Terahertz metamaterials with electromagnetically induced transparency (EIT) have attracted extensive attention recently due to the broad application prospects in communication, optical storage, slow light effect, and biosensing. Here, we have studied the EIT effect caused by the interlayer coupling of two asymmetric split ring resonators with four gaps. The upper and the lower layers spaced by the intermediate Si have the same metastructures with the rotated angle of 90° to each other. By varying the length of the metallic arm, we find that the EIT effect becomes increasingly apparent as the asymmetry coefficient decrease. The simulation results indicate that with the increase of the thickness of Si layer, the EIT phenomenon will first emerge, gradually become the strongest with the thickness of 5μm, and finally tend to be weakened after further increasing the Si thickness. Meanwhile, the frequency of the transparency peak exhibits redshift with the Si thickness. It is also found that the EIT effect can be further optimized by adjusting the microstructure width of the split ring resonators. When the asymmetry coefficient and the thickness of the intermediate layer is determined, the EIT effect becomes most obvious with the width of 3 μm, and will gradually weaken with the increase of metallic width. The transparency peak frequency presents blue shift simultaneously. Our designed metastructure could provide the optional approach to modify the EIT behaviors and play an important role in the sensors and modulators.
Coding metasurface has attracted much attention due to its flexible design of coding sequences and powerful control ability of light beams. However, the traditional coding metasurfaces with pin-diode switches between two metallic patches are usually used in the microwave band. Few studies have been carried out in the terahertz (THz) region with tunable metastructures. In order to realize the dynamic modulation of terahertz metasurface, in this paper we use the phase change material vanadium dioxide (VO2) to activate modulation coding metasurface in the terahertz band. We designed a VO2 embedded hybrid structure with metallic patches as the metasurface unit, which can produce a 180- degree phase change near 0.69 THz during the phase transition of VO2 from an insulating state to the metallic state. Meanwhile, we have constructed a metasurface array with the above designed tunable VO2 components and non-tunable metallic units to realize the dynamic switching of the far-field beam at that frequency. Our simulated results indicate that when the VO2 conductivity increases from 200 to 200000 S/m, the far-field reflected beams of the metasurface array can change from the separation of about 41° apart to close together. Notably, this coding metasurface will remain the reflectivity higher than 0.76 at the working frequency and exhibit polarization insensitive feature to the incident light. The active coding metasurface we designed provides a new idea for flexible beam control and has broad application prospects in terahertz functional devices.
Optical regulation strategy with the aid of hybrid materials can significantly optimize the performance of terahertz devices so as to be used in the real world. Gold nanobipyramids (AuNBPs) with synthetical tunability to the nearinfrared band show strong local electric field enhancement, which improves the optical coupling at the interface and benefits the modulation performance of all-optical devices. Here we design AuNBPs-integrated terahertz modulators with multiple structured surfaces and indicate that introducing AuNBPs can effectively enhance their modulation depths. In particular, an ultrahigh modulation enhancement with one order of magnitude can be achieved in the AuNBPs hybrid metamaterials accompanied with the multifunctional modulation characteristics. Applying the coupled Lorentz oscillator model, the theoretical calculation suggests that the optical regulation with AuNBPs originates from increased damping rate and higher coupling coefficient under pump excitation. With the help of excellent modulation enhancement in the AuNBPs integrated metastructures, a prototype of novel spatial light modulator is constructed. As a novel terahertz photonic device with the low-power consumption and multifunctionality, this modulator is promising for the potential application in spatial and frequency selective imaging.
