Tunable Terahertz Wave Research Articles

Inter-/intra-pulse dual-wavelength-operated mid-infrared optical parametric oscillator.

We demonstrate a pulsed mid-infrared (MIR) optical parametric oscillator (OPO) with both inter-pulse and intra-pulse dual-wavelength operation capability. A fiber master oscillator power amplifier incorporating an acousto-optic tunable filter (AOTF) was employed as the pump for the OPO. By finely adjusting the drive wave packets for the AOTF, dual-wavelength pump can be realized within each pulse or between two adjacent pulses. These special temporal-spectral behaviors of the pump can be transferred to MIR via an OPO. In the proof-of-principle experiments, two pump wavelengths at ∼1065 and ∼1076 nm were generated and amplified to ∼31.2 W with equivalent spectral intensities for both pulsation modes. At the highest pump power, total idler power of ∼3.5 W was achieved at ∼3.45 and ∼3.55 µm under both pulsation modes. To the best of our knowledge, this is the first demonstration of both inter-pulse and intra-pulse dual-wavelength MIR generation via an OPO with an identical configuration. It is believed that our design may provide a promising solution to many practical applications including differential absorption lidar and tunable terahertz wave generation.

Generation of Tunable Terahertz Waves from Tailored Versatile Spintronic Meta-Antenna Arrays.

Broadband spintronic terahertz (THz) radiation can be efficiently generated by spin-to-charge current conversion in a ferromagnetic/nonmagnetic heterostructure. There had been many studies on realizing the enhancement or the modulation of spintronic terahertz waves. However, reported devices so far focus on implementing certain specific modulation methods, either related to the spintronic stacks or related to the metamaterial structures. In this study, a set of femtosecond laser-driven versatile spintronic terahertz devices are proposed by integrating meta-antenna structures with W/CoFeB/Pt nanolayer stacks. These monolithic integrated devices exhibit spintronic terahertz wave emission, spectral modulation, and polarization manipulation simultaneously. The terahertz pulses are generated within the ferromagnetic heterostructure interfaces and transmitted along the metallic structures, leading to the modulation of the spintronic terahertz waves. Results have shown that the center-frequency shift is up to 140 GHz and the value of ellipticity can reach 0.6, demonstrating a set of integrated and efficient spintronic terahertz devices to modulate the emitted wave. In addition, compared with the slotline antenna, the maximum peak value of the bandpass band is enhanced up to 1.63 times by carefully designing the metamaterial structure. The spintronic meta-antenna array proposed here paves an integrated way for the manipulation of spintronic terahertz optoelectronic devices.

Generation of terahertz waves based on nonlinear frequency conversion with stimulated Raman adiabatic passage.

In recent years, high-power, tunable terahertz (THZ) radiation sources have become the key areas of research in the world. The method of THZ waves by nonlinear optical difference frequency generation (DFG) has the advantages of wide tuning, high power, room temperature operation, and compact structure. However, the conversion efficiency of the current difference frequency method is low, which needs a trade-off between conversion efficiency and tuning range. We apply the nonlinear optical cascade difference frequency conversion theory based on stimulated Raman adiabatic passage (STIRAP) and propose a theoretical scheme to generate THZ waves. Numerical simulation investigates the cascaded difference frequency process of generating THZ waves with the help of the nonlinear medium lithium niobate (LN) crystal. The theoretical analysis shows that the maximum quantum conversion efficiency from signal laser to THZ waves is 43.2 % when the wavelength of the tuned signal laser varies between 1.044 - 1.065 µm with the fixed two pump laser wavelengths constant. The tunable THZ waves of 0.48 - 5.0 THz can be obtained and the maximum output intensity of THZ waves is 2.17 MW/cm2, and the method is robust to temperature variations. It also provides a novel idea for the cascaded difference frequency generation of THZ waves.

Incident-Angle-Dependent Extraordinary Transmission of the Terahertz Bull’s-Eye Structure

The bull's-eye structure in the terahertz (THz) frequency region has ample applications owing to its ability to focus free-propagating waves into subwavelength apertures, resulting in enhanced transmission, that is, extraordinary transmission. However, its coupling properties have been primarily discussed in terms of the normal plane-wave incidence to the structure. In this study, we investigate the multiple resonances in extraordinary transmission with normal and oblique incident waves. The experiment using a widely tunable and high-power THz wave source revealed two types of resonances. The main resonance split depends on the incident angle, and the other corresponds to the side lobe of the main resonances. The results are explained by a simple analytical model using a finite number of scattering media. The analysis is supported by the full-wave simulation using the finite-element method, which agrees with the experimental results. The coupling mechanisms will be applicable to design devices, such as THz biosensing devices or THz antennas for rapid communication systems.

