Recent Advances of Efficient Design of Terahertz Quantum-Cascade Lasers

Terahertz (THz) quantum cascade lasers (QCLs) are electrically pumped and heterostructure based semiconductor laser sources with intersubband transitions of electrons in different layers of the quantum wells and barriers. The THz QCLs have high output power in THz region which make them important from application point of view. Recently intensive research has been carried out by researchers for obtaining efficient designs of THz sources. Most of the researchers have investigated the THz frequency range between 0.1 and 3 THz; however, the output power of the THz sources in the frequency range 3–5 THz is small because of transit time and resistance-capacitance effects. Nevertheless, the present review is focused for the development of efficient THz QCL sources in the frequency range from 3 to 5 THz where one of the major problem of thermal backfilling of the carriers has to be overcome by engineering the heterostructure.

     

Nanoscale biomaterials for terahertz imaging: A non-invasive approach for early cancer detection

Terahertz (THz) technology is developing a non-invasive imaging system for biosensing and clinical diagnosis. THz medical imaging mainly benefits from great sensitivity in detecting changes in water content and structural variations in diseased cells versus normal tissues. Compared to healthy tissues, cancerous tumors contain a higher level of water molecules and show structural changes, resulting in different THz absorption. Here we described the principle of THz imaging and advancement in the field of translational biomedicine and early detection of pathologic tissue, with a particular focus on oncology. In addition, although the main forte of THz imaging relies on detecting differences in water content to distinguish the exact margin of tumor, THz displays limited contrast in living tissue for in-vivo clinical imaging. In the last few years, nanotechnology has attracted attention to aid THz medical imaging and various nanoparticles have been investigated as contrast enhancements to improve the accuracy, sensitivity, and specificity of THz images. Most of these multimodal contrast agents take advantage of the temperature-dependent of THz spectrum to the conformational variation of the water molecule. We discuss advances in developing THz contrast agents to accelerate the advancement of non-invasive THz imaging with improved sensitivity and specificity for translational clinical oncology.     

Effect of Boundary Condition and Periodical Extension on Transmission Characteristics of Terahertz Filters with Periodical Hole Array Structure Fabricated on Aluminum Slab

Terahertz (THz) filters based on extraordinary optical transmission from periodical hole array structures fabricated on aluminum slab have been experimentally investigated by using THz time-domain spectroscopy. The incident THz pulses with frequency from 0.1 to 2.7 THz could be partly filtered, and the central peak was at ～0.26. The high frequency signal could be observed to decrease, especially for the frequency above ～1 THz. Moreover, the transmission peak from small-size sample with less hole arrays shifts to high frequency at ～0.53 THz due to both the effects of boundary condition and insufficient periodical extension. Furthermore, finite element method with surface plasmon polariton theory is employed to analyze this extraordinary optical transmission and filter phenomena.     

Do hydration dynamics follow the structural perturbation during thermal denaturation of a protein: a terahertz absorption study

We investigate the thermal denaturation of human serum albumin and the associated solvation using terahertz (THz) spectroscopy in aqueous buffer solution. Far- and near-ultraviolet circular dichroism spectroscopy reveal that the protein undergoes a native (N) to extended (E) state transition at temperature ≤55°C with a marginal change in the secondary and tertiary structure. At 70°C, the protein transforms into an unfolded (U) state with significant irreversible disruption of its structures. We measure the concentration- and temperature-dependent THz absorption coefficient (α) of the protein solution using a p-Ge THz difference spectrometer (2.1–2.8 THz frequency range), thereby probing the collective protein-water network dynamics. When the solvated protein is heated up to 55°C and cooled down again, a reversible change in THz absorption is observed. When increasing the temperature up to 70°C, we find a dramatic irreversible change of THz absorption. The increase in THz absorption compared to bulk water is attributed to a blue shift in the spectrum of the solvated protein compared to bulk water. This is supported by measurements of THz absorption coefficients using THz time-domain spectroscopy (0.1–1.2 THz frequency range). We also use picosecond-resolved fluorescence spectroscopy of the tryptophan 214 moiety of human serum albumin. All experimental observations can be explained by a change in the hydration dynamics of the solvated protein due to the additional exposure of hydrophobic residues upon unfolding.     

