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.

Enhanced NO2 Gas Sensing Properties of WO3-Coated Multiwall Carbon Nanotube Sensors.

WO3-coated multiwall carbon nanotubes (MWCNTs) were fabricated by sputter-deposition of WO3 on MWCNT paste. The outer diameters of WO3-coated MWCNTs ranged from 20 to 40 nm and the lengths ranged up to a few tens of micrometers. The low-magnification TEM image of a typical WO3-coated CNT showed a CNT with an inner diameter of ~20 nm and a tube wall thickness of ~7 nm and WO3 shells with a thickness up to 10 nm at both edges of the tube. The WO3 shells were very nonuniform in thickness not only along the axis of the nanotube but also from one nanotube to the other. The sensing properties of multiple networked WO3-coated CNT sensors toward NO2 gas were examined. The WO3-coated MWCNT sensors showed responses of 120-221% over an NO2 concentration range of 1 to 5 ppm at room temperature. The responses were 1-2 fold higher than those of the pristine MWCNT sensor over the same NO2 concentration range. The origin of the enhancement of the MWCNTs in the response to NO2 by coating them with WO3 is discussed.

Surface Plasmon Resonance-Enhanced Luminescence in Pd-Functionalized ZnO Nanowires.

Palladium (Pd)-functionalized ZnO nanowires were synthesized by thermal evaporation of a ZnO/graphite powder mixture followed by solution method. The ZnO nanowires had a rod-like morphology with relatively uniform width and length. The widths and lengths of the nanowires ranged from 30 to 100 nm and 5-10 µm, respectively. The diameters of the Pd particles on the nanowires ranged from 5 to 50 nm. Effects of postannealing on the photoluminescence properties of Pd-functionalized ZnO nanowires were examined. Thermal annealing resulted in an increase and decrease in the near-band edge (NBE) and deep level (DL) emission intensities of Pd-capped ZnO nanowires, respectively, whereas both the NBE and DL emission intensities of uncapped ZnO nanowires were increased by annealing. The intensity ratio of NBE emission to DL emission of the Pd-capped ZnO nanowires was increased ~18 fold by annealing in a hydrogen atmosphere. The underlying mechanism for NBE emission enhancement and DL emission suppression of Pd-capped ZnO nanorwires by postannealing is discussed based on the surface plasmon resonance effect of Pd.

NO2 Gas Sensing Properties of Multiple Networked ZnGa2O4 Nanorods Coated with TiO2.

The NO2 gas sensing properties of ZnGa2O4-TiO2 heterostructure nanorods was examined. ZnGa2O4-core/TiO2-shell nanorods were fabricated by the thermal evaporation of a mixture of Zn and GaN powders and the sputter deposition of TiO2. Multiple networked ZnGa2O4-core/TiO2-shell nanorod sensors showed the response of 876% at 10 ppm NO2 at 300 degrees C. This response value at 10 ppm NO2 is approximately 4 times larger than that of bare ZnGa2O4 nanorod sensors. The response values obtained by the ZnGa2O4-core/TiO2-shell nanorods in this study are more than 13 times higher than those obtained previously by the SnO2-core/ZnO-shell nanofibers at 5% NO2. The significant enhancement in the response of ZnGa2O4 nanorods to NO2 gas by coating them with TiO2 can be explained based on the space-charge model.

Enhanced Ethanol Sensing Properties of TeO2 Nanorods Functionalized with Co3O4 Nanoparticles.

TeO2 nanorods functionalized with Co3O4 nanoparticles were fabricated using thermal evaporation and hydrothermal methods. X-ray diffraction and transmission electron microscopy showed that the cores and shells were single crystal TeO2 and polycrystalline Co304, respectively. The multiple networked TeO2/Co3O4 composite nanorod sensor showed responses of 333-1,276%, response times (75-110 sec), and recovery times (20-40 sec) ethanol (C2H5OH) concentrations of 50-250 ppm at 300 degrees C. The response values are 3-10 times higher and the response and recovery times are also far shorter than those of the pristine TeO2 nanorod sensor over the same C2H5OH concentration range. The origin of the enhanced ethanol sensing properties of the composite nanorod sensor is discussed.

Fabrication, Structure, and Gas Sensing of Multiple-Networked Pt-Functionalized Bi2O Nanowires.

Bi2O3 is highly sensitive to low concentrations of NO2 in ambient air and are almost insensitive to most other common gases. However, it still remains a challenge to enhance their sensing performance and detection limit. This study examined the influence of the functionalization of Bi2O3 nanorods with Pt on the NO2 gas sensing properties. Pt-functionalized ß-Bi2O3 nanorods were fabricated by the thermal evaporation of Bi powder followed by sputter-deposition of Pt and annealing. The Pt-functionalized nanorods ranged from 100 to 300 nm in diameter and from a few tens to a few hundreds of micrometers in length. Multiple networked Pt-functionalized ß-Bi2O3 nanorod sensors showed the responses of ~141 to ~313% at 1-5 ppm NO2 at 300 °C. These response values were 1.23-1.93 times higher than those of pristine Bi2O3 nanorod sensors at 1-5 ppm NO2. The origin of the enhancement of the sensing properties of the Bi2O3 nanorods by Pt functionalization is discussed.

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.

Role of the interfaces in multiple networked one-dimensional core-shell nanostructured gas sensors.

