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Terahertz technology related to generation and detection of terahertz radiation, as well as its interaction with matter has been gaining much attention making it one of the most attractive research fields in turn of the 21st century and covering such major sectors as the semiconductor, medical, space and defense industries. With the advent of intense terahertz sources providing subpicosecond terahertz pulses with photon energies of 0.4–40 meV and maximum field strengths greater than 1 MV/cm, nonlinear effect studies of matter are now available. High-power terahertz sources open up a range of potential applications both in characterization and controlling of solid-state material properties that are elusive for other frequency domains. Recently, a new high-power terahertz source with a record-breaking electric field strength up to 100 MV/cm in the 0.1–5 THz range has been developed at JIHT RAS. In this paper, recent advances in terahertz technology related to intense terahertz field-matter interactions at field strengths above 1 MV/cm are discussed for the first time.

Generation and application of energetic, broadband terahertz pulses (bandwidth ~0.1–50 THz) is an active and contemporary area of research. The main thrust is toward the development of efficient sources with minimum complexities—a true table-top setup. In this work, we demonstrate the generation of terahertz radiation via ultrashort pulse induced filamentation in liquids—a counterintuitive observation due to their large absorption coefficient in the terahertz regime. The generated terahertz energy is more than an order of magnitude higher than that obtained from the two-color filamentation of air (the most standard table-top technique). Such high terahertz energies would generate electric fields of the order of MV cm-1, which opens the doors for various nonlinear terahertz spectroscopic applications. The counterintuitive phenomenon has been explained via the solution of nonlinear pulse propagation equation in the liquid medium., Developing simple and efficient table-top sources of intense terahertz radiation is an ongoing pursuit. Here, Dey et al. demonstrate broadband terahertz generation from laser filamentation in liquids with an order of magnitude higher energy than from conventional two-color filamentation in air.

Terahertz radiation has a growing number of applications in material characterization, where spectral fingerprinting and diffractive effects are the carriers of information. On the other hand, electromagnetic waves in the range of millimeters exhibit strong unwanted specular reflections, resulting in uncontrolled interferences. This problem is especially disturbing in the goniometric time-domain spectroscopy (TDS) configuration, where angular distribution of the field modified by the sample is altered by unwanted reflections. For this reason, low-cost anti-reflection layers are desired. Here, we present a simple way of designing and manufacturing one-sided and two-sided anti-reflection polyamide layers for the THz range. The structures were fabricated using 3-D printers based on selective laser sintering. We demonstrate experimentally in the goniometric time-domain spectroscopy the significant reduction of wavelength-dependent oscillations in Fabry-Perot configuration in the range between 0.1 and 0.3 THz. We also examine the influence of the anti-reflection layers on the distribution of THz energy in reflected, transmitted, and diffracted fields.

Terahertz applications urgently require high performance and room temperature terahertz sources. The gyrotron based on the principle of electron cyclotron maser is able to generate watt-to-megawatt level terahertz radiation, and becomes an exceptional role in the frontiers of energy, security and biomedicine. However, in normal conditions, a terahertz gyrotron could generate terahertz radiation with high efficiency on a single frequency or with low efficiency in a relatively narrow tuning band. Here a frequency tuning scheme for the terahertz gyrotron utilizing sequentially switching among several whispering-gallery modes is proposed to reach high performance with broadband, coherence and high power simultaneously. Such mode-switching gyrotron has the potential of generating broadband radiation with 100-GHz-level bandwidth. Even wider bandwidth is limited by the frequency-dependent effective electrical length of the cavity. Preliminary investigation applies a pre-bunched circuit to the single-mode wide-band tuning. Then, more broadband sweeping is produced by mode switching in great-range magnetic tuning. The effect of mode competition, as well as critical engineering techniques on frequency tuning is discussed to confirm the feasibility for the case close to reality. This multi-mode-switching scheme could make gyrotron a promising device towards bridging the so-called terahertz gap.

We present the direct measurements of terahertz meta-atoms, an elementary unit of metamaterials, by using locally generated terahertz waves in the near-field region. In contrast to a conventional far-field terahertz spectroscopy or imaging, our technique features the localized emission of coherent terahertz pulses on a sub-wavelength scale, which has a potential for visualizing details of dynamics of each meta-atom. The obtained data show the near-field coupling among the meta-atoms and the impact of the electric field distribution from the excited meta-atom to neighbor meta-atoms. The observable LC resonance response is enhanced with an increase of numbers of meta-atoms. Furthermore, our approach also has a potential for visualizing the individual mode of meta-atom at different terahertz irradiation spots. These data can help us to understand the important role of the meta-atom in metamaterials and develop the novel terahertz components and devices such as active terahertz metamaterial and compact, high-sensitive bio-sensor devices.

