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.

Efficient Gate Modulation in a Screening-Engineered MoS2/Single-Walled Carbon Nanotube Network Heterojunction Vertical Field-Effect Transistor.

In this report, a screening-engineered carbon nanotube (CNT) network/MoS2/metal heterojunction vertical field effect transistor (CNT-VFET) is fabricated for an efficient gate modulation independent of the drain voltage. The gate field in the CNT-VFET transports through the empty space of the CNT network without any screening layer and directly modulates the MoS2 semiconductor energy band, while the gate field from the Si back gate is mostly screened by the graphene layer. Consequently, the on/off ratio of CNT-VFET maintained 103 in overall drain voltages, which is 10 times and 1000 times higher than that of the graphene (Gr) VFET at Vsd = 0.1 (ratio = 81.9) and 1 V (ratio = 3), respectively. An energy band diagram simulation shows that the Schottky barrier modulation of CNT/MoS2 contact along the sweeping gate bias is independent of the drain voltage. On the other hand, the gate modulation of Gr/MoS2 is considerably reduced with increased drain voltage because more electrons are drawn into the graphene electrode and screens the gate field by applying a higher drain voltage to the graphene/MoS2/metal capacitor.

Terahertz continuous wave system using phase shift interferometry for measuring the thickness of sub-100-µm-thick samples without frequency sweep.

A terahertz continuous wave system is demonstrated for thickness measurement using Gouy phase shift interferometry without frequency sweep. One arm of the interferometer utilizes a collimated wave as a reference, and the other arm applies a focused beam for sample investigation. When the optical path difference (OPD) of the arms is zero, a destructive interference pattern is produced. Interference signal intensity changes induced by the OPD changes can be easily predicted by calculations. By minimizing the difference between the measured and the calculated signal against the OPD, the thicknesses of sub-100-µm-thick samples are determined at 625 GHz.

Polarization-selective control of nonlinear optomechanical interactions in subwavelength elliptical waveguides.

Photonic devices that exhibit all-optically reconfigurable polarization dependence with a large dynamic range would be highly attractive for active polarization control. Here, we report that strongly polarization-selective nonlinear optomechanical interactions emerge in subwavelength waveguides. By using full-vectorial finite element analysis, we find, at certain core ellipticities (or aspect ratios), that the forward simulated light scattering mediated by a specific acoustic resonance mode is eliminated for one polarization mode. Whereas, that for the other polarization mode is rather enhanced. This intriguing phenomenon can be explained by the interplay between the electrostrictive force and radiation pressure and turns out to be tailorable by the choice of waveguide materials.

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.

Direct growth of doping controlled monolayer WSe2 by selenium-phosphorus substitution.

Although many studies have been carried out on the doping of transition metal dichalcogenides (TMDCs), introducing controllable amounts of dopants into a TMD lattice is still insufficient. Here we demonstrate doping controlled TMDC growth by the replacement of selenium with phosphorus during the synthesis of the monolayer WSe2. The phosphorus doping density was precisely controlled by fine adjustment of the amount of P2O5 dopant powder along the pre-annealing time. Raman spectroscopy, photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and high-angle annular bright field scanning tunneling electron microscopy (HAADF STEM) provide evidence that P doping occurs within the WSe2 crystal with P occupying the substitutional Se sites. With regard to its electrical characteristics, the hole majority current of P-doped WSe2 is 100-times higher than that of the intrinsic WSe2. The measured doping concentration ranged from ∼8.16 × 1010 to ∼1.20 × 1012 depending on the amount of P2O5 dopant powder by pre-annealing.

Two-terminal floating-gate memory with van der Waals heterostructures for ultrahigh on/off ratio.

Concepts of non-volatile memory to replace conventional flash memory have suffered from low material reliability and high off-state current, and the use of a thick, rigid blocking oxide layer in flash memory further restricts vertical scale-up. Here, we report a two-terminal floating gate memory, tunnelling random access memory fabricated by a monolayer MoS2/h-BN/monolayer graphene vertical stack. Our device uses a two-terminal electrode for current flow in the MoS2 channel and simultaneously for charging and discharging the graphene floating gate through the h-BN tunnelling barrier. By effective charge tunnelling through crystalline h-BN layer and storing charges in graphene layer, our memory device demonstrates an ultimately low off-state current of 10(-14) A, leading to ultrahigh on/off ratio over 10(9), about ∼10(3) times higher than other two-terminal memories. Furthermore, the absence of thick, rigid blocking oxides enables high stretchability (>19%) which is useful for soft electronics.

