Terahertz Photonics III

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 [1]. Recent concepts employing angular momentum of THz light, e.g. for driving chiral excitations, rely on the undistorted measurement of intense THz fields and on the precise knowledge about sophisticated THz helicity states. Here, we establish z-cut α-quartz as a precise electrooptic THz detector for full amplitude, phase and polarization measurement of intense THz fields, all at a fraction of costs of conventional THz detectors [2]. We experimentally determine its complex detector response function, in excellent agreement with our modeling. These insights allow us to develop a swift and reliable protocol to precisely measure arbitrary THz polarization and helicity states, thereby turning any THz spectrometer into an affordable THz ellipsometer. Thereupon, we highlight pathways for driving chiral modes and the exact preparation of coherent phonon angular momentum. [1] M. Frenzel et al., Sci. Adv. 9, eadg3856 (2023) [2] M. Frenzel et al., arXiv:2309.08286 (2023)

Plasmonic structures like ribbons or disks can strongly enhance the light-matter interaction of two-dimensional electron gases at resonance. This can be exploited to fabricate devices with improved performance. In addition, also the optical nonlinear properties of the material is enhanced. Here we present a series of pump-probe measurements on various samples, mostly based on graphene, to disentangle the different effects. Besides the optical nonlienarities, circular plasmons can be excited with circularly polarized radiation, resulting in a uni-polar magnetic field in the vicinity of the plasmonic element, due to the inverse Faraday effect. This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Project No. 278162697-SFB 1242.

We report on hybrid THz cavities based on ultrastrong coupling between a Tamm cavity and an LC circuit metamaterial and demonstrate that these hybrid THz cavities combine high quality factors of up to Q∼37 with a deep-subwavelength mode volume of V=3.2x10−4λ3. These Tamm cavity-LC metamaterial coupled resonators open a route toward the development of single photon THz emitters and detectors and to the exploration of ultrastrong THz light–matter coupling with a high degree of coherence in the few to single electron limit.

The aim of this study is to investigate the simultaneous influence of the laser chirp and the phase difference caused by air dispersion on the properties of THz radiation generated by a two-colour plasma in gas. The results show that these two factors have a significant influence on both the THz waveform and the emitted power. In terms of THz pulse shape, positive and negative chirps have different effects. A positively chirped pump laser produces a THz waveform with a negative monopolar shape, while a negatively chirped pump laser leads to a positive THz pulse shape. Furthermore, our results show that the relationship between THz power and chirp changes with variations in the specific phase difference between the first and second harmonics. These results underscore the importance of carefully tuning both parameters to achieve maximum generation efficiency for a given waveform.

Bismuth chalcogenides have emerged as an intriguing option in nanophotonics due to their unique properties, including a significant dielectric function and topologically protected surface states in the terahertz (THz) range. Our study focuses on the optical response of Bi2Se3 topological-insulator nanospheres, where topological properties arise, influencing electromagnetic modes and field enhancement. Within a fully electrodynamic picture, we explore the emergence of unreported magnetic modes induced by Dirac plasmon polaritons resulting from terahertz photon and Dirac electron interactions. We observe their profound impact on electric and magnetic transitions of quantum emitters near Bi2Se3 nanospheres, resulting in record - high Purcell factors. Our research highlights the rich optical response of Bi2Se3 nanospheres, involving contributions from both phonon polaritons and topologically protected surface states. Our findings confirm the emergence of topological optical modes in Bi2Se3 spherical TIs, positioning them as promising candidates for advancements in strong light-matter interactions in topological nanophotonics and THz technologies.

In terahertz system design, the integration of reconfigurable components, and especially switches, is indispensable for routing terahertz signal flow. While on-off switching in the terahertz range has been demonstrated in various topologies, the realization of 1-to-N switching has remained unexplored. This significant gap in dynamically reconfigurable routing capabilities has fundamentally limited the achievable complexity of terahertz systems. To address this, we present 1-to-N switches using cascaded disk resonators in an integrated substrateless silicon waveguide platform. These switches are individually controlled through photoexcitation, achieved by directing a low-power 658-nm laser onto the disk resonator. This action suppresses the resonance and impedes signal coupling into a specific output port. Furthermore, we integrate a 1-to-3 switch monolithically together with a Luneburg lens to realize beam switching. This integration is a major step in terahertz technology, as it is the first demonstration of dynamic reconfigurable beam control in a low-loss terahertz integration platform, opening up promising possibilities for sensing, imaging, and communications.