The scope and potential uses of time-domain terahertz imaging and spectroscopy are mainly limited by the low optical-to-terahertz conversion efficiency of photoconductive terahertz sources. State-of-the-art photoconductive sources utilize short-carrier-lifetime semiconductors to recombine carriers that cannot contribute to efficient terahertz generation and cause additional thermal dissipation. Here, we present a novel photoconductive terahertz source that offers a significantly higher efficiency compared with terahertz sources fabricated on short-carrier-lifetime substrates. The key innovative feature of this source is the tight three-dimensional confinement of the optical pump beam around the terahertz nanoantennas that are used as radiating elements. This is achieved by means of a nanocavity formed by plasmonic structures and a distributed Bragg reflector. Consequently, almost all of the photo-generated carriers can be routed to the terahertz nanoantennas within a sub-picosecond time-scale. This results in a very strong, ultrafast current that drives the nanoantennas to produce broadband terahertz radiation. We experimentally demonstrate that this terahertz source can generate 4 mW pulsed terahertz radiation under an optical pump power of 720 mW over the 0.1–4 THz frequency range. This is the highest reported power level for terahertz radiation from a photoconductive terahertz source, representing more than an order of magnitude of enhancement in the optical-to-terahertz conversion efficiency compared with state-of-the-art photoconductive terahertz sources fabricated on short-carrier-lifetime substrates.

In order to increase the terahertz output power of the photoconductive antenna, a new multiple terahertz power synthesis technology based on terahertz time domain spectroscopy waveform consistency is proposed. The dipole photoconductive antenna with 500-μm aperture and a plurality of delay lines are used to build multiple terahertz power synthesis system. A synthesis efficiency evaluation method is proposed on the basis of the temporal coherence degree. The terahertz time-domain spectroscopy signals can be generated synchronously by virtue of the precise control of multiple delay lines, which can be realized the maximizing frequency domain peak power. Experimental results showed that the synthesized time-domain spectroscopy signal peaks and frequency domain power peaks are maximized, when the terahertz optical path difference is consistent. Two channel, three channel, and four channel synthetic temporal coherence values are 99.47%, 90.45%, and 83.37%, respectively, and the terahertz radiation power peaks can reach 1.94, 2.27, and 2.45 times the peaks before synthesis, respectively.

A tunable beat source has been made using an optical switch module. A stably-tunable beat source for continuous wave terahertz spectroscopy system was implemented by simply connecting 16 coaxial distributed feedback laser diodes to an optical switch. The terahertz frequency was rapidly changed without frequency drifts by changing the optical path. The continuous wave terahertz frequency was tuned from 0.05 to 0.8 THz in steps of 50 GHz or 0.4 nm. We measured continuous wave terahertz waveforms emitted from the photomixers using the switched optical beat source. We also calculated the terahertz frequency peaks by taking fast Fourier transforms of the measured terahertz waveforms. By equipping the implemented tunable beat source with an optical switch, a continuous wave terahertz spectroscopy system was constructed and used to demonstrate the feasibility of continuous wave terahertz spectroscopy for nondestructive tests using the spectra of two type of Si wafers with different resistivity.

The extraordinary electronic, thermal and optical properties of graphene, together with its two dimensional nature, and with the possibility of facile integration, have enabled its application into a new generation of high-performance devices capable of extending the performance of existing terahertz communications technologies. Although promising for wireless communications applications, the terahertz region of the spectrum, i.e. the frequency range between 300 GHz and 10 THz, is in fact still characterized by a lack of efficient, compact, solid state components capable of operating at room temperature. In this regard, graphene-based terahertz components have shown very promising results in terms of modulation, detection, as well as generation of terahertz waves. This paper will review and discuss recent progress on graphene based devices for modulation, detection and generation of terahertz waves, which are among the key components for future terahertz band communications systems.

Advances in terahertz technology rely on the combination of novel materials and designs. As new devices are demonstrated to address the terahertz gap, the ability to perform high-efficiency beam control will be integral to making terahertz radiation a practical technology. Here, we use a metasurface composed of nonuniform dielectric resonator antennas on a ground plane to achieve efficient beam focusing at 1 THz. The dielectric resonators are made of high-resistivity silicon, which is a low-loss, nondispersive material for terahertz waves. The resonators operate around the resonance of the displacement current in the silicon, which is crucial to attaining high efficiency. The reflectarray’s capacity to focus terahertz radiation is experimentally verified, and hence by the principle of antenna reciprocity, it can also be employed as a terahertz collimator. The demonstrated device can therefore be deployed for high-gain terahertz antennas. Further measurements show that the loss of the reflectarray is negli...

A polarization dependent dual-band EIT-like effect (0.35 THz and 0.7 THz) has been applied to optimize the FSS-based terahertz devices. The simulated and experimental results show that such devices can act as a polarization dependent narrowband THz wave filter. Under modulation signal, the optical controllable design can be adopted as a flexible polarization modulator. These results can be further applied in polarization controllable terahertz source for nonlinear terahertz optics.

