Showing papers on "Terahertz radiation published in 2014"
TL;DR: An overview and evaluation of state-of-the-art photodetectors based on graphene, other two-dimensional materials, and hybrid systems based on the combination of differentTwo-dimensional crystals or of two- dimensional crystals and other (nano)materials, such as plasmonic nanoparticles, semiconductors, quantum dots, or their integration with (silicon) waveguides are provided.
Abstract: Graphene and other two-dimensional materials, such as transition metal dichalcogenides, have rapidly established themselves as intriguing building blocks for optoelectronic applications, with a strong focus on various photodetection platforms The versatility of these material systems enables their application in areas including ultrafast and ultrasensitive detection of light in the ultraviolet, visible, infrared and terahertz frequency ranges These detectors can be integrated with other photonic components based on the same material, as well as with silicon photonic and electronic technologies Here, we provide an overview and evaluation of state-of-the-art photodetectors based on graphene, other two-dimensional materials, and hybrid systems based on the combination of different two-dimensional crystals or of two-dimensional crystals and other (nano)materials, such as plasmonic nanoparticles, semiconductors, quantum dots, or their integration with (silicon) waveguides
3,025 citations
IBM1
TL;DR: The basic properties of graphene plasmons are reviewed: their energy dispersion, localization and propagation, plasmon-phonon hybridization, lifetimes and damping pathways, and emerging and potential applications.
Abstract: In recent years, we have seen a rapid progress in the field of graphene plasmonics, motivated by graphene's unique electrical and optical properties, tunabilty, long-lived collective excitation and their extreme light confinement. Here, we review the basic properties of graphene plasmons; their energy dispersion, localization and propagation, plasmon-phonon hybridization, lifetimes and damping pathways. The application space of graphene plasmonics lies in the technologically significant, but relatively unexploited terahertz to mid-infrared regime. We discuss emerging and potential applications, such as modulators, notch filters, polarizers, mid-infrared photodetectors, mid-infrared vibrational spectroscopy, among many others.
1,056 citations
IBM1
TL;DR: In this paper, the basic properties of graphene plasmons are reviewed, including energy dispersion, localization and propagation, plasmon-phonon hybridization, lifetimes and damping pathways.
Abstract: In recent years, we have seen a rapid progress in the field of graphene plasmonics, motivated by graphene’s unique electrical and optical properties, tunability, long-lived collective excitation and its extreme light confinement. Here, we review the basic properties of graphene plasmons: their energy dispersion, localization and propagation, plasmon–phonon hybridization, lifetimes and damping pathways. The application space of graphene plasmonics lies in the technologically significant, but relatively unexploited terahertz to mid-infrared regime. We discuss emerging and potential applications, such as modulators, notch filters, polarizers, mid-infrared photodetectors, and mid-infrared vibrational spectroscopy, among many others.
951 citations
TL;DR: In this article, a phase-stable, high-harmonic transients over the whole frequency range of 0.1-675 THz were obtained. But the frequency of 30 THz was not considered.
Abstract: Terahertz waveforms with peak fields of 72 MV cm−1 and a central frequency of 30 THz drive interband polarization in bulk GaSe off-resonantly and accelerate excited electron–hole pairs, inducing dynamical Bloch oscillations. This results in the emission of phase-stable, high-harmonic transients over the whole frequency range of 0.1–675 THz.
841 citations
TL;DR: Active metamaterials have been used to realize terahertz imaging with a single-pixel detector Compressive techniques permit high-fidelity images to be acquired at high frame rates as discussed by the authors.
Abstract: Active metamaterials have been used to realize terahertz imaging with a single-pixel detector Compressive techniques permit high-fidelity images to be acquired at high frame rates The technique involves no moving parts and yields improved signal-to-noise ratios over standard raster scanning techniques
695 citations
TL;DR: It is shown that a widely tunable metasurface composed of optical antennas on graphene can be incorporated into a subwavelength-thick optical cavity to create an electrically tunable perfect absorber.
