Active terahertz metamaterial devices

The ever increasing demand for bandwidth in wireless communication systems will inevitably lead to the extension of operating frequencies toward the terahertz (THz) band known as the 'THz gap'. Towards closing this gap, we present a multi-level amplitude shift keying (ASK) terahertz wireless communication system using terahertz spatial light modulators (SLM) instead of traditional voltage mode modulation, achieving higher spectral efficiency for high speed communication. The fundamental principle behind this higher efficiency is the conversion of a noisy voltage domain signal to a noise-free binary spatial pattern for effective amplitude modulation of a free-space THz carrier wave. Spatial modulation is achieved using an an active metamaterial array embedded with pseudomorphic high-electron mobility (pHEMT) designed in a consumer-grade galium-arsenide (GaAs) integrated circuit process which enables electronic control of its THz transmissivity. Each array is assembled as individually controllable tiles for transmissive terahertz spatial modulation. Using the experimental data from our metamaterial based modulator, we show that a four-level ASK digital communication system has two orders of magnitude improvement in symbol error rate (SER) for a degradation of 20 dB in transmit signal-to-noise ratio (SNR) using spatial light modulation compared to voltage controlled modulation.

Terahertz (THz) radiation has a frequency of the order of 10<SUP>12</SUP>12 Hz. This lies between the infrared and microwave regions of the electromagnetic spectrum; a section labeled the 'THz gap'. Infrared and microwave radiation is used in the medical field; research is underway for an application for THz radiation. At present no formal safety analysis of a THz pulsed imaging (TPI) system has been performed. This will be necessary for future in vivo studies. The radiation is delivered in a train of femtosecond pulses. International guidelines on exposure to non-ionizing radiation, and supporting literature, were reviewed to determine the Maximum Permissible Exposure (MPE) for radiation of this range of wavelengths, both for a single pulse and continuous wave exposure. Two methods of deriving the MPE were identified. Current guidelines for infrared and microwave regions of the electromagnetic spectrum incorporate the THz region. Using conservative parameter estimation an MPE per pulse, over the area of the beam, of 94 W was calculated. At present THz pulsed imaging systems produce pulses of power approximately 1 mW; this lies within the limit calculated using the published guidelines. There are, however, areas requiring further investigation before the technique becomes routine in clinical practice.

We have demonstrated the phase-locking of a THz quantum cascade laser (THz-QCL) toward the realization of an accurate and stable local oscillator for a high-resolution receiver. The beat note between the THz-QCL and a THz reference was obtained by heterodyne mixing in a superconducting hot electron bolometer mixer (HEBM) and used for stabilizing the phase of 3.1 THz radiation from the THz-QCL. The phase-locked 3.1 THz radiation was fully evaluated with a superlattice harmonic mixer operating in the THz band, and this revealed that the THz-QCL synchronized with the microwave reference with a fractional frequency instability of 3×10 -15 at an averaging time of 100 s, corresponding to a center frequency deviation within 1 mHz, and the imposed phase noises during the heterodyne mixing were negligibly small.

Graphene nano-optics at terahertz (THz) frequencies (ν) is theoretically anticipated to feature extraordinary effects. However, interrogating such phenomena is nontrivial, since the atomically thin graphene dimensionally mismatches the THz radiation wavelength reaching hundreds of micrometers. Greater challenges happen in the THz gap (0.1-10 THz) wherein light sources are scarce. To surpass these barriers, we use a nanoscope illuminated by a highly brilliant and tunable free-electron laser to image the graphene nano-optical response from 1.5 to 6.0 THz. For ν < 2 THz, we observe a metal-like behavior of graphene, which screens optical fields akin to noble metals, since this excitation range approaches its charge relaxation frequency. At 3.8 THz, plasmonic resonances cause a field-enhancement effect (FEE) that improves the graphene imaging power. Moreover, we show that the metallic behavior and the FEE are tunable upon electrical doping, thus providing further control of these graphene nano-optical properties in the THz gap.

The terahertz (THz) region of the electromagnetic spectrum spans frequencies roughly from 300 GHz to 20 THz, corresponding to wavelengths from 1 mm to 15 μm. Although this range overlaps the vibrational levels of many materials (for example, phonon modes of semiconductors), difficulties in developing adequate sources have slowed its exploitation, despite promising applications in imaging, chemistry, biology, and materials science. Recent progress in sources driven by both lasers and electron beams has started to fill this “terahertz gap.” Accelerator-based sources are now opening a new high-field regime with the potential to control materials with THz radiation by resonantly driving bonds, biasing a material with ultrashort electric or magnetic fields, or using the light as a catalyst. Some new sources can produce pulsed fields of sufficient intensity to drive nonlinear responses in matter.