We design an electrically controllable terahertz wave attenuator by using graphene. We show that terahertz wave can be confined and propagate on S-shaped graphene waveguide with little radiation losses, and the confined terahertz wave is further manipulated and controlled via external applied voltage bias. The simulated results show that, when chemical potential changes from 0.03 into 0.05 eV, the extinction ratio of the terahertz wave attenuator can be tuned from 1.28 to 39.42 dB. Besides the simplicity, this novel terahertz wave attenuator has advantages of small size (24 × 30 μm2), a low insertion loss, and good controllability. It has a potential application for forthcoming planar terahertz wave integrated circuit fields.

Terahertz radiation (THzR) consists of electromagnetic waves within the band of frequencies from 0.3 to 3 terahertz with the wavelengths of radiation in the range from 0.1 mm to 1 mm, respectively. The technology for generating and manipulating THzR is still in its initial stage. Herein, we demonstrate that the wrinkled Si1–xGex/Si1–yGey films can be used as radiation sources, which emit electromagnetic waves (EMW) in a very wide range of the frequencies including the terahertz band from 0.3 to 3 THz and far IR from 3 THz to 20 THz. These findings provide the theoretical foundation for the wrinkled nanofilm radiation emission and may allow, to some extent, to fill the terahertz gap.

We demonstrate ultrafast optical switching of electromagnetic waves through a metallic slot antenna in the terahertz frequency regime. The screening of the terahertz field due to the optical excitation of carriers in GaAs/AlGaAs multiple quantum wells is further enhanced due to the strong field confinement near the antenna, which results in the complete suppression of the far-field transmission of terahertz waves. The ultrafast rising time of the carrier population enables picosecond switching of terahertz transmission. A simulation based on a modified Rayleigh diffraction theory combined with the Drude model confirms that the fast decay of electromagnetic field within GaAs is responsible for the ultrafast optical switching.

To extend the usage of the terahertz wave, we present a simple method for variable focus length terahertz wave lens based on graphene. The focus length of the graphene-based terahertz lens can be tunable by changing the applied electric field without change the configuration. To demonstrate the feasibility of the approach, numerical simulation performed with the aid of the finite element method is used to evaluate the terahertz performance of the proposed device. With an appropriate design, the focal length of the proposed device can be tuned from 7.3 μm to 15.2 μm. The total size of the present graphene lens is only 3.5 μm×13 μm. It is believed to be applicable for future communication, imaging and sensing in terahertz range.

The combination of terahertz technology and photonic crystal provides a new approach to realize compact terahertz wave devices. Relying on a conventional photonic crystal waveguide and photonic crystal surface-mode waveguides, a tunable multi-channel terahertz-wave power splitter is proposed. The mechanism of such a power splitter is further theoretically analyzed and numerically investigated with the aid of the plane-wave-expansion method and the finite-difference time-domain method. With an appropriate design, the proposed device can split the input terahertz wave energy equally into six output ports at the frequency of 0.6 THz. When changing the external magnetic field, the input terahertz wave can be equally divided into four output ports with the aid of a magnetic-sensitive material. Furthermore, the present device is very compact and the total size is of 4.4×6.0 mm 2 .

The development of efficient, robust, broadband, and coherent Terahertz (THz) sources is constantly pushing the frontier of THz sciences and technologies, enabling new THz related research and application, especially within the new categories of nonlinear phenomena that require ultra-intense THz radiations. Aside from discussing THz-matter interactions and different THz applications, it is important to introduce how THz waves are generated. This article focuses on broadband THz pulse generations that involve many different ultrafast optical methods.

Terahertz electromagnetic radiation, its generation, detection and material interaction is an emerging field of study. Diverse range of researches is going on to explore and understand this frequency range and its potential applications. Situated between microwave and optical frequency domain, THz radiation is non-responding to the techniques commonly used for these two domains, thus insufficiency of supporting technology, high atmospheric absorption, lack of high power source have kept this spectrum away from technical and commercial interest for a long time. Advancements in Photonics like high power ultrafast lasing devices, high-power THz generation by means of nonlinear effects, band-engineered hetero-structures have greatly extended its potential. In order to explore all its merits and realize it as a full-fledged functional frontier, more high power sources, more sensitive sensors and efficient waveguides need to be developed. In order to achieve that, the main focus of the research world has moved ...

Electromagnetic polarization conveys valuable information for signal processing. Manipulation of terahertz wavelength demultiplexer exhibits tremendous potential in developing application of terahertz science and technology. We propose an approach to separate efficiently four frequencies terahertz waves based on three cascaded directional coupling two-dimensional photonic crystal waveguides. Both plane wave expansion method and finite-difference time-domain method are used to calculate and analyze the characteristics of the proposed device. The simulation results show that the designed terahertz wavelength demultiplexer can split four different wavelengths of terahertz wave into different propagation directions with high transmittance and low crosstalk. The present device is very compact and the total size is 6.8×10.6 mm 2 . This enables the terahertz wavelength demultiplexer to be used in terahertz wave system and terahertz wave integrated circuit fields.

