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The operation of THz quantum cascade laser in the region of negative differential resistance
摘要： We investigate the light-current-voltage characteristics and emission spectra of 2.3 THz quantum cascade laser operating in the negative differential resistance (NDR) region. It was shown that the formation of electric ?eld domains (EFDs) leads to a large number of discontinuities on the current-voltage and the total optical power on current characteristics. Measurements of emission spectra at different current (before the NDR region and in the NDR region) shows that the formation of EFDs results in decrease of the output intensity, but does not in?uence on lasing frequencies. The performed calculations qualitatively explain the experimental results.
关键词： THz region,Resonant-phonon design,Quantum-cascade laser,Negative differential resistivity
更新于2025-09-11 14:15:04
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Metamaterials and Metasurfaces || Electron Beam-Induced Directional Terahertz Radiation from Metamaterials
摘要： We have numerically analyzed an electron beam (e-beam)-induced directional terahertz (THz) radiation from metamaterials. Here, we used metallic grating structures with graded depths, in which only one-way surface mode can be supported based on the spoof surface plasmon polariton (spoof SPP) concept and gives unique directional THz radiation. For numerical analysis, we used a simplified particle-in-cell (PIC) finite-difference time-domain (FDTD) method. First, we describe our simplified PIC-FDTD method in detail. Then, we show our results on the e-beam-induced directional THz radiation from graded grating with graded depths. By passing pulsed (bunched) e-beam along the grating surface, directional THz radiations are obtained from one side of the grating with shallower grooves. The direction of these radiations can be switched backward or forward by making the groove depth deeper or shallower. The spectra of these directional radiations are wide band and contain multiple sharp peaks. The deepest and the shallowest groove depths determine the lowest and the highest frequency of the radiation band, respectively. These unique radiation characteristics cannot be explained by the conventional Smith-Purcell radiation and should be attributed to the spoof SPP that originates from different locations on the graded grating.
关键词： graded grating,Smith-Purcell radiation,THz,particle-in-cell finite-difference time-domain,metamaterial,PIC-FDTD,electron beam,terahertz
更新于2025-09-11 14:15:04
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Control of THz field waveform emitted from air plasma by chirping two-color laser pulses
摘要： Few-cycle terahertz pulses can be effectively generated in air plasma by mixing an ultrashort laser pulse with its second harmonic. The temporal overlap between these two pulses is critical, which can degrade with the mismatch of group velocities. However, the overlap between the fundamental and second harmonic can be controlled by pre-stretching the temporally separated pulses. In this paper, we study the role of chirp and the relative phase between the fundamental and its second harmonic in the formation of the terahertz waveform. We demonstrate experimentally and explain within the context of transient photocurrent model, that the combination of chirping and relative phase shifting can provide a powerful tool to control the waveform of the terahertz pulse.
关键词： Dispersion,Plasma physics,Nonlinear optics,THz generation,Ultrashort pulses
更新于2025-09-11 14:15:04
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[IEEE 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Paris, France (2019.9.1-2019.9.6)] 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Whispering-Gallery Resonators for Highly Integrated Plasmonic THz Circuits
摘要： Whispering-gallery resonators based on spoof plasmonic rings are presented. The resonators exhibit exceptionally high quality factors for THz metasurfaces (in excess of 240). As the resonator is coupled to two waveguides, it can serve as narrowband band-stop and band-pass filter in an integrated plasmonic circuit. The complicated mode structure shows frequency comb-like behaviour.
关键词： Whispering-gallery resonators,THz metasurfaces,integrated plasmonic circuit,high quality factors,spoof plasmonic rings
更新于2025-09-11 14:15:04
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Dynamically Tunable and High-Contrast Graphene-Based Terahertz Electro-Optic Modulator
摘要： We propose and discuss terahertz (THz) electro-optic modulator induced by periodically patterned graphene microcavity. Due to the joint effect of graphene plasmon resonances and Fabry-Perot (F-P) oscillations, plasmon-induced transparency (PIT) effect is achieved and the operation frequency can be dynamically tuned by graphene Fermi energies and structural parameters. The results reveal that modulation depth (MD) larger than 80% is obtained across a wide frequency range from 4.2 THz to 9.4 THz, and the largest MD and Q factor reaches 95% and 15.8, respectively. In addition, operation frequency range and MD can also be tuned by optimizing the structure parameters. This investigation promises the development of high-performance widely tunable THz modulator in chip integrated photonic circuits.
关键词： modulator,graphene,THz
更新于2025-09-11 14:15:04
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Characteristics of Coplanar Waveguide of Small Cross Section on BCB with Coplanar Ground to Conductor-Backed Plane Interconnection
摘要： Conductor-backed coplanar waveguide on 30-μm-thick BCB polymer is fabricated. In order to eliminate unwanted propagation modes for these lines designed for frequencies up to 1 THz, electroplated vias interconnect each coplanar ground to the backed conductor. Transmission line properties are extracted here up to 67 GHz, showing a characteristic impedance of about 50 Ω, but with high conduction losses for this frequency range due to the thin metal layer used. Thru-reflect-line correction and a de-embedding with electrical circuit of accesses are applied to extract line properties. The comparison with a coplanar waveguide model leads to a relative permittivity of 2.676 for the BCB in this low-frequency range.
