Terahertz Near-field Microscopy Research Articles

Additive and subtractive hybrid manufacturing assisted by femtosecond adaptive optics

Advanced micro–nano devices commonly require precise three-dimensional (3D) fabrication solutions for pre-designing and integrating 0D to 3D configurations. The additive–subtractive hybrid manufacturing strategy dominated by femtosecond laser direct writing has become an increasingly interesting technical route for material processing. In this study, a novel approach termed femtosecond adaptive optics-assisted hybrid manufacturing was proposed, which integrates subtractive (femtosecond laser ablation) and additive (two-photon polymerization) fabrication. In this hybrid manufacturing method, the introduction of adaptive optics offers parallel direct writing and wide-area material processing capabilities. To demonstrate the validity of the hybrid approach, on-chip surface plasmon polariton waveguides with strong sub-wavelength field confinement and enhanced functionality were successfully fabricated. In comparison with the terahertz-wave devices fabricated based on the focused ion beam technique, the functional tests in terahertz near-field microscopy show a rival performance fabricated with our hybrid approach. Besides, our cost-effective solution also dramatically reduces the fabricating time of excitation regions by a factor >16. Our work provides a new inspiration in integrated photonics.

Near-field imaging and spectroscopy of terahertz resonators and metasurfaces [Invited

Terahertz (THz) metasurfaces have become a key platform for engineering light-matter interaction at THz frequencies. They have evolved from simple metallic resonator arrays into tunable and programmable devices, displaying ultrafast modulation rates and incorporating emerging quantum materials. The electrodynamics which govern metasurface operation can only be directly revealed at the scale of subwavelength individual metasurface elements, through sampling their evanescent fields. It requires near-field spectroscopy and imaging techniques to overcome the diffraction limit and provide spatial resolution down to the nanoscale. Through a series of case studies, this review provides an in-depth overview of recently developed THz near-field microscopy capabilities for research on metamaterials.

Rough surface effect in terahertz near-field microscopy: 3D simulation analysis.

Terahertz scattering-type scanning near-field optical microscopy (THz-s-SNOM) has emerged as a powerful technique for high-resolution imaging. However, most previous studies have focused on simplified smooth surface models, overlooking the realistic surface roughness induced by contamination during sample preparation. In this work, we present a novel 3D model, to the best of our knowledge, that combines the point dipole model with the finite element method to investigate the influence of sample morphology on scattered signals. We explore surfaces with a protrusion, a depression, and random roughness, characterizing the variations in scattered signals and highlighting the role of higher-order scattering in mitigating surface roughness effects. Our findings provide valuable insights into the impact of sample morphology on THz-s-SNOM imaging.

Terahertz Nanoscopy of Metal and Gallium Implanted Silicon

The Drude model successfully quantifies the optical constants for bulk matter, but it is not suitable for subwavelength objects. In this paper, terahertz near-field optical microscopy and finite element simulation are used to study gallium-etching-fabricated gold patches. Electron transport is discovered to determine the optical signal strength. However, the signal from the substrate is complicated and not fully understood. When the etching area decreases, the near-field interaction is not dominated by the doping concentration, and a higher signal is observed near connected metals. Simulation is used to discuss the anomalous enhancement phenomenon, which includes electric field distribution and electromagnetic heating. This work lays the foundation for more sophisticated simulations and further experimental verification in the future.

Quantitative Modeling of Near-Field Interactions in Terahertz Near-Field Microscopy

Terahertz scattering-scanning near-field optical microscopy (THz s-SNOM), combining the best features of terahertz technology and s-SNOM technology, has shown unique advantages in various applications. Consequently, building a model to characterize near-field interactions and investigate practical issues has become a popular topic in THz s-SNOM research. In this study, a finite element model (FEM) is proposed to quantify the near-field interactions, and to investigate the edge effect and antenna effect in THz s-SNOM. Our results indicate that the proposed model can give us a better understanding of the near-field interactions and direct the parameter design of the probe for THz s-SNOM.

