Terahertz Wave Modulation Research Articles

Generation of Tunable Terahertz Waves from Tailored Versatile Spintronic Meta-Antenna Arrays.

Broadband spintronic terahertz (THz) radiation can be efficiently generated by spin-to-charge current conversion in a ferromagnetic/nonmagnetic heterostructure. There had been many studies on realizing the enhancement or the modulation of spintronic terahertz waves. However, reported devices so far focus on implementing certain specific modulation methods, either related to the spintronic stacks or related to the metamaterial structures. In this study, a set of femtosecond laser-driven versatile spintronic terahertz devices are proposed by integrating meta-antenna structures with W/CoFeB/Pt nanolayer stacks. These monolithic integrated devices exhibit spintronic terahertz wave emission, spectral modulation, and polarization manipulation simultaneously. The terahertz pulses are generated within the ferromagnetic heterostructure interfaces and transmitted along the metallic structures, leading to the modulation of the spintronic terahertz waves. Results have shown that the center-frequency shift is up to 140 GHz and the value of ellipticity can reach 0.6, demonstrating a set of integrated and efficient spintronic terahertz devices to modulate the emitted wave. In addition, compared with the slotline antenna, the maximum peak value of the bandpass band is enhanced up to 1.63 times by carefully designing the metamaterial structure. The spintronic meta-antenna array proposed here paves an integrated way for the manipulation of spintronic terahertz optoelectronic devices.

Feasibility Demonstration of THz Wave Generation/Modulation Based on Photomixing Using a Single Wavelength-Tunable Laser

The photomixing of two lightwaves is one of the promising methods of generating a terahertz (THz) wave. The conventional photomixing system consisting of two lasers and a modulator results in large transmitter volumes and high power consumption. To solve this issue, we devised a novel THz wave generation and modulation system based on photomixing using a single wavelength-tunable laser in combination with delayed self-multiplexing. We successfully demonstrated the feasibility of 300-GHz wave generation and modulation.

High-Performance Dual-Channel Photonic Crystal Terahertz Wave Modulator Based on the Defect Mode Disappearance of a Combined Microcavity

With the working frequency of wireless communication systems moving to a higher terahertz (THz) band, the design of high-performance THz wave modulators has become a pivotal issue to be tackled urgently in THz communication. In this paper, we design a high-performance dual-channel photonic crystal modulator to enable ON–OFF regulation of the THz wave based on the defect mode disappearance of combined microcavities. The modulator introduces Y-type line defects into silicon rod photonic crystals as a dual-channel waveguide and the point defects and ring resonator form a combined microcavity. Due to the refractive index of the ring resonator filler, gallium arsenide, it is tunable with pump light excitation, and the defect mode frequency of the combined microcavity can be dynamically changed. Under pump excitation with a wavelength of 810 nm and an intensity of 0.4 μJ/cm2, 1.34 THz and 1.83 THz dual-channel waves can be OFF due to the defect mode disappearance of the combined microcavity. This is simulated by the time-domain steady-state response and steady-state THz wave field intensity distribution of the modulator by the finite-difference time-domain method. The results indicate that the dual-channel modulator has large modulation depths of 100% and 99.7%, high modulation rates of 4.05 GHz and 4.17 GHz, and low insertion losses of 0.31 dB and 0.18 dB, which lays foundation for the development of high-speed and low-loss THz communication technology.

Ultrafast Modulation of THz Waves Based on MoTe2-Covered Metasurface.

The sixth generation (6G) communication will use the terahertz (THz) frequency band, which requires flexible regulation of THz waves. For the conventional metallic metasurface, its electromagnetic properties are hard to be changed once after being fabricated. To enrich the modulation of THz waves, we report an all-optically controlled reconfigurable electromagnetically induced transparency (EIT) effect in the hybrid metasurface integrated with a 10-nm thick MoTe2 film. The experimental results demonstrate that under the excitation of the 800 nm femtosecond laser pulse with pump fluence of 3200 μJ/cm2, the modulation depth of THz transmission amplitude at the EIT window can reach 77%. Moreover, a group delay variation up to 4.6 ps is observed to indicate an actively tunable slow light behavior. The suppression and recovery of the EIT resonance can be accomplished within sub-nanoseconds, enabling an ultrafast THz photo-switching and providing a promising candidate for the on-chip devices of the upcoming 6G communication.

