We report on spontaneous parametric down-conversion (SPDC) in periodically poled lithium niobate (PPLN) using 660 nm pump wavelength and the type 0 phase-matching condition to the terahertz and even sub-terahertz frequency range. Detection of the frequency-shifted signal photons is achieved by using highly efficient and narrowband volume Bragg gratings and an uncooled sCMOS camera. The acquired frequency-angular spectrum shows backward and forward generation of terahertz and sub-terahertz photons by SPDC, as well as up-conversion and higher order quasi phase-matching (QPM). The frequency-angular spectrum is theoretically calculated using a Monte-Carlo integration scheme showing a high agreement with the measurement. This work is one important step toward quantum sensing and imaging in the terahertz and sub-terahertz frequency range.

Terahertz time-domain spectroscopy as well as all optical pump-probe techniques with ultrashort pulses relies on the exact knowledge of an optical delay between related laser pulses. Classical realizations of the measurement principle vary the optical path length for one of the pulses by mechanical translation of optical components. Most commonly, only an indirect measurement of the translation is carried out, which introduces inaccuracies due to imprecise mechanics or harsh environment. We present a comprehensive study on the effect of delay inaccuracies on the quality of terahertz spectra acquired with time-domain spectroscopy systems and present an interferometric technique to directly acquire the optical delay simultaneously to the terahertz measurement data. This measurement principle enables high-precision terahertz spectroscopy even in harsh environment with non-systematic disruptions.

We present a novel terahertz spectroscopy principle by using incoherent light from a super luminescent diode for terahertz cross-correlation spectroscopy. The combless nature of this light source leads to a truly continuous terahertz spectrum. We demonstrate the possibility to influence the terahertz spectral bandwidth of the system by changing the bandwidth of different bandpass filters in the system. Depending on the employed bandpass filter we achieve peak dynamic ranges of 60 dB or a terahertz spectral width of about 1.7 THz. The applicability of the measurement system to spectroscopic terahertz measurement tasks is demonstrated.

The acquisition speed of terahertz time-domain spectroscopy systems has undergone a significant improvement in recent years. With the development of dual-laser-based sampling techniques such as ASOPS or ECOPS, waveform acquisition rates in the kilohertz range have become feasible. Here, we present measurements of sub-wavelength layers in multilayer systems at a rate of 1.6 kHz, where the individual layer thicknesses are analyzed in real time. For demonstration, we image layers on metallic and non-metallic disks with a 300 mm diameter, acquiring 240,000 pixels in only 2.5 min. By combining a rotation of the samples with a linear translation of the measurement head, we maximize the measurement yield. Owing to a “single-shot” (625 µs) dynamic range of 40 dB and a bandwidth above 3 THz of the ECOPS system, we achieve sub-micron repeatability for each layer.

The detection of terahertz photons by using silicon-based devices enabled by visible photons is one of the fundamental ideas of quantum optics. Here, we present a classical detection principle using optical upconversion of terahertz photons to the near-infrared spectral range in the picosecond pulse regime, which finally enables the detection with a conventional sCMOS camera. By superimposing terahertz and optical pump pulses in a periodically poled lithium-niobate crystal, terahertz photons at 0.87 THz are converted to optical photons with wavelengths close to the central pump wavelength of 776 nm. A tunable delay between the pulses helps overlap the pulses and enables time-of-flight measurements. Using a sCMOS camera, we achieve a dynamic range of 47.8 dB with a signal to noise ratio of 23.5 dB at a measurement time of one second, in our current setup.

Terahertz cross-correlation spectroscopy enables phase-sensitive measurements without the need for a laser source and, hence, presents a cost-efficient and versatile alternative to common terahertz time-domain spectroscopy approaches. This review article presents the development of this technique over the past two decades as well as applications of this approach. It is completed by a detailed mathematical description proving the irrelevance of the optical phases of the employed pump light modes. Numerical investigations of the resulting signal demonstrate the applicability and are compared to state-of-the-art measurements. Terahertz cross-correlation spectroscopy is a valuable alternative for moderate-demand applications already. Further possible improvements are discussed.