Nonlinear Optical Benzoindolium-Based Single Crystals: H-Contacts Induced Noncentrosymmetric Alignment Approaching Efficient Terahertz-Wave Generation

Ionic-type nonlinear optical (NLO) crystals, which belong to noncentrosymmetric (NCS) space groups, have been the core device materials in the field of laser technology, especially in the field of terahertz-wave generation. In this study, three iodide benzoindolium-based crystals, namely, MBI (C24H24NOI, Cc), m-OHBI (C23H22NOI, P21), and p-OHBI (C23H22NOI, P21/c), with different substitution sites of oxygen-containing donor groups have been successfully synthesized and grown. Acentric MBI and m-OHBI crystals exhibit transparent edges below 639 nm, good environmental stabilities, and a strong second-harmonic generation (SHG) response close to typical OH1 (d33 = 120 pm/V at 1.9 μm), and the oxygen-containing donor groups are analyzed to be effective for inducing noncentrosymmetric alignment of the functional building units. Notably, for the first time, a widely tunable terahertz wave has been realized through an MBI prototype device by the difference frequency generation method. This work provides a feasible strategy for a new NLO crystal design to achieve strong SHG intensity and efficient terahertz-wave generation.

A Band-Switchable and Tunable THz Metamaterial Based on an Etched Vanadium Dioxide Thin Film

A band-switchable and tunable terahertz (THz) metamaterial based on a vanadium dioxide (VO2) thin film was proposed in the THz frequency regime. The VO2 thin film with a high conductivity change rate and smooth phase transition characteristics was deposited. To obtain band switching characteristics and reduce THz wave loss, the VO2 thin film was etched in the form of a line. Two rectangular C-shaped resonators were configured to face each other, with an etched VO2 thin film line in between. When the VO2 thin film was in the insulator phase, the two resonators individually resonated, and when the VO2 thin film was in the metal phase, they were connected and resonated as one, showing band switching characteristics. According to the state of the VO2 thin film, the fabricated metamaterial resonated at 1.29 THz when the two resonators were electrically separated and resonated at 0.65 THz when the two resonators were electrically connected. In the band-switching process, the THz wave transmission characteristics were continuously tunable. The measurement results of the proposed structure clearly showed that the rectangular C-shaped metamaterial based on the etched VO2 thin film is capable of band switching and continuous transmission control. In the near future, band-switchable and tunable THz metamaterials based on etched VO2 thin films can be employed as key devices in THz wave 6G wireless communication technology.

Widely Tunable Terahertz-Wave Parametric Oscillator with a Shallow Surface Cross-pump Configuration

Widely Tunable Terahertz-Wave Parametric Oscillator with a Shallow Surface Cross-pump Configuration

Tunable Terahertz Free Spectra Range Using Electric Split-Ring Metamaterial

A design of electric split-ring metamaterial (eSRM) with tunable terahertz (THz) free spectra range (FSR) characteristic is presented. The proposed eSRM is composed of double semicircle-shaped ring resonators with two connected metallic bars. By changing the radius of the semicircle-shaped ring resonators, the tuning ranges of FSR are 0.187 THz and 0.024 THz in transverse electric (TE) and transverse magnetic (TM) modes, respectively, which indicates eSRM could operate stably in TE and TM modes. Furthermore, eSRM exhibits switching and polarization-dependent characteristics by exploiting comb driver to control the distances between ring resonators along ${x}$ -and ${y}$ -axis directions. The electromagnetic responses could be tuned and switched between dual- and quad-resonance in both TE and TM modes. eSRM shows polarization angle-sensitive characteristic and the corresponding FSR could be tuned from 919 GHz to 336 GHz by changing the polarization angle of incident THz wave. In view of the above optical characteristics of eSRM, it is used for the sensing application exposed on different surrounding refraction index. The sensitivity, quality factor (Q-factor), and figure of merit (FOM) are calculated. The sensitivity is 0.457 THz/RIU and the average Q-factor and FOM are 40.65 and 35.47, respectively. These results indicate the proposed eSRM could be used in widespread applications, such as dual-/quad-resonance switches, polarization switches, and high-efficient environmental sensors. [2021-0006]

