Multifunctional active terahertz metasurface with electromagnetically induced transparency, perfect absorption, and circular dichroism

Driven by the pressing demand for integration and miniaturization within the terahertz (THz) spectrum, this research introduces an innovative approach to construct chiral structures using dichroism as the target function. This initiative aims to tackle the prevalent issues of single-functionality, narrow application scope, and intricate design in conventional metasurfaces. The proposed multifunctional tunable metasurface employs a graphene-metal hybrid structure to address the critical constraints found in existing designs. When circularly polarized light is incident, the metasurface exhibits broadband circular dichroism (CD) function, generating CD intensities greater than 0.5 at 3.44-6.0 THz, and polarization switching function at 3.43-6.17 THz. When linearly polarized light is incident, the proposed design exhibits a broadband linear dichroism (LD) function, producing LD intensities greater than 0.5 at 2.25-5.65 THz and a polarization conversion function at 3.32-6.05 THz. Its broad bandwidth ensures that each function is competitive and effective. A noteworthy feature is the capability to adjust the graphene's chemical potential and the state of the incident light to finely calibrate the intensity of each functional aspect. This innovation makes the proposed multifunctional metasurface a significant reference for the development of chiral photodetectors, CD supermirrors, smart switches, and polarization digital imaging systems within the THz range.

<sec>The phenomenon of electromagnetically induced transparency (EIT)-like in terahertz (THz) metasurfaces facilitates agile manipulation of electromagnetic wave transmission windows and the deceleration of light, rendering it suitable for applications in modulators, absorbers, slow light devices, and more. Traditional design methodologies focus on the coupling between bright-dark modes and bright-bright modes within the unit cell, leveraging interference cancellation effects to regulate electromagnetic wave transmission. Notably, the periodicity of the array structure also plays a pivotal role in modulating the amplitude and resonance intensity of the transparent window, a phenomenon termed lattice-induced transparency (LIT). In this paper, we introduce a gold nanorod structure and an S-shaped gold split-ring resonator supported on a vanadium dioxide (<inline-formula><tex-math id="M1">\begin{document}${\rm{VO}}_{2}$\end{document}</tex-math></inline-formula>) thin film to investigate LIT. Unlike conventional structures that solely consider single bright-bright or bright-dark mode coupling, our proposed structure incorporates both bright-bright and bright-dark modes coupling. Furthermore, the dark mode in our structure is not a conventional multipolar mode but rather a surface lattice resonance (SLR) arising from the coupling between lattice modes and the localized surface plasmon resonance (LSPR) of the structure itself.</sec><sec>Through the analysis of simulated transmission spectra for the individual gold nanorod and S-shaped split-ring structures, we observed that the gold nanorod exhibits LSPR at 0.985 THz, whereas the S-shaped split-ring structure demonstrates LSPR and SLR at 0.51 THz and 1.025 THz, respectively. When combined, these structures form transparent windows with transmission rates of 66.03% and 59.4% at 0.643 THz and 1.01 THz due to the interplay of bright-bright and bright-dark modes coupling. Upon examining the electric field distribution in the x-y plane, we found that the electric field energy is predominantly concentrated on the S-shaped split-ring.</sec><sec>To gain deeper insights into each resonance mode, we employed multipolar decomposition to quantify resonance scattering energy. Our findings revealed that both transparent windows are predominantly governed by electric dipole scattering energy. Further investigations showed that as the array structure’s period varies from 60 μm to 95 μm, the lattice mode progressively couples into the high frequency transmission valley (1.031 THz), giving rise to a high frequency hybrid mode (HFHM). The <i>Q</i> value of this mode initially increases and then decreases, peaking at 27 when the period is 84 μm. Similarly, as the period continues to increase, the lattice mode couples into the low frequency resonance valley (0.76 THz), forming a low frequency hybrid mode (LFHM) with a <i>Q</i> value that reaches a maximum of 51 at 115 μm—approximately an order of magnitude higher than that at a period of 60 μm. Additionally, as the periodicity increases, the near field coupling effect between adjacent units diminishes, leading to the gradual disappearance of the two transparent windows.</sec><sec>To achieve active control over these transparent windows, we varied the conductivity of <inline-formula><tex-math id="M2">\begin{document}${\rm{VO}}_{2}$\end{document}</tex-math></inline-formula> from 20 S/m to 30000 S/m, resulting in a decrease in the transmission amplitudes of the two transparent windows to 37.58% and 3.39%, respectively. Finally, we investigated the slow light effect of the two transparent windows, comparing the maximum group delay between them, which was found to be 8.1 ps. The terahertz metasurface proposed in this study opens up avenues for the design of dynamically tunable sensing and slow light devices in the future.</sec>

