Controllable Terahertz Research Articles

High-efficiency and frequency-controllable terahertz pulses driven by two-color relativistic laser pulses

The two-color laser filamentation technique serves as a high-quality, laser-based strong-field terahertz source. However, the terahertz energy obtained from this method tends to saturate at high laser intensity, limiting its ability to fully exploit the advantage of the extremely strong field offered by relativistic lasers. Here, we proposed a novel approach based on photon deceleration in plasma to obtain controllable terahertz radiations with two-color relativistic femtosecond laser pulses. In our method, an 800-nm laser drives the plasma wave and controls the plasma density distribution, while the other 10.6-μm laser experiences a strong frequency downshift in the plasma wave and converts to a multi-cycle terahertz pulse, whose central frequency is adjustable by altering the parameters of laser and plasma. The conversion efficiency from the 10.6-μm laser to terahertz radiation can be over 12%, potentially unlocking novel applications in the terahertz domain.

Ultrafast All-Optical Switching Modulation of Terahertz Polarization Conversion Metasurfaces Based on Silicon.

With the development of ultrafast optics, all-optical control of terahertz wave modulation based on semiconductors has become an important technology of terahertz wave regulation. In this article, an ultrawideband terahertz linear polarization converter consisting of a double-layered metasurface is first proposed. The polarization conversion ratio of the device is ∼ 100% at 0.2-2.2 THz, and the transmission of copolarization approaches zero in the full band, which demonstrates the ability of high-purity output with rotating input linear polarization of 90° over an ultrawideband. By analysis of the surface current and electric field distribution, the physical mechanism of polarization conversion is elucidated. In addition, the influence of important geometric parameters on the device is discussed and analyzed in detail, which provides theoretical support for the design of high-performance polarization converters. More importantly, by introducing semiconductor silicon to construct an actively controllable metasurface, we design all-optical polarization converters based on a meta-atomic molecularization metasurface and all-dielectric metasurface; the dynamically tunable ultrawideband linear polarization conversion is realized under optical pumping, which solves the inherent problem of the performance of the metasurface polarization converters. Numerical simulation shows that the switching response of the two types of actively controllable devices under optical pumping is about 700 and 1800 ps, respectively, and can manipulate polarized wave conversion ultrafast, which brings new opportunities for all-optical controlled ultrafast terahertz polarization converters. Our results provide a feasible scheme for the development of state-of-the-art active and controllable ultrafast terahertz metasurface polarization converters, which have great application potential in short-range wireless terahertz communication, ultrafast optical switches, the transient spectrum, and optical polarization control devices.

Controllable terahertz intersubband absorptions in ZnO/(Sb,N) co-doped ZnO quantum wells: First-principles study

ZnO-based heterostructures have emerged as promising materials for infrared and terahertz (THz) optoelectronic technologies capable of operating at room temperatures. This is attributed to the intrinsically high longitudinal-optical (LO) phonon energy which suppresses the phonon scattering to boost the temperature performance. Numerous high-performance THz sources rely on single doping (e.g., ZnO/MgxZn1–xO, ZnO/CdxZn1–xO hybrids). Herein, the combined theoretical approaches based on density functional theory (DFT) and the numerical matrix method were employed to unravel the electronic and optical properties of a quantum well made from ZnO/(Sb,N) co-doped ZnO heterostructures. The findings reveal that (Sb,N) co-doped relatively requires the lower formation energy than the single doping counterparts. Both un-doped and (Sb,N) co-doped ZnO are characterized as semiconductors of which the energy gap of the latter is relatively lower. The combination of these materials consequently creates type-I band lineups with the valence and conduction band offsets of 1.03 eV and 0.61 eV, respectively. Intriguingly, a confined electron in the resultant quantum well derived from the conduction band offset exhibits the intersubband optical transitions in the energy range of 6.0–10.0 THz which is the challenging regime in THz technology. Moreover, the optical absorptions can be effectively modulated by the width of the quantum well and the external electric fields. Specifically, the increases in the well width and field strength result in the redshift and blueshift in the absorption spectrum, respectively. Hence, ZnO/(Sb,N) co-doped ZnO/ZnO heterostructure is conclusively a potential candidate for THz optoelectronic devices.

