Near-field Terahertz Sensing of HeLa Cells and Pseudomonas Based on Monolithic Integrated Metamaterials with a Spintronic Terahertz Emitter

Terahertz label-free detection of nicotine-induced neural cell changes and the underlying mechanisms

Nicotine exposure can lead to neurological impairments and brain tumors, and a label-free and nondestructive detection technique is urgently required by the scientific community to assess the effects of nicotine on neural cells. Herein, a terahertz (THz) time-domain attenuated total reflection (TD-ATR) spectroscopy approach is reported, by which the effects of nicotine on normal and cancerous neural cells, i.e., HEB and U87 cells, are successfully investigated in a label/stain-free and nondestructive manner. The obtained THz absorption coefficients of HEB cells exposed to low-dose nicotine and high-dose nicotine are smaller and larger, respectively, than the untreated cells. In contrast, the THz absorption coefficients of U87 cells treated by nicotine are always smaller than the untreated cells. The THz absorption coefficients can be well related to the proliferation properties (cell number and compositional changes) and morphological changes of neural cells, by which different types of neural cells are differentiated and the viabilities of neural cells treated by nicotine are reliably assessed. Collectively, this work sheds new insights on the effects of nicotine on neural cells, and provides a useful tool (THz TD-ATR spectroscopy) for the study of chemical-cell interactions.

Strain versus Tunable Terahertz Nanogap Width: A Simple Formula and a Trench below

A flexible zerogap metallic structure is periodically formed, healing metal cracks on a flexible substrate. Zerogap is continuously tunable from nearly zero to one hundred nanometers by applying compressive strains on the flexible substrate. However, there have been few studies on how the gap width is related to the strain and periodicity, nor the mechanism of tunability itself. Here, based on atomic force microscopy (AFM) measurements, we found that 200 nm-deep nano-trenches are periodically generated on the polymer substrate below the zerogap owing to the strain singularities extant between the first and the second metallic deposition layers. Terahertz and visible transmission properties are consistent with this picture whereby the outer-bending polyethylene terephthalate (PET) substrate controls the gap size linearly with the inverse of the radius of the curvature.

Research Progress in Surface-Enhanced Infrared Absorption Spectroscopy: From Performance Optimization, Sensing Applications, to System Integration

Infrared absorption spectroscopy is an effective tool for the detection and identification of molecules. However, its application is limited by the low infrared absorption cross-section of the molecule, resulting in low sensitivity and a poor signal-to-noise ratio. Surface-Enhanced Infrared Absorption (SEIRA) spectroscopy is a breakthrough technique that exploits the field-enhancing properties of periodic nanostructures to amplify the vibrational signals of trace molecules. The fascinating properties of SEIRA technology have aroused great interest, driving diverse sensing applications. In this review, we first discuss three ways for SEIRA performance optimization, including material selection, sensitivity enhancement, and bandwidth improvement. Subsequently, we discuss the potential applications of SEIRA technology in fields such as biomedicine and environmental monitoring. In recent years, we have ushered in a new era characterized by the Internet of Things, sensor networks, and wearable devices. These new demands spurred the pursuit of miniaturized and consolidated infrared spectroscopy systems and chips. In addition, the rise of machine learning has injected new vitality into SEIRA, bringing smart device design and data analysis to the foreground. The final section of this review explores the anticipated trajectory that SEIRA technology might take, highlighting future trends and possibilities.

Generation of Tunable Terahertz Waves from Tailored Versatile Spintronic Meta-Antenna Arrays

Broadband spintronic terahertz (THz) radiation can be efficiently generated by spin-to-charge current conversion in a ferromagnetic/nonmagnetic heterostructure. There had been many studies on realizing the enhancement or the modulation of spintronic terahertz waves. However, reported devices so far focus on implementing certain specific modulation methods, either related to the spintronic stacks or related to the metamaterial structures. In this study, a set of femtosecond laser-driven versatile spintronic terahertz devices are proposed by integrating meta-antenna structures with W/CoFeB/Pt nanolayer stacks. These monolithic integrated devices exhibit spintronic terahertz wave emission, spectral modulation, and polarization manipulation simultaneously. The terahertz pulses are generated within the ferromagnetic heterostructure interfaces and transmitted along the metallic structures, leading to the modulation of the spintronic terahertz waves. Results have shown that the center-frequency shift is up to 140 GHz and the value of ellipticity can reach 0.6, demonstrating a set of integrated and efficient spintronic terahertz devices to modulate the emitted wave. In addition, compared with the slotline antenna, the maximum peak value of the bandpass band is enhanced up to 1.63 times by carefully designing the metamaterial structure. The spintronic meta-antenna array proposed here paves an integrated way for the manipulation of spintronic terahertz optoelectronic devices.

