Subwavelength Terahertz Research Articles

Spatial and spectral beam characteristics in a terahertz broadband sub-wavelength imaging system using a solid immersion lens

This study focuses on the spatial and spectral beam characteristics in a terahertz (THz) broadband sub-wavelength imaging system using a solid immersion lens (SIL). Previously, we demonstrated a broadband sub-wavelength THz imaging system by integrating a SIL with a THz time-domain spectrometer (TDS). Key parameters that influence beam characteristics and, consequently, imaging performance, such as SIL misalignment tolerances and beam propagation from the SIL, constitute the primary focus of this investigation. Numerical simulations demonstrate that the system can tolerate millimeter-level transverse and longitudinal SIL position displacements, underscoring its robustness for sub-wavelength imaging in a wide frequency range. Additionally, numerical simulations of beam propagation characteristics reveal that the system achieves sub-wavelength imaging resolution up to 1 mm from the SIL at 0.5 THz, highlighting its potential for non-destructive testing of subsurface structures. These findings gain experimental validation through imaging stacked utility knife blades with sub-wavelength structures ranging from 0.2 to 2 THz.

Near-field detection of gate-tunable anisotropic plasmon polaritons in black phosphorus at terahertz frequencies

Polaritons in two-dimensional layered crystals offer an effective solution to confine, enhance and manipulate terahertz (THz) frequency electromagnetic waves at the nanoscale. Recently, strong THz field confinement has been achieved in a graphene-insulator-metal structure, exploiting THz plasmon polaritons (PPs) with strongly reduced wavelength (λp ≈ λ0/66) compared to the photon wavelength λ0. However, graphene PPs propagate isotropically, complicating the directional control of the THz field, which, on the contrary, can be achieved exploiting anisotropic layered crystals, such as orthorhombic black-phosphorus. Here, we detect PPs, at THz frequencies, in hBN-encapsulated black phosphorus field effect transistors through THz near-field photocurrent nanoscopy. The real-space mapping of the thermoelectrical near-field photocurrents reveals deeply sub-wavelength THz PPs (λp ≈ λ0/76), with dispersion tunable by electrostatic control of the carrier density. The in-plane anisotropy of the dielectric response results into anisotropic polariton propagation along the armchair and zigzag crystallographic axes of black-phosphorus. The achieved directional subwavelength light confinement makes this material system a versatile platform for sensing and quantum technology based on nonlinear optics.

Broadband sub-wavelength terahertz subsurface imaging using a solid-immersion lens

We demonstrate a broadband sub-wavelength terahertz (THz) imaging system by combining a solid-immersion lens (SIL) with a THz time-domain spectrometer (TDS). The imaging resolution of the system is enhanced in a wide spectral range (from 0.3 to 2.0 THz) with the aid of the SIL. The beam size is within 120–280 μm (0.28–0.8 λ) in the measured frequency range. Experimentally achieved imaging resolution enhancement results are consistent with the numerical simulation results. Unlike the previous reports on imaging systems using SIL, the beam size reduction by the SIL is more than a factor of nSIL (3.42) for all frequencies near the focal spot. An analytical calculation is employed to obtain insights into the beam size reduction mechanism and estimate the beam size in a given experimental condition. THz image of the WR1.9 waveguide is measured to highlight the broadband sub-wavelength imaging capability. The increased reflection from the waveguide near the cutoff frequency for TE10 and TE01 modes, inaccessible with other THz-TDS-based or continuous wave (CW) source-based THz imaging systems, is experimentally observed. In addition, we show the multispectral sub-wavelength images of subsurface structures by selecting the appropriate measurement time window of the temporal THz signal. THz images of a credit card show the sub-wavelength details of the card, including an IC chip, invisible due to the plastic cover. The experimental results highlight the potential applications of the imaging system in non-contact, non-destructive testing of the sub-wavelength scale structures in various fields.