Polarization-selective modulation of meandered-line metamaterials with graphene surface plasmonics

Periodic metal lines are the first kind of metamaterials, which typically exhibit plasmonic resonances to the normally incident waves polarized perpendicularly to them, i.e., transverse magnetic (TM) waves. Meandered lines can obtain resonances for both TM and transverse electric (TE) incidences and have found important applications in chirality and polarization conversion. Recently, several stimuli have been investigated to dynamically modulate the optical properties of periodic lines, which simultaneously tune both TE and TM resonances and show little polarization dependence. In this work, metal-graphene hybrid meandered-line metamaterials are designed for polarization-selective modulation at terahertz frequencies for the first time. The periodic lines meandered in S-shape resonate at 103 GHz for TE waves and at 195 and 320 GHz for TM waves, respectively. Graphene strips are patterned in pairs right at the bending corners along the polarization direction of TE waves, which produce even plasmonic dipole mode for TE incidence. In contrast, they are asymmetric with respect to the TM waves and generate odd coupling mode under TM incidence. These modes with various field concentrations exhibit considerable modulation difference to the graphene conductivity variation. As the graphene chemical potential sweeps, TE transmittance is modulated from 0.7 to 0.47 at 100 GHz, and TM transmittance is modulated just from 0.4 to 0.39 at 190 GHz and from 0.57 to 0.52 at 289 GHz, respectively. The corresponding modulation axial ratio is as large as 23 and 4.6 under a low bias from -1.2 to 1.2 V. This work paves a new way to control terahertz waves polarization-selectively.

Enhanced polarization conversion and giant Faraday rotation in patterned terahertz graphene metamaterials with combined electrical and magnetic tuning

Graphene metamaterials (MMs) have the potential to reconfigure and dynamically control terahertz (THz) waves. In this study, we conducted numerical investigations to explore the effects of externally applied magnetic fields up to 20 Tesla on the transmission properties of graphene patterned split ring resonator (GSRR) MMs in the THz region. We quantitatively compared the tunability of resonance amplitude and frequency in the co-polarized transmission component between the magnetic method and the traditional electrical approach. Our results demonstrate that magnetic tuning can effectively modulate the resonant properties of GSRR MMs. Furthermore, when combining electrical and magnetic tuning, we observed an enhancement in the polarization conversion ratio, as well as the achievement of a significant Faraday rotation angle of nearly 90 degrees in GSRR MMs. These findings indicate the potential of functional graphene-based THz devices, including switches, modulators, polarization converters, and sensors.

Coherent generation and control of tunable narrowband THz radiation from a laser-induced air-plasma filament.

We report on the proof-of-principle experiment of generating carrier-envelope phase (CEP)-controllable and frequency-tunable narrowband terahertz (THz) radiation from an air-plasma filament prescribed by the beat of a temporally stretched two-color laser pulse sequence. The pulse sequence was prepared by propagating the fundamental ultrafast laser pulse through a grating stretcher and Michelson interferometer with variable inter-arm delay. By partially frequency-doubling and focusing the pulse sequence, an air-plasma filament riding a beat note was created to radiate a THz wave with primary pulse characteristics (center frequency and CEP) under coherent control. To reproduce experimental results and elucidate complex nonlinear light-matter interaction, numerical simulation has been performed. This work demonstrates the feasibility of generating coherently controlled narrowband THz wave with high tunability in laser-induced air plasma.

Thermally controllable electromagnetically induced transparency in a coupled terahertz metamaterial

We investigate the thermally controllable electromagnetically induced transparency (EIT) effect in a planar terahertz (THz) metamaterial configuration comprising metallic semi-circular split ring resonators (MSCSRR) integrated with indium antimonide (InSb). In transmission configuration, a transparency window is observed by placing a horizontal cut wire (HCW) in close proximity to the MSCSRR. It develops by frequency detuning and strong near-field coupling between both the bright resonators. The transparency window is found to be controllable with temperature along with the structural parameters of the unit cell. The blue shift of 0.028 THz is observed when the temperature increases from 260 K to 320 K. Moreover, the group delay of the designed EIT structure can also dynamically tune by varying temperature of InSb. The shift from 3.19 ps to 1.98 ps is observed when the temperature increases from 260 K to 320 K. We used a coupled harmonic oscillator model to thoroughly understand the coupling between resonators and interpret the transparency effect. Our findings can be closely predicted from the model. The present study can be significant for the development of active and controllable terahertz photonic components such as slow light systems, buffers, etc.

