Charge Carriers Localization Effect Revealed through Terahertz Spectroscopy of MXene: Ti3C2Tx.

The transport properties of charge carriers in MXene, a promising material, have been studied using terahertz time-domain spectroscopy (THz-TDS) to examine its potential applications in optical and electronic devices. However, previous studies have been limited by narrow frequency ranges, which have hindered the understanding of the intrinsic mechanisms of carrier transport in MXenes. To address this issue, ultrabroadband THz-TDS with frequencies of up to 15 THz to investigate the complex photoconductances of MXene (Ti3C2Tx) films with different thicknesses are employed. The findings indicate that the electronic localization is substrate-dependent, and this effect decreases with an increase in the number of layers. This is attributed to the screening effect of the high carrier density in Ti3C2Tx. Additionally, the layer-independent photocarrier relaxations revealed by optical pump THz probe spectroscopy (OPTP) provide evidence of the carrier heating-induced screening effect. These results are significant for practical applications in both scientific research and various industries.

Investigation of nonlinear optical properties in α-A2BB'O6 (A = Li, Na, K; B = Ti, Zr, Hf; B' = Se, Te) by first-principles calculations.

Nonlinear optical (NLO) crystals based on oxides typically have wide bandgaps and large laser damage thresholds (LDTs), which are important for generating high-power and continuous terahertz radiation. Recently, a new family of NLO materials α-A2BB'O6 including Li2TiTeO6 (LTTO) with a strong second harmonic generation (SHG) efficiency of 26 × KH2PO4 (KDP) and a large LDT of 550 MW cm-2 were reported. Herein, we systematically study the electronic structures and NLO properties of α-A2BB'O6 (A = Li, Na, K; B = Ti, Zr, Hf; B' = Se, Te) to explore the relationship between the structure and SHG coefficient. First, 15 members of the A2BB'O6 family are demonstrated to be highly stable and NLO materials, excluding K2TiTeO6, K2TiSeO6 and K2ZrSeO6. Then, the electronic band structure, dipole moment and distortion of BO6/B'O6 octahedrons, SHG coefficient and terahertz absorption spectrum are calculated comprehensively with the element variation of A-site, B-site and B'-site. Finally, the magnitude of the SHG coefficient is found to be directly proportional to the value of total dipole moment and distortion, and inversely proportional to the bandgap value. Most importantly, among the A2BB'O6 materials, K2HfSeO6 shows the smallest direct bandgap of 2.99 eV, the largest SHG coefficient d33 of about 5 × LTTO and low terahertz absorbance from 0.1 to 9 THz. Our results provide new NLO crystals that may have potential application in terahertz radiation sources and other nonlinear electronics.

Tunable linear-to-circular terahertz polarization convertor enabled by a plasmonic nanocomposite metasurface.

We proposed and demonstrated a metasurface based terahertz polarizer consisting of an optically responsive nanocomposite and a flexible base body, which fulfilled the function of linear-to-circular polarization conversion in transmission mode. Meanwhile, as the dynamic and stretchable materials enable the active manipulation of conversion points, evident frequency shifts for circular polarization transformation were discovered by applying laser irradiation and tension. Hence the modulation of conversion points covered a broadband with combination of those two external excitations. This THz polarization convertor may find its applications in polarization controls and beam steering, which also provides a low-cost and large-scale manufacturable method to achieve versatile active THz devices.

Flexible and transparent metadevices for terahertz, microwave, and infrared multispectral stealth based on modularization design.

Multispectral stealth technology including terahertz (THz) band will play an increasingly important role in modern military and civil applications. Here, based on the concept of modularization design, two kinds of flexible and transparent metadevices were fabricated for multispectral stealth, covering the visible, infrared (IR), THz, and microwave bands. First, three basic functional blocks for IR, THz, and microwave stealth are designed and fabricated by using flexible and transparent films. And then, via modular assembling, that is, by adding or removing some stealth functional blocks or constituent layers, two multispectral stealth metadevices are readily achieved. Metadevice 1 exhibits THz-microwave dual-band broadband absorption, with average measured absorptivity of 85% in 0.3-1.2 THz and higher than 90% in 9.1-25.1 GHz, suitable for THz-microwave bi-stealth. Metadevice 2 is for IR and microwave bi-stealth, with measured absorptivity higher than 90% in 9.7-27.3 GHz and low emissivity around 0.31 in 8-14 µm. Both metadevices are optically transparent and able to maintain good stealth ability under curved and conformal conditions. Our work offers an alternative approach for designing and fabricating flexible transparent metadevices for multispectral stealth, especially for applications in nonplanar surfaces.

Dual-band terahertz all-silicon metasurface with giant chirality for frequency-undifferentiated near-field imaging.