Thickness-dependent Auger scattering in a single WS2 microcrystal probed with time-resolved terahertz near-field microscopy.

Time-resolved terahertz (THz) spectroscopy has been shown as a powerful technique to non-invasively determine the charge carrier properties in photoexcited semiconductors. However, the long wavelengths of terahertz radiation reduce the applicability of this technique to large samples. Using THz near-field microscopy, we show THz measurements of the lifetime of 2D single exfoliated microcrystals of transition metal dichalcogenides (WS2). The increased spatial resolution of THz near-field microscopy allows spatial mapping of the evolution of the carrier lifetime, revealing Auger assisted surface defect recombination as the dominant recombination channel. THz near-field microscopy allows for the non-invasive and high-resolution investigation of material properties of 2D semiconductors relevant for nanoelectronic and optoelectronic applications.

Terahertz near-field microscopy based on an air-plasma dynamic aperture

Terahertz (THz) near-field microscopy retains the advantages of THz radiation and realizes sub-wavelength imaging, which enables applications in fundamental research and industrial fields. In most THz near-field microscopies, the sample surface must be approached by a THz detector or source, which restricts the sample choice. Here, a technique was developed based on an air-plasma dynamic aperture, where two mutually perpendicular air-plasmas overlapped to form a cross-filament above a sample surface that modulated an incident THz beam. THz imaging with quasi sub-wavelength resolution (approximately λ/2, where λ is the wavelength of the THz beam) was thus observed without approaching the sample with any devices. Damage to the sample by the air-plasmas was avoided. Near-field imaging of four different materials was achieved, including metallic, semiconductor, plastic, and greasy samples. The resolution characteristics of the near-field system were investigated with experiment and theory. The advantages of the technique are expected to accelerate the advancement of THz microscopy.

Subcycle contact-free nanoscopy of ultrafast interlayer transport in atomically thin heterostructures

Tunnelling is one of the most fundamental manifestations of quantum mechanics. The recent advent of lightwave-driven scanning tunnelling microscopy has revolutionized ultrafast nanoscience by directly resolving electron tunnelling in electrically conducting samples on the relevant ultrashort length- and timescales. Here, we introduce a complementary approach based on terahertz near-field microscopy to perform ultrafast nano-videography of tunnelling processes even in insulators. The central idea is to probe the evolution of the local polarizability of electron–hole pairs with evanescent terahertz fields, which we detect with subcycle temporal resolution. In a proof of concept, we resolve femtosecond interlayer transport in van der Waals heterobilayers and reveal pronounced variations of the local formation and annihilation of interlayer excitons on deeply subwavelength, nanometre scales. Such contact-free nanoscopy of tunnelling-induced dynamics should be universally applicable to conducting and non-conducting samples and reveal how ultrafast transport processes shape functionalities in a wide range of condensed matter systems.

Antenna-coupled field-effect transistors as detectors for terahertz near-field microscopy.

We report the successful implementation of antenna-coupled terahertz field-effect transistors (TeraFETs) as homodyne detectors in a scattering-type scanning near-field optical microscope (s-SNOM) operating with radiation at 246.5 GHz. The devices were fabricated in Si CMOS foundry technology with two different technologies, a 90 nm process, which provides a better device performance, and a less expensive 180 nm one. The high sensitivity enables s-SNOM demodulation at up to the 10th harmonic of the cantilever's oscillation frequency. While we demonstrate application of TeraFETs at a fixed radiation frequency, this type of detector device is able to cover the entire THz frequency range.

