Institute of Superhigh-Frequency Semiconductor Electronics of the Russian Academy of Sciences

Abstract Over the past few decades, significant attention has been paid to the biomedical applications of terahertz (THz) technology. Nowadays, THz spectroscopy and imaging have allowed numerous demanding problems in the biological, medical, food, plant and pharmaceutical sciences to be solved. Among the biomedical applications, the label-free diagnosis of malignant and benign neoplasms represents one of the most attractive branches of THz technology. Despite this attractiveness, THz diagnosis methods are still far from being ready for use in medical practice. In this review, we consider modern research results in the THz diagnosis of malignant and benign neoplasms, along with the topical research and engineering problems which restrain the translation of THz technology to clinics. We start by analyzing the common models of THz-wave–tissue interactions and the effects of tissue exposure to THz waves. Then, we discuss the existing modalities of THz spectroscopic and imaging systems, which have either already been applied in medical imaging, or hold strong potential. We summarize the earlier-reported and original results of the THz measurements of neoplasms with different nosology and localization. We pay attention to the origin of contrast between healthy and pathological tissues in the THz spectra and images, and discuss the prospects of THz technology in non-invasive, minimally invasive and intraoperative diagnosis, as well as in aiding histology. Finally, we review the challenging problems of THz diagnosis, as well as attempts to solve them, which should bring THz technology much closer to medical practice. This review allows one to objectively uncover the benefits and weaknesses of THz technology in the diagnosis of malignant and benign neoplasms.

We have developed a method of terahertz (THz) solid immersion (SI) microscopy for continuous-wave reflection-mode imaging of soft biological tissues with a sub-wavelength spatial resolution. In order to achieve strong reduction in the dimensions of the THz beam caustic, an electromagnetic wave is focused into the evanescent field volume behind a medium with a high refractive index. We have experimentally demonstrated a 0.15λ-resolution of the proposed imaging modality at λ = 500 μm, which is beyond the Abbe diffraction limit and represents a considerable improvement over the previously-reported arrangements of SI imaging setups. The proposed technique does not involve any sub-wavelength near-field probes and diaphragms, thus, avoiding the THz beam attenuation due to such elements. We have applied the developed method for THz imaging of various soft tissues: a plant leaf blade, cell spheroids, and tissues of the breast ex vivo. Our THz images clearly reveal sub-wavelength features in tissues, therefore, promising applications of THz SI microscopy in biology and medicine.

This review highlights recent and novel trends focused on metallic (plasmonic) and dielectric metasurfaces in photoconductive terahertz (THz) devices. We demonstrate the great potential of its applications in the field of THz science and technology, nevertheless indicating some limitations and technological issues. From the state-of-the-art, the metasurfaces are, by far, able to force out previous approaches like photonic crystals and are capable of significantly increasing the performance of contemporary photoconductive devices operating at THz frequencies.

Unique properties of graphene are combined to enable graphene plasmonic devices that could revolutionize the terahertz (THz) electronic technology. A high value of the carrier mobility allows us to excite resonant plasma waves. The graphene bipolar nature allows for different mechanisms of plasma wave excitation. Graphene bilayer and multilayer structures make possible improved THz device configurations. The ability of graphene to form a high quality heterostructure with h-BN, black phosphorus, and other materials systems supports advanced heterostructure devices comprised of the best properties of graphene and other emerging materials. In particular, using black phosphorus compounds for cooling electron–hole plasma in graphene could dramatically improve the conditions for THz lasing. High optical phonon energy allows for reaching higher plasma frequencies that are supported by high sheet carrier densities in graphene. Recent improvements in graphene technology combined with a better understanding of the device physics of graphene THz plasmonics and graphene plasmonic device designs hold promise to make graphene THz plasmonic technology one of the key graphene applications. Commercialization of plasmonic graphene technology is facing the same challenges as other graphene applications, which have difficulties in producing uniform large graphene layers, bilayers, and heterostructures of high quality and making good low resistance stable Ohmic contacts. The time projection for large scale graphene electronic device applications now extends into the 2030s. However, emerging graphene mass production technologies might bring commercial applications of the graphene plasmonic terahertz technology closer.

