Compact Terahertz Source Research Articles

A High-Power Terahertz Source Over 10 mW at 0.45 THz Using an Active Antenna Array With Integrated Patch Antennas and Resonant-Tunneling Diodes

We propose a compact, high-power, and high-directivity surface-emitting terahertz (THz) source based on an array of active antennas with integrated patch antennas and resonant-tunneling diodes (RTDs). An array configuration of active antennas, each with an integrated patch antenna and two RTDs and coupled by microstrip lines, enables spatial power combining and improves directivity through coherent oscillation. We confirmed a maximum radiation power of 11.8 mW in a prototype 6 × 6 array at an oscillation frequency of 0.45 THz. Parasitic oscillation at low frequencies was suppressed by use of a bias stabilization circuit consisting of series-connected resistors and capacitors, and the dc to RF efficiency of this device was estimated to be approximately 1%. The radiant intensity of 210 mW/sr and the 3-dB beamwidth of 13° for the measured 6 × 6 array confirmed that directivity was improved by coherent oscillation based on mutual injection locking. The directivity of the patch antenna capable of surface emission can be controlled only by the number of antennas, even if there is no secondary radiator, such as an Si lens. The obtained results suggest that RTDs are promising as practical THz sources for realizing applications of THz imaging and 6G communication.

Phase Control of Terahertz Waves Using Injection-Locked Resonant Tunneling Diode Oscillator

Controlling the phase of terahertz (THz) waves is necessary for various applications, particularly beam steering with phased arrays. Resonant tunneling diodes (RTDs) are promising candidates for compact THz sources, and injection-locking phenomenon can be used to control the phase of RTD oscillators. In this study, we experimentally demonstrate the phase control of an injection-locked RTD oscillator. We fabricated an RTD oscillator with a slot antenna and developed a device package that reduces external THz feedback to obtain continuous frequency tuning. The free-running frequency varied from 380.5 to 386.7 GHz over a bias range of 0.61 to 0.65 V. To measure the phase of RTD oscillator, we constructed a heterodyne detection system in which all signals were well synchronized to achieve good phase measurement accuracy. The frequency of the RTD oscillator was locked to an injection signal of 383.2 GHz from an external source, and the phase was controlled by the bias voltage under the injection locking. The measured phase changed from −81° to +73° within the locking range. The bias-voltage dependence of the phase change reasonably agreed with the theoretical expectation.

A Microelectronic Terahertz Source Using Multiple Field Emitter Cathodes With an Array of Coupled Cavities

Compact, high-power, and broad-spectral coverage terahertz sources have application prospects in diverse scientific and technological fields, which attract great interest from researchers. The recently developed microelectronic device, a so- called terahertz laser diode (TLD), affords a promising candidate to develop desirable terahertz sources. However, the performances and the accessibility of TLD have unavoidable restrictions in the practices. In this article, we propose an enhanced TLD, which uses multiple field-emitter cathodes with an array of coupled cavities. The power and efficiency can be several times higher than that in previous TLDs. The required electric field intensity and the emission current density from the field-emitter cathode can be significantly reduced, which enables established field-emitting materials to be applicable and decreases the manufacturing difficulty as well. In addition, the fields in different cavities can be coupled, forming a series of unified oscillation modes, which can work as operation modes with various output frequencies. This improved TLD is attractive for developing compact, high-power, and broad-spectrum coverage terahertz sources for practices.

