Time-domain Spectroscopy Setup Research Articles

Non-destructive evaluation of artillery combustible cartridge case using terahertz radiation

In this study, terahertz radiation was employed to determine the radius, diameter, and thickness of the nitrocellulose combustible cartridge case used for tank ammunition. The construction of the sample was outlined, and the refractive index of the case material was measured. A terahertz rotary scanner with a time-domain spectroscopy setup in reflection configuration was developed, and its performance and parameters were determined. A wire polariser was introduced as a reference to determine the stable time difference. The radius and diameter measurements of the case were based on a reference terahertz pulse reflected from its metal base. The thickness of the case was calculated based on the time difference between pulses reflected from its inner and outer surfaces. These measurements were in agreement with the reference measurements. The results prove that the developed setup and measurement methods are suitable for the non-destructive evaluation of cases and provided satisfactory performance.

Terahertz nondestructive testing of alumina-based ceramic ballistic protection armor

We prepared an alumina-based ceramic ballistic protection armor sample consisting of hexagonal alumina tiles arranged in a honeycomb structure, glued with Kevlar®, and covered with a polyurethane-based resin. The sample was shot with four projectiles, strongly hit from the back, and locally covered with water. The terahertz complex refractive index of the materials constituting the sample was measured. A terahertz time-domain spectroscopy (THz-TDS) setup in a reflection configuration was used to raster scan the XY sample. The results in the form of time signals, top views, and cross-sections enabled an understanding the internal structure of the sample. THz-TDS was proved to be suitable for non-destructive testing of the alumina-based armors with depth-discriminant 3D imaging. Transfer matrix method (TMM) correctly simulated terahertz signals reflected from the sample, calculated the thickness of the sample layers, and help determine the type of hidden defects.

Rapid sensing of hidden objects and defects using a single-pixel diffractive terahertz sensor

Terahertz waves offer advantages for nondestructive detection of hidden objects/defects in materials, as they can penetrate most optically-opaque materials. However, existing terahertz inspection systems face throughput and accuracy restrictions due to their limited imaging speed and resolution. Furthermore, machine-vision-based systems using large-pixel-count imaging encounter bottlenecks due to their data storage, transmission and processing requirements. Here, we report a diffractive sensor that rapidly detects hidden defects/objects within a 3D sample using a single-pixel terahertz detector, eliminating sample scanning or image formation/processing. Leveraging deep-learning-optimized diffractive layers, this diffractive sensor can all-optically probe the 3D structural information of samples by outputting a spectrum, directly indicating the presence/absence of hidden structures or defects. We experimentally validated this framework using a single-pixel terahertz time-domain spectroscopy set-up and 3D-printed diffractive layers, successfully detecting unknown hidden defects inside silicon samples. This technique is valuable for applications including security screening, biomedical sensing and industrial quality control.

Terahertz spectra of selected low-temperature cofired ceramics: A comparative study

Low-temperature cofired ceramics (LTCC) which exhibit low dielectric permittivity and low dielectric loss are promising platforms for emerging terahertz devices and systems including high-speed telecommunications. This study focuses on the analysis of the terahertz spectra of novel low dielectric permittivity LTCC materials, such as copper and zinc borates, zinc silicates, and lithium-containing dielectrics. Furthermore, their sintering conditions and THz characteristics were compared with the results presented for other materials in our previous works and those reported in literature, including selected microwave dielectric properties. The effect of sintering aids (AlF3-CaB4O7 and CuBi2O4) on the dielectric properties of Li2WO4, LiBO2, and 75 wt.% CuB2O4–25 wt.% Cu3B2O6 composite was also investigated. XRD patterns and SEM images illustrate single phase composition and dense microstructure of the tested ceramics. We describe in details the time-domain spectroscopy setup used to measure the dielectric properties of the developed materials in the 0.1–3 THz range.

400 kHz repetition rate THz-TDS with 24 mW of average power driven by a compact industrial Yb-laser.

We demonstrate a high average power terahertz time-domain spectroscopy (THZ-TDS) set-up based on optical rectification in the tilted-pulse front geometry in lithium niobate at room temperature, driven by a commercial, industrial femtosecond-laser operating with flexible repetition rate between 40 kHz - 400 kHz. The driving laser provides a pulse energy of 41 µJ for all repetition rates, at a pulse duration of 310 fs, allowing us to explore repetition rate dependent effects in our TDS. At the maximum repetition rate of 400 kHz, up to 16.5 W of average power are available to drive our THz source, resulting in a maximum of 24 mW of THz average power with a conversion efficiency of ∼ 0.15% and electric field strength of several tens of kV/cm. At the other available lower repetition rates, we show that the pulse strength and bandwidth of our TDS is unchanged, showing that the THz generation is not affected by thermal effects in this average power region of several tens of watts. The resulting combination of high electric field strength with flexible and high repetition rate is very attractive for spectroscopy, in particular since the system is driven by an industrial and compact laser without the need for external compressors or other specialized pulse manipulation.

