Detection Of Terahertz Radiation Research Articles

Large area Terahertz digitated photoconductive antennas based on a single high resistivity metal and nanoplasmonic electrode

Optical excited photoconductive antennas are a central technology for the Terahertz (THz) domain, crucial for both emitting and detecting THz radiation. This work proposes and experimentally realises a new approach in digitated photoconductive antennas (d-PCAs) based on a single digitated high resistivity metal contact with integrated resistances as voltage dividers. This permits a uniform applied electric field over a large surface area and a single step device processing procedure, simplifying the device realisation. This concept is further combined with digitated plasmonic nano-antennas that permits to enhance the light-matter interaction. Through femtosecond optical excitation of such structures, THz pulses can be generated efficiently through this device. Further, for the plasmonic d-PCA, the detected THz electric field of the device shows the effect of polarisation of the incident IR beam, highlighting the role of the nanostructured digitated contacts. This work is supported by electromagnetic simulations showing the optical and THz response of this new type of photoconductive antenna with integrated resistances.

Two-dimensional topological semimetals: an emerging candidate for terahertz detectors and on-chip integration.

The importance of terahertz (THz) detection lies in its ability to provide detailed information in a non-destructive manner, making it a valuable tool across various domains including spectroscopy, communication, and security. The ongoing development of THz detectors aims to enhance their sensitivity, resolution and integration into compact and portable devices such as handheld scanners or integrated communication chips. Generally, two-dimensional (2D) materials are considered potential candidates for device miniaturization but detecting THz radiation using 2D semiconductors is generally difficult due to the ultra-small photon energy. However, this challenge is being addressed by the advent of topological semimetals (TSM) with zero-bandgap characteristics. These semimetals offer low-energy excitations in proximity to the Dirac point, which is particularly important for applications requiring a broad detection range. Their distinctive band structures with linear energy-momentum dispersion near the Fermi level also lead to high electron mobility and low effective mass. The presence of topologically protected dissipationless conducting channels and self-powered response provides a basis for low-energy integration. In order to establish paradigms for semimetal-based THz detectors, this review initially offers an analytical summary of THz detection principles. Then, the review demonstrates the distinct design of devices, the excellent performance derived from the topological surface state and unique band structures in TSM. Finally, we outline the prospective avenues for on-chip integration of TSM-based THz detectors. We believe this review can promote further research on the new generation of THz detectors and facilitate advancements in THz imaging, spectroscopy, and communication systems.

Superstrate-lens integration using paraffin wax on top of semiconductor-based THz detector chips

A detector of electromagnetic radiation may benefit – if its sensor area is small – from the application of a substrate lens, which focusses the radiation onto the active sensing area of the device and thus enhances its responsivity. The use of such a lens, attached directly onto the detector backside in order to avoid reflection losses, requires that the detector substrate and backside be transparent to the radiation. However, if this is not the case, one may like to place instead a superstrate lens onto the front side of the detector. It may even be of interest to use both a substrate and a superstrate lens if the detector needs to be illuminated with two beams, e.g. for heterodyne detection, where one beam provides the local-oscillator signal. The use of a superstrate lens is, however, often hindered or impeded by an uneven surface topography or by the presence of bonding wires on the front side of the detector. Here, we address this issue and explore the use of paraffin wax to form or attach superstrate lenses. In the first case, which is the main topic of this contribution, we exploit the surface tension of liquid paraffin, brought onto the detector, to sculpt the wax itself into a lens. In the second case, only addressed conceptually here, we use paraffin to form a thin intermediate layer which also acts as an adhesive for the attachment of a plastic or silicon lens. In both cases, the application of liquid paraffin allows one to fill out an uneven detector surface and to embed wires without breaking them. We investigate the use of wax for the case of CMOS TeraFETs – detectors of terahertz radiation based on field-effect transistors – embedded into antenna structures. We describe the processing steps and analyze the performance of a TeraFET equipped with such a wax superstrate lens for front-side beam coupling.

Micromechanical field-effect transistor terahertz detectors with optical interferometric readout

We investigate the response of the micromechanical field-effect transistors (MMFETs) to the impinging terahertz (THz) signals. The MMFET uses the microcantilevers (MC) as a mechanically floating gate and the movable mirror of the Michelson optical interferometer. The MC mechanical oscillations are transformed into optical signals and the MMFET operates as the detector of THz radiation with the optical output. The combination of the mechanical and plasmonic resonances in the MMFET with the optical amplification enables an effective THz detection.