As the communication band gradually approaches the terahertz (THz) range, there is an urgent need to explore materials with ideal dielectric properties for THz communication devices. Nevertheless, most polymers present a low dielectric constant (Dk), and the regulation of their dielectric properties in the THz range has rarely been reported. In this work, the isotactic polypropylene (iPP)/α-Al2O3 whisker composites were synthesized and their THz dielectric parameters were optimized. The Dk values increased from 2.23 to 3.13 with filler (α-Al2O3 whisker) concentration, ranging from 0 to 20 vol%, but were almost independent of the test frequency. The loss tangent (Df) values presented an increasing tendency along with both filler concentrations and test frequency. All composites exhibited Df values of less than 4.0 × 10−3. Particularly, the dielectric properties of composites can be further regulated by adjusting the orientation direction of the whisker fillers. The orientation of the whisker fillers was adjusted via the injection molding method. Along the direction of the whisker orientation distribution, the composites exhibit higher Dk values and lower Df values. This work presented a schematic to design polymer composites with higher Dk but controlled Df in the THz range and is significant for the development of advanced materials-based THz devices.

A tunable bifunctional terahertz (THz) metamaterial device based on Dirac semimetal films (DSFs) and VO2 is presented. The insulator-to-metal phase transition of VO2 enables bifunctional asymmetric transmission and dual-directional absorption to be switched in the THz range. When VO2 serves as a dielectric, tunable broadband asymmetric transmission of linearly polarized THz waves can be achieved. When VO2 is in a metallic state, the proposed device acts as a tunable dual-directional absorber with perfect absorption in both illumination directions. In each case, the response can be tuned by varying the Fermi energy of the DSFs. This offers a new pathway for the development of tunable multifunctional THz metamaterial devices.

Terahertz (THz) range holds between infrared light and millimeter wave or microwave radiation. Moreover, THz waves is highly attenuated by the metal object or sensitive to an inter-molecular binding force. Therefore, imaging using THz range is attracted much attention for security, manufacturing, chemical imaging, and so on. In our research, the THz detector composed of Indium arsenide (InAs) high electron mobility transistor (HEMT) and one-sided directional slot antenna on a chip will be developed. In this paper, we focused on the antenna on a chip. The proposed antenna has three layers, namely, top antenna metal, dielectric substrate (BCB, benzocyclobutene) and bottom floating metal layer. There are a coplanar (CPW) feed lines and slots on the top antenna metal. By optimizing the size of the bottom floating metal layer, the radiation toward the back side is suppressed. The CPW feed line is connected the gate electrode on the InAs HEMT. In order to maximize the receiving THz signal form the antenna to InAs HEMT, antenna and gate input impedance is characterized by using the 3D electromagnetic simulator. It has been found that when the input impedance of the gate electrode changes from 10 ohms to 50 ohms, the voltage generated at the gate electrodes is tripled. The antenna was fabricated by the conventional photolithography process. The size of the radiation metal is 290 μm x 210 μm on the top metal with probe pads. The measured antenna gain is 5.57 dBi at 0.93 THz compared with the 5.96 dBi antenna gain at 1 THz from the simulation.

Vanadium dioxide-based terahertz metasurface device with switchable broadband absorption and beam steering functions

The metasurfaces based on nanostructure film play an important role in many fields. Usually, the properties and functions of metasurfaces are limited by their structure. Once the metasurface samples are processed, their functions have already been restricted. The dual-function device designed in this work utilizes the phase transition characteristic of vanadium dioxide (VO2). The entire layer of VO2 film is inserted between the double metal micro-nano structure. When VO2 film is in the metallic state after phase change, an isotropic narrow absorber is obtained in the terahertz (THz) region, which consists of a top Z-shaped meta-atom, a middle dielectric layer, and a bottom VO2 film. By adjusting structure parameters of VO2 film, perfect absorption is realized at the frequency of 0.525 THz with the overall absorption beyond 91%. When VO2 is in insulating state, the top Z-shaped meta-atom will interact with the bottom Z-shaped structure, and the resonance coupling leads to the appearance of electromagnetically induced transparency (EIT). The designed metal-VO2 hybrid metamaterial opens possible avenues for switchable functionalities in a single device.