Terahertz near-field microscopy of ductal carcinoma in situ (DCIS) of the breast

Imaging with terahertz (THz) waves has been expected as a non-invasive/non-staining visualization tool for breast cancer margins during surgeries. Breast cancer is a generic name for a heterogeneous lesion comprising invasive adenocarcinoma, in situ adenocarcinoma, most frequently in the form of ductal carcinoma in situ (DCIS) and benign tissues. Until now, THz imaging has focused on invasive adenocarcinoma; however, THz analysis of DCIS has hardly been performed. One of the reasons is that the size of an individual DCIS lesion, ranging from 50 to 500 µm, is typically much smaller than that of an invasive carcinoma. This makes it difficult to identify these lesions by THz imaging, which has only a diffraction-limited spatial resolution of several millimeters. To overcome this drawback, we have developed a scanning point terahertz source (SPoTS) microscope with a resolution of 20 µm, in which a near-infrared-pump-laser-induced two-dimensionally-scannable point THz source (φ THz ≈ φ Pump) generated in a GaAs crystal contacts a sample. In this study, utilizing this state-of-the-art microscope, we mainly performed THz near-field transmission imaging of a paraffin-embedded human breast cancer sample containing invasive carcinoma and DCIS, as a preliminary study. Consequently, for the first time, we succeeded in clearly visualizing a DCIS lesion of ∼φ500 µm in the THz images. It was also found that the THz attenuation by DCIS was higher than that by invasive ductal carcinoma. Furthermore, also in a reflection-mode measurement, we successfully obtained a similar outcome to the above transmission-mode one. These results can be caused by the interaction between the THz waves and the cellular density, indicating that SPoTS microscopy may be suitable for DCIS diagnosis.

Ghost spintronic THz-emitter-array microscope

Terahertz (THz) waves show great potential in nondestructive testing, biodetection and cancer imaging. Despite recent progress in THz wave near-field probes/apertures enabling raster scanning of an object’s surface, an efficient, nonscanning, noninvasive, deep subdiffraction imaging technique remains challenging. Here, we demonstrate THz near-field microscopy using a reconfigurable spintronic THz emitter array (STEA) based on the computational ghost imaging principle. By illuminating an object with the reconfigurable STEA followed by computing the correlation, we can reconstruct an image of the object with deep subdiffraction resolution. By applying an external magnetic field, in-line polarization rotation of the THz wave is realized, making the fused image contrast polarization-free. Time-of-flight (TOF) measurements of coherent THz pulses further enable objects at different distances or depths to be resolved. The demonstrated ghost spintronic THz-emitter-array microscope (GHOSTEAM) is a radically novel imaging tool for THz near-field imaging, opening paradigm-shifting opportunities for nonintrusive label-free bioimaging in a broadband frequency range from 0.1 to 30 THz (namely, 3.3–1000 cm−1).

Spintronic terahertz emitter: Performance, manipulation, and applications

Spintronic terahertz (THz) emitter, which is based on ultrafast spin-to-charge current conversion in ferromagnetic/nonmagnetic heterostructures, provides excellent advantages such as ultra-broadband, tunable polarization, and ultra-thin structure, thereby attracting increasing interests recently. In this review article, we first introduce the fundamental concepts of THz wave, THz spintronics and spintronic THz emitter. Next, we focus on the recent progress of spintronic THz emitter by closely looking at the performances, manipulations and applications. Performance improvement is presented based on the three fundamental processes: optical excitation, ultrafast spin transport, and THz emission. The active manipulation of polarization and spectral response, as well as the relevant applications such as ultra broadband measurements, magnetic structure detection and imaging, and THz near-field microscopy, are reviewed comprehensively. Finally, a brief summary and outlook are given.