Abstract: Dynamically reconfigurable metasurfaces open up unprecedented opportunities in applications such as high capacity communications, dynamic beam shaping, hyperspectral imaging, and adaptive optics. The realization of high performance metasurface-based devices remains a great challenge due to very limited tuning ranges and modulation depths. Here we show that a widely tunable metasurface composed of optical antennas on graphene can be incorporated into a subwavelength-thick optical cavity to create an electrically tunable perfect absorber. By switching the absorber in and out of the critical coupling condition via the gate voltage applied on graphene, a modulation depth of up to 100% can be achieved. In particular, we demonstrated ultrathin (thickness < λ0/10) high speed (up to 20 GHz) optical modulators over a broad wavelength range (5–7 μm). The operating wavelength can be scaled from the near-infrared to the terahertz by simply tailoring the metasurface and cavity dimensions.
671 citations
TL;DR: In this paper, a planar terahertz metamaterial was used for ultra-sensitive sensing in the fingerprint region of the tera-hertz regime, where the low-loss quadrupole and Fano resonances with extremely narrow linewidths were used to measure the minute spectral shift caused due to the smallest change in the refractive index of the surrounding media.
Abstract: High quality factor resonances are extremely promising for designing ultra-sensitive refractive index label-free sensors, since it allows intense interaction between electromagnetic waves and the analyte material. Metamaterial and plasmonic sensing have recently attracted a lot of attention due to subwavelength confinement of electromagnetic fields in the resonant structures. However, the excitation of high quality factor resonances in these systems has been a challenge. We excite an order of magnitude higher quality factor resonances in planar terahertz metamaterials that we exploit for ultrasensitive sensing. The low-loss quadrupole and Fano resonances with extremely narrow linewidths enable us to measure the minute spectral shift caused due to the smallest change in the refractive index of the surrounding media. We achieve sensitivity levels of 7.75 × 103 nm/refractive index unit (RIU) with quadrupole and 5.7 × 104 nm/RIU with the Fano resonances which could be further enhanced by using thinner substrates. These findings would facilitate the design of ultrasensitive real time chemical and biomolecular sensors in the fingerprint region of the terahertz regime.
565 citations
TL;DR: In this paper, the authors review the recent developments in nonlinear optical (NLO) phenomena such as second harmonic and difference frequency generation (SHG and DFG), which are effective at producing a coherent laser beam in difficult to reach frequency regions of the electromagnetic spectrum.
Abstract: Materials chemistry and the pursuit of new compounds through exploratory synthesis are having a strong impact in many technological fields. The field of nonlinear optics is directly impacted by the availability of enabling materials with high performance. Nonlinear optical (NLO) phenomena such as second harmonic and difference frequency generation (SHG and DFG, respectively) are effective at producing a coherent laser beam in difficult to reach frequency regions of the electromagnetic spectrum. Such regions include the infrared (IR), far-infrared, and terahertz frequencies. High performance NLO crystals are critical for applications utilizing these coherent light sources, and new materials are continuously sought for better conversion efficiency and performance. The class of metal chalcogenides is the most promising source of potential NLO materials with desirable properties particularly in the IR region where most classes of materials face various fundamental challenges. We review the recent developments...
516 citations
TL;DR: Time-resolved measurements indicate that the graphene detector is eight to nine orders of magnitude faster than the best room-temperature terahertz detectors for an optimally coupled device, and a simple model of the response reproduces the qualitative features of the data, and indicates that orders-of-magnitude sensitivity improvements are possible.
Abstract: Hot-electron effects in graphene can be used to detect terahertz radiation at room temperature with high sensitivity, low noise-equivalent power and a fast response time.
509 citations
TL;DR: The direct observation of and control over quantum plasmon resonances at length scales in the range 0.4 to 1.3 nanometers across molecular tunnel junctions made of two plAsmonic resonators bridged by self-assembled monolayers (SAMs) are described.