Infrared and Terahertz Radiation Detectors Based on Opto-Acoustic Converters (a Review)

Infrared and Terahertz Radiation Detectors Based on Opto-Acoustic Converters (a Review)

Resonant plasmonic detection of terahertz radiation in field-effect transistors with the graphene channel and the black-As_xP_{1-x} gate layer

We propose the terahertz (THz) detectors based on field-effect transistors (FETs) with the graphene channel (GC) and the black-Arsenic (b-As) black-Phosphorus (b-P), or black-Arsenic-Phosphorus (b-As_xP_{1-x}) gate barrier layer. The operation of the GC-FET detectors is associated with the carrier heating in the GC by the THz electric field resonantly excited by incoming radiation leading to an increase in the rectified current between the channel and the gate over the b-As_xP_{1-x} energy barrier layer (BLs). The specific feature of the GC-FETs under consideration is relatively low energy BLs and the possibility to optimize the device characteristics by choosing the barriers containing a necessary number of the b-As_xP_{1-x} atomic layers and a proper gate voltage. The excitation of the plasma oscillations in the GC-FETs leads to the resonant reinforcement of the carrier heating and the enhancement of the detector responsivity. The room temperature responsivity can exceed the values of 10^3 A/W. The speed of the GC-FET detector’s response to the modulated THz radiation is determined by the processes of carrier heating. As shown, the modulation frequency can be in the range of several GHz at room temperatures.

Observation of Magnetic State Dependent Thermoelectricity in Superconducting Spin Valves.

Superconductor-ferromagnet tunnel junctions demonstrate giant thermoelectric effects that are being exploited to engineer ultrasensitive terahertz radiation detectors. Here, we experimentally observe the recently predicted complete magnetic control over thermoelectric effects in a superconducting spin valve, including the dependence of its sign on the magnetic state of the spin valve. The description of the experimental results is improved by the introduction of an interfacial domain wall in the spin filter layer interfacing the superconductor. Surprisingly, the application of high in-plane magnetic fields induces a double sign inversion of the thermoelectric effect, which exhibits large values even at applied fields twice the superconducting critical field.

Semiconductor Terahertz Physics

AbstractA rich range of physical phenomena exhibit frequencies in the range of 0.1–10 THz. Within semiconductors, these include phononic, electronic, and mixed modes of oscillations. These fundamental phenomena then serve as the basis for specific devices to produce, manipulate, and detect terahertz radiation.

The Use of Brunel Harmonics of Elliptically Polarized Laser Pulses for High-Resolved Detection of Terahertz and Mid-Infrared Radiation

We study the generation of low-order harmonics during the interaction of helium atoms with intense elliptically-polarized laser pulses in the presence of a static electric field. Based on the solution of the three-dimensional time-dependent Schrödinger equation, the spectra of generated radiation are calculated for various pulse ellipticities, intensities, and static-field strengths. We show the possibility of using the second harmonic generation in the free-electron current density by laser pulses with arbitrary (even circular) polarization for high-resolved measuring the waveforms of terahertz and mid-infrared radiation.

Ultralow-noise Terahertz Detection by p–n Junctions in Gapped Bilayer Graphene

Graphene shows strong promise for the detection of terahertz (THz) radiation due to its high carrier mobility, compatibility with on-chip waveguides and transistors, and small heat capacitance. At the same time, weak reaction of graphene's physical properties on the detected radiation can be traced down to the absence of a band gap. Here, we study the effect of electrically induced band gap on THz detection in graphene bilayer with split-gate p-n junction. We show that gap induction leads to a simultaneous increase in current and voltage responsivities. At operating temperatures of ∼25 K, the responsivity at a 20 meV band gap is from 3 to 20 times larger than that in the gapless state. The maximum voltage responsivity of our devices at 0.13 THz illumination exceeds 50 kV/W, while the noise equivalent power falls down to 36 fW/Hz1/2.

Theory of the in-plane photoelectric effect in quasi-one-dimensional electron systems

The in-plane photoelectric (IPPE) effect is a recently discovered [Sci. Adv. 8, eabi8398 (2022)] quantum phenomenon, which enables efficient detection of terahertz (THz) radiation in semiconductor structures with a two-dimensional (2D) electron gas. Here, we develop a theory of the IPPE effect in quasi-one-dimensional electron systems in which the width of the 2D conducting channel is so small that the transverse quantization energy is larger than the thermal energy. We calculate the THz photoresponse of such a system as a function of the THz frequency, control gate voltages, and geometrical parameters of the detector. We show that the transverse quantization of the electron motion manifests itself in oscillating gate-voltage dependences of the photocurrent, if the THz photon energy is less than the one-dimensional quantization energy. Results of the theory are applicable to any semiconductor systems with 2D electron gases, including III--V structures, silicon-based field effect transistors, and the novel 2D layered graphene-related materials.

