Motivated by the recent advances in 5G and 6G technologies, we developed an optoelectronic hybrid system capable of high-resolution spectrum analysis in the terahertz range. The concept involves the phase-locking of three continuous-wave (CW) distributed-feedback diode lasers by mixing the laser light with the radiation of a high-power, electronic narrow-band emitter. To synchronize the laser beat signals with the reference source, two heterodyne optical phase-locked loops consisting of a photoconductive antenna and a PID controller are implemented in the setup. This approach eliminates the need for frequency extenders currently required in state-of-the art spectrum analyzers, resulting in a simpler and more cost-effective architecture. Using our hybrid system, we are able to generate laser difference frequencies ranging from 10 MHz to over 1 THz, with precision in the range of several Hz. Moreover, the frequency can be easily extended to higher frequencies by incorporating suitable electronic emitters or by cascading even more lasers with our concept. To demonstrate the system’s resolution and stability, we measure various spurious harmonics of electronic emitters, by implementing the laser frequency synchronization setup in a CW free-space terahertz spectrum analyzer.
We proposed and fabricated a high-power high-directivity terahertz (THz) oscillator, which consists of two coupled offset-fed slot-ring antennas based on resonant tunneling diodes (RTDs). From an electromagnetic simulation, the two-slot-ring antenna geometry enables a high directivity over 11dBi downwards. The simulation results also suggested a high output power of over 1 mW at around 500 GHz with the offset-fed structure by increment in radiation conductance. We obtained ~500 GHz radiation with the fabricated device, but the output power (~100 μW) was lower than expected, because of the unexpected series resistance around the RTD mesa. This undesired series resistance can be avoided by fabrication optimization, thus higher output power can be anticipated in future works.
It is shown that structured nonparaxial terahertz (THz) light in the form of Gaussian, Bessel and Airy beams can be generated using exclusively silicon diffractive optics elements fabricated by femtosecond laser ablation technology. The accelerating nature of the structured light is demonstrated via THz imaging of objects partially obscured by a metallic obstacle. Lensless nonparaxial beam generation and application in THz imaging of single objects and stacked graphene layers are presented and discussed. Benchmarking revealed that the use of structured THz light in imaging consistently outperforms the conventional one in terms of resolution and contrast.
Spintronic terahertz emitters have attracted considerable interest within the last decade. They promise terahertz sources with unmatched broad frequency bandwidth which are easy to manufacture and operate, and therefore easy to scale at low cost. However, the experiments and proofs of concept rely on free-space ultrashort-pulsed pump lasers and rather complex benchtop setups. This contrasts with the requirements of widespread industrial applications, where robust, compact, and safe footprints are needed. To meet these requirements, we present a novel fiber-tip spintronic terahertz emitter solution that allows spintronic terahertz systems to become fully fiber-coupled.
We present the first demonstration of a “high frequency” backward THz-wave parametric oscillator designed for a center frequency of 0.87 THz by quasi-collinear phase matching in a slant-stripe-type periodically poled lithium niobate crystal when pumped with sub-nanosecond pulsed source of $\lambda=$ 1064.44 nm. Along with the THz wave, an idler wave appears simultaneously with $\lambda=1067.75\mathrm{n}\mathrm{m}$. We also show that the oscillation frequency is tunable by a simple crystal rotation, spanning the range of 0.836–0.905 THz based on the change in the spectral line separation between the pump and idler beams. The threshold pump energy for BW-TPO was determined to be 12.4 mJ, equivalent to a pump intensity of 7.24 $\mathrm{G}\mathrm{W}/\mathrm{c}\mathrm{m}^{2}$ while obtaining a conversion efficiency as high as 8.39% at a pump energy (intensity) of 15.5 $\mathrm{m}\mathrm{J}(9.05\mathrm{G}\mathrm{W}/\mathrm{c}\mathrm{m}^{2})$.
A novel unsupervised deep neural network framework driven by a physics model is introduced to design metasurface-based holograms. The proposed framework shows perfect reconstructions of holographic images with a shorter prediction time, higher peak signal-to-noise ratio and better structural similarity compared with the conventional Gerchberg-Saxton algorithm. An end-to-end design of metasurface-based holograms without requirements of complete light modulation is demonstrated. The proposed framework opens up a new approach to inverse design of metasurface-based photonic devices.
