A Dual‐Resonant Metamaterial in Terahertz Regime

The ever-growing number of interconnected devices and the emergence of high-bandwidth technologies such as holographic communications in mixed reality are driving an exponential growth in the demand for terabits-per-second (Tbps) data rates with sub-milliseconds latency. For this reason, the sixth-generation (6G) wireless communications are moving towards the terahertz (THz) frequency band because of its wider spectral bandwidth availability to enable higher channel capacity and data rates according to the Shannon-Hartley theorem. However, the efficiency of wireless THz signal transmission is reduced by the increased path loss and atmospheric attenuation due to water vapor absorption at THz frequencies, which significantly decreases the signal-to-noise ratio and the spectral efficiency of THz wireless communications. The realization of THz band communications is further impeded by the limited power output of existing compact THz sources due to the extremely low conversion efficiencies. Therefore, it is crucial to develop broadband THz integrated circuits with waveguiding and beamforming functionalities for supporting high data-rate on-chip communications and to provide sufficient gains to counteract the path loss and limited power of THz wireless transmissions. This thesis aims to improve the spectral efficiency of 6G wireless communications via the spatiotemporal shaping of THz waves using silicon photonics. Spatiotemporal light shaping is the basis of nearly all THz photonic devices, ranging from THz wavefront manipulation using metasurfaces to THz planar waveguiding using photonic crystal waveguides. Silicon photonics is a promising material platform for THz wave shaping due to the low-loss propagation of THz waves in high-resistivity silicon and the prospect of a monolithic integrated circuit due to the complementary metal-oxide semiconductor (CMOS) fabrication process compatibility. Silicon metasurfaces allow the wavefront modulation of incident THz radiation by exploiting the generalized Snell’s law of refraction and reflection to beamform highly directional THz waves in a dynamic multi-user communication environment for enhancing the spectral efficiency. Likewise, broadband silicon photonic crystal waveguides allow the in-plane and lossless transport of THz waves to support on-chip high data-rate THz communications that can also seamlessly integrate multiple THz photonic devices in a THz integrated circuit. In addition, silicon electron emitters and silicon gratings potentially allow the on-chip generation of highly intense and broadband THz radiation to counteract the significant THz attenuation. In this thesis, spatiotemporal wave shaping techniques using silicon photonics for enhancing the spectral efficiency of THz band communications is presented. First, a self-adaptive and deep reinforcement learning model for THz beamforming is proposed, which can beamform multiple highly directional THz radiation beam in real-time to serve a dynamically changing multi-user communication environment and improve the signal-to-noise ratio of THz wireless transmission while potentially reducing the communication latency. Second, a broadband THz topological silicon waveguide designed based on valley photonic crystals is proposed for the robust and lossless transport of broadband THz waves for the construction of THz integrated circuits with broadband channel capacity for enabling higher data-rate on-chip communication. Last, the versatility of free-electron radiation from silicon gratings is investigated for the generation of highly intense spatiotemporal wave packets with user-designed group velocities and tunable intensity profile, with the potential to realize integrated high-power THz sources. The deep learning beamforming model, the broadband topological waveguide designs, and the free-electron radiation intensity shaping techniques presented in this thesis extends beyond the THz regime and are scalable across the entire electromagnetic spectrum. The CMOS process compatibility of silicon metasurfaces, silicon planar waveguides, along with silicon electron emitters and silicon gratings offers the opportunity to realize a broadband THz photonic integrated circuit with waveguiding, wave generation and beamforming functionalities for enhancing the spectral efficiency of THz communications in 6G wireless networks.

An optimization-based metasurface is proposed and employed to design a wideband single-layer reflectarray antenna for use in the terahertz (THz) regime. The random hill-climbing optimization method is utilized to obtain a wideband unit cell. A multiobjective fitness function is defined to consider three required design aims, whereas an established link between MATLAB and HFSS software is employed to simulate the required periodic metasurface. The final created unit cell has a remarkable bandwidth of 44.89% together with −0.44 dB average variation of the magnitude of the reflection coefficient. Moreover, the phase variation versus various scales of the metallic surface of the cell is more than 300° at all of the desired frequencies between 0.95 THz and 1.5 THz. Two different reflectarray antennas are proposed based on the optimized cell, and two excitation scenarios are implemented to investigate the performance of the designed reflectarray antennas. In the first scenario, the metasurface is designed to deflect a plane wave with angle of incidence of $$30^{^\circ }$$ to the normal direction. In the second one, a THz feeding horn illuminates a metasurface reflector including 201 elements. According to the simulation results, 3-dB and 1-dB gain bandwidths of 39.67% (0.99–1.48 THz) and 20.51% (1.05–1.29 THz), respectively, are achieved, whereas the maximum gain is 19.8 dBi at 1.2 THz.