The lack of suitable sources and detectors of THz radiation with sufficient power and sensitivity, small footprint and portability is the biggest obstacle for many scientific and commercial applications. Current photonic THz systems have already shown great potential in terms of high tunability, standard room temperature operation and signal quality. However, these systems still have several drawbacks, such as large device size (requiring an optical stage), mechanical interference (besides noise and alignment), high power consumption (electrical and optical), and low system flexibility (each application requires a new setup). Therefore, we have proposed and developed new THz devices towards a new THz system platform based on photonic integrated circuits (PICs) and nanotechnology to overcome all these drawbacks.

Terahertz lightwave scanning tunneling microscopy (THz-STM) enables the imaging of ultrafast dynamics of elementary excitations on surfaces with angstrom spatial and THz-sub-cycle temporal resolution. Spintronic THz emitters (STE) are attractive sources for THz-STM because they allow the generation of ultra-broadband single-cycle THz pulses with easy polarity and polarization control by magnetic fields. However, the ultrathin metal film of the STE makes it highly sensitive to laser-induced heating, limiting the THz fields achievable with multi-watt laser systems operating at MHz repetition rates as required for THz-STM. We show that average power heating can be effectively reduced by rotating the STE at sub-kHz speed. The rotating STE can be operated at optimal excitation fluences of ~0.1-1 mJ/cm2 using laser pulses of a few tens of µJ energy and up to 18 W power at MHz repetition rates collimated in a pump spot of a few mm diameter, resulting in power densities that would normally cause immediate thermal damage to the STE. Finally, we highlight the ability of the STE-driven THz-STM to drive plasmonic STM-luminescence and to probe ultrafast electron and phonon dynamics in 1T-TaS2.

We report on the THz harmonic generation in an integrated graphene/metamaterial device by using ultrafast table top powerful THz-TDS systems. An absorption peak at the third harmonic signal is detected on top of the main resonant one at 0.65 THz, for E-field pulses in the range 1-20 kV/cm. By electrostatically changing the Dirac point of graphene it was possible to observe a positive correlation with the carriers’ density, consistently with literature. The nonlinear process is believed to be supported by the ps-long metamaterial resonance, allowing an efficient build up of the nonlinear process.

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 channel and draws a comparative analysis between the two optical channels in term of nonlinear effects. Bit error rate 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.

I will explore various facets of photonic quantum systems and their application in photonic quantum technologies. Firstly, I will focus into quantum foundations and by discuss quantum interference, a key element in photonic quantum technologies. I will highlight how the distinguishability and mixedness of quantum states influence the interference of multiple single photons – and demonstrate novel schemes for generating multipartite entangled quantum states. I will then address photonic quantum computing, specifically focusing on the building blocks of photonic quantum computers. This includes the generation of resource states essential for photonic quantum computing. I will then shift to photonic quantum networks, covering both their hardware aspects and showcasing quantum-network applications that extend beyond bi-partite quantum communication. Lastly, I will outline how photonic integration facilitates the scalability of these systems and discuss the associated challenges.

Joining the rich photophysics of organic light-emitting materials with the exquisite sensitivity of optical resonances to geometry and refractive index enables a plethora of devices with unusual and exciting properties. Examples from my team include biointegrated microlasers for real time sensing of cellular activity and long-term cell tracking, as well as the development of photonic implants with extreme form factors and wireless power supply that support thousands of individually addressable organic LEDs and thus allow optogenetic targeting of neurons deep in the brain with unprecedented spatial control. Very recently, by driving the interaction between excited states in organic materials and resonances in thin optical cavities into the strong coupling regime, we unlocked new tuning parameters which may play a crucial role in the next generation of TVs and computer displays to achieve even more saturated colour while retaining angle-independent emission characteristics.