Subwavelength terahertz imaging via virtual superlensing in the radiating near field

Imaging with resolutions much below the wavelength λ – now common in the visible spectrum – remains challenging at lower frequencies, where exponentially decaying evanescent waves are generally measured using a tip or antenna close to an object. Such approaches are often problematic because probes can perturb the near-field itself. Here we show that information encoded in evanescent waves can be probed further than previously thought, by reconstructing truthful images of the near-field through selective amplification of evanescent waves, akin to a virtual superlens that images the near field without perturbing it. We quantify trade-offs between noise and measurement distance, experimentally demonstrating reconstruction of complex images with subwavelength features down to a resolution of λ/7 and amplitude signal-to-noise ratios < 25dB between 0.18–1.5 THz. Our procedure can be implemented with any near-field probe, greatly relaxes experimental requirements for subwavelength imaging at sub-optical frequencies and opens the door to non-invasive near-field scanning.

Room-temperature-grown tungsten oxide hybridized dipole cavities to realize thermally tunable terahertz surface plasmons

Recent developments in the field of terahertz (THz) subwavelength structures have inspired novel discoveries and innovations involving the use of multidimensional materials in fields such as spectroscopy, photonics, biomedical imaging, nonlinearity, and communication technology. In this context, metal oxide materials with adaptable electronic and optical properties have emerged as promising candidates to realize dynamically reconfigurable THz subwavelength devices. Therefore, we experimentally demonstrated temperature-tunable surface plasmon resonances (SPRs) in an array of tungsten oxide (WO3) hybridized subwavelength cavities. By using THz time-domain spectroscopy, we examined the thermally tunable dielectric properties of a WO3 thin film over a temperature range of 25 °C–110 °C. On the basis of the tunable optoelectronic properties of WO3, we implement a THz subwavelength dipole cavity that can exhibit a temperature-dependent dynamic modification in SPR characteristics. Such tunability could be used to fabricate novel active SPR-based plasmonic devices like temperature sensors, spatial modulators, filters, and integrated THz optoelectronic elements.

High Q and sub-wavelength THz electric field confinement in ultrastrongly coupled THz resonators

The control of light–matter coupling at the single electron level is currently a subject of growing interest for the development of novel quantum devices and for studies and applications of quantum electrodynamics. In the terahertz (THz) spectral range, this raises the particular and difficult challenge of building electromagnetic resonators that can conciliate low mode volume and high quality factor. Here, we report on hybrid THz cavities based on ultrastrong coupling between a Tamm cavity and an LC circuit metamaterial and show that they can combine high quality factors of up to Q=37 with a deep-subwavelength mode volume of V=3.2×10−4λ3. Our theoretical and experimental analysis of the coupled mode properties reveals that, in general, the ultrastrong coupling between a metamaterial and a Fabry–Perot cavity is an effective tool to almost completely suppress radiative losses and, thus, ultimately limit the total losses to the losses in the metallic layer. These Tamm cavity-LC metamaterial coupled resonators open a route toward the development of single photon THz emitters and detectors and to the exploration of ultrastrong THz light–matter coupling with a high degree of coherence in the few to single electron limit.

Terahertz Sub-wavelength Focusing and Negative Refraction Assisted Beam Transferring Based on 3-D Metamaterial Flat Lens Configurations

Terahertz Sub-wavelength Focusing and Negative Refraction Assisted Beam Transferring Based on 3-D Metamaterial Flat Lens Configurations

Subwavelength Terahertz Resonance Imaging (STRING) for Molecular Fingerprinting.

Subwavelength terahertz (THz) imaging methods are highly desirable for biochemical sensing as well as materials sciences, yet sensitive spectral fingerprinting is still challenging in the frequency domain due to weak light-matter interactions. Here, we demonstrate subwavelength THz resonance imaging (STRING) that overcomes this limitation to achieve ultrasensitive molecular fingerprinting. STRING combines individual ring-shaped coaxial single resonators with near-field spectroscopy, yielding considerable sensitivity gains from both local field enhancement and the near-field effect. As an initial demonstration, we obtained spectral fingerprints from isomers of α-lactose and maltose monohydrates, achieving sensitivity that was enhanced by up to 10 orders of magnitude compared to far-field THz measurements with pelletized samples. Our results show that the STRING platform could enable the development of THz spectroscopy as a practical and sensitive tool for the fingerprinting and spectral imaging of molecules and nanoparticles.