Chiral metasurfaces are widely used in imaging and biosensing due to their powerful light field control capabilities. Most of the work is devoted to achieving the goals of chirality enhancement and tunability, but lacks consideration of design complexity, loss, cost, and multi-band operation. In order to alleviate this situation, we propose a pair of dual-frequency giant chiral structures based on all-silicon, which can achieve excellent and opposite spin-selective transmission around 1.09 THz and 1.65 THz. The giant chirality derives from the in-plane electric and magnetic dipole moments excited in different degrees. Theoretically, the maximum circular dichroism at the two frequencies are both as high as 0.34, and the coverage bandwidths of the two giant chirality are 85.5 GHz and 41.4 GHz, respectively. The experimental results are in good agreement with the simulation results. Based on the dual-band giant chiral patterns, the terahertz near-field imaging of different Chinese character images is demonstrated at two frequencies. The frequency-undifferentiated characteristics, good intensity contrast and three-dimensional imaging information are shown by the results. This work provides new ideas for the design of terahertz devices with simple structure and multi-functions, which are expected to be applied in the field of terahertz imaging or multi-channel communication.

Vacancy tuned coupling in terahertz metamaterial arrays.

Metamaterials have shown great potential for modulation on the amplitude, phase and polarization of the terahertz wave. Here vacancies were introduced into the metamaterial arrays to tune the mutual interaction between the constituent resonators, which could heavily affect the electromagnetic response of the whole metamaterial arrays. We show that the introduced vacancies in the metamaterial arrays can effectively affect the resonance mode of the metamaterial arrays. Based upon the vacancy mediated coupling, a silicon-metal hybrid metamaterial arrays were designed to achieve active modulation of propagating terahertz waves.

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.

All-optical spatial terahertz modulator with surface-textured and passivated silicon.

For a Si-based all-optical spatial terahertz modulator (STM), an enhanced modulation efficiency under low illumination density would be of great significance to exploit the competence of THz technology in real-world applications. We presented here an implementation of such a device by microtexturing and passivating the Si surface, forming a truncated pyramidal array (TPA). This TPA structure with SiO2 passivating coatings not only decreases light reflectance and expands the active area for THz modulation but also remarkably increases the photogenerated carrier lifetime. These 3-fold benefits render Si-TPA superior to bare-Si with respect to the achievable modulation efficiency, especially at low irradiation power. Furthermore such a Si-TPA device is also more applicable than its counterpart that is only passivated by SiO2 nanocoatings, even though the Si-SiO2 has a slightly increased modulation efficiency. These periodically aligned pyramids resembled as a mesa array significantly suppress the lateral diffusion induced by longer diffusion, resulting in an equivalent resolution of bare-Si. This novel Si-TPA based STM is highly desired for realizing a high-performance THz imager and provides a feasible approach to breaking the trade-off between resolution and modulation efficiency.

Terahertz magneto-optical response of bismuth-gadolinium-substituted rare-earth garnet film.

We report the magneto-optical Faraday response of bismuth-gadolinium-substituted rare-earth iron garnet at terahertz frequencies ranging from 100 GHz to 1.2 THz. The maximum transmittance of ±45° component is about 60% near the frequency point of 0.63 THz. When the external magnetic field change from -100 mT to +100 mT, the Faraday rotation angle is between -6° and +7.5°. The overall change of ellipticity is relatively small. The maximum value of the Verdet constant is about 260 °/mm/T at 0.1 THz and then gradually decreases to 80 °/mm/T at 1.2 THz. Within the considered frequency range, the thick film exhibits magnetically tunable, non-reciprocal characters and a strong magneto-optical effect within a small external magnetic field at room temperature, which will be widely used for the terahertz isolators, circulators, nonreciprocal phase shifters, and magneto-optical modulators.

Terahertz magneto-optical effect of wafer-scale La: yttrium iron garnet single-crystal film with low loss and high permittivity.

The wafer-scale La:YIG single crystal thick films were fabricated on a three-inch gadolinium gallium garnet (GGG) substrate by liquid phase epitaxy method. The terahertz (THz) optical and magneto-optical properties of La:YIG film were demonstrated by THz time domain spectroscopy (THz-TDS). The results show that a high refractive index of approximately 4.09 and a low absorption coefficient of 10-50 cm-1 from 0.1 to 1.6 THz for this La:YIG film. Moreover, the THz Faraday rotation effect of La:YIG film was measured by the orthogonal polarization detection method in THz-TDS system, which can be actively manipulated by a weak longitudinal magnetic field of up to 0.155 T. With 5 samples stacked together, the Faraday rotation angle varies linearly from -15° to 15°, and the Verdet constant of La:YIG is about 100 °/mm/T within the saturation magnetization. This magneto-optical single crystal thick film with large area shows low loss, high permittivity and strong magneto-optical effect in the THz regime, which will be widely used in magneto-optical polarization conversion, nonreciprocal phase shifter and isolator for THz waves.