Highly conformable terahertz metasurface absorbers via two-photon polymerization on polymeric ultra-thin films

Abstract The continuously increasing interest in flexible and integrated photonics requires new strategies for device manufacturing on arbitrary complex surfaces and with smallest possible size, respectively. Terahertz (THz) technology can particularly benefit from this achievement to make compact systems for emission, detection and on-demand manipulation of THz radiation. Here, we present a novel fabrication method to realize conformable terahertz metasurfaces. The flexible and versatile character of polymeric nanomembranes is combined with direct laser writing via two-photon polymerization to develop free-standing ultra-thin quasi-perfect plasmonic absorbers with an unprecedentedly high level of conformability. Moreover, revealing new flexible dielectric materials presenting low absorption and permittivity in the THz range, this work paves the way for the realization of ultra-thin, conformable hybrid or all-dielectric devices to enhance and enlarge the application of THz technologies, and flexible photonics in general.

Advances in terahertz technology for cancer detection applications.

Currently, there is an increasing demand for the diagnostic techniques that provide functional and morphological information with early cancer detection capability. Novel modern medical imaging systems driven by the recent advancements in technology such as terahertz (THz) and infrared radiation-based imaging technologies which are complementary to conventional modalities are being developed, investigated, and validated. The THz cancer imaging techniques offer novel opportunities for label free, non-ionizing, non-invasive and early cancer detection. The observed image contrast in THz cancer imaging studies has been mostly attributed to higher refractive index, absorption coefficient and dielectric properties in cancer tissue than that in the normal tissue due the local increase of the water molecule content in tissue and increased blood supply to the cancer affected tissue. Additional image contrast parameters and cancer biomarkers that have been reported to contribute to THz image contrast include cell structural changes, molecular density, interactions between agents (e.g., contrast agents and embedding agents) and biological tissue as well as tissue substances like proteins, fiber and fat etc. In this paper, we have presented a systematic and comprehensive review of the advancements in the technological development of THz technology for cancer imaging applications. Initially, the fundamentals principles and techniques for THz radiation generation and detection, imaging and spectroscopy are introduced. Further, the application of THz imaging for detection of various cancers tissues are presented, with more focus on the in vivo imaging of skin cancer. The data processing techniques for THz data are briefly discussed. Also, we identify the advantages and existing challenges in THz based cancer detection and report the performance improvement techniques. The recent advancements towards THz systems which are optimized and miniaturized are also reported. Finally, the integration of THz systems with artificial intelligent (AI), internet of things (IoT), cloud computing, big data analytics, robotics etc. for more sophisticated systems is proposed. This will facilitate the large-scale clinical applications of THz for smart and connected next generation healthcare systems and provide a roadmap for future research.

Terahertz radiation detection with a cantilever-based photoacoustic sensor.

We report the photoacoustic (PA) response in the terahertz (THz) range by employing a detection process actuated with a silicon cantilever pressure sensor and a carbon-based radiation absorber. The detection relies on the mechanical response of the cantilever, when the volume of the carrier gas inside the PA cell expands with the heat produced by the radiation absorber. The detector interferometrically monitors the movement of the cantilever sensor to generate the PA signal. We selected the absorber material with the highest THz responsivity for detailed studies at 1.4 THz (214 µm wavelength). The observed responsivities of two different radiation absorbers are nearly the same at 1.4 THz and agree within 10% with responsivity values at 0.633 µm wavelength. The results demonstrate the potential of covering with a single PA detector a broad spectral range with approximately constant responsivity, large dynamic range, and high damage threshold.

A room-temperature polarization-sensitive CMOS terahertz camera based on quantum-dot-enhanced terahertz-to-visible photon upconversion.

Detection of terahertz (THz) radiation has many potential applications, but presently available detectors are limited in many aspects of their performance, including sensitivity, speed, bandwidth and operating temperature. Most do not allow the characterization of THz polarization states. Recent observation of THz-driven luminescence in quantum dots offers a possible detection mechanism via field-driven interdot charge transfer. We demonstrate a room-temperature complementary metal-oxide-semiconductor THz camera and polarimeter based on quantum-dot-enhanced THz-to-visible upconversion mechanism with optimized luminophore geometries and fabrication designs. Besides broadband and fast responses, the nanoslit-based sensor can detect THz pulses with peak fields as low as 10 kV cm-1. A related coaxial nanoaperture-type device shows a to-date-unexplored capability to simultaneously record the THz polarization state and field strength with similar sensitivity.