We report on the first experimental observation of terahertz emission and detection in a double graphene layered (GL) heterostructure which comprises a thin hexagonal-boron nitride tunnel-barrier layer sandwiched between two separately contacted GLs. Inter-GL population inversion is induced by electrically biasing the structure. Resonant tunneling and negative differential resistance is expected when the two graphene band structures are perfectly aligned. However, in the case of small misalignments we demonstrate that the photon-absorption/emission-assisted non-resonant- and resonant-tunneling causes all excess charges in the n-type GL to recombine with the holes in the p-type GL giving rise to an increased measured dc current. This work highlights a novel strategy for the realization of efficient voltage-tunable terahertz emitters and detectors.

Abstract In this paper, we report on an approach for shaping the spectra of THz pulse generation in photoconductive antennas (PCAs) by frequency-dependent impedance modulation. We introduce a theoretical model describing the THz pulse generation in PCAs, and accounting for impedances of the photoconductor and of the antenna. Additionally we account a detector response for comparative analysis with experimental data. In order to showcase an impact of frequency-dependent impedance modulation on the spectra of THz pulse generation, we applied this model to simulating broadband PCAs with log-spiral topology. Finally, we fabricated two different log-spiral PCAs and characterized them experimentally using the THz pulsed spectroscopy. The observed results demonstrate agreement between the theoretical model and experiment, justifying the potential of shaping the spectra of THz pulse generation in PCAs by modulation of frequency-dependent impedances. This approach makes it possible to optimize PCA performance and thus accommodes the needs of THz pulsed spectroscopy and imaging in fundamental and applied branches of THz science and technology.

We report on the design, optimization and fabrication of a plasmon-assisted terahertz (THz) photoconductive antenna (PCA) for THz pulse generation at low-power optical pumps. The PCA features a high aspect ratio dielectric-embedded plasmonic Au grating placed into the photoconductive gap. Additionally, Si3N4-passivation of the photoconductor and the Al2O3-antireflection coating are used to further enhance antenna performance. For comparative analysis of the THz photocurrents, THz waveforms and THz power spectra we introduced the THz photocurrent δi and the THz power enhancement δTHz factors, which are defined as ratios between the THz photocurrents and the THz power spectra for the plasmon-assisted and conventional PCAs. We demonstrated superior performance of the plasmon-assisted PCA δi=30 and δTHz=3 ⋅ 103 at the lowest optical pump power of P=0.1 mW. Nevertheless the increase to P=10 mW lead to monotonically decrease in the both values to δi=2 and δTHz=102 due to screening effects. These results demonstrate a strong potential of the plasmonic PCA for operation with low-power lasers, thus, opening opportunities for the development of portable and cost-effective THz spectrometers and imaging systems.

Ever increasing demands of data traffic makes the transition to 6G communications in the 300 GHz band inevitable. Short-channel field-effect transistors (FETs) have demonstrated excellent potential for detection and generation of terahertz (THz) and sub-THz radiation. Such transistors (often referred to as TeraFETs) include short-channel silicon complementary metal oxide (CMOS). The ballistic and quasi-ballistic electron transport in the TeraFET channels determine the TeraFET response at the sub-THz and THz frequencies. TeraFET arrays could form plasmonic crystals with nanoscale unit cells smaller or comparable to the electron mean free path but with the overall dimensions comparable with the radiation wavelength. Such plasmonic crystals have a potential of supporting the transition to 6G communications. The oscillations of the electron density (plasma waves) in the FET channels determine the phase relations between the unit cells of a FET plasmonic crystal. Excited by the impinging radiation and rectified by the device nonlinearities, the plasma waves could detect both the radiation intensity and the phase enabling the line-of-sight terahertz (THz) detection, spectrometry, amplification, and generation for 6G communication.

Terahertz (THz) waves feature high sensitivity to the content and state of water in biological tissues. Therefore, during the past decades, THz technology has attracted significant attention in biophotonics, including diagnosis of malignant and benign neoplasms with different nosologies and localizations. The pathophysiological features of malignant tumors of the central nervous system determine appearance of several morphological phenomena, such as increased vascularity, edema, necrosis. These phenomena cause water content increase in the studied tissues and, thus, open new ways for the THz technology applications in the intraoperative neurodiagnosis, including delineation of tumor margins. This research area is rather novel and, despite the small amount of accumulated research material, is undoubtedly extremely promising for creation of new diagnostic approaches. In this review, available results in the considered exciting branch of THz technology are summarized, and potential projections of this topic into the future are constructed.

