Terahertz Near-field Imaging Research Articles

Real-time near-field terahertz imaging with atomic optical fluorescence

Terahertz (THz) near-field imaging is a flourishing discipline [1], with applications from fundamental studies of beam propagation [2,3] to the characterisation of metameterials [4,5] and waveguides [6,7]. Beating the diffraction limit typically involves rastering structures or detectors with length scale shorter than the radiation wavelength; in the THz domain this has been achieved using a number of techniques including scattering tips [8,9] and apertures [10]. Alternatively, mapping THz fields onto an optical wavelength and imaging the visible light removes the requirement for scanning a local probe, speeding up image collection times [11,12]. Here we report THz to optical conversion using a gas of highly excited `Rydberg' atoms. By collecting THz-induced optical fluorescence we demonstrate a real-time image of a THz standing wave and we use well-known atomic properties to calibrate the THz field strength. The mono-atomic gas does not distort the THz field and offers the potential to immerse structures within the THz-to-optical conversion medium.

Improving time and space resolution in electro-optic sampling for near-field terahertz imaging

In this Letter, we present a significant improvement to time and space resolutions in electro-optic sampling (EO) for two-dimensional terahertz (THz) near-field imaging. Using a THz microscope, we readapt a recent EO sampling scheme based on optical probe spectrum filtering. Combined with an ultra-thin EO crystal, we achieve record broadband video-rate THz near-field imaging. Particularly, this new scheme improves the THz bandwidth, the imaging contrast, and the spatial resolution. To validate our method, we show THz near-field images ranging from 100 GHz to 4 THz with a spatial resolution up to λ/600 at 100 GHz. This demonstration positively affects the detection of intense THz pulses derived from the tilted-pulse-front excitation of lithium niobate and will accelerate our understanding of the interaction processes between electromagnetic waves and the conducting electrons of metallic interfaces.

Noninvasive, near-field terahertz imaging of hidden objects using a single-pixel detector.

Terahertz (THz) imaging can see through otherwise opaque materials. However, because of the long wavelengths of THz radiation (λ = 400 μm at 0.75 THz), far-field THz imaging techniques suffer from low resolution compared to visible wavelengths. We demonstrate noninvasive, near-field THz imaging with subwavelength resolution. We project a time-varying, intense (>100 μJ/cm(2)) optical pattern onto a silicon wafer, which spatially modulates the transmission of synchronous pulse of THz radiation. An unknown object is placed on the hidden side of the silicon, and the far-field THz transmission corresponding to each mask is recorded by a single-element detector. Knowledge of the patterns and of the corresponding detector signal are combined to give an image of the object. Using this technique, we image a printed circuit board on the underside of a 115-μm-thick silicon wafer with ~100-μm (λ/4) resolution. With subwavelength resolution and the inherent sensitivity to local conductivity, it is possible to detect fissures in the circuitry wiring of a few micrometers in size. THz imaging systems of this type will have other uses too, where noninvasive measurement or imaging of concealed structures is necessary, such as in semiconductor manufacturing or in ex vivo bioimaging.

Chip-Based Near-Field Terahertz Microscopy

Near-field terahertz (THz) imaging is vital to investigations in subwavelength regions down to the micro/nanometer scale. This technology is particularly useful for examining objects, such as nanomaterials, polymers, cells, and biomolecules. The first part of this paper briefly explains two types of near-field THz imaging technologies: aperture type and apertureless type. The second part explains our chip-based near-field imaging technology. We perform collection-mode near-field THz imaging by scanning an evanescently coupled THz detector closely across a sample surface. The implementation of a magnetically tunable THz detection method further enables us to obtain frequency-selective near-field THz images. As an application of this technology, cryogenic THz-emission imaging is presented, wherein THz radiation associated with electron injection into a semiconductor device is imaged with no THz pump source. This technique provides direct information about the spatial distribution of electrons injected from an electrode into a semiconductor channel.