Robust coupling between a pulsed laser beam and a photoelectric circuit is an important issue in the development of miniaturized, integrated, and embedded terahertz instrumentation. Here, we present a study of the effect of varying the focus and alignment parameters of an excitation laser pulse on the emission characteristics of a standard Hertzian- dipole type terahertz photoelectric source. The objective is to quantify the tolerance of a terahertz time-domain spectroscopy system, and we study the variation of peak amplitude, waveform, phase, and energy distribution as a function of excitation position and defocus. We find that a terahertz source can be made relatively tolerant to variations in focus, alignment, and details of the geometry of the photoelectric system, providing a window for a more robust field operation.

Coherent sources of electromagnetic waves in terahertz frequency range is very promising for various applications, including biology and medicine. In this paper we propose a scheme of compact terahertz source, in which terahertz radiation is generated due to effective interaction of electrons in a quantum well with an electromagnetic wave of a corrugated waveguide. We have shown that the generation of electromagnetic waves with a frequency of 1012 s–1 and an output power of up to 25 mW is possible in the proposed scheme.

Terahertz technology is one of the research fronts in the microwave society. Among many technical challenges, achieving high-power terahertz radiation has been attracting many efforts. In this paper, we investigate the possibility of power synthesis at low-end frequencies of the terahertz gap based on discrete sources. We show that by applying precision digital phase control, such a power synthesis can be achieved, overcoming the difficulty of phase alignment at these frequencies. For demonstration, we implement a 110-GHz prototype system employing solid-state impact avalanche and transit time diodes. Using a simulation- and measurement-based design methodology, the impedance matching of the designed cavity is able to be simultaneously obtained at both the RF bias and the 110-GHz frequency without using any absorbing material. Detailed design, simulation, and measurement of the prototype are introduced and the experimental results comply well with analytical expectations. Analysis shows that with increased wide digital bit width, the proposed approach is able to provide sufficient phase control precision, making it possible to be used in the power synthesis applications at low terahertz frequencies.

High power sources and high sensitivity detectors are highly in demand for terahertz imaging and sensing systems. Use of nano-antennas and nano-plasmonic light concentrators in photoconductive terahertz sources and detectors has proven to offer significantly higher terahertz radiation powers and detection sensitivities by enhancing photoconductor quantum efficiency while maintaining its ultrafast operation. This is because of the unique capability of nano-antennas and nano-plasmonic structures in manipulating the concentration of photo-generated carriers within the device active area, allowing a larger number of photocarriers to efficiently contribute to terahertz radiation and detection. An overview of some of the recent advancements in terahertz optoelectronic devices through use of various types of nano-antennas and nano-plasmonic light concentrators is presented in this article.

An investigation of beam propagation properties for terahertz ultrashort pulses emitted from a photoconductive switch antenna is presented. The propagation of terahertz pulsed beams is studied based on the diffraction theory of electromagnetic fields. It is shown that under the assumption that the pulse’s spectrum radiates from a common initiating beam waist, the terahertz temporal pulse shape can be predicted elsewhere without the need for paraxial or far-field approximations. An algorithm is given that enables simple calculation of a terahertz pulse and its spatial distribution, based on simplifications of previous works. A numerical simulation model is presented that predicts spatiotemporal behavior and shows excellent agreement with experimental results. The presented algorithm provides a practical tool for many relevant applications. It also assists in designing and understanding practical pulsed terahertz laboratory setups based on photoconductive switches and can be generalized to predict t...

The terahertz high rate wireless communication system plays an important role in terahertz technology. There are two typical occasions for terahertz communication to be applied, the satellite communication and short-range communication on the ground. In this paper, the link budget calculations of terahertz communication in different scenes have been finished, which prove the terahertz wave source is the key of terahertz system. Among the terahertz source devices, mixer and resonant tunneling oscillator are more appropriate for terahertz communication in space and indoors. The simulation and analysis of sub-harmonic mixer and resonant tunneling oscillator characteristics is discussed according to different application scenes. At last, the prospect of terahertz communication is given.

The recently revealed special Smith-Purcell radiation (S-SPR) is quite attractive since it has better monochromaticity and higher intensity in comparison with the ordinary SmithPurcell radiation. It offers effective ways of generating terahertz radiations. Based on the S-SPR mechanism, we proposed and investigated a series of terahertz sources. They substantially enhanced the operation frequency and radiation power, which are main defects of traditional vacuum electronic devices and Smith-Purcell free-electron-lasers in the terahertz region. These proposed schemes can promisingly be developed as compact and high power terahertz sources in practice.