A Cost-Effective Approach for Achieving Subwavelength THz Imaging Using Photoinduced Coded-Apertures on Mesa-Array Structures

A novel and cost-effective approach for high-resolution terahertz (THz) imaging using photoinduced (PI) coded-apertures on micromachined mesa-array structures is reported. The mesa-array structures are designed to have subwavelength electrically isolated mesas for generation of high-fidelity photopatterns for THz imaging. An Si mesa-array prototype structure was successfully fabricated and tested. Measurements show that a modulation depth of ∼22 dB was obtained in the frequency range of 740–750 GHz under a light intensity of 11.7 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which agrees well with theoretical analysis and full-wave simulation. Initial imaging experiments were first performed using 64 pixels (over different areas) for validation and performance evaluation. More advanced imaging using quadrupled pixel number (256 pixels) was conducted in order to resolve subwavelength features. As a result, a wavelength-scale spatial resolution of 400 μm has been successfully demonstrated, and a subwavelength resolution as small as ∼250 μm (0.625 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> at 740 GHz) has been shown to be potentially achievable. The proposed PI coded-aperture imaging using mesa arrays is promising for developing real-time high-resolution THz imaging with simple system and low cost.

Enhanced confinement of infrared surface plasmon polaritons in borophene waveguides

In this paper, a sub-wavelength terahertz plasmonic waveguide based on a borophene–insulator–metal (BIM) structure is investigated by using the 2D finite-difference time-domain method. It is proved there is a plasmonic mode of high confinement and low propagation loss in an optimized wavelength range. A dynamically adjustable filter, formed by introducing a borophene nanoribbon into the BIM waveguide, is computationally and theoretically explored, which can achieve tailoring terahertz surface plasmon polaritons in deep-subwavelength scale. These results can further provide a broader path for dynamic regulation of light transmission in the borophene-based waveguide and the development of adjustable borophene-based optoelectronic filtering.

Novel Terahertz Properties of Nanostructured Mn3+0.53Sn Films with Different Crystalline Orientations Driven by Ostwald Ripening on (0001) c-Al2O3

The characteristic energies of elementary excitations and collective modes in many quantum materials lie mostly in the terahertz (THz) frequency range, which provides a wide space for the development of THz optical materials and devices. In particular, topological Weyl semimetal Mn3Sn is a noncollinear antiferromagnet with anomalous THz properties, which is strongly affected by thermal energy and external magnetic fields. Despite the explosive growth of the research on magnetic Weyl semimetals recently, its nanoscale structure for applications in THz optical devices remains to be explored. Here, we fabricated nanostructured Mn3+0.53Sn films with different crystal orientations, driven by Ostwald Ripening (OR) on (0001) c-Al2O3. A huge anisotropic THz response manifested a firm link between the optical properties of Weyl antiferromagnet Mn3Sn and its contrivable spin structures. The topological properties of Mn3Sn are robustly protected in its nanostructures. This work can provide a new horizon for the fabrication of a nanostructured magnetic Weyl alloy and for its potential applications in subwavelength high-performance THz devices.

Terahertz subwavelength sensing with bio-functionalized germanium fano-resonators

Abstract Localized Surface Plasmon Resonances (LSPR) based on highly doped semiconductors microstructures, such as antennas, can be engineered to exhibit resonant features at THz frequencies. In this work, we demonstrate plasmonic antennas with increased quality factor LSPRs from Fano coupling to dark modes. We also discuss the advances in the biofunctionalization of n-doped Ge antennas for specific protein immobilization and cell interfacing. Finally, albumin biolayers with a thickness of a few hundred nanometers are used to demonstrate the performance of the fano-coupled n-Ge antennas as sensors. A resonant change of over 10% in transmission, due to the presence of the biolayer, can be detected within a bandwidth of only 20 GHz.