We propose, fabricate, and evaluate strain-induced InGaAs/InAlAs superlattice (SL), which can efficiently radiate broadband terahertz (THz) waves. By means of optical pump-probe measurements, we demonstrate ultrashort photocarriers relaxation times of τ∼1.7ps without Be-doping of InGaAs photoconductive layers. We assume two dominant mechanisms to be responsible for a sharp reduction of τ in strained SL, which are photocarriers scattering at InGaAs/InAlAs heterointerface roughness and the decrease in the energy bandgap of InGaAs photoconductive layers due to the residual strain. The THz time-domain spectroscopic measurements reveal the rise in both emitted THz waveform and spectrum amplitudes with an increase of the residual strain in SL, in particular, at the low-frequency region. We refer this to the band structure engineering due to the residual strain in SL—since InGaAs photoconductive layers become compressively strained, this reduces the semiconductor’s energy bandgap, thus more photocarriers can contribute to the THz emission. The results might be of specific interest for the development of portable THz pulsed spectroscopic and imaging systems and other fundamental and applied aspects of the THz science and technology.

We propose and theoretically substantiate the concept of terahertz and infrared photodetectors using the resonant radiative transitions between graphene layers (GLs) in double-GL structures. The calculated absorption spectrum and the spectral characteristics of the photodetector responsivity exhibit sharp resonant maxima at the photon energies in a wide range. The resonant maxima can be tuned by the applied voltage. We compare the photodetector responsivity with that of the GL p-i-n photodiodes and quantum-well infrared photodetectors. Weak temperature dependences of the photocurrent and dark current enable the effective operation of the proposed photodetector at room temperature.

Abstract Today the surface plasmon‐polariton (SPP) waves propagating along the curve trajectory are the only plasmonic beams of the Airy family. Herein, a new class of curved surface plasmon wave, the photonic hook plasmon (PHP), is introduced. The PHP is created using the in‐plane focusing of the SPP wave through an asymmetric dielectric particle. This is fundamentally simpler than the generation of the SPP Airy‐family beams. The PHP propagates along a wavelength‐scaled curved trajectory with radius less than the SPP wavelength, which represents the smallest radius of curvature ever recorded for an SPP beam, and can exist despite the strong energy dissipation at metal surface.

Abstract Beginning from the 1990s, an ever-lasting interest in the terahertz (THz) spectroscopy and THz instruments has produced wide progress in the development of high-speed THz detectors. The constantly growing requirements aimed at the increase of spectral resolution, sensitivity, and acquisition rate of THz detectors have attracted much attention in this field. In the present review, we summarize the most recent advances in the THz photodetectors based on semiconductor structures with quantum confinement of an electron gas. Their main advantages over existing detectors are fast response time, increased spectral resolution, and multicolor operation thanks to the variability of their designs and band structure engineering. These all allow for using them in various important applications such as single photon detection, THz heterodyne detection, continuous monitoring of toxic gases, THz free space communications and radio astronomy.

Unique effects of terahertz (THz)-wave–matter interaction push rapid progress in THz optoelectronics aimed at bridging the problematic THz gap. However, majority of modern methods of THz spectroscopy and imaging are still hampered by low spatial resolution. Common lens/mirror-based THz optics fails to overcome the Abbe barrier and usually provides resolution larger than a free-space wavelength λ (i.e., hundreds of micrometers or even few millimeters). To mitigate this difficulty, supperresolution THz imaging modalities were introduced recently, among which we particularly underline different methods of THz scanning-probe near-field microscopy. They not only rely on strong light confinement on sub-wavelength probes and provide resolution down to ∼10−1–10−3λ but also suffer from small energy efficiency or presume an interplay among imaging resolution, signal-to-noise ratio, and performance. In this paper, we consider reflection-mode THz solid immersion (SI) microscopy that offers some compromise between the high imaging resolution of 0.15λ and high energy efficiency, which is due to the absence of any subwavelength probe in an optical scheme. Recent achievements, challenging problems, and prospects of SI microscopy are overviewed with an emphasis on resolving the inverse problem and applications in THz biophotonics.

We develop the model for the terahertz (THz) and infrared (IR) photoconductivity of graphene layers (GLs) at room temperature. The model accounts for the linear GL energy spectrum and the features of the energy relaxation and generation-recombination mechanisms inherent at room temperature, namely, the optical phonon absorption and emission and the Auger interband processes. Using the developed model, we calculate the spectral dependences of the THz and IR photoconductivity of the GLs. We show that the GL photoconductivity can change sign depending on the photon frequency, the GL doping and the dominant mechanism of the carrier momentum relaxation. We also evaluate the responsivity of the THz and IR photodetectors using the GL photoconductivity. The obtained results along with the relevant experimental data might reveal the microscopic processes in GLs, and the developed model could be used for the optimization of the GL-based photodetectors.

