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Signals in the terahertz range are mainly wireless, waveguide-bound or on-chip signals. To measure their spectral power, linewidth and harmonics, usually spectrum analyzers are used. Here we present a photonic spectrum analyzer for waveguide-bound signals for the frequency range from 450 GHz to 1.05 THz with the possibility to extend its operation to on-chip signals. Therefore, a photonically generated local oscillator frequency is used to downconvert the signal under test within a photoconductor. A Vivaldi antenna feeds the signal from a dielectric waveguide to the photoconductor. This allows the measurement of any signal that can be transported via a dielectric waveguide. We show details on the transition from a rectangular metallic hollow waveguide to a dielectric waveguide and spectral measurements of a rectangular metallic hollow waveguide source. The same transition can also be used to attach metallic waveguide-coupled ground-source-ground probes in order to characterize on-chip sources.

The biodiesel content in petroleum diesel-biodiesel blends is determined using broadband terahertz time-domain spectroscopy. We model the spectra of the blend components based on the nonpolar characteristics of the samples. The biodiesel content in diesel is determined from the coefficients that best fit the measured spectra to the optimal linear combination of the model for each individual component (pure diesel and pure biodiesel). This method proves to be valid for real-time applications. A simpler approach based on the maximum observed dispersion is also demonstrated to be a distinctive characteristic of a given blend.

Material characterisation and imaging applications using terahertz radiation have gained interest in the past few years due to their enormous potential for industrial applications. The availability of fast terahertz spectrometers or multi-pixel terahertz cameras has accelerated research in this domain. In this work, we present a novel vector-based implementation of the gradient descent algorithm to fit the measured transmission and reflection coefficients of multilayered objects to a scattering parameter-based model, without requiring any analytical formulation of the error function. We thereby extract thicknesses and refractive indices of the layers within a maximum 2% error margin. Using the precise thickness estimates, we further image a 50 nm-thick Siemens star deposited on a silicon substrate using wavelengths larger than 300 µm. The vector-based algorithm heuristically finds the error minimum where the optimisation problem cannot be analytically formulated, which can be utilised also for applications outside the terahertz domain.

Terahertz (THz) domain is quickly developing with various applications such as beam diagnostics at particle accelerators, spectroscopy, communications, space science, etc. However, often requiring fast intermediate frequency (IF) electronics. We present the design of a double mirror stub (DMS) based a planar broadband bias tee having an isolation port S 31 with 14.45 GHz bandwidth below −10 dB and S 33 with 13.1 GHz bandwidth above −2 dB. CST simulation and measured results are in very good agreement. The bias tee will be a part of a new generation of on-chip THz detectors based on zero-bias Schottky diode and high electron mobility field effect transistor (HEMT).

Strontium molybdate (SrMoO3) thin films are grown epitaxially by pulsed laser deposition onto gadolinium scandate (GdScO3) substrates and characterized in the terahertz (THz) and visible part of the electromagnetic spectrum. X-ray diffraction measurements prove a high crystallinity and phase-pure growth of the thin films. The high-quality SrMoO3 thin films feature a room temperature DC conductivity of around $3{\frac {1}{\mu {\Omega } m}}$ 3 1 μ Ω m . SrMoO3 is characterized in the THz frequency range by time domain spectroscopy. The resulting AC conductivity is in excellent agreement with the DC value. A Lorentz-Drude oscillator approach models the THz and visible conductivity of SrMoO3 very well. We compare the results of the SrMoO3 thin films to a standard, sputtered gold film, with a resulting THz conductivity of $8{\frac {1}{\mu {\Omega } m}}$ 8 1 μ Ω m . The comparison demonstrates that oxide thin film–based devices can play an important role in future THz technology.

THz time-domain spectroscopy (TDS) is a promising tool for quality control purposes in industrial applications, but the high cost and the relatively large laser sources still make it difficult to use the full potential of the technology for a decent price. In this work, a THz TDS system, which uses a commercially available Fabry&ndash, Perot laser diode emitting at 1550 nm, is presented. By dispersion compensation, pulses with a duration of 544 fs were generated, resulting in THz radiation with a bandwidth of 1.4 THz and a peak dynamic range of 56 dB with state-of-the-art ErAs:In(Al)GaAs photoconducting antennas. These results are compared with those of a conventional and expensive fiber laser system with a 90 fs pulse duration.

We present an investigation of an ErAs:InAlGaAs photoconductive antenna receiver optimized for 1030 nm probe pulses at 24 mW THz power and show a dynamic range of 92 dB in 10 s measurement time. The saturation behavior under high THz average power is studied as well.