We present photoconductive emitters for broadband THz emission based on Ge and GeSn. Ge is attractive for this purpose due to its high carrier mobility and because of the absence of infrared-active phonons that suppress the THz emission. Emitters based on Ge, with Au trapping centers for carrier lifetime reduction, excited with 11 fs near-infrared pulses emit gapless THz spectra extending up to 70 THz. For excitation at the telecom wavelength 1550 nm a slight reduction of the bandgap of Ge is desirable. We show that this can be achieved in layers of GeSn with 2 % of Sn, grown by molecular beam epitaxy on Si. THz devices on a silicon platform offer the perspective to develop CMOS compatible THz systems with photonic integration both for the near-infrared radiation and the broadband THz radiation.

Spintronic THz emitters, consisting of Ta/Co/Pt trilayers patterned into rectangles of size 10 µm, have been integrated in planar electromagnetic antennas of various types (dipole, bow-tie, spiral). Antenna dimensions and shapes have been optimized with the help of electromagnetic simulations so as to maximize antenna efficiency in both narrow-band and broad-band geometries at/around 1 THz. The THz emission has been characterized using a pump-probe free space electro-optic sampling set-up, both for a single emitter geometry and for arrays of emitters. An increase of the detected THz signal is observed for all antenna geometries, with an enhancement ratio up to a factor of ten, together with changes of the emission bandwidth consistent with simulated characteristics.

In this work, we performed temperature-dependent studies of the THz transient amplitude FeCo waveformes, from a FeCo/graphene nanobilayer sample, triggered by fs pulsed laser in the 80–400 K range. We show that a due-twofold extension, in the range 80–300 K the amplitude increases with temperature and tends to saturate above this range. This dependence contrasts sharply with the temperature dependence of the FeCo film's magnetization, which shows a typical ferromagnetic (FM) trend with Curie temperature well above 400 K. We explain this discrepancy, as the presence of an antiferromagnetic (AFM) at the FeCo/graphene interface, which is associated with the native oxide formed at the FeCo surface. The Angle Resolved X-ray Photoelectron Spectroscopy studies of a bare FeCo film revealed coexistence of the metallic [Co(0), Fe(0)] and antiferromagnetic [Co(II)O and the Fe(III)2O3] phases, at the sample surface. The observation of the exchange bias in our magnetization hysteresis loop of a FeCo film confirms presence of an FM/AFM interface layer at the FeCo surface. In summary, we conclude that the temperature dependence of the THz transient amplitude is governed by the AFM phase.

THz photonics-based sources are attractive as they offer room-temperature solutions that rely on mature photonics technology and provide broadband tunability and large modulation bandwidth to address specific THz applications such as high-data-rate communications or spectroscopy. We will present an overview of our recent results on coherent and structured light emitted from III-V semiconductor lasers and we will focus on THz generation based on these original near-infrared lasers operating at 1064 nm. Vertical external-cavity surface-emitting lasers that exploit parity symmetry breaking together with integrated meta-surfaces can generate unconventional light states such as vortex light, spatially modeless laser, transverse multiplexing, non-linear structured light... Coherent THz emission has been obtained from a dual-mode laser, that operates simultaneously on two Laguerre-Gauss transverse modes, using either uni-traveling-carrier photodiodes and plasmonic photo-conductive antennas. We will discuss the ongoing work towards multiplex structured coherent photonic sources that offer high potential for powerful THz emission.

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 photonics-enabled 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 instruments operating in the range of a few terahertz for deep space explorations. In a spectrometer development, it is essential to have a diffraction grating capable of withstanding the demanding conditions of space. However, the dimensions and morphology required make obtaining a diffraction grating challenging. For the first time, it has been possible to manufacture a metallic diffraction grating with adequate dimensions and a sawtooth profile by laser micro-structuring with a 5-axis femtosecond laser system on an aluminum substrate. The grating operates only with the first diffraction order and favors the transverse magnetic (TM) over the transverse electric (TE) polarization components. We have been able to have the necessary equipment to experimentally verify that the value of the diffraction efficiency of the manufactured diffraction grating is the predicted one by theoretical simulations. Diffraction efficiencies greater than 70% have been achieved.