Terahertz time-domain attenuated total reflection spectroscopy integrated with a microfluidic chip

The integration of a microfluidic chip into terahertz time-domain attenuated total reflection (THz TD-ATR) spectroscopy is highly demanded for the accurate measurement of aqueous samples. Hitherto, however little work has been reported on this regard. Here, we demonstrate a strategy of fabricating a polydimethylsiloxane microfluidic chip (M-chip) suitable for the measurement of aqueous samples, and investigate the effects of its configuration, particularly the cavity depth of the M-chip on THz spectra. By measuring pure water, we find that the Fresnel formulae of two-interface model should be applied to analyze the THz spectral data when the depth is smaller than 210 μm, but the Fresnel formula of one-interface model can be applied when the depth is no less than 210 μm. We further validate this by measuring physiological solution and protein solution. This work can help promote the application of THz TD-ATR spectroscopy in the study of aqueous biological samples.

Revolutionary approaches for cancer diagnosis by terahertz-based spectroscopy and imaging

Most tumors are easily missed and misdiagnosed due to the lack of specific clinical signs and symptoms in the early stage. Thus, an accurate, rapid and reliable early tumor detection method is highly desirable. The application of terahertz (THz) spectroscopy and imaging in biomedicine has made remarkable progress in the past two decades, which addresses the shortcomings of existing technologies and provides an alternative for early tumor diagnosis. Although issues such as size mismatch and strong absorption of THz waves by water have set hurdles for cancer diagnosis by THz technology, innovative materials and biosensors in recent years have led to possibilities for new THz biosensing and imaging methods. In this article, we reviewed the issues that need to be solved before THz technology is used for tumor-related biological sample detection and clinical auxiliary diagnosis. We focused on the recent research progress of THz technology, with an emphasis on biosensing and imaging. Finally, the application of THz spectroscopy and imaging for tumor diagnosis in clinical practice and the main challenges in this process were also mentioned. Collectively, THz-based spectroscopy and imaging reviewed here is envisioned as a cutting-edge approach for cancer diagnosis.

Small-molecule fluorogenic probes for mitochondrial nanoscale imaging

Mitochondria are inextricably linked to the development of diseases and cell metabolism disorders. Super-resolution imaging (SRI) is crucial in enhancing our understanding of mitochondrial ultrafine structures and functions. In addition to high-precision instruments, super-resolution microscopy relies heavily on fluorescent materials with unique photophysical properties. Small-molecule fluorogenic probes (SMFPs) have excellent properties that make them ideal for mitochondrial SRI. This paper summarizes recent advances in the field of SMFPs, with a focus on the chemical and spectroscopic properties required for mitochondrial SRI. Finally, we discuss future challenges in this field, including the design principles of SMFPs and nanoscopic techniques.

Unidirectional spintronic terahertz emitters with high efficiency

Due to the high performance and low cost, spintronic terahertz emitters (STEs) have been a hot topic in the field of terahertz sources. However, most of the research focuses on the THz generation process and little attention has been paid to the control and modulation of the THz wave generated by the STE. In this Letter, a unidirectional spintronic terahertz emitter (USTE) integrating a common STE with a metal grating is proposed to manipulate the THz emission process. The dyadic Green's function method and finite element method are adopted to survey the characteristics of the USTE. Simulations show that the metal grating not only has a transmission larger than 97% in the optical band, but also has a higher reflectivity larger than 99% in the THz band. As a result, the USTE has a unidirectional THz emission along the direction of the pump beam with a larger than 4-fold enhancement in intensity. Moreover, the USTE has the capability of tuning the central frequency and THz wave steering by adjusting the distance and angle between the STE and the metal grating. We believe that this USTE can be used in THz wireless communications and holographic imaging, especially in the field of THz bio-sensing, which needs some resonance frequencies to sense.

Ultra-sensitive terahertz metamaterials biosensor based on luxuriant gaps structure

Fast, simple, and label-free detections and distinctions are desirable in cell biology analysis and diagnosis. Here, a biosensor based on terahertz metamaterial has luxuriant gaps, which can excite dipole resonance is designed. Filling the gaps with various analytes can change the biosensor's capacitance resulting in electromagnetic properties changing. The idea is verified by simulations and experiments. The theoretical sensitivity of the biosensor approaches 290 GHz/RIU, and the experimental concentration sensitivity of the biosensor is ≥ 275 kHz mL/cell. Candida Albicans, Escherichia Coli, and Shigella Dysenteriae were selected as analytes, and the measurement frequency shift is 270 GHz, 290 GHz, and 310 GHz, respectively, which indicates that the biosensor can detect and distinguish these bacteria. Successfully detection of low-concentration glioblastoma (200 cells/mL), showing great potential for the early diagnosis of glioblastoma of the biosensor. This biosensor supplies a new horizon for cell detection, which will significantly benefit cell biology investigation.