Apertureless near-field terahertz imaging using the self-mixing effect in a quantum cascade laser

We report two-dimensional apertureless near-field terahertz (THz) imaging using a quantum cascade laser (QCL) source and a scattering probe. A near-field enhancement of the scattered field amplitude is observed for small tip-sample separations, allowing image resolutions of ∼1 μm (∼λ/100) and ∼7 μm to be achieved along orthogonal directions on the sample surface. This represents the highest resolution demonstrated to date with a THz QCL. By employing a detection scheme based on self-mixing interferometry, our approach offers experimental simplicity by removing the need for an external detector and also provides sensitivity to the phase of the reinjected field.

Near-field terahertz imaging using sub-wavelength apertures without cutoff.

We demonstrate near-field imaging capabilities of a conical waveguide without cutoff using broadband terahertz (THz) radiation. In contrast to conventional conically tapered waveguides, which are characterized by strong suppression of transmission below the cutoff frequency, the proposed structure consists of two pieces, such that there is an adjustable gap along the length of the waveguide. We also ensure that the sidewalls are thin in the vicinity of the gap. The combination of these geometrical features allow for significantly enhanced transmission at frequencies below the cutoff frequency, without compromising the mode confinement and, consequently, the spatial resolution when used for imaging applications. We demonstrate near-field imaging with this probe simultaneously at several frequencies, corresponding to three regimes: above, near and below the cutoff frequency. We observe only mild degradation in the image quality as the frequency is reduced below the cutoff frequency. These results suggest that further refinements in the probe structure will allow for improved imaging capabilities at frequencies well below the cutoff frequency.

Terahertz near-field imaging of surface plasmon waves in graphene structures

We introduce a near-field scanning probe terahertz (THz) microscopy technique for probing surface plasmon waves on graphene. Based on THz time-domain spectroscopy method, this near-field imaging approach is well suited for studying the excitation and evolution of THz plasmon waves on graphene as well as for mapping of graphene properties at THz frequencies on the sub-wavelength scale.

Diagnose human colonic tissues by terahertz near-field imaging.

Based on a terahertz (THz) pipe-based near-field imaging system, we demonstrate the capability of THz imaging to diagnose freshly surgically excised human colonic tissues. Through THz near-field scanning the absorbance of the colonic tissues, the acquired images can clearly distinguish cancerous tissues from healthy tissues fast and automatically without pathological hematoxylin and eosin stain diagnosis. A statistical study on 58 specimens (20 healthy tissues and 38 tissues with tumor) from 31 patients (mean age: 59 years; range: 46 to 79 years) shows that the corresponding diagnostic sensitivity and specificity on colonic tissues are both 100%. Due to its capability to perform quantitative analysis, our study indicates the potential of the THz pipe-based near-field imaging for future automation on human tumor pathological examinations.

Sub-wavelength terahertz beam profiling of a THz source via an all-optical knife-edge technique.

Terahertz technologies recently emerged as outstanding candidates for a variety of applications in such sectors as security, biomedical, pharmaceutical, aero spatial, etc. Imaging the terahertz field, however, still remains a challenge, particularly when sub-wavelength resolutions are involved. Here we demonstrate an all-optical technique for the terahertz near-field imaging directly at the source plane. A thin layer (<100 nm-thickness) of photo carriers is induced on the surface of the terahertz generation crystal, which acts as an all-optical, virtual blade for terahertz near-field imaging via a knife-edge technique. Remarkably, and in spite of the fact that the proposed approach does not require any mechanical probe, such as tips or apertures, we are able to demonstrate the imaging of a terahertz source with deeply sub-wavelength features (<30 μm) directly in its emission plane.

Scanning laser THz imaging system

The laser terahertz (THz) emission microscope (LTEM) is a unique inspection tool which can directly two-dimensionally map the THz pulse emission from a variety of electric materials and devices. Thus, two-dimensional mapping directly yields various physical information by visualizing the distributions of electric field, supercurrent, ferroelectric domain structures, magnetic fluxes, etc. Based on techniques created for conventional LTEM, we have developed a more highly functional laser THz imaging system, including a scanning laser THz (near-field) imaging system having the performance of high-speed and high-resolution, a scanning probe LTEM coupled with an atomic force microscope for high-resolution imaging, and a dynamic THz emission microscope to investigate the ultrafast carrier dynamics two-dimensionally in the sample. These systems offer diverse characteristics, making them suitable for various applications. We take a look at these systems and some typical applications.

