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Terahertz band is envisaged to provide substantially higher capacity and much lower latency for wireless communications in contrast to microwave frequencies. Moving to higher frequencies comes with its own unique challenges to be addressed, such as poor coupling efficiency from free space into and out of planar air-core waveguides. Here, we propose a framework for rapid design and low-cost fabrication of terahertz horn couplers. The horn couplers are first designed by maximizing the field overlap integral on apex and aperture interfaces, then fabricated exploiting 3D printing technique, and finally sputtered with a thin layer of gold. A 28~μm standard deviation of the surface roughness height of the 3D printed horn couplers is calculated. Experimental demonstrations show that the proposed horn coupler improves the transmittance of a hybrid photonic crystal waveguide by 20 dB in comparison with the previous pinhole-based coupling configuration. This work provides a fast, convenient and economical approach for design and fabrication of customized couplers for any waveguide size, with a cost of only 5% of commercially available counterparts, and could be integrated in 3D-printed terahertz devices during fabrication.

We propose and demonstrate a narrow-bandwidth few-mode Fabry-Perot (FP) filter based on the tilted fiber Bragg gratings (TFBGs). The proposed filter is based on the few mode fiber which possesses ring-core structure. The two TFBGs function as the reflecting mirrors in the FP cavity as well as the mode converters. Therefore, compared with the traditional FP filter and mode converter, the proposed FP filter can realize the conversion of vector modes at specific wavelengths with narrow-bandwidth characteristic. The proposed FP filter can find applications in fiber lasers, mode/wavelength division multiplexing system.

Terahertz radiation as an upcoming carrier frequency for next-generation wireless communication systems has great potential to enable ultra-high-capacity transmissions with several tens of gigahertz bandwidths. Nevertheless, dispersion is one of the main impairments in achieving a higher bit rate. Here, we experimentally demonstrate a compact terahertz dispersion compensator based on subwavelength gratings. The gratings are fabricated from the low-loss cyclic olefin copolymer exploiting micro-machining fabrication techniques. With the strong index modulation introduced in the subwavelength grating, the high negative group velocity dispersion of -188 (-88) ps/mm/THz is achieved at 0.15 THz for x-polarization (y-polarization), i.e., 7.5 times increase compared to the state-of-the-art reported to date for terahertz. Such high negative dispersion is realized in a grating of 43 mm length. The asymmetric cross-section and periodic-structural modulation along propagation direction lead to considerable birefringence that maintains and filters two orthogonal polarization states, respectively. These polymer-based birefringent gratings can be integrated into terahertz communication systems for dispersion compensation of both long-haul wireless links and waveguide-based interconnect links.

We experimentally demonstrate a multi-wavelength fiber laser based on dual-Sagnac comb filter with LP11 modes output. The filter is used to generate comb spectrum, which is consisted of three optical couplers, two polarization controllers and two segments of polarization maintaining fibers. The proposed fiber laser can stably output up to quintuple-wavelength with the side-mode-suppression ratio higher than 40 dB. Sextuple-wavelength lasing is also obtained, but due to the strong mode competition, the stability should be further improved. A mode selective coupler (MSC) is utilized as mode converter in the cavity. Thus, the multi-wavelength LP11 modes is feasible to generate, owing to the combination of MSC and the filter. The mode purity of the output LP11 modes is higher than 95%. This work has potential applications in optical communication and optical sensing system.

We report a dual-wavelength fiber laser sensor based on a uniform fiber Bragg grating (UFBG) and a Polyvinyl alcohol (PVA) film-coated long-period grating (LPG) as sensor probe for simultaneous detection of relative humidity (RH) and temperature. Two UFBGs and the LPG were inscribed in three single mode fibers utilizing phase/amplitude mask technology and mobile scanning technique via the 248 nm UV laser. In our experiment, the RH function based on the differential intensity measurement at dual-wavelength output from 30% to 85% is in a quadratic equation, and the RH sensitivity coefficient of 0.358 dB/% shows good linear relationship and stability from 55% to 85%. Meanwhile, the temperature sensitivity coefficients based on wavelength detection and differential intensity detection are 9.1 pm/°C and 0.21 dB/°C, respectively. The structure of dual-wavelength fiber laser sensor with high signal to noise ratio (SNR), narrow spectral width and good stability enables simultaneous measurement of the RH and temperature with high accuracy and good repeatability.

