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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Cherenkov Generation of Wideband Terahertz Radiation using a Sub-Micron LiNbO <sub/>3</sub> Slab Waveguide
摘要： Terahertz (THz) radiation is critical to the areas of security, medicine, communications, and consumer applications. THz generation is often achieved using lithium niobate, LiNbO3, since it has a high nonlinear coefficient in this regime (d33=180 pm/V [1]). Dramatic improvement to the generation efficiency has been achieved by incorporating this material into waveguiding configurations [2,3], since waveguides allow the pump pulse to remain confined over large distance (millimeters or longer). To date, waveguides investigated for THz generation utilize transverse core dimensions larger than a few microns. However, when the waveguide core is sub-wavelength with respect to the pump wavelengths, as well as the generated THz wavelengths, another benefit is observed: substantial broadening of the generated THz bandwidth. Here, we report on a sub-wavelength SiO2-LiNbO3-SiO2 slab waveguide that produces wideband THz radiation and emits it in the form of Cherenkov waves. The investigated LiNbO3 slab waveguide has transverse core dimensions of 720 nm × 4 mm and a length of 100 μm. This waveguide is excited using an electric field pulse having a duration of 50 fs and a central-wavelength of 780 nm. Figure 1(a) shows the generated THz electric field pulse, which has the short duration of <1 ps. The power spectrum of the generated THz pulse is shown in Fig. 1(b), having a central-frequency of 6.9 THz and a full-width half-maximum (FWHM) bandwidth of 2.1 THz. Notably, LiNbO3 has a strong phonon resonance at 7.6 THz [4]. However, since the generated THz radiation is emitted as Cherenkov waves, it exits the sub-wavelength LiNbO3 core after propagating a distance much less than a wavelength. This characteristic minimizes the high loss of the LiNbO3 phonon resonance, allowing for generation to occur directly at this phonon resonance. As such, the sub-wavelength nature of the LiNbO3 core allows for dramatic improvement to the THz radiation bandwidth. The data is Fig. 1 is obtained by performing finite-difference time-domain simulations. Experimental verification of this result is currently underway. In this experimental work, the LiNbO3 slab waveguide instead has a length of 1 cm and the excitation pulses will have energies ranging up to 200 nJ.
关键词： wideband THz generation,Terahertz radiation,LiNbO3,Cherenkov waves,sub-wavelength waveguide
更新于2025-09-12 10:27:22
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Enhanced terahertz radiation of photoconductive antenna fabricated on GaAs-on-sapphire
摘要： The terahertz (THz) radiation properties of a photoconductive antenna (PCA) fabricated on a GaAs-on-sapphire (GoS) substrate are reported at sub-THz band. The GaAs layer with a thickness of approximately 1 μm was directly deposited on a sapphire wafer by means of molecular beam epitaxy. A butterfly-shaped antenna structure was then fabricated on the GoS substrate by photolithography, and the device was tested as the emitter of an in-house built THz time-domain spectrometer. The performance of this antenna was compared with a commercial one, which had an identical antenna structure but was fabricated on low-temperature-grown GaAs (LT-GaAs). The results showed that the GoS-based PCA radiated an enhanced THz field, which could be as much as 1.9 times that of the LT-GaAs-based PCA, indicating that GoS could be a promising photoconductive material. In addition, the optical transparency of the sapphire substrate allows the device to be illuminated from the backside, which is crucial for THz near-field imaging applications where the sample is usually in close proximity to the front surface of the PCA device.
关键词： photoconductive antenna,molecular beam epitaxy,THz time-domain spectrometer,terahertz,GaAs-on-sapphire
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - DSTMS-Based Ultrabroadband Terahertz Time-Domain Spectroscopy
摘要： DSTMS organic electro-optic crystal is a very promising and efficient THz-wave generation material. It allows for both very high THz electric fields generation, exceeding several GV/m using optical rectification of femtosecond laser pulses, as well as the possibility for extremely broad bandwidth extending well beyond 10 THz. However, the usual laboratory THz systems based on these crystals exploit very bulky and expensive fs laser systems, which are not desired for industrial THz imaging and spectroscopy applications. Recently, some compact system employing organic crystals were demonstrated, however they use a conventional photoconductive antenna as a THz-wave detector. Here we present a compact THz time-domain spectroscopy system based on a relatively low-cost and small-size femtosecond fiber lasers operating at 1560 nm. As the THz generation and detection material, we choose the organic crystal DSTMS (4-N,N-dimethylamino-4’-N’-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate), since it reaches the highest overall efficiency and the flattest spectrum among the presently known state-of-the-art organic crystals. The developed terahertz system is capable of achieving frequencies up to 20 THz with a high dynamic range of more than 60 dB. We use a commercial femtosecond laser with pulse length of 18 fs, 200 mW average power, 40 MHz repetition rate and 1560 nm wavelength as pump beam. We achieve a very large frequency bandwidth reaching 20 THz, profiting from using DSTMS as both THz generator and THz coherent detector material.