Fast THz-TDS Reflection Imaging with ECOPS-Point-by-Point versus Line-by-Line Scanning.

We built a high-speed TDS setup with the use of electronically controlled optical sampling (ECOPS), which can measure up to 1600 terahertz pulses per second. The movement of the sample was provided by two fast-speed motorized linear stages constituting the gantry. We developed a flat-bar-based metal marker approach for the synchronization of continuous line-by-line scans. We carefully compared the performance of the terahertz reflection time-domain spectroscopy (TDS) scanner operating in a slow point-by-point and a one-hundred-times faster line-by-line imaging scheme. We analyzed images obtained for both schemes for a uniform metallic breadboard with holes, as well as a glass composite sample with defects. Although the measurement time was reduced by 100 times in terms of the line-by-line scheme, the overall performance in both schemes was almost identical in terms of the defects' sizes, shapes and locations. The results proved that the proposed ECOPS TDS system can provide uniform and extremely fast scanning without any deterioration in image quality.

Broadband terahertz time-domain polarimetry based on air plasma filament emissions and spinning electro-optic sampling in GaP.

We report on a time-domain polarimetry (TDP) system for generating and detecting broadband terahertz (THz) waves of different polarization angles. We generate THz waves from two-color laser filaments and determine their polarization states with a detection bandwidth of up to 8 THz using a spinning gallium phosphide crystal. The polarization of THz emission can be controlled by adjusting the position and tilt angle of the β-barium borate crystal. We characterize the precision of this system for polarimetric measurements at fixed time delay to be and for complete time-domain waveforms. We also demonstrate the feasibility of our TDP system by measuring broadband optical properties of anisotropic samples in both transmission and reflection geometries. The THz-TDP technique can be easily integrated in conventional THz time-domain spectroscopy setups using nonlinear crystal detectors.

A porous core Zeonex THz fiber with low loss and small dispersion

A fabricated microstructured polymer optical fiber (MPOF), with porous core and porous cladding consisting of a hexagonal array of circular air-holes, is experimentally investigated via a terahertz (THz) time-domain spectroscopy (TDS) setup. The fiber features a low propagation loss ranged from 0.0180 to 0.0345 cm−1, in the frequency regime of 0.3–0.5 THz with diameter of 3 mm. Meanwhile, the dispersion and fabrication deformation are also discussed. Experimental results show there is a flat and near-zero dispersion band in the range of 0.39 to 0.45 THz with a value of −0.285 ± 0.02 ps/THz/cm The simulation and experimental results of transmission loss and dispersion are in good agreement within 0.3–0.5 THz. There are a few deviations regarding air porosity between theoretical and actual structure, and the absolute variation of air hole fraction in core and cladding is 0.485% and 1.657%, respectively. The proposed MPOF is potentially useful for efficient and convenient transmission of broadband THz radiation.

Spintronic terahertz emission from Ni/Pt bilayer grown on MgO

Spintronic THz emission from Ni/Pt bilayer grown on MgO is reported based on the novel THz emitter using metallic structures. The Ni metal was deposited first on a MgO substrate and capped with a thin Pt metal via electron beam deposition. The THz emission data was obtained using a standard terahertz time-domain spectroscopy setup using a Ti: sapphire laser excitation source. Initial measurements were done using 800nm excitation with 7 mW and 185 mW pump powers under upward and downward magnetic field orientations. Polarity reversal of the terahertz signal was observed upon changing the orientation of the magnetic field. Maximum amplitude was found at 0.5 THz with bandwidth up to ~6 THz. A saturation fluence of 85.04 mJ/cm2 was calculated from the pump fluence-dependence plot of the THz peak-to-peak signal. The results are consistent with the spintronic THz emission due to the inverse spin-Hall effect and provide insights for future development and optimizations.

Development of a terahertz time-domain scanner for topographic imaging of spherical targets.

Topographical abnormality in corneal tissue is a common diagnostic marker for many eye diseases and injuries. Using an asynchronous optical sampling terahertz time-domain spectroscopy setup, we developed a non-contact and normal-incidence imaging system to measure topographic changes along the surface of spherical samples. We obtained orthogonal 1D scans of calibration spheres to evaluate the minimum axial resolution of our system. We determined the axial and spatial resolution of the scanner using 3D-printed spherical cross and Boehler star targets. Furthermore, we characterized the asymmetrical performance of the scanner due to the use of an off-axis parabolic mirror. Finally, we developed an edge-detection filter to aid with improving the topographic scans. We showed that when imaging samples were comparable in size to the human cornea, the axial and spherical spatial resolutions were limited to about 15 µm (∼λ/67) and 1 mm, respectively.