Efficient tunable terahertz generation via noncollinear phase matching in the ZnGeP2 crystal

A novel noncollinear phase-matching (PM) scheme was proposed by introducing a small tunable angle between two pump beams to maximize the effective nonlinear coefficient ( d e f f ) in tunable terahertz wave difference frequency generation in the Z n G e P 2 crystal. Compared with the collinear PM condition, the noncollinear geometry transfers the PM angle to be around θ = 90 ∘ for type-II ( o − e → o ) PM to fully use the maximum nonlinear coefficient d 36 , leading to a high conversion efficiency improved by tens of times in theory. Based on the theory of noncollinear PM, the angle-tuning characteristics were calculated and the crystal design was provided for efficient outcoupling. A rigorous theoretical model was built under small-signal approximation to show the affecting factors of noncollinear PM and reveal its superiority compared with collinear PM.

Light-control splitting ratio tunable Y-type Terahertz waves to beam splitter based on two-dimensional photonic crystal

Light-control splitting ratio tunable Y-type Terahertz waves to beam splitter based on two-dimensional photonic crystal is proposed.In this scenario,the feature that coupling with line defects and the point defects of nonlinear polymer materials which have the third-order nonlinear Kerr effect is utilized.The performance of Terahertz waves splitter with tunable splitting ratio is studied by Plane Wave Method(PWM)and Finite-Difference Time-Domain(FDTD).The research results show that the Ytype Terahertz wave splitter with tunable splitting ratio of two output ports by changing the light intensity of the control light can be realized.This beam splitter which the transmittance reaches over 98%can achieve Terahertz waves splitting with a specific wavelength.At the same time,the additional loss is less than 0.087dB and the response time reaches the order for ps.The splitter with tunable splitting ratio will have broad application prospects in future Terahertz wave communication.

Graphene-Based High-Efficiency Broadband Tunable Linear-to-Circular Polarization Converter for Terahertz Waves

Broadband tunable terahertz (THz) polarization converter is a vivid necessity due to its great significance in functional utilization. Graphene is a promising material for the evolution of tunable terahertz devices, thanks to its tunable surface conductivity and extraordinary electrical properties. In this paper, we present a high-efficiency graphene-based ultra-thin metasurface manifesting linear-to-circular (LTC) polarization conversion for THz wave. A broadband LTC conversion with ellipticity over 0.95 can be achieved for the entire operating bandwidth of 14 THz to 40 THz for any graphene chemical potential from 0.4 eV to 0.6 eV. It is also shown that tuning the graphene chemical potential may provide much improved ellipticity and efficiency for different frequencies. The proposed LTC converter also provides over 90% efficiency for the complete operating bandwidth for graphene chemical potential from 0 eV to 0.7 eV. Simulation results also reveal that a perfect LTC converter (ellipticity = 1) with more than 90% efficiency can be realized from 16 THz to 37 THz by tuning the graphene chemical potential between 0 eV to 0.7 eV. Altogether, the proposed LTC polarization converter provides the advantage of dynamic regulation, simpler structure, higher efficiency, enhanced ellipticity and greater relative bandwidth.

A novel tunable terahertz wave modulator based on graphene and frequency selective surface (FSS)

In this paper, a high-performance modulator based on FSS and double layers graphene has been designed and numerically investigated. The proposed structure is composed of graphene, dielectric, FSS, dielectric and graphene. The amplitude of terahertz signal which transmit from the modulator can be dynamically controlled by the graphene, and the FSS is used to enhance the light-matter interaction in graphene. The advantages of the proposed modulator are high modulation depth and a broad bandwidth. The maximum modulation depth can reach 100%, and the modulation depth can reach more than 90% even if the wavelength ranges from 500 um to 1300 um (0.23 THz to 0.6 THz). Moreover, the performance is demonstrated by the equivalent circuit theoretically. The numerically results are in well agreement with equivalent circuit results. The proposed modulator shows enormous potential in terahertz imaging and communications systems.