Terahertz metasurface absorption materials, which absorb terahertz wave through subwavelength artificial structures, play a key role in terahertz wave shielding and stealth technology, etc. However, most of the metasurface absorption materials in terahertz suffer from limited tuning range and narrow incident angle characteristics. Here, we demonstrate a liquid-metal-based metasurface through microfluidic technology, which functions as a terahertz absorption material with broadband tunability and wide-angle features. The proposed terahertz metasurface absorption material exhibits an experimental tuning range from 0.246 THz to 0.415 THz (the tuning range of central frequency reaches 51.1%), and the tuning range maintains at high level with wide-angle response up to 60°. NRF (Natl Research Foundation, S’pore) Published version

We present a novel photoconductive terahertz detector, which offers significantly higher detection bandwidths and sensitivities compared to the state-of-the art. The detector is based on 2D plasmonic nano-antenna arrays, which are designed to concentrate a major fraction of an incident terahertz and optical pump beam at the same nanoscale regions in their close proximity. When the optical pump beam excites the detector, a large number of photocarriers are drifted to the plasmonic nano-antennas by the induced terahertz field to form the output current of the detector. The nano-antennas are also designed to offer high terahertz electric field enhancement factors over a broad terahertz frequency range. As a result, high-sensitivity and broadband operation is achieved simultaneously. We experimentally demonstrate pulsed terahertz detection with record-high signal-to-noise ratios as high as 107 dB over a 5 THz frequency range, which is 50 times higher compared to the state-of-the art.

We present a novel photoconductive terahertz emitter, which offers significantly high terahertz radiation power levels through three-dimensional light confinement near terahertz radiating elements. Arrays of plasmonic nano-antennas fabricated on a photo-absorbing semiconductor substrate are used as the terahertz radiating elements. The plasmonic nano-antenna arrays are designed to offer high radiation resistance over a broad terahertz frequency range. An optical reflector layer is embedded inside the substrate to spatially confine and absorb a major portion of an incident optical pump beam near the plasmonic nano-antennas. Therefore, very efficient ultrafast photocurrent can be generated and coupled to the plasmonic nano-antennas for high-efficiency terahertz radiation generation. We experimentally demonstrate record-high terahertz radiation powers as high as 11.4 mW over 0.1–5 THz frequency range with 2.3% optical-to-terahertz conversion efficiency.

In the first part of this talk, I will give an overview of the unique applications of terahertz waves for chemical identification, material characterization, biomedical sensing and diagnostics and describe the state of the existing terahertz sensors. In the second part of the talk, I will introduce a game changing technology that enables high performance, low cost, and compact terahertz spectroscopy and imaging systems for various applications. More specifically, I will introduce a new generation of optically driven terahertz sources and detectors that offer three orders of magnitude higher terahertz radiation power levels and two orders of magnitude higher terahertz detection sensitivity levels compared to the existing technologies. This leap-frog performance enhancement is achieved by funneling the laser light through specifically engineered metallic nanostructures into the device active area, enhancing light matter interaction at nanoscale. Moreover, this technology is optimized for operation at near infrared optical wavelengths, where very high performance, compact and cost-effective optical sources are commercially available, paving the way to compact and low-cost terahertz sensors that could offer numerous opportunities for e.g., medical imaging and diagnostics, atmospheric sensing, pharmaceutical quality control, and security screening systems.

Heterodyne terahertz spectrometers are highly in demand for space explorations and astrophysics studies. A conventional heterodyne terahertz spectrometer consists of a terahertz mixer that mixes a received terahertz signal with a local oscillator signal to generate an intermediate frequency signal in the radio frequency (RF) range, where it can be easily processed and detected by RF electronics. Schottky diode mixers, superconductor-insulator-superconductor (SIS) mixers and hot electron bolometer (HEB) mixers are the most commonly used mixers in conventional heterodyne terahertz spectrometers. While conventional heterodyne terahertz spectrometers offer high spectral resolution and high detection sensitivity levels at cryogenic temperatures, their dynamic range and bandwidth are limited by the low radiation power of existing terahertz local oscillators and narrow bandwidth of existing terahertz mixers. To address these limitations, we present a novel approach for heterodyne terahertz spectrometry based on plasmonic photomixing. The presented design replaces terahertz mixer and local oscillator of conventional heterodyne terahertz spectrometers with a plasmonic photomixer pumped by an optical local oscillator. The optical local oscillator consists of two wavelength-tunable continuous-wave optical sources with a terahertz frequency difference. As a result, the spectrometry bandwidth and dynamic range of the presented heterodyne spectrometer is not limited by radiation frequency and power restrictions of conventional terahertz sources. We demonstrate a proof-of-concept terahertz spectrometer with more than 90 dB dynamic range and 1 THz spectrometry bandwidth.