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.

Frequency modulation based continuous-wave terahertz homodyne system.

In this study, inspired by the frequency-modulated continuous-wave (FMCW) method, an operation scheme of continuous-wave (CW) terahertz (THz) homodyne system is proposed and evaluated. For this purpose, we utilized the fast and stable wavelength tuning characteristics of a dual-mode laser (DML) as a beating source. Using the frequency-modulated THz waves generated by DML, a cost-effective and robust operation of CW THz system to be applicable to the measurements of thickness or refractive index of a sample is demonstrated. We believe that the proposed scheme shows a potential to the implementations of compact and fast CW THz measurement systems that can be useful in many THz applications.

Real-time continuous-wave terahertz line scanner based on a compact 1 × 240 InGaAs Schottky barrier diode array detector.

We demonstrate real-time continuous-wave terahertz (THz) line-scanned imaging based on a 1 × 240 InGaAs Schottky barrier diode (SBD) array detector with a scan velocity of 25 cm/s, a scan line length of 12 cm, and a pixel size of 0.5 × 0.5 mm². Foreign substances, such as a paper clip with a spatial resolution of approximately 1 mm that is hidden under a cracker, are clearly detected by this THz line-scanning system. The system consists of the SBD array detector, a 200-GHz gyrotron source, a conveyor system, and several optical components such as a high-density polyethylene cylindrical lens, metal cylindrical mirror, and THz wire-grid polarizer. Using the THz polarizer, the signal-to-noise ratio of the SBD array detector improves because the quality of the source beam is enhanced.

Hybrid states of propagating and localized surface plasmons at silver core/silica shell nanocubes on a thin silver layer.

Hybrid characteristics of propagating surface plasmons (PSPs) and localized surface plasmons (LSPs) appear at a combined structure of a thin silver (Ag) layer and silver core/silica shell nanocubes (AgNC@SiO(2)s) in the Kretschmann configuration, because the resonant condition of PSPs on the thin Ag layer is significantly modified by LSPs of the AgNC@SiO(2)s. We investigate theoretically and experimentally that due to the hybrid property, the slope and position of the minimum reflectance band can be controlled on a graph of incident angle versus wavelength of reflected light, by changing structural parameters. The hybrid properties of PSPs and LSPs have a potential to simultaneously detect surface plasmon resonance signals and fluorescence images.

Intermediate plasmonic characteristics in a quasi-continuous metallic monolayer.

There has been a significant interest on plasmonics in a metallic structure with very narrow gaps for studies of nanophotonics. However, little attention has been paid to the behavior of surface plasmons (SPs) in quasi-continuous metallic structures. This study observes and analyzes intermediate characteristics between propagating SPs (PSPs) and localized SPs (LSPs) in a quasi-continuous metallic monolayer of core-shell nanocubes. We reveal that, in a very narrow region of few-nanometer gaps among the nanocubes, the intrinsic energy bands of PSPs and LSPs intersect each other to generate two hybrid bands and an anti-crossing. Using a self-assembly method instead of the lithographic techniques which have several limitations as of now, we materialize the quasi-continuous metallic layer with plenty of nano-gaps that exhibit intermediate plasmonic characteristics. The intermediate plasmonic characteristics observed in this study will lead to interesting subjects, such as band engineering and slow SPs, in nanophotonics.

Low-temperature-grown InGaAs terahertz photomixer embedded in InP thermal spreading layer regrown by metalorganic chemical vapor deposition.

A novel buried photomixer for integrated photonic terahertz devices is proposed. The active region of the mesa-structure InGaAs photomixer is buried in an InP layer grown by metalorganic chemical vapor deposition (MOCVD) to improve heat dissipation, which is an important problem for terahertz photomixers. The proposed photomixer shows good thermal properties compared to a conventional planar-type photomixer. The MOCVD regrowth process indicates the possibility for THz photomixers to be integrated monolithically with conventional photonic devices.