Electromagnetically induced transparency (EIT) can be analogically achieved by terahertz (THz) metamaterial, which has extensive applications in sensing, filtering, and slow light devices. Here, we firstly construct a metastructure that can modulate THz transmission, consisting of an outer symmetrical split ring resonator (SRR) embedded with two inner closed ring resonators. The simulated THz transmission spectrum presents a simple lineshape superposition of two resonances, corresponding to the low frequency dipole mode at 1.184 THz from the external SRR and the high frequency dipole mode at 1.757 THz from the closed ring resonators, respectively. However, the EIT phenomenon can be observed by replacing the inner part with two asymmetric split ring resonators. We have attributed this to that the inner metastructure can induce an extremely weak LC resonance at 1.074 THz due to the breaking of structure asymmetry. This mode will couple with the above dipole resonance of the outer SRR to accomplish the EIT effect through the near-field coupling of the weakly accessible bright-mode and the strongly excited bright-mode in this system. By varying different parameters, we have found that the impact of the rings distance on the EIT effect is more obvious. To further modulate the EIT window, the semiconductor silicon was placed at the opening gaps of the two inner asymmetric split ring resonators. Our simulated results indicate that with the increasing of the silicon conductivity from 0 to 9000 S/m, the EIT peak will gradually weaken and finally vanish, which is consistent with the results of closed ring resonators and shows the switch on/off of EIT phenomenon. Our work provides a design approach to control the electromagnetic transparent peak and manipulate EIT effect, for the potential applications in versatile THz devices.
Metamaterials are artificial materials with unique electromagnetic properties and could exhibit a strong electromagnetic resonance. Resonant metamaterials are widely used in the field of biosensors because they are sensitive to the changes of surface dielectric environment. In this study, we demonstrated a highly sensitive detection method of 4-Mercaptohydrocinnamic acid using terahertz metamaterials fabricated on silicon substrate with metal arrays of the three bars structure. We measured the transmission spectra of the metamaterial to detect the 4-Mercaptohydrocinnamic acid at three different densities by the terahertz time-domain spectroscopy system. With the density of 4-Mercaptohydrocinnamic acid increasing, resonance dips exhibit the red-shift phenomena and transmission at the resonance frequency simultaneously reduce. The movement of resonance dips and the change of transmission show that the three bars resonators are sensitive to the density of analyte. The finite-difference time-domain simulation shows a good agreement with the experimental data, and the simulation of surface current and electric field distributions at resonance dips can further understand the resonance modes in transmission spectra. Our study provides new prospect into the application of terahertz metamaterials in biosensing.
Highly sensitive response of anisotropic terahertz metamaterials to electromagnetic waves has attracted considerable attention due to its potential applications in terahertz modulators and biosensing devices. We designed three microstructure samples with split ring arrays. The terahertz transmission spectra are experimentally measured at different rotation angle. It is found that as the rotation angle increases, the resonance mode has gradually evolved from a single dipole oscillation mode to the double oscillation modes accompanied with LC and dipole oscillations, we further simulated the polarization conversion characteristics of the single splitting rings. It shows that the highest conversion efficiency appears at 45 degrees. But it doesn’t have polarization conversion effect at 0° and 90°. Subsequently, we have utilized ultrashort pulse laser to optically control the electromagnetic response by exciting the photogenerated carriers in the metamaterial samples. The experimental data show that the transmittance change obviously with the pump light. Additionally, when the angle is 90 degrees, it can be observed that the LC resonance annihilates earlier than the resonance of the dipole resonance with the increase of pump power, implying the former is particularly sensitive to the variation of the dielectric environment. To further explore the influence of dielectric environment on the resonance characteristics of the terahertz metamaterials, we have further performed the simulations with the applied surface analyte of different refractive index. The simulated data show that with the increase of the refractive index of the surface analyte, the resonance frequency of the dipole oscillation has a more significant blue shift than the resonance frequency of the LC oscillation. Our obtained results could provide the idea for designing terahertz modulators and sensitive biosensing devices.