Controllable terahertz beam focusing and rotating by graphene-assisted microring metasurfaces

This research proposes a terahertz (THz) metalens for focusing THz waves with an electrically controllable focal length. The dynamic of tuning of THz metalenses with graphene provides high controllability, fast responses, and compact structures for beam focusing. An array of graphene microrings has been presented with significant phase variations possibility. The three-dimensional finite-difference time-domain method has been employed for the analysis of the proposed structure. Applying a voltage to the graphene layer changes the graphene Fermi level, and the focal length can be controlled. As the chemical potential of graphene varies from 0.1 to 0.3 eV, the focal length changes 20.5λ (from 4 to 12.2 mm at the frequency of 0.75 THz). In addition, by biasing every array of graphene microrings with different gate voltages, we can electrically rotate the focal beam up to 32.02 deg. An equivalent circuit model (CM) is proposed for fast analysis and the verification of the FDTD results as well. The achieved results of the CM comply very well with those derived by the FDTD method. The proposed structure has the potential to be used in THz imaging.

Narrowband, Angle-Tunable, Helicity-Dependent Terahertz Emission from Nanowires of the Topological Dirac Semimetal Cd3As2.

All-optical control of terahertz pulses is essential for the development of optoelectronic devices for next-generation quantum technologies. Despite substantial research in THz generation methods, polarization control remains difficult. Here, we demonstrate that by exploiting band structure topology, both helicity-dependent and helicity-independent THz emission can be generated from nanowires of the topological Dirac semimetal Cd3As2. We show that narrowband THz pulses can be generated at oblique incidence by driving the system with optical (1.55 eV) pulses with circular polarization. Varying the incident angle also provides control of the peak emission frequency, with peak frequencies spanning 0.21-1.40 THz as the angle is tuned from 15 to 45°. We therefore present Cd3As2 nanowires as a promising novel material platform for controllable terahertz emission.

Controllable transmissive-reflective multifunction terahertz metasurface by different polarization and operating frequencies

Due to the limitations of frequency bands and control methods, it is difficult to achieve multi-functional integration and real-time regulation of full-space metasurfaces. In this paper, we proposed a switchable transmissive-reflective mode terahertz metasurface independently depending on the incident wave frequencies and polarizations. The unit cell consists of four metallic layers, which are separated by three silicon dioxide layers. When the x-polarized wave is incident along the ± z-axis, the meta-device achieves focusing function at 1.62 THz. When the y-polarized wave is incident along the ± z-axis, the structure generates the reflective two and four splitting beams at a frequency of 0.82 THz and realizes a focused beam with a topological charge of l=±1 at a frequency of 1.65 THz. The full wave simulation results are in good agreement with the theoretical calculation predictions. The metasurface provides a new idea for the control of terahertz devices, and has a broad application prospect in the field of terahertz systems.

Controllable waveform terahertz generation using rippled plasma driven by an inhomogeneous electrostatic field.

We theoretically present the waveform controls of terahertz (THz) radiations generated from homogeneous and rippled plasma within inhomogeneous external electrostatic field. The Particle-in-cell (PIC) simulations is implemented to demonstrate generation and controllability of three types of THz pulses: single frequency THz pulse in homogeneous plasma, broadband THz pulse and dual frequency THz pulse in rippled plasma. The single frequency THz pulse can be tuned via shifting the knob of electron density of homogeneous plasma. Waveform of broadband THz pulse can be regulated into an envelope-like shape by varying amplitude of electron density of rippled plasma. The two center frequencies' interval of dual frequency THz pulse can be controlled by wave numbers of density distribution of rippled plasma. This work provides a potential means to generate the dual frequency THz pulses with two harmonic frequencies (ω+Ωω, Ω=2) or incommensurate frequencies (ω+Ωω, Ω=1.7,1.8, 2.2…).

Actively Controllable Terahertz Metal-Graphene Metamaterial Based on Electromagnetically Induced Transparency Effect.

