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.

Gate-tuned conductance of graphene-ribbon junctions with nanoscale width variations.

To utilize graphene as interconnection electrodes in high-density nanoelectronic structures, the electrical stability of graphene should be guaranteed under nanometer-scale deviations. Graphene-ribbon (GR) junctions with accessible dimensions (i.e., sub-micrometer widths) are used in diverse interconnection electrode applications and should be characterized properly if they are to be applied in high-density nanoelectronics. Analyzing the effects of nanoscale GR width variations on the conductance of the entire graphene electrode is necessary for their proper characterization. Here, we diagnose the conductance and thermal effect of graphene electrode junctions constructed from GRs of various widths and directions under gate-tuned voltages. On applying partial gate voltages, we identify the effect of local potential variance on the entire graphene electrode junction. As a result, we were able to perceive precise and minute conductance variations for the entire graphene electrode, arising mainly from different sub-micrometer-scale widths of the GRs, which could not be distinguished using conventional global gating methods.

Terahertz rectifier exploiting electric field-induced hot-carrier effect in asymmetric nano-electrode.

Rectifiers have been used to detect electromagnetic waves with very low photon energies. In these rectifying devices, different methods have been utilized, such as adjusting the bandgap and the doping profile, or utilizing the contact potential of the metal-semiconductor junction to produce current flow depending on the direction of the electric field. In this paper, it is shown that the asymmetric application of nano-electrodes to a metal-semiconductor-metal (MSM) structure can produce such rectification characteristics, and a terahertz (THz) wave detector based on the nano-MSM structure is proposed. Integrated with a receiving antenna, the fabricated device detects THz radiation up to a frequency of 1.5 THz with responsivity and noise equivalent power of 10.8 V/W and [Formula: see text] respectively, estimated at 0.3 THz. The unidirectional current flow is attributed to the thermionic emission of hot carriers accelerated by the locally enhanced THz field at the sharp end of the nano-electrode. This work not only demonstrates a new type of THz detector but also proposes a method for manipulating ultrafast charge-carrier dynamics through the field enhancement of the nano-electrode, which can be applied to ultrafast photonic and electronic devices.

Electrically controllable terahertz square-loop metamaterial based on VO2 thin film.

An electrically controllable square-loop metamaterial based on vanadium dioxide (VO2) thin film was proposed in the terahertz frequency regime. The square-loop shaped metamaterial was adopted to perform roles not only as a resonator but also as a micro-heater for the electrical control of the VO2. A dual-resonant square-loop structure was designed to realize band-pass characteristics in the desired frequency band. The measured Q-factors of the basic and scaled-down metamaterials fabricated on VO2 thin films were 2.22 and 1.61 at the center frequencies of 0.44 and 1.14 THz in the passbands, respectively. The transmittances of the proposed metamaterial were successfully controlled by applying a bias voltage without an external heater. The measured transmittance on-off ratios of the metamaterials were over 40 at the center frequencies in the passbands. In the future, electrically controllable terahertz metamaterial based on VO2 metamaterial could be employed as high-performance active filters or sensors.

Two-Dimensional Semiconductor Optoelectronics Based on van der Waals Heterostructures.

Two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDCs) and black phosphorous have drawn tremendous attention as an emerging optical material due to their unique and remarkable optical properties. In addition, the ability to create the atomically-controlled van der Waals (vdW) heterostructures enables realizing novel optoelectronic devices that are distinct from conventional bulk counterparts. In this short review, we first present the atomic and electronic structures of 2D semiconducting TMDCs and their exceptional optical properties, and further discuss the fabrication and distinctive features of vdW heterostructures assembled from different kinds of 2D materials with various physical properties. We then focus on reviewing the recent progress on the fabrication of 2D semiconductor optoelectronic devices based on vdW heterostructures including photodetectors, solar cells, and light-emitting devices. Finally, we highlight the perspectives and challenges of optoelectronics based on 2D semiconductor heterostructures.

Bias field tailored plasmonic nano-electrode for high-power terahertz photonic devices.

Photoconductive antennas with nano-structured electrodes and which show significantly improved performances have been proposed to satisfy the demand for compact and efficient terahertz (THz) sources. Plasmonic field enhancement was previously considered the dominant mechanism accounting for the improvements in the underlying physics. However, we discovered that the role of plasmonic field enhancement is limited and near-field distribution of bias field should be considered as well. In this paper, we clearly show that the locally enhanced bias field due to the size effect is much more important than the plasmonic enhanced absorption in the nano-structured electrodes for the THz emitters. Consequently, an improved nano-electrode design is presented by tailoring bias field distribution and plasmonic enhancement. Our findings will pave the way for new perspectives in the design and analysis of plasmonic nano-structures for more efficient THz photonic devices.

Metal-VO2 hybrid grating structure for a terahertz active switchable linear polarizer.