Terahertz photonic chips play an important role in next-generation information systems, such as high-speed inter/intrachip connections. Here we report a hybrid hollow-core terahertz topological planar waveguide where topological silicon pillars (assembled as expanded and compressed hexagonal clusters) are sandwiched between parallel gold plates. Assisted by numerical optimization of topological waveguide unit cells and supercells, we achieve a wide topological bandgap (relative bandwidth of 16.1%) in which two low-loss (below 0.1 dB/mm) topological pseudospin states exist and cover 81.8% of the bandgap. The dual unidirectional channel in a single waveguide path can be used for short-range (below 100 mm) 10-Gbps full-duplex on-chip transmissions under the forward error correction limit (bit error rate less than 10−3). Simulation results reveal that the communication performance of the proposed topological waveguide is largely group velocity dispersion dependent. This work may pave an avenue for high-speed integrated circuit design and applications in both electric and photonic fields.

Broadband, low-loss and low-dispersion propagation of terahertz pulses in compact waveguide chips is indispensable for terahertz integration. We successfully fabricated and demonstrated an air-channel hybrid (gold parallel planes and silicon pillars) photonic crystal terahertz waveguide chip using silicon microfabrication techniques, which exhibits a better performance in terms of bandwidth compared to single-mode all-dielectric photonic crystal waveguide and lower loss compared to all-metallic photonic crystal waveguide. In our primary design, a row of photonic crystal pillars is removed to achieve the air-channel for guiding terahertz waves. Here, we investigate the effect of air-channel width and height on the overall performance of the hybrid waveguide. To ensure a strict single-mode propagation, we estimate the maximum height values using the cut-off frequency of the first high-order mode of a parallel metallic waveguide. Moreover, due to the different lateral confinement feature between the hybrid waveguide and the metallic rectangular waveguide, we determine the maximum channel width by numerical simulations. The simulation results confirm that the optimal waveguide with air-channel size of 335 μm × 550 μm provides single-mode, low-loss (below 0.05 dB/mm) propagation bandwidth up to 0.149 THz, which is 26.27% wider compared to that of the initial design (0.118 THz).

We experimentally demonstrate a robust topological terahertz planar air-channel waveguide using a simple and innovative low-cost stereolithography 3D printing. Our mechanically reconfigurable waveguide chip supports a wider relative topological bandgap (over 1.6 times) with respect to the only other reported THz topological waveguide, which was based on a silicon-channel. Our work provides a significant step toward THz topological waveguides becoming the physical platform for the next generation of broadband short-range THz wireless communications.

We present a terahertz horn coupler to enhance the coupling efficiency from a free-space wave into and out of a hybrid photonic crystal waveguide. The optimized horn structure could ideally offer 84.5% coupling efficiency of free-space radiation into a tightly confined photonic crystal waveguide mode in the frequency range of 0.359-0.430 THz. The horn coupler enables adiabatic mode-matching and is fabricated using 3D printing and gold sputtering. The proposed horn coupler could be applied for terahertz chip integrations.

The resonances with narrow linewidths generated in metamaterials are valuable for numerous applications ranging from bio-sensing to narrow-band filter. And breaking structural symmetry of metamaterials is one of the common and efficient approaches to achieve such sharp resonances. In this paper, we present symmetry breaking introduced in vertical split ring resonators (VSRR) metamaterials to excite narrow resonance. Different from the previously reported planar asymmetric metamaterials, the three-dimensional metamaterials are dominantly excited by the magnetic field component of the terahertz illumination together with the electric component. In this case, sharp resonances with characteristic magnetic field distributions are excited. And the sharpest resonance shows up with an ultra-narrow line-width (full width at half maximum, FWHM is 5.90 GHz) and high Q of 327 at 1.93 THz. Through systematic theoretical analysis and numerical mode implementation, we reveal the sharp resonance dips are produced by resonance hybridization between a resonance mode induced by the destruction of the structural symmetry and another special resonance inherent in the symmetric VSRR metamaterials. In addition, the fabrication robustness and sensing potential of the asymmetric VSRR metamaterials are explored in this paper. The results show that the AVSRR design is insensitive to the errors in the fabrication process, like rough metal surface and smooth metal corner, but highly sensitive to the approaching substance, which prove the excellent sensing performance.