Metasurfaces hold tremendous promise for various innovative sensing applications, thanks to their remarkable ability to manipulate light. A recent significant advancement in this research direction is using quasi‐BIC (bound states in the continuum) metasurfaces for mid‐infrared molecular sensing, which relies on creating a series of nearly uniformly spaced, sharp resonances on a single device. Although many studies have highlighted the potential of adopting this method in the terahertz (THz) regime, experimental demonstration is lacking. In this work, the first experimental demonstration of such frequency comb‐like, quasi‐BIC resonances on a single device is presented in the THz regime. By pixelating a 6‐inch Si wafer with 25 different metasurfaces that possess both high‐order quasi‐BIC modes and a geometric scaling, a set of 241 sharp resonances ranging from 350 to 750 GHz is produced based on numerical simulation. By using a high‐resolution vector network analyzer, 140 peaks that fall into the detection range from 500 to 750 GHz are confirmed experimentally, and their frequencies match well with the simulated results. By experimentally demonstrating frequency comb‐like, quasi‐BIC THz resonances on a single device, this work shows a new path for metasurface‐based sensing in the THz regime.

Vanadium dioxide (VO2), due to its characteristic semiconductor–metal transition (SMT) at 341 K, is a promising material for optical modulators in the terahertz (THz) regime. In this study, doping Co ions into epitaxial VO2 films have been demonstrated to acquire SMT with a reduced critical temperature, a large modulation depth, and a narrow transition region in the THz regime, which would benefit the applications of VO2 film in THz switches and memory devices. Utilizing Co ions, a 77% THz modulation ratio within a 3 °C transition region appeared when the doping content was 4.0 at. %. Moreover, more doping content induced two different phases in VO2 films, identified by X-ray diffraction (XRD), which would decrease the THz modulation ratio. Theoretical analysis based on the Drude-Smith model and the Bruggeman effective medium theory predict that the phase containing more Co ions might be transparent to THz waves and void of switching ability in the temperature range from 30 °C to 80 °C. This phase separation may be responsible for the complex correlation between the Co doping concentration and the SMT properties.

A dielectric metasurface with line-square compound lattice structure has been fabricated and demonstrated in the terahertz (THz) regime by the THz time-domain spectroscopy and numerical simulation. A polarization dependent electromagnetically induced transparency (EIT) effect is achieved in this metasurface due to the mode coupling and interference between the resonance modes in line and square subunits of the metasurface. Accompany with the EIT effect, a large artificial birefringence effect between two orthogonal polarization states is also observed in this compound metasurface, of which birefringence is over 0.6. Furthermore, the liquid crystals are filled on the surface of this dielectric metasurface to fabricate an electrically tunable THz LC phase shifter. The experimental results show that its tunable phase shift under the biased electric field reaches 0.33π, 1.8 times higher than the bare silicon, which confirms the enhancement role of THz microstructure on the LC phase shift in the THz regime. The large birefringence phase shift of this compound metasurface and its LC tunable phase shifter will be of great significance for potential applications in THz polarization and phase devices.

