Search

We experimentally and numerically demonstrate resonant behaviors of terahertz metamaterials can be tuned significantly by their asymmetry. For the ring structure cut symmetrically, transmission spectra are nearly the same as those of their individual single-ring resonators. However, three resonant dips appear in the spectra due to the broken symmetry in single-ring resonator cut asymmetrically from the top. For the asymmetrically-cut double-ring structures, the mutual coupling between four arcs results in multi-resonance dips, especially a low-frequency Fano-like resonance accompanied with a sharp peak. Our asymmetric structures could offer potential applications in terahertz frequency selectors and bio-sensors.

We present a broadband terahertz (THz) polarizer based on the stacks of aligned Ni nanowire (NW) arrays. We demonstrated that the polarizer has an extinction ratio of 58.8 dB and an average extinction ratio of 46.6 dB throughout a frequency range of 0.3–2.3 THz. Compared to carbon-nanotube and metallic wire-grid polarizers, our Ni-NW polarizers with rapid, reliable, low-cost fabrication processes are ideal candidates for emerging THz technologies.

We experimentally and numerically demonstrate that resonant behaviors of asymmetric terahertz metamaterials can be excited and tuned by altering the position of the intermediate wire. For the parallel polarization of the terahertz E -field to the bottom bar, we attribute the low frequency to the superposition of two inductive–capacitive circular currents, which are generated by the shared intermediate wire and lateral arms. In perpendicular polarization, the observed low-frequency resonance modes are from coupling effect between the bottom bar and the vertical shortest arm under the broken symmetry of the structures. Our asymmetric structures offer the potential applications in terahertz frequency selectors and filters.

In recent years, terahertz metamaterials have attracted extensive attention because of their high sensitivity to electromagnetic waves in biosensing application. In order to obtain high-quality factor resonance, we designed and fabricated single split-ring structures with the radius of 25.5 μm. In the experiment, we measured the terahertz transmission spectra when the angle between the open gap direction and the incident wave horizontal polarization is set to be 0, 30, 45, 60 and 90 degrees, respectively, and compared the effects of different angles on the transmission characteristics. With the increase of the rotation angle, the frequency of sharpest resonance dip in the terahertz transmission spectra has a remarkable blue-shift. Furthermore, we further simulated the polarization conversion characteristics of the single splitting rings with different rotation angles. It is found that there is no polarization conversion effect when the sample is placed at 0 and 90 degrees. When the sample is rotated by 30 degrees, 45 degrees and 60 degrees, polarization conversion is observed, with the highest conversion efficiency at 45 degrees. Our obtained results indicate that the terahertz metamaterials have great application prospects in biosensors.

We demonstrate the asymmetric U-shaped array exhibits different spectral patterns in response to incident terahertz wave with horizontal and vertical polarizations. In the horizontal incident polarization, where terahertz electric-field Ein is in parallel with the bottom bar of U-shaped structure, the two-absorption-dip transmission spectrum is similar to that of a typical split-ring resonator. When Ein is perpendicular to bottom bar, an interesting phenomenon comes up with three dips. This differs from the split-ring resonator which only has one dip in the transmission spectrum. The bottom-bar length also has significant impacts on frequency-shift and absorptive depth related to this resonant phenomenon.

We have grown VO2 films and combined with terahertz metamaterials to manipulate the memory effect during the insulator-to-metal transition. The temperature-dependent resonant frequency of hybrid structure shows a thermal hysteresis accompanied with frequency shift and bandwidth variation due to the presence of a VO2 dielectric layer. This frequency memory effect significantly depends on the metallic micro-structure. Further theoretical calculation demonstrates this phenomenon mainly originates from the different coupling strength between VO2 and metallic structures. Our findings could facilitate the application of VO2 films in the smart window and dynamical terahertz modulators.

We investigate the nonlinear optical third harmonic generation of sinusoidal curved graphene which is placed on the dielectric substrate. Under transverse magnetic polarization, the third harmonic generation (THG) from graphene is dramatically enhanced at fundamental-mode and several higher-order modes of surface plasmon of graphene. The enhancement factor of THG is up to106 compared to that from graphene on a planar dielectric. We study in detail THG intensity affected by the Fermi level of graphene, grating period and amplitude, and the angle of incidence of light. The tunable and enhanced THG response at multiple wavelengths show that the sinusoidal-shaped graphene grating has potential applications for graphene based broadband nonlinear photonics.

