Terahertz Time-domain Spectrometer Research Articles

Terahertz Multi-Spectral Mueller Matrix Polarimetry on Leaf Using Only Orthogonal-Polarization Measurements

There has been a surge of emphasis on polarimetric imaging modalities recently, in which Mueller matrix polarimetry (MMP) is at the heart of our understanding of polarization properties of an imaged sample. However, the historiography of MMP, predominantly undertaken in optics, largely lost sight of the potential of terahertz radiation on interactions with the imaged samples, especially biological ones. In addition, studies of MMP are generally concerned with intensity-only measurements requiring at least 16 polarization combinations, which dramatically add to the time cost in the process of measurements. The proposed study sets out to examine the complete 4 × 4 MMP from 0.2 to 1 THz using only orthogonal polarizations for two-dimensional (2-D) and 3-D imaging on leaf. A modified THz time domain spectrometer with self-designed linear polarizers is utilized for measurements and data acquisition. The Mueller matrix polar decomposition and Mueller matrix transform are employed to facilitate extraction of polarization characteristics of the imaged leaf. The time-of-flight technique is adopted to assist in the 3-D reconstruction of the leaf. The results demonstrate the effectiveness and feasibility of the novel measurement synthesis approach and the modified system for accurate characterization of the sample. Meanwhile, the findings indicate that MMP is superior in feature extraction as well as contrast enhancement of the sample. The insights gained from this article may be of assistance to the characterization of biological samples with MMP at higher frequencies for future research.

Study of starch aging characteristics based on Terahertz technology.

Traditional methods for the determination of starch aging indicators often have a series of shortcomings such as time‐consuming, high cost, large human error, damage to samples, environmental pollution, and high requirements for inspectors. Therefore, it is meaningful to find or establish a dynamic fingerprint identification pattern that can detect the aging degree of starch during the process of processing or storage quickly and accurately. It not only provides guidance for starch food processing but also saves a lot of human, material resources, and time. Terahertz technology is an emerging molecular spectroscopy technology in the 21st century. It is with low energy and basically harmless to the human body. It can also realize nondestructive testing of samples. In the experiment, the samples were prepared by the tableting method and the samples containing 20% of 50 mg samples were prepared with polyethylene as the diluent. The thickness of the samples was 1 mm and the diameter was 13 mm. The terahertz time‐domain spectrometer was used to obtain the spectral information of aging starch at different aging times. After the pretreatment of the spectrum by vector normalization, first derivative, and multiple scattering correction, the prediction models of aging days, crystallinity, and resilience of aging starch were established, respectively. The determination coefficient (R 2) of the established models is all greater than 95%, indicating that the established models are highly reliable and can be used to predict the aging days, crystallinity, and retrogradation degree of starch. And the R 2 of the prediction model based on the refractive index spectrum is greater than that of the absorption coefficient spectrum. The experimental method obtains the dynamic fingerprint identification map of starch in the aging process, realizes the real‐time monitoring and detection of the starch aging process, and provides an effective means for the production and processing of starch‐related industries.

Terahertz time-domain derivative spectrometer using a large-aperture piezoelectric micromachined device.

The engineering of optomechanical systems has exploded over the past decades, with many geometries and applications arising from the coupling of light with mechanical motion. The modulation of electromagnetic radiation in the terahertz (THz) frequency range through optomechanical systems is no exception to this research effort. However, some fundamental modulation capabilities for THz communications and/or high-speed data processing applications are yet to be established. Here, we demonstrate a THz time-domain derivative spectrometer based on a piezoelectric micromachined (PM) device. Insertion of the PM device into the THz beam path provides reference modulation for the lock-in detection unit, which in turn provides access to the nth-order derivative information of the incoming THz signal. Strikingly, the integration of the recorded derived signal leads to a recovered reference signal with an equivalent or even better signal-to-noise ratio, opening the door to a new type of highly sensitive THz measurements in the time domain.

Tunable polarization-induced Fano resonances in stacked wire-grid metasurfaces

Stacked metasurfaces are being investigated in light of exploring exotic optical effects that cannot be achieved with single-layered metasurfaces. In this article, we theoretically demonstrate that metallic wire-grid metasurfaces with specific polarization properties have the ability to induce tunable Fano resonances when they are stacked. The developed original model—combining a circulating field approach together with an extended Jones formalism—reveals the underlying principle that gives rise to the polarization-induced Fano resonances. The theoretical frame is validated in an experimental proof of concept using commercially available wire-grids and a terahertz time domain spectrometer. This unexplored possibility opens an alternative path to the realization and control of Fano resonances by using stacked metallic metasurfaces. Furthermore, these findings suggest that the polarization can be used as an additional degree of freedom for the design of optical resonators with enhanced and tunable properties.

