Electrically tunable liquid crystal terahertz device based on double-layer plasmonic metamaterial.

In order to suit the increasingly complex electromagnetism environment, multifunctional and continuously tunable terahertz device is becoming a research focus. In this paper, a frequency and amplitude tunable dual-band terahertz metamaterial absorber is proposed. Continuous tuning of absorption frequency and amplitude are completed by dynamic control of the conductivity of photosensitive silicon and the polarization angle, respectively. The tuning range of 55 GHz and amplitude modulation depth up to 100% are obtained. It can be concluded that the structure provides a new approach for the realization of future terahertz metamaterial absorbers possessing the characteristics of multifunction, portability, and intelligence.

We experimentally demonstrate a dynamically tunable terahertz (THz) metamaterial absorber based on an electrostatic microelectromechanical systems (MEMS) actuator and electrical dipole resonator array. The absorption of the THz wave is mainly a result of the electrical dipole resonance, which shows a tunable performance on demand. By preforming the finite integral technique, we discovered that the central absorption frequency and the amplitude can be simultaneously tuned by the applied voltage U. Characterized by a white light interferometer and a THz time domain spectroscopy system, our THz absorber is measured to show a modulation of the central frequency and the amplitude to about 10% and 20%, respectively. The experimental results show good agreement with the simulation. This dynamically tunable absorber has potential applications on THz filters, modulators and controllers.

We proposed plasmonic effect-based narrow band tunable terahertz switches consisting of multilayered graphene metamaterial. Though several terahertz optical switches based on metamaterials were previously reported, these switches had complicated fabrication processes, limited tunability, and low modulation depths. We designed and simulated ingenious four- and eight-state terahertz optical switch designs that can be functional for multimode communication or imaging using the finite-difference time-domain simulation technique. The plasmonic bright modes and transparency regions of these structures were adjusted by varying the chemical potential of patterned graphene layers via applying voltage in different layers. The structures exhibited high modulation depth and modulation degree of frequency, low insertion loss, high spectral contrast ratio, narrow bandwidth, and high polarization sensitivity. Moreover, our proposed simple fabrication process will make these structures more feasible compared to previously reported terahertz switches. The calculated modulation depths were 98.81% and 98.71%, and the maximum modulation degree of frequencies were $$\sim$$ 61% and $$\sim$$ 29.1% for four- and eight-state terahertz switches, respectively. The maximum transmittance in transparency regions between bright modes and the spectral contrast ratio were enumerated to be 95.9% and $$\sim$$ 96%, respectively. The maximum insertion losses were quite low with values of 0.22 dB and 0.33 dB for four- and eight-state terahertz switches, respectively. Our findings will be beneficial in the development of ultra-thin graphene-based multistate photonic devices for digital switching and sensing in the terahertz regime.

A theoretical method is presented that facilitates the analysis and design of graphene-based tunable terahertz polarization rotators. Most previous designs are based on a three-dimensional (3-D) full-wave electromagnetic simulation; thus, it is time-consuming to get well-tuned structural parameters. Using the proposed method, the transmission response of the polarization rotator is directly calculated for a given set of structural parameters. Hence, the need of the electromagnetic simulation is lifted. The accuracy of the proposed method is rigorously validated, as excellent agreement between the theoretical and simulation results is observed. Using the method, a rotator of 12THz central frequency with a small magnetic bias field of 0.5T and a small unit cell of 0.5 by 0.5(µm)2 is designed. It is shown that the center frequency can be increased to any desired frequency, without the need of a large magnetic bias, by reducing the unit cell size. The method presented in this work can be extended for the analysis and design of other tunable terahertz nonreciprocal devices, such as isolators, circulators, phase shifters, and switches.

Fully reconfigurable terahertz devices enabled by T-shaped graphene two-parallel-sheet

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.

In recent years, the limited tuning ranges and selections of active materials in tunable terahertz metamaterials severely restrict their wide applications in tunable terahertz devices. In this paper, we reported an in-plane reconfigurable terahertz metamaterial which consists of a couple of T-shape structures. One is fixed to the substrate while the other is patterned on a movable frame actuated though a micromachined actuator. By adjusting the distance between T-shapes, the terahertz metamaterial exhibits a large tuning range up to 43%. In addition, the terahertz metamaterial can also realize switching between the polarization dependent and polarization independent state. Therefore, the proposed terahertz metamaterial shows potential applications in designing tunable terahertz devices.

