Terahertz Frequency Radiation Research Articles

Spiral room-temperature thin-filmed photo-thermoelectric THz detector based on Bi88Sb12 solid solutions

A terahertz (THz) room-temperature photo-thermoelectric (PTE) detector based on elements of spiral shape has been proposed. The units have been fabricated on a 20 μm mica substrate from 150 nm thin films of Bi88Sb12 and copper by a thermal deposition technique. Thermoelectric Bi88Sb12 leg acted as a sensitive element due to its high absorption and spiral shape that enable heating. It also acts as a radiator to maintain a temperature difference and a thermo-electromotive force formation. A prototype of a PTE detector has been designed, fabricated, and tested at the 0.14 THz radiation frequency. Measurements showed a voltage signal of 275 μV from 1 unit, the responsivity of 50.7 mV/W. It showed a noise equivalent power of 158 nW Hz−1/2, which is 1–2 orders of magnitude higher than for bolometers and Golay cells, but is comparable with pyroelectric detectors. The response time was 2.21 s and 1.75 s according to the experiment and the simulation, respectively, which is a bit better than for pyroelectric detectors. Based on numerical simulation an improved detector design has been proposed allowing the voltage up to 800 μV from 1 unit. The proposed PTE detector possesses several significant advantages resulting from the usage of Bi88Sb12 and comprehensive design. The fabrication process is fast, low-cost, and CMOS-compatible. The design is compact and simple and requires only two materials that simultaneously act as an absorber, thermocouple, electrical contacts, and radiator. The detector operates at room temperature and can be applied in 6 G communication systems, imaging and medical diagnostics.

Enhanced Delivery and Detection of Terahertz Frequency Radiation from a Quantum Cascade Laser Within Dilution Refrigerator

We report on significant enhancements to the integration of terahertz (THz) quantum cascade lasers (QCL) and THz detection with a two-dimensional electron gas (2DEG) within a dilution refrigerator obtained by the inclusion of a multi-mesh 6 THz low-pass filter to block IR radiation, a Winston cone to focus light output, and gating the 2DEG for optimised sensitivity. We show that these improvements allow us to obtain a > 2.5 times reduced sample electron temperature (160 mK compared with 430 mK previously), during cyclotron resonance (CR) measurements of a 2DEG under QCL illumination. This opens up a route to performing sub-100 mK experiments using excitation by THz QCLs.

Generating High-Power, Frequency Tunable Coherent THz Pulse in an X-ray Free-Electron Laser for THz Pump and X-ray Probe Experiments

Precisely synchronized X-ray and strong-field coherent terahertz (THz) enable the coherent THz excitation of many fundamental modes (THz pump) and the capturing of X-ray dynamic images of matter (X-ray probe), while the generation of such a light source is still a challenge for most existing techniques. In this paper, a novel X-ray free-electron laser based light source is proposed to produce a synchronized high-powered X-ray pulse and strong field, widely frequency tunable coherent THz pulse simultaneously. The technique adopts a frequency beating laser modulated electron bunch with a Giga-electron-volt beam energy to generate an X-ray pulse and a THz pulse sequentially by passing two individual undulator sections with different magnetic periods. Theoretical analysis and numerical simulations are carried out using the beam parameters of the Shanghai soft X-ray free-electron laser facility. The results show that the technique can generate synchronized 4 nm X-ray radiation with a peak power of 1.89 GW, and narrow-band THz radiation with a pulse energy of 1.62 mJ, and the frequency of THz radiation can be continuously tuned from 0.1 to 40 THz. The proposed technique can be used for THz pump and X-ray probe experiments for dynamic research on the interaction between THz pulse and matter at a femtosecond time scale.

Directed delivery of terahertz frequency radiation from quantum cascade lasers within a dry 3He dilution refrigerator.

We present a scheme for the full integration of terahertz (THz) frequency quantum cascade lasers (QCLs) within a dilution refrigerator in order to provide a directed delivery of THz power into the sample space. We describe a successful operation of a 2.68THz QCL located on the pulse tube cooler stage of the refrigerator, with its output coupled onto a two-dimensional electron gas (2DEG) located on a milli-kelvin sample stage via hollow metal waveguides and Hysol thermal isolators, achieving a total loss from QCL to the sample of ∼-9 dB. The thermal isolators limit heat leaks to the sample space, with a base temperature of ∼210 mK being achieved. We observe cyclotron resonance (CR) induced in the 2DEG by the QCL and explore the heating impact of the QCL on all stages of the refrigerator. The CR effect induced by the THz QCL is observable at electron temperatures as low as ∼430 mK. The results show a viable route for the exploitation of THz QCLs within the environment of a dilution refrigerator and for the THz power delivery in very low-temperature (<0.5K) condensed matter experiments.