With strong optical response from subwavelength metamaterial structures in terahertz, plasmon-induced transparency (PIT) has attracted considerable attention in terahertz modulators and biosensing devices. Here, we tune PIT effect by the destructive interference of two bright modes. We design a terahertz metamaterials structure with triple U-shaped resonators (TUR) arrays. In the vertical direction, double U-shaped resonators (DUR) are arranged downward, single-U resonator (SUR) is upward. We change the length of the SUR, the inner arms of the DUR and the horizontal distance of the DUR in order to observe the terahertz transmission spectrum. It is found from the results that with the length of the SUR increases, the resonant frequency has an obvious red-shift, the absorption of the low-frequency resonance increases and the nonresonance absorption peak gradually decreases. As the arms of the DUR increase and the distance between them decreases in horizontal, the frequency of resonance dip has a red-shift, and the transmittance of the non-resonant region increases slightly. To explore the influence of dielectric environment on the resonance characteristics of the terahertz metamaterials, we have further performed the simulations with the applied surface analyte of different refractive index. The results show that with the increase of the refractive index of the surface of the analyte, the resonance frequency has a more significant red-shift. Our obtained results could provide the idea for designing terahertz modulators and sensitive biosensing devices.
Metamaterials has shown outstanding flexibility and functionality in optics and electromagnetics, which attracted plenty of interest. Nested ring resonators have a wide range of applications in terahertz (THz) spectroscopy, sensing and communication. Hence, we design a kind of metamaterials structure which can modulate THz transmission and resonance. It consists of a circular split ring resonator (CSRR) inside a closed square ring resonator. The single CSRR has an inductive-capacitive (LC) resonance and the single closed square ring resonator has a dipole resonance. After nesting two resonators, the resonance mode is changed from single mode to double modes. The results show that the amplitude transmission of non-resonant region is related to the gap opening and the asymmetry of structure. The amplitude transmission of resonance region depends on the conductivity of substrate. With the gap opening of CSRR increases, the amplitude transmission of non-resonant region increase. Meanwhile, the frequency of resonance has an obvious blue-shift. With the bottom edge distance of the two resonators decreases(the asymmetry increases), the amplitude transmission of the non-resonant region increases gradually and the low frequency of resonance has a red-shift and the high frequency of resonance has a blue-shift. To further analyze the influence of conductivity of substrate on amplitude transmission, we change the conductivity of substrate during the simulation. The results demonstrate that with the conductivity of the substrate increases, the resonance absorption peak decreases until disappears, the amplitude transmission of the non-resonant region decreases. Our results may have the potential applications in THz modulator.
We modulate the electromagnetic response of terahertz metamaterials by exciting photogenerated carriers with ultra-short pulsed laser, and realize the active control of terahertz transmission and resonances in a wide band. The experimental results show that when the terahertz electric vector is perpendicular and parallel to the bar direction with the open gap in the circular split ring resonators, respectively, the transmission of the non-resonant region and resonance absorption are significantly tuned by the irradiated pump pulse. With the increase of the pump power, the resonance absorption peak decreases and the frequency of resonance dip has a remarkable blue-shift. Meanwhile, the transmission of the non-resonant region decreases correspondingly. It is also found that the blue-shift of the resonance is mainly determined by the change of conductivity and dielectric constant in the photoexcited layer of the substrate. Through the numerical simulation, we have further proved the introduction of pump light has a significant modulation effect on the electromagnetic properties of terahertz metamaterials. Additionally, it can be observed that the LC resonance annihilates earlier than the resonance of the dipole resonance with the increase of pump power, implying the former is particularly sensitive to the variation of the active control. In the theoretical analysis, based on the Lorenz oscillator model, we have derived and calculated the resonance response intensity of the coupled oscillators under an external field. The calculated results indicate the coupling coefficient and damping rate have an impact on the spectra evolution, showing that the resonant response peak has a remarkable blue-shift with the increase of coupling coefficient and the resonance response intensity decreases with the increase of damping rate.