Abstract: Quantum tunneling between two plasmonic resonators links nonlinear quantum optics with terahertz nanoelectronics. We describe the direct observation of and control over quantum plasmon resonances at length scales in the range 0.4 to 1.3 nanometers across molecular tunnel junctions made of two plasmonic resonators bridged by self-assembled monolayers (SAMs). The tunnel barrier width and height are controlled by the properties of the molecules. Using electron energy-loss spectroscopy, we directly observe a plasmon mode, the tunneling charge transfer plasmon, whose frequency (ranging from 140 to 245 terahertz) is dependent on the molecules bridging the gaps.
400 citations
TL;DR: In this article, frequency combs based on terahertz quantum cascade lasers are demonstrated, which combine the high power of lasers with the broadband capabilities of pulsed sources. But their performance was limited to 3.5 THz.
Abstract: Frequency combs based on terahertz quantum cascade lasers, which combine the high power of lasers with the broadband capabilities of pulsed sources, are demonstrated. The frequency combs generate 5 mW of terahertz power covering a frequency range of almost 500 GHz and produce more than 70 lines at 3.5 THz.
TL;DR: In this article, a planar terahertz metamaterial has been used for ultra-sensitive sensing with high quality factor resonances in order to measure the minute spectral shift caused by the smallest change in the refractive index of the surrounding media.
Abstract: High quality factor resonances are extremely promising for designing ultra-sensitive refractive index label-free sensors since it allows intense interaction between electromagnetic waves and the analyte material. Metamaterial and plasmonic sensing has recently attracted a lot of attention due to subwavelength confinement of electromagnetic fields in the resonant structures. However, the excitation of high quality factor resonances in these systems has been a challenge. We excite an order of magnitude higher quality factor resonances in planar terahertz metamaterials that we exploit for ultrasensitive sensing. The low-loss quadrupole and Fano resonances with extremely narrow linewidths enable us to measure the minute spectral shift caused due to the smallest change in the refractive index of the surrounding media. We achieve sensitivity levels of 7.75 x 10^3 nm/ RIU with quadrupole and 5.7 x 10^4 nm/ RIU with the Fano resonances which could be further enhanced by using thinner substrates. These findings would facilitate the design of ultrasensitive real time chemical and biomolecular sensors in the fingerprint region of the terahertz regime.
TL;DR: The method presented here paves a way toward nonlinear quantum optics in superconductor with driving the pseudospins collectively and can be potentially extended to exotic superconductors for shedding light on the character of order parameters and their coupling to other degrees of freedom.
Abstract: Superconductors host collective modes that can be manipulated with light. We show that a strong terahertz light field can induce oscillations of the superconducting order parameter in NbN with twice the frequency of the terahertz field. The result can be captured as a collective precession of Anderson's pseudospins in ac driving fields. A resonance between the field and the Higgs amplitude mode of the superconductor then results in large terahertz third-harmonic generation. The method we present here paves a way toward nonlinear quantum optics in superconductors with driving the pseudospins collectively and can be potentially extended to exotic superconductors for shedding light on the character of order parameters and their coupling to other degrees of freedom.
TL;DR: In this paper, an ultra-broadband, polarization-insensitive, and wide-angle metamaterial absorber for terahertz (THz) frequencies using arrays of truncated pyramid unit structure made of metal-dielectric multilayer composite was demonstrated.
Abstract: We demonstrated an ultra-broadband, polarization-insensitive, and wide-angle metamaterial absorber for terahertz (THz) frequencies using arrays of truncated pyramid unit structure made of metal-dielectric multilayer composite. In our design, each sub-layer behaving as an effective waveguide is gradually modified in their lateral width to realize a wideband response by effectively stitching together the resonance bands of different waveguide modes. Experimentally, our five layer sample with a total thickness 21 μm is capable of producing a large absorptivity above 80% from 0.7 to 2.3 THz up to the maximum measurement angle 40°. The full absorption width at half maximum of our device is around 127%, greater than those previously reported for THz frequencies. Our absorber design has high practical feasibility and can be easily integrated with the semiconductor technology to make high efficient THz-oriented devices.