Structures, dynamics, and phenomena in the chemical and biological sciences are based on equilibrium descriptions in the absence of an external stimulus. However, an ever-growing swathe of processes involving energetic stimuli – for example, light excitation in photocatalysis or strong-field THz radiation –drive these systems out of equilibrium to render new states or behavioral dynamics along with new properties in health-related applications. In this talk, we will anchor ultrafast terahertz excitation as a generalized approach to theoretically and experimentally study basic nonequilibrium states of water and water-based systems to orchestrate solvation (bio)chemistry to capable of tackling real-world societal challenges, such as paving the path to water remediation of environmental pollutants and understanding biological function of proteins under external stimuli for personalized medicine.

The amalgamation of metamaterial-based sensing with a biochemical functionalization of the corresponding active sensor surface offers a selective or even specific detection pathway. In this technique, a combination of surface immobilization reactions and inherently selective biochemical binding events enables the fixation of one target biomarker at the active sensor interface. We present in this contribution a versatile, metamaterial-based sensing platform which is capable of selectively detecting transcription factor EGR2 (early growth response protein 2), highly sensitive detection of reverse transcribed MIA (melanoma inhibitory activity) DNA and responding to exosomes and other vesicular or supermolecular structures as well. Insights from measurements like these have the potential to deliver novel information on melanoma, a deadly type of skin cancer as a prime model of many cancer diseases.

We achieved ultrafast ps THz switches with Fano resonant with low pump threshold 200uJ/cm2 [1]. Pixelated frequency-agile THz metasurfaces for molecular fingerprint sensing are proposed [2]. We design photon-tunable THz sensor, shifted from the Lorentz mode to EIT mode [3]. We exploit laser controlled two EIT modes dividing spectra to realize calibration free for monitoring cancer [4]. We explore quasi-BICs with AuNPs to 674 GHz/RIU and 1 picoM [5]. We discovered THz accelerates DNA unwinding and reduces transition temperature [6,7]. We found THz enhances permeability of the voltage-gated ion channel [8], and THz-infrared light have nonthermal, long distance, and reversible modulatory effects on action potential [9]. THz modulates the brain behavior, and accelerates associative learning [10]. [1]. J.Lou, et al., Small 2021. [2]. L. Sun, et al., Nanoscale 2022. [3]. Y. Jiao, et al., Materials Horizons,2022. [4]. J.Lou, et al. PNAS 2022. [5]. R. Wang, et al., Small 2023. [6]. K.Wu, et al, JPCL 2020. [7]. C.Zhang, et al., Nano letter 2022. [8]. Y.Li, et al., JACS 2021. [9]. X.Liu, et al., PNAS 2021. [10]. J.Zhang, et al., Nature Communication 2021.

High-frequency Terahertz waves regulate the dynamic network of mitochondria in neuropathic pain model of mice Yuchen Tian1, Wenyu Peng2# 1The college of life sciences, Northwest University, Xi'an 710032, China 2Department of Biochemistry and Molecular Biology, Air Force Medical University, Xi'an 710032, China In this research, we developed a neuropathic pain model using spared nerve injury (SNI) and observed that a the frequency of ~36 THz could alleviate neuropathic pain, as assessed through pain behavior tests. By comparing the morphology and network of mitochondria in the anterior cingulate cortex (ACC) brain region under two physiological conditions using electron microscopy, we discovered that this frequency of Terahertz waves can regulate mitochondrial dynamic network and enhance ATP production. However, the specific mechanism linking mitochondrial morphology and neuropathic pain remains unclear. In conclusion, this article highlights the potential mechanism of high-frequency Terahertz waves alleviate the neuropathic pain through mitochondria, which may a potential interventionstrategy for other mitochondrial diseases.