Field-resolved high-order sub-cycle nonlinearities in a terahertz semiconductor laser

The exploitation of ultrafast electron dynamics in quantum cascade lasers (QCLs) holds enormous potential for intense, compact mode-locked terahertz (THz) sources, squeezed THz light, frequency mixers, and comb-based metrology systems. Yet the important sub-cycle dynamics have been notoriously difficult to access in operational THz QCLs. Here, we employ high-field THz pulses to perform the first ultrafast two-dimensional spectroscopy of a free-running THz QCL. Strong incoherent and coherent nonlinearities up to eight-wave mixing are detected below and above the laser threshold. These data not only reveal extremely short gain recovery times of 2 ps at the laser threshold, they also reflect the nonlinear polarization dynamics of the QCL laser transition for the first time, where we quantify the corresponding dephasing times between 0.9 and 1.5 ps with increasing bias currents. A density-matrix approach reproducing the emergence of all nonlinearities and their ultrafast evolution, simultaneously, allows us to map the coherently induced trajectory of the Bloch vector. The observed high-order multi-wave mixing nonlinearities benefit from resonant enhancement in the absence of absorption losses and bear potential for a number of future applications, ranging from efficient intracavity frequency conversion, mode proliferation to passive mode locking.

Study of compact terahertz source with parallel multi-beam

Study of compact terahertz source with parallel multi-beam

Novel mechanism for terahertz radiation by oblique colliding of two electron beams in plasma

A new scheme for generating high power terahertz (THz) radiation by using two obliquely colliding electron beams in plasma is proposed. The particle-in-cell (PIC) simulation finds that a small beam intersection of high electron density is formed within a plasma wavelength when two thin electron beams collide. Meanwhile, the beam intersection can oscillate in a strong plasma wave driven by the two beams to produce THz radiation at double plasma frequency. A theoretical model is established showing that the radiation mechanism is the second-order current generated by the beam intersection oscillation. The angular distribution of the THz radiation is calculated by the theory, which agrees well with the PIC simulation. Compared with the existing high power THz sources, the scheme provided in this article has the advantages of a simple structure and no need for extreme parameter conditions, which is hopeful for applications in kilowatt-level coherent compact THz sources.

Intra-oscillator broadband THz generation in a compact ultrafast diode-pumped solid-state laser.

We demonstrate broadband and powerful terahertz (THz) generation at megahertz repetition rate based on intra-oscillator optical rectification (OR) in gallium phosphide (GaP). By placing the nonlinear crystal directly inside the cavity of a Kerr-lens mode-locked ultrafast diode-pumped solid-state laser (DPSSL) oscillator, we demonstrate a compact and single-stage THz source. Using only 7 W of diode-pump power, we drive OR in a GaP crystal with 22 W of average power at ∼80 MHz repetition rate. In a first configuration, using a 0.3-mm-thick GaP and 105 fs driving pulses, we generate up to 150 µW of THz radiation with a spectrum extending to 5.5 THz. In a second configuration allowing for sub-50-fs pulse duration, we generate up to 7 THz inside a 0.1-mm-thick GaP crystal. This performance is well suited for THz time-domain spectroscopy and THz imaging. Intra-oscillator THz generation in sub-100-fs DPSSLs is a promising way to scale down footprint, complexity and cost of powerful broadband THz sources.

Terahertz Emitter Using Resonant-Tunneling Diode and Applications.

A compact source is important for various applications utilizing terahertz (THz) waves. In this paper, the recent progress in resonant-tunneling diode (RTD) THz oscillators, which are compact semiconductor THz sources, is reviewed, including principles and characteristics of oscillation, studies addressing high-frequency and high output power, a structure which can easily be fabricated, frequency tuning, spectral narrowing, different polarizations, and select applications. At present, fundamental oscillation up to 1.98 THz and output power of 0.7 mW at 1 THz by a large-scale array have been reported. For high-frequency and high output power, structures integrated with cylindrical and rectangular cavities have been proposed. Using oscillators integrated with varactor diodes and their arrays, wide electrical tuning of 400–900 GHz has been demonstrated. For spectral narrowing, a line width as narrow as 1 Hz has been obtained, through use of a phase-locked loop system with a frequency-tunable oscillator. Basic research for various applications—including imaging, spectroscopy, high-capacity wireless communication, and radar systems—of RTD oscillators has been carried out. Some recent results relating to these applications are discussed.