This paper investigates fully integrated Terahertz (THz) Schottky detectors using silver (Ag) metallic nanowires (NWs) with 120 nm diameter as bridge contacts for zero-bias operating THz detectors based on highly doped Gallium Arsenide (GaAs) and Indium Gallium Arsenide (InGaAs) layers. The combination of InGaAs and metallic NWs shows improved performance at zero-bias than a GaAs based detector with a simulated capacitance of 0.5 fF and a series resistance of 29.7 $\Omega $ . Thus, the calculated maximum cut-off frequency of 2.6 THz was obtained for a NW contacted vertical InGaAs THz detector. Initial THz measurements were carried out using a common THz setup for frequencies up to 1.2 THz. A responsivity of 0.81 A/W and a low noise-equivalent power (NEP) value of 7 pW/ $ {~\sqrt {Hz}}$ at 1 THz were estimated using the measured IV-characteristics of the zero-bias NW-InGaAs based THz Schottky detector.

To date, exploiting the full bandwidth of state-of-the-art InGaAs photomixers for generation and detection of continuous-wave (CW) THz radiation (typ. ~50 GHz to ~3 THz) required complex and costly external-cavity diode lasers with motorized resonator control. Distributed feedback (DFB) lasers, by contrast, are compact and inexpensive, but the tuning range per diode is limited to ~600 GHz at 1.5 ��m. In this paper, we show that a combination of three DFB diodes covers the complete frequency range from 0 ��� 2750 GHz without any gaps. In combination with InGaAs-based photomixers for terahertz generation and detection, the system achieves a dynamic range of > 100 dB at 56 GHz, 64 dB at 1000 GHz, and 26 dB at 2500 GHz. A field-programmable gate array (FPGA)-based lock-in amplifier permits a flexible adjustment of the integration time from 0.5 ms to 600 ms. Employing an optimized ���fast scan��� mode, a spectrum of ~1200 GHz ��� the bandwidth of each subset of two lasers ��� and 40 MHz steps is acquired in less than one minute, still maintaining a reasonable dynamic range. To the best of our knowledge, the bandwidth of 2.75 THz presents a new record for DFB-based CW-terahertz systems.

Dielectric waveguide architectures enable low loss, miniaturized terahertz systems-on-chip with extreme bandwidth. However, transfer of power from active devices to the waveguides presents a severe challenge. In this paper, we present a comparison of two Vivaldi end-fire antenna designs with losses of 2 ��� 5 dB per coupling interface to silicon waveguides over a frequency range as large as 0.6 ��� 0.9 THz. We demonstrate lower losses between 0.63 ��� 0.83 THz as compared to far field in-coupling with silicon lenses, despite being orders of magnitude smaller in size.

We report on the selection of contact metallisations for ZnSe-based metal-semiconductor-metal ultraviolet photo­de­tectors. Our evaluation is based on Ni/Au, Cr/Au, and hybrid Ag-nanowire contacts. Low values of dark current of 0.32 nA, 0.82 nA and 1.64 nA at bias voltage of 15 V were achieved for photodetectors with Ag-NW, Ni/Au and Cr/Au interdigital contacts, respectively. The best performance of our ZnSe-based ultraviolet photodetectors is observed for Ni/Au interdigital contacts. This is due to the higher Schottky barrier height, which is equal to ~ 1.49 eV for Ni/Au contacts in comparison with ~ 1.26 eV for Cr/Au contacts.A very high responsivity of 5.40 AW-1at bias voltage of 15 V for light with a wave­length of 325 nm is obtained for devices withNi/Auinterdigital contacts. Moreover, the maximum of photocurrent on/off ratio of 20342 and minimum of NEP of ~ 3 × 10-15 W Hz-1/2 at bias voltage of 15 V was achieved for this type of device.

Field effect transistor (FET) based terahertz rectifiers are promising candidates for sensitive, room-temperature operated high speed (THz) detectors, e.g. in communication, medical, biochemical, security, quality control applications, or beam diagnostic applications at particle accelerators. This paper investigates the equivalent circuit in the intermediate frequency band from 0.1 to 65 GHz with S-parameter measurements in order to enable implementation with high speed post detection electronics. Preliminary results are obtained by de-embedding On-Wafer measurements and compared with theoretical expectations from hall measurements and a simplified equivalent circuit. The knowledge of the channel behavior and impedance is mandatory for impedance matching to IF circuitry in advanced detector designs.