We investigate two dipoles which are attached or separated with the orthogonal arrangement in the terahertz frequency. These results show that the metasurface could achieve the resonance coupling and polarization conversion effect. There are two resonance dips in the transmission spectra, when these two dipoles are attached to form the L-shaped structure. With the spacing between vertical and horizontal dipoles separated, the broadband of the resonator becomes narrower and resonance dips merge into one deeper dip due to the superposition of the interaction of two dipoles. The loss of the energy is not only coupled to the free space but also converted to the cross-polarization. The broadband and the strength of the crosspolarization are modulated by changing the distance between the vertical and horizontal dipoles. Tuning the spacing, we control the co- and cross polarization of the broadband and the strength at the same time. This modulation provides the functionally potential applications in the terahertz modulators and filters.

By utilizing optical pump terahertz probe spectroscopy we have investigated four samples with terahertz subwavelength structures, which derived from split ring resonators. The experimental results show that adding the same length bar in the middle of the symmetric samples has less influence on resonant properties. However, asymmetric samples could present coupling and splitting effect in transmission spectra. Additionally, compared with the symmetric structures the asymmetry structures are more sensitive to pump excitation. Under pump excitation for different resonant mechanisms, we find the LC resonance has higher sensitivity than dipole resonance to the pump excitation. Our obtained results indicate that the photogenerated carriers in the substrate play a very important role in optical modulation properties.

Artificial metamaterials with appropriate design can exhibit unique electromagnetic phenomena which do not exist in natural materials. Some studies have shown that the method of breaking the geometric symmetry is capable to modify the electromagnetic responses. Here, we simulated and measured the transmission spectra of period arrays of subwavelength double-bar structure. The obtained results show the trapped-mode resonance with Fano-shaped spectrum can be induced in terahertz metamaterial with asymmetric double-bar structure, accompanied with a metamaterial induced transparency window between two resonant dips. And the bar spacing and lattice constant have great impact on the coupling strength concerned with the transparency position and spectral lineshape. We attribute there are two mechanisms together determine the coupling pattern between the bar array and the terahetz wave, the coupling between the bars of the same unit cell and the coupling between the bars of the neighbouring cells. Our obtained results indicate that such metamaterial with very simple configuration could also provide the potential application in the field of terahertz slow-light devices, amplitude and phase modulators.

Compared with the wide application of liquid crystals (LCs) in the visible frequency band, their properties in the terahertz band have not been investigated extensively yet. In this paper, we have investigated the optical anisotropy of LCs TEB30A and 9023 at room temperature using terahertz time-domain spectroscopy (THz-TDS). The extraordinary and ordinary refraction indices of LC TEB30A are ne≈1.84 and no≈1.65, or a birefringence of 0.19 from 0.5 to 2.2 THz. The extraordinary and ordinary refraction indices of LC 9023 are ne≈1.83 and no≈1.62, or a birefringence of 0.21 from 0.5 to 2.2 THz. LC 9023 exhibits a little larger terahertz birefringence than that of LC TEB30A.

Terahertz subwavelength dipole array designed and fabricated by us could display characteristic resonance in transmission spectra, which also shows resonance redshift with increase in the dipole length. We use this property to realize double-bands filter by fabricating double-dipoles structure. Additionally it is theoretically and experimentally demonstrated that polarization state of terahertz pulse changes with the rotation of sample at specific frequency. This phenomenon originates from dipole resonance, showing that dipole reflects one terahertz-electric-field component along dipole direction while the other perpendicular component transmits dipole without loss. The transmission spectra of our fabricated split-“L” structures further verify our conclusion.

Structures consisting of a planar array of subwavelength metallic bars fabricated on a dielectric substrate are theoretically investigated in order to rotate the polarization of a normally incident terahertz wave in the resonance frequency band. We attribute this phenomenon to one component of the terahertz electric field parallel to inclined metallic bars which is reflected strongly due to the terahertz re-radiation from accelerated electrons in the metallic bars, while the other component perpendicular to the bars transmits almost entirely through the structures. By theoretical analysis, we obtain the transmitted-terahertz components polarized in the x- and y-directions, respectively. Further studies indicate polarization of a terahertz wave can be rotated within two specific frequency bands when the terahertz impinges on the double-bar sample.