Spin-VCSELs with Local Optical Anisotropies: Toward Terahertz Polarization Modulation

Putting a different spin on photonics: Spin-injected vertical-cavity surface-emitting lasers (Spin-VCSELs) with strong anisotropies are important in particular for ultrafast and terahertz applications, but the field is being held back in part because of lack of sufficiently predictive physical models. This study uses temporal coupled-mode theory with the transfer-matrix formalism to make progress by extending the spin-flip model. Additionally, the authors propose a spin-VCSEL with an anisotropic intracavity grating. This approach points to engineering solutions for ultrafast optical communication and compact terahertz sources.

Assessing frost damage in barley using terahertz imaging.

Frost is estimated to cost Australian grain growers $ 360 million in direct and indirect losses every year. Assessing frost damage manually in barley is labor intensive and involves destructive sampling. To mitigate against significant economic loss, it is crucial that assessment decisions on whether to cut for hay or continue to harvest are made soon after frost damage has occurred. In this paper, we propose a non-destructive technique by using raster-scan terahertz imaging. Terahertz waves can penetrate the spike to determine differences between frosted and unfrosted grains. With terahertz raster-scan imaging, conducted in both transmission and reflection at 275 GHz, frosted and unfrosted barley spikes show significant differences. In addition, terahertz imaging allows to determine individual grain positions. The emergence of compact terahertz sources and cameras would enable field deployment of terahertz non-destructive inspection for early frost damage.

Graphene plasmonic devices for terahertz optoelectronics

Abstract Plasmonic excitations, consisting of collective oscillations of the electron gas in a conductive film or nanostructure coupled to electromagnetic fields, play a prominent role in photonics and optoelectronics. While traditional plasmonic systems are based on noble metals, recent work has established graphene as a uniquely suited materials platform for plasmonic science and applications due to several distinctive properties. Graphene plasmonic oscillations exhibit particularly strong sub-wavelength confinement, can be tuned dynamically through the application of a gate voltage, and span a portion of the infrared spectrum (including mid-infrared and terahertz (THz) wavelengths) that is not directly accessible with noble metals. These properties have been studied in extensive theoretical and experimental work over the past decade, and more recently various device applications are also beginning to be explored. This review article is focused on graphene plasmonic nanostructures designed to address a key outstanding challenge of modern-day optoelectronics – the limited availability of practical, high-performance THz devices. Graphene plasmons can be used as a means to enhance light–matter interactions at THz wavelengths in a highly tunable fashion, particularly through the integration of graphene resonant structures with additional nanophotonic elements. This capability is ideally suited to the development of THz optical modulators (where absorption is switched on and off by tuning the plasmonic resonance) and photodetectors (relying on plasmon-enhanced intraband absorption or rectification of charge-density waves), and promising devices based on these principles have already been reported. Novel radiation mechanisms, including light emission from electrically excited graphene plasmons, are also being explored for the development of compact narrowband THz sources.

Depopulation mechanism for incoherent terahertz source – THz torch – based on GaAsBi/GaAs quantum well in GaAs/AlGaAs parabolic quantum well

Parabolic quantum wells (PQWs) are known as a promising candidate for a compact terahertz (THz) source. PQWs have equidistant subbands that can be designed to be separated by few meV to meet the THz frequency range. To enhance the efficiency and power of THz emission from PQWs, a new approach is proposed by employing depopulation of the lowest subbands of PQW. In this work, the theoretical analysis of an incoherent THz torch device is presented. The findings suggest that the introduction of narrower band-gap GaAsBi/GaAs rectangular quantum well within the GaAs/AlGaAs PQW can alter subbands arrangement to enable a faster depopulation mechanism exploiting LO phonon scattering. The calculated radiative power spectra show the increase of oscillator strength between the rearranged subbands of PQW due to the added GaAsBi rectangular potential. The increased intersubband radiative transition probability can lead to an efficient compact incoherent THz source – THz torch.