Molecular vibration mode assignment of nematic liquid crystal 5CB on Terahertz spectra

The infrared region is a range of the electromagnetic spectrum split into three parts, the near-infrared (NIR), the mid-infrared (MIR) and the far-infrared (FIR). Previously, it was usual to refer to the terahertz (THz) range as the range of frequencies below FIR. Nowadays, the THz range is attributed to the electromagnetic spectrum which spans the FIR to the millimeter wave. This THz range of the electromagnetic spectrum starts at 1000 µm and finishes at 50 µm (0.3 THz to 6 THz). So, these wavelengths are commonly named THz, far-infrared or submillimeter wave. THz waves are located between the optical region (infrared) and the microwaves. Only a few compact, easy to use, room temperature sources are available.

Owing to the unique characteristics of terahertz (THz) waves and their interaction with matter, THz imaging has significant potential in diverse fields, including life sciences, non-destructive quality control, and security. However, today’s THz technology faces major obstacles in providing the cost, compactness, and functional scope that is required to facilitate a wide-reaching use outside the laboratory. Therefore, the general aim of this thesis is to increase the societal impact of THz technology by exploring novel THz imaging modalities with integrated circuits (ICs) based on conventional silicon technolgy. Previous research in THz ICs mostly focused on building compact integrated source and detector components for far-field imaging systems that exploit the ability of THz waves to penetrate through diverse dielectric materials. One part of this thesis joins this research and demonstrates low-cost volumetric THz imaging based on computed tomography with silicon components for the first time. To this end, a high-power 0.43 THz source in 0.13 μm silicon-germanium heterojunction bipolar transistor (SiGe HBT) technology is developed in this thesis. However, the resolution of THz far-field imaging is limited by diffraction to the millimeter range, whereas some of the most promising THz applications in life sciences, such as intraoperative imaging of cancerous tissue, are in need for a microscopic resolution to resolve material properties on the cellular level. This thesis presents the realization of microscopic THz imaging with a silicon-based sensor system. The central contribution of this work is the development and analysis of a 128-pixel THz near-field sensor System-on-a-Chip (SoC) operating at 0.55 THz and showing a spatial resolution around 10 μm. The sensor exploits the capacitive near-field interaction between split-ring-resonator probes and imaging objects, giving an imaging contrast based on the dielectric permittivity. To simultaneously enable real-time image acquisition, high sensor sensitivity, and an integration level that is comparable to conventional consumer electronics, the full integration capabilities of a high-speed 0.13 μm SiGe bipolar CMOS (SiGe-BiCMOS) technology are exploited. In particular, the presented SoC employs cointegration of a chip-scale one-dimensional THz sensor front-end, analog signal processing, and digital circuitry for controlling the chip and external communication. The achieved results in terms of imaging speed, system cost, and integration level reach well beyond the state of the art in THz near-field imaging. The ability to rapidly acquire THz images with micrometer-scale resolution will be of benefit to fundamental research into material properties in the THz range and lays the foundation for the exploration of THz bioimaging applications on the cellular level. In particular, the sensor enables the conduction of large-scale clinical studies on the relevance of microscopic THz imaging to ex vivo tumor margin assessment in breast cancer surgeries for the first time.