One of the main limitations for realizing high-performance time-domain terahertz imaging and spectroscopy systems is the low responsivity and narrow bandwidth of the existing pulsed terahertz detectors. In this work, we present a high-responsivity and broadband large-area terahertz detector that incorporates a two-dimensional array of plasmonic nanoantennas fabricated on a low-temperature-grown GaAs substrate. By using a large-area device architecture, large optical spot sizes can be used, mitigating the carrier screening effect at high optical pump powers. Using a large-area device architecture also makes the device less sensitive to changes in optical and terahertz alignment. The two-dimensional array of plasmonic nanoantennas is designed to offer a broad terahertz detection bandwidth. It is also designed to enhance optical absorption in close proximity to the nanoantennas by exciting surface plasmon waves. This allows drifting a large portion of photo-generated electrons and holes to the nanoantennas in presence of an incident terahertz pulse, offering high responsivity levels. We experimentally demonstrate detection of terahertz pulses with more than 5 THz bandwidth with high responsivity and signal-to-noise ratio levels exceeding that of electro-optic detectors. Such terahertz detectors would play a critical role in realization of the next generation time-domain terahertz imaging and spectroscopy systems.

Background As a significant terahertz functional device, terahertz beam splitters with high performance are highly required to meet the need for terahertz communication, terahertz image, and terahertz sensor systems. Results The proposed 1×6 power splitter becomes 1×4 power splitter with the aid of the localized temperature change at the frequency of 1.0THz. The total output power is equivalent to 97.8% of the input power for the six-channel splitter and 95.4% for the four-channel splitter. The dimension of the device is of 35a×27a. Conclusions A temperature-controlled terahertz power divider based on photonic crystal multimode interference structure and Y-junction photonic crystal waveguides is an efficient mechanism for the power divider of terahertz waves. The proposed device paves a promising way for the realization of terahertz wave integrated device.

Terahertz technology is getting fast development in scientific research and the characteristics of terahertz beam is of great importance when using a terahertz laser. In this paper we scan the THz beam along its diameter by a slit and a circular aperture to measure the THz beam’s power distribution and thus get its spot’s size. The results show that the beam spot of the THz source we employed is satisfied with Gaussian distribution. The value of the peak power would affect the determination of the spot boundary, the influence of environmental noise will increases when the the peak power become weak and the measured spot diameter will be too large ultimately.

A non-invasive, unstaining tissue measurement method is expected to be an important tool for regenerative medical. As THz wave do not affect biological tissue because of their low energy, THz measurement method is expected to become a new modality of biomedical analysis as a noninvasive diagnosis. Due to the fact that biological tissues possess high level of hydration, it results in strong absorption at terahertz frequencies. A ridge waveguide LiNbO3 based nonlinear terahertz generator was used to achieve high output power for THz time domain spectroscopy (THz-TDS). A ridge waveguide was designed for high efficiency emission from the LiNbO3 crystal by the electro-optic Cherenkov effect. This THz-TDS system has realized six orders of dynamic range, and the bandwidth of the spectrum reaches 7 THz in the upper limit. We measured a reflected terahertz pulse shape at the interface between a plastic culture dish and biological tissues. By studying the gradient of phase spectroscopy of reflected THz pulse, we have been able to differentiate between human fibroblast tissue and cancer tissue. We demonstrate the application of terahertz time domain spectroscopy pulse in reflection geometry for the non-distractive measurement of biological tissues cultured on a plastic culture dish. These results demonstrate the potential of terahertz phase information for the study of biological tissues.

We proposed a novel planar terahertz (THz) plasmonic waveguide with folded stub arrays to achieve excellent terahertz propagation performance with tight field confinement and compact size based on the concept of spoof surface plasmon polaritons (spoof SPPs). It is found that the waveguide propagation characteristics can be directly manipulated by increasing the length of the folded stubs without increasing its lateral dimension, which exhibits much lower asymptotic frequency of the dispersion relation and even tighter terahertz field confinement than conventional plasmonic waveguides with rectangular stub arrays. Based on this waveguiding scheme, a terahertz concentrator with gradual step-length folded stubs is proposed to achieve high terahertz field enhancement, and an enhancement factor greater than 20 is demonstrated. This work offers a new perspective on very confined terahertz propagation and concentration, which may have promising potential applications in various integrated terahertz plasmonic circuits and devices, terahertz sensing and terahertz nonlinear optics.

Metamaterials can give rise to unprecedented refractive indices and drive the rapid development of metadevices with on-demand electromagnetic properties. Recent advances in terahertz science demand high-performance optical elements beyond conventional designs of naturally occurring materials in the terahertz wave band. However, how an epsilon-near-zero (ENZ) structure can exploit terahertz metadevices is still not fully demonstrated based on a physical analysis. Here, inspired by the ENZ concept, we demonstrate a design guideline of a terahertz ENZ cut-through metal-slit array antenna. Measurements by a terahertz imager visualize the beam profile of a terahertz wave, and the measured permittivity of 0.26 agrees well with that of 0.27 obtained by simulation and theory. The terahertz ENZ antenna provides a wide range of potential applications such as high-directivity antennas, beam dividers, beam-steering elements, phase-control devices, and novel filters.