As continuous wave (cw) THz spectroscopy advances rapidly, its high potential for sensing and non-destructive testing is becoming increasingly apparent. This is demonstrated in two recent developments: First, I will present our novel terahertz receivers for frequency-domain spectroscopy based on rhodium-doped InGaAs grown by molecular beam epitaxy. These new devices provide a peak dynamic range of 130 dB, an 18 dB improvement over the state of the art. Secondly, I will present a compact terahertz spectroscopy system with a measurement rate of 1 kHz, made possible by a single photonic integrated circuit that acts as the optical driver engine. These new results demonstrate the excellent performance and flexibility of frequency-domain spectroscopy, paving the way for compact and task-specific terahertz systems for science and industry.

Solid-state electrolytes based on the poly(ethylene oxide) (or PEO) polymer containing lithium (Li) salts paved ways for the development of a new generation of Li-ion batteries featuring high energy storage capacity and enhanced safety. Enabled by the flexibly vibrating polymer chains, the ionic conduction in these electrolytes is a result of intra- and inter-chain hopping of the Li+ ions within the PEO matrix. Mixing different types of Li salts in PEO-based electrolytes was found to enhance the Li+ ion mobility significantly, resulting in an improved performance of the developed Li-ion batteries. Here we study the conduction properties and the associated temperature-dependent ionic and molecular dynamics of single- and dual-salt PEO-based electrolytes in the THz frequency range. This is accomplished by using THz time-domain spectroscopy and applying a physical model that reproduces the experimental results.

Overcoming technical performance limitations in the detection and characterization of terahertz (THz) radiation will enable ground-breaking scientific advances from the study of fast and non-reproducible phenomena to enabling THz quantum applications that require single-photon sensitivity. Electro-optic sampling techniques intrinsically rely on the acquisition of multiple data points to reconstruct the full THz waveform, which leads to long data acquisition times, and prevents the detection of single photons. We have developed two distinct and highly sensitive detection techniques for pulsed THz radiation: i) a single-pulse measurement which employs chirped-pulse spectral encoding and a dispersive Fourier transform method for time-resolved THz spectroscopy at a demonstrated rate of 50 kHz; and ii) a single-THz-photon detection technique based on parametric frequency conversion and single-photon counting technology capable of detecting THz pulses at the zeptojoule level. These extreme detection schemes will lay the foundation for THz applications in the single-pulse and single-photon regimes.

For several years now, terahertz technologies have been sufficiently developed to offer new techniques and ideas for detecting, probing or characterizing materials in various fields of research. These new photomixers-based instruments provide access to the amplitude and phase of the signal with sufficient SNR over the frequency range from 100 GHz to 5 THz. In this presentation, we present a new terahertz bench that exploits this acquisition technique (TERASCAN 1550) with a configuration that enables reflection and transmission measurements to be carried out for a variable angle of incidence (0 to 50 degrees). This installation is one of the first that aims to collect a complete set of data (reflection, transmission absorption and phase) in this terahertz frequency range, offering scientists a plateform with a comprehensive and robust method for characterizing layered materials. Demonstration is given with measurements made on a silicon wafer sample for angles of incidence from 10 to 40 degree in reflection and transmission mode. It shows that for this non-absorbing media, the relation R+T=1 is satisfied over this frequency range and for every angle of incidence.

Terahertz non-destructive testing offers a highly attractive solution for inline testing of electrode thicknesses in battery production for electric vehicles. Measuring systems with high spectral bandwidths are required to address the thin layers of typically less than 100 µm. In addition, multiple measuring heads are desired at different location on a production line to ensure adequate control at high throughput. We solve this by means of a highly scalable photonic terahertz radar. Its measuring principle is based on frequency-modulated continuous wave technology in conjunction with two-color laser radiation. The number of measuring heads can be easily scaled through the use of laser amplifiers. Another advantage of a photonic continuous wave system is the simple possibility of distributing fiber-coupled measuring heads over long fiber lengths of even more than 100 m. In this article, we show the potential of the system concept by the implementation of an 8-channel system and demonstration of relevant thickness measurements.

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.