The plasmonic metamaterial perfect absorber (MPA) is a recently developed branch of metamaterial which exhibits nearly unity absorption within certain frequency range.[1-6] The optically thin MPA possesses characteristic features of angular-independence, high Q-factor and strong field localization that have inspired a wide range of applications including electromagnetic wave absorption,[3, 7, 8] spatial[6] and spectral[5] modulation of light,[9] selective thermal emission,[9] thermal detecting[10] and refractive index sensing for gas[11] and liquid[12, 13] targets. In this work, we demonstrate a MPA working at terahertz (THz) regime and characterize it using an ultrafast THz time-domain spectroscopy (THz-TDS). Our study reveal an ultra-thin Fabry-Perot cavity mechanism compared to the impedance matching mechanism widely adopted in previous study [1-6]. Our results also shows higher-order resonances when the cavities length increases. These higher order modes exhibits much larger Q-factor that can benefit potential sensing and imaging applications. [1] C. M. Watts, X. L. Liu, and W. J. Padilla, "Metamaterial Electromagnetic Wave Absorbers," Advanced Materials, vol. 24, pp. 98-120, Jun 19 2012. [2] M. Hedayati, F. Faupel, and M. Elbahri, "Review of Plasmonic Nanocomposite Metamaterial Absorber," Materials, vol. 7, pp. 1221-1248, 2014. [3] N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, "Perfect metamaterial absorber," Physical Review Letters, vol. 100, p. 207402, May 23 2008. [4] H. R. Seren, G. R. Keiser, L. Cao, J. Zhang, A. C. Strikwerda, K. Fan, et al., "Optically Modulated Multiband Terahertz Perfect Absorber," Advanced Optical Materials, vol. 2, pp. 1221-1226, 2014. [5] D. Shrekenhamer, J. Montoya, S. Krishna, and W. J. Padilla, "Four-Color Metamaterial Absorber THz Spatial Light Modulator," Advanced Optical Materials, vol. 1, pp. 905-909, 2013. [6] S. Savo, D. Shrekenhamer, and W. J. Padilla, "Liquid Crystal Metamaterial Absorber Spatial Light Modulator for THz Applications," Advanced Optical Materials, vol. 2, pp. 275-279, 2014. [7] H. Tao, N. I. Landy, C. M. Bingham, X. Zhang, R. D. Averitt, and W. J. Padilla, "A metamaterial absorber for the terahertz regime: Design, fabrication and characterization," Optics Express, vol. 16, pp. 7181-7188, May 12 2008. [8] J. Hao, J. Wang, X. Liu, W. J. Padilla, L. Zhou, and M. Qiu, "High performance optical absorber based on a plasmonic metamaterial," Applied Physics Letters, vol. 96, p. 251104, 2010. [9] X. Liu, T. Tyler, T. Starr, A. F. Starr, N. M. Jokerst, and W. J. Padilla, "Taming the Blackbody with Infrared Metamaterials as Selective Thermal Emitters," Physical Review Letters, vol. 107, p. 045901, 07/18/ 2011. [10] T. Maier and H. Brückl, "Wavelength-tunable microbolometers with metamaterial absorbers," Optics Letters, vol. 34, pp. 3012-3014, 2009/10/01 2009. [11] A. Tittl, P. Mai, R. Taubert, D. Dregely, N. Liu, and H. Giessen, "Palladium-Based Plasmonic Perfect Absorber in the Visible Wavelength Range and Its Application to Hydrogen Sensing," Nano Letters, vol. 11, pp. 4366-4369, 2011/10/12 2011. [12] N. Liu, M. Mesch, T. Weiss, M. Hentschel, and H. Giessen, "Infrared Perfect Absorber and Its Application As Plasmonic Sensor," Nano Letters, vol. 10, pp. 2342-2348, Jul 2010. [13] G. H. Li, X. S. Chen, O. P. Li, C. X. Shao, Y. Jiang, L. J. Huang, et al., "A novel plasmonic resonance sensor based on an infrared perfect absorber," Journal of Physics D-Applied Physics, vol. 45, p. 205102, May 23 2012.

Active broadband terahertz (THz) polarization manipulation devices are challenging to realize, but also of great demand in broadband terahertz systems. Vanadium dioxide (VO2) shows a promising phase transition for active control of THz waves and provides broadband polarization characteristics when integrated within grating-type structures. We creatively combine a VO2-based grating structure with a total internal reflection (TIR) geometry providing a novel interaction mechanism between the electromagnetic waves and the device, to realize a powerful active broadband THz polarization-controlling device. The device is based on a Si-substrate coated with a VO2 layer and a metal grating structure on top, attached to a prism for generating the TIR condition on the Si-VO2-grating interface. The grating is connected to electrodes for electrically switching the VO2 between its insulating and conducting phases. By properly selecting the incident angle of the THz waves, the grating direction, and the incident polarization state, we first achieved a broadband intensity modulator under a fused silica prism with an average modulation depth of 99.75% in the 0.2-1.1 THz region. Additionally, we realized an active ultra-broadband quarter-wave converter under a Si prism that can be switched between a 45° linear rotator and a quarter wave converter in the 0.8-1.5 THz region. This is the first demonstration of an active quarter-wave converter with ultra-broad bandwidth performance. Our work shows a highly flexible and multifunctional polarization-controlling device for broadband THz applications.