Highly sensitive terahertz sensing with 3D-printed metasurfaces empowered by a toroidal dipole

Highly sensitive terahertz (THz) sensing with metasurfaces has attracted considerable attention recently. However, ultrahigh sensing sensitivity remains a huge challenge for practical applications. To improve the sensitivity of these devices, herein we have proposed an out-of-plane metasurface-assisted THz sensor consisting of periodically arranged bar-like meta-atoms. Benefiting from elaborate out-of-plane structures, the proposed THz sensor with high sensing sensitivity of 325 GHz/RIU can be easily fabricated via a simple three-step fabrication process, and the maximum sensing sensitivity can be ascribed to toroidal dipole resonance-enhanced THz-matter interactions. The sensing ability of the fabricated sensor is experimentally characterized by the detection of three types of analytes. It is believed that the proposed THz sensor with ultrahigh sensing sensitivity and its fabrication method might provide great potential in emerging THz sensing applications.

Flexible Control of Broadband Polarization in a Spintronic Terahertz Emitter Integrated with Liquid Crystal and Metasurface

Flexible polarization control of the terahertz wave in the wide bandwidth is crucial for numerous applications, such as terahertz communication, material characterization, imaging, and biosensing diagnosis. However, this promise is impeded by the operating bandwidth of circular polarization states, control modes, and the efficiency of the regulation. Here, we report a spintronic terahertz emitter integrated with phase complementary elements, consisting of a liquid crystal and metasurface, to achieve broadband polarization control with high flexibility. This strategy allows the broadband conversion between linear, elliptical, and circular polarization by changing the rotation angle to modulate the space-variant Pancharatnam-Berry phase. The device is characterized with a terahertz time-domain spectroscopy system, demonstrating that the ellipticity of the circular polarization state could keep greater than 0.9 in 0.60-0.99 THz. In the case of an external electro-magnetic field, further polarization modulation experiments are carried out to provide multiple conversion approaches for multi-azimuth. We first propose a method of full broadband polarization state control of the terahertz emitter based on Pancharatnam-Berry phase modulation and an external electro-magnetic field. We believe that such integrated devices with broadband working bandwidth and multiple control modes will make valuable contributions to the development and multi-scene applications of ultrafast terahertz technologies.

Magnetization Precession at Sub-Terahertz Frequencies in Polycrystalline Cu2 Sb-Type (Mn-Cr)AlGe Ultrathin Films

A ferromagnetic metal nanolayer with a large perpendicular magnetic anisotropy, small saturation magnetization, and small magnetic damping constant is a crucial requirement for high-speed spintronic devices. Fabrication of these devices on Si/SiO₂ amorphous substrates with polycrystalline structure is also strongly desired for the mass production industry. This study involves the investigation of sub-terahertz (THz) magnetization precessional motion in a newly developed material system consisting of Cu₂ Sb-type MnAlGe and (Mn-Cr)AlGe films by means of an all-optical pump-probe method. These materials exhibit large perpendicular magnetic anisotropy in regions of a few nanometers in size. The pseudo-2D crystal structures are clearly observed in the high-resolution transmission electron microscopy (TEM) images for the film samples grown on thermally oxidized silicon substrates. The TEM images also show a partial substitution of Cr atoms for the Mn sites in MnAlGe. A magnetization precession frequency of 0.164 THz with a relatively small effective magnetic damping constant of 0.012 is obtained for (Mn-Cr)AlGe. Theoretical calculation infers that the modification of the total density of states by Cr substitution decreases the intrinsic magnetic damping constant of (Mn-Cr)AlGe.

