The advent of high-field terahertz (THz) sources opened the field of nonlinear THz physics and unlocked access to fundamental low energy excitations for ultrafast control of quantum materials and other correlated systems. Recent concepts employing angular momentum of THz light, e.g. for driving chiral excitations, rely on the undistorted measurement of THz fields and on the precise knowledge about sophisticated THz helicity states. Both of these requirements pose experimental challenges when employing intense single-cycle pulses, because THz attenuators commonly do not completely conserve polarization states and the few broadband THz polarization optics suffer from significant transmission losses. Here, we resolve both challenges by employing z-cut α-quartz as a versatile and costefficient electro-optic THz detector. Quartz’s moderate electro-optic coefficient in conjunction with the threefold 𝜒⁽²⁾ symmetry enables direct retrieval of the vectorial THz field trajectory. The retrieved amplitude- and phase-resolved time domain information can be easily mapped into a circular basis or on a Poincaré sphere in frequency domain. By this approach, we provide a convenient procedure to measure and map sophisticated THz polarization states relevant for interpreting THz ellipsometry data and for THz pulse shaping towards coherent control of chiral low-energy excitations in future.

The generation of Terahertz (THz) radiation using two-color plasma in gases has garnered significant interest within the scientific community for its ability to produce powerful waves with very broad and tunable spectra. Understanding how to modify the characteristics of the generated THz radiation paves the way for optimizing its performance for targeted applications. To achieve this, various approaches have been explored, however, the combined impact of chirp and wavelength dispersion has not yet been thoroughly investigated. Our study aims to understand how the laser chirp and the phase difference caused by air dispersion influence the shape of the THz pulse. The findings highlight that these factors significantly alter the shape of the THz pulse. Specifically, the nature of the chirp (positive or negative) in the pump laser distinctly affects the pulse shape. For instance, a pump laser with positive chirp results in a THz waveform with a negative monopolar configuration, while a laser with negative chirp generates a THz pulse with a positive configuration.

We report on the THz harmonic generation in a graphene/metamaterial device by using ultrafast table top powerful THz-TDS systems. A complex nonlinear transmission spectra, which includes a peak at the third harmonic signal is detected on top of the main resonant features at 0.65 THz, for E-field pulses in the range 1-30 kV/cm. Whilst these results are consistent with acquainted literature, they offer a novel perspective for the exploitation of graphene nonlinearity in integrated devices.

A photonic sub-THz transceiver design for use in the remote radio head (RRH) for future high capacity 6G networks that employ THz frequencies is demonstrated. The work studies the backscattering effects when a single fiber is used for fronthauling to and from the RRH. The work also demonstrates replacing the fiber link with free space optical (FSO) channel and draws a comparative analysis between the two optical channels in term of nonlinear effects. Bit error rate (BER) performance for 10Gb/s NRZ signals is measured by varying the received optical power at the uplink photodiode (for a fixed optical launch power) and also by varying the launched optical power at the transmitter side (for a fixed received optical power at the uplink photodiode) for 25 km bidirectional fibre transmission and 25 km of FSO transmission respectively. At +5 dBm optical launch power (received optical power at photodiode being 0.080 dBm), BER is found to be 10^-5 while using a single fibre and 10^-7 while using FSO link. This is due to the presence of backscattering effects in single mode fibre when used for bi-directional transmission, which signifies that FSO may be more suitable than optical fiber for sub-THz transceiver designs.

We present an optoelectronic mixer for the terahertz (THz) frequency-domain based on an iron-doped InGaAs layer integrated in a plasmonic microcavity. We show that this structure, under 1550-nm-wavelength illumination, allows for more than 70% absorption efficiency in a 220 nm-thin InGaAs absorber and very high Roff/Ron >1000. It leads to THz mixers driven by 1550-nm lasers showing conversion loss as low as ~30 dB at 300 GHz. Therefore, this design is very promising for application as receivers in high-data-rate wireless telecom, in cw-THz spectrometers, or in photonic-senabled THz spectrum analyzers.

Terahertz (THz) quantum cascade lasers (QCL) may operate as harmonic frequency combs, exhibiting a mode-separation of multiple times the round trip frequency. This work aims to shed light on the coupled field-electron dynamics that lead to harmonic mode-locking in defect-engineered THz QCLs. Therefore, we use the Maxwell-Bloch equations describing a medium of two-level quantum systems interacting with the electric field in the laser cavity. We find that both the amplitude and phase of the electric field are coupled to the introduced defects, and the system quickly reaches a locked state. Despite the presence of the reflectors, spatial hole burning is necessary to enable multimode operation.