Six-dimensional phase space preservation in a terahertz-driven multistage dielectric-lined rectangular waveguide accelerator

Staged acceleration, driven by terahertz (THz) frequency radiation pulses in a lattice with alternating orientation dielectric-lined waveguides and intervening matching optics, is shown to mitigate transverse emittance and energy spread growth, opening a route to multistage THz linacs. Decomposition of the longitudinal THz field into the multipolar components reveals a quadrupole field component with strong radial dependence. As such, it induces a transverse energy correlation in the beam during acceleration due to the large variation in the electric field with radius and azimuthal position of the electrons. An alternating orientation of stages separated by a matching section provides a compensation of transverse energy spread correlation induced in the beam during its interaction with the THz field. Furthermore, the monopolar component of the accelerating ${\mathrm{LSM}}_{11}$ mode was found to be constant with respect to transverse position, entailing zero monopolar transverse voltage and preventing emittance growth, unlike conventional radio-frequency structures. We demonstrate in a rectangular dielectric-lined waveguide structure that, when used for the acceleration of relativistic electrons, the slice transverse emittance is conserved and the growth in the slice energy spread is reduced by 70%--80% simultaneously over a system of two stages, each providing an interaction length of 4 mm and an energy gain of up to 2 MeV.

Chip‐Scale Terahertz Frequency Combs through Integrated Intersubband Polariton Bleaching

AbstractQuantum cascade lasers (QCLs) represent a fascinating accomplishment of quantum engineering and enable the direct generation of terahertz (THz) frequency radiation from an electrically biased semiconductor heterostructure. Their large spectral bandwidth, high output powers, and quantum‐limited linewidths have facilitated the realization of THz pulses by active mode‐locking and passive generation of optical frequency combs (FCs) through intracavity four‐wave‐mixing, albeit over a restricted operational regime. Here, an integrated architecture is conceived for the generation of high power (5.5–8.0 mW) THz FCs comprising an ultrafast THz polaritonic reflector, exploiting intersubband (ISB) cavity polaritons, and a broad bandwidth (2.3–3.8 THz) heterogenous THz QCL. By tuning the group‐delay‐dispersion in an integrated geometry, through the exploitation of light‐induced bleaching of the ISB‐based THz polaritons, spectral reshaping of the QCL emission and stable FC operation over an operational range up to 38%, characterized by a single and narrow (down to 700 Hz) intermode beatnote are demonstrated. This concept provides design guidelines for a new generation of compact, cost‐effective, electrically driven chip‐scale FC sources based on ultrafast polariton dynamics, paving the way toward the generation of mode‐locked THz microlasers that can strongly impact a broad range of applications in ultrafast sciences, data storage, high‐speed communication, and spectroscopy.

Terahertz radiation generation from metallic electronic structure manipulated by inhomogeneous DC-fields

We propose a feasible, high-efficiency scheme of primary terahertz (THz) radiation source through manipulating electronic structure (ES) of a metallic film by targeted-designed DC-fields configuration. The DC magnetic field is designed to be of a spatially inhomogeneous strength profile, and its direction is designed to be normal to the film, and the direction of the DC electric field is parallel to the film. Strict quantum theory and numerical results indicate that the ES under such a field configuration will change from a 3D Fermi sphere into a highly-degenerate structure whose density-of-state curve has pseudogap near Fermi surface. Wavefunctions’ shapes in this new ES are space-asymmetric, and the width of pseudogap near Fermi surface, as well as magnitudes of transition matrix element, can be handily controlled by adjusting parameter values of DC fields. Under available parameter values, the width of the pseudogap can be at milli-electron-volt level (corresponding to THz radiation frequency), and the magnitude of oscillating dipole can be at 10^{-9} C*m-level. In room-temperature environment, phonon in metal can pump the ES to achieve population inversion.

Computational absorption and reflection studies of normal human skin at 0.45 THz.

Applications using terahertz (THz) frequency radiation will inevitably lead to increased human exposure. The power density and specific absorption rate (SAR) simulations of thin skin at 0.45 THz show the bulk of the energy being absorbed in the upper stratum spinosum, and the maximal temperature rise is in the lower stratum spinosum. There are regions of SAR increase of 100% above the local average at the stratum spinosum/stratum basale boundary. The dead Stratum Corneum layer protects underlying tissues in thick skin. Reflection studies suggest that acute angles and the use of polarised incident radiation may enhance the assessment of diabetic neuropathy.