Computed terahertz near-field mapping of molecular resonances of lactose stereo-isomer impurities with sub-attomole sensitivity

Terahertz near-field microscopy (THz-NFM) could locally probe low-energy molecular vibration dynamics below diffraction limits, showing promise to decipher intermolecular interactions of biomolecules and quantum matters with unique THz vibrational fingerprints. However, its realization has been impeded by low spatial and spectral resolutions and lack of theoretical models to quantitatively analyze near-field imaging. Here, we show that THz scattering-type scanning near-field optical microscopy (THz s-SNOM) with a theoretical model can quantitatively measure and image such low-energy molecular interactions, permitting computed spectroscopic near-field mapping of THz molecular resonance spectra. Using crystalline-lactose stereo-isomer (anomer) mixtures (i.e., α-lactose (≥95%, w/w) and β-lactose (≤4%, w/w)), THz s-SNOM resolved local intermolecular vibrations of both anomers with enhanced spatial and spectral resolutions, yielding strong resonances to decipher conformational fingerprint of the trace β-anomer impurity. Its estimated sensitivity was ~0.147 attomoles in ~8 × 10−4 μm3 interaction volume. Our THz s-SNOM platform offers a new path for ultrasensitive molecular fingerprinting of complex mixtures of biomolecules or organic crystals with markedly enhanced spatio-spectral resolutions. This could open up significant possibilities of THz technology in many fields, including biology, chemistry and condensed matter physics as well as semiconductor industries where accurate quantitative mappings of trace isomer impurities are critical but still challenging.

Terahertz Time‐Domain Spectroscopy and Near‐Field Microscopy of Transparent Silver Nanowire Networks

AbstractTransparent conductive layers are key components of optoelectronic devices. Here, a polyol method is used to synthesize large quantities of monodisperse silver nanowires (AgNWs) and these are used to fabricate transparent conducting networks over large areas. The optical extinction and terahertz (THz) conductance of these networks are simultaneously investigated, using optical and THz spectroscopy, and THz near‐field microscopy. The combination of optical and THz measurements allows the identification of transparent regions with high conductance. The THz near‐field measurements reveal local variations in the THz transmission and conductance that are averaged in far‐field measurements. These results demonstrate that THz near‐field microscopy is a powerful tool for the quantitative investigation of new conductive transparent electrodes.

Hyperspectral time-domain terahertz nano-imaging.

Terahertz (THz) near-field microscopy has wide and unprecedented application potential for nanoscale materials and photonic-device characterization. Here, we introduce hyperspectral THz nano-imaging by combining scattering-type scanning near-field optical microscopy (s-SNOM) with THz time-domain spectroscopy (TDS). We describe the technical implementations that enabled this achievement and demonstrate its performance with a heterogeneously doped Si semiconductor sample. Specifically, we recorded a hyperspectral image of 40 by 20 pixels in 180 minutes and with a spatial resolution of about ~170 nm by measuring at each pixel with a time domain spectrum covering the range from 0.4 to 1.8 THz. Fitting the spectra with a Drude model allows for measuring-noninvasively and without the need for Ohmic contacts-the local mobile carrier concentration of the differently doped Si areas. We envision wide application potential for THz hyperspectral nano-imaging, including nanoscale carrier profiling of industrial semiconductor structures or characterizing complex and correlated electron matter, as well as low dimensional (1D or 2D) materials.

Diffraction enhanced transparency in a hybrid gold-graphene THz metasurface

Terahertz (THz) near-field microscopy is used to investigate the underlying physics that leads to diffraction enhanced transparency (DET) in a periodic array of detuned metallic rods. At the transparency frequency, the system is highly dispersive and THz radiation is delayed for several tens of picoseconds before being re-emitted into the forward direction. Using polarization sensitive measurements of the electric THz near-field spectrum, we demonstrate that an out-of-phase field distribution is formed in the unit cell leading to a reduced coupling to the far-field. This quadrupolar field distribution originates from the excitation and interference of surface lattice modes produced by the enhanced radiative coupling through the lattice of localized resonances in the metallic rods. These results represent the first experimental near-field investigation of DET, shedding light onto this phenomenon and providing important information for its further development. Implementing DET in applications requires control over the transparency window. We demonstrate that adding a monolayer of graphene, absorbing at THz frequencies, is sufficient to fully suppress the DET despite its monoatomic thickness. This efficient suppression is due to the diffractive wave character of DET and a metal insulator metal resonance formed between the localized resonance of the gold resonator and the extended graphene layer. The possibility to exert control over the transparency window by changing the conductivity of graphene by altering the Fermi level opens the possibility of active THz devices.