Spectrum discretization of Smith–Purcell radiation for efficient multicolor terahertz emission

The Smith–Purcell radiation (SPR) is a promising way for developing compact and high power terahertz sources with broad application prospects. Yet, the conventional SPR is largely restricted by its low efficiency and incoherency. Here we illustrate an enhanced coherent SPR from a compound array of deep-narrow-rectangular-grooves (DNRGs). We show, by detailed theoretical analysis and simulations, that each DNRG is an effective resonant cavity with low radiating Q-factor and a cluster of closely adjacent DNRGs can get enhanced coherent emission. More notably, by setting clusters of DNRGs with gradient shapes to be periodically arranged in an array, we can change the conventional incoherent SPR to be a coherent emission with a series of specified frequencies at specified directions, namely, the radiation is discretized. The total radiation efficiency can be increased by more than an order of magnitude. Thus, it affords an efficient way for high power multicolor terahertz generations.

Frequency increase in resonant-tunneling diode cavity-type terahertz oscillator by simulation-based structure optimization

Terahertz (THz) radiation, in the range between the light waves and millimeter waves, has gained much attention because of its various promising applications. Resonant tunneling diodes (RTDs) have the potential for use as compact and coherent THz sources operating at room temperature. In this paper, oscillation frequency limitations and structure dependences for a RTD THz oscillator with a cylindrical cavity resonator are analyzed. Based on the dependences of the oscillation frequency on resonant cavity dimensions, a method of the oscillator structure optimization and matching with an RTD structure was proposed. It is revealed that there is only one combination of the resonant cavity dimensions which gives the maximum possible oscillation frequency for this type of structure. Analysis of the calculation results has shown that fundamental oscillation up to 2.77 THz could be expected for the RTD device considered in the present study by using the proposed optimization technique.

Spatiotemporal visualization of the terahertz emission during high-power laser-matter interaction.

Single-cycle pulses with multimillion volts per centimeter field strengths and spectra in the terahertz (THz) band have attracted great interest due to their ability to coherently manipulate molecular orientations and electron spins resonantly and nonresonantly. The tremendous progress made in the development of compact and powerful terahertz sources have identified intense laser-thin foil interaction as a potential candidate for high-power broadband terahertz radiation. They are micrometers in size and deliver radially polarized terahertz pulses with millijoule energy and gigawatt peak power. Although several works have been carried out to investigate the terahertz generation process, their origin and angular distribution are still debated. We present here an indisputable study on their spatiotemporal characteristics and elaborate the underlying physical processes via recording the three-dimensional beam profile along with transient dynamics. These results are substructured with the quantitative visualization of the charge particle spectra.

A Compact 4.75-THz Source Based on a Quantum-Cascade Laser With a Back-Facet Mirror

We report on a compact terahertz (THz) source based on a quantum-cascade laser (QCL). The source is intended for a local oscillator in a THz heterodyne receiver for the detection of the fine structure line of neutral atomic oxygen at 4.7448 THz. The laser is mounted in a mechanical cryocooler. Laser operation is realized in continuous-wave mode in the range from −2.7 to +9.4 GHz around the target frequency. Special care has been taken on the laser design, which employs a GaAs/AlAs active region heterostructure and a resonator with an additional back-facet mirror. At a typical QCL operating point ( I = 200 mA, T = 60 K, ν = 4.75 THz, P opt = 5 mW) close to the target frequency, this system with a mass of only 4 kg requires an electrical input power of less than 130 W for a cooling capacity of up to 4 W. The maximum optical output power of the QCL reaches up to 8 mW with an electrical pump power of less than 1.8 W. The application of such a system in a spaceborne mission appears to be feasible.

Investigation of terahertz radiation based on forward and backward wave feedback circuit

A forward and backward wave feedback circuit for a vacuum terahertz radiation source is proposed and demonstrated. In the feedback circuit, two bunched electron beams with different velocities interact with two spatial harmonics, respectively. The simulation reveals that the forward wave and the backward wave form a feedback circuit and affect each other through beam-wave interaction. Furthermore, the maximum gain of the forward wave of the feedback circuit is about 12.4 dB higher than the single beam traveling wave tube. When observing the excitation of the backward wave, it is found that the starting current is 44.4% lower than the traditional backward wave oscillator. The forward and backward wave feedback circuit is a promising choice for the watt-level tunable compact terahertz source that can be potentially used in a transmitter for Terahertz radar detection.