Near-field terahertz imaging of a discontinuity in split ring resonator array

We investigated the spatiotemporal evolution of single cycle terahertz pulses transmitted through a split ring resonator array including a void. Using a large-field-of- view terahertz microscope, we revealed the confinement and enhancement of the defect mode.

Scanning laser terahertz near-field imaging system

We have proposed and developed a scanning laser terahertz (THz) near-field imaging system using a 1.56 μm femtosecond fiber laser for high spatial resolution and high-speed measurement. To obtain the two-dimensional (2D) THz images of samples, the laser pulses are scanned over a 2D THz emitter plate [DASC: 4'-dimenthylamino-N-methyl-4- stilbazolium p-chlorobenzenesulfonate] by a galvano meter. In this system, THz wave pulses locally generated at the laser irradiation spots transmit through the sample set on the emitter, and the amplitude of the transmitted THz wave pulse is detected by using a typical THz time-domain spectroscopy (THz-TDS) technique. Using this system, we have succeeded in obtaining THz transmission images of a triangle shaped metal sheet of millimeter-size and a human hair sample with a spatial resolution of sub-wavelength order up to ~27 μm (~λTHz/28) at an imaging speed of about 47 seconds/image for 512 x 512 pixels.

Reducing Transmission Losses in Hollow THz Waveguides

Research on reducing material absorption in Terahertz (THz) waveguides has lead to development of guiding structures with transmission losses as low as 1 dB/m. Among waveguides that exhibit low loss at THz frequencies are the dielectric-lined hollow cylindrical metallic waveguides. Loss reduction in this waveguide is attributed to an ideal profile of the dominant hybrid HE11 mode. This mode profile also results in relatively low dispersion and very high coupling efficiency. In this contribution we overview properties of dielectric-lined hollow cylindrical metallic waveguides for THz waves, their design principles and the fabrication process. The impact of the mode profile on losses and dispersion at THz frequencies is confirmed experimentally by THz near-field imaging and THz time-domain spectroscopy and numerically by the finite element method.

Introduction to the Special Issue on “Terahertz Near-field Optics”

Terahertz science and technology are progressing at a tremendous speed as evidenced by the ever increasing numbers of publications in this field. The field is becoming large enough that we can begin to sub-divide it into various specializations. One of these is “Terahertz near-field microscopy”. Pure and simple, it is the science and technology of beating the terahertz diffraction limit, in other words, imaging with a spatial resolution better than about half-of-a wavelength. In this issue we have four invited contributions giving a flavor of what this subfield is currently about. The first paper, “Review of Near-Field Terahertz Measurement Methods and Their Applications; How to Achieve Sub-wavelength Resolution at THz Frequencies” by A. J. L. Adam from the University of Technology Delft, the Netherlands, provides an extensive overview of state-of-the-art terahertz nearfield microscopy techniques. M. Walther, from the University of Freiburg, Germany, and A. Bitzer, currently at the University of Bern, Switzerland, wrote a paper entitled “Electromagnetic Wave Propagation Close to Microstructures Studied by Time and Phase-Resolved THz Near-field Imaging”, in which they show results of measurements of the terahertz electromagnetic field near metal structures, such as sub-wavelength sized holes and wires. R. Mueckstein and co-workers, from University College London, UK, and Rutgers University USA, in their paper “Imaging and analysis of THz surface plasmon polariton waves with the integrated subwavelength aperture probe” discuss the intricacies of measuring terahertz surface plasmon polaritons using a near-field probe, showing results of measurements of surface waves near small apertures and bow-tie antennas. Finally, A. Doi and co-workers, from Olympus Corporation, CREST, and Kyoto University, Japan, in “Improving Spatial Resolution of Real-Time Terahertz Near-Field Microscope” discuss the limitations of their recently developed real-time terahertz near-field imaging setup. All papers show the fantastic progress that has been made in the past few years in THz near-field microscopy. Many groups at various places in the world are working in this area and it is thus to be expected that in the coming years more spectacular results with emerge. Until then I hope you’ll enjoy reading the papers in this special issue!