The terahertz frequency region of the electromagnetic spectrum lies between the lower frequency electronic and the higher frequency photonic regions. It is an area of vast scientific and technological interest and potential but is relatively unexplored due to the technical challenges involved in producing efficient, inexpensive, robust, compact, and reliable sources and detectors of terahertz radiation. Terahertz imaging is accomplished by raster scanning or by using focal-plane arrays. Terahertz tomography has also been demonstrated. Terahertz spectroscopy is accomplished using either tunable sources or broadband sources and dispersive spectrometers. The recent development of time-domain terahertz spectroscopy is revolutionizing the field.

For terahertz (THz) wave generators based on organic electrooptic crystals, their intrinsic phonon modes are playing an essential role in THz generation characteristics. Here, this study proposes an effective design strategy for THz phonon mode engineering of organic electrooptic salt crystals for efficient optical-to-THz frequency conversion. To reduce phonon-mode intensity, strongly electronegative trifluoromethyl group acting as strong hydrogen-bond acceptor is incorporated into molecular anions. New 2-(4-hydroxy-3-methoxystyryl)-1-methylquinolinium 4-(trifluoromethyl)benzenesulfonate (HMQ-4TFS) crystals exhibit a relatively small absorption coefficient in the THz spectral range between 0.5 and 4 THz, which is attributed to suppressed molecular vibrations due to strong hydrogen bonds involving the 4TFS anion. In addition, HMQ-4TFS crystals possess a very large macroscopic optical nonlinearity, comparable (or even higher) to benchmark stilbazolium crystals. Based on the low-intensity THz phonon modes and the large optical nonlinearity, a 0.37 mm thick HMQ-4TFS crystal pumped with 150 fs infrared laser pulses facilitates very efficient THz wave generation by optical rectification, delivering 23 times higher peak-to-peak THz electric field than the widely used standard inorganic ZnTe crystal (1.0 mm thick) and a broader spectral bandwidth. Therefore, strongly electronegative groups introduced into molecular salt electrooptic crystals provide a very promising design strategy of THz phonon mode engineering for developing intense broadband THz sources.

Terahertz band lying between infrared and microwave is a very promising part of electromagnetic spectrum for spectroscopy, space applications and different kind of imaging applications such as medical and security. Recently, this special band has attracted the attention of communication society because of its huge bandwidth and therefore ability of very high speed wireless communications. One of the challenges still on the table is to obtain efficient terahertz sources. In this paper, recent studies on the topic of efficient terahertz sources are reviewed comparatively. Especially for the communication concept, taking into account the efficiency, it can be said that the trend goes to the optical photoconductive materials method including plasmonic electrodes.

An innovative external cavity dual-mode laser has been developed for continuous-wave terahertz generation via photomixing. Photomixing is a method of generating terahertz radiation at the beat frequency developed by two input optical light modes. Dual-mode lasers are an intriguing option for photomixing sources because the differential frequency produced is stabilized by the common mode rejection ratio effect. The laser presented here utilizes an acousto-optic tunable filter for frequency selection, which creates an ultra-wideband tunable range and provides the potential for tuning speed on the order of microseconds. The unique design of this laser enables 100% solid state operation resulting in a robust and compact system for applications in terahertz imaging or spectroscopy. Operating at around 1550 nm, this laser is capable of producing differential frequencies between 300 GHz and 18 THz with a side-mode-suppression-ratio of 50 dB and peak amplitudes of over 0 dBm. The device presented here represents a significant achievement in the advancement of dual mode lasing technology for tunable terahertz generation with high phase and amplitude stability.

Monte-Carlo simulations are used to show that the transit time in ∼100 nm gap photoconductive emitters of terahertz radiation is short enough to produce broad-bandwidth pulses. Furthermore, with these calculations we demonstrate that nanostructured contacts remove the need for low-temperature-grown or ion-implanted materials for broad-band terahertz devices.

Compared to the visible and infrared regions of the electromagnetic spectrum, development of optical components in the terahertz region has progressed slowly even though the feature size of structures makes many of them easily attainable by many conventional manufacturing techniques. Of these, the performance of components with machined metal surfaces, such as filters, typically has suffered from the errors and inconsistencies in the manufacturing which has led many to manufacture these using more expensive deposition and processing tools. Here we show that by using a novel, high power Yb:doped pulsed nanosecond fiber laser system with exceptional beam quality, aluminum metal surfaces can be machined with high precision leading to a high quality band pass filter working in the terahertz frequency range. The filter was laser machined on a 0.05 mm thick aluminum substrate over a 1 cm $$^2$$ area where the unit cell in the pattern had an equilateral triangular geometry in which the base length between the holes and the diameters were machined with better than 4 % accuracy. The produced structures are modeled by utilizing the obtained structural parameters in a waveguide configuration and then characterized by the existing home-built time-domain terahertz spectrometers in our laboratories. The results show a near 100 % power transmission at the desired terahertz frequency range which suggests that these manufacturing techniques can be used to produce low-cost THz filters.