Thermal and mechanical THz modulation of flexible all-dielectric metamaterial.

The implementation of Terahertz (THz) modulation is critical for applications in high-speed wireless communications, security screening and so on. Therefore, it is particularly significant to obtain THz wave modulation devices with stable and flexible performance, easy manipulation of the modulation method, and multi-functionality. Here, we propose a flexible all-dielectric metamaterial by embedding zirconia (ZrO2) microspheres into a vanadium dioxide/polydimethylsiloxane (VO2/PDMS) composite, which can achieve thermal and mechanical tuning of THz wave transmission. When the temperature of the ZrO2/VO2/PDMS metamaterial increases, VO2 changes from the insulating phase to the metallic phase, and the 1st (at 0.304 THz) and 2nd (at 0.414 THz) order magnetic resonances exhibit the tunability of 20 GHz and 15 GHz, respectively. When stretched, the 1st and 2nd order magnetic resonances show the tunability of 12 GHz and 10 GHz, respectively. In the meantime, there are accompanying changes in transmittance at the resonances. The ZrO2/VO2/PDMS all-dielectric metamaterial presented in this work provides an alternative strategy for developing actively tunable, flexible, and versatile THz devices. In addition, it has the merits of simple preparation and low cost, promising large-area and rapid preparation of meta-arrays.

Photo-reconfigurable and electrically switchable spatial terahertz wave modulator [Invited

Spatial terahertz wave modulators that can arbitrarily tailor the electromagnetic wavefront are in high demand in nondestructive inspections and high-capacity wireless communications. Here, we propose a liquid crystal integrated metadevice. It modulates the terahertz wave based on the adjustable electromagnetically induced transparency analog when spatially changing the environmental refractive index. The functions of the device can be arbitrarily programmed via photo-reorienting the directors of liquid crystals with a digital micromirror device-based exposing system. The thin liquid crystal layer can be further driven by an electric field, and thus the function can be rapidly switched. Amplitude modulation and the lens effect are demonstrated with modulation depths over 50% at 0.94 THz.

Terahertz multibeam modulation reflection-coded metasurface

Most of reported coding metasurfaces only use phase encoding or amplitude encoding to regulate electromagnetic waves, which limits the flexibility of terahertz wave regulation. In this work, a metasurface element structure is proposed. The metasurface element is composed of three layers, i.e. metal pattern structure layer, intermediate medium layer, and metal base layer. According to the geometric phase principle, the phase coverage in the 2π range can be achieved by rotating the metal pattern structure layer under the incidence of the circular-polarized terahertz wave. The metasurface element structure is arranged reasonably by using the phase coding, and the 1-bit and 2-bit phase coding metasurface are designed. First of all, the coding metasurface with interlacing “0” and “1” is designed to generate a double beam reflection under the vertical incidence of circular polarized terahertz waves, while the two-dimensional checkerboard coding metasurface with “0” and “1” generates a symmetrical four-beam reflection. In addition, the metasurface is designed to deflect the reflected beam, and the coding period is changed to design the metasurface to deflect the reflected beam to the specified angle, showing good flexibility. Finally, the convolutional operation is introduced to flexibly regulate the circular polarized beam, and the functions of beam splitting and reflection beam deflection are obtained. The amplitude coded metasurface is designed under theincidence of the online polarized terahertz wave, and the near-field imaging effect can be realized by the amplitude differentiation of polarization reflection. The designed amplitude coded metasurface realizes the function of imaging in space, presenting the designed “CJLU” pattern, which has different imaging effects at different observation locations. When the observation plane distance is 80 μm at the observation frequency of 1.22 THz, the near-field imaging effect is best. In conclusion, we propose a terahertz multibeam modulation reflection-coded metasurface, which combines geometric phase and amplitude variation to achieve different terahertz wave modulation functions under different polarization incident terahertz waves. The results from the simulated near-field radiation model and the far-field radiation model are both in agreement with the theoretical calculation predictions. The designed metasurface provides a degree of freedom method for terahertz wave polarization and phase manipulation, which greatly improves the efficiency of terahertz wave manipulation and has potential applications in terahertz systems.