We introduce a double near-field THz probe microscope using micro-structured photoconductive antennas that allows the local excitation and detection of THz transients. When the THz probes for emission and detection are at distances much shorter than THz wavelengths, this setup effectively detects the complex THz field at the position of the source. The imaginary component of this field corresponds to the partial local density of optical states (partial LDOS), which defines the strength of interaction of the local source with its surrounding photonic medium. We use this novel technique to perform the first direct measurement of the partial LDOS of a dipole source close to a planar interface, the so-called Drexhage configuration, achieving an excellent agreement with theory. Our direct determination of the partial LDOS by measuring the complex field at the position of the source illustrates the potential of THz near-field microscopy for the precise investigation of photonic media and can be easily applied to more complex resonant media.
Ultra-fast, GaAs-based multi-quantum well photodetectors operating in the 9.5-11.5um range are demonstrated, with a 3dB-cutoff bandwidth of 100GHz at room temperature. The multi quantum-well active region is sandwiched between a bottom metallic ground-plane and a top two-dimensional array of patch antennas connected in parallel. We study arrays of different dimensions and measure their frequency response in the range 0-220GHz. Beyond 100GHz we find a roll-off dominated by the 2.5ps-long, intrinsic capture time of the photo-excited electrons. To demonstrate the potential of these detectors for fast sensing applications, we show that they can be used to measure electronically and in real-time the time-dependent emission frequency of a quantum cascade laser operated in pulsed mode over a frequency range $\gt60$ GHz. By exploiting broadband electronics, and thanks to its high signal-to-noise ratio, this technique allows the acquisition, in a single-shot, of frequency-calibrated, high resolution heterodyne mid-infrared spectra spanning up to 100GHz and beyond.
Based on its high sensitivity to the presence of water, terahertz radiation has introduced itself as a suitable source for the study of biological tissue and the diagnosis of some medical conditions. As shown in previous publications, the water content of a given system can be calculated by approximating the tissue with effective medium theories. Within these models, the optical behavior of water is a well-known temperature dependent parameter, whereas the behavior of the dry tissue has been traditionally assumed to be temperature independent. In this study we show with the usage of a self-designed heat chamber that the dielectric functions of various biological tissues as porcine skin, oak leaf and brown mushroom show between $20 ^{\circ}\mathrm{C}$ and $36.5 ^{\circ}\mathrm{C}$ indeed some variations based on their respective temperatures, which might indicate that this parameter cannot always be neglected in the analysis models.
We present a broadband, monolithic outcoupler based on planarized double metal waveguides that addresses both the reflectivity design and broadband narrow beam emission in THz quantum cascade lasers. The waveguide mirror reflectivity can be adjusted by shaping the end facet, obtained with an efficient inverse design algorithm. The THz laser radiation is coupled to a broadband patch-array antenna for surface emission. All the components are optimized for octave-spanning spectra between 24 THz. We implement these concepts to demonstrate a broadband surface-emitting THz quantum cascade laser frequency comb featuring output powers of 13 mW, optical bandwidths in excess of 800 GHz and a single-lobed far-field pattern with a beam divergence below $( 20 ^{\circ} \mathrm{x}20 ^{\circ})$.
Photonic sources for terahertz (THz) generation demonstrate unrivaled phase noise. The resulting THz waves can be used for precision spectroscopy of molecular rotations. Here, we show that a rotational transition of nitrous oxide (N 2 O) can be used to stabilize the THz frequency of a photonic oscillator to a few parts per trillion,, after only 10 seconds of averaging time. The core of the photonic oscillator utilizes a dissipative Kerr soliton generated in a micro-ring resonator, and the THz spectrometer is purely built in a waveguide. With these techniques combined, this demonstration represents a low-noise, low-drift THz source that is feasibly miniaturized.