Real-Time High Resolution THz Imaging with a Fiber-Coupled Photo Conductive Antenna and an Uncooled Microbolometer Camera.

We present a real-time THz imaging method using a commercial fiber-coupled photo conductive antenna as the THz source and an uncooled microbolometer camera for detection. This new combination of state-of-the-art components is very adaptable due to its compact and uncooled radiation source, whose fiber coupling allows for a flexible placement. Using a camera with high sensitivity renders real-time imaging possible. As a proof-of-concept, the beam shape of a THz Time Domain Spectrometer was measured. We demonstrate real time imaging at nine frames per second and show its potential for practical applications in transmission geometry covering both material science and security tasks. The results suggest that hidden items, complex structures and the moisture content of (biological) materials can be resolved. We discuss the limits of the current setup, possible improvements and potential (industrial) applications, and we outline the feasibility of imaging in reflection geometry or extending it to multi-spectral imaging using band pass filters.

Simulation and Experimental Study of Terahertz Wave Transmission Characteristics Based on Periodic Metal Open Resonant Ring Structures

Different open resonant ring structures with substrate of polyimide were designed. The transmission characteristics of the structures for terahertz wave were investigated by simulation and experiment. The results show that the transmission peak of the structures moves to high frequency with increase of thickness of the metal layer. With increase of substrate thickness, the transmission peak moved to low frequency and the transmissivity decreased. The influence of number of “C” shape open resonant rings in the unit structure on the transmission characteristics of terahertz wave was also studied. It is found that when the number of “C” shape open resonant rings increases from one to two, more transmission peaks appeared in the frequency of 0.2–2 THz. The transmissivity of the designed structures was tested by terahertz time-domain spectrometer (THz-TDS). The experimental results showed good agreement to the simulation results.

Phase-contrast THz-CT for non-destructive testing.

A new approach for image reconstruction in THz computed tomography (THz-CT) is presented. Based on a geometrical optics model containing the THz signal amplitude and phase, a novel algorithm for extracting an average phase from the measured THz signals is derived. Applying the algorithm results in a phase-contrast sinogram, which is further used for image reconstruction. For experimental validation, a fast THz time-domain spectrometer (THz-TDS) in transmission geometry is employed, enabling CT measurements within several minutes. Quantitative evaluation of reconstructed 3D printed plastic profiles reveals the potential of our approach in non-destructive testing of plastic profiles.

Comparison of terahertz time-domain spectroscopy based on photoconductive antenna and electro-optic sampling and the optical parameter measurement of GaSe nanofilm in THz band

Due to the advantages of THz wave in nondestructive examination (NDE) of materials, terahertz time-domain spectrometer (THz-TDS) has become a mature device for THz detection. Thus, improving signal-to-noise ratio (SNR) and reliability of THz-TDS are considered as the vital work. In this paper, photoconductive antenna (PCA) and electro-optic sampling (EOS) were used to measure the intensity of THz wave. Under the same laser conditions, the residual pump laser and probe laser polarization in THz-TDS were studied for influence on the SNR of THz wave. The experimental results demonstrated the significance of the existing THz-TDS. In THz band, the refractive index of GaSe nanofilm varied from 3.3 to 4.1 with the frequency range from 0.3 THz to 0.6 THz.

High-Power Lensless THz Imaging of Hidden Objects

The potential of pulsed THz radiation for time-of-flight imaging applications is well recognized. However, advances in this field are currently severely limited by the low average power of ultrafast THz sources. Typically, this results in impractically long acquisition times and a loss in resolution and contrast. These limitations make imaging of the objects in real-life scenarios impossible. Here, conclusively, the potential of state-of-the-art high-average power THz time-domain spectrometer (TDS), driven by a 100-W class, one-box ultrafast oscillator for imaging applications is shown by demonstrating lensless THz imaging in reflection mode of a dielectric sample with low reflectivity. Images obtained with our home-built 20-mW average power THz-TDS system show a significant contrast enhancement compared to a state-of-the-art commercial THz-TDS with less than $200~\mu \text{W}$ of average power. Our unique setup even allows us to obtain images of such an object with high-contrast hidden inside a medium-density fiberboard (MDF) box. This opens the door to THz time-of-flight imaging of concealed objects of unknown shape and orientation in various real-life scenarios which were so far impossible to realize.