Efficient transition between photonic and plasmonic guided modes at abrupt junction of MIM plasmonic waveguide.

We propose a novel metal-insulator-metal (MIM) waveguide mode transition scheme by the use of the abrupt junction of MIM plasmonic waveguide. Power coupling between anti-symmetric plasmonic mode and fundamental photonic mode can be easily done by reflection at the waveguide junction with an oblique MIM mode incidence due to the field intersection between those modes. With numerical simulation we find that mode conversion efficiency can be obtained up to 60% for single junction geometry, and it can be further increased up to 82% with the suppression of non-transited mode by adapting Bragg grating structure composed of periodical arranges of MIM junctions.

Generation of finite power Airy beams via initial field modulation.

We investigate the finite power Airy beams generated by finite extent input beams such as a Gaussian beam, a uniform beam of finite extent, and an inverse Gaussian beam. Each has different propagation behavior: A finite Airy beam generated by a uniform input beam keeps its Airy profile much longer than the conventional finite Airy beam. Also, an inverse Gaussian beam generates a finite Airy beam with a good bent focusing in free space. In this paper, the analysis and experimental results of finite Airy beams are presented.

Tunable subwavelength hot spot of dipole nanostructure based on VO2 phase transition.

We propose a novel approach to generate and tune a hot spot in a dipole nanostructure of vanadium dioxide (VO2) laid on a gold (Au) substrate. By inducing a phase transition of the VO2, the spatial and spectral distributions of the hot spot generated in the feed gap of the dipole can be tuned. Our numerical simulation based on a finite-element method shows a strong intensity enhancement difference and tunability near the wavelength of 678 nm, where the hot spot shows 172-fold intensity enhancement when VO2 is in the semiconductor phase. The physical mechanisms of forming the hot spots at the two-different phases are discussed. Based on our analysis, the effects of geometric parameters in our dipole structure are investigated with an aim of enhancing the intensity and the tunability. We hope that the proposed nanostructure opens up a practical approach for the tunable near-field nano-photonic devices.

Role of magnetic induction currents in nanoslit excitation of surface plasmon polaritons.

We present a method for exciting surface plasmon polaritons (SPPs) caused by a magnetic field component perpendicular to the direction of slit. The excitation mechanism is based on the spatially oscillating induced current along the edges of the slit under obliquely incident electromagnetic waves. Our finding distinguishes itself from previous mechanisms based on transverse electric fields and unveils the missing point of the SPP-excitation problem in a nanoslit. The use of a magnetic field for SPP excitation can be highly efficient and even comparable to that with an electric field, so that their composition can lead to selective unidirectional excitation.

Site-selective synthesis of silver nanoparticles in pre-patterned trenches and their localized surface plasmon resonances.

A method for depositing silver nanoparticles in a pre-patterned trench by site-selective synthesis is described. In the trench patterns with various shapes, silver nanoparticles can be selectively nucleated and grown only on polyvinylpyrrolidone (PVP) domains by attraction (or repulsion) between silver ions and the hydrophilic PVP island domains in a silica matrix of the trench (or the hydrophobic fluorosilane layer). Regarding the silver nanoparticles in the trench, localized surface plasmon resonance (LSPR) could be excited by obliquely incident light, reradiating the enhanced electromagnetic field in the far- and near-fields. Even in the case of a large angle incidence in total internal reflection (TIR), the patterned silver nanoparticle clusters underwent strong scattering with a high intensity, due to the LSPR effect.

Dynamic switching of the chiral beam on the spiral plasmonic bull's eye structure [Invited].

A polarization-dependent switchable plasmonic beaming structure composed of metallic hole surrounded by double spiral dielectric gratings is proposed. The main mechanism of the proposed structure is based on the angular momentum change of surface plasmon caused by the spiral geometry. On- and off-states of the proposed device are determined by the condition whether the rotating direction of incident polarization is the same as or opposite of the direction of the spiral rotations. Qualitative analytical expressions of the switching mechanisms and full-vectorial numerical results are presented.