A novel detection technique for capillary electrophoresis (CE) is presented using terahertz (THz) waves, namely “THz-CE,” which enables to follow the separated substances in a solution flowing in a hollow of capillary whose inner diameter (ID) is smaller than 100 μm. We placed a narrow open-tubular capillary on the surface of a GaAs semiconductor substrate as a “localized” THz-emitter. By focusing femtosecond pulsed laser beams at the surface of GaAs closest to the capillary, THz waves were locally generated to pass through the capillary, so that THz absorbance spectra were obtained from the capillary which has narrower ID than the diffraction limit. We successfully achieved the CE separation between acetic acid and n-propionic acid around neutral pH and obtained the electropherograms with THz-time domain spectroscopy (TDS), which are similar to those obtained with conventional contactless conductivity detection. Our proposed THz-CE showed the potential for the systematic analysis of inter/intra-molecular weak interactions like hydrogen bonds, which are unable to obtain with conventional detectors. This technique will be able to be used for the analysis of biological samples containing many chemical components combined with various separation techniques.
This work evaluated the effects of humidity and temperature on organic nonlinear optical crystals. To assess the effect of humidity, we compared THz wave generation before and after exposure to a temperature of $30^{\circ}\mathrm{C}$ and a humidity of 80 % for 10 weeks. THz wave output decreased for the DAST crystal but not for the DASC or OH1 crystals. Regarding the effect of temperature, we evaluated the temperature dependence on the electro-optic (EO) effect. When evaluated using one DAST crystal, the EO signal changed periodically with temperature fluctuations. In comparison, when the a-axes of the two DAST crystals were orthogonal, the EO signal did not fluctuate. These results show that the properties of organic nonlinear optical crystals need to be understood and optimized.
Graphene is an exciting candidate for the detection of high-frequency electromagnetic radiation. Here, we have studied the bolometric performance of epitaxial graphene quantum dots (Q.D.s) on the silicon carbide (SiC) substrate in the terahertz (THz) range. The graphene Q.D. having a diameter in the 200 nm range, exhibited an extremely high resistance variation with temperature up to 4.7 MΩ K-1, a crucial parameter for the hot electron bolometers. The graphene Q.D.s bolometers have been fabricated in different geometrical configurations, such as variations in electrode spacing (2.5 and 5.0 μm) and parallelly connected arrays of 4 and 8 Q.D.s. It is demonstrated that the absorbed power can be improved by tuning the bolometer geometrical configuration and the active graphene area, and the electrical responsivity is still very high for an extensive range of absorbed power. Additionally, we report that the photo response of graphene Q.D. bolometer devices is meagerly affected by the presence of a magnetic field as high as 15 T. The results presented here open ways to continue to optimize and realize the chip-scale matrix of the graphene Q.D.s bolometers for THz imaging and magneto-optical spectroscopy applications.
This work presents the results of the initial acquisition of a multi-modal dataset that will be utilized to train and test a neural network for wood sorting. The aim of the project is to improve wood recycling from bulky waste by using four complementary sensing systems: visual, infrared, terahertz, and thermography. The four systems were combined to capture 57 multi-modal images of bulky waste samples moving on the conveyor belt at a speed of 10 cm/s. Early fusion results on THz show 0.77 accuracy, whereas the best multi-modal data fusion accuracy equals 0.921.
Sparse decomposition method is used to locate the layer interface pulse with terahertz transmission signal as prior knowledge. Then take advantage of time of flight (TOF) method to realize multi thin layer coating thickness detection on anisotropic substrates without considering the complex properties of the anisotropic materials. This method is validated on the coated carbon-fiber reinforced polymers (CFRP) samples with two coating layers successfully.
We use multi-terahertz (THz) light fields to force electron–hole pairs in crystalline semiconductors onto closed trajectories and clock the delay between separation and recollision with a 300 as precision. This value corresponds to 0.7% of the driving field’s oscillation period. The strong Coulomb correlations emergent in atomically thin WSe 2 are found to shift the optimal timing of recollisions by up to 1.2 ± 0.3 fs compared to the bulk material. A quantitative analysis with quantum-dynamic many-body computations yields a direct and intuitive view on how the Coulomb interaction, non-classical aspects, the strength of the driving field, and the valley polarization influence the dynamics, opening unprecedented views of quantum many-body correlations and phase transitions.
The subject of latex film formation has been studied for many years and it is known to be affected by many environmental conditions such as evaporation rate, polymer glass transition temperature, T $_{\mathbf{g}}$, and particle size. Understanding latex film formation is particularly relevant to the paint industry, to ensure even coated films. In this study, THz-TDS was used to analyze various latex solutions with different polymer glass transition temperatures and particle sizes. 2D water distribution maps were produced, as a function of drying time, to monitor latex drying processes such as the ‘coffee-ring effect’.