We propose a few-mode erbium-ring-doped fiber laser with high slope efficiency towards an individual transverse vector mode output. Based on the rate equation, specific transverse vector mode output is achieved through numerical investigations of the position and the thickness of the transverse erbium-doped region. Our results show that a thinner erbium-doped region is favorable for increasing the number of available output transverse vector mode, and the position selects desired transverse vector mode to excite. Moreover, high-order vector modes up to the third-order can be excited with appropriate erbium-doped locations. The laser output using the proposed method has merit of high slope efficiency and high mode purity. The slope efficiency and the mode purity can reach 61.25% and 99.99%, respectively. Our work may find potential applications in high-power lasers and high-order transverse mode operation of fiber lasers.

We propose polarization-maintaining few-mode elliptical hollow-core optical fibers with simple triple-layer structure toward space-division-multiplexing based high-capacity optical communication systems. The proposed fibers include a central elliptical air hole, a concentric elliptical ring core and a circular cladding. The structure-dependent polarization-maintaining performance is comprehensively investigated via numerical simulations. With appropriate structural parameters, 10 distinct polarization-maintaining modes can be guided in this fiber. The effective refractive index differences (Δneff) between all adjacent modes are beyond 1.93 × 10−4 at 1550 nm, and the values are higher than 1.8 × 10−4 over 1510–1630 nm. The modal chromatic dispersions are analyzed subsequently. Furthermore, the number of guided polarization-maintaining modes can be expanded to 14 with modified structural parameters meanwhile keeping all Δneff values no less than 1.33 × 10−4 in range of 1510–1630 nm.

Magnetic resonators based on metamaterials are valuable for numerous applications including perfect absorber, sensing and medical imaging. However, due to the existence of the difficulty in magnetic excitation in planar metasurface, the study of magnetic metamaterial resonators is less reported, particularly in contrast to the electrical resonators. In this paper, the three-dimensional split ring resonator (SRR) metamaterials featuring dual-band magnetic plasmonic resonance modes are proposed and numerically analyzed. We calculate the electromagnetic field distributions of the three-dimensional metamaterials at the resonant frequencies to elucidate the resonant characteristics of the dual modes (fundamental LC mode and high-order magnetic plasmonic resonance mode). The influences of geometric parameters of the constituent meta-atoms on the resonance frequencies of the two resonance modes are analyzed by numerical calculations. The results show that the resonance frequency of the high-order magnetic resonance mode is nearly independent of the standing columns of the metamaterials owing to the special resonant characteristic of the high-order mode. Benefiting from the independence, the frequency interval between the dual modes can be customized by adjusting the dimensions of the column. In addition, by tuning the dimension of the columns, the quality factors of the resonances can reach the highest value of 176 for LC mode and 270 for the high-order mode, respectively. In light of practical application, we evaluate refractive index sensing performance of the proposed metamaterials, the maximum sensitivity can reach 474 GHz/RIU for LC mode and 446 GHz/RIU for high-order resonance, respectively. We also explore the universality of the special resonant characteristic of the high-order mode in other similar three-dimension SRR metamaterials, which shows all of these three-dimension magnetic plasmonic metamaterials provide potential platforms for multiband magnetic metamaterial applications.

We report a compact terahertz polarization maintaining subwavelength filter featuring large negative group velocity dispersions (over -200 ps/mm/THz) at around 0.15 THz. Such waveguide grating has recently been fabricated exploiting micro-machining technique. Simulation results indicate that the filter can be applied for dispersion compensation in terahertz communication links. Refereed/Peer-reviewed

We demonstrate an all-metallic terahertz topological waveguide using cost-effective 3D printing technique. The experimental results indicate guiding of the mode within the topological band gap in the bianisotropic medium while no transmission in gapless state.

We present an experimental demonstration of hybrid terahertz waveguide chips for terahertz integration, which support a single-mode, low-loss and low-dispersion bandwidth over 0.367-0.411 THz. Waveguide bends can also be integrated inside the chip.