A low loss platform for subwavelength terahertz graphene plasmon propagation

Terahertz (THz) waves are assumed to be the candidate for next generation wireless communication owing to the broad frequency band and high channel capability. In order to accommodate the high data transfer rate, THz communication devices with high efficiency are in urgent need. Besides, the seamless connections between optical fibers and THz devices are also of great importance for on-chip THz propagating and manipulation. Here we propose a semiconductor nanowire (GaAs) based hybrid graphene plasmon waveguide, and realize a platform supporting graphene plasmon modes with low loss and strong optical energy confinement. Investigation on the influence of modal properties on nanowire geometrical shapes and orientations, key geometric parameter, Fermi energy level and carrier mobility of graphene revealed an achievement of a good balance between mode confinement and propagation loss. Higher than 50% modulation in the propagation length is reported. The proposed structure is capable of realizing future THz communication components and tunable devices.

All-optical terahertz logic gates based on coupled surface plasmon polariton sub-wavelength waveguiding in bulk Dirac semimetal

Bulk Dirac semimetals (BDS) are quantum fermionic system also identified as three-dimensional counterpart of graphene exhibit rich plasmonic response at terahertz (THz) frequencies. In this work, we report THz waveguiding in the BDS curved geometries, which confine electromagnetic field in the form of symmetrically coupled surface plasmon polariton (SPP) modes. We demonstrate how the sub-wavelength BDS-Air-BDS multilayer system with various bending and corners efficiently confines THz beam with minimal bending losses. The realized sub-wavelength THz waveguiding in BDS system is advantageous than other approaches like zero-refractive index metamaterials. Based on the symmetric coupled SPP waveguiding in BDS multilayers, all-optical logic gates including OR, AND, XOR, NOR and NAND are realized at THz waveband. The realized THz logic gates exhibit optimal threshold transmission (at least 50%) for defining ON-OFF states. We anticipate that the illustrated waveguiding and logic gate applications in BDS system could open an avenue for optoelectronic applications at THz frequencies.

Investigations on Practical Issues in Solid Immersion Lens Based Sub-Wavelength Terahertz Imaging Technique: System Stability Verification and Interference Pattern Removal.

Terahertz (THz) imaging techniques are attractive for a wide range of applications, such as non-destructive testing, biological sensing, and security imaging. We investigate practical issues in THz imaging systems based on a solid immersion lens (SIL). The system stability in terms of longitudinal misalignment of the SIL is experimentally verified by showing that the diffraction-limited sub-wavelength beam size (0.7 λ) is maintained as long as the SIL is axially located within the depth-of-focus (~13 λ) of the objective lens. The origin of the fringe patterns, which are undesirable but inevitable in THz imaging systems that use continuous waves, is analytically studied, and a method for minimizing the interference patterns is proposed. By combining two THz images obtained at different axial positions of the object and separated by λ/4, the interference patterns are significantly reduced, and the information hidden under the interference patterns is unveiled. The broad applicability of the proposed method is demonstrated by imaging objects with different surface profiles. Our work proves that the resolution of conventional THz imaging systems can easily be enhanced by simply inserting a SIL in front of the object with high tolerance in the longitudinal misalignment and provides a method enabling THz imaging for objects with different surface profiles.

Sub-wavelength continuous THz imaging system based on interferometric detection.

We have developed a continuous wave sub-wavelength terahertz (THz) imaging system that combines two prominent classical optical techniques: solid immersion microscopy and interferometric detection. This combination allows for resolution beyond the diffraction limit at 703 GHz. We experimentally demonstrate sub-wavelength spatial resolution working with a relatively low-cost pyroelectric detector and with both high and low contrast samples.

Terahertz subwavelength edge detection based on dispersion-induced plasmons.