Terahertz time-domain spectroscopy characterization of nitrocellulose in transmission and reflection configurations

In this work, we report the first terahertz characterization of nitrocellulose, a workhorse energetic material widely employed in many civilian and military applications, especially as a propellant or explosive ingredient. The aim behind this study is to find the main spectral features and to draw the absorption and refractive index profiles of nitrocellulose using a terahertz time-domain spectroscopy setup over the frequency range from 0.3 to 3 THz. We particularly highlight the interest of carrying out combined measurements in transmission and in reflection configurations in order to increase the identification capabilities using the terahertz spectroscopy. Thus, five spectral absorption lines have been clearly observed in transmission mode, namely 0.41, 0.69, 1.01, 2.15 and 2.76 THz. These spectral resonances constitute fingerprints of nitrocellulose. When spreading a few particles of NC on an adhesive ribbon and performing characterization in reflection, the lines at 0.69 and 2.15 THz are still easily observed. These spectral fingerprints allow a simple, easy and highly sensitive remote detection and identification of nitrocellulose even if it is concealed behind barrier materials like papers, textiles and so forth.

Observation of enhanced terahertz emission in two-dimensional metal line arrays on GaAs surfaces

Enhanced terahertz (THz) emission was observed from semi-insulating GaAs with square and diamond two-dimensional metal line arrays (2DMLA’s) of subwavelength periodicity. An externally-applied magnetic field was used to generate elliptically-polarized THz radiation from the samples and the THz emission characteristics were measured using a THz time-domain spectroscopy setup in transmission geometry. Finite difference time-domain simulations verified the enhancement as due to confinement of the THz electric field at the 2DMLA apertures. These results demonstrating spoof plasmonic enhancement effects may be useful for future photoconductive antenna emitter designs incorporating 2D micro-scale integrated optics, THz metamaterials, without the need for expensive electron beam nanolithography.

Towards Fingerprint Spoofing Detection in the Terahertz Range.

Spoofing attacks using imitations of fingerprints of legal users constitute a serious threat. In this study, a terahertz time domain spectroscopy (TDS) setup in a reflection configuration was used for the non-intrusive detection of fingerprint spoofing. Herein, the skin structure of the finger pad is described with a focus on the outermost stratum corneum. We identified and characterized five representative spoofing materials and prepared thin and thick finger imitations. The complex refractive index of the materials was determined in TDS in the transmission configuration. For dataset collection, we selected a group of 16 adults of various ages and genders. The reflection results were analyzed both in the time (reflected signal) and frequency (reflectivity) domains. The measured signals were positively verified with the theoretical calculations. The signals corresponding to samples differ from the finger-related signals, which facilitates spoofing detection. Thanks to deconvolution, we provide a basic explanation of the observed phenomena. We propose two spoofing detection methods, predefined time–frequency features and deep learning based. The methods achieved high true detection rates of 87.9% and 98.8%. Our results show that the terahertz technology can be successfully applied for spoofing detection with high detection probability.

Terahertz optical fibers [Invited

Lying between optical and microwave ranges, the terahertz band in the electromagnetic spectrum is attracting increased attention. Optical fibers are essential for developing the full potential of complex terahertz systems. In this manuscript, we review the optimal materials, the guiding mechanisms, the fabrication methodologies, the characterization methods and the applications of such terahertz waveguides. We examine various optical fiber types including tube fibers, solid core fiber, hollow-core photonic bandgap, anti-resonant fibers, porous-core fibers, metamaterial-based fibers, and their guiding mechanisms. The optimal materials for terahertz applications are discussed. The past and present trends of fabrication methods, including drilling, stacking, extrusion and 3D printing, are elaborated. Fiber characterization methods including different optics for terahertz time-domain spectroscopy (THz-TDS) setups are reviewed and application areas including short-distance data transmission, imaging, sensing, and spectroscopy are discussed.

Monitoring of air voids at plastic-metal interfaces by terahertz radiation

The time domain spectroscopy (TDS) setup in a reflection configuration was exploited for non-destructive measurements of four samples with a 10-mm (approx.) leaf-shaped air void at the interface between the plastic and metal structures. We identified and measured the material parameters of the samples required for precise determination of the voids’ thicknesses in the range of 125–500 μm. For high resolution measurements we selected the reflection TDS setup with a short focal length and developed a method for the accurate calculation of the incidence angle. We used a deconvolution-based method for thickness monitoring of voids and discussed its limitations. The two-dimensional thickness, amplitude and binary maps were exploited for imaging of the void areas. We also showed that the multiplication of the deconvolved and binarised maps can provide relevant information about the real thickness of the layer and the presence of the void in the single image. The correlation coefficient between the real and TDS measured shapes of the leaf in the range 0.8–0.9 suggests a good perspective for this technique in industrial applications.