We propose a novel polarization-maintaining few-mode fiber by introducing four identical circular side-holes surrounding an elliptical ring core. In addition to the parameters of ring core, this kind of fiber endows new degrees of freedom to adjust the modal birefringence of guided modes. Numerical simulations indicate that such fiber could support 10 distinct polarization modes, including the effectively separated fundamental modes. The influences of side-hole size and positions on the polarization-maintaining property are investigated for the 10-mode fiber. With appropriate parameters of side-holes, the minimum effective refractive difference (Δneff) between adjacent modes is 1.65 × 10−4 at 1550 nm. Compared with the fiber excluding side-holes, the modal effective refractive indexes are decreased while most Δneff values are enlarged in the proposed fiber. Moreover, all the Δneff values could be higher than 1.52 × 10−4 over a bandwidth ranging from 1510 nm to 1630 nm. The chromatic dispersion covering the broadband is analyzed subsequently. Furthermore, 16 vector modes could be guided in the proposed fiber structure with modified parameters. The proposed fiber is capable of supporting extended modes and might be a promising candidate toward high–capacity spatial-division-multiplexing communications.

A novel optical fiber magnetic field sensor based on D-shaped fiber modal interferometer and magnetic fluid is proposed and experimentally investigated. Thanks to the etched cladding of D-shaped fiber, the resulting interference is strongly influenced by the surrounding magnetic fluid, which leads to a high magnetic field sensitivity. Simultaneous measurement of the magnetic field and temperature was realized by monitoring the wavelength shift of the two resonance dips at the same time. In the experiment, the magnetic field and temperature sensitivity can reach 99.68 pm/Oe and −77.49 pm/°C, respectively. The proposed magnetic field sensor based on D-shaped fiber is featured with high sensitivity, low cost and simple configuration.

We present a polarization-maintaining photonic bandgap fiber (PBGF) in terahertz (THz) regime, consisting of a quasi-elliptical hollow-core (QEHC) and triangularly arranged hexagonal holes in cladding. Thanks to the specially designed core shape, the QEHC-PBGF can support two stable polarization-maintaining fundamental modes. From numerical simulations, birefringence, confinement loss and group velocity dispersion of the fundamental mode group are 9.4 × 10−4, 3 × 10−3 cm−1 and 0.39 psTHz−1 cm−1 at 0.9 THz, respectively. Moreover, we investigate the fiber fabrication tolerance in the context of polarization-maintaining and confinement performances. The proposed QEHC-PBGF is anticipated to be useful in polarization sensitive THz applications.

We propose a vector mode conversion approach based on tilted fiber Bragg grating (TFBG) written in ring-core fiber with effective separation of eigenmodes. The mode coupling properties of TFBG are numerically investigated. It is shown that under the constraint of phase matching, the conversion of high-order vector modes could be achieved at specific wavelengths. Moreover, the polarization of incident light and tilt angle of TFBG play critical roles in mode coupling process. The proposed TFBG provides an efficient method to realize high-order vector mode conversion, and it shows great potential for fibers based OAM beam generation and fiber lasers with vortex beams output.

We propose a terahertz (THz) hollow-core photonic bandgap fiber (HC-PBGF) supporting few-mode operation with large birefringence. The periodic arrangement of square lattice with round corners in fiber cladding offers bandgap guidance. Numerical simulation indicates that a 21-cell HC-PBGF could support six vector modes in the bandgap. Characteristics of guided modes are comprehensively investigated, suggesting modal confinement loss of the order of 10−3 cm−1, group velocity dispersion under 1 psTHz−1cm−1, and modal birefringence higher than 10−4 for all modes at around 0.92 THz. Moreover, 24-cell and 32-cell HC-PBGFs are discussed to explore the influence of HC geometry on guided mode number and modal birefringence.

Compact and robust waveguide chips are crucial for new integrated terahertz applications, such as high-speed interconnections between processors and broadband short-range wireless communications. Progress on topological photonic crystals shows potential to improve integrated terahertz systems that suffer from high losses around sharp bends. Robust terahertz topological transport through sharp bends on a silicon chip has been recently reported over a relatively narrow bandwidth. Here, we report the experimental demonstration of topological terahertz planar air-channel metallic waveguides which can be integrated into an on-chip interconnect. Our platform can be fabricated by a simple, cost-effective technique combining 3D-printing and gold-sputtering. The relative size of the measured topological bandgap is ~12.5%, which entails significant improvement over all-silicon terahertz topological waveguides (~7.8%). We further demonstrate robust THz propagation around defects and delay lines. Our work provides a promising path towards compact integrated terahertz devices as a next frontier for terahertz wireless communications.