3D Printed Al2O3 for Terahertz Technology

In this work we demonstrate that 3D printed Al2O3 is a promising material for prototyping and precise fabrication of quasi-optical devices in the terahertz frequency range. The 3D printed Al2O3 exhibits a low absorption coefficient ( $\alpha at 1 THz) and a high refractive index ( $n >3$ ). The printing resolution in the sub $50~\mu \text{m}$ range allows for the implementation of structures in the 0.3-3.0 THz range on the subwavelength scale. Furthermore, the printing process enables the realization of crystalline solids, which allows the use of the Al2O3 birefringence effect. Here, a $\Delta n \approx ~0.05$ was achieved and used for the implementation of $\lambda $ /2-wave plates working at ~1 THz. The material properties and wave plates were characterized using a terahertz time-domain spectrometer.

Application of terahertz time-domain spectroscopy combined with support vector machine to determine tea and pesticide samples

The transmission-type terahertz spectrometer can rapidly identify samples in a nondestructive and non-contact way. This method can also identify the important components in these samples. To test the application of the Terahertz Time-Domain Spectrometer (THz-TDS), our work focused on the identifying isomers of catechin (an important compound in tea) and the difference among green, black, and white tea. We detected the properties of pesticide residues such as thiamethoxam. Thiamethoxam with similar structure was added into the tea samples to test the resolution of the system. In this work, Support Vector Machine (SVM) and K-means clustering algorithms were employed to identify green, white, and black tea. The terahertz plus identification spectra of these structures were obtained from the experiment. The results demonstrate that THz-TDS has strong physical recognition abilities and supports the theory and application of terahertz spectrum and terahertz response.

All-optical tunable terahertz modulator based on a BiFeO3/Si heterostructure

An all-optical tunable terahertz modulator based on a BiFeO3/Si heterostructure is proposed. Optical and transmission properties of the BiFeO3/Si sample are characterized by the terahertz time-domain spectrometer. Under an external optical pumping, the modulator demonstrates an optical power-dependent modulation effect. A maximum modulation depth of 91.13% can be acquired when the optical pumping power is 700mW in the observed frequency. Due to the separation and localization of photogenerated carriers caused by the BiFeO3/Si heterostructure, the conductivity of the device can be changed and finally resulting in a modulation of the incident terahertz wave. In addition, the photoconductive property of the BiFeO3 thin film on Si substrate is investigated to further explore and interpret the working mechanism of the proposed modulator.

Detection of gene mutation responsible for Huntington's disease by terahertz attenuated total reflection microfluidic spectroscopy.

Terahertz absorption spectroscopy based on attenuated total reflection (ATR) from a microfluidic sample cell was designed and implemented to detect gene mutations leading to Huntington's disease (HD). The self-developed compact ATR microfluidic system was employed to detect two groups of base-repeated DNA molecules combined with a terahertz time-domain spectrometer in a marker-free manner. The first group featured different repetition patterns of oligonucleotide fragments, and the second group included the HD gene. For the oligonucleotides of different repetition patterns, there were significant differences among the three oligonucleotides with three repeats of the double bases, which could be unambiguously classified and identified; For the HD gene, it was found that the magnitude of the terahertz absorption coefficients of the four oligonucleotide solutions was, in ascending order, CAG-4 < CAG-16 < CAG-32 < CAG-40 (the numbers are the repeat times of the CAG base segment, with 40 repeats belonging to the HD gene), when the concentration of oligonucleotide was 1 mg/mL. Principal component analysis result indicated that the spectral differences of the four oligonucleotide solutions with different CAG repeat times were statistically significant and clearly distinguishable. These results demonstrate the potential of terahertz spectroscopy as a noninvasive, unmarked, fast and low-cost assay for gene diagnosis and clinical disease detection.

Broadband terahertz spectroscopy of paper and banknotes

Paper and paper products banknotes play an important role in daily life. Two terahertz (THz) time-domain spectrometers (TDS) with combined spectral coverage from 0.5 to 10 THz were used to characterize the optical properties of several types of paper and banknotes. We found that the paper and banknotes show remarkable fingerprints in the terahertz regime, which are highly related to the vibrational modes of the main component of paper and banknotes — cellulose. Meanwhile, THz transmission imaging measurements had been performed and shown the capability of imaging the key anti-counterfeit labels of banknotes such as watermarks, security threads and optically variable inks. The presented study here indicates that the THz spectroscopy together with the imaging method could provide a visible, contactless and nondestructive alternative to delineating paper and paper product banknotes.