In recent years, the field of Terahertz (THz) science and technology has entered a completely new phase of unprecedented expansion that is generating ever-growing levels of broadbased international attention. Indeed, the plethora of activities that have arisen recently in both the technology and scientific arenas associated with the THz frequency domain i.e., between 1 millimeter (300 GHz) and 100 micrometers (3 THz), suggest that the field might be attempting to undergo a dramatic transition that could lead to long-awaited payoffs in a number of application areas. The inherent advantages and potential payoffs of the THz regime for military & security as well as industry relevant applications have long stood as an important driver of interest in this science and technology area. This extremely expansive and spectrally unique portion of the EM spectrum had initial application in space-based communications, upper atmospheric sensing and potentially for short-range terrestrial communications and non-intrusive package screening. However, the very rapid growth in more recent years is arguably most closely linked to the potential payoffs of THz sensing and imaging for an array of military, security and industrial applications. These applications include the spectroscopic-based detection identification and characterization of chemical and biological agents and materials, remote and standoff early-warning for chemicalbiological warfare threats, and imaging of concealed weapons and explosives, just to name a few. In addition, THz-regime finds its application possibilities in industry and private-sector areas as food-industry process control, pharmaceutical industry, biological science, medical diagnostics and security screening. Systems for rapidly emerging applications at THz frequencies thus require reliable highpower sources. In the last few years, the development of suitable sources for this frequency regime is being extensively explored worldwide. There are broadly two technology roadmaps for THz semiconductor devices. Approaching from the lower frequency range in the THz regime, electronic devices such as, Gunn diode, Resonant Tunneling diode (RTD) and nanometer Field Effect Transistors (FET) based on plasma wave have been widely investigated for THz frequency generation. From higher portion of the THz frequency spectrum, the photonics-based device Quantum Cascade Laser (QCL) extends the emission 7

The -carotene is a short chain polyene molecule containing nine -electron conjugated double-bonds. Because of its special molecular structure, -carotene has been used widely in many fields, including functional materials, optoelectronic devices and biological applications of light collection, light protection, anti-cancer, etc. Recently, new applications of -carotene in generation and detection of terahertz (THz) wave have also attracted great attention. In this work, all-trans -carotene films are prepared by spray coating, and the THz spectra in a wavenumber range of 30-400 cm-1 (a frequency range of 0.9-12 THz) of the as-prepared products are experimentally measured at room temperature by Fourier transform infrared spectroscopy. For comparison, the THz spectra in 0.5-3.0 THz are also characterized at the same temperature by THz time-domain spectroscopy. Based on these measurements, the fingerprint peaks of all-trans -carotene in the THz region are experimentally identified to be located at 54 cm-1 (1.62 THz), 57 cm-1 (1.71 THz), 64 cm-1 (1.91 THz), 77 cm-1 (2.32 THz), 90 cm-1 (2.69 THz), 98 cm-1 (2.95 THz), 115 cm-1 (3.45 THz), 124 cm-1 (3.72 THz), 134 cm-1 (4.02 THz), 170 cm-1 (5.11 THz), 247 cm-1 (7.42 THz), and 279 cm-1 (8.38 THz), respectively. It is worth noting that the recent results about the THz spectra of palm leaves are thus verified. Particularly, the B3 LYP method of density functional theory is further utilized in this work to theoretically simulate the THz spectra of all-trans -carotene molecule. It is revealed that the theoretical simulation results accord well with those experimentally measured data. In addition, we also find that the absorption peaks are caused by the torsion, deformation and rocking vibration of the molecules. Accordingly, the vibrational modes of the measured THz characteristic peaks at 148 cm-1 (4.44 THz), 132 cm-1 (3.96 THz), 115 cm-1 (3.45 THz), 76 cm-1 (2.28 THz) and 52 cm-1 (1.56 THz) are theoretically assigned, which provides a reference to explain the formation mechanism of the THz spectra. The valuable results presented in this work will be helpful for promoting the studies of the THz spectral features and response mechanisms of the organics.

Despite significant advancements in terahertz (THz) generation and detection technologies, the practical deployment of portable THz systems remains constrained by existing modulator limitations, particularly stringent temperature requirements, inefficient thermal management, and high power consumption. Here, we demonstrate a breakthrough in magnetically controlled THz modulation using a CoFe2O4/MgO/F-Mica composite structure. The tunable optical response and dielectric properties of the composite under varying magnetic fields were systematically investigated via THz time-domain spectroscopy. Experimental results revealed a magnetic field-dependent THz transmission attenuation, with systematic suppression of transmission spectra proportional to the applied magnetic field intensity. A theoretical model accounting for external magnetic field variations was proposed, which agrees well with the experimental results regarding the imaginary component. Remarkably, the composite material realizes phase modulation simultaneously under magnetic field. This exceptional magnetic responsiveness significantly broadens the application potential of ferrites in the THz regime. These findings provide critical insights for designing tunable multifunctional THz magnetic devices in 6G communications, medical diagnostics, and nondestructive testing applications.