TL;DR: In this paper, an overview of the range of available terahertz sources, emphasizing recent developments, is given, and the focus then narrows to the sources that rely on surface phenomena.
Abstract: Bibliometric data set the scene by illustrating the growth of terahertz work and the present interest in terahertz science and technology. After locating terahertz sources within the broader context of terahertz systems, an overview is given of the range of available sources, emphasizing recent developments. The focus then narrows to terahertz sources that rely on surface phenomena. Three are highlighted. Optical rectification, usually thought of as a bulk process, may in addition exhibit a surface contribution, which, in some cases, predominates. Transient surface currents, for convenience often separated into drift and diffusion currents, are well understood according to Monte Carlo modelling. Finally, terahertz surface emission by mechanical means—in the absence of photoexcitation—is described.
TL;DR: The proposed polarization modulator is able to electrically control the reflected wave with a linear polarization of continuously tunable azimuth angle of the major axis from 0° to 90° at the working frequency and proposes a polarization modulation scheme of terahertz wave by applying similar polarization dependent absorbers.
Abstract: Graphene can be utilized in designing tunable terahertz devices due to its tunability of sheet conductivity. In this paper, we combine the metamaterial having unit cell of cross-shaped metallic resonator with the double layer graphene wires to realize polarization independent absorber with spectral tuning at terahertz frequency. The absorption performance with a peak frequency tuning range of 15% and almost perfect peak absorption has been demonstrated by controlling the Fermi energy of the graphene that can be conveniently achieved by adjusting the bias voltage on the graphene double layers. The mechanism of the proposed absorber has been explored by a transmission line model and the tuning is explained by the changing of the effective inductance of the graphene wires under gate voltage biasing. Further more, we also propose a polarization modulation scheme of terahertz wave by applying similar polarization dependent absorbers. Through the proposed polarization modulator, it is able to electrically control the reflected wave with a linear polarization of continuously tunable azimuth angle of the major axis from 0° to 90° at the working frequency. These design approaches enable us to electrically control the absorption spectrum and the polarization state of terahertz waves more flexibly.
TL;DR: In this paper, a terahertz spatial light modulator implemented with metamaterial absorbers (MMAs) functionalized with isothiocyanate-based liquid crystals (LCs) is experimentally demonstrated.
Abstract: A terahertz (THz) spatial light modulator implemented with metamaterial absorbers (MMAs) functionalized with isothiocyanate-based liquid crystals (LCs) is experimentally demonstrated. The device is designed to work in refl ection mode and is arranged in a 6 × 6 pixel matrix where the response of each pixel is modulated by electronically controlling the orientation of liquid crystal dimers covering the entire metamaterial absorber landscape. Experiments show that each pixel can be controlled independently and that pixelated absorption patterns can be created at will. The SLM shows an overall modulation depth of 75%. Furthermore, computational results show that losses arising from LCs impose a severe limitation on the overall performance and that consequently the modulation depth of each pixel could be improved with liquid crystal mixtures designed primarily for THz frequencies. This work demonstrates the viability of liquid crystal-based reconfi gurable metamaterials and highlights their great potential use for future state-of-the-art THz devices.
TL;DR: High-field terahertz transients with 0.9-mJ pulse energy produced in a 400 mm² partitioned organic crystal by optical rectification of a 30-m J laser pulse centered at 1.25 μm wavelength are reported on.
Abstract: We report on high-field terahertz transients with 0.9-mJ pulse energy produced in a 400 mm² partitioned organic crystal by optical rectification of a 30-mJ laser pulse centered at 1.25 μm wavelength. The phase-locked single-cycle terahertz pulses cover the hard-to-access low-frequency range between 0.1 and 5 THz and carry peak fields of more than 42 MV/cm and 14 Tesla with the potential to reach over 80 MV/cm by choosing appropriate focusing optics. The scheme based on a Cr:Mg₂SiO₄ laser offers a high conversion efficiency of 3% using uncooled organic crystal. The collimated pump laser configuration provides excellent terahertz focusing conditions.