This work discusses simulation results for signal deteriorations arising from a higher carrier in a 3-line optical frequency comb based high bandwidth sampling system in terms of the key performance indicators such as root-mean-square error (RMSE), signal-to-noise and distortion ratio (SINAD), and the effective number of bits (ENOB).

There is actual demand from astronomers for space instrumentation operating in the range of a few terahertz (between 1 and 10 THz) for deep space explorations. Analyzing this radiation coming from space is possible thanks to the development of a customized spectrometer, which splits the light collected at the terahertz spectral range using a diffraction grating. This diffraction grating has to be capable of withstanding the demanding conditions of space. However, obtaining the required dimensions, morphology, and wide spectral range for the grating is challenging. This work presents the outcomes of the diffraction efficiency verification for a metallic grating provided with a sawtooth profile and manufactured by laser micro-structuring with a 5-axis femtosecond laser system on an aluminum, fulfilling those requirements. The grating operates only with the first diffraction order (m=-1), favoring the transverse magnetic (TM) polarization over the transverse electric (TE) component, with a view to its incorporation in a polarization-selective optical system. In the course of the work, the grating characteristics, fabrication methods, and experimental verification of the grating properties, morphology, and diffraction efficiency will be presented. Diffraction efficiencies greater than 85% have been achieved.

This work is dedicated to present a versatile, metamaterial-based sensing platform for biomarker detection in the THz frequency regime. The sensor can be classified as frequency selective surface and exhibits a characteristic dual resonance feature in the transmission frequency response which has been proven to perform well in biosensing applications. The platform is able to selectively detect proteins such as transcription factor EGR2 (early growth response protein 2) from a complex sample matrix with more than 7 · 10³ other protein species, to sensitively detect reverse-transcribed MIA (melanoma inhibitory activity) DNA from a complex sample matrix and to respond to extracellular vesicular structures as well.

High-frequency Terahertz waves, operating at the level of molecular group vibrations, hold promise in regulating the function of biomolecules and potentially interfering with the progression of associated diseases. In this research, we established a neuropathic pain (NP) mice model using spared nerve injury (SNI) and observed that a frequency of approximately 36 THz could alleviate NP. Furthermore, we noted that this frequency can regulated the mitochondrial dynamic network in the anterior cingulate cortex (ACC) region, leading to enhanced ATP production. This article tries to explore the potential mechanism between high-frequency terahertz waves and NP through mitochondria, which may be a promising strategy for the therapy of other mitochondria diseases.

The polyethylene oxide (PEO)-based polymer electrolytes containing lithium salts pave the way for the development of safe lithium batteries with high energy storage capacity. Enabled by the flexibly vibrating polymer matrix, the Li-ion transport occurs via hopping along and between the polymer chains. Here, terahertz (THz) time-domain spectroscopy over a frequency range extending from 0.1 THz to 7 THz is applied to investigate the conduction properties of PEO-based electrolytes with lithium salts, and to elucidate the associated dependence of the THz conductivity on the content of the added lithium salt and the electrolyte temperature. It is remarkably found that the higher the observed THz vibrational activity of the electrolyte, the higher its technologically relevant ionic conductivity.

We present the design and characterization of dielectric slot waveguides fabricated in highly resistive silicon for operation in the THz frequency range. We designed rib and slot waveguides with straight and curved beams using the mode solvers from BeamPROP and CST Studio Suite. The transmission parameters of the free space measurement is about -30 dB. Using the slot waveguide, the transmission is about -15 dB demonstrating the ability to guide fields in our structures. At 300 GHz, the measured losses are in the range of 0.15 dB/mm. Solvents are injected into the slot of the waveguide by capillary force changing the transmission. Each solvent realizes a different change in transmission, so that we aim to use the slot waveguides for THz spectroscopy in future.