We present a photoconductive terahertz emitter that incorporates three-dimensional plasmonic contact electrodes to offer record high optical-to-terahertz power conversion efficiencies. By use of three-dimensional plasmonic contact electrodes the majority of photocarriers are generated within nanoscale distances from the photoconductor contact electrodes and drifted to the terahertz radiating antenna in a sub-picosecond time-scale to efficiently contribute to terahertz radiation. We experimentally demonstrate 105 μW of broadband terahertz radiation in the 0.1-2 THz frequency range in response to a 1.4 mW optical pump, exhibiting a record high optical-to-terahertz power conversion efficiency of 7.5%.

The terahertz radiation, also known as THZ light or T-rays, lies between the microwave and infrared regions of the electromagnetic spectrum typically defined as 0.1–10 THZ. It is strongly attenuated by water and very sensitive to water content. Terahertz radiation has very low photon energy and thus it does not pose any ionization hazard for biological tissues. These unique features make terahertz imaging very attractive for dental applications in order to provide complimentary information to existing imaging techniques. There has been an increasing interest in terahertz imaging of biologically related applications within the last few years and more and more terahertz spectra are being reported. This article highlights terahertz technology and provides a short review of recent advances in terahertz imaging.

Bridging of the terahertz gap in the electromagnetic spectrum between the microwave and infrared ranges requires a variety of new technological developments from basic elements, such as emitters and detectors, to complete systems, such as spectrum analyzers and imagers. As an example of these developments, Hilbert-transform spectral analysis of terahertz radiation sources has been demonstrated. A spectrum analyzer based on a high- $T_{c}$ square-law Josephson detector has been developed and characterized in the frequency range from 50 to 1800 GHz. Spectra of output terahertz radiation from optically-pumped lasers and frequency multipliers have been studied, and their regimes were optimized for a single-frequency operation. Starting from the optimized multipliers, a polychromatic source has been synthesized and characterized with Hilbert-transform spectrum analyzer.

The active control of matter by strong electromagnetic fields is of growing importance, with applications all across the optical spectrum from the extreme-ultraviolet to the far-infrared. In recent years, phase-stable terahertz fields have shown tremendous potential for observing and manipulating elementary excitations in solids. In the gas phase, on the other hand, driving free charges with terahertz transients provides insight into ultrafast ionization dynamics. Developing such approaches for locally enhanced terahertz fields in nanostructures will create new means to govern electron currents on the nanoscale. Here, we use single-cycle terahertz transients to demonstrate extensive control over nanotip photoelectron emission. The terahertz near-field is shown to either enhance or suppress photocurrents, with the tip acting as an ultrafast rectifying diode6. We record phase-resolved sub-cycle dynamics and find spectral compression and expansion arising from electron propagation within the terahertz near-field. These interactions produce rich spectro-temporal features and offer unprecedented control over ultrashort free electron pulses for imaging and diffraction.

This paper studies laser air-photonics used for remote sensing of short pulses of electromagnetic radiation at terahertz frequency. Through the laser ionization process, the air is capable of generating terahertz field strengths greater than 1 MV/cm, useful bandwidths over 100 terahertz, and highly directional emission patterns. Following ionization and plasma formation, the emitted plasma acoustic or fluorescence can be modulated by an external terahertz field to serve as omnidirectional, broadband, electromagnetic sensor. These results help to close the “terahertz gap” once existing between electronic and optical frequencies, and the acoustic and fluorescence detection methodologies developed provide promising new avenues for extending the useful range of terahertz wave technology. Our experimental results indicate that by hearing the sound or seeing the fluorescence, coherent detection of broadband terahertz wave at remote distance is feasible.

We present an ultra-broadband metamaterial absorber in the terahertz regime based on the proposed 4 x 4 cascaded metal-dielectric pairs, which would help to reduce the demand for fabrication precision. The generation mechanism of electromagnetic resonances and the formation process of the ultra-broadband absorption were investigated by finite-difference time-domain (FDTD) method. Numerical results showed that absorptivity of similar to 92 % was obtained between 3.2 and 11.8 THz using the 4 x 4 structure. Such absorber was one of potential candidates for rough terahertz frequency estimator with an average detection resolution of 0.56 THz within a 9-THz range, which could be a supplement to the active terahertz spectrum analyzers with high resolution but limited measurement range (similar to 1 THz).

We report the generation of 3.3-mW single-cycle terahertz (THz) pulses at 1-kHz repetition rate via optical rectification in MgO-doped prism-cut stoichiometric LiNbO3. Efficient pulse-front tilting of 800-nm pulses was realized by an optimized single-lens focusing scheme for radially-symmetric propagation of THz beams. In this geometry, nearly-diffraction-limited THz Gaussian beams with electric field strength as high as 350 kV/cm were generated. The pump-to-THz energy conversion efficiency of $1.36{\times}10^{-3}$ and the extremely high signal-to-noise ratio of ~1:15000 achieved are among the best results for 1-kHz single-cycle terahertz pulse generation ever demonstrated in room temperature operation.