Near-field direction control of terahertz metasurface provides a new paradigm in photonic devices for terahertz (THz) communications. The control of near-field patterns and polarization states is the major challenge for THz metasurface devices. In this work, the hybrid MXene (Ti3C2Tx)/ vanadium dioxide (VO2) metasurface structure is explored for the tuning of transmission amplitude modulation, polarization control, and near-field pattern switching. The hybrid metasurface device is comprised of Ti3C2Tx/VO2 hexagon resonators arranged in a periodic array having silicon (Si) as a substrate with a thin silicon dioxide (SiO2) layer. The response of Ti3C2Tx/VO2 hybrid structure has been investigated under the influence of gate biasing and different optical pump intensities. The enhanced bandwidth tuning has been realized under the polarization and phase control. The rise of MXene conductivity with increasing gate bias is also investigated for transmission tuning. The transition of VO2 from an insulator to a conductor is specifically induced by an increase in optical pump intensity, with 95% of the transmission achieved at the 1-4 THz range. Furthermore, π/2 rotation of near-field is realized with a change in near-field pattern symmetry at various planes. VO2 phase shifting, switch the near-field dipolar pattern from the z-axis to the x-axis into a twisted Gaussian pattern, when TMyx (p-polarized) incidence source is used. The low-cost MXene material can serve as a promising material for terahertz devices.

The equivalent circuit model (ECM) is developed by using a MATLAB code to analyze graphene-based multi-band chiral metamaterial absorbers composing graphene-based square split-ring resonator arrays in the terahertz (THz) range. The absorbers are simulated numerically by the finite element method (FEM) in CST Software to verify the ECM results. Our introduced multi-band absorbers can be used as suitable platforms in polarization-sensitive devices and systems in the THz range. We have designed four tunable graphene-based chiral metamaterial absorbers containing one, two, three, and four gaps in their arms, respectively. The absorber with one gap has four absorption bands (two for TE and three for TM, one band of both modes approximately overlaps) with absorption > 50%. The absorber with two gaps has three absorption bands (two for TE and two for TM, one band of both modes approximately overlaps). The absorber with three gaps has four absorption bands (three for TE and two for TM, one band of both modes approximately overlaps). The absorber with four gaps has three absorption bands (three for TE and two for TM, two bands of both modes approximately overlap). They work in the 1-5.5 THz with maximum linear dichroism (LD) responses of 98, 99, 89, and 77%, respectively. The designed absorbers are dynamically tunable. Additionally, by a 90° rotation of the incident electromagnetic fields, it is possible to switch between the number and/or location of absorption bands making these absorbers a promising candidate for future THz systems. ECM results are following the FEM ones. The proposed ECM procedure is a simple and fast way to recognize the characteristics of the designed absorbers. Our proposed absorbers could be promising enablers in future THz systems.

Metal gratings formed by parallel conductors are widely used as elements of various devices in the terahertz (THz) frequency range. With a change in the angle of incidence of radiation on the grating, the reflection and transmission coefficients of this grating change. Moreover, with certain grating parameters, the nature of the change in the reflection and transmission coefficients can differ significantly from the linear dependence. Especially when the period is comparable to the wavelength. Theoretical models for calculating grating parameters exist and used. However, in real gratings there are a number of design and technological features, the influence of which is theoretically extremely difficult to take into account. Therefore, along with theoretical calculations, comparative measurements are required. This is especially true in the THz range, where there is a shortage of experimental data. The aim of the work is the experimental determination of the properties of one-dimensional metal gratings at different angles of incidence of a plane electromagnetic wave in them in the THz range. An experimental determination was carried out of the propagation of laser radiation with a wavelength of 337 μm through wire gratings with a pitch of 400 and 200 μm at various incidence angles of THz radiation. The analysis of the experimental data and their comparison with theoretical calculations obtained. The experimental results correlate with theoretical data and most fully characterize the properties of the gratings, taking into account their design and technological features. Which are extremely difficult to consider in theoretical calculations. The resonance maxima in the dependences of the transmitted radiation power on its angle of incidence practically coincide with the calculated data. This makes possible to develop a number of measurement methods. The recommendations have been developed to improve the accuracy of measurements and the practical use of the obtained data. The obtained experimental results contribute to the study of the electrodynamic properties of one-dimensional metal gratings in the THz range.