The ratio of direct to reverberant sound energy (D/R) has been shown to be a primary acoustic cue to perceived sound source distance. Because it is unclear exactly how D/R might be encoded in the auditory system, a variety of more physiologically plausible correlates to D/R have been identified, including: spectral variance, interaural correlation, and temporal cues. Here, following recent neural work by Kuwada and Kim [ARO, 2014], we describe a new correlate to D/R and perceived distance related to the amplitude modulation (AM) depth of the signal at the listener's location. This cue is caused by the change in the modulation transfer characteristics of the room as a function of source distance. Results from an apparent distance estimation task confirm the efficacy of this AM depth cue in a reverberant soundfield (approximate broadband T60 = 3 s), when level cues are made ineffective. Distance estimates to near sources (≤2m) were found to be more accurate when the source signal (1-octave band of noise centered at 4 kHz) had AM (32 Hz, 100% depth), and this facilitation was only observed in reverberation. The facilitation was most evident for monaural input, indicating that the AM depth cue is likely processed monaurally.

Design of a tunable multiband terahertz waves absorber

Actively tunable ultrafast broadband terahertz (THz) polarimetry using a reconfigurable phase‐change material holds great potentials and prospects for the achievement of next‐generation versatile integrated THz components and systems in a variety of THz applications. Here, an optically tunable ultrafast broadband THz polarimetric device (THz‐PoD) composed of a phase‐change material Ge2Sb2Te5 (GST) and a thin mica substrate is demonstrated. This proposed novel THz‐PoD is verified for a frequency range of 0.1–2.5 THz, exhibiting broadband and ultrafast determination of polarization states for linearly polarized THz waves at polarization angles from −90° to 90°. It is shown that optical excitation with ns pulses allows easy and efficient control of the polarimetric properties of such THz‐PoD. The essential role of the GST film in switching the phase transition between the amorphous and crystalline phases is emphasized by the theoretical investigation of the optically tunable ultrafast polarimetric mechanism of the device. This phase transition allows optically changing the THz‐PoD's properties by ns‐pulsed laser in a controlled way to achieve THz polarimetry for linearly polarized THz waves. The combined advantages of this strategy can open up a new and promising way for realizing versatile reconfigurable and integrated THz devices, which may further promote the development of novel THz systems and applications.

A highly tunable terahertz (THz) filter with magneto-optical Bragg grating formed in semiconductor-insulator-semiconductor waveguides is proposed and demonstrated numerically by means of the Finite Element Method. The results reveal that a sharp peak with high Q-value presents in the band gap of Bragg grating waveguide with a defect, and the position of the sharp peak can be modified greatly by changing the intensity of the transverse magnetic field applied to the device. Compared to the situation without magnetic field applied, the shift of the filtered frequency (wavelength) reaches up to 36.1 GHz (11.4 μm) when 1 T magnetic field is applied. In addition, a simple model to predict the filtered frequency and an effective way to improve the Q-value of the filter are proposed by this paper.

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.

Here, we consider that a novel regime of propagation occurs for Transverse Magnetic (TM) waves propagating, in the long wavelength regime, through rapidly modulated stratified media with a large dielectric modulation depth (Kapitza media). The considered electromagnetic situation is conceptually equivalent to that of the mechanical inverted pendulum whose pivot point is subjected to high-frequency vertical oscillations (Kapitza pendulum) [1], since these oscillations produce a rapidly varying contribution to the lagragian function with a large modulation depth. We consider a monochromatic TM electromagnetic field of complex amplitudes E = Ex(x, z)ex + Ez(x, z)ez, H = Hy(x, z) ey (assuming the time dependence e-iωt) in a dielectric medium modulated along the z-axis whose dielectric permittivity admits the Fourier series expansion e = em + Σn0 (an + bn/η) ein K/η z, where em and (an + bn/η) are the Fourier coefficients whereas 2pη/K is the spatial period.

Coupled resonance via localized surface plasmon polaritons in Iridium-based refractory metamaterials

Tunable terahertz (THz) angular/frequency filters in the modified Kretschmann–Raether configuration with the insertion of single-layer graphene have been numerically demonstrated. Due to the excitation of the transverse magnetic (TM) polarized surface plasmons at the interface of two dielectrics with the insertion of the graphene sheet, the transmission resonance occurs at some range incident angles and frequencies in the THz frequency range, which can be adopted for designing the angular/frequency filters. It is shown that the resonant angle and the resonant wavelength can be tuned by varying the Fermi energy of the graphene sheets via electrostatic biasing. Moreover, we show that the resonant behaviors can be engineered by changing the gap thickness or the incident angle.