Terahertz time-domain spectroscopy (THz-TDS) systems enable a wide range of applications from fundamental research to industrial non-destructive testing. However, the comparative bulk, mechanical sensitivity, and high cost have hindered their widespread use in applications outside of research labs. Solutions for constructing more compact and lower cost THz-TDS systems based on semiconductor lasers have been around for more than 20 years. However, their performance has lagged far behind conventional systems. A type of light source that promises to fill this gap are monolithic mode-locked laser diodes (MLLDs). These sources emit stable pulse trains in the 1550 nm telecom band with sub-picosecond pulse durations and a repetition rate of a few dozen gigahertz. In this contribution, we review the theoretical background of THz-TDS using MLLDs and the steps that are necessary for achieving good system performance. We discuss promising applications and present first results as well as suitable data evaluation techniques.

Organic nonlinear optical (NLO) crystals have emerged as highly efficient sources and field-sensitive detectors of broadband THz radiation. They provide an exciting alternative to conventional inorganic or semiconducting materials due to their inherently high second-order nonlinearity and broadband phase matching, both tailorable via molecular engineering. We will discuss our recent results on ultra-broadband time-domain spectroscopy from 1 THz to 40 THz (300 µm to 7.5 µm of wavelength) based on a compact femtosecond laser operating at MHz repetition rates. We explore new and nearly forgotten organic NLO crystals that enable high-dynamic range time-domain measurements at room temperature and sub-Watt levels of near-IR pumping through optical rectification and terahertz-induced lensing. We will also provide an outlook for future research directions to improve the spectral flatness and dynamic range.

High speed terahertz imaging based on optimized galvanometric illumination Yuzhe Zhang, Ran Ning, Jie Zhao, Shufeng Lin, Lu Rong, Dayong Wang The pursuit of high-resolution, high-fidelity, real-time imaging is receiving significant attention in terahertz community. In this study, a versatile illumination approach based on a dual-mirror galvanometer is proposed and optimized for terahertz full-field imaging and computed terahertz tomography. We analyzed the mechanism of galvanometric illumination and elucidated three main factors affecting its homogeneity properties. In this illumination module, the terahertz beam is deflected rapidly by the galvanometer which is driven by triangular voltage signals, and then focused by a self-designed aspherical f-θ lens to illuminate the object at an equal lateral scanning velocity. A homogeneous illumination field with a speckle contrast of 0.11 and isotropic imaging resolution is recorded by an array detector in the form of non-correlated accumulation in a single integration time. By virtue of leading illumination homogeneity and parallelism, a compact imaging system is built for 2D and 3D terahertz imaging with high imaging speed and fidelity.

Terahertz (THz) radiation has already been applied to many technologies and devices in various areas, including medical diagnostics, non-destructive testing, data transmission or imaging. However, one must remember that an extremely low ratio of wavelength to the aperture diameter in this spectral range poses unique challenges. This work investigates two types of spatial filtering methods – positive and negative phase contrast and dark field. The sets of phase objects and filters, have been manufactured by means of FDM 3D printing. The possibility of realizing the phase contrast and dark field imaging has been evaluated in the 4f optical setup, consisting of two HDPE lenses having 300-mm-diameter and 300-mm-focal length. The experimental data has proved without a doubt the usefulness of this method but also shown some unexpected effects connected with the very long wavelengths (in comparison to the dimensions of the setup and its elements).

Spatial and temporal control of thermally emitted terahertz radiation could pave the way for a new family of devices in imaging, spectroscopy and communication systems. We devise an all-optically controlled THz emissivity modulator and use it to generate structured THz illumination patterns. Using this unconventional THz source, we present a completely new approach to THz imaging based on computational (ghost) imaging with a single-pixel detector with potential to be competitive with commercial systems without the need for femtosecond lasers.

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, serves as a zone plate, allowing precise manipulation of 100 GHz radiation. Investigations involved imaging beam shape with different geometrical parameters of the CSRR and examining the metasurface's functionality under mechanical bending which revealed a minimal reduction in beam intensity. Additionally, the metasurface demonstrated the ability to control polarization based on its rotational angle, enhancing THz imaging capabilities. Practical demonstrations, including imaging a plastic card with a key, showcase the metasurface's suitability for real-life scenarios, highlighting its value in THz imaging systems.