Metamaterials with subwavelength structural features show unique electromagnetic responses that are unattainable with natural materials. Recently, the research on these artificial materials has been pushed forward to the terahertz (THz) region because of potential applications in biological fingerprinting, security imaging, and high frequency magnetic and electric resonant devices. Furthermore, active control of their properties could further facilitate and open up new applications in terms of modulation and switching. In our work, we will first present our studies of dipole arrays at terahertz frequencies. Then in experimental and theoretical studies of terahertz subwavelength L-shaped structure, we proposed an unusual-mode current resonance responsible for low-frequency characteristic dip in transmission spectra. Comparing spectral properties of our designed simplified structures with that of split-ring resonators, we attribute this unusual mode to the resonance coupling and splitting under the broken symmetry of the structure. Finally, we use optical pump–terahertz probe method to investigate the spectral and dynamic behaviour of optical modulation in the split-ring resonators. We have observed the blue-shift and band broadening in the spectral changes of transmission under optical excitation at different delay times. The calculated surface currents using finite difference time domain simulation are presented to characterize these resonances, and the blue-shift can be explained by the changed refractive index and conductivity in the photoexcited semiconductor substrate.

We demonstrate that the trapped-mode resonance with Fano-shaped spectrum can be induced in terahertz metamaterial with asymmetric double-bar structure. Spacing between two bars permits the tuning of resonant position and lineshape in a controlled manner, showing an anomalous increased coupling strength with spacing. The steep phase change around transparency region indicates slow-light effect proved by the retrieved group delays. Simulated results verify the coupling that exists between the bars of the same unit cell and those of the neighbouring cells. Our simplified structure offers the potential application in terahertz modulators and slow-light devices.

The frequency-dependent absorption coefficients and refractive indexes of three selected pour point depressants (PPDs) are extracted within the spectral range of 0.2–2.5 THz using terahertz time-domain spectroscopy (THz-TDS). The selected PPDs are also characterized by the middle-infrared spectrum. The experimental results reveal that PPD is more sensitive in the THz range than that in the middle-infrared range. Moreover, the different compositions of PPD can be identified according to their different spectral features in the THz range. Due to its properties of better repeatability, shorter testing time, and easier operation, THz-TDS can be used as a complement for identifying the chemical compositions of PPD. OCIS codes: 300.6495, 300.6250. doi: 10.3788/COL201109.S10505.

The optical property and spectroscopy of selected kinds of lubricating oil are studied based on the terahertz time-domain spectroscopy (THz-TDS) in the spectral range of 0.3–1.6 THz. The samples are classified by their characteristics via the near-infrared spectrum. The experimental results reveal that lubricating oil is more sensitive in the range of terahertz than in the near-infrared, and the specific kinds of lubricating oil can be identified according to their different spectral features in the terahertz range. The THz-TDS technology applied to lubricating oil analysis has potentially significant impact on the petroleum field.

Metamaterials with subwavelength structural features show unique electromagnetic responses that are unattainable with natural materials. Modulation property is regarded as one of the most important features of metamaterials. At present, the development of such terahertz switches and modulators are relatively slow. So the research of the terahertz metamaterial is very meaningful. The light-control modulation, for example, could control the resonance characteristics of split ring resonators (SRRs) by changing dielectric property of the light layer. Due to the complicated effect in photo-excited layer, we could simplify the research to firstly study the influence of substrate’s refractive index on the resonant behaviors, providing the way to further the investigation of complex problems. In addition, the shape and size of metal microstructure can produce important effect on electromagnetic response. Therefore, based on the finite-difference time-domain method, we have also simulated several SRR structures with different geometry. We find the calculated terahertz transmission spectra exhibit remarkable change, showing that the resonant dips have a red-shift phenomenon and the bandwidth gets narrow with the increased refractive index as well as the structure size. Compared with the geometry effect, the red-shift is more sensitive to the change in refractive index. This work could help us to choose the suitable substrate materials for sample fabrication to realize the specific features.

We have studied the terahertz (THz) radiation in superlattices (SLs) in moderate electric field region (between 12 kV/cm and 25 kV/cm) from both theoretical and experimental aspects in this work. The THz emission intensity has been calculated for three samples of GaAs/Al 0.3 Ga 0.7 As SLs by using Kane model. The theoretical results show that the radiation intensity increases until the electric field F achieves a certain value between 14 kV/cm and 16 kV/cm for various samples, and then starts to roll off. The agreement of the calculated results with the experimental data demonstrates that the rolling of THz radiation intensity is due to the competition of field induced localization of wave functions and the increasing of the photon energy in THz radiation with increased electric fields. Furthermore, it is also shown that THz radiation intensity can be affected by the scattering of the randomly distributed Al atoms in the barriers.