A metal–graphene metamaterial device exhibiting a tunable, electromagnetically induced transparency (EIT) spectral response at terahertz frequencies is investigated. The metamaterial structure is composed of a strip and a ring resonator, which serve as the bright and dark mode to induce the EIT effect. By employing the variable conductivity of graphene to dampen the dark resonator, the response frequency of the device shifts dynamically over 100 GHz, which satisfies the convenient post-fabrication tunability requirement. The slow-light behavior of the proposed device is also analyzed with the maximum group delay of 1.2 ps. The sensing performance is lastly studied and the sensitivity can reach up to 100 GHz/(RIU), with a figure of merit (FOM) value exceeding 4 . Therefore, the graphene-based metamaterial provides a new miniaturized platform to facilitate the development of terahertz modulators, sensors, and slow-light applications.

Ultra‐Broadband Terahertz Polarization Conversion Enabled by All‐Dielectric Grating Structures

All-Dielectric Grating Structures Based on the all-dielectric grating structure, Liang Wu and co-workers propose a controllable terahertz polarization converter with ultra-broadband bandwidth. By simply tilting the grating structure, it can switch arbitrarily between the quarter-waveplate and the half-waveplate, which greatly enriches the way to control the polarization states in terahertz band. More details can be found in article number 2200033.

Multichannel terahertz quasi-perfect vortex beams generation enabled by multifunctional metasurfaces

Abstract Vortex beams carrying orbital angular momentum (OAM) open a new perspective in various terahertz research. Multichannel and divergence controllable terahertz vortex beam generation holds the key to promoting the development of OAM related terahertz research. Here, we introduced and experimentally demonstrated quasi-perfect vortex beam (Q-PVB) with a controllable divergence angle independent of the topological charge and multichannel Q-PVBs generation with all-dielectric multifunctional metasurfaces. By superimposing specific phase functions together into the metasurfaces, multiple vortex beams and four-channel Q-PVBs with different topological charges are generated as well as focused at separated positions. High resolution characterization of terahertz electric field shows the good quality and broadband properties of Q-PVBs. Interestingly, compared with conventional perfect vortex beam; Q-PVB displays a smaller divergence angle and thinner ring width. The metasurfaces proposed here provide a promising avenue to realize multichannel vortex beams generation in compact terahertz systems; benefiting OAM related researches such as mode division multiplexing, vortex beam related plasmonic enhancement and spinning objective detection.

Electrothermally controllable terahertz metamaterial for sensing application

A design of electrothermally controllable terahertz (THz) metamaterial (ECTM) is presented, which is composed of split-ring resonator (SRR) on Si substrate. All SRR unit cells are connected to form ECTM with a current loop. By driving a direct-current (DC) bias voltage on ECTM, the heating energy is generated and then changed the surrounding refraction index. Therefore, it results in a resonant shift. While the DC bias voltage is applied from 0 volt to 0.95 volts in transverse electric (TE) mode, ECTM shows a tunable triple-resonance with the tuning ranges of 24 GHz, 40 GHz, and 32 GHz, respectively. The maximum sensitivity is 42 GHz/volt. Meanwhile, the maximum Q-factor is 24.5. These results pave a way for ECTM to be applied in polarization-dependent filters and sensors for further tunable and controllable optoelectronics applications.

Dynamic terahertz anisotropy and chirality enhancement in liquid-crystal anisotropic dielectric metasurfaces

To enhance and actively control terahertz (THz) anisotropy and chirality, we have designed and fabricated a THz composite device with a liquid crystal (LC) layer and Si anisotropic metasurface. By initial anchoring and electrically rotating the spatial orientation of the LC optical axis, the different symmetry relationships are obtained in this hybrid device. When the optical axis of LC is parallel or perpendicular to the optical axis of the Si metasurface, the anisotropy of the device will be enhanced or offset, which leads to a tunable phase-shift range of more than 180°. When there is an angle between the two optical axes, due to the destruction of the mirror symmetry in the LC-Si anisotropic medium, the highest circular dichroism of the device reaches 30 dB in the middle orientation state of the LC optical axis, and the active modulation can be realized by changing the bias electric field on the LC layer. This composite device demonstrates rich characteristics for the feasible manipulation of THz polarization conversion and chiral transmission, which can be applied in THz polarization imaging and chiral spectroscopy.

Electrically controllable terahertz metamaterials with large tunabilities and low operating electric fields using electrowetting-on-dielectric cells.