We present the application of THz monocycles to the Atom Probe Tomography (APT), an analytical microscope that allows the three-dimensional mapping of chemical heterogeneities in a material at the atomic scale. When a positive electric field of several volts per angstrom is applied to the surface of a material, the surface atoms evaporate as ions even at cryogenic temperatures, this is the work principle of APT. We prove that THz transient can induce the controlled evaporation of surface atoms due to the strong increase in the THz field in the near field of the sample. In addition, the use of THz pulses reduces the thermal effects reported when using laser pulses in the visible or near ultraviolet domain. We are also studying the effect of the THz pulses on the energy of the evaporated ions.

THz digital holography with camera sensor in off-axis configuration suffers from difficult recording conditions.In order to obtain a high resolution on the object, the latter must be close to the sensor, yielding high recording angles. It was already shown that iterative reconstruction methods can be used to limit the impact of spurious fringes obtained in such schemes. In this paper, we propose an inverse problem-based reconstruction technique that jointly reconstructs the object field and the amplitude of the reference field. Regularization in the wavelet domain promotes a sparse object solution. A single objective function combining the data-fidelity and regularization terms is optimized with a dedicated algorithm based on an ADMM framework. Each iteration alternates between two consecutive optimizations using projections operating on each solution and one soft thresholding operator applying to the object solution. The method is preceded by a windowing process to alleviate artifacts due to the mismatch between camera frame truncation and periodic boundary conditions assumed to implement convolution operators. Experiments demonstrate the effectiveness of the method.

Because of the unique properties of terahertz (THz) radiation, the development of THz imaging has attracted considerable attention. With the introduction of near-field technology, the resolution of THz imaging was significantly improved, which has greatly broadened the application fields of this technique. Here, a new approach was developed for THz near-field microscopy based on a cross-filament. The cross-filament was formed by two crossed air-plasmas, which opened a dynamic aperture to modulate the intensity of a THz beam on a sample surface. By using this technique, THz near-field information can be acquired without approaching the sample surface. To demonstrate the feasibility of this technique, sub-wavelength THz imaging of four different materials was achieved, including metallic, semiconductor, plastic, and greasy samples. The advantages of the technique are expected to accelerate the advancement of THz microscopy.

Optical cavities with nonlinear elements and delayed self-coupling are widely explored candidates for photonic reservoir computing (RC). For time series prediction applications that appear in many real-world problems, energy efficiency, robustness and performance are key indicators. With this contribution I want to clarify the role of internal dynamic coupling and timescales on the performance of a photonic RC system and discuss routes for optimization. By numerically comparing various delay-based RC systems e.g., quantum-dot lasers, spin-VCSEL (vertically emitting semiconductor lasers), and semiconductor amplifiers regarding their performance on different time series prediction tasks, to messages are emphasized: First, a concise understanding of the nonlinear dynamic response (bifurcation structure) of the chosen dynamical system is necessary in order to use its full potential for RC and prevent operation with unsuitable parameters. Second, the input scheme (optical injection, current modulation etc.) crucially changes the outcome as it changes the direction of the perturbation and therewith the nonlinearity. The input can be further utilized to externally add a memory timescale that is needed for the chosen task and thus offers an easy tunability of RC systems.

Programmable photonic circuits manipulate the flow of light on a chip by electrically controlling a set of tunable analog gates connected by optical waveguides. Light is distributed and spatially rerouted to implement various linear functions by interfering signals along different paths. A general-purpose photonic processor can be built by integrating this flexible hardware in a technology stack comprising an electronic monitoring and controlling layer and a software layer for resource control and programming. This processor can leverage the unique properties of photonics in terms of ultra-high bandwidth, high-speed operation, and low power consumption while operating in a complementary and synergistic way with electronic processors. This talk will review the recent advances in the field and it will also delve into the potential application fields for this technology including, communications, 6G systems, interconnections, switching for data centers and computing.