We investigate the diffusion interaction and quantitative analysis of zinc dialkyldithiophosphate (ZDDP) mixed with lube base oil (LBO) at different concentrations using terahertz time-domain spectroscopy (THz-TDS). When the concentration exceeds 6.78%, the characteristic absorption peaks exhibit significantly shift, and the absorption coefficient peak value is nonlinear against concentration. Moreover, the absorption coefficients of mixed samples follow the Beer's law at a concentration below 6.78%. The quantitative analysis enables a strategy for monitoring the formulation of lubricating oil in real time.

Metamaterials with subwavelength structural features show unique electromagnetic responses that are unattainable with natural materials. Recent research on these artificial materials has been pushed forward to the terahertz region because of potential applications in biological fingerprinting, security imaging, remote sensing, and high frequency magnetic and electric resonant devices. Active control of their properties could further facilitate and open up new applications in terms of modulation and switching. Liquid crystals, which have been the subject of research for more than a century, have the unique properties for the development of many other optical components such as light valves, tunable filters and tunable lenses. In this paper, we investigated the transmitted spectral modulation in terahertz range by using liquid crystals (5CB and TEB300) covering on the fabricated double-ring resonators to realize the shift of the resonance frequency. Our obtained results indicate the low frequency resonance shows the obvious blue-shift, while the location of high frequency resonance is nearly unchanged. We believe this phenomenon is related to not only the refractive index of the covering liquid crystals but also the resonant mechanism of both resonances.

Resonance and absorption peak of transmission spectrum can be achieved when terahertz pulses transmit through the split-ring resonators (SRRs). Using the theory of LC oscillating circuit and the model of half wave resonance, we can account for the phenomenon. We once fabricate metal subwavelength dual-ring structure which is different from the SRRs and there aren't splits in. The resonance and absorption peak, however, still appear in the THz frequency range. In this time, we respectively fabricate the dual-ring structure and reverse dual-ring structure on quartz crystal substrate and silicon. When the metal structure is fabricated on quartz crystal, Using terahertz time-domain spectroscopy (TDS), we find the characteristic spectra exist as well as time domain waveform and frequency domain spectra of transmitted terahertz pulses present obviously periodic variation with rotating the sample round the normal of its surface. However, if silicon is used as substrate, the phenomena will not appear. In this article, we will analyze the phenomena and obtain the reason resulting in it. We hold that the metal dual-ring structure in itself can't lead to the variation of the terahertz pulses by changing the sample's angle due to isotropy of the dual-ring structure in surface of the sample. We think the phenomenon may originate from quartz crystal substrate.

The spatiotemporal and spectral characteristics of ultrawide-band terahertz pulses after passing through a Fresnel lens are studied by using scalar diffraction theory. The simulation shows that the transmitted terahertz waveforms compress with increasing propagation distance, and the multi-frequency focusing phenomenon at different focal points is observed. Additionally, the distribution of terahertz fields in planes perpendicular to the axis is also discussed, and it is found that the diffraction not only induces focusing on axis but also inhibit focusing at off-axis positions. Therefore, the Fresnel lens may be a useful alternative approach to be a terahertz filter. Moreover, the terahertz pulses travelling as basic mode of Gaussian beam is discussed in detail.

Femtosecond pump-terahertz probe studies of carrier dynamics in semi-insulating GaAs have been investigated in detail under various pump powers. The pump-induced attenuation of terahertz transmission and the carrier relaxation time are found to be correlated with photo-injected carrier density. These effects can be fully explained in terms of the enhanced photoconductivity and surface states filling in GaAs. Moreover, abnormal reversal and increase of terahertz reflected waves transmitted from GaAs are observed, and these phenomena arise primarily from the pump injected carriers. One-dimensional pump scans of terahertz transmission of both the main peaks and the reflection peaks are discussed to reveal the more complete studies of the dynamics of carrier transfer in GaAs.

Huygens-Fresnel principle and half-wave method are usually used to explain the diffraction effect as the light passing through a hole. We present a new simple method to enhance the terahertz signal intensity by using a small hole. We found that in the terahertz time-domain spectroscopy, the terahertz signal enhances by 13.87% as a hole with certain size is put in a certain position of the terahertz beam path. This effect can be fully explained by using Huygens Fresnel principle. Therefore, in the condition of terahertz emitter and detector are unchanged, by using this method to enhance the terahertz signals is of great significance.