The unrestricted control of circularly polarized (CP) terahertz (THz) waves is important in science and applications, but conventional THz devices suffer from issues of bulky size and low efficiency. Although Pancharatnam–Berry (PB) metasurfaces have shown strong capabilities to control CP waves, transmission-mode PB devices realized in the THz regime are less efficient, limiting their applications in practice. Here, based on Jones matrix analysis, we design a tri-layer structure (thickness of ~λ/5) and experimentally demonstrate that the structure can serve as a highly efficient transmissive meta-atom (relative efficiency of ~90%) to build PB metadevices for manipulating CP THz waves. Two ultrathin THz metadevices are fabricated and experimentally characterized with a z-scan THz imaging system. The first device can realize a photonic spin Hall effect with an experimentally demonstrated relative efficiency of ~90%, whereas the second device can generate a high-quality background-free CP Bessel beam with measured longitudinal and transverse field patterns that exhibit the nondiffracting characteristics of a Bessel beam. All the experimental results are in excellent agreement with full-wave simulations. Our results pave the way to freely manipulate CP THz beams, laying a solid basis for future applications such as biomolecular control and THz signal transportation.

The terahertz (THz) regime of the electromagnetic spectrum is rich with the emerging possibilities in imaging applications with unique characteristics to screening for weapons, explosives and bio-hazards, imaging of concealed objects, water content, and skin, and these advantages can be harnessed by using the effective THz sources and detectors. In THz imaging systems, the pulsed THz sources and detectors find unique applications and thus we have emphasized on re-visiting these kinds of systems. Several novel imaging techniques which exploit the distinctive properties of the THz systems have been presented. Moreover, the THz antenna is one of the most important components of a THz imaging system as it plays a significant role in both impedance matching and power source. Therefore, the recent developments in THz antenna design for imaging applications are reviewed and the potential challenges of such THz systems are investigated. The photoconductive antennas form the basis of many THz imaging and spectroscopy systems and finds promising applications in various scientific fields. However, for the imaging applications, there is a requirement of planar and compact THz antenna sources with on-chip fabrication and high directivity in order to achieve large depth-of-field for better image resolution. Therefore, the key modalities of improving photoconductive dipole antennas performance are identified for imaging applications. Also, the ways to improve the directivity of the photoconductive dipole antenna are discussed. The main purpose of this review is to provide an assortment of all relevant literature to bring researchers up-to-date on the current state-of-the-art and potential challenges of THz antenna technology for imaging applications.

Terahertz (THz) spectroscopy is a nondestructive method that has the ability to identify many hazardous materials by investigating their low frequency vibrational modes (0.1-6.0 THz). Ammonium nitrate (AN), often used in improvised explosives, exhibits featureless reflection/transmission spectrum at THz frequencies at room temperature. However, the low frequency vibrational modes exhibit strong temperature dependence below room temperature (<240k) due to the polymorphic phase transitions. In this work, we study the effective dielectric properties of AN embedded in a polytetrafluoroethylene (PTFE) host medium using terahertz time domain spectroscopy in the temperature ranging from 5K to 300K. The dielectric properties of pure AN were extracted using three different effective medium theories (EMT): (i) the simple effective medium approach, (ii) the Maxwell-Garnett (MG) model, and (iii) the Bruggeman (BR) model. The dielectric properties obtained from theoretical approximations agree well with the experimental values. We identified six lattice vibrational modes between 0.2-3.0 THz that are associated with the polymorphic phase transitions at low temperature.

This paper presents two new reflection type linear cross converters at the terahertz (THz) waveband. The proposed converter possesses the advantages of compact and simple unit cell geometry, which can be optimized easily for single wideband and dual broadband polarization conversion. The unit cell of the converter comprises anisotropic geometry of double E-shaped resonators. The designed structure converts a linear polarized incident wave into cross-polarization over the frequency range of 0.67 to 1.65 THz (84.5 &#x0025;) with above 90 &#x0025; polarization conversion ratio (PCR). At three frequencies of 0.73, 1.11, and 1.55 THz near 100 &#x0025; polarization conversion is achieved. The converter exhibits stable polarization conversion characteristics for incidence angle &#x003E; 35&#x00B0;. A pair of parallel slits are placed along the length of the E-shaped resonators to obtain a notch in the converter&#x0027;s response. The modified metasurface exhibits dual-band cross-polarization conversion between the frequency ranges of 0.67-1.06 THz (45.1 &#x0025;) and 1.37-1.74 THz (23.8 &#x0025;).