We propose a novel M-shaped single mode fiber (M-SMF) sensor based on stimulated Brillouin scattering of higher-order acoustic modes for simultaneous temperature and strain measurement. Numerical investigation of longitudinal acoustic modes in the M-SMF shows that multi-peak Brillouin spectrum in M-SMF is contributed by the couplings between the LP01 optical mode and multiple acoustic modes. The Brillouin characteristics in the M-SMF are compared with those of standard single mode fiber. Utilizing the multi-peak feature of Brillouin gain spectrum, sensing characteristics of the M-SMF under different temperature and strain conditions were studied. Thanks to the widely spaced $L_{01}$ and $L_{02}$ acoustic mode peaks in the Brillouin spectrum of M-SMF, the proposed fiber enables discriminative sensing of temperature and strain at an accuracy of 0.47 °C and $12.3~{\mu \varepsilon }$ , respectively.

A novel sensor is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature by splicing a 9.5 mm claddingless fiber to a 20.3 mm polarization-maintaining fiber in a Sagnac loop. Only two extinct dips are observable from 1400 nm to 1700 nm in the transmission spectrum of the sensor. One dip induced by Sagnac interference of the Sagnac loop is located near 1520 nm, while the other one near 1630 nm is the result of multimode interference of the single mode-multimode-polarization maintaining fiber (SMP) structure. Since the dip induced by Sagnac interference is immune to RI, temperature can be solely measured and the RI information can be extracted from the wavelength shift of the other dip with temperature effect compensated. Thanks to the high temperature-sensitivity of Sagnac loop and the high RI sensitivity of the SMP structure, a temperature-sensitivity of 1.929 nm/°C is achieved and an average RI-sensitivity reaches up to 235.3 nm/RIU.

We propose a novel polarization-maintaining supermode fiber containing quasi-elliptically arranged high-index cores. Numerical simulation indicates that such a fiber could support up to 20 distinct modes with different polarization states and spatial orientations. At 1550 nm, all the 20 modes are separated from their adjacent modes with effective refractive differences beyond 10- 4 . We also investigate the influence of parameters on mode number and effective area of the proposed fiber. Wavelength-dependent performance is analyzed subsequently ranging from 1500 to 1630 nm. The proposed fiber could guide 20 modes with low dispersion over the entire C-band. This letter illustrates that the proposed fiber might be a promising candidate for mode division multiplexing to enhance optical transmission capacity.

We present a theoretical study on the mode coupling between core modes and cladding modes of the tilted long period fiber grating (TLPFG) in dual-mode fiber. The TLPFG can be applied as mode filters at several specific wavelengths. Using the full vector complex coupled mode theory, numerical simulation shows the maximum coupling coefficients between core modes (LP01 and LP11) and cladding modes can be found near the tilted angle of 86°. The transmission peaks of modes coupling are sensitive to the titled angle. Furthermore, several peak amplitude between the LP11 mode and same azimuthal order modes increase with the growth of the titled angle. With the band-stop characteristic of specific mode, the TLPFG written in dual-mode fiber could be exploited as mode filters.

We present a method to generate tunable orbital angular momentum (OAM) based on linearly polarized (LP) modes in optical fibers. The tunable OAM is produced by combining two even (odd) LP modes with orthogonal polarization directions. The tunability of OAM is realized by controlling the power proportion of the two LP modes through a polarizer. From another perspective, the generated mixed vortex can also be regarded as a result of overlapping two orthogonal optical vortices of equal helicity but opposite chirality, then the tunable OAM can be achieved through filtering the mixed mode using a polarizer.

An ultralow bending-loss trench-assisted single-mode optical fiber is designed and fabricated in this letter. The experimental results show that the fiber can support an ultralow bending loss of 0.05 dB/turn at a sharp bend radius of 5 mm at 1550 nm. This letter is considered to be of significance for the design and fabrication of bend insensitive fibers (BIFs), specifically for verifying the value of trench-assisted BIF designs. Furthermore, the fast growth of fiber-to-the-home applications will benefit from this letter.