Terahertz imaging has recently attracted great attention owing to the abilities of high penetration and low ionizing damages. However, the low resolution and low contrast resulting from the diffraction limit and unwanted background illumination significantly hinder the extensive usage. In this Letter, we propose and numerically demonstrate a terahertz subwavelength imaging method capable of extracting only the edges and fine features of the targets. The underlying physics is the efficient transmission of the scattering evanescent waves related to key geometric information while blocking the propagating components. By exploiting the structurally induced plasmons in a bounded metallic waveguide, the transmission channel for evanescent waves is realized by hyperbolic metamaterials through periodically stacking dielectric layers. On this basis, high-contrast edge detection with a resolution up to ${0.1}\lambda$ is demonstrated at terahertz wavelengths. The proposed terahertz imaging method may find important applications in non-destructive testing, weak scattering object detection, and high-contrast microscopy.

Dispersion-limited versus power-limited terahertz communication links using solid core subwavelength dielectric fibers

Terahertz (THz) band (0.1–10 THz) is the next frontier for ultra-high-speed communication systems. Currently, most of communications research in this spectral range is focused on wireless systems, while waveguide/fiber-based links have been less explored. Although free space communications have several advantages such as convenience in mobility for the end user, as well as easier multi-device interconnectivity in simple environments, fiber-based communications provide superior performance in certain short-range communication applications such as multi-device connectivity in complex geometrical environments (ex., intra-vehicle connectivity) and secure communications with low probability of eavesdropping, as well as secure signal delivery to hard-to-reach or highly protected environments. In this work, we present an in-depth experimental and numerical study of the short-range THz communications links that use subwavelength dielectric fibers for information transmission and define the main challenges and trade-offs in the link implementation. Particularly, we use air or foam-cladded polypropylene-core subwavelength dielectric THz fibers of various diameters (0.57–1.75 mm) to study link performance as a function of the link length of up to ∼ 10 m , and data bit rates of up to 6 Gbps at the carrier frequency of 128 GHz (2.34 mm wavelength). We find that depending on the fiber diameter, the quality of the transmitted signal is mostly limited either by the modal propagation loss or by the fiber velocity dispersion (GVD). An error-free transmission over 10 m is achieved for the bit rate of 4 Gbps using the fiber of smaller 0.57 mm diameter. Furthermore, since the fields of subwavelength fibers are weakly confined and extend deep into the air cladding, we study the modal field extent outside of the fiber core, as well as fiber bending loss. Finally, the power budget of the rod-in-air subwavelength THz fiber-based links is compared to that of free space communication links, and we demonstrate that fiber links offer an excellent solution for various short-range applications.

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.

Super-Resolution Orthogonal Deterministic Imaging Technique for Terahertz Subwavelength Microscopy

Terahertz subwavelength imaging aims at developing THz microscopes able to resolve deeply subwavelength features. To beat the diffraction limit, the current trend is to use various subwavelength probes to convert the near-field to the far-field. These techniques offer significant gains in spatial resolution but suffer from low light throughput and are slow due to the necessity of a slow pixel-by-pixel raster scan. In parallel, in the visible spectral range, super-resolution imaging techniques enhance the image resolution by statistically correlating multiple frames of an object backlit by stochastically blinking fluorophores. In this work, we develop a super-resolution imaging technique for the THz range, that we name super-resolution orthogonal deterministic imaging (SODI). Since there are no natural THz fluorophores, we design artificial fluorophores in the form of optimal mask sets brought close to the object. By deterministically controlling the blinking, we avoid statistical averages and reconstruct high resolution images using very few frames. After developing the theoretical basis of SODI, we experimentally demonstrate the second-order resolution improvement using only eight phase and binary amplitude masks. We then show how to extend the SODI technique to higher orders to further improve the resolution. Our methodology can be readily adapted with existing THz phase-sensitive single-pixel imaging systems or any THz amplitude imaging arrays. Finally, this work can be of interest to the optical community in other wavelengths, as our technique can be used to deterministically structure light at a subwavelength scale in order to improve the image resolution with few frames and achieve real-time super-resolution microscopy.