A well-known empirical rule for the demand of wireless communication systems is that of Edholm's law of bandwidth. It states that the demand for bandwidth in wireless short-range communications doubles every 18 months. With the growing demand for bandwidth and the decreasing cell size of wireless systems, terahertz (THz) communication systems are expected to become increasingly important in modern day applications. With this expectation comes the need for protecting users' privacy and security in the best way possible. With that in mind, we show that quantum key distribution can operate in the THz regime and we derive the relevant secret key rates against realistic collective attacks. In the extended THz range (from 0.1 to 50 THz), we find that below 1 THz, the main detrimental factor is thermal noise, while at higher frequencies it is atmospheric absorption. Our results show that high-rate THz quantum cryptography is possible over distances varying from a few meters using direct reconciliation, to about 220m via reverse reconciliation. We also give a specific example of the physical hardware and architecture that could be used to realize our THz quantum key distribution scheme.

Spin Hall effect and its reciprocal, the inverse spin Hall effect (ISHE), provide the means for conversion between spin and charge currents [1, 2]. In particular, the ISHE transforms a pure spin current into a transverse charge current due to spin-orbit coupling. Recently, the decisive role of the ISHE effect on extending the field of spintronics into the terahertz (THz) regime was revealed [3, 4]. Here, we demonstrate the generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. The experimentally determined optimum layer thicknesses of Fe (2 nm) / Pt (3 nm) were in qualitative agreement with simulations of the spin current induced in the ferromagnetic layer [5]. Our model takes into account generation of spin polarization, spin diffusion and accumulation in Fe and Pt and electrical as well as optical properties of the bilayer samples. Using the device in a counterintuitive orientation a Si lens was attached to increase the collection efficiency of the emitter. The optimized emitter provided a bandwidth of up to 8 THz which was mainly limited by the low-temperature-grown GaAs (LT-GaAS) photoconductive antenna used as detector and the pulse length of the pump laser. The THz pulse length was as short as 220 fs for a sub 100 fs pulse length of the 800 nm pump laser. Average pump powers as low as 25 mW (at a repetition rate of 75 MHz) have been used for terahertz generation. This and the general performance make the spintronic terahertz emitter compatible with established emitters based on optical rectification in nonlinear crystals. Furthermore, we correlate the interface structural properties with the THz-E-field amplitude and the bandwidth of the THz radiation. By allowing the Pt layers to grow along different crystallographic directions on top of Fe we modify the THz emission characteristics of the optimized emitters. We present a theoretical model which correlates the loss of energy of the hot electrons and the electron-phonon/defect scattering lifetime at the Fe/Pt interface with the ISHE current that causes the THz emission. By taking into account the response function of the THz detector we describe the influence of the crystal structure of Pt onto the THz signal shape and spectrum. The demonstration of the role of the individual layer thicknesses and of the interface quality in the THz emission spectra paves the way for more efficient spintronic THz emitters.

In this study, a design of transmissive linear-to-circular polarization conversion (LCPC) related to electromagnetically induced transparency (EIT) by arranging four sets of asymmetric resonators on a metasurface in a cross-shaped distribution is theoretically reported in the terahertz (THz) regime. It has numerically demonstrated that the bright modes and quasi-dark modes can also be mutually coupled when the x-polarized (TM) and y-polarized (TE) waves are incidents, resulting in a famous EIT phenomenon. Subsequently, when the direction of the electric fields of the electromagnetic (EM) waves are incident along the x-axis with an angle of 45°, the electric fields can be decomposed into the x- and y-directions. At this time, the corresponding amplitude and phase meet the conditions of the LCPC. Furthermore, for the TM and TE waves, the values of the maximum transmission coefficients respectively reach up to 0.910 at 1.384 THz and 0.890 at 1.277 THz. Correspondingly, two brand-new bright transparent windows are located in 1.276-1.454 THz for the TM waves and 1.206-1.373 THz for the TE waves. Concurrently, the band of the maximum 3 dB axial ratio is obtained from 1.308 THz to 1.343 THz with a relative bandwidth of 3%.