Because of its unique nature, terahertz radiation has wide application prospects in physics, chemistry, biomedicine, communications, radar and security checks, etc. Liquid crystal, which is a kind of macromolecule soft material with special properties in physical and chemical, has been widely used in planar displays. Recently, much attention has been paid to non-display studies of liquid crystals, covering the fields of biology, chemistry, physics, material and engineering. In this paper, the transmission spectra of several nematic liquid crystals such as 5CB, TEB300, RDP-92975 are measured by THz time-domain spectroscopy technique and free-space electro-optic sampling method. The absorption coefficients of liquid crystals in the THz range are calculated. Furthermore, the optical parameters are compared and analyzed, expecting to fill the spectrum gap of liquid crystals in the THz range and provide the experimental and theoretical foundation for the application of liquid crystals.

The carrier dynamics and terahertz photoconductivity in the n-type silicon (n-Si) as well as in the p-type Silicon (p- Si) have been investigated by using femtosecond pump-terahertz probe technique. The measurements show that the relative change of terahertz transmission of p-Si at low pump power is slightly lower than that of n-Si, due to the lower carrier density induced by the recombination of original holes in the p-type material and the photogenerated electrons. At high pump power, the bigger change of terahertz transmission of p-Si originates from the greater mobility of the carriers compared to n-Si. The transient photoconductivities are calculated and fit well with the Drude-Smith model, showing that the mobility of the photogenerated carriers decreases with the increasing pump power. The obtained results indicate that femtosecond pump-terahertz probe technique is a promising method to investigate the carrier dynamics of semiconductors.

Ultrafast carrier dynamics in semiconductors has attracted much attention due to the application in high speed devices. Compared to the conventional experimental techniques, such as the time-resolved optical transmission technique and the all-optical pump-probe spectroscopy, the optical pump-terahertz probe spectroscopy has a plethora of advantages to provide the ability to temporally resolve phenomena at the fundamental timescales of carrier motion. The distinct advantage of OPTP is being able to directly measure the photo-induced changes in the photoconductivity, which contains the information of carrier density and mobility, with a temporal resolution of sub-picosecond. The ultrafast carrier dynamics and transient terahertz photoconductivity in semi-insulating GaAs have been investigated under electric field by using optical pump-terahertz probe technique with an unchanged pump power irradiating on the GaAs surface. One-dimensional pump scan at the maximum value of the THz pulse under electric fields of 0 kV/cm, 6 kV/cm, and 15 kV/cm, respectively. The measurements indicate that the terahertz transmission change induced by the pump pulses at high electric field is smaller than that without electric field. It is obvious that the threshold value of E, which begins to enhance the transmission, is about 3 - 4 kV/cm. We attribute this phenomenon to carrier scattering into the L valley or even X valley, which leads to a drop in carrier mobilities due to the large effective masses in those satellite valleys. The calculated transient photoconductivities fit well with the Drude-Smith model, which attributes the negative imaginary conductivity to the backward scattering of electrons. The negative value of c1 in our fitting implies that a fraction, but not all, of the backward scattering is a result of the electron reflecting from surfaces. It could also result from a Coulombic scattering between carriers. Due to the low mobilities of electrons in the L valley, the average mobility of all electrons will decrease under high E. These fitting results are consistent with our intervalley scattering model. Our investigation suggests that the OPTP technique is a very promising method for detecting the ultrafast dynamics in those materials.

Using the terahertz time-domain spectroscopy (THz-TDS), the frequency-dependent absorption coefficient(s) and refractive index (indices) of selected lube base oil (LBO) were extracted in the spectral range of 0.3-1.6 THz. The selected LBO was also characterized by near-infrared spectrum. The experimental results reveal that LBO is more sensitive in the THz range than near-infrared, and the different viscosity index of LBO can be identified according to their different spectral features in THz range. Due to better repeatability, shorter testing time, and easy operation, THz- TDS can be used as a complementary for identifying the chemical composition of LBO.

The ultrafast photoconductive characteristics of GaAs bulk were investigated by the optical-pump terahertz-probe spectroscopy (OPTP) at room temperature. In our experiment, a significant decrease of the terahertz transmittance has been observed when the time delay between the optical pump pulse and the terahertz probe pulse was adjusted. It can be concluded that the electronic states of the GaAs became metallic, when the optical excitation occurred on the surface of the GaAs material. Since we expect only the free carrier response in the terahertz range, the decrease of the transmittance is obviously assigned to the appearance of the high electronic conductivity due to the increasing free carriers. Furthermore, it was found that transmittance of the terahertz radiation decreased with the increase of the optical pump power due to the more optical generated carriers under the higher power. We can also find that the carrier recombination time of the GaAs became shorter while the power of the optical pump was lowered.

