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.     

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.     

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.     

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.

     

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.     

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.     

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.     

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.     

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.     

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.

     

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.     

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.     

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.     

Molecular dynamics simulations of conformation and chain length dependent terahertz spectra of alanine polypeptides

Terahertz absorption spectra of alanine polypeptides in water were simulated with classical molecular dynamics at 310 K. Vibrational modes and oscillator strengths were calculated based on a quasi-harmonic approximation. Absorption spectra of Ala_n (n = 5, 15, 30) with different chain lengths and Ala₁₅ in coiled and helical conformations were studied in 10–40 cm^? 1 bandwidth. Simulation results indicated both the chain length and the conformation have significant influences on THz spectra of alanine polypeptides. With the increase of chain length, the average THz absorption intensity increases. Compared with the helical Ala₁₅ polypeptide, the THz spectra of coiled one shows stronger absorption peaks. These results were explained from different numbers of hydrogen bonds formed between polypeptides and the surrounding water molecules.     

A Hybrid Terahertz Metamaterial Sensor Using a Hexagonal Ring Resonator with Bio-medical Applications

Plasmonics - In recent years, novel terahertz (THz) metamaterial sensors are being actively explored by the research community. These structures absorb the incident THz waves, and their absorption...     

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.     

Nano-Scale THz Wave Propagating with Ultra-Low Loss

A metal nanowire placed in a dielectric hole is proposed to guide THz modified surface plasmon polaritons (MSPPs). In theory, the MSPP waveguide can guide THz wave with nano-scale mode width (570 nm) and simultaneously ultra-long propagation distance (2.4 m). Compared with conventional surface plasmon polaritons (SPPs) on a bare metal nanowire, the MSPPs’ mode nanoconfinement can be maintained by keeping a part of the mode field nearly unchanged. On the other hand, by modifying the rest of the mode field, the THz power inside the metal nanowire can be significantly reduced for MSPPs, which dramatically decreases the propagation loss (3 orders of magnitude).

     

Terahertz Plasmonic Quantum Cascade Lasers: a Comprehensive Physics Study

Generation of terahertz (THz) radiation has been a hot research topic in recent years. Plasmonic quantum cascade lasers (QCLs) are among the most compact and efficient sources to generate THz radiation. In this paper, we comprehensively study plasmonic QCLs designed based on the antenna-feedback structure to generate efficient radiation about the center frequency of 3 THz. By changing the geometric structure of the plasmonic cavity and using two-dimensional simulation, a minimum loss less than 5.9 cm^?1 is achieved at the lasing frequency. It is also possible to control the orientation of the output beam either vertically or tilted by changing the geometry of the antenna design via chirped or non-chirped grating scheme. Moreover, the output characteristics of the QCL are simulated based on the three-level rate equations through which the dynamics of the laser, as well as the P-I curve, are investigated. Also, the gain spectra for two laser designs (with chirped and non-chirped gratings) are simulated and compared to each other. The results of this paper may provide deep insight into designing efficient laser sources in the THz region.

     

Tunable Salisbury Screen Absorber Using Square Lattice of Plasmonic Nanodisk

We propose a novel polarization independent Salisbury screen absorber to provide tunable resonant absorption at terahertz (THz) frequencies. The Salisbury screen absorber is designed by using a planar array of thin gold nanodisks arranged in a square lattice. Certain configurations of Salisbury screen have multiple distinctive absorption bands that support near-unity/FWHM absorption bandwidth reaching 36 THz/169 THz, respectively. Moreover, the absorption bandwidth depends upon the optical thickness of the dielectric spacer between the metasurface and the metallic ground plane. The proposed tunable Salisbury screen absorber can find practical applications in photonic detection, imaging, sensing, and solar cells at optical frequencies.