The lack of radiation sources in the frequency range of 7-10 THz is associated with strong absorption of the THz waves on optical phonons within the GaAs Reststrahlen band. To avoid such absorption, we propose to use HgCdTe as an alternative material for THz quantum cascade lasers thanks to a lower phonon energy than in III-V semiconductors. In this work, HgCdTe-based quantum cascade lasers operating in the GaAs phonon Reststrahlen band with a target frequency of 8.3 THz have been theoretically investigated using the balance equation method. The optimized active region designs, which are based on three and two quantum wells, exhibit the peak gain exceeding 100 cm −1 at 150 K. We have analyzed the temperature dependence of the peak gain and predicted the maximum operating temperatures of 170 K and 225 K for three- and two-well designs, respectively. At temperatures exceeding 120 K, the better temperature performance has been obtained for the two-well design, which is associated with a larger spatial overlap of weakly localized lasing wavefunctions, as well as, a higher population inversion. We believe that the findings of this work can open a pathway towards the development of THz quantum cascade lasers featuring a high level of optical gain due to the low electron effective mass in HgCdTe.

We propose and study a microstructure based on a dielectric cuboid placed on a thin metal film that can act as an efficient plasmonic lens allowing the focusing of surface plasmons at the subwavelength scale. Using numerical simulations of surface plasmon polariton (SPP) field intensity distributions, we observe high-intensity subwavelength spots and formation of the plasmonic nanojet (PJ) at the telecommunication wavelength of 1530 nm. The fabricated microstructure was characterized using amplitude and phase-resolved scattering-type scanning near-field optical microscopy. We show the first experimental observation of the PJ effect for the SPP waves. Such a novel, to the best of our knowledge, and simple platform can provide new pathways for plasmonics, high-resolution imaging, and biophotonics, as well as optical data storage.

This paper reviews recent advances in the research and development of graphene-based plasmonic metamaterials for terahertz (THz) laser transistors. The authors' theoretical discovery on THz laser transistors in 2007 was realized as a distributed-feedback dual-gate graphene-channel field-effect transistor (DFB-DG-GFET) in 2018, demonstrating ∼0.1 µW single-mode emission at 5.2 THz and ∼80 µW amplified spontaneous 1-7.6 THz emission at 100 K. To realize room-temperature, dry-cell-battery operating intense THz lasing with fast direct modulation, various approaches based on graphene plasmonic metamaterials are investigated and introduced as real device implementations, including (i) replacement of the laser photonic cavity with plasmonic cavity enormously improving the THz photon field confinement with larger gain overlapping, (ii) introduction of THz amplification of stimulated emission via current-driven graphene Dirac plasmons (GDPs), and (iii) controlling the parity and time-reversal symmetry of GDPs enabling ultrafast direct gain-switch modulation. Possible real device structures and design constraints are discussed and addressed toward coherent light sources applicable to future 6G- and 7G-class THz wireless communication systems.

We report on the experimental observation of the curved plasmonic beam, a plasmonic hook (PH), for surface plasmon-polariton (SPP) waves. The SPP PH effect could be obtained with a cuboid particle with broken shape symmetry fabricated with a relatively simple routine. This has a pronounced difference with fabrication of the structure for generation of the Airy SPPs, which require complex techniques to compensate the wave vector mismatch. We confirmed the existence of SPP PH by amplitude scattering scanning near-field optical microscopy. The experimental results agree well with our predictions. Importantly, the SPP PH demonstrates the smallest curvature of the beam ever recorded for SPPs compared to that for the Airy-family plasmonic beams, which potentially can strongly impact many useful applications from nanoparticle manipulation to nanoscale bio-sensing.

We propose and evaluate the heterostructure based on the graphene layer (GL) with the lateral electron injection from the side contacts and the hole vertical injection via the black phosphorus layer (BL) $({\mathrm{P}}^{+}\text{\ensuremath{-}}\mathrm{PL}$-PL-GL heterostructure). Due to a relatively small energy of the holes injected from the PL into the GL (about 100 meV, smaller than the energy of optical phonons in the GL which is about 200 meV), the hole injection can effectively cool down the two-dimensional electron-hole plasma in the GL. This simplifies the realization of the interband population inversion and the achievement of the negative dynamic conductivity in the terahertz (THz) frequency range enabling the amplification of the surface plasmon modes. The latter can lead to the plasmon lasing. The conversion of the plasmons into the output radiation can be used for new types of the THz sources.