The Spoof Surface Plasmon Polariton (SSPP) phenomenon, which is a metal having corrugations on both sides and shows low return loss and high performance at terahertz frequencies, is utilized for the terahertz antenna design. The design of the antenna features uneven corrugations to create dual confinements, mimicking two distinct traveling surface waves. As a result of the asymmetrical design, the designed antenna has end-fire radiation. At the bottom end of the SSPP structure, a taper is integrated to reduce the overall return loss of the antenna. The parts of the antenna have been optimized, including the transition circuit from the coplanar waveguide (CPW) to SSPP, the asymmetrical SSPP, and the taper, to increase the radiation. Consequently, the advanced SSPP antenna records a return loss of -13 dB in the range of 0.5 THz to 2 THz and peaks with a gain of 14.8 dBi at 1 THz.

We conducted THz emission measurements on the bulk MoS2 layered crystal, employing two femtosecond excitation wavelengths: 480 nm for above-bandgap excitation and 1300 nm for below-bandgap excitation.

Water molecules absorb THz radiation, which makes it an excellent candidate for use in a variety of medical research and diagnostic applications. To develop such technologies, it is crucial to use realistic tissue-mimicking phantoms to understand how the radiation interacts with tissue. In this study, we have characterised a gelatine phantom that dries over time with continually varying moisture content. The widely accepted model for determining the optical characteristics of tissue samples and phantoms assumes that only the free water changes as hydration increases, while the bound water stays constant. Our comparison of this model with measured permittivities of the gelatine phantoms, obtained using THz time-domain spectroscopy, yielded results that didn’t agree. The phantom samples were simultaneously investigated using Raman spectroscopy to further investigate the water molecules, with varied coordination levels evolving with hydration. These measurements also indicated a continual variation in the bound water concentration in the phantoms.

The confocal microscopy in visible has been widely studied which offers many advantages, including the ability to control depth of field, elimination or reduction of background information away from the focal plane, and the capability to collect serial optical sections from thick specimens. In this paper, we propose THz confocal scanning imaging based on three types of lenses, i.e., terahertz super-oscillating lens (TSOL), fibonacci lens (FBL) and transmissive convex lenses (TCL). TSOL can produce large focal depth and far-field subwavelength focused beam. FBL is a diffractive lens that produces multiple foci along the axial coordinate. TCL produces one focal point, which is the typical THz lens applied in THz imaging system. The implementation and calibration of three types of THz confocal imaging system will be detailed and the corresponding experimental results will be evaluated and shown. The application will be discussed.

This study introduces an innovative real-time methodology for detecting concealed metallic objects using a state-of-the-art on going SiGe 300 GHz radar source. Achieving remarkable precision at 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 new ground with millimeter-level accuracy. Equipped with evaluation kits featuring a user-friendly graphical interface (WebGUI), users can fine-tune baseboard parameters, visualize radar data, and make real-time adjustments. The reflected signal strength exhibits a nuanced second-order polynomial nature intricately correlated with signal bandwidth and the target's distance. Notably, the system showcases maximum and minimum power peaks at 289 GHz and 329 GHz, respectively, achieving a Signal-to-Noise Ratio (SNR) of 3.78 at a detection distance of 2.0 m. A detailed analysis reveals the system's capabilities, with the maximum target distance spanning 2.0 m for a 20 GHz

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 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

In this study, we investigated the effects of chirp (C) and phase differences caused by air dispersion (Δϕ) on the polarization and spatio-temporal evolution of Terahertz (THz) radiation generated in a two-color plasma within a nitrogen atmosphere. Our laser emits 800 nm radiation at 1 kHz, with 3 mJ energy and a 35 fs duration. Setting C=-0.83 and Δϕ=π/2 maximizes THz generation efficiency and yields elliptical polarization. Further exploration into different chirp and wavelength dispersion settings revealed an interesting phenomenon: the THz polarization transitions from elliptical to either circular or to a ‘flower-like’ shape. The observed polarization shift corresponds to alterations in the spatio-temporal trajectory of the THz signal, demonstrating characteristic patterns. Supported by the mathematical photocurrent model, our results open up new avenues for applications in various fields.

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. Amplification of exited slow SPPs (with a phase velocity down to ~10^6 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.