This study examined the gas sensing mechanism of multiple networked core-shell nanowire sensors. The ethanol gas sensing properties of In2O3/ZnO core-shell nanowires synthesized by the thermal evaporation of indium powder in an oxidizing atmosphere followed by the atomic layer deposition of ZnO were examined as an example. The pristine In2O3 nanowires and In2O3-core/ZnO-shell nanowires exhibited responses of ∼30% and ∼196%, respectively, to 1000 ppm ethanol at 300 °C. The response of the core-shell nanostructures to ethanol also showed a strong dependence on the shell layer width. The strongest response to ethanol was obtained with a shell layer thickness of ∼44 nm corresponding to 2λD, where λD is the Debye length of ZnO. The enhanced sensing properties of the core-shell nanowires toward ethanol can be explained based on the potential barrier-controlled carrier transport model combined with the surface depletion model; the former is predominant over the latter.

Light assisted room temperature ethanol gas sensing of ZnO-ZnS nanowires.

ZnO-core/ZnS-shell nanowires were synthesized by the thermal evaporation of a mixture of ZnO and graphite powders followed by the thermal evaporation of ZnS powders. Multiple networked nanowire gas sensors were then fabricated using the core-shell nanowires. The morphology, crystal structure and sensing properties of the ZnO-core/ZnS-shell nanowires to C2H5OH gas at room temperature under ultraviolet (UV) illumination were examined. The responses of the ZnO-core/ZnS-shell nanowires to C2H5OH gas were 1.2-2.1 times higher than those of the ZnO nanowires at C2H5OH concentrations ranging from 50 to 250 ppm under UV illumination. On the other hand, the resistance of the ZnO nanowires in the dark at room temperature was too high to measure. The underlying mechanism for the enhanced gas sensing properties of the ZnO-core/ZnS-shell nanowires toward C2H5OH gas is discussed.

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.

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.

Simple and cost-effective thickness measurement terahertz system based on a compact 1.55 µm λ/4 phase-shifted dual-mode laser.

A simple thickness measurement method based on the coherent homodyne CW THz system was demonstrated; it does not require precise control of the frequencies of the beat source, and only accurate scanning of the optical delay line is needed. Three beat frequencies are sufficient for measuring the thickness of a sample without considering the modulo 2π ambiguity. A novel compact 1.55 µm λ/4 phase-shifted dual-mode laser (DML) was developed as an optical beat source for the CW THz system. The thickness of a sample was accurately estimated from the measurements using the proposed method. Our results clearly show the possibility of a compact, simple, and cost-effective CW THz system for practical applications.

Distributed feedback laser diode integrated with distributed Bragg reflector for continuous-wave terahertz generation.

A widely tunable dual mode laser diode with a single cavity structure is demonstrated. This novel device consists of a distributed feedback (DFB) laser diode and distributed Bragg reflector (DBR). Micro-heaters are integrated on the top of each section for continuous and independent wavelength tuning of each mode. By using a single gain medium in the DFB section, an effective common optical cavity and common modes are realized. The laser diode shows a wide tunability of the optical beat frequency, from 0.48 THz to over 2.36 THz. Continuous wave THz radiation is also successfully generated with low-temperature grown InGaAs photomixers from 0.48 GHz to 1.5 THz.

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.

Synthesis of nanograined ZnO nanowires and their enhanced gas sensing properties.

Polycrystalline ZnO nanowires with grain sizes ranging from 20 to 100 nm were synthesized using a newly designed two-step process: (first step) synthesis of ZnSe nanowires by vapor transportation of a mixture of ZnSe powders; and (second step) thermal oxidation of the ZnSe nanowires at 650 °C. Compared to the single-crystal ZnO nanowire gas sensors and other nanomaterial gas sensors reported previously, the multiple networked nanowire gas sensors fabricated from the nanograined ZnO nanowires showed substantially enhanced electrical responses to NO2 gas at 300 °C. The NO2 gas sensing properties of the nanograined ZnO nanowires increased dramatically with increasing NO2 concentration. The multiple-networked nanograined ZnO nanowire sensor showed a response value of 237,263% at 10 ppm NO2 and 300 °C, whereas the single-crystal ZnO nanowire sensors showed a response of only 6.5% under the same conditions. The recovery time of the nanograined ZnO nanowire sensor was much shorter than that of the normal ZnO nanowire sensor over the NO2 concentration range of 1-10 ppm, even though the response time of the former was somewhat longer than that of the latter. The origin of the enhanced NO2 gas sensing properties of the nanograined ZnO nanowire sensor is discussed.

Tunable continuous-wave terahertz generation/detection with compact 1.55 µm detuned dual-mode laser diode and InGaAs based photomixer.

We demonstrate a tunable continuous-wave (CW) terahertz (THz) homodyne system with a novel detuned dual-mode laser diode (DML) and low-temperature-grown (LTG) InGaAs photomixers. The optical beat source with the detuned DML showed a beat frequency tuning range of 0.26 to over 1.07 THz. Log-spiral antenna integrated LTG InGaAs photomixers are used as THz wave generators and detectors. The CW THz radiation frequency was continuously tuned to over 1 THz. Our results clearly show the feasibility of a compact and fast scanning CW THz spectrometer consisting of a fiber-coupled detuned DML and photomixers operating in the 1.55-µm range.

Rapidly frequency-swept optical beat source for continuous wave terahertz generation.

We propose a rapidly frequency-swept optical beat source for continuous wave (CW) THz generation using a wavelength swept laser and a fixed distributed feedback (DFB) laser. The range of the sweeping bandwidth is about 17.3 nm (2.16 THz), 1541.42-1558.72 nm. The achieved side mode suppression ratio for both wavelengths within the full sweeping range is more than 45 dB. We observe CW THz signals