High-power terahertz emission in magnetized plasma via optical rectification of a super-Gaussian laser beam

The paper discusses the generation of terahertz (THz) wave via the optical rectification of an amplitude-modulated super-Gaussian laser beam in the preformed periodic density plasma when a magnetic field is applied transversely. The non-uniform spatial variation in intensity of the laser beam drives a strong ponderomotive force. Such ponderomotive force generates nonlinear oscillatory drift velocity to plasma electrons that produce a strong transverse nonlinear current at the modulation frequency (the frequency of the radiation lies in the THz domain). Moreover, the presence of the periodic density plasma structure and magnetic field can shift the condition of resonance and can produce the tunable THz frequency radiation. In the current scheme, the THz field amplitude increases with the profile index of the laser, modulation index, ripple parameters, and transversely applied static magnetic field. As a result, the efficiency of the generated THz wave can be achieved of the order of for the exact phase-matching condition.

Black phosphorus nanodevices at terahertz frequencies: Photodetectors and future challenges

The discovery of graphene triggered a rapid rise of unexplored two-dimensional materials and heterostructures having optoelectronic and photonics properties that can be tailored on the nanoscale. Among these materials, black phosphorus (BP) has attracted a remarkable interest, thanks to many favorable properties, such as the high carrier mobility, the in-plane anisotropy, the possibility to alter its transport via electrical gating, and the direct band-gap, which can be tuned by thickness from 0.3 eV (bulk crystalline) to 1.7 eV (single atomic layer). When integrated in a microscopic field effect transistor, a few-layer BP flake can detect Terahertz (THz) frequency radiation. Remarkably, the in-plane crystalline anisotropy can be exploited to tailor the mechanisms that dominate the photoresponse; a BP-based field effect transistor can be engineered to act as a plasma-wave rectifier, a thermoelectric sensor, or a thermal bolometer. Here we present a review on recent research on BP detectors operating from 0.26 THz to 3.4 THz with particular emphasis on the underlying physical mechanisms and the future challenges that are yet to be addressed for making BP the active core of stable and reliable optical and electronic technologies.

Monolithic Semiconductor Lasers with Dynamically Tunable Linear-to-Circular Polarization

The ability to control the polarization state of emission from semiconductor lasers is essential for many applications in spectroscopy, imaging, and communications, inter alia, with monolithic integration approaches being extremely beneficial. Although manipulating the output polarization of radiation from a laser can be achieved through a number of approaches, obtaining continuous dynamic control, e.g., from linear to circular, remains extremely challenging. In this paper, we demonstrate that the polarization of terahertz (THz) frequency radiation can be continuously tuned electronically from linear to circular polarization by monolithically integrating in-plane metasurfaces with two phase-locked semiconductor-based THz quantum cascade lasers. Moreover, the metasurfaces—metal antenna arrays in this case—also act as efficient beam collimators, yielding a collimated beam divergence of ∼10° × 10°. Our results, however, have broad applicability to a wide range of semiconductor lasers operating from the visib...

Generation of continuous wave terahertz frequency radiation from metal-organic chemical vapour deposition grown Fe-doped InGaAs and InGaAsP

We demonstrate the generation of continuous wave terahertz (THz) frequency radiation from photomixers fabricated on both Fe-doped InGaAs and Fe-doped InGaAsP, grown by metal-organic chemical vapor deposition. The photomixers were excited using a pair of distributed Bragg reflector lasers with emission around 1550 nm, and THz radiation was emitted over a bandwidth of greater than 2.4 THz. Two InGaAs and four InGaAsP wafers with different Fe doping concentrations were investigated, with the InGaAs material found to outperform the InGaAsP in terms of emitted THz power. The dependencies of the emitted power on the photomixer applied bias, incident laser power, and material doping level were also studied.

Generation of Terahertz Radiation from Fe-doped InGaAsP Using 800 nm to 1550 nm Pulsed Laser Excitation

We demonstrate efficient generation of terahertz (THz) frequency radiation by pulsed excitation, at wavelengths between 800 and 1550 nm, of photoconductive (PC) switches fabricated using Fe-doped InGaAsP wafers, grown by metal organic chemical vapor deposition (MOCVD). Compared to our previous studies of Fe-doped InGaAs wafers, Fe:InGaAsP wafers exhibited five times greater dark resistivity to give a value of 10 kΩ cm, and Fe:InGaAsP PC switches produced five times higher THz power emission. The effect of Fe-doping concentration (between 1E16 and 1.5E17 cm−3) on optical light absorption (between 800 and 1600 nm), on resistivity, and on THz emission is also discussed.