Highly sensitive broadband terahertz modulator based on MAPbI<sub>3</sub>/Graphene/Si composite structure

A high-performance silicon-based terahertz modulator is one of the key devices for building an ultrawideband terahertz-fiber hybrid communication system. In this paper, an ultrawideband terahertz modulator with large modulation depth based on a chalcogenide/graphene/silicon (MAPbI<sub>3</sub>/Graphene/Si) composite structure driven by near-infrared light (NIR) is proposed. The experimental results show that the graphene thin film and the chalcogenide hole transport layer can effectively promote the interfacial charge separation, increase the carrier complex lifetime, significantly enhance the surface conductivity of the device, further modulate the terahertz wave transmission amplitude, and realize the function of the light-controlled terahertz wave modulator under the NIR light drive. The terahertz transmission characteristics of the device are characterized by an 808 nm NIR modulation excitation source, and a large modulation depth of up to 88.3% is achieved in an ultra-wide frequency range of 0.2–2.5 THz and a low power density of 6.1 mW/mm<sup>2</sup> driven by NIR light, which is much higher than that of the bare silicon substrate (14.0%), with the significant advantages of high sensitivity, broadband, and large modulation depth. The corresponding semi-analytical device model is established and the experimental results are verified by simulation. The proposed MAPbI<sub>3</sub>/Graphene composite thin film is effective in enhancing the silicon-based modulator performance and provides a new strategy for the future integration of silicon-based terahertz modulators in NIR terahertz-fiber hybrid communication systems.

Ultrafast modulation of terahertz waves using on-chip dual-layer near-field coupling

As a key potential component of future sixth-generation (6G) communication systems, terahertz (THz) technology has received much attention in recent years. However, a lack of effective high-speed direct modulation of THz waves has limited the development of THz communication technology. Currently, most high-speed modulators are based on photonic systems that can modulate electromagnetic waves with high speed using sophisticated optoelectronic conversion techniques. Yet, they usually suffer from low conversion efficiency of light to the THz range, resulting in low output power of the modulated THz waves. Here, we describe a guided-wave modulator for THz signals whose performance nearly matches that of existing in-line fiber-optic modulators. Our results demonstrate a maximum modulation depth greater than 20 dB (99%) and a maximum sinusoidal modulation speed of more than 30 GHz, with an insertion loss around 7 dB. We demonstrate the capabilities of this modulator in a point-to-point communication link with a 25 Gbit/s modulation speed. Our modulator design, based on near-field coupling of a THz transmission line to a single resonant meta-element, represents a powerful improvement for on-chip integrated high-performance THz devices.

Ultrafast terahertz transparency boosting in graphene meta-cavities

Abstract As an exceptional nonlinear material, graphene offers versatile appealing properties, such as electro-optic tunability and high electromagnetic field confinement in the terahertz regime, spurring advance in ultrashort pulse formation, photodetectors and plasmonic emission. However, limited by atomic thickness, weak light–matter interaction still limits the development of integrated optical devices based on graphene. Here, an exquisitely designed meta-cavities combined with patterned graphene is used to overcome this challenge and promote THz-graphene interaction via terahertz location oscillation. By using an 800 nm pump laser, the local field-induced strong interaction allows sensitive responses to the ultrafast energy transfer from the ultrafast optical pump to graphene electron heat, enabling 46.2% enhancement of terahertz transparency. Such optical modulation of terahertz waves shows ultrafast response in delay less than 10 ps. Moreover, thanks to the nature of graphene, the device shows unique potential for electrically dynamic tuning and further bandwidth broadening.

Electrically addressable integrated intelligent terahertz metasurface.

Reconfigurable intelligent surfaces (RISs) play an essential role in various applications, such as next-generation communication, uncrewed vehicles, and vital sign recognizers. However, in the terahertz (THz) region, the development of RISs is limited because of lacking tunable phase shifters and low-cost sensors. Here, we developed an integrated self-adaptive metasurface (SAM) with THz wave detection and modulation capabilities based on the phase change material. By applying various coding sequences, the metasurface could deflect THz beams over an angle range of 42.8°. We established a software-defined sensing reaction system for intelligent THz wave manipulation. In the system, the SAM self-adaptively adjusted the THz beam deflection angle and stabilized the reflected power in response to the detected signal without human intervention, showing vast potential in eliminating coverage dead zones and other applications in THz communication. Our programmable controlled SAM creates a platform for intelligent electromagnetic information processing in the THz regime.