Surface states generally degrade semiconductor device performance by raising the charge injection barrier height, introducing localized trap states, inducing surface leakage current, and altering the electric potential. Therefore, there has been an endless effort to use various surface passivation treatments to suppress the undesirable impacts of the surface states. We show that the giant built-in electric field created by the surface states can be harnessed to enable passive wavelength conversion without utilizing any nonlinear optical phenomena. Photo-excited surface plasmons are coupled to the surface states to generate an electron gas, which is routed to a nanoantenna array through the giant electric field created by the surface states. The induced current on the nanoantennas, which contains mixing product of different optical frequency components, generates radiation at the beat frequencies of the incident photons. We utilize the unprecedented functionalities of plasmon-coupled surface states to demonstrate passive wavelength conversion of nanojoule optical pulses at a 1550 nm center wavelength to terahertz regime with record-high efficiencies that exceed nonlinear optical methods by 4-orders of magnitude. The presented scheme can be used for optical wavelength conversion to different parts of the electromagnetic spectrum ranging from microwave to infrared regimes by using appropriate optical beat frequencies., Comment: Manuscript: 8 pages, 4 figures Supplementary materials: 21 pages, 11 figures

In this manuscript, the use of dielectric rod waveguide antenna (DRW) with an embedded planar lens is proposed as a highly directional alternative to an electrically large hyper-hemispheric silicon ...

Precise real-time detection of Terahertz pulses is a key requirement in Terahertz communication technology, non-destructive testing, and characterization of pulsed Terahertz sources. We experimentally evaluate the speed limits of Terahertz rectification in field-effect transistors using the example of pulses from a free-electron laser. We develop an improved model for the description of these Terahertz pulses and demonstrate its validity experimentally by comparison to spectroscopic data as well as to expectation values calculated from free-electron laser physics. The model in conjunction with the high speed of the detectors permits the detection of an exponential rise time of the pulses as short as 5 ps despite a post detection time constant of 11 and 14 ps for a large area and an antenna-coupled detector, respectively. This proves that field-effect transistors are excellent compact, roomtemperature Terahertz detectors for applications requiring an intermediate frequency bandwidth of several tens of GHz.

The resolution along the propagation direction of far field imagers can be much smaller than the wavelength by exploiting coherent interference phenomena. We demonstrate a height profile precision as low as 31 nm using wavelengths between 0.375 mm and 0.5 mm (corresponding to 0.6 THz–0.8 THz) by evaluating the Fabry-Pérot oscillations within surface-structured samples. We prove the extreme precision by visualizing structures with a height of only 49 nm, corresponding to 1:7500 to 1:10000 vacuum wavelengths, a height difference usually only accessible to near field measurement techniques at this wavelength range. At the same time, the approach can determine thicknesses in the centimeter range, surpassing the dynamic range of any near field measurement system by orders of magnitude. The measurement technique combined with a Hilbert-transform approach yields the (optical) thickness extracted from the relative phase without any extraordinary wavelength stabilization., The authors demonstrate a far-field approach for height profile measurements with sub-millimetre waves. By evaluating Fabry-Pérot oscillations within surface-structured samples, combined with a Hilbert-transform approach, they visualize structures with a height of 49 nm, with 31 nm precision.

We demonstrate a free space two-port photonic vector network analyzer capable of measuring the scattering parameters of devices and materials in the terahertz range with a frequency coverage of 0.2 - 2 THz in a single system. It is based on photoconductive terahertz sources and detectors driven by a telecom-wavelength femtosecond laser. Being able to cover a bandwidth of one order of magnitude, the system is capable of performing S-parameter measurements deep into the terahertz range, beyond frequencies reachable by their electronic counterparts. We demonstrate high performance at three application examples, namely S-parameter measurements of a split ring resonator array and a distributed Bragg reflector, as well as material parameter extraction of several materials.

We demonstrate a Photonic Vector Network Analyzer (PVNA) capable of measuring a complete set of scattering parameters within the frequency range of 0.2 ��� 2 THz. The PVNA shows superiority in terms of bandwidth compared to electrical VNAs and is also free from the errors introduced by standing waves, metal waveguides, misaligned flanges, and features less stringent requirements on rigorous and time-consuming calibration routines. As a proof of principle, we characterize a crossed dipole resonator array using the free space two-port PVNA.

The conductivity and dielectric function of a different SrMoO 3 (SMO) oxide thin film is characterized in the THz frequency range. The dielectric constant is determined to $\epsilon_{2}= 1.6\cdot 10^{5}$ at 200 GHz and $\epsilon_{2}=0.3\cdot 10^{5}$ at 1.0 THz with almost vanishing real part $\epsilon_{1}$ for a 20 nm SMO film on a GdScO 3 (GSO) substrate. We compare THz properties of different growth runs in order to evaluate influence of the film quality. All investigated SMO thin films show metal-like conduction similar to that of Titanium.