Polarization Management of Terahertz Extraordinary Optical Transmission through Ultracompact L-Shaped Subwavelength Patterns on Metal Films

Compact and efficient terahertz (THz) polarization conversion components are of importance for applications where the small dimension of the laser device/system is critical. Here, we propose an ultracompact L-shaped subwavelength patterns on metal films to realize the THz polarization management. By optimizing the geometric parameters of single-layered and double-layered patterns, the linear-polarized THz incidence can be converted to elliptical polarized output or rotated by 90° efficiently due to the THz extraordinary optical transmission phenomenon. The physical mechanism is explored by mode analysis using numerical and analytical modeling.     

Terahertz Selective Emission Enhancement from a Metasurface-Coupled Photoconductive Emitter in Quasi-Near-Field Zone

We present a THz emission enhancement of 41 times at 0.92 THz from a metasurface made of T-shaped resonators excited in a quasi-near-field zone. Such a metasurface has an intrinsic transmission minimum with Q factor of 4 at 1.25 THz under far-field excitation. When this metasurface is coupled onto the backside of a 625-μm-thick photoconductive emitter, the metasurface is below the Fraunhofer distance to the excitation source. As such, one broad enhancement around 0.47 THz and another extremely narrow enhancement at 0.92 THz in the emission spectrum are observed owing to a quasi-near-field excitation. Theoretically, the Q factor of the latter is up to 307, which is limited by the spectral resolution in experiment. The numerical simulations indicate that the T-shaped resonators serve as an array of plasmonic antennas resulting in the aforementioned emission enhancement of THz radiation.

     

太赫兹(THz)光谱在生物大分子研究中的应用

太赫兹(THz)辐射是一种新型的远红外相干辐射源,近年来,在生物大分子研究中得到了广泛的应用,特别是在生物分子的结构和动力学特性等方面有着巨大的应用潜力.结合THz光谱的特点,介绍了利用THz光谱对蛋白质、糖类及DNA等生物大分子的探索研究,以及THz技术在测定水环境与生物分子相互作用等方面的应用.探讨了该技术在生物学领域应用中有待解决的问题及发展前景.     

In vivo sensing of rabbit cornea by terahertz technology

A Novel scalable approach using Terahertz (THz) waves together with the electromagnetic field simulation was applied to investigate four rabbits of eight rabbit corneas in vivo. One eye of each rabbits’ corneas was edema induced; the other eye of the corneas served as the control. The simulation revealed the propagation of THz waves at a certain distance along the sub-surface of the cornea. THz spectra have been collected close to the corneal surface by deviating the direct reflection of the THz beam for the edema cornea, the reflected wave intensity for edema corneas is generally larger compared with the control cornea. Upon edema becomes severe at the end of the observation, the reflected wave intensities obtained by detector corresponding to the corneal deep stroma layer approach to the same value for all observed corneas. Good correlation is observed between central corneal thickness measurements and THz wave reflection signal intensities. Our results demonstrated that THz spectroscopy technique could obtain the information from different corneal sublayers.     

A first principle study of the structural,electronic, and temperature-dependent thermodynamic properties of graphene/MoS2 heterostructure

Journal of Molecular Modeling - Based on the unique advantages of terahertz (THz) spectrum on the detection of energetic cocrystals, the low-temperature dependent THz spectra of CL-20/TNT cocrystal...     

Active Focal Length Control of Terahertz Slitted Plane Lenses by Magnetoplasmons

Active plasmonic devices are mostly designed at visible frequencies. Here, we propose an active terahertz (THz) plasmonic lens tuned by an external magnetic field. Unlike other tunable devices where the tuning is achieved by changing the plasma frequency of materials, the proposed active lens is tuned by changing the cyclotron frequency through manipulating magnetoplasmons (MPs). We have theoretically investigated the dispersion relation of MPs of a semiconductor?Cinsulator?Csemiconductor structure in the Voigt configuration and systematically designed several lenses realized with a doped semiconductor slab perforated with sub-wavelength slits. It is shown through finite?Cdifference time?Cdomain simulations that THz wave propagating through the designed structure can be focused to a small size spot via the control of MPs. The tuning range of the focal length under the applied magnetic field (up to 1?T) is ??3??, about 50% of the original focal length. Various lenses, including one with two focal spots and a tunable lens for dipole source imaging, are realized for the proposed structure, demonstrating the flexibility of the design approach. The proposed tunable THz plasmonic lenses may find applications in THz science and technology such as THz imaging.     