Photoconductor-metadevices: an enabling platform for dual-optical/electrical modulation of terahertz waves

We report a photoconductor-based terahertz metadevice whose transmission characteristics can be comprehensively controlled by a combination of an optical excitation and electrical bias. A metasurface with interdigitated electrodes is proposed to simultaneously support a terahertz resonance and the photoconductive effect, allowing efficient local and global dual-tuning of the carriers in the semiconductor and thus the terahertz wave, based on the simultaneous external optical and electrical stimuli. Experimental results reveal the opposite tuning trend of optical and electrical excitations, leading to the comprehensive transmission modulation of terahertz wave. Our study demonstrates a proof-of-concept device for sophisticated manipulation of terahertz radiation.

MAPbI3 perovskite hybrid metasurface for agile, multidimensional THz wave modulation with 3D Dirac semimetal-like Fermi model optically induced by three wavelength bands

Organic–inorganic hybrid three-dimensional (3D) perovskite materials have emerged as potential candidates for terahertz (THz) modulators. In this study, the metasurface of electromagnetically induced transparency resonance was integrated with 3D perovskite to achieve a high-efficiency dynamic THz modulator. The integration was achieved by modulating the intrinsic optoelectronic properties of perovskite with three wavelengths of optical pump and external voltage excitation. The maximum modulation depth was 466.8 %, with a pump flux of 11.5 mW/cm2 and excitation at 532 nm. The Dirac-like semimetal-Fermi energy band model was proposed to explain the dynamic transitions of free carriers in perovskites and reveal the internal mechanism of the dynamic modulation. In addition, we verified our proposed theoretical model by simulation and theoretical fitting, and calculated the conductivity and number of free carriers of perovskite under different excitations. The system thoroughly explains the observed differences under three wavelengths of optical pump excitation. The active control of THz waves based on Dirac semimetal materials with optical and electrical inputs could have many potential applications that have been proposed by few studies.

Picosecond mode switching and Higgs amplitude mode in superconductor-metal hybrid terahertz metasurface

Abstract The ultrafast modulation of terahertz (THz) waves is essential for numerous applications, such as high-rate wireless communication, nonreciprocal transmission, and linear frequency conversion. However, high-speed THz devices are rare due to the lack of materials that rapidly respond to external stimuli. Here, we demonstrate a dynamic THz metasurface by introducing an ultrathin superconducting microbridge into metallic resonators to form a superconductor-metal hybrid structure. Exploiting the susceptibility of superconducting films to external optical and THz pumps, we realized resonance mode switching within a few picoseconds. The maximum on/off ratio achieved is 11 dB. The observed periodic oscillation of transmission spectra both in the time and frequency domain under intense THz pump pulse excitation reveals the excitation of Higgs amplitude mode, which is used to realize picosecond scale THz modulation. This study opens the door to ultrafast manipulation of THz waves using collective modes of condensates, and highlights an avenue for developing agile THz modulation devices.

Tune the resonance of VO2 joined metamaterial dimers by adjacent cut wires.

Two terahertz metamaterials were joined by a conductivity variable VO2 patch to obtain a metamaterial dimer. By applying voltage or heat to the VO2 patches, active modulation of terahertz wave could be achieved. A cut-wire metamaterial was placed adjacent to the VO2 joined dimer to affect its electromagnetic response. It was found that the cut wire could heavily impact the resonance mode of the VO2 joined dimer, which gives dual resonance dips in transmission spectrum for both insulating and conducting states of VO2 patches. As a result, by tuning the conductivity of VO2, active dual band phase modulation could be achieved with high transmission window by this dimer-cut wire coupling system.

Switchable vortex beam polarization state terahertz multi-layer metasurface

We propose a switchable vortex beam polarization state terahertz multi-layer metasurface, which consists of three-layer elliptical metal crosses, four-layer dielectrics, and two-layer hollow metal circles, which are alternately superimposed. Under the normal incidence of left-handed circularly polarized (LCP) wave and the right-handed circularly polarized (RCP) waves, the proposed structure realizes three independent control functions, i.e., focused and vortex beam, vortex beam with different topological charges, and polarization states switching, and azimuth switching of two vortex beams with different polarization states. The results show that the proposed metasurface provides a new idea for investigating the multifunctional terahertz wave modulation devices.

Reconfigurable photoinduced terahertz wave modulation using hybrid metal-silicon metasurface.