Evaluating the Hydrocarbon Yield of Oil Shale Using Electrically Tunable Terahertz Wave

Abstract Hydrocarbons in oil shale are significant for the output of fossil fuels and petrochemical materials; thus, the oil yield characterization is of great significance for efficient utilization and commercial exploitation of these resources. In this paper, we propose an evaluating means combined with electrical testing and terahertz (THz) measurements, named as resistivity-THz analysis (RTA), to characterize the oil shale from different places in China. Electrical and THz measurements were performed together to characterize the oil yield-dependent resistivity and THz absorption. Owing to the divergence in structures and compositions, both the electrical conductivity and THz parameters varied non-monotonic with the oil yield. However, electrically tunable THz wave absorption of oil shale can be realized by the linear correlation between the resistivity and THz attenuation coefficient, with the tunability varies monotonously with the oil yield. The results demonstrate that the carbon structures in kerogens are not only the conductive medium in oil shale but also the main source of THz absorption. As a non-contacting means for organic content characterization in oil shale, RTA is helpful to optimize the comprehensive utilization of this unconventional resource.

Actively tunable terahertz coding metasurfaces

Lately, programmable metasurfaces have been proposed by adding a PIN diode between two metallic patches in microwave region. However, the pin-diode switches can only operate at relatively lower frequencies (¡ 60 GHz), the conventional electronic method of device design used in the microwave band cannot easily be extended to the terahertz frequency regime. Here, we describe a new design principle for the actively tunable terahertz wave coding metasurfaces using vanadium dioxide embedded metallic patch. The radar cross section (RCS) reduction and radiation patterns of our new coding scheme can be altered using the metal–insulator transition of vanadium dioxide at around 68 °C. To the best of our knowledge, this is the first time that an active control of programmable coding metasurfaces was demonstrated at terahertz regime.

Tunable dual-color terahertz wave parametric oscillator based on KTP crystal.

A tunable dual-color KTP terahertz (THz) wave parametric oscillator (TPO) pumped by a dual-wavelength laser was proposed in this Letter. Theoretical analysis denotes that the emission of a tunable dual-color THz wave can be achieved by the simultaneous stimulated polariton scattering processes from multiple A1-symmetry phonon modes of the KTP crystal. The tunable dual-color THz wave emitted from KTP TPO was demonstrated in our experiment, where the THz frequencies simultaneously tuned from 3.15THz to 11.63THz and from 1.47THz to 6.03THz with some gaps. The maximum dual-color THz output energy of 1.31μJ was obtained under the THz frequencies of 5.94THz and 4.42THz. Moreover, at a certain phase-matching angle, the THz output energies for the two frequencies were independent, which means that the dual-color THz wave emission with any energy ratio can be achieved by adjusting the pump energy ratio between a dual-wavelength laser.

Reconfigurable and tunable terahertz wrench-shape metamaterial performing programmable characteristic.

A design of a tunable terahertz (THz) programmable device by using wrench-shape metamaterial (WSM) is presented, which is composed of a Au layer fabricated on a Si substrate. The size of the WSM unit cell is 30 μm×30 μm, and the distance between each WSM is 20μm. The electromagnetic response of the THz programmable device exhibits the switch function for single-band resonance and dual-band resonance at the transverse electric mode and transverse magnetic mode. The resonance spans from 3.00 to 8.00THz and is insensitive to the angle of the incident THz wave. While changing the incident angle of the THz wave, the bandwidth of the resonance becomes broader and the transmission spectrum decreases gradually. By configuring the unit cell from single-atom to quad-atom, WSM exhibits optical-logic behaviors with programmable characteristics and anti-interference. Such results are very suitable for use for an ultra-narrowband filter, single-/dual-band switch, polarization switch, and programmable device. It could potentially provide all-optical logical devices with multichannel data processing at higher bit rates.

Terahertz dielectric anisotropy enhancement in dual-frequency liquid crystal induced by carbon nanotubes

Dual-frequency liquid crystals (DFLCs), the mixture of positive and negative liquid crystal (LC) molecules, exhibit unique alternating (AC) frequency dependent anisotropic properties. Carbon nanotube (CNT), as a novel nanomaterial with strong anisotropy, has attracted much attention in recent years. Herein, we investigate the tunable terahertz birefringence and phase shift characteristics of the DFLC doped with CNT (CNT-LC) by using the terahertz time-domain polarization spectroscopy. The results show that the CNT-LC (1.5 wt%) can reach 0.5π at 0.793 THz, which can be used as a tunable THz phase shifter that is 0.12π higher than the pure DFLCs. Furthermore, through measuring the output polarization state, it is confirmed that the dielectric anisotropy enhancement mechanism of CNT-LC originates from the surface interaction between CNTs and LC molecules. Therefore, a tunable quarter-wave plate with the active polarization conversion from linearly polarized (LP) to LP or LP to circularly polarized (CP) can be realized at 0.925 THz in the CNT-LC, while the pure DFLC cannot be achieved at the same frequency. The dielectric anisotropy enhancement of CNT-LC shows its utility in the improvement of various tunable terahertz LC phase shifter and wave plate.