TL;DR: In this paper, a sinusoidally modulated graphene leaky-wave antenna with beam scanning capabilities at a fixed frequency is proposed, which is composed of a graphene sheet transferred onto a back-metallized substrate and a set of polysilicon DC gating pads located beneath it.
Abstract: This paper proposes the concept, analysis and design of a sinusoidally modulated graphene leaky-wave antenna with beam scanning capabilities at a fixed frequency. The antenna operates at terahertz frequencies and is composed of a graphene sheet transferred onto a back-metallized substrate and a set of polysilicon DC gating pads located beneath it. In order to create a leaky-mode, the graphene surface reactance is sinusoidally modulated via graphene's field effect by applying adequate DC bias voltages to the different gating pads. The pointing angle and leakage rate can be dynamically controlled by adjusting the applied voltages, providing versatile beamscanning capabilities. The proposed concept and achieved performance, computed using realistic material parameters, are extremely promising for beamscanning at THz frequencies, and could pave the way to graphene-based reconfigurable transceivers and sensors.
TL;DR: In this article, a terahertz (THz) frequency quantum cascade laser with peak powers of > 1 W from a single facet in the pulsed mode is demonstrated.
Abstract: Terahertz (THz) frequency quantum cascade lasers emitting peak powers of >1 W from a single facet in the pulsed mode are demonstrated. The active region is based on a bound-to-continuum transition with a one-well injector, and is embedded into a surface-plasmon waveguide. The lasers emit at a frequency of ∼3.4 THz and have a maximum operating temperature of 123 K. The maximum measured emitted powers are ∼1.01 W at 10 K and ∼420 mW at 77 K, with no correction made to allow for the optical collection efficiency of the apparatus.
TL;DR: A modulation and channel access scheme for nanonetworks in the Terahertz Band is developed based on the transmission of one-hundred-femtosecond-long pulses by following an asymmetric On-Off Keying modulation Spread in Time (TS-OOK).
Abstract: Nanonetworks consist of nano-sized communicating devices which are able to perform simple tasks at the nanoscale. Nanonetworks are the enabling technology of long-awaited applications such as advanced health monitoring systems or high-performance distributed nano-computing architectures. The peculiarities of novel plasmonic nano-transceivers and nano-antennas, which operate in the Terahertz Band (0.1-10 THz), require the development of tailored communication schemes for nanonetworks. In this paper, a modulation and channel access scheme for nanonetworks in the Terahertz Band is developed. The proposed technique is based on the transmission of one-hundred-femtosecond-long pulses by following an asymmetric On-Off Keying modulation Spread in Time (TS-OOK). The performance of TS-OOK is evaluated in terms of the achievable information rate in the single-user and the multi-user cases. An accurate Terahertz Band channel model, validated by COMSOL simulation, is used, and novel stochastic models for the molecular absorption noise in the Terahertz Band and for the multi-user interference in TS-OOK are developed. The results show that the proposed modulation can support a very large number of nano-devices simultaneously transmitting at multiple Gigabits-per-second and up to Terabits-per-second, depending on the modulation parameters and the network conditions.
TL;DR: In this paper, the multipolar spoof localized surface plasmons (LSPs) on a planar textured metallic disk are proposed and experimentally demonstrated at microwave frequencies.
Abstract: The multipolar spoof localized surface plasmons (LSPs) on a planar textured metallic disk are proposed and experimentally demonstrated at microwave frequencies. Based on ultrathin metal film printed on a thin dielectric substrate, the designed plasmonic metamaterial clearly shows multipolar plasmonic resonances, including the dipole, quadrupole, hexapole, octopole, decapole, dodecapole, and quattuordecpole modes. Both numerical simulations and experiments are in good agreement. It is shown that the spoof LSP resonances are sensitive to the disk's geometry and local dielectric environments. Hence, the ultrathin textured metallic disk may be used as plasmonic sensors and find potential applications in the microwave and terahertz frequencies.
TL;DR: In this article, a compact and efficient laser-based THz generation in the terahertz frequency gap (1-10 THz) was reported. But the authors did not consider the effect of the laser-to-THz energy conversion efficiency.