Studying the genotoxic effects of high intensity terahertz radiation on fibroblasts and CNS tumor cells

The data is obtained on the effect of high-intensity pulses of terahertz (THz) radiation with a broad spectrum (0.2–3 THz) on cell cultures. We have evaluated the threshold exposure parameters of THz radiation causing genotoxic effects in fibroblasts. Phosphorylation of histone H2AX at Ser 139 (γH2AX) was chosen as a marker for genotoxicity and a quantitative estimation of γH2AX foci number in fibroblasts was performed after cell irradiation with THz pulses for 30 min. No genotoxic effects of THz radiation were observed in fibroblasts unless peak intensity and electric field strength exceeded 21 GW cm⁻² and 2.8 MV cm⁻¹, respectively. In tumor cell lines (neuroblastoma (SK-N-BE (2)) and glioblastoma (U87)), exposure to THz pulses with peak intensity of 21 GW cm⁻² for 30 min caused no morphological changes as well as no statistically significant increase in histone phosphorylation foci number.     

Sensing minute changes in biological cell monolayers with THz differential time-domain spectroscopy

We used terahertz differential time-domain spectroscopy (THz-DTDS) to measure minute changes of bovine lung microvessel endothelial cells (BLMVEC) in response to vascular endothelial growth factor (VEGF). These changes were reflected by alterations in THz wave attenuations and THz dielectric properties of the treated cells. The VEGF-induced THz attenuations of cell monolayers correlated well with changes in transendothelial resistance, as measured using electric cell-substrate impedance sensing (ECIS). However, the morphological differences that gave rise to these changes were not observed with standard optical phase contrast microscopy. We conclude that THz-DTDS is a highly sensitive, non-invasive, powerful new tool to measure minute changes in the morphology of live, cultured cell monolayers. This method enables spectroscopic investigations of cells in the THz band, providing information unavailable through other conventional methods such as optical phase contrast microscopy and ECIS.     

Analysis of the inter-molecular interactions between amino acids and acetone by THz spectroscopy

The THz spectra of amino acids after application of spots of acetone were measured. The 0.6 THz band was commonly observed in many amino acids that formed the intra-molecular salt structure. The band can be attributed to the interaction vibration from the common structural configuration of amino acids and acetone molecules. The evidence suggests that the vibration between the amino acids with intra-molecular salt structure and acetone has a peak at 0.6 THz. A model of the interaction vibration of acetone and the functional groups of amino acids is proposed.     

Imaging brain tissue slices with terahertz near-field microscopy

Photoconductive antenna microprobe (PCAM)-based terahertz (THz) near-field imaging technique is promising for biomedical detection due to its excellent biocompatibility and high resolution; yet it is limited by its imaging speed and the difficulty in the control of the PCAM tip-sample separation. In this work, we successfully realized imaging of mouse brain tissue slices using an improved home-built PCAM-based THz near-field microscope. In this system, the imaging speed was enhanced by designing and applying a voice coil motor-based delay-line. The tip-sample separation control was implemented by developing an image analysis-based technique. Compared with conventional PCAM-based THz near-field systems, our improved system is 100 times faster in imaging speed and the tip-sample separation can be controlled to a few micrometers (e.g., 3 μm), satisfying the requirements of THz near-field imaging of biological samples. It took about ~30 min (not the tens of hours it took to acquire the same kind of image previously) to collect a THz near-field image of brain tissue slices of BALb/c mice (500 μm × 500 μm) with pixel size of 20 μm × 20 μm. The results show that the mouse brain slices can be properly imaged and different regions in the slices (i.e., the corpus callosum region and the cerebrum region) can be identified unambiguously. Evidently, the work demonstrated here provides not only a convincing example but a useful technique for imaging biological samples with THz near-field microscopy. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2741, 2019.     

