Free-space Terahertz Radiation Research Articles

Terahertz Spoof Surface Plasmonic Logic Gates.

SummaryLogic gates are important components in integrated photonic circuitry. Here, a series of logic gates to achieve fundamental logic operations based on linear interference in spoof surface plasmon polariton waveguides are demonstrated at terahertz frequencies. A metasurface-based plasmonic source is adopted to couple free-space terahertz radiation into surface waves, followed by a funnel-shaped metasurface to efficiently couple the surface waves to the waveguides built on a domino structure. A single Mach-Zehnder waveguide interferometer can work as logic gates for four logic functions: AND, NOT, OR, and XOR. By cascading two such interferometers, NAND and NOR operations can also be achieved. Experimental investigations are supported by numerical simulations, and good agreement is obtained. The logic gates have compact sizes and high intensity contrasts for the output “1” and “0” states. More complicated functions can be envisioned and will be of great value for future terahertz integrated computing.

Terahertz Transmission Characteristics of Free-Standing Fractal Jesus-Cross Structure

We have fabricated Jesus-cross structure on aluminum foil by using the femtosecond laser technique. By using terahertz time-domain spectroscopy (THz-TDS) system, the transmission properties of free-standing double-layer crossing fractal structure are tested. The parameters of the proposed structure were optimized by using finite element frequency domain technology of commercial software CST Microwave Studio package. The dimensions of the aluminum foil periodically patterned with crossing fractal structure are 1.5×1.5cm2. The resonant frequencies of the proposed structure are 0.216THz and 0.735THz with 3-dB bandwidths of 62GHz and 15GHz, respectively. The transmission ratio can reach to 0.89 and 0.57. It indicates the structure having dual-band filtering performance. This work has the potential to open a new avenue as a filter working for free-space terahertz radiation.

Muti-band terahertz modulator based on double metamaterial/perovskite hybrid structure

We proposed a novel multi-frequency-band terahertz modulator working for free-space terahertz radiation. The unit cell of the metamaterial/perovskite hybrid structure, which consists of split-ring resonator, is patterned on silica with a dielectric constant of ε=3.75, a loss tangent of tan δ=0.01 and concentric double rings are fabricated on the bottom of the silica with the thickness of 200 nm. The split gaps of split-ring resonators are filled with perovskite quantum dots thin film. The terahertz wave modulation mechanism of our structure is numerically investigated by using CST Microwave Studio. We have also employed an equivalent circuit model (ECM) to validate our numerically simulated observations. The results exhibit four different frequency carrier wave signal modulation at 0.42THz, 0.83THz, 1.18THz and 1.62THz with modulation depth (>99%) and megahertz modulation speed. This work demonstrates a new approach for realizing multi-band terahertz amplitude shift keying (ASK) modulator for terahertz wavelength division multiplex (WDM) communication.

Broadband Terahertz Power Detectors Based on 90-nm Silicon CMOS Transistors With Flat Responsivity Up to 2.2 THz

We present broadband high sensitivity terahertz (THz) detectors based on 90 nm CMOS technology with the state-of-the-art performance. The devices are based on bow-tie and log-spiral antenna-coupled field-effect transistors (FETs) for the detection of free-space THz radiation (TeraFETs). We report on optimized performance, which was achieved by employing an in-house developed physics-based model during detector design and thorough device characterization under THz illumination. The implemented detector with bow-tie antenna design exhibits a nearly flat frequency response characteristic up to 2.2 THz with an optical responsivity of 45 mA/W (or 220 V/W). We have determined a minimum optical noise-equivalent power as low as 48 pW/ $\sqrt {\textsf {Hz}}$ at 0.6 THz and 70 pW/ $\sqrt {\textsf {Hz}}$ at 1.5 THz. The results obtained at 1.5 THz are better than the best narrowband TeraFETs reported in the literature at this frequency and only up to a factor of four inferior to the best narrowband devices at 0.6 THz.

Terahertz Metasurface Quantum-Cascade VECSELs: Theory and Performance

Achieving both high power and high-quality beam pattern has been a longstanding challenge for terahertz (THz) quantum-cascade (QC) lasers largely due to their use of subwavelength metallic waveguides. Recently, the vertical-external-cavity surface-emitting laser (VECSEL) concept was demonstrated for the first time in the THz range and for a QC-laser. This is enabled by the development of an amplifying metasurface reflector capable of coupling incident free-space THz radiation to the QC-laser material such that it is amplified and reradiated. The THz metasurface QC-VECSEL initiates a new approach for making QC-lasers with high power and excellent beam pattern. Furthermore, the ability to engineer the electromagnetic phase, amplitude, and polarization response of the metasurface enables lasers with new functionality. This paper provides an overview of the fundamental theory, design considerations, and recent results for high-performance THz QC-VECSELs.