Utilizing terahertz time domain spectroscopy system, we have characterized the electromagnetic response of a planar array of split ring resonators (SRRs) fabricated upon a high resistivity GaAs substrate. We discuss the phenomena of resonant transmission and absorption experimentally, and analyze the factors that affect the frequency of the resonant transmission peak. Free charge in metal rings occurs directional moving with effects of electric and magnetic fields when terahertz wave is incident in split ring (SR). It makes transmission and absorption peaks at certain frequencies by electric and magnetic responses between metal rings. There is another kind of absorption which is made by its structure at natural frequency of SR. We demonstrate, SRR structure has anisotropic property. The directions of split will decide transmission mode of terahertz in samples. Different modes of dipole oscillating are made by changing electric wave vector, and its absorption is also affected. However, natural frequency of SRR is decided by lattice size, independent of transmission mode. Magnetic response occurs when magnetic wave vector has component which is normal to sample surface. It makes phenomenon of absorption more apparently. In addition, decreasing component of which is parallel to sample surface can make transmission peak move to low frequency. Infections to resonance properties of SRR by terahertz electric and magnetic wave vectors are discussed in this letter. It can be a reference for researching terahertz devices.

Recently, more and more groups are concentrating on learning surface plasma on metal split with sub-wavelength of terahertz. There is a new material which is made of two materials named metamaterial. It is combined with semiconductor and metal materials. Metal parts are made with subwavelength microstructure. Abnormal phenomenon occurred when it is excited by an intense light, showing an enhancement transmission of terahertz wave. In this paper, we design a planar structure composed of gallium arsenide (substrate) and copper (metal). Copper array is plating on a GaAs wafer on order of micron with photoetching technology. The thickness of GaAs and copper layer are 650 and 0.5 micros individually. THz incidence has an enhanced transmission like surface plasmonic resonance with terahertz time-domain system when another exciting lighted on, which made current carriers in photoconductor. This resonance arises from an inductor-capacitor circuit resonance. Comparison is performed between results with and without excitation. The angle of sample plane with terahertz polarization and incidence can affect this electromagnetic response in some degree. Its transmittance with back-incidence is much stronger than that with front-incidence at certain frequencies. This kind of artificial structure has potential abilities in terahertz devices in the future. It is helpful to develop terahertz filter, polarizing film, beam reflector, phase retarder and so on.

This kind of plate that has an imaging layer, comprising an admixture of (1) a resole resin, (2) a novolac resin, (3) a latent Bronsted acid (4) an infrared absorber, and (5) colorant, relates to an image recording material which can be used as a material for a lithographic printing plate. The radiation-sensitive composition is sensitive to both ultraviolet and infrared radiation. The plate is intended for digital exposure with an infrared laser with 830 nm in a computer to plate (CTP) machine. The dot density of the printing plate ranges from 2 to 98 percent, and the resolution can reach 6μ. On the aspect of making printing plate these characteristics achieve acceptance criterion of printing. In addition, the level of laser energy needed to image a thermal plate is substantially higher than that needed to image today's conventional plates.

In experimental and theoretical studies of terahertz subwavelength “L”-shaped structure, we proposed an unusual-mode current resonance responsible for low-frequency characteristic dip in transmission spectra. Comparing spectral properties of our designed simplified structures with that of split-ring resonators, we attribute this unusual mode to the resonance coupling and splitting under the broken symmetry of the structure. Furthermore, it is found that this phenomenon is also related to not only the spacing between horizontal and vertical sides but also their relative position in “L” structure, showing the weakened coupling and spectral splitting with the increased distance and structure symmetry.

In this letter, we describe detailed femtosecond-pump–terahertz-probe studies of the electromagnetic response of arrays of double-ring and complementary double-ring resonators. A surprising result is that an increase in the terahertz transmission is observed when a double-ring structure is excited by a 400 nm pump pulse. To explain this anomalous transmission, we propose a mechanism in which the photogenerated carriers play two competing roles. In other cases, for instance, when the wavelength of the pump pulse is 800 nm or in the case of a complementary double-ring structure, the normal transmission can also be understood in terms of this mechanism.