Terahertz radiation enhancement in dipole photoconductive antenna on LT-GaAs using a gold plasmonic nanodisk array

This study proposes a new-fashioned plasmonic photoconductive antenna (PCA) with high optical-to-terahertz (THz) conversion efficiency. Finite element method was used to investigate and optimize the interaction of 800 nm femtosecond laser with the designed nanodisk array in the antenna’s gap using its geometrical parameters. According to the simulation results, our optimized nanoplasmonic structure showed more than 38% enhancement in the absorption efficiency compared to the conventional structure without any nanostructure. Measuring the THz radiation of the fabricated PCAs using a time domain spectroscopy setup exhibited an exceptional 5.6 times higher electric field in 0.1–2.5 THz range compared to a similar PCA but without nanoplasmonic structure.

Experimental Demonstration of Ultrafast THz Modulation in a Graphene-Based Thin Film Absorber through Negative Photoinduced Conductivity.

We present an experimental demonstration and interpretation of an ultrafast optically tunable, graphene-based thin film absorption modulator for operation in the THz regime. The graphene-based component consists of a uniform CVD-grown graphene sheet stacked on an SU-8 dielectric substrate that is grounded by a metallic ground plate. The structure shows enhanced absorption originating from constructive interference of the impinging and reflected waves at the absorbing graphene sheet. The modulation of this absorption, which is demonstrated via a THz time-domain spectroscopy setup, is achieved by applying an optical pump signal, which modifies the conductivity of the graphene sheet. We report an ultrafast (on the order of few ps) absorption modulation on the order of 40% upon photoexcitation. Our results provide evidence that the optical pump excitation results in the degradation of the graphene THz conductivity, which is connected with the generation of hot carriers, the increase of the electronic temperature, and the dominant increase of the scattering rate over the carrier concentration as found in highly doped samples.

Terahertz radiation of a butterfly-shaped photoconductive antenna(invited)

The terahertz(THz) far-field radiation properties of a butterfly-shaped photoconductive antenna (PCA) were experimentally studied using a home-built THz time-domain spectroscopy(THz-TDS) setup. To distinguish the contribution of in-gap photocurrent and antenna structure to far-field radiation, polarization-dependent THz field was measured and quantified as the illuminating laser beam moved along the bias field within the gap region of electrodes. The result suggests that, although the far-field THz radiation originates from the in-gap photocurrent, the antenna structure of butterfly-shaped PCA dominates the overall THz radiation. In addition, to explore the impact of photoconductive material, radiation properties of butterfly-shaped PCAs fabricated on both low-temperature-grown GaAs(LT-GaAs) and semi-insulating GaAs(Si-GaAs) were characterized and compared. Consistent with previous experiments, it is observed that while Si-GaAs-based PCA can emit higher THz field than LT-GaAs-based PCA at low laser power, it would saturate more severely as laser power increased and eventually be surpassed by LT-GaAs-based PCA. Beyond that, it is found the severe saturation effect of Si-GaAs was due to the longer carrier lifetime and higher carrier mobility, which was confirmed by the numerical simulation.

Statistical Models for Averaging of the Pump–Probe Traces: Example of Denoising in Terahertz Time-Domain Spectroscopy

In this paper, we first discuss the main types of noise in a typical pump–probe system, and then focus specifically on terahertz time domain spectroscopy (THz-TDS) setups. We then introduce three statistical models for the noisy pulses obtained in such systems, and detail rigorous mathematical algorithms to denoise such traces, find the proper averages, and characterize various types of experimental noise. Finally, we perform a comparative analysis of the performance, advantages, and limitations of the algorithms by testing them on the experimental data collected using a particular THz-TDS system available in our laboratories. We conclude that using advanced statistical models for trace averaging results in fitting errors that are significantly smaller than those obtained with only a simple statistical average.

Three-Dimensional Phase Modulator at Telecom Wavelength Acting as a Terahertz Detector with an Electro-Optic Bandwidth of 1.25 Terahertz

We report a thin film phase modulator employing organic nonlinear optical molecules, with an electro-optic bandwidth of 1.25 THz. The device acts as a polarization sensitive broadband Pockels medium for coherent electric field detection in a dual wavelength terahertz time-domain spectroscopy setup in the telecom band at 1550 nm. To increase the sensitivity, we combine a three-dimensional bow-tie antenna structure with strongly electro-optically active molecules JRD1 in poly(methyl methacrylate) supporting polymer. The antenna provides subwavelength field confinement of the terahertz wave with its waveguide gap with lateral dimensions of 2.2 μm × 5 μm × 4 μm. In the gap, the electric field is up to 150× stronger than in a diffraction limited space-time volume, such that an interaction length of only 4 μm suffices for the detection of fields below 10 V/m. This device is promising in the growing field of quantum optics in the terahertz, single photon terahertz detection, nonlinear imaging, and on-chip teleco...