Terahertz Field Confinement in Nonlinear Metamaterials and Near-Field Imaging

This article reviews recent advances in terahertz science and technology that rely on confining the energy of incident terahertz radiation to small, very sub-wavelength sized regions. We focus on two broad areas of application for such field confinement: metamaterial-based nonlinear terahertz devices and terahertz near-field microscopy and spectroscopy techniques. In particular, we focus on field confinement in: terahertz nonlinear absorbers, metamaterial enhanced nonlinear terahertz spectroscopy, and in sub-wavelength terahertz imaging systems.

Imaging brain tissue slices with terahertz near-field microscopy.

Photoconductive antenna microprobe (PCAM)-based terahertz (THz) near-field imaging technique is promising for biomedical detection due to its excellent biocompatibility and high resolution; yet it is limited by its imaging speed and the difficulty in the control of the PCAM tip-sample separation. In this work, we successfully realized imaging of mouse brain tissue slices using an improved home-built PCAM-based THz near-field microscope. In this system, the imaging speed was enhanced by designing and applying a voice coil motor-based delay-line. The tip-sample separation control was implemented by developing an image analysis-based technique. Compared with conventional PCAM-based THz near-field systems, our improved system is 100 times faster in imaging speed and the tip-sample separation can be controlled to a few micrometers (e.g., 3 μm), satisfying the requirements of THz near-field imaging of biological samples. It took about ~30 min (not the tens of hours it took to acquire the same kind of image previously) to collect a THz near-field image of brain tissue slices of BALb/c mice (500 μm × 500 μm) with pixel size of 20 μm × 20 μm. The results show that the mouse brain slices can be properly imaged and different regions in the slices (i.e., the corpus callosum region and the cerebrum region) can be identified unambiguously. Evidently, the work demonstrated here provides not only a convincing example but a useful technique for imaging biological samples with THz near-field microscopy. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2741, 2019.

Resonant terahertz probes for near-field scattering microscopy.

We propose and characterize a scattering probe for terahertz (THz) near-field microscopy, fabricated from indium, where the scattering efficiency is enhanced by the dipolar resonance supported by the indium probe. The scattering properties of the probe were evaluated experimentally using THz time-domain spectroscopy (TDS), and numerically using the finite-difference time-domain (FDTD) method in order to identify resonant enhancement. Numerical measurements show that the indium probes exhibit enhanced scattering across the THz frequency range due to dipolar resonance, with a fractional bandwidth of 0.65 at 1.24 THz. We experimentally observe the resonant enhancement of the scattered field with a peak at 0.3 THz. To enable practical THz microscopy applications of these resonant probes, we also demonstrate a simple excitation scheme utilizing a THz source with radial polarization, which excites a radial mode along the length of the tip. Strong field confinement at the apex of the tip, as required for THz near-field microscopy, was observed experimentally.

Transmission and plasmonic resonances on quasicrystal metasurfaces

The control of light-matter interaction in metasurfaces offers an unexplored potential for the excitation and manipulation of light. Here, we combine experimental terahertz time-domain spectroscopy and near-field scanning terahertz microscopy to demonstrate the role of reciprocal vectors in the transmission and plasmonic resonances of quasicrystal metasurfaces. An investigation of two-dimensional metasurface structures with different rotationally symmetric quasicrystal arrangements demonstrates that the transmission minima resulting from Wood's anomaly are directly related to the surface plasmon resonances. We also find that the surface plasmon resonances of the quasicrystal metasurface were determined by the reciprocal vectors, which could be well explained by the coupling condition of the resonances, and the characteristic frequencies remain un-shifted under various slit sizes. Our findings demonstrate a new potential in developing novel plasmonic metasurfaces.