Metamaterials are artificial materials with unique electromagnetic properties not found in nature and could exhibit a strong electromagnetic resonance with frequency. These characteristics depending on the geometry, dimensions, compositions of the structure, and even the symmetry. In this study, we demonstrated a highly sensitive detection method of glycerol-water solutions using terahertz metamaterials fabricated on silicon substrate with metal arrays of the asymmetric double split-ring resonator structure. We measured the transmitted spectra of the metamaterial to detect the water-glycerol solutions at different concentrations by the terahertz time-domain spectroscopy system. With the increasing of glycerol concentration, resonant dips of metamaterial exhibit the blue-shift phenomena and transmittance at the resonance frequency simultaneously enhance. The movement of resonant dips and the change of transmittance shows that these resonators are sensitive to the variation of glycerol concentration. The finite-difference time-domain simulation agree well with the experimental data, and the simulation of surface current and electric field distribution at resonance dips can further understand the resonance modes in transmission spectra. Our study provides new prospect into the application of terahertz metamaterials in detecting glycerol proportions of solution.
Graphene, made of carbon atoms arranged in a honeycomb lattice, has already attracted intense research and commercial interest in recent years. Early research focused on its remarkable electronic properties, such as the observation of massless Dirac fermions and the half-integer quantum Hall effect. Now graphene is finding application in touch-screen displays, as channels in high-frequency transistors and in graphene-based integrated circuits. The unique properties of graphene have also attracted various researches on carrier dynamics using THz spectroscopy. Here we present an experimental demonstration of monolayer CVD grown graphene via THz time-domain spectroscopy, as well as optical pump terahertz probe system. We observe that the maximum transmittance of the graphene is nearly 96% compared to the ambient signal. However, under the excitation of different optical pump fluences, it is found that unlike the semiconductor material, its transmitted amplitude is enhanced accordingly. We observed a wide-band modulation of the terahertz transmission at the range of 0.3-1.6 THz and a large modulation depth of 16.4% with a certain optical excitation. We attributed it to suppression of the air-adsorbed graphene photoconductivity due to an increase in the carrier scattering rate induced by the increase in the free-carrier concentration by photoexcitation. The obtained results not only highlight the influence of air conditions on how THz characterizations would guide the design and fabrication of graphene-based terahertz modulators and optoelectronic devices, but also show that graphene exhibits the potential for terahertz broadband transmission enhancement with photoexcitation.
In recent years, terahertz metamaterials have attracted great attention due to their widely application in frequency selector, filter, and other optical devices. Vanadium dioxide as the promising thermal memory material, has been used in making terahertz modulation device because of its well-known insulator-to-metal transition triggered by thermal, optical, and electrical. Here, we designed a composite metamaterial with three rings structure, where outer and inner rings are set to be ideal metallic gold and the middle ring is replaced by VO2. Using finite-difference time-domain method, we theoretically studied the variety of THz transmitted spectra during the insulator-to-metal transition of VO2 and its influence on the resonance coupling effect. When VO2 is in insulating phase, there are two resonant dips in spectrum. However, as VO2 undergoes an insulator-to-metal transition, the low-frequency resonance gradually moves to high frequency and finally splits into two. Our obtained results indicate the reversible terahertz modulation can be achieved by combining metamaterials with phase transition material.
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.
The hydrogen bond (H-bond) in organic-water molecules is essential in nature. Combining with the charge - transfer analyses, we investigated the penetrating molecular-orbitals in glycine-water clusters, which give evidences of the covalent-like characteristics of H-bonds in this system. Besides, the infrared spectral features provide a rare opportunity to discover the exceedingly-evident redshifts of symmetric stretching modes (Symst) in water on forming H-bond, in contrast to the slightly-redshifted asymmetric stretching modes (Asyst) in water. To explain these intriguing behaviors, we further analyzed the nuclear vibrating patterns, which clearly reveal that H-bond retains two unexpected effects on nuclear motions in water: (i) Intensifying donor Symst, and (ii) Inhibiting donor Asyst. Furthermore, we also quantified the impact of anharmonic quantum fluctuations on each hydrogen bond. For the stretching modes involved in H-bonds, red shifts up to more than one hundred wave numbers are observed under anharmonic vibration, explicitly indicating the increased ‘covalency’ of H-bonds. These finds shed light on the essential understanding of H-bonding comprehensively, and should provide incentives for future experimental studies.