Abstract: We report on compact and efficient laser-based THz generation in the terahertz frequency gap (1-10 THz). The radiation is generated by optical rectification of a midinfrared laser in a large-size, partitioned nonlinear organic crystal assembly. This enables up-scaling of presently field-limited tabletop THz sources to GV/m electric and several tesla magnetic field at millijoule pulse energy. In agreement with simulations, the THz beam properties at focus are shown to be not deteriorated by the discontinuity of the emitter surface. The high laser-to-THz energy conversion efficiency exceeds the Manley-Rowe limit and is explained by a cascaded chi((2)) process in the organic crystals accompanied by a significant redshift of the pump spectrum. The scheme provides a compact, tabletop THz source for single-cycle transients at field strength equivalent or even higher to linear accelerator and FEL-based THz sources. This opens an avenue toward novel nonlinear THz applications.
TL;DR: This work reports on the development of a powerless, compact, broadband, flexible, large-area, and polarization-sensitive carbon nanotube THz detector that works at room temperature and suggests that judicious design of thermal management and quantum engineering of Seebeck coefficients will lead to further enhancement of device performance.
Abstract: Terahertz (THz) technologies are promising for diverse areas such as medicine, bioengineering, astronomy, environmental monitoring, and communications. However, despite decades of worldwide efforts, the THz region of the electromagnetic spectrum still continues to be elusive for solid state technology. Here, we report on the development of a powerless, compact, broadband, flexible, large-area, and polarization-sensitive carbon nanotube THz detector that works at room temperature. The detector is sensitive throughout the entire range of the THz technology gap, with responsivities as high as ∼2.5 V/W and polarization ratios as high as ∼5:1. Complete thermoelectric and opto-thermal characterization together unambiguously reveal the photothermoelectric origin of the THz photosignal, triggered by plasmonic absorption and collective antenna effects, and suggest that judicious design of thermal management and quantum engineering of Seebeck coefficients will lead to further enhancement of device performance.
TL;DR: A ∼50% amplitude modulation of THz waves with gated single-layer graphene is demonstrated by the use of extraordinary transmission through metallic ring apertures placed right above the graphene layer, promising complementary metal-oxide-semiconductor compatible THz modulators with tailored operation frequencies, large on/off ratios, and high speeds.
Abstract: Gate-controllable transmission of terahertz (THz) radiation makes graphene a promising material for making high-speed THz wave modulators. However, to date, graphene-based THz modulators have exhibited only small on/off ratios due to small THz absorption in single-layer graphene. Here we demonstrate a ∼50% amplitude modulation of THz waves with gated single-layer graphene by the use of extraordinary transmission through metallic ring apertures placed right above the graphene layer. The extraordinary transmission induced ∼7 times near-filed enhancement of THz absorption in graphene. These results promise complementary metal–oxide–semiconductor compatible THz modulators with tailored operation frequencies, large on/off ratios, and high speeds, ideal for applications in THz communications, imaging, and sensing.
TL;DR: In this paper, the authors demonstrate ultrabroadband time-resolved terahertz spectroscopy on a single InAs nanowire with 10nm spatial resolution and sub-100 fs time resolution.
Abstract: The authors demonstrate ultrabroadband time-resolved THz spectroscopy on a single InAs nanowire with 10 nm spatial resolution and sub-100 fs time resolution. Phase-locked ultrashort pulses in the rich terahertz spectral range1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18 have provided key insights into phenomena as diverse as quantum confinement7, first-order phase transitions8,12, high-temperature superconductivity11 and carrier transport in nanomaterials1,6,13,14,15. Ultrabroadband electro-optic sampling of few-cycle field transients1 can even reveal novel dynamics that occur faster than a single oscillation cycle of light4,8,10. However, conventional terahertz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielectric functions averaged over the size, structure, orientation and density of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved terahertz spectroscopy to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM)16,17,18,19,20,21,22,23,24,25,26. We trace the time-dependent dielectric function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast (<50 fs) formation of a local carrier depletion layer.