Proposal and numerical studies of a terahertz Cherenkov free electron laser driven by a non-ultra relativistic bunched electron beam

Corrugated and dielectric structures have been widely used for producing terahertz radiation source. Recently, a novel scheme of sub-terahertz free electron laser (FEL) from a metallic pipe with corrugated walls driven by a non-ultra-relativistic ( < 10 MeV) picosecond electron beam was proposed and analyzed by using the Vlasov–Maxwell equations which can only provide crude estimates of the expected parameters of the FEL. It offers a promising candidate for compact accelerator-based high power terahertz source which can be realized with relatively low energy and low peak-current electron beams. In this paper, we propose a terahertz Cherenkov free electron laser scheme by using the cylindrical dielectric loaded waveguide (DLW) instead, and the new device scheme is studied numerically using both one-dimensional (1D) numerical model and particle in cell (PIC) simulations. The 1D model is developed in specialty for the longitudinal beam dynamics and electromagnetic simulations of the new Cherenkov FEL scheme. The basic idea of the approach is to use the analytic expressions of the longitudinal mode fields steady-state solution using Green function) to compute the wakefields at each time step using the macro particle currents as sources, while the Lorentz equations of the particles can be integrated numerically. The transitive effects in electromagnetic field (related to the group velocity of the wakefield) are modeled only approximately even in 1D case. It allows fast and accurate time-dependent calculation of the interaction between the wakefields and the electron beam. Good agreement is obtained comparing with particle in cell (PIC) simulation results.

Ultrafast two-dimensional field spectroscopy of terahertz intersubband saturable absorbers.

Intersubband (ISB) transitions in semiconductor multi-quantum well (MQW) structures are promising candidates for the development of saturable absorbers at terahertz (THz) frequencies. Here, we exploit amplitude and phase-resolved two-dimensional (2D) THz spectroscopy on the sub-cycle time scale to observe directly the saturation dynamics and coherent control of ISB transitions in a metal-insulator MQW structure. Clear signatures of incoherent pump-probe and coherent four-wave mixing signals are recorded as a function of the peak electric field of the single-cycle THz pulses. All nonlinear signals reach a pronounced maximum for a THz electric field amplitude of 11 kV/cm and decrease for higher fields. We demonstrate that this behavior is a fingerprint of THz-driven carrier-wave Rabi flopping. A numerical solution of the Maxwell-Bloch equations reproduces our experimental findings quantitatively and traces the trajectory of the Bloch vector. This microscopic model allows us to design tailored MQW structures with optimized dynamical properties for saturable absorbers that could be used in future compact semiconductor-based single-cycle THz sources.

Boosting Terahertz Photoconductive Antenna Performance with Optimised Plasmonic Nanostructures.

Advanced nanophotonics penetrates into other areas of science and technology, ranging from applied physics to biology, which results in many fascinating cross-disciplinary applications. It has been recently demonstrated that suitably engineered light-matter interactions at the nanoscale can overcome the limitations of today’s terahertz (THz) photoconductive antennas, making them one step closer to many practical implications. Here, we push forward this concept by comprehensive numerical optimization and experimental investigation of a log-periodic THz photoconductive antenna coupled to a silver nanoantenna array. We shed light on the operation principles of the resulting hybrid THz antenna, providing an approach to boost its performance. By tailoring the size of silver nanoantennas and their arrangement, we obtain an enhancement of optical-to-THz conversion efficiency 2-fold larger compared with previously reported results for similar structures, and the strongest enhancement is around 1 THz, a frequency range barely achievable by other compact THz sources. We also propose a cost-effective fabrication procedure to realize such hybrid THz antennas with optimized plasmonic nanostructures via thermal dewetting process, which does not require any post processing and makes the proposed solution very attractive for applications.