We investigate photoconductive terahertz (THz) emitters compatible with 1550 nm excitation for THz time-domain spectroscopy (TDS). The emitters are based on rhodium (Rh) doped InGaAs grown by molecular beam epitaxy. InGaAs:Rh exhibits a unique combination of ultrashort trapping time, high electron mobility, and high resistivity. THz emitters made of InGaAs:Rh feature an emitted THz power of 637 μW at 28 mW optical power and 60 kV/cm electrical bias field. In particular for a fiber coupled photoconductive emitter, this is an outstanding result. When these emitters are combined with InGaAs:Rh based receivers in a THz TDS system, 6.5 THz bandwidth and a record peak dynamic range of 111 dB can be achieved for a measurement time of 120 s.

Broadband terahertz spectroscopy enables many promising applications in science and industry alike. However, the complexity of existing terahertz systems has as yet prevented the breakthrough of this technology. In particular, established terahertz time-domain spectroscopy (TDS) schemes rely on complex femtosecond lasers and optical delay lines. Here, we present a method for optoelectronic, frequency-modulated continuous-wave (FMCW) terahertz sensing, which is a powerful tool for broadband spectroscopy and industrial non-destructive testing. In our method, a frequency-swept optical beat signal generates the terahertz field, which is then coherently detected by photomixing, employing a time-delayed copy of the same beat signal. Consequently, the receiver current is inherently phase-modulated without additional modulator. Owing to this technique, our broadband terahertz spectrometer performs (200 Hz measurement rate, or 4 THz bandwidth and 117 dB peak dynamic range with averaging) comparably to state-of-the-art terahertz-TDS systems, yet with significantly reduced complexity. Thickness measurements of multilayer dielectric samples with layer-thicknesses down to 23 µm show its potential for real-world applications. Within only 0.2 s measurement time, an uncertainty of less than 2 % is achieved, the highest accuracy reported with continuous-wave terahertz spectroscopy. Hence, the optoelectronic FMCW approach paves the way towards broadband and compact terahertz spectrometers that combine fiber optics and photonic integration technologies.

In future wireless communication networks at terahertz frequencies, the directivity and the beam profile of the emitters are highly relevant since no additional beam forming optics can be placed in free-space between the emitter and receiver. We investigated the radiation pattern and the polarization of broadband continuous-wave (cw) terahertz emitters experimentally and by numerical simulations between 100 GHz and 500 GHz. The emitters are indium phosphide (InP) photodiodes with attached planar antenna, mounted on a hyper-hemispherical silicon lens and integrated into a fiber-pigtailed module. As both packaging and material of the emitter was identical for all devices, similarities and differences can be directly linked to the antenna structure. We found that the feeding point structure that connects photodiode and antenna has a large influence on the radiation pattern. By optimizing the feeding point, we could reduce side lobes from −2 dB to −13 dB and narrow the 6dB beam angle from ±14° to ±9° at 300 GHz.

We present photomixers made of iron doped indium gallium arsenide (InGaAs:Fe) as broadband receivers in optoelectronic continuous wave (cw) terahertz (THz) systems. InGaAs:Fe shows higher resistivity and shorter carrier lifetimes compared to the state-of-the-art low-temperature-grown material. These improved material properties translate into an improved frequency response and lower noise level of the InGaAs:Fe photomixers. We were able to measure a bandwidth of 4.5 THz with a peak dynamic range of 112 dB at 30 mW laser excitation around 1550 nm. To the best of our knowledge, these are record high values for cw THz spectroscopy. Furthermore we achieved an increased dynamic range by up to 10 dB for frequencies above 1 THz compared to state-of-the-art photomixing receivers. These improvements enable faster and more precise spectroscopy with higher bandwidth. In industrial non-destructive testing, the measurement rate may be increased by a factor of ten posing a valuable contribution to inline process monitoring.

We compare a state-of-the-art terahertz (THz) time domain spectroscopy (TDS) system and a novel optoelectronic frequency domain spectroscopy (FDS) system with respect to their performance in layer thickness measurements. We use equal sample sets, THz optics, and data evaluation methods for both spectrometers. On single-layer and multi-layer dielectric samples, we found a standard deviation of thickness measurements below 0.2 µm for TDS and below 0.5 µm for FDS. This factor of approx. two between the accuracy of both systems reproduces well for all samples. Although the TDS system achieves higher accuracy, FDS systems can be a competitive alternative for two reasons. First, the architecture of an FDS system is essentially simpler, and thus the price can be much lower compared to TDS. Second, an accuracy below 1 µm is sufficient for many real-world applications. Thus, this work may be a starting point for a comprehensive cross comparison of different terahertz systems developed for specific industrial applications.