In polarization-sensitive terahertz (THz) communication systems, grating is one of the essential components that filters THz waves at desired frequencies. Based on dielectric rectangular subwavelength waveguide, we propose a THz grating structure to filter two orthogonally polarized states. From numerical simulations, the proposed filter has two passbands with over 20.9 dB extinction ratio and around 21.1 GHz full-width half-maximum. Moreover, by introducing a π-phase shift, the grating can operate as a polarization-maintaining narrow bandpass filter with over 12.3 dB extinction ratio and less than 1.7 GHz full-width half-maximum.

A novel terahertz hybrid waveguide chip consisting of silicon photonic crystals sandwiched in parallel gold plates is developed. Both simulation and experimental results demonstrate that the hybrid waveguide offers a wide single-mode transmission window with low group velocity dispersion and low loss. This compact, substrate-free terahertz chip would play a significant role in broadband, dense-integrated, multi-functional terahertz systems.

We propose and numerically analyze a tunable nanomechanical plasmonic filter based on Bragg-grating-assisted gold–air–gold gap plasmon (GP) waveguide. Bragg grating is formed by introducing periodical grooves on lower gold layers, and the air-gap size is mechanically changeable by electrostatically actuating the upper gold layer. Using the sensitivity of the GP mode to the gap size, the tunability of the proposed filter is realized by changing the gap size. By introducing a phase shift in the Bragg grating, sharp transmission peak with a quality factor of 35 is achieved within the stopband, which could be tuned over a wide wavelength range by a small change in the gap size. The proposed filter can be applied in the tunable plasmonic devices for future integrated optical circuits.

We present a single-mode multilayer-core fiber with a large mode area (LMA) and a low bending loss in this Letter. A low equivalent core-cladding refractive index difference is achieved by exploiting the multilayer structure. The multilayer structure has a better bending performance than a traditional step-index core and this structure also contributes to realizing different curved refractive index profiles that have a better bending performance. An index trench is also introduced to dramatically reduce the bending loss. The experimental results show that, at a wavelength of 1550 nm, the mode area of the fabricated fiber is about 215.5 μm2 and the bending loss is 0.58 dB/turn at a 10 mm bending radius. The LMA and excellent bending performance can be obtained simultaneously with the proposed fiber.

We propose a permittivity-tunable metamaterial channel, which is composed of alternative layers of graphene and silica. Optical waves can pass through the metamaterial channel only if its permittivity is tuned to zero. Taking advantage of the permittivity tunable property of the metamaterial, a multi-functional optical device, which can act as a wavelength demultiplexer, switch, and optical splitter without changing the geometric parameters has been proposed and numerically investigated by using the Finite Element Method. Owing to the permittivity tunable property of graphene, the working wavelength of the multi-functional device can be flexibly controlled by tuning the gate voltage applied on the metamaterial. This tunable ultracompact multi-functional optical device may find potential applications in highly integrated photonic circuits.

Metamaterial absorbers (MAs) serve as important electromagnetic wave-absorbing devices that have captured the attention of researchers for a long term. Functioning as sensitive detectors to determine perturbations in an ambient environment is another significant subsidiary function. Here, we theoretically propose an optimized fabrication method to implement terahertz MAs with fewer steps and also evaluate both absorption and sensing performances of such MAs realized by the new method. Simulation findings demonstrate that such MAs can basically maintain the original absorption features perfectly, including near-complete absorption at resonance as well as strong robustness to wide incident angles. Specifically, the full width at half-maximum and quality factor of the absorption resonances attenuate less than 26% and 8% with this new method, remaining in the ranges of ∼ 0.03 – 0.04 THz and ∼ 20 – 27 for two selected example MAs. More significantly, sensing capacities of this type of MA, in terms of maximum detection range (enhancing at least 9%), observable spectral modulation (increasing at least 6.3%), and refractive index sensitivity, are improved to a large extent because of more intense coupling between resonant field and matter in the case of surface-relief MAs. This stronger coupling results from exposing more spots of the resonantly high field to direct contact with an approaching analyte, which is illustrated by field profiles of the MAs at resonance in this work. Additionally, other desirable absorber features are also explored with such MAs, like functioning as building blocks to configure multiband MAs and strong robustness against fabrication errors. Such new-style terahertz MAs shown in the paper, acting as good examples, not only prove that terahertz MAs can be fabricated by the proposed time- and cost-saving route in contrast to the traditional MA fabrication process, but also can serve as novel platforms to explore other intriguing terahertz photonic effects, such as the field enhancement effect.