An active terahertz (THz) wave hybrid grating structure of Au/Ti metallic grating on VO2/Al2O3 (0001) was fabricated and evaluated. In our structure, it is shown that the metallic gratings on the VO2 layer strengthen the metallic characteristics to enhance the contrast of the metallic and dielectric phases of a VO2-based device. Especially, the metal grating-induced optical conductivity of the device is greatly enhanced, three times more than that of a metallic phase of bare VO2 films in the 0.1-2.0 THz spectral range. As an illustrative example, we fabricated an actively on/off switchable THz linear polarizer. The fabricated device has shown commercially comparable values in degree of polarization (DOP) and extinction ratio (ER). A high value of 0.89 in the modulation depth (MD) for the transmission field amplitude, superior to other THz wave modulators, is achieved. The experimental results show that the fabricated device can be highly useful in many applications, including active THz linear polarizers, THz wave modulators and variable THz attenuators.

Continuous-wave terahertz system based on a dual-mode laser for real-time non-contact measurement of thickness and conductivity.

Terahertz (THz) waves have been exploited for the non-contact measurements of thickness and refractive index, which has enormous industrial applicability. In this work, we demonstrate a 1.3-µm dual-mode laser (DML)-based continuous-wave THz system for the real-time measurement of a commercial indium-tin-oxide (ITO)-coated glass. The system is compact, cost-effective, and capable of performing broadband measurement within a second at the setting resolution of 1 GHz. The thickness of the glass and the sheet conductivity of the ITO film were successfully measured, and the measurements agree well with those of broadband pulse-based time domain spectroscopy and Hall measurement results.

InGaAs Schottky barrier diode array detector for a real-time compact terahertz line scanner.

We present a terahertz (THz) broadband antenna-integrated 1 × 20 InGaAs Schottky barrier diode (SBD) array detector with an average responsivity of 98.5 V/W at a frequency of 250 GHz, which is measured without attaching external amplifiers and Si lenses, and an average noise equivalent power (NEP) of 106.6 pW/√Hz. The 3-dB bandwidth of the SBD detector is also investigated at approximately 180 GHz. For implementing an array-type SBD detector by a simple fabrication process to achieve a high yield, a structure comprising an SiN(x) layer instead of an air bridge between the anode and the cathode is designed. THz line beam imaging using a Gunn diode emitter with a center frequency of 250 GHz and a 1 × 20 SBD array detector is successfully demonstrated.

Compact fiber-pigtailed InGaAs photoconductive antenna module for terahertz-wave generation and detection.

We propose a compact fiber-pigtailed InGaAs photoconductive antenna (FPP) module having an effective heat-dissipation solution as well as a module volume of less than 0.7 cc. The heat-dissipation of the FPP modules when using a heat-conductive printed circuit board (PCB) and an aluminium nitride (AlN) submount, without any cooling systems, improve by 40% and 85%, respectively, when compared with a photoconductive antenna chip on a conventional PCB. The AlN submount is superior to those previously reported as a heat-dissipation solution. Terahertz time-domain spectroscopy (THz-TDS) using the FPP module perfectly detects the absorption lines of water vapor in free space and an α-lactose sample.

Photoinduced phase transitions by time-resolved far-infrared spectroscopy in V2O3.

Using time-resolved far-infrared spectroscopy, we observe multiple routes for photoinduced phase transitions in V(2)O(3). This includes (i) a photothermal antiferromagnetic to paramagnetic transition and (ii) an incipient strain-generated paramagnetic metal to paramagnetic insulator transition, which manifests as coherent oscillations in the far-infrared conductivity. The ∼100 ps conductivity oscillation results from coherent acoustic phonon modulation of the bandwidth W. Our results indicate that poor metals are particularly amenable to coherent strain control of their electronic properties.

Novel approach of predicting fracture load in the human proximal femur using non-invasive QCT imaging technique.

This paper presents an analysis of predicting the load-bearing capacities of human femurs using quantitative computer tomography (QCT)-based beam theory. Cross-sectional images of 12 human cadaver femurs (intact bones, age: 39-77 years; male = 8, female = 4) were scanned in conjunction with a calcium hydroxyapatite phantom which has five chambers of known densities. The apparent densities obtained from the scans were used to evaluate the Young's modulus (E) by applying the established empirical relationships. The fracture load of a configuration that simulated single-legged stance was measured experimentally and compared with the predicted failure load using a composite beam theory, plane stress model of the femur. In this model, the failure was assumed to occur at the weakest cross-section through the bone determined from QCT-based structural analysis. In contrast to the other experimental investigations, the setup used in this study considers the entire length of a human femur and also incorporates a novel mechanical jig to mimic the realistic physiological scenario. In one of our earlier studies, simulated lytic defects of varying size were created at the inter-trochanteric region of femurs and their load-bearing capacities were calculated based on their structural properties. Both the results obtained from the current study as well as the ones from our previous study were used to assess the viability of the methodology. A high degree of correlation was observed when the predicted failure loads obtained from the intact femurs and previously studied defective femurs were compared with the ex vivo fracture loads. The coefficients of determination (R(2)) of QCT-derived predicted loads with respect to the measured failure loads were 0.80 for the intact femurs and 0.87 for the defective femurs. The results suggest that the QCT-derived beam analysis provides a viable approach for the assessment of load-bearing capacity in various clinical scenarios.