Ultra-extraordinary optical transmission induced by cascade coupling of surface plasmon polaritons in composite graphene-dielectric stack.

Surface plasmon polaritons have been extensively studied owing to the promising characteristics of near fields. In this paper, the cascade coupling of graphene surface plasmon polaritons (GSPPs) originating from cascading excitation and multiple coupling within a composite graphene-dielectric stack is presented. GSPPs confined to graphene layers are distributed in the entire stack as waveguide modes. Owing to the near-field enhancement effect and large lifetime of the GSPPs, the terahertz wave-graphene interaction is significantly enhanced, which induces an ultra-extraordinary optical transmission (UEOT) together with the reported negative dynamic conductivity of graphene. Furthermore, owing to cascade coupling, the UEOT exhibits considerable transmission enhancement, up to three orders of magnitude, and frequency and angle selections. Based on the key characteristics of cascade coupling, the mode density and coupling intensity of GSPPs, the dependences of the number of graphene layers in the stack, the thickness of dielectric buffers, and the effective Fermi levels of the graphene on the UEOT are also analyzed. The proposed mechanism can pave the way for using layered plasmonic materials in electric devices, such as amplifiers, sensors, detectors, and modulators.

Photo-induced enhanced negative absorption in the graphene-dielectric hybrid meta-structure.

Recently, the negative absorption in graphene-based metamaterials became a very attractive direction of THz electronic devices. Here we propose a graphene-dielectric hybrid meta-structure to realize photo-induced enhanced negative absorption in the THz regime, which results from strong graphene-light interaction. The negative absorption is derived from the degradation of the conductivity of graphene under optical pump. Meanwhile, the graphene-dielectric hybrid meta-structure introduces dispersion relation and resonance mode, which can couple with the incident wave to construct a strong resonance. In this case, both the dispersion of the propagating waves and resonance are contributed to the graphene-light interaction and enhance the negative absorption, in which the resonance coupling determines the distribution of negative absorption, and the maximum is dominated by dispersion. More importantly, compared with the previous work, the negative absorption is increased by nearly 100 times by adopting this meta-structure.

An ultrathin MoSe2 photodetector with near-perfect absorption.

An ultrathin near-perfect MoSe2 absorber working in the visible regime is demonstrated theoretically and experimentally, and it consists of a MoSe2/Au bi-layer film. The polymer-assisted deposition method is used to synthesize MoSe2 films, which can reduce the roughness and thus improve the film absorption. Simulation results show that the absorption of the absorber with 22 nm MoSe2 reaches to larger than 90% between 628.5 nm and 718 nm with a peak value up to 99.5% at 686 nm. Moreover, the measured absorption also shows near-perfect absorption of this simple absorber. Finally, an ultrathin photodetector is fabricated based on this perfect absorber and shows on/off reproducibility and remarkable photocurrent, which is three orders of magnitude higher than the dark current.

An Optically Tunable THz Modulator Based on Nanostructures of Silicon Substrates.

Nanostructures can induce light multireflection, enabling strong light absorption and efficient photocarrier generation. In this work, silicon nanostructures, including nanocylinders, nanotips, and nanoholes, were proposed as all-optical broadband THz modulators. The modulation properties of these modulators were simulated and compared with finite element method calculations. It is interesting to note that the light reflectance values from all nanostructure were greatly suppressed, showing values of 26.22%, 21.04%, and 0.63% for nanocylinder, nanohole, and nanotip structures, respectively, at 2 THz. The calculated results show that under 808 nm illumination light, the best modulation performance is achieved in the nanotip modulator, which displays a modulation depth of 91.63% with a pumping power of 60 mW/mm2 at 2 THz. However, under shorter illumination wavelengths, such as 532 nm, the modulation performance for all modulators deteriorates and the best performance is found with the nanohole-based modulator rather than the nanotip-based one. To further clarify the effects of the nanostructure and wavelength on the THz modulation, a graded index layer model was established and the simulation results were explained. This work may provide a further theoretical guide for the design of optically tunable broadband THz modulators.

Semiconductor terahertz modulator arrays: the size and edge effect.

A terahertz spatial modulator is the critical component for active terahertz imaging using compressive sensing. Here small silicon pieces were put in arrays on flexible polymer substrate to fabricate semiconductor terahertz spatial modulators. By doing this, the inter-diffusion of photo-generated charge carriers is prevented for better resolution, and flexibility is achieved. Since the size of silicon is comparable to the wavelength of the terahertz wave, and the dielectric properties of the gap are very different from silicon, the optical modulation of each element is very different from the large silicon. In this Letter, the terahertz wave interaction and optical modulation of the small silicon are systematically studied by time domain spectroscopy. Notably, a strong resonance-like absorption peak was observed in a transmittance spectrum for the small silicon due to the size and edge effect. The spatial modulation of the terahertz wave was also compared between the silicon array and the large silicon samples.