Current-induced enhancement of photo-response in graphene THz radiation detectors

Thermoelectric readout in a graphene terahertz (THz) radiation detector requires a p-n junction across the graphene channel. Even without an intentional p-n junction, two latent junctions can exist in the vicinity of the electrodes/antennas through the proximity to the metal. In a symmetrical structure, these junctions are connected back-to-back and therefore counterbalance each other with regard to rectification of the ac signal. Because of the Peltier effect, a small dc current results in additional heating in one and cooling in another p-n junction, thereby breaking the symmetry. The p-n junctions then no longer cancel, resulting in a greatly enhanced rectified signal. This allows simplifying the design and controlling the sensitivity of THz radiation detectors.

Theoretical investigation of the spectroscopic properties of diatomic systems: ZnTe, ZnTe+, and ZnTe-

The Zinc telluride (ZnTe) system presents appealing optoelectronic properties used in radiology as well as in the detection of terahertz radiation (THz). In this work, we report a theoretical investigation of the ground and excited states of ZnTe and its ions ZnTe+ and ZnTe- using different ab inito methods. The potential energy curves (PEC) are computed using the CASSCF/LANL2DZ and MRCI/LANL2DZ methods. These curves allow inferring the different spectroscopic constants such as the internuclear distance (Re), the harmonic vibration frequency (ωe), the rotation constant (Be) and the dissociation energy (De). Our results indicate that the electronic states of ZnTe and its ions converge to several lower dissociation limits. The existence of unstable and crossings states is due to the presence of rotational and spin–orbit interactions. The results obtained are compared to the existing experimental data and have generally shown a reasonable agreement.

The Coming Age of Pnictide and Chalcogenide Ternary Crystals in the Terahertz Frequency Regime

The last decade has witnessed explosive growth in terahertz radiation fields, ranging from fundamental research to applications. This progress has been marked by the introduction of various terahertz radiation sources and detectors. Here, we review the progress of nonlinear pnictide and chalcogenide ternary crystals for use in the terahertz frequency regime. At the center of this topical review are the various physical and optical properties of numerous emerging pnictide and chalcogenide nonlinear crystals, which are anchored to state-of-the-art coherent terahertz radiation sources based on optical rectification and difference frequency generation, as well as terahertz radiation phase-resolved detectors based on electro-optic sampling. This review aims to provide the foundation necessary to continue advancing pnictide and chalcogenide ternary crystals for the generation, detection, and manipulation of terahertz radiation.

Theory of the in-plane photoelectric effect in two-dimensional electron systems

A new photoelectric phenomenon, the in-plane photoelectric (IPPE) effect, has been recently discovered at terahertz (THz) frequencies in a GaAs/Al$_x$Ga$_{1-x}$As heterostructure with a two-dimensional (2D) electron gas (W. Michailow et al., Sci. Adv. \textbf{8}, eabi8398 (2022)). In contrast to the conventional PE phenomena, the IPPE effect is observed at normal incidence of radiation, the height of the in-plane potential step, which electrons overcome after absorption of a THz photon, is electrically tunable by gate voltages, and the effect is maximal at a negative electron ``work function'', when the Fermi energy lies above the potential barrier. Based on the discovered phenomenon, efficient detection of THz radiation has been demonstrated. In this work we present a detailed theory of the IPPE effect providing analytical results for the THz wave generated photocurrent, the quantum efficiency, and the internal responsivity of the detector, in dependence on the frequency, the gate voltages, and the geometrical parameters of the detector. The calculations are performed for macroscopically wide samples at zero temperature. Results of the theory are applicable to any semiconductor systems with 2D electron gases, including III-V structures, silicon-based field effect transistors, and the novel 2D layered, graphene-related materials.

Terahertz time-domain ellipsometry: tutorial.

Ellipsometry is extensively used in the optical regime to investigate the properties of many materials as well as to evaluate with high precision the surface roughness and thickness of thin films and multilayered systems. Due to the inherent non-coherent detection technique, data analyses in optical ellipsometry tend to be complicated and require the use of a predetermined model, therefore indirectly linking the sample properties to the measured ellipsometric parameters. The aim of this tutorial is to provide an overview of terahertz (THz) time-domain ellipsometry, which is based instead on a coherent detection approach and allows in a simple and direct way the measurement of the material response. After giving a brief description of the technology presently used to generate and detect THz radiation, we introduce the general features of an ellipsometric setup operating in the time domain, putting in evidence similarities and differences with respect to the classical optical counterpart. To back up and validate the study, results of THz ellipsometric measurements carried out on selected samples are presented.