Integrated Terahertz Surface Plasmon Resonance on Polyvinylidene Fluoride Layer for the Profiling of Fluid Reflectance Spectra

We design terahertz (THz) surface-plasmon-resonance (SPR) sensors using a ferroelectric polyvinylidene fluoride (PVDF) thin layer for biological sensing. The reflectivity properties based on SPR are described using transfer matrix method (TMM) and numerically simulated using finite-difference time domain (FDTD) method. The sensing characteristics of the structure are systematically analyzed through the examination of the reflectivity spectrum. The results reveal that the pronounced SPR resonance peak has quasi-linear relationship with the refractive index variation of the material under investigation. Through analyzing and optimizing the structural parameters of the THz SPR sensor, we achieved the theoretical value of the refractive index detection sensitivity as high as 0.393 THz/unit change of refractive index (RIU) for a 20-μm-thick liquid sample with a 10-μm PVDF layer. This work shows great promise toward realizing a THz SPR sensor with high sensitivity for identifying the signatures of biological fluid sample.     

Simulation,Fabrication and Characterization of THz Metamaterial Absorbers

Metamaterials (MM), artificial materials engineered to have properties that may not be found in nature, have been widely explored since the first theoretical¹ and experimental demonstration² of their unique properties. MMs can provide a highly controllable electromagnetic response, and to date have been demonstrated in every technologically relevant spectral range including the optical³, near IR⁴, mid IR⁵ , THz⁶ , mm-wave⁷ , microwave⁸ and radio⁹ bands. Applications include perfect lenses¹⁰, sensors¹¹, telecommunications¹², invisibility cloaks¹³ and filters^14,15. We have recently developed single band¹⁶, dual band¹⁷ and broadband¹⁸ THz metamaterial absorber devices capable of greater than 80% absorption at the resonance peak. The concept of a MM absorber is especially important at THz frequencies where it is difficult to find strong frequency selective THz absorbers¹⁹. In our MM absorber the THz radiation is absorbed in a thickness of ~ λ/20, overcoming the thickness limitation of traditional quarter wavelength absorbers. MM absorbers naturally lend themselves to THz detection applications, such as thermal sensors, and if integrated with suitable THz sources (e.g. QCLs), could lead to compact, highly sensitive, low cost, real time THz imaging systems.     

First principles investigation of L-alanine in terahertz region

Terahertz absorption spectrum (0.5–4.0 THz) of L-alanine in the solid phase was measured by terahertz time-domain spectroscopy at room temperature. Simulations utilizing gaseous-state and solid-state theory were performed to determine the origins of the observed vibrational features. Our calculations showed that the measured features in solid-state materials could be well understood by considering the crystal packing interactions in a solid-state density functional theory calculation. Furthermore, intermolecular vibrations of L-alanine are found to be the dominating contributions to these measured spectral features in the range of 0.5–4.0 THz, except that located at 3.11 THz.     

Light-induced Difference Terahertz Spectroscopy

Visible/near-infraredlaser-induced difference spectroscopy basedon a time-domain terahertz system has beendeveloped, and used to study copperpathancyonine. We find that the absorptionpeak of this molecule at 1.08 THz changessignificantly under 790 nm laserexcitation, suggesting that we haveobserved the first evidence of vibrationalmode changes in the THz range induced byvisible/near-infrared light.     

Two Methods for Modelling the Propagation of Terahertz Radiation in a Layered Structure

Modelling the interaction of terahertz(THz) radiation with biological tissueposes many interesting problems. THzradiation is neither obviously described byan electric field distribution or anensemble of photons and biological tissueis an inhomogeneous medium with anelectronic permittivity that is bothspatially and frequency dependent making ita complex system to model.A three-layer system of parallel-sidedslabs has been used as the system throughwhich the passage of THz radiation has beensimulated. Two modelling approaches havebeen developed a thin film matrix model anda Monte Carlo model. The source data foreach of these methods, taken at the sametime as the data recorded to experimentallyverify them, was a THz spectrum that hadpassed though air only.Experimental verification of these twomodels was carried out using athree-layered in vitro phantom. Simulatedtransmission spectrum data was compared toexperimental transmission spectrum datafirst to determine and then to compare theaccuracy of the two methods. Goodagreement was found, with typical resultshaving a correlation coefficient of 0.90for the thin film matrix model and 0.78 forthe Monte Carlo model over the full THzspectrum. Further work is underway toimprove the models above 1 THz.