Recently, the terahertz biosensors based on metamaterials have attracted much attention due to the fact that metamaterials are sensitive to the local enhancement of electromagnetic field and the changes of the surrounding dielectric environment. In order to obtain the resonances with the high quality factor for biosensing, here we designed, simulated, and fabricated the metamaterial structures composed of a series of the asymmetric “double” circular arc (DASR) structures. The experimental data show there are three sharp resonance dips named Fano resonance in the terahertz transmission spectra. In the previous study of asymmetric double rings, we studied the effect of different cutting widths on the transmission characteristics of terahertz when the samples were placed at 0 degrees. Here, the spectral characteristics and polarization conversion characteristics of the samples after 90 degrees were studied. We found that when the sample rotated by 90 degrees only two resonance dips exist in the transmission spectrum. As the separation of asymmetric arcs gradually increases, two resonance frequencies also show blue-shift. To further analyze the reasons for the changes in the transmission spectrum at different angles of sample placement, we present the surface currents and the electromagnetic field distributions in those structures. Our obtained results indicate the terahertz metamaterial has great potential in application of biosensing field.
The spectral characteristics of the asymmetric double-wire structures have been investigated in the terahertz (THz) range. Based on the THz time-domain spectroscopy system, the spectral of the samples has been measured. It is shown that a transparent peak appears in the transmission spectrum which indicate that a resonance coupling exist between the two metal bars. Furthermore, we study the optical modulation properties when the pump light irradiates on the sample by using optical pump-terahertz probe technique (OPTP). The measured results indicate that the pump light can realize an optical switch effect to modulate the transmittance of THz wave. When adjusting the time delay between the optical pump and the terahertz probe pulses, the transmittance of THz wave varies, indicating the photoexcited carriers in the substrate have great influence on the resonance characteristics of the structure. Our obtained results indicate optical modulation method could provide the functionally potential applications in the terahertz modulators and filters.
In this paper, we use the finite-difference time-domain (FDTD) method to simulate and study the electromagnetic response characteristics of ring and arc-shaped resonators. Firstly, we study the terahertz transmission properties of two single-ring resonators with different radii. Either the single-ring resonator with a large radius or with a small radius only has one resonance in the transmission spectra. Then, we combine those resonators into a double-ring resonator structure. The results conclude that the two resonant frequencies of the double-ring resonator are caused by the simple superposition of the resonances of the large and small radius single-ring resonators, respectively. Additionally, on the basis of a single-ring resonator, we also study the influence of the symmetrical and asymmetric arc-shaped resonators on electromagnetic response characteristics. The ring resonator is split from the middle into two symmetrical arc-shaped resonators. As the width of the middle gap gradually increases, the resonant frequency shows blueshift and the intensity of the surface current distribution gradually weakens. Finally, the direction of the current is reversed. In order to further compare the relationship between the single-ring resonators and the double-ring resonators, we study the double arc-shaped resonators. The main purpose of this paper is to study the frequency response characteristics of the ring resonator in the terahertz band and to control the terahertz spectrum by changing the symmetry of the ring resonator. In the future, we can further study the coupling response between the ring structures and the multi-frequency response modulation of the multi-ring structures.