Tunable terahertz wave broadband absorber based on metamaterial

With the increasing scarcity of spectrum resources, terahertz wave technologies have attracted more and more attention in recent decades, and have made tremendous progress. Terahertz wave referring to electromagnetic waves with a frequency in a range of 0.1－10 THz has a wide range of applications in wireless communication, nondestructive imaging and remote sensing. Due to the advantages of high absorption, ultra-thin thickness, frequency selectivity and design flexibility, metamaterial absorbers have attracted more attention in terahertz band. In this paper, two terahertz metamaterial absorbers with different performances are designed which are named “T” terahertz multi-band absorber and “T” terahertz tunable broadband absorber, respectively. The absorbers are both comprised of three layers: metal substrate, matched dielectric layer and surface metamaterial layer. The main structures of these two absorbers are composed of four T-shape Au plates on the top of polyimide dielectric layer and an Au sheet acting as a bottom layer. The only difference between these two absorbers is that the terahertz broadband tunable absorber possesses a square photosensitive silicon in the metamaterial layer. The simulations results show that the terahertz multi-band absorber has six absorption peaks at 2.918, 3.7925, 4.986, 6.966, 7.2685, and 7.4665 THz, with the absorptivity peaks of 95.631%, 99.508%, 96.34%, 94.835%, 96.485%, 94.732%, respectively, and the average absorption rate is 96.26%. Terahertz tunable broadband absorber has the characteristics of broadband absorption. When the conductivity of silicon is 1600 S/m, the absorber reaches its absorption peak at 0.786 THz with the absorptivity of 99.998%, and the frequency bandwidth with the absorption rate exceeding 90% reaches 240 GHz. The more interesting thing is that by changing the conductivity of silicon, the terahertz tunable broadband absorber shows the ability to dynamically control the existence of absorption band and adjust the frequency position of absorption peak. For terahertz tunable broadband absorber, the frequency of absorption peak can be regulated in a bandwidth of about 30 GHz. The terahertz wave absorbers designed in this paper possess rather simple structures, therefore the proposed absorbers are easy to fabricate. Because of these excellent properties, the absorbers may have potential applications in optical switch, optical detection, optical imaging, band-stop devices, and other fields.

Optical control of polarized terahertz waves using dye-doped nematic liquid crystals

We propose an optically tunable terahertz (THz) polarization converter based on use of a dye-doped nematic liquid crystal (LC). The dye-doped LC exhibits a photothermally-induced nematic-isotropic phase transition under irradiation by a laser beam with a wavelength that lies within the absorption band of the dye. We investigate the transmission spectra and the polarization-conversion properties of the dye-doped LC cell with twisted alignment using THz time-domain spectroscopy. The polarization state of a THz wave transmitted through the proposed dye-doped LC cell can be controlled using the laser beam irradiation based on the photothermal effect. The experimental results also demonstrate that optical control of this dye-doped LC cell offers an advantage in terms of response speed when compared with voltage-based control of LC cells via their electrodes. The dye-doped LC proposed in this study presents the possibility that a variety of LC-based THz elements with optical tunability may be realized.

Theoretical Study of Organic Crystal-Based Terahertz-Wave Difference Frequency Generation and Up-Conversion Detection

Optical pumped quasi-monochromatic and tunable terahertz (THz) wave generation and detection system is investigated theoretically. Calculations are performed on an organic nonlinear medium: 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST), due to the superior properties (large dynamic range and extremely wide tunability). The behavior of pulses during the nonlinear interaction is characterized. Previous experimental results are well explained. Our theoretical model provides an approach to improve the performance of the coherent THz system from two aspects. For the source part, an optimal crystal thickness that maximizes the THz output is obtained. For the detector part, a linear conversion relation (used for calibration) and the parameter dependence of responsivity are provided. It was predicted that a dynamic range of ten orders could be achieved with the DAST-based THz system. The presented model can also be applied to guide the design of THz systems with other nonlinear mediums in collinear phase-matching geometry.