A simple method that is compatible with all geometrical structures of terahertz (THz) metamaterials for increasing their frequency tunabilities and decreasing their operating electric fields is proposed. This method uses the displacement of glycerol droplets with various volumes to tune the resonance frequency of a THz metamaterial in an electrowetting-on-dielectric (EWOD) cell. The experimental results reveal that the THz metamaterial has a large frequency tunability of 28% at an operating electric field that is smaller than 0.2 V/µm as the glycerol droplets move in and out of the path of a THz beam. The frequency tunability is large because the near field of the metamaterial "experiences" a large difference between the refractive indices of glycerol and air. The EWOD cell with the THz metamaterial is a great achievement for developing electrically controllable band-stop filters with large frequency tunabilities and small operating electric fields.

Ultrabroadband electrically controllable terahertz modulation based on GaAs Schottky diode structure

We demonstrate an ultrabroadband electrically controllable terahertz modulator based on a Schottky diode structure formed with periodic metal microslits on an n-doped GaAs substrate. The mechanism of our design is different from that of the traditional Schottky diode-based THz electrical modulator, which uses free charge carriers in a substrate to control the resonant behavior of metamaterials. In our device, the modulation is based on free-carrier absorption on the THz wave and therefore broadband. The charge carrier concentration between the metal microslits is actively modified by applying a reverse bias voltage to generate a direct modulation of THz waves. The modulation performance is enhanced by the THz non-resonant electric field enhancement effect from the metal microslits. The experimental results indicate that the modulation depth is positively correlated with the electric field enhancement ratio at the depletion region in the gap and the number of microslits in the THz light spot-covered area. An averaged modulation depth of ∼40% in the measurable frequency range from 0.4 to 1.4 THz was achieved by the device with a metal microslits gap width of 2 µm and a period of 20 µm. A maximum modulation depth of ∼75% was achieved by stacking two devices back-to-back with a 3-dB down bandwidth modulation speed of ∼100 kHz. Further improvements of the device can be achieved by optimizing the parameters such as the free-carrier density in the doping layer, the active area size, and the specifications of the metal microslits.

Highly efficient terahertz emission from layered ZrTe5 crystal with strong anisotropy and high in-plane carrier mobility

Intense, polarization controllable, and single-cycle terahertz pulses as light source are important tools to investigate physical and biological properties of materials. In this paper, we have detected efficient terahertz emission from layered transition-metal pentatelluride ZrTe5 excited by femtosecond laser pulse due to its strong anisotropy and high mobility of electrons. The physical mechanism of terahertz generation from this material is studied and the absolute predominance of diffusion current induced by photo-Dember effect is proved. In addition, we have achieved the modulation of terahertz polarization by adjusting the polarization states of the pump pulse. The generated elliptically polarized THz pulse is crucial for the investigation of vibration modes of chiral biological molecules, such as proteins and DNAs. Our findings indicate that strong and polarization-controllable THz emission can be generated by ZrTe5.

Active terahertz metamaterials electrically modulated by InGaZnO Schottky diodes

Active metamaterials (MTMs) are artificially engineered structures with tunable and exceptional properties that are absent in natural materials. Recently, InGaZnO (IGZO), a widely used semiconductor for large-area and flexible display backplane drivers, has gained interest for active control of MTMs due to its large-area uniformity, ease of thin film deposition, and low cost. In this paper, IGZO Schottky barrier diodes (SBDs) are proposed to reconfigure electric-field-coupled inductor-capacitor (ELC) MTMs and actively control terahertz (THz) waves for the first time. The SBDs are designed to bridge the capacitors of the ELC resonators so that the average conductivity within the capacitor gap can be modulated by bias voltage while keeping the capacitance value almost unchanged. To precisely simulate this mechanism, two U-shaped resistive sheet models beside the gap are built for IGZO SBD in 3-D simulation for maintaining the same capacitance and resonant frequency. Furthermore, a device with 14400 MTM cells is fabricated and characterized using frequency-domain spectroscopy. The measured transmission shows a continuous modulation from -14.2 to -9.4 dB at 0.39 THz, which corresponds to a modulation depth of 14.3%. This work paves a new way for active THz MTMs using industrial compatible thin-film technology.