Amplifiers of free-space terahertz radiation

Amplifiers of free-space radiation are quite useful, especially in spectral ranges where the radiation is weak and sensitive detectors are hard to come by. A preamplification of the said weak radiation signal will significantly boost the S/N ratio in remote sensing and imaging applications. This is especially true in the terahertz (THz) range where the radiation signal is often weak and sensitive detectors require the cooling of liquid helium. Although quantum cascade structures are promising for providing amplification in the terahertz band from 2 to 5 THz, a THz amplifier has been demonstrated in an integrated form, in which the source is in close proximity to the amplifier, which will not be suitable for the aforementioned applications. Here we demonstrate what we believe is a novel approach to achieve significant amplification of free-space THz radiation using an array of short-cavity, surface-emitting THz quantum cascade lasers operating marginally below the lasing threshold as a Fabry–Perot amplifier. This free-space “slow light” amplifier provides 7.5 dB(×5.6) overall gain at ∼3.1 THz. The proposed devices are suitable for low-noise pre-amplifiers in heterodyne detection systems and for THz imaging systems. With the sub-wavelength pixel size of the array, the reflective amplifier can also be categorized as active metasurface, with the ability to amplify or absorb specific frequency components of the input THz signal.

Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter.

We report on large-area photoconductive terahertz (THz) emitters with a low-temperature-grown GaAs (LT-GaAs) active layer fabricated on quartz substrates using a lift-off transfer process. These devices are compared to the same LT-GaAs emitters when fabricated on the growth substrate. We find that the transferred devices show higher optical-to-THz conversion efficiencies and significantly larger breakdown fields, which we attribute to reduced parasitic current in the substrate. Through these improvements, we demonstrate a factor of ~8 increase in emitted THz field strength at the maximum operating voltage. In addition we find improved performance when these devices are used for photoconductive detection, which we explain through a combination of reduced parasitic substrate currents and reduced space-charge build-up in the device.

Three-dimensional shaping of sub-micron GaAs Schottky junctions for zero-bias terahertz rectification

We demonstrate a rectification mechanism based on quantum tunneling through the narrow Schottky barrier of a sub-micrometric Au/Ti/n-GaAs junction, which is capable of efficient power detection of free-space terahertz radiation beams even without an applied dc bias. Three-dimensional shaping of the junction geometry provides an enhanced zero-bias tunneling probability due to increased electric fields at the junction, resulting in cutoff frequencies up to 0.55 THz, responsivity up to 200 V/W, and noise equivalent power better than 10−9 W/Hz0.5 without applied dc bias.

Terahertz meta-atoms coupled to a quantum well intersubband transition

We present a method of coupling free-space terahertz radiation to intersubband transitions in semiconductor quantum wells using an array of meta-atoms. Owing to the resonant nature of the interaction between metamaterial and incident light and the field enhancement in the vicinity of the metal structure, the coupling efficiency of this method is very high and the energy conversion ratio from in-plane to z field reaches values on the order of 50%. To identify the role of different aspects of this coupling, we have used a custom-made finite-difference time-domain code. The simulation results are supplemented by transmission measurements on modulation-doped GaAs/AlGaAs parabolic quantum wells which demonstrate efficient strong light-matter coupling between meta-atoms and intersubband transitions for normal incident electromagnetic waves.

High-Power THz Generation, THz Nonlinear Optics, and THz Nonlinear Spectroscopy

It is now possible to generate terahertz (THz) pulses with sufficient energy and field amplitude to enable versatile applications in THz nonlinear optics and spectroscopy. In addition, THz waveform shaping at high intensities promises wide ranging new capabilities in THz coherent control. We review recent progress in generation of high-power THz phonon-polariton waves in lithium niobate that can be coupled into free space THz radiation. A ldquopolaritonicsrdquo toolset for control and processing of the THz waves is also reviewed briefly. Recent demonstrations of THz nonlinear optics and spectroscopy are then presented.

Guided propagation of terahertz pulses on metal wires

We demonstrate a new waveguiding structure for terahertz (THz) radiation in which broadband THz pulses are confined and guided along a bare metal wire. The propagation of THz pulses on such a waveguide is characterized with a fiber-coupled terahertz time-domain spectroscopy system. Free-space THz radiation is coupled onto the waveguide at different positions along the wire, and spatially resolved detection of the electric field of the guided wave is performed at the end of the wire. This waveguide exhibits the lowest attenuation of any waveguide for broadband THz pulses reported so far because of the minimal exposed metallic surface area. It also supports propagation of broadband radiation with negligible group-velocity dispersion, making it especially suitable for use in THz sensing and diagnostic systems. In addition, the structural simplicity lends itself naturally to the facile manipulation of the guided pulses, including coupling, directing, and beam splitting. These results can be described in terms of a model developed by Sommerfeld for waves propagating along the surface of a cylindrical conductor.

Terahertz surface plasmon polariton coupling on metallic gratings

Terahertz time-domain spectroscopy is used to study the nature and efficiency of coupling between surface plasmon polaritons and free-space terahertz radiation by metallic gratings made from brass rods and a grooved aluminum plate. Reflection and transmission mode measurements indicate very rapid coupling and decoupling with > 70% efficiency after accounting for scattering. Results indicate a good match to theoretical coupling frequencies within the accuracy of the experimental setup.

Enhancement of narrow-band terahertz radiation from photoconducting antennas by optical pulse shaping

We report the use of optical pulse shaping to convert single femtosecond pulses into lower-intensity pulse sequences for excitation of photoconductive dipole antennas, which results in the generation of bursts of tunable narrow-band free-space terahertz radiation. Our experiments demonstrate that the use of such pulse sequences can significantly enhance the spectral amplitude of the narrow-band terahertz radiation by avoiding the saturation effects that occur with single-pulse excitation. Our technique also provides the capability for terahertz wave-form shaping.