Generation of tailored terahertz waves from monolithic integrated metamaterials onto spintronic terahertz emitters

Recently emerging spintronic terahertz (THz) emitters, featuring many appreciable merits such as low-cost, high efficiency, ultrabroadband, and ease of integration, offer multifaceted capabilities not only in understanding the fundamental ultrafast magnetism physics but also for exploring multifarious practical applications. Integration of various flexible and tunable functions at the source such as polarization manipulation, amplitude tailoring, phase modulation, and radiation beam steering with the spintronic THz emitters and their derivatives can yield more compact and elegant devices. Here, we demonstrate a monolithic metamaterial integrated onto a W/CoFeB/Pt THz nanoemitter for a purpose-designed functionality of the electromagnetically induced transparency analog. Through elaborate engineering the asymmetry degree and geometric parameters of the metamaterial structure, we successfully verified the feasibility of monolithic modulations for the radiated THz waves. The integrated device was eventually compared with a set of stand-alone metamaterial positioning scenarios, and the negligible frequency difference between two of the positioning schemes further manifests almost an ideal realization of the proposed monolithic integrated metamaterial device with a spintronic THz emitter. We believe that such adaptable and scalable devices may make valuable contributions to the designable spintronic THz devices with pre-shaping THz waves and enable chip-scale spintronic THz optics, sensing, and imaging.

Polarization control of THz emission using spin-reorientation transition in spintronic heterostructure

Polarization of electromagnetic waves plays an extremely important role in interaction of radiation with matter. In particular, interaction of polarized waves with ordered matter strongly depends on orientation and symmetry of vibrations of chemical bonds in crystals. In quantum technologies, the polarization of photons is considered as a "degree of freedom", which is one of the main parameters that ensure efficient quantum computing. However, even for visible light, polarization control is in most cases separated from light emission. In this paper, we report on a new type of polarization control, implemented directly in a spintronic terahertz emitter. The principle of control, realized by a weak magnetic field at room temperature, is based on a spin-reorientation transition (SRT) in an intermetallic heterostructure TbCo₂/FeCo with uniaxial in-plane magnetic anisotropy. SRT is implemented under magnetic field of variable strength but of a fixed direction, orthogonal to the easy magnetization axis. Variation of the magnetic field strength in the angular (canted) phase of the SRT causes magnetization rotation without changing its magnitude. The charge current excited by the spin-to-charge conversion is orthogonal to the magnetization. As a result, THz polarization rotates synchronously with magnetization when magnetic field strength changes. Importantly, the radiation intensity does not change in this case. Control of polarization by SRT is applicable regardless of the spintronic mechanism of the THz emission, provided that the polarization direction is determined by the magnetic moment orientation. The results obtained open the prospect for the development of the SRT approach for THz emission control.

Magnetic-Nanosensor-Based Virus and Pathogen Detection Strategies before and during COVID-19

The novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes coronavirus disease 2019 (COVID-19), is a threat to the global healthcare system and economic security. As of July 2020, no specific drugs or vaccines are yet available for COVID-19; a fast and accurate diagnosis for SARS-CoV-2 is essential in slowing the spread of COVID-19 and for efficient implementation of control and containment strategies. Magnetic nanosensing is an emerging topic representing the frontiers of current biosensing and magnetic areas. The past decade has seen rapid growth in applying magnetic tools for biological and biomedical applications. Recent advances in magnetic nanomaterials and nanotechnologies have transformed current diagnostic methods to nanoscale and pushed the detection limit to early-stage disease diagnosis. Herein, this review covers the literature of magnetic nanosensors for virus and pathogen detection before COVID-19. We review popular magnetic nanosensing techniques including magnetoresistance, magnetic particle spectroscopy, and nuclear magnetic resonance. Magnetic point-of-care diagnostic kits are also reviewed aiming at developing plug-and-play diagnostics to manage the SARS-CoV-2 outbreak as well as preventing future epidemics. In addition, other platforms that use magnetic nanomaterials as auxiliary tools for enhanced pathogen and virus detection are also covered. The goal of this review is to inform the researchers of diagnostic and surveillance platforms for SARS-CoV-2 and their performances.

Secure Deep Learning for Intelligent Terahertz Metamaterial Identification

Metamaterials, artificially engineered structures with extraordinary physical properties, offer multifaceted capabilities in interdisciplinary fields. To address the looming threat of stealthy monitoring, the detection and identification of metamaterials is the next research frontier but have not yet been explored. Here, we show that the crypto-oriented convolutional neural network (CNN) makes possible the secure intelligent detection of metamaterials in mixtures. Terahertz signals were encrypted by homomorphic encryption and the ciphertext was submitted to the CNN directly for results, which can only be decrypted by the data owner. The experimentally measured terahertz signals were augmented and further divided into training sets and test sets using 5-fold cross-validation. Experimental results illustrated that the model achieved an accuracy of 100% on the test sets, which highly outperformed humans and the traditional machine learning. The CNN took 9.6 s to inference on 92 encrypted test signals with homomorphic encryption backend. The proposed method with accuracy and security provides private preserving paradigm for artificial intelligence-based material identification.