关键词： Transmission lines,Submillimeter wave waveguides,THz coplanar waveguides,Polymer dielectric
更新于2025-09-11 14:15:04
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[IEEE 2017 IEEE 44th Photovoltaic Specialists Conference (PVSC) - Washington, DC (2017.6.25-2017.6.30)] 2017 IEEE 44th Photovoltaic Specialist Conference (PVSC) - High resolution THz scanning for optimization of dielectric layer opening process on doped Si surfaces
摘要： Diffused IBC solar cells based on non-fire-through (NFT) metallization can achieve higher performance with respect to FT metallization, thanks to lower contact resistance. Less resistive contacts allows for lower metal fraction and thus lower contact recombination. For the application of NFT contacts, the rear side dielectric passivation layer needs to be locally opened. Standard processes for dielectric opening include laser ablation, use of etching pastes or masking and etching. In all this cases it is important that the opening process does not cause any damage to the diffusion below the contacts. We present a fast inspection method based on THz near-field scanning that allows investigating the diffusion layers after dielectric opening with spatial resolution suited for the small feature size of standard IBC solar cells. This method can be used to optimize crucial parameters of the dielectric opening without processing full cells. In this paper, we use THz scanning to optimize the curing temperature of the etch paste used to open the dielectric passivation layers on solar cells half-fabricates. After optimization, full cells are processed with top efficiency of 20.8%.
关键词： THz scanning,NFT metallization,dielectric layer opening,solar cells,doped Si surfaces
更新于2025-09-11 14:15:04
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Effect of cross-focusing of two Laser beams on THz radiation in graphite nanoparticles with density ripple
摘要： The present work explores the study of terahertz (THz) wave generation by the cross-focusing of two Gaussian laser beams in a bulk medium containing graphite nanoparticles (NPs). This system acts as a nonlinear medium behaving like plasma. The ripple is applied on the density of NPs to increase the conversion efficiency. Using the perturbative technique, a wave equation is derived which describes the nonlinear interaction of the laser beam with NPs. The nonlinear current at THz frequency arises due to the ponderomotive force exerted on the electrons of graphite NPs by the incident laser beams in the presence of density ripple. Numerical results show that the normalized amplitude of the generated THz wave increases by 3.5 times with the consideration of the phenomenon of cross-focusing. The results show the enhancement in the efficiency of generated THz wave on application of density ripple, due to the fulfillment of the phase matching condition.
关键词： plasma,cross-focusing,nanoparticles,density ripple,THz radiation
更新于2025-09-11 14:15:04
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[IEEE 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Paris, France (2019.9.1-2019.9.6)] 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Exceptionally high THz energy densities generated from organic crystals pumped with mid-infrared pulses
摘要： Optical Parametric Chirped-Pulse Amplification (OPCPA) is an emerging laser technology, providing a flexibility in operating wavelengths and power upscaling. Here, we investigate THz generation in organic crystals (DAST) by optical rectification (OR) of intense Mid–Infrared (MIR) pulses. The broadband spectrum exceeds several octaves when pumped by 100 fs, 3.9 μm OPCPA pulses. We report on an extraordinarily high THz energy density of 3.1 mJ/cm2 and crystal damage threshold > 150 mJ/cm2.
关键词： optical rectification,OPCPA,mid-infrared pulses,THz generation,organic crystals
更新于2025-09-11 14:15:04
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Energy losses and transition radiation in graphene traversed by a fast charged particle under oblique incidence
摘要： We perform fully relativistic calculations of the energy loss channels for a charged particle traversing a single layer of graphene under oblique incidence in a setting pertinent to a scanning transmission electron microscope (STEM), where we distinguish between the energy deposited in graphene in the form of electronic excitations (Ohmic loss) and the energy emitted in the far field in the form of transition radiation (TR). Our formulation of the problem uses a definition of two in-plane, dielectric functions of graphene, which describe the longitudinal and transverse excitation processes that contribute separately to those two energy loss channels. Using several models for the electric conductivity of graphene as the input in those dielectric functions enables us to discuss the effects of oblique incidence on several processes in a broad range of frequencies, from the terahertz (THz) to the ultraviolet (UV). In particular, at the THz frequencies, we demonstrate that the nonlocal effect in the graphene’s conductivity is not important in the retarded regime, and we show that the longitudinal and transverse contributions to the emitted TR spectra exhibit strongly anisotropic angular patterns that are readily distinguishable in a cathodoluminescence measurement in a STEM. Moreover, we explore the possibility of exciting the so-called transverse mode in the optical response of graphene at the mid-infrared (MIR) range of frequencies by means of a fast charged particle under oblique incidence. Finally, we demonstrate that, aside from the usual high-energy peaks in the longitudinal contribution to the Ohmic energy loss in the MIR to the UV frequency range, there may arise strongly directional features in the in-plane distribution of the transverse contribution to the Ohmic energy loss for an oblique trajectory, which could be possibly observed via momentum- and angle-resolved electron energy loss spectroscopy of graphene in STEM.
关键词： MIR,energy loss,STEM,THz,transition radiation,UV,graphene,cathodoluminescence,oblique incidence,Ohmic loss
更新于2025-09-11 14:15:04