Single-cycle THz pulses with electric field strength of MV/cm are required for wide range of applications from physics to biology and medicine [1]. Presently, high fields are still challenging to produce in the THz gap (0.1-10 THz) where important condensed matter resonances are expected (i.e. magnons, phonons, electromagnons). We present our latest results on the generation of 1.5 MV/cm electric field in this frequency range. Broadband THz pulse are achieved by optical rectification (OR) in large organic crystals such as DAST, OH1 and DSTMS [2-4]. These materials present an OR susceptibility one order of magnitude larger than the room-temperature inorganic nonlinear crystals. The THz radiation is phase-matched with IR pump allowing for photon conversion larger than 200% and pulse energy of tens of μJ. The beam can be focused to nearly diffraction-limit for the realization of the highest field. For 0.5 mm thick DSTMS, electric field larger than 1.5 MV/cm is achieved in 350 μm FWHM spot. The electro-optical sampling trace, the corresponding 5-octave spectrum (0.15-5 THz) and the THz focus are shown in Fig. 1. Fields of the 1 MV/cm are obtained also in DAST and OH1. We show that proper dispersive materials can be used to control absolute phase of the THz transient without significant losses.

Among all kinds of electromagnetic waves, terahertz (THz) waves are those with frequencies varying from 0.1 to 10 THz (with wavelengths ranging from 3000 to 30x10-6 m). Millimeter waves in the long wavelength range overlap with electromagnetic waves, as well as infrared light in the short wavelength range. The THz gap in the electromagnetic spectrum is the crossover between electronics and photonics, and between macroscopic classical theory and microscopic quantum theory. The THz wave band has the same characters as spectrum of substances such as semiconductors, plasmas, organisms, and biomolecules, having the characteristics of quantum energy and low blackbody temperature. The time-domain spectral signal-to-noise ratio of THz is very high, making it excellent for applications involving images. Because of the extremely large instantaneous bandwidth (0.1-10THz), high-speed transmission is possible. Therefore, utilizing THz can deepen and broaden humans’ knowledge of some fundamental scientific issues in physics, chemistry, astronomy, informatics, and life sciences. At present, technology related to THz wave is widely used in temperature sensors, biosensors, communication, astronomy, and safety inspection fields.

Not long ago, the terahertz frequency range was considered the last remaining gap of the electromagnetic spectrum. The main reason was the difficulty of generating intense, directional terahertz radiation. During the past few years, optoelectronic approaches based on modern laser technologies have helped to bridge the “terahertz gap”. This report discusses some of the scenarios in which terahertz technologies are just about to make their industrial entrance — in applications as varied as trace gas detection, process control, and the imaging of airbag safety covers.

A tunable continuous-wave source of a far-infrared and THz radiation based on a semiconductor nanoheterostructure with asymmetric quantum wells is suggested. It utilizes Rabi oscillations at a transition between quantum well subbands excited by external femtosecond pulses of a mid-infrared electromagnetic field. Due to quantum well broken inversion symmetry the subbands possess different average dipole moments, which enables the creation of polarization at the Rabi frequency as the subband populations change. It is shown that if this polarization is excited so that it is periodic in space, then, though being pulsed, it can produce continuous-wave output radiation. Changing the polarization space period and the time intervals between the exciting pulses, one can tune the frequency of this radiation throughout the far-infrared and THz range. In the present work a concrete multiple quantum well heterostructure design and a scheme of its space-periodic polarization are suggested. It is shown that for existing sources of mid-infrared femtosecond pulses the proposed scheme can provide a continuous-wave output power of order the power of far-infrared and THz quantum cascade lasers. Being added to the possibility of its output frequency tuning, this can make the suggested device attractive for fundamental research and various applications.

Strong, monochromatic, coherent and continuous terahertz (THz) radiation was generated from the intrinsic Josephson junctions in a cylindrical stand-alone mesa sandwich structure fabricated from a single crystal of the high-temperature superconductor Bi2Sr2CaCu2O8+δ. By varying the base temperature and the dc bias current-voltage characteristic (IVC) points, the emission frequency is tunable from 0.5 to a record high 2.4 THz observed on two inner IVC branch points. Strong emission power peaks were observed at 1.0 THz and 1.6 THz. This device is hence an excellent candidate to fill the “THz gap” between ∼1.4 and 2.0 THz.