We present a photoinduced reconfigurable metasurface to enable high spatial resolution terahertz (THz) wave modulation. Conventional photoinduced THz wave modulation uses optically induced conductive patterns on a semiconductor substrate to create programmable passive THz devices. The technique, albeit versatile and straightforward, suffers from limited performance resulting from the severe lateral diffusion of the photogenerated carriers that undermines the spatial resolution and conductivity contrast of the photoinduced conductive patterns. The proposed metasurface overcomes the limitation using a metal-jointed silicon mesa array with subwavelength-scaled dimensions on an insulator substrate. The structurephysically restrains the lateral diffusion of the photogenerated carriers while ensuring the electrical conductivity between the silicon mesas , which is essential for THz wave modulation. The metasurface creates high-definition photoconductive patterns with dimensions smaller than the diffusion length of photogenerated carriers. The metasurface provides a modulation depth of -20 to -10 dB for the THz waves between 0.2 to 1.2 THz and supports a THz bandpass filter with a tunable central frequency. The new, to the best of our knowledge, design concept will benefit the implementation of reconfigurable THz devices.

Physicochemical Insights on Terahertz Wave Diminished Side Effects of Drugs from Slow Dissociation.

Dopamine D2 receptors (D2Rs) are one of the most intensely investigated and well-established drug targets for neuropsychiatric disorders. Selective D2R antagonists have been developed as efficacious antipsychotic drugs. Nevertheless, the potent drugs with necessarily high affinity are prone to slow dissociation, which invokes a plethora of severe side effects such as extrapyramidal symptoms, substantial weight gain, associated diabetes, etc. This has become a major barrier in treating psychiatric patients. In this work, we propose a physical method, terahertz wave modulation, to promote the dissociation of high-affinity antipsychotics and thus diminish the side effects. We have proven that a 4.0 THz wave could reduce the affinity by 71% between the D2R and a risperidone ligand and meanwhile expand the exit via conformation modulation, which promises an accelerated dissociation of risperidone. In addition, it is estimated that the enhancement of the dissociation rate due to lowered binding by terahertz irritation could constitute up to 8 orders of magnitude, which is fairly impressive and resembles the enzyme's catalysis. Also, acceleration of the dissociation rate could be adjusted by the irritation strength. This work elaborates the terahertz wave-modulated noncovalent interactions critical in cell signaling pathways. Most importantly, it demonstrates the feasibility of terahertz technologies intervening in receptor-ligand complex regulated diseases such as neurodegenerative disorders, metabolic diseases, etc.

Broadband terahertz tunable multi-film absorber based on phase-change material.

Based on the impedance matching method, we have numerically demonstrated a broadband tunable multilayer structure in a terahertz (THz) regime. The switchable functional characteristics of the absorber can be achieved by utilizing the phase transition property of vanadium dioxide (VO2). When VO2 is in the metallic state, the designed device behaves as a broadband absorber with an absorbance greater than 90% under normal incident from a 4.5 to 10 THz range. When VO2 is in the insulating state, the absorption in this band is down to near 0%. Moreover, this high absorption band shows a good polarization insensitive property and can be maintained over a range of incident angles up to 45°. Our proposed device exhibits the merits of wideband reconfigure absorbance in THz, and the absorber can be easily fabricated without involving any lithographic process, both of which are very attractive to potential THz applications such as sensing, camouflaging, and modulation of THz waves.

Minimal-invasive enhancement of auditory perception by terahertz wave modulation

Hearing impairment is a common disease affecting a substantial proportion of the global population. Currently, the most effective clinical treatment for patients with sensorineural deafness is to implant an artificial electronic cochlea. However, the improvements to hearing perception are variable and limited among healthy subjects. Moreover, cochlear implants have disadvantages, such as crosstalk derived from the currents that spread into non-target tissue between the electrodes. Here, in this work, we describe terahertz wave modulation, a new and minimally invasive technology that can enhance hearing perception in animals by reversible modulation of currents in cochlear hair cells. Using single-cell electrophysiology, guinea pig audiometry, and molecular dynamics simulations, we show that THM can reversibly increase mechano-electrical transducer currents (∼ 50% higher) and voltage-gated K+ currents in cochlear hair cells through collective resonance of −C=O groups. In addition, measurement of auditory brainstem response (ABR) in guinea pigs treated with THM indicated a ~ 10 dB increase in hearing sensitivity. This study thus reports a new method of highly spatially selective hearing enhancement without introducing any exogeneous gene, which has potential applications for treatment of hearing disorders as well as several other areas of neuroscience.