-based phase-sensitive terahertz spectrometer concept that utilizes FET as a detector was proposed [25]. Here, we present experimental evidence supporting this theory. We employ a THz detector based on 90 nm Si CMOS FET and broadband bow-tie antenna (also called TeraFET).