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 response, such as the metamaterial induced transparency in the Fano resonators. In this work, by using the finite-difference time-domain method, we firstly simulate the process that terahertz wave interacts with double-bar structures, in which one bar length is fixed at 36 μm and the other bar length is set to be 12, 24, 36, 48, and 56 μm, respectively. The incident terahertz polarization is along the bar direction. Simulated results show when the variable bar length is less than 36 μm, there is only one obvious resonant dip in transmission spectrum. Meanwhile, with the decreased bar length, this dip frequency presents a slight blueshift. Additionally, by tuning the spacing vertical to bar direction between these two bars, it still exhibits one dip. This result indicates the short bar less than 36 μm does not play important role and the coupling between vertical bars is weak. However, when the variable bar length is larger than 36 μm there are two obvious Fano-shaped resonant dips. With the increased bar length, the low-frequency dip shows a remarkable redshift, while the high-frequency one is almost unchanged. By further tuning the bar spacing vertical to the bar direction, two dips always exist. This phenomenon implies that the coupling between horizontal bars is dominated in this process. Moreover, the metamaterial induced transparency window is found between two resonant dips. The appearance of the resonances is attributed to the excitation of trapped mode. Our obtained results indicate that such metamaterials with very simple configuration could also provide the potential application in the field of terahertz slow-light devices, amplitude and phase modulators.
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.
Asymmetric split-ring resonators (SRRs) exhibit different resonant modes and phenomena that do not have in symmetric structure, such as Fano resonance, electromagnetic induced transparency, and plasma resonance hybridization. The asymmetric SRR was first confirmed to produce narrow linewidth resonance and has a high quality factor. Then it extends to the terahertz and near infrared bands. It has been found that the most common way to modulate the electromagnetic response characteristics is to change the asymmetry of the SRR and the coupling strength between the resonators. Here we use the finite-difference time-domain (FDTD) method to simulate the electromagnetic response characteristics of asymmetric structures. When the polarization direction along the bottom bar of the U-shaped structure, there are two similar resonance dips like those in typical SRR structure. When the incident wave is perpendicular to the bottom bar, there is only one dipole resonance. However, with the broken of the symmetry, the resonant behaviors will change. In horizontal direction, both the resonant frequency and transmittance has changed. In the vertical polarization, there are three resonant dips in transmission spectrum. Meanwhile a sharp window appears in transmission spectrum. In addition, when we turn the bottom bar of the U-shaped structure into the arc shape, we just find very slight change in frequency-shift and modulation depth in both cases, showing the impact of the short arc is nearly equivalent to the linear dipole resonance. Our obtained results indicate that we could tune the electromagnetic resonances in metamaterials and the interaction mechanism with terahertz wave.
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.
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.
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.
Ultrafast carrier dynamics in Schottky barriers is an extremely active area of research in recent years. The observation of the generation of terahertz pulses from metal/semiconductor interfaces provides a technique to characterize electronic properties of these materials. However, a detailed analysis of these phenomena has not been performed satisfactorily. In this work, the measurements of optically generated terahertz emission from Au/GaAs interfaces are investigated in detail. We observe that, under high laser power excitation, terahertz signals from bare GaAs wafers and Au/GaAs samples exhibit an opposite polarity. The polarity-flip behaviors in the terahertz beams are also observed in the temperature-dependent measurements and the femtosecond pump-generation studies of the Au/GaAs interfaces. These effects can be fully explained in terms of the dynamics of carrier transfer in the Au/GaAs Schottky barriers, which involves the internal photoelectric emission and the electron tunneling effect, and picosecond time constants are found for these processes.
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.
We have demonstrated a light control terahertz modulation system based on the semiconductor material, the system is composed of a commercial 850nm 10Gbps light source and a continuous THz source. The semiconductor device is the epitaxial growth Si layer on Si substrate wafer. When the 850nm light and continuous terahertz waves incident simultaneously to this semiconductor wafer, the modulated light can excite photogenerated carriers. And the carrier determines the absorption of terahertz waves, so the amplitude of output terahertz waves will be modulated. The tested modulation depth is 35%. The semiconductor material carriers lifetime determines both the modulation depth and modulation speed. So the carrier lifetime should be trade off considered. The modulation speed has been theoretical calculated and experimental tested. The carriers lifetime is tested about 2ns, and the modulation speed is calculated beyond gigabit per second.
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.