Ultrabroadband and Low-Dispersion Multiple-Joint Transmission-Line for Terahertz Region

A multiple-joint transmission line based on the principle of the periscope was developed to realize the flexible guidance of terahertz (THz) waves. The transmission line has a slightly larger diameter than the standard plastic hollow-pipe waveguide, sufficient stability, low transmission loss, ultrabroadband spectrum, and low dispersion. The transmission line is evaluated using a THz time-domain spectrometer consisting of reflective two-dimensional electro-optic sampling with a high-speed complementary metal-oxide semiconductor camera.

Active metamaterial polarization modulators for the Terahertz frequency range

Active control of chirality in the terahertz frequency range is of great importance in many scientific areas, which include research into fundamental optical phenomena, investigation of novel materials, spectroscopy, imaging, wireless communications and chemistry. The lack of efficient, integrated and fast-reconfigurable polarization modulators has hindered, so far, the full exploitation of applications in all the aforementioned fields. Metamaterials are artificial resonant elements possessing unique remarkable properties such as high efficiency and miniaturization capability. The interplay of metallic metamaterial arrays with electrostatically tunable monolayer graphene has been demonstrated to be a valid approach for the realization of a novel class of THz devices. In this work, the realization of active chiral graphene/metamaterial modulator is presented. The versatility of this experimental approach allowed the device integration with broadband sources such as terahertz time domain spectrometers as well as with quantum cascade lasers. A continuous rotation of the polarization plane > 30° has been reported with a reconfiguration speed > 5 MHz. These results pave the way to the integration of fast terahertz polarization modulators in all the applications where these devices are in great demand.

Titanium-Based Microbolometers: Control of Spatial Profile of Terahertz Emission in Weak Power Sources

Terahertz (THz) imaging and spectroscopy set-ups require fine optical alignment or precise control of spatial mode profile. We demonstrate universal, convenient and easy-to-use imaging—resonant and broadband antenna coupled ultrasensitive titanium-based—dedicated to accurately adjust and control spatial mode profiles without additional focusing optical components of weak power THz sources. Versatile operation of the devices is shown using different kinds of THz—electronic multiplier sources, optical THz mixer-based frequency domain and femtosecond optoelectronic THz time-domain spectrometers as well as optically pumped molecular THz laser. Features of the microbolometers within 0.15–0.6 THz range are exposed and discussed, their ability to detect spatial mode profiles beyond the antennas resonances, up to 2.52 THz, are explored. Polarization-sensitive mode control possibilities are examined in details. The suitability of the resonant antenna-coupled microbolometers to resolve low-absorbing objects at 0.3 THz is revealed via direct, dark field and phase contrast imaging techniques as well.

Application of metasurface-based low-pass filters for improving THz-TDS characteristics

We propose an approach to improve the technical characteristics of terahertz time-domain spectrometers at low-frequency measurements. The approach is based on applying low-pass THz filters to narrow the frequency band of the THz signal that allows increasing the sampling interval in accordance with the Nyquist–Shannon theorem. This concept was verified by studying the transmission spectra of low-frequency band-pass THz filters centered at 156 and 376 GHz. We confirm that the high-quality low-pass filters can improve accuracy of THz measurements and significantly reduce data acquisition time. The reduction up to 12 times was experimentally demonstrated in our case.

In situ Determination of the State of Conservation of Paint Coatings on the Kiosk of Guadalajara Using Terahertz Time-Domain Spectroscopy

On site measurements of historically accumulated paint layers on the kiosk of Guadalajara were taken using a terahertz time-domain spectrometer. Sub-surface air gaps produced by delamination were found on various regions of the structure. The occurrence of the air gaps correlates with the position in the structure, being more frequent on the southeastern side.

Enhanced terahertz radiation of photoconductive antenna fabricated on GaAs-on-sapphire

The terahertz (THz) radiation properties of a photoconductive antenna (PCA) fabricated on a GaAs-on-sapphire (GoS) substrate are reported at sub-THz band. The GaAs layer with a thickness of approximately 1 µm was directly deposited on a sapphire wafer by means of molecular beam epitaxy. A butterfly-shaped antenna structure was then fabricated on the GoS substrate by photolithography, and the device was tested as the emitter of an in-house built THz time-domain spectrometer. The performance of this antenna was compared with a commercial one, which had an identical antenna structure but was fabricated on low-temperature-grown GaAs (LT-GaAs). The results showed that the GoS-based PCA radiated an enhanced THz field, which could be as much as 1.9 times that of the LT-GaAs-based PCA, indicating that GoS could be a promising photoconductive material. In addition, the optical transparency of the sapphire substrate allows the device to be illuminated from the backside, which is crucial for THz near-field imaging applications where the sample is usually in close proximity to the front surface of the PCA device.