Near-field signature of electromagnetic coupling in metamaterial arrays: a terahertz microscopy study

Using terahertz near-field imaging we experimentally investigate the interaction between split-ring resonators (SRRs) in metamaterial arrays. Depending on the inter-SRR spacing two regimes can be distinguished for which strong coupling between SRRs occurs. For dense arrays SRRs couple via their electric and magnetic near-fields. In this case distinct deformations of the SRRs' characteristic near-field patterns are observed as a signature of their strong interaction. For larger separations with a periodicity matching the resonance wavelength, the SRRs become diffractively coupled via their radiated fields. In this regime hybridization between plasmonic and lattice modes can be clearly identified in the experimentally obtained near-field maps.

Terahertz near-field imaging using subwavelength plasmonic apertures and a quantum cascade laser source

The first demonstration, to our knowledge, of near-field imaging using subwavelength plasmonic apertures with a terahertz quantum cascade laser source is presented. "Bull's-eye" apertures, featuring subwavelength circular apertures flanked by periodic annular corrugations were created using a novel fabrication method. A fivefold increase in intensity was observed for plasmonic apertures over plain apertures of the same diameter. Detailed studies of the transmitted beam profiles were undertaken for apertures with both planarized and corrugated exit facets, with the former producing spatially uniform intensity profiles and subwavelength spatial resolution. Finally, a proof-of-concept imaging experiment is presented, where an inhomogeneous pharmaceutical drug coating is investigated.

Real-time terahertz near-field microscope

We report a terahertz near-field microscope with a high dynamic range that can capture images of a 370 x 740 μm2 area at 35 frames per second. We achieve high spatial resolution (14 μm corresponding to λ/30 for a center frequency at 0.7 THz) on a large area by combining two novel techniques: terahertz generation by tilted-pulse-front excitation and electro-optic balanced imaging detection using a thin crystal. To demonstrate the microscope capability, we reveal the field enhancement at the gap position of a dipole antenna after the irradiation of a terahertz pulse.

Terahertz near-field microscopy of complementary planar metamaterials: Babinet's principle

Using terahertz near-field imaging we experimentally investigate the resonant electromagnetic field distributions behind a split-ring resonator and its complementary structure with sub-wavelength spatial resolution. For the out-of-plane components we experimentally verify complementarity of electric and magnetic fields as predicted by Babinet's principle. This duality of near-fields can be used to indirectly map resonant magnetic fields close to metallic microstructures by measuring the electric fields close to their complementary analogues which is particularly useful since magnetic near-fields are still extremely difficult to access in the THz regime. We find excellent agreement between the results from theory, simulation and two different experimental near-field techniques.

Highly Sensitive Detector for On-Chip Near-Field THz Imaging

We present novel terahertz (THz) detectors, using nanoelectronic devices, such as 2-D semiconductor and carbon nanotube (CNT). Our detector is useful for high-resolution THz imaging, which requires highly sensitive detection. We have developed a frequency-tunable THz-photon detector and an on-chip near-field imaging detector. The detection mechanism of the former is that the CNT transistor detects electrical polarization generated by THz-excited electron-hole pairs in the 2-D semiconductor. In the latter, we have achieved THz imaging beyond the diffraction limit with an all-integrated device, including an aperture, a probe, and a detector. We further show applications of the THz imaging to semiconductor research. We have successfully visualized and clarified spatial properties of electrons in the 2-D semiconductors, which have not been revealed by conventional transport measurements.

Near-field terahertz imaging of ferroelectric domains in barium titanate

We report on results of a near-field imaging experiment with broadband terahertz pulses applied to a BaTiO3 multidomain single crystal. Data with a spatial resolution below 15 µm were collected by our scanning time-domain terahertz imaging setup. We performed the scanning by an asymmetric skewed sapphire probe metallised from two sides ended with a 60 µm × 140 µm facet. The imaging scans were performed with various angles of the polarisation of the probing field with respect to the direction of the domain walls in order to identify the orientations with optimal sensitivity. The raw experimental data were prewhitened and processed by the singular value decomposition. In this way, independent components belonging to experimental features (polarisation, noise and external perturbations) were found. The spatial resolution of the sensing probe was found significantly higher than that allowed by the outer dimensions of the probe end facet. This result was also confirmed by electromagnetic-field simulations.