关键词： DSTMS,organic crystal,terahertz time-domain spectroscopy,THz-wave generation,femtosecond fiber lasers
更新于2025-09-12 10:27:22
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[IEEE 2019 Second International Workshop on Mobile Terahertz Systems (IWMTS) - Bad Neuenahr, Germany (2019.7.1-2019.7.3)] 2019 Second International Workshop on Mobile Terahertz Systems (IWMTS) - Endfire Transition from Coplanar Waveguide-to-WR3 Rectangular Waveguide for Monolithic Integration with THz Photodiodes
摘要： In this paper, an endfire transition from coplanar-to-rectangular waveguide (CPW-to-WR3) for operation in the WR3-band (220-320 GHz) is presented. The transition is designed for providing connectivity between a monolithically integrated InP-based terahertz photodiode (PD) to a WR3 rectangular waveguide. The presented transition is based on a CPW-fed planar dipole antenna designed on InP substrate which placed in the E-plane of the rectangular waveguide. The dipole-based transition converts the direct coupled quasi-TEM CPW mode of the PD output to the TE10 mode of the rectangular waveguide. The proposed design provides a simple approach avoiding complex integration and packaging techniques like air bridges, ribbon-bonds, or via holes. Numerical analysis of the designed transition shows a wideband performance with a 3 dB insertion loss (IL) over 88 GHz bandwidth and a 10 dB return loss (RL) over 53 GHz bandwidth within the WR3-band.
关键词： monolithic integration,Coplanar waveguide (CPW),WR3-band,THz photodiode.,endfire transition
更新于2025-09-12 10:27:22
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An advanced approach to control the electro-optical properties of LT-GaAs-based terahertz photoconductive antenna
摘要： This work reports on an advanced approach to the design of THz photoconductive antenna (PCA). The LT-GaAs thin films used for the PCA fabrication were synthesized by MBE method on GaAs (100) substrate by adjusting the As pressure, As/Ga fluxes ratio, growth/annealing temperatures and annealing time. These parameters crucially affect electro-optical properties of the PCA samples as evidenced by the THz radiation power and time-domain spectroscopy measurements. The annealing temperature of 670 °C was found to be optimal for constructing a PCA possessing high amplitude of the THz radiation over the spectral range up to 1 THz at the resonance of 0.1 THz. The comparison of this PCA with the reference ZnTe crystal reveals a 2-fold increase in THz power. Furthermore, this antenna attains a 1.5-, 3-, and 2-fold increase in THz power, photocurrent efficiency, and actuating dc BV, as compared with the commercial ZOMEGA antenna. These results pave the way towards the creation of highly efficient LT-GaAs-based PCAs.
关键词： THz-antenna,Terahertz (THz) radiation,Photoconductive antenna (PCA),Low temperature-grown gallium arsenide (LT-GaAs),THz-spectroscopy
更新于2025-09-11 14:15:04
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[IEEE 2019 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications (IMWS-AMP) - Bochum, Germany (2019.7.16-2019.7.18)] 2019 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications (IMWS-AMP) - Pulse-resolved Data Acquisition System for THz Pump Laser Probe Experiments at TELBE using Super-radiant Terahertz Sources
摘要： the terahertz (THz) frequency range lies between the frequency range of radio and infrared. The development of suitable detectors, detection techniques, and sources for this frequency range has much interest over the past decade. THz pulses of sufficient strength that act as an excitation of dynamics in the matter have only been available, due to the development in 4th generation superconducting radiofrequency (SRF) technology. In the THz pump laser probe experiments the electric or magnetic field in the THz pump pulse acts as the excitation of dynamics in the matter. Ultra-short laser pulses then probe this dynamic in turn. In this contribution, we will outline the pulse-resolved data acquisition scheme of the TELBE user facility based on the characterization of a new class of accelerator-based light sources, which provide a unique combination of high pulse energies and high repetition rates.