TL;DR: In this article, a high efficiency, broadband, tunable and flexible quarter-wave plate based on a multilayer metamaterial is presented, with excellent achromatic π/2 phase retardance with high transmission.
Abstract: Metamaterials offer exciting opportunities that enable precise control of light propagation, its intensity and phase by designing an artificial medium of choice. Inducing birefringence via engineered metamolecules presents a fascinating mechanism to manipulate the phase of electromagnetic waves and facilitates the design of polarimetric devices. In this paper, a high-efficiency, broadband, tunable and flexible quarter-wave plate based on a multilayer metamaterial is presented. Excellent achromatic π/2 phase retardance with high transmission is observed upon terahertz propagation through the quarter-wave plate. The calculated Stokes parameter represents the output polarization state numerically, indicating an excellent broadband conversion of linearly polarized light into circularly polarized light. The metamaterial-based quarter-wave plate demonstrated in this work could be an important step forward in the development of functional terahertz polarization conversion devices for practical applications.
TL;DR: In this paper, the principles of tomography for terahertz Computed tomography (CT), tomosynthesis (TS), synthetic aperture radar (SAR), and time-of-flight (TOF) tomography are established.
Abstract: Terahertz and millimeter waves penetrate various dielectric materials, including plastics, ceramics, crystals, and concrete, allowing terahertz transmission and reflection images to be considered as a new imaging tool complementary to X-Ray or Infrared. Terahertz imaging is a well-established technique in various laboratory and industrial applications. However, these images are often two-dimensional. Three-dimensional, transmission-mode imaging is limited to thin samples, due to the absorption of the sample accumulated in the propagation direction. A tomographic imaging procedure can be used to acquire and to render three-dimensional images in the terahertz frequency range, as in the optical, infrared or X-ray regions of the electromagnetic spectrum. In this paper, after a brief introduction to two dimensional millimeter waves and terahertz imaging we establish the principles of tomography for Terahertz Computed tomography (CT), tomosynthesis (TS), synthetic aperture radar (SAR) and time-of-flight (TOF) terahertz tomography. For each technique, we present advantages, drawbacks and limitations for imaging the internal structure of an object.
TL;DR: An overview on terahertz metrology, including parameter estimation, signal processing, measurement characteristics, uncertainties, and calibrations is provided, which serves as guidance for metrology and further developments of THz-TDS systems.
Abstract: Terahertz time-domain spectroscopy (THz-TDS) has emerged as a main spectroscopic modality to fill the frequency range between a few hundred gigahertz to a few terahertz. This spectrum has been known as “terahertz gap” owing to limited accessibility by conventional electronic and optical techniques. Over the past two decades, THz-TDS has evolved substantially with enhanced compactness and stability. Since THz-TDS is becoming an industrial standard, the performance and precision of the system are of prime importance. This article provides an overview on terahertz metrology, including parameter estimation, signal processing, measurement characteristics, uncertainties, and calibrations. The overview serves as guidance for metrology and further developments of THz-TDS systems.
TL;DR: In this paper, an ultra-wideband plasmonic waveguide based on designer surface Plasmon polaritons (DSPPs) with double gratings was proposed.
Abstract: We propose an ultra-wideband plasmonic waveguide based on designer surface plasmon polaritons (DSPPs) with double gratings. In such plasmonic metamaterials, the DSPP waves in the region of lower frequencies of the dispersion curve can be tightly confined and hence effectively broaden the operating bandwidth. Based on such features, we design and fabricate a high performance DSPP filter, in which a transducer consisting of microstrip, slotline, and gradient corrugations is employed to feed electromagnetic energies into the plasmonic waveguide with high efficiency. The simulated and measured results on reflection and transmission coefficients in the microwave frequency demonstrate the excellent filtering characteristics such as low loss, wide band, and high square ratio. The high performance DSPP waveguide and filter pave a way to develop advanced plasmonic integrated functional devices and circuits in the microwave and terahertz frequencies.