The method of spectral dynamics analysis (SDA-method) is used for obtaining the2D THz signature of drugs. This
signature is used for the detection and identification of drugs with similar Fourier spectra by transmitted THz signal. We
discuss the efficiency of SDA method for the identification problem of pure methamphetamine (MA),
methylenedioxyamphetamine (MDA), 3, 4-methylenedioxymethamphetamine (MDMA) and Ketamine.
Engine oil, most of which is extracted from petroleum, consist of complex mixtures of hydrocarbons of molecular
weights in the range of 250-1000. Variable amounts of different additives are put into them to inhibit oxidation,
improve the viscosity index, decrease the fluidity point and avoid foaming or settling of solid particles among others.
Terahertz (THz) spectroscopy contains rich physical, chemical, and structural information of the materials. Most
low-frequency vibrational and rotational spectra of many petrochemicals lie in this frequency range. In recent years,
much attention has been paid to the THz spectroscopic studies of petroleum products. In this paper, the optical properties
and spectroscopy of selected kinds of engine oil consisting of shell HELIX 10W-40, Mobilube GX 80W-90, GEELY
ENGINE OIL SG 10W-30, SMA engine oil SG 5W-30, SMA engine oil SG 10W-30, SMA engine oil SG 75W-90 have
been studied by the terahertz time-domain spectroscopy (THz-TDS) in the spectral range of 0.6-2.5 THz. Engine oil with
different viscosities in the terahertz spectrum has certain regularity. In the THz-TDS, with the increase of viscosity, time
delay is greater and with the increase of viscosity, refractive indexes also grow and their rank is extremely regular. The
specific kinds of engine oil can be identified according to their different spectral features in the THz range. The
THz-TDS technology has potentially significant impact on the engine oil analysis.
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.
KEYWORDS: Terahertz radiation, Terahertz spectroscopy, Near field diffraction, Signal detection, Spectroscopy, Sensors, Signal to noise ratio, Detection theory, Mirrors, Modulation
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.
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.
The ultrafast carrier dynamics and surface photoconductivity of unbiased semi-insulating GaAs have been investigated in
detail by using terahertz pump-emission technique. Through theoretical modeling based on Hertz vector potential, it is
found that transient photoconductivity plays a very important role in the temporal waveform of terahertz radiation pulse.
Anomalous enhancement in both terahertz radiation and transient photoconductivity is observed subsequent to the
excitation of pump pulse, and our modeling gives successful analyses for the dynamics of photogenerated carriers in the
GaAs. We attribute these phenomena to carrier capture in the EL2 centers. Moreover, the pump power- and temperaturedependent
measurements are also performed to verify this model.
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.
THz spectra of two pure materials and some mixed explosive materials and the theoretical analysis results are introduced.
The refractive index and absorption coefficient of the samples are measured in the region of 0.2-2.5 terahertz by
time-domain spectroscopy (THz-TDS). The absorption spectra are obtained correspondingly. The obtained results
indicate the THz-TDS technology can identify the different explosives and roughly analysis the main ingredients of the
mixed samples.
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.
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.
The sensing of the explosives and the related compounds is very important for homeland security and defense. Based on
the non-invasive terahertz (THz) technology, we have studied some pure and mixed explosives by using the THz
time-domain spectroscopy and have obtained the absorption spectra of those samples. The obtained results show that
those explosives can be identified due to their different characterized finger-prints in the terahertz frequency region of
0.2-2.5 THz. Furthermore, the spectra analyses indicate that the shape and peak positions of the spectra for these mixed
explosive are mainly determined by their explosive components. In order to identify those different kinds of explosives,
we have applied the artificial neural network, which is a mathematical device for modeling complex and non-linear
functionalities, to our present work. After the repetitive modeling and adequate training with the known input-output data,
the identification of the explosive is realized roughly on a multi-hidden-layers model. It is shown that the neural network
analyses of the THz spectra would positively identify the explosives and reduce false alarm rates.
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.
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