Megahertz repetition rate fiber lasers at 1.5 μm of wavelength have reached technological maturity and robustness that permits their integration into tablet-size self-starting devices with battery operation capabilities. When subsequently amplified by an erbium-doped fiber amplifier (EDFA), optical pulses with sub-20-fs durations and nanojoule energies can be obtained via in-fiber self-phase modulation and dispersive compression. While such sources are of relevance for many fields ranging from material processing to nonlinear microscopy, optical sensing offers more advantages at longer wavelengths. In our work, we explore the feasibility of frequency conversion of telecommunication-wavelength optical pulses using new organic nonlinear optical crystals in the nanojoule pulse energy regime for spectroscopy. In particular, we study the broadband terahertz (THz) emission and detection capabilities of PNPA ((E)-4-((4- nitrobenzylidene)amino)-N-phenyl-aniline) compared with DSTMS (4-N,N-dimethylamino-4'-N'-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate). Not only can the crystals be used for room-temperature broadband THz generation, but also detection via the optical Pockels effect – a linear change of the refractive index in the presence of an external electric field. This in turn lifts the requirement for cooled infrared detectors because a conventional uncooled InGaAs photodiode for near-infrared wavelengths can be used instead to detect far-infrared waveforms. Currently, we obtain a spectroscopic coverage of 2–25 THz but future improvements in application-tailored organic crystals should offer even broader optical bandwidths.

The fast development of terahertz (THz) technology holds significant importance in numerous applications where imaging systems are indispensable. Moreover, advanced imaging systems increasingly demand the capability to image transparent objects. This is particularly challenging due to the relatively low efficiency of THz sources, leading to low power in THz systems. Additionally, parasitic reflections in mostly very coherent THz optical systems result in the occurrence of high speckle noise. One of the solutions for the enhancement of the imaging contrast of transparent objects is the application of spatial filtering (SF) methods in the system, which we implement here via the following methods: positive phase contrast (PPC), negative phase contrast (NPC), and dark field (DF). In this study, the authors report the implementation of all three methods in the 4f THz system for relatively long-distance imaging of 1200 mm. The system was tested with the numerical simulation. The detailed analysis of the results allowed for the selection of the recommended size of the spatial phase filter for the implementation of the SF methods in the experimental optical setup. The filter and the object were manufactured using fused deposition modeling (FDM) 3D printing technology. Subsequently, the 4f system facilitating the SF methods was built and examined experimentally. The analysis of the experimental results indicates the enhancement of the signal-to-noise ratio (SNR) by almost 14 times with the use of the PPC method compared to the system without the inserted filter. Thus, the improvement of the image contrast by the implementation of SF method is unquestionable.

Spatial and temporal control of thermally emitted terahertz (THz) radiation could pave the way for a new family of devices in imaging, spectroscopy and communication systems. We demonstrate a computational THz imaging method enabled by a structured illumination provided by a spatio-temporal emissivity modulation. We apply a surface passivation stack composed of ZnO and Al₂O₃ layers to a high-resistivity silicon wafer to increase the effective carrier lifetime of the electron-hole pairs by a factor of 16. The emitted power is further boosted by increasing the temperature of the modulator to 390 K. Using a low power LED and a digital micromirror device we optically control the local THz emission from the modulator and use it to produce structured THz beams in a single-pixel imaging setup. We employ a ghost imaging procedure with a single-pixel detector and sequential illumination with patterns from a Hadamard basis set to allow for computational reconstruction of the object from the temporal signal. We evaluate the performance of the technique and its potential trade-offs with respect to resolution and acquisition time and apply a simple compressed sensing protocol to speed up the imaging process.

This study introduces a stainless steel-based complementary C-shaped single split ring resonator (CSRR) metasurface designed for Terahertz (THz) imaging applications. The CSRR metasurface was created from a 25 μm thick stainless-steel foil using laser ablation, and serves as a zone plate, allowing precise manipulation of 100 GHz radiation. The investigation involved beam-shaping characterization of the metasurface comprising CSRRs of different geometrical parameters and examining the bending influence on the metasurface's functionality, revealing a minimal reduction in beam intensity. Additionally, the proposed metasurface demonstrated the ability to control polarization by sweeping its rotational angle, enhancing THz polarization-resolved imaging capabilities. Practical demonstrations showcase the metasurface's suitability for real-life scenarios, highlighting its value in THz imaging systems.

Terahertz (THz) waves have many important characteristics, e.g., fingerprint spectrum, low energy and penetrability, thus show great potential and wide application value in many fields such as biomedical imaging, industrial nondestructive testing, cultural relics protection, security inspection and anti-terrorism. With the development of THz sources and detectors, the progress of THz imaging is also constantly advancing. THz confocal imaging realizes high axial resolution by removing stray light of the non-focus planes by point illumination and small aperture diaphragm. This imaging approach obtains object depth information layer-by-layer. In this paper, we illustrated a THz confocal scanning imaging system which three types of lenses are designed and used to verify its focal capability for imaging illumination, i.e., THz super-oscillating lens (TSOL), Fibonacci lens (FBL) and transmissive convex lenses (TCL). TSOL can produce long focal depth and subwavelength focused beam. FBL is a diffractive lens that produces multiple foci along the axial coordinate. TCL produces one focal point in far-field, which is the typical THz lens applied in various THz imaging systems. The implementation and calibration of three types of THz confocal imaging systems are introduced in detail, thereafter the corresponding results are shown and compared.