关键词： Superradiant Terahertz Sources,Data Acquisition System,THz Radiation Detection Techniques,THz pump Laser Probe Experiments
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - THz Pulse Generation Emitted from Slant-Stripe-Type PPLN Via Optical Rectification Effect
摘要： To generate THz wave efficiency, phase-matching condition and small absorption of THz waves in nonlinear crystal are required. PPLN has attracted attention as a promising candidate for nonlinear crystal for THz wave generation, since periodically poled lithium niobate (PPLN) can be controlled its phase matching condition by designing periodically-poled structure. In case of ultrashort pulse pumping, multi-cycle THz pulses are obtained from PPLNs via optical rectification effect [1]. In this case, THz pulses with an arbitrary frequency shift are expected to be obtained by optimizing the periodic structures of the PPLNs [1,2]. Utilizing this feature, it is expected that multi-cycled THz pulses can be applied to radar techniques with high resolution. However, the pulse length is limited by 10~30 ps owing to absorption of PPLN. Note that PPLNs have absorption coefficient of approximately a few tens cm-1 at room temperature [3]. To prevent absorption of the generated THz wave, slant-stripe-type PPLN crystal was proposed by Sasaki et-al [4]. The slanted-PPLN is designed that propagation direction of generated THz wave is perpendicular to the direction of pump beam. In this case, generated THz wave is emitted from side surface of the crystal. By setting pump beam as close to the lateral face of the crystal as possible, damping of the THz wave is small, due to the short path length in the nonlinear crystal. In case of two color ns pulse pumping (via difference frequency generation, DFG), efficient THz wave generation have been demonstrated. However, multi-cycled THz pulse by ultra-short pulse pumping (via optical rectification effect, OR) emitted from the slanted PPLN have not been demonstrated. As THz wave generation mechanisms are different in cases of DFG and OR, it is not certain whether multi-cycled THz pulse generate efficiently and propagate in the direction perpendicular to the pump beam via OR effect, similarly to the case of DFG. Our experiments were performed at room temperature. The slanted PPLN was pumped by femtosecond pulse (pulse width ~ 170 fs, wavelength ~ 1035 nm, repetition rate ~ 30 MHz). THz pulse was emitted from the side surface of the PPLN and detected by bolometer. Fig. 1 (a) shows the signal intensity of the bolometer as a function of intensity of pump power. It is confirmed that THz signal increased proportional to the square of the pump intensity increased. Fig. 1(b) shows THz wave intensity as a function of the polarization angle of the incident pump pulse. It is confirmed that the signal is maximized at 0 degree (parallel to polarization direction of the PPLN) and is minimized at 90 degree (perpendicular to the polarization) and is well fitted by cos4(cid:84) ((cid:84): polarization angle). Owing to these results, it is confirmed that the signal detected by the bolometer is THz pulse.
关键词： slant-stripe-type,THz pulse generation,multi-cycled THz pulses,optical rectification effect,PPLN
更新于2025-09-11 14:15:04
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Comparison of the Features of Electron Transport and Subterahertz Generation in Diodes Based on 6-, 18-, 70-, and 120-Period GaAs/AlAs Superlattices
摘要： A comparison of the features of electron transport in diodes based on 6-, 18-, 30-, 70-, and 120-period GaAs/AlAs superlattices with a similar design is performed. However, the number of periods and diode areas are different. The values of the parasitic resistances of the near-contact diode regions are correlated, and the specific voltage drop across one superlattice period is determined for all special points in the current–voltage characteristics of the diodes. The mechanism of the appearance of stable current oscillations in diodes based on 6-, 18-, 30-, 70-, and 120-period GaAs/AlAs superlattices with a high doping level is investigated.
关键词： diodes,THz generation,electron transport,superlattices
更新于2025-09-11 14:15:04
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Parasitic-coupled high-gain graphene antenna employed on PBG dielectric grating substrate for THz applications
摘要： In THz frequency region, the high-gain miniaturized patch antenna is essential to overcome the atmospheric losses. A novel dual-band miniaturized graphene antenna array with parasitic-coupled feed and mounted on PBG dielectric grating substrate is proposed in the operating frequency band from 0.85 to 1.04 THz. The analysis of graphene surface conductivity, surface wave polariton (SPP) of photonic band gap (PBG) substrate, and the dielectric grating substrate has been done for the verification of a projected design. Parametric analysis of PBG radius, periodic perturbation of dielectric grating substrate, and mutual coupling distance has been verified to achieve the enhanced radiation characteristics of the antenna. The minimum return loss of ?52.58 dB, VSWR of 1.005, impedance bandwidth of 33.34 GHz, the peak gain of 16.4 dB, and peak directivity of 17 dBi the projected design. The parasitic-coupled method is suitable to reduce the complexity of the antenna array and proposed design is useful in imaging, security using spectroscopy, and wireless communication applications.
关键词： parasitic coupling,PBG,dielectric grating,graphene,THz band
更新于2025-09-11 14:15:04
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[IEEE 2019 Second International Workshop on Mobile Terahertz Systems (IWMTS) - Bad Neuenahr, Germany (2019.7.1-2019.7.3)] 2019 Second International Workshop on Mobile Terahertz Systems (IWMTS) - Wideband Characterization of 3D Printed THz Rectangular Dielectric Waveguides by THz Frequency-Domain Spectroscopy
摘要： In this paper single-mode dielectric waveguides for 140 GHz are designed and fabricated by 3D printing technology. The waveguides are simulated using the finite difference time domain method and characterized by determining their frequency-dependent attenuation constant and frequency-dependent effective refractive index from 100 GHz to 180 GHz using THz frequency-domain spectroscopy. Furthermore, the mode patterns at different frequencies are observed by performing transversal scans of the far-field radiation pattern at the output of the waveguide.
关键词： Dielectric Waveguide,Frequency Domain Spectroscopy,THz
更新于2025-09-11 14:15:04