3-D Numerical Analysis of Smith–Purcell-Based Terahertz Wave Radiation Excited by Effective Surface Plasmon

At lower frequency regime, altering the surface structure modifies the characteristics of propagating effective surface plasmons (ESPs), particularly their interaction with light. Movement of an electron beam in the proximity of the surface of a structured metallic grating excite these ESPs, which due to existence of a velocity phase match, they are able to emit terahertz (THz) waves known as Smith–Purcell (SP) radiation. By introducing a femtosecond perturbation (a single electron bunch), the important characteristics of a desired grating are revealed. Through our 3-D investigations, we find the incoherent THz radiation frequency span, and moreover we confirm that the frequency of the ESP is always less than the minimum SP frequency. Additionally, we find that the maximum of signal amplitude is distributed around 90°, only if the grating width is comparable to the longest SP wavelength. Also we learned that by increasing the grating length, the magnitude and spectral resolution of the radiation increases too. In order to study the coherent (superradiant) radiation, we use a train of electron bunches with variable bunch-to-bunch distances, and we calculate the radiation angle of the coherent SP signal. Simulations on the generation of SP radiation at THz frequencies are performed with the help of the 3-D particle-in-cell (PIC) finite integral (FI) method, in which the results agree very well with previously reported 2-D simulations.

The effect of beam pre-bunching on the excitation of terahertz plasmons in a parallel plane guiding system

The excitation of terahertz (THz) plasmons by a pre-bunched relativistic electron beam propagating in a parallel plane semiconducting guiding system is studied. It is found that the n-InSb semiconductor strongly supports the confined surface plasmons in the terahertz frequency range. The growth rate and efficiency of the THz surface plasmons increase linearly with modulation index and show the largest value as modulation index approaches unity. Moreover, the growth rate of the instability scales as one-third power of the beam density and inverse one-third power of the THz radiation frequency.

Tuning of the Terahertz Radiation Frequency From Superconductors

The terahertz radiation due to the flux flow in a superconductor slab is investigated, and the radiated power spectrum is calculated. The slab of the superconductor is immersed inside a magnetic field and biased with an ac component added to the dc current. The results show that, by adding an ac component, the peaks of radiation could be manipulated. Therefore, by adjusting the amplitude and frequency of the ac current, we can achieve the desired terahertz frequency. The results also indicate that, by increasing the amplitude of ac component and for appropriate ac frequency, the power peaks could increase.

On-chip picosecond pulse detection and generation using graphene photoconductive switches.

We report on the use of graphene for room temperature on-chip detection and generation of pulsed terahertz (THz) frequency radiation, exploiting the fast carrier dynamics of light-generated hot carriers, and compare our results with conventional low-temperature-grown gallium arsenide (LT-GaAs) photoconductive (PC) switches. Coupling of picosecond-duration pulses from a biased graphene PC switch into Goubau line waveguides is also demonstrated. A Drude transport model based on the transient photoconductance of graphene is used to describe the mechanism for both detection and generation of THz radiation.

Terahertz response of a polymer composite with high concentration of silicon micro- and nanoparticles

The methods used for the formation of composite materials with a definite high refractive index in a terahertz (THz) frequency range due to the incorporation of silicon micro- or nanoparticles into a polymer-thermoplast matrix are considered. The dielectric properties of the composites are studied versus the concentration of silicon particles and THz radiation frequency. The maximum achievable limits of refraction, absorption, and scattering are determined. The possibility to use a composite material based on polyethilene impregnated with Si-nanoparticles for manufacturing THz optics elements is demonstrated.

Near-Field Analysis of Terahertz Pulse Generation From Photo-Excited Charge Density Gradients

Excitation of photo-current transients at semiconductor surfaces by subpicosecond optical pulses gives rise to emission of electromagnetic pulses of terahertz (THz) frequency radiation. To correlate the THz emission with the photo-excited charge density distribution and the photo-current direction, we mapped near-field and far-field distributions of the generated THz waves from GaAs and Fe-doped InGaAs surfaces. The experimental results show that the charge dynamics in the plane of the surface can radiate substantially stronger THz pulses than the charge dynamics in the direction normal to the surface, which is generally regarded as the dominant origin of the emission.

Laser beat wave excitation of terahertz radiation in a plasma slab

Terahertz (THz) radiation generation by nonlinear mixing of lasers, obliquely incident on a plasma slab is investigated. Two cases are considered: (i) electron density profile is parabolic but density peak is below the critical density corresponding to the beat frequency, (ii) plasma boundaries are sharp and density is uniform. In both cases, nonlinearity arises through the ponderomotive force that gives rise to electron drift at the beat frequency. In the case of inhomogeneous plasma, non zero curl of the nonlinear current density gives rise to electromagnetic THz generation. In case of uniform plasma, the sharp density variation at the plasma boundaries leads to radiation generation. In a slab width of less than a terahertz wavelength, plasma density one fourth of terahertz critical density, laser intensities ∼1017 W/cm2 at 1 μm, one obtains the THz intensity ∼1 GW/cm2 at 3 THz radiation frequency.