Broadband and high modulation-depth THz modulator using low bias controlled VO2-integrated metasurface.

An active vanadium dioxide integrated metasurface offering broadband transmitted terahertz wave modulation with large modulation-depth under electrical control is demonstrated. The device consists of metal bias-lines arranged with grid-structure patterned vanadium dioxide (VO2) film on sapphire substrate. Amplitude transmission is continuously tuned from more than 78% to 28% or lower in the frequency range from 0.3 THz to 1.0 THz, by means of electrical bias at temperature of 68 °C. The physical mechanism underlying the device's electrical tunability is investigated and found to be attributed to the ohmic heating. The developed device possessing over 87% modulation depth with 0.7 THz frequency band is expected to have many potential applications in THz regime such as tunable THz attenuator.

Flexible terahertz modulator based on coplanar-gate graphene field-effect transistor structure.

The terahertz (THz) modulators, as an essential component of the THz system, have been developed by many efforts until now. However, the development of flexible THz modulators is hindered due to the lack of flexible THz modulating materials. Herein, for the first time to the best of our knowledge, we demonstrated the feasibility of flexible THz modulators based on the coplanar-gate field-effect transistor (FET) structure of ion-gel/graphene/polyethylene terephthalate. The THz transmittance through this THz graphene modulator can be well controlled with a modulation depth up to 22% by tuning the carrier concentration of graphene via electrical gating. Furthermore, because of the integration of high flexibilities of graphene, ion-gel, and polyethylene terephthalate (PET), the proposed THz graphene modulator shows superior flexible performance, where the modulation properties can be maintained almost unchanged, not only under bending deformations, but also before and after bending 1000 times. In addition, due to the unique structure of ion-gel/graphene/PET, the flexible THz graphene modulator has a low insertion loss (1.2 dB). Therefore, this Letter is expected to be beneficial for the potential applications, ranging from the traditional compact THz system to a new flexible THz technology.

High-speed and broadband terahertz wave modulators based on large-area graphene field-effect transistors.

We present a broadband terahertz wave modulator with improved modulation depth and switch speed by cautiously selecting the gate dielectric materials in a large-area graphene-based field-effect transistor (GFET). An ultrathin Al2O3 film (∼60 nm) is deposited by an atomic-layer-deposition technique as a high-k gate dielectric layer, which reduces the Coulomb impurity scattering and cavity effect, and thus greatly improves the modulation performance. Our modulator has achieved a modulation depth of 22% and modulation speed of 170 kHz in a frequency range from 0.4 to 1.5 THz, which is a large improvement in comparison to its predecessor of SiO2-based GFET.

Organohydrogel-based transparent terahertz absorber via ionic conduction loss.

The fast-growing terahertz technologies require high-performance terahertz absorber for suppressing electromagnetic interference. Since the dissipation mechanism in terahertz band usually focuses on electronic conduction loss, almost all terahertz absorbers are constructed with electronically conducting materials being opaque, which limits their applications in scenarios requiring high visible transmittance. Here, we demonstrate a transparent terahertz absorber based on permittivity-gradient elastomer-encapsulated-organohydrogel. Our organohydrogel-based terahertz absorber exhibits a high absorbing property (average reflection loss of 49.03 dB) in 0.5-4.5 THz band with a thin thickness of 700 µm and a high average visible transmittance of 85.51%. The terahertz absorbing mechanism mainly derives from the ionic conduction loss of the polar liquid in organohydrogel. Besides, the hydrophobic and adhesive elastomer coating endows this terahertz absorber high absorbing stability and interfacial adhesivity. This work paves a viable way to designing transparent terahertz absorbers.

Switchable Broadband Terahertz Absorbers Based on Conducting Polymer-Cellulose Aerogels.

Terahertz (THz) technologies provide opportunities ranging from calibration targets for satellites and telescopes to communication devices and biomedical imaging systems. A main component will be broadband THz absorbers with switchability. However, optically switchable materials in THz are scarce and their modulation is mostly available at narrow bandwidths. Realizing materials with large and broadband modulation in absorption or transmission forms a critical challenge. This study demonstrates that conducting polymer-cellulose aerogels can provide modulation of broadband THz light with large modulation range from ≈ 13% to 91% absolute transmission, while maintaining specular reflection loss < -30 dB. The exceptional THz modulation is associated with the anomalous optical conductivity peak of conducting polymers, which enhances the absorption in its oxidized state. The study also demonstrates the possibility to reduce the surface hydrophilicity by simple chemical modifications, and shows that broadband absorption of the aerogels at optical frequencies enables de-frosting by solar-induced heating. These low-cost, aqueous solution-processable, sustainable, and bio-friendly aerogels may find use in next-generation intelligent THz devices.