Terahertz (THz) technology is becoming a spotlight of scientific interest due to its promising myriad applications including imaging, spectroscopy, industry control and communication. However, one of the major bottlenecks for advancing this field is due to lack of well-developed solid-state sources and detectors operating at THz gap which serves to mark the boundary between electronics and photonics. Here, we demonstrate exceptionally wide tunable terahertz plasma-wave excitation can be realized in the channel of micrometer-level graphene field effect transistors (FET). Owing to the intrinsic high propagation velocity of plasma waves (>~108 cm/s) and Dirac band structure, the plasma-wave graphene-FETs yield promising prospects for fast sensing, THz detection, etc. The results indicate that the multiple guide-wave resonances in the graphene sheets can lead to the deep sub-wavelength confinement of terahertz wave and with Q-factor orders of magnitude higher than that of conventional 2DEG system at room temperature. Rooted in this understanding, the performance trade-off among signal attenuation, broadband operation, on-chip integrability can be avoided in future THz smart photonic network system by merging photonics and electronics. The unique properties presented can open up the exciting routes to compact solid state tunable THz detectors, filters, and wide band subwavelength imaging based on the graphene-FETs.

DSTMS organic electro-optic crystal is a very promising and efficient THz-wave generation material. It allows for both very high THz electric fields generation, exceeding several GV/m using optical rectification of femtosecond laser pulses, as well as the possibility for extremely broad bandwidth extending well beyond 10 THz [1]. However, the usual laboratory THz systems based on these crystals exploit very bulky and expensive fs laser systems [2], which are not desired for industrial THz imaging and spectroscopy applications [3,4]. Recently, some compact system employing organic crystals were demonstrated [5], however they use a conventional photoconductive antenna as a THz-wave detector. Here we present a compact THz time-domain spectroscopy system based on a relatively low-cost and small-size femtosecond fiber lasers operating at 1560 nm. As the THz generation and detection material, we choose the organic crystal DSTMS (4-N,N-dimethylamino-4‘-N‘-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate), since it reaches the highest overall efficiency and the flattest spectrum among the presently known state-of-the-art organic crystals (Figure 1).

The terahertz (THz) gap lying between the microwave and optical parts of the electromagnetic spectrum has attracted immense attention due to its applications between radiofrequency (RF)/microwave and photonic systems. A structure consisting of a graphene sheet sandwiched between two graphene disks is introduced herein to control the reflection, absorption, and transmission of THz waves. The proposed metamaterial structures are designed analytically using transmission line theory. Also, the dimensions of the structure and the electrical gating of the graphene are optimized utilizing a genetic algorithm. The structure is simulated using two different methods: (1) a circuit model based on transmission line theory and (2) commercial full-wave software based on the finite element method, which are verified by the agreement between their results. Finally, the proposed method is used to design a THz filter and THz wave absorber, which are in great demand for application in modulators, sensors, detectors, and imaging systems.

Transmission of a normally incident plane wave through a pixelated metasurface with meta-atoms made up of graphene-patched and InSb-patched pixels on a silicon substrate was simulated using CST Microwave StudioTM2019 software. This metasurface is electrically controllable due to the graphene-patched pixels, and it is magnetically as well as thermally controllable due to the InSb-patched pixels. A tricontrollable stopband with minimum transmittance as low as 0.05 can be achieved for operation in the terahertz gap.

The Terahertz gap in EM spectrum has found potential applications in high data rate communication, electronic materials, spectroscopy, medicines, biosciences, space research and remote sensing of public and strategic areas. For the generation of THz radiation with medium power many compact vacuum microelectronics/ MEMS technology based devices such as TWTs, Klystrons etc are being tried out. CEERI jointly with Technical University Berlin has been pursuing design and development of W-Band folded Waveguide TWT. Folded Waveguide structure has been chosen because of its robust construction, high power handling capability, simpler coupling and reasonable wide bandwidth. Design of W-band Folded Wave Guide RF structure has been completed using CST-MWS <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> and GdfidL <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> simulation codes. As well in house developed SUNRAY 1-D3Large Signal Code. Simple Waveguide Coupler with reference to its construction through micro-EDM as well as LIGA techniques has been designed with the excellent cold test performance. Initially a moderate circuit length (55mm) has been taken with reasonable gain.