The present study introduces an innovative approach for a real-time detection of concealed metallic objects using a SiGe 300 GHz radar source. It has achieved remarkable precision distances up to 2.0 m. The system employs a miniaturized 300 GHz Industrial, Scientific, and Medical (ISM) band frequency-modulated continuous wave (FMCW) radar system. This setup includes a SiGe single-chip radar sensor and Liquid Crystal Polymer (LCP) off-chip antennas integrated into a Quad Flat No-Lead (QFN) package, breaking a new ground with millimeter-level accuracy. The system is linked to a user-friendly graphical interface (WebGUI) software, which help the users for fine-tuning the baseboard parameters to visualize radar data, and make real-time adjustments. The strength of reflected signal exhibits a nuanced second-order polynomial nature intricately correlated with signal bandwidth and the target's distance. Notably, the system provides maximum and minimum peak power at 289 and 329 GHz, respectively and helping us to achieve a Signal-to-Noise Ratio (SNR) of the order of 3.78 at a detection distance of 2.0 m. A detailed frequency bandwidth-based analysis reveals the system’s detection range. The maximum target distance was obtained 2.0 m at 20 GHz bandwidth range. Similarly, bandwidth of 30, 40 50 and 60 GHz were able to achieve the target range of 146, 100, 80 and 63 cm range, respectively. The 300 GHz SiGe radar source outperforms the lower-frequency (in microwave range) in terms of excellent spatial resolution and minimum interference from microwave devices which are some essentially requirements for radar based imaging and sensing applications. This adaptive radar system act as a powerful tool for enhancing the homeland security and defense system where concealed objects pose threats to public safety.

In the present study, we have studied the applicability of terahertz (THz) metamaterials for sensing low concentrations of premium explosives like RDX and TNT. A parallel metal-pair-based metamaterial has been investigated. The structure exhibits a Fano resonance at 0.627 THz in reflection geometry within the 0.1 to 1 THz range. The unit cell of the metamaterial comprises two asymmetric aluminium rod-like structures on an intrinsic silicon wafer with dimensions of 100 μm and 80 μm, respectively, and a silicon wafer thickness of 40 μm. The structure's periodicity is 120 μm along the x and y directions. We have also performed COMSOL-based simulations of metamaterial structures with different analyte thicknesses in conjunction with experimental verification. In the experiment, for an analyte thickness of 0.5 μm, the structure exhibits a refractive index-dependent sensitivity (S) of 5.1 GHz/RIU. For explosives, resonance peak shifts of 0.031 THz for TNT (refractive index 1.61) and 0.043 THz for RDX (refractive index 1.85) were observed from their respective resonance positions at 0.627 THz. These findings underscore the efficacy of THz metamaterials for detecting trace amounts of explosives.

The generation of Terahertz (THz) waves via two-color plasma in gas has captured the interest of the research community due to its capability to create intense waves characterized by a wide and adjustable spectrum. Efforts aimed at improving the performance of THz radiation for particular applications entail examining and adjusting several variables. In this study, we introduce a simple method for manipulating THz polarization through the adjustment of chirp and wavelength dispersion. Specifically, we will show that by managing these characteristics, it is possible to produce THz waves with polarizations that can be elliptical, circular, or resemble a "flower" pattern. The implications of these changes on the spatiotemporal path of THz radiation will also be examined.

A technique enabling the generation of THz radiation using an ordered array of double-walled carbon nanotubes (DWCNTs) pumped by a direct electric current is proposed. The initial excitation of surface plasmon polaritons (SPPs) in the DWCNTs is performed by two laser beams with slightly different frequencies. The amplification of exited slow SPPs (with a phase velocity down to ~10⁶ m/s) is provided by a drift current flowing through the DWCNTs. The DWCNTs with SPPs act as sources of THz radiation and emit coherent electromagnetic waves into free space. The proposed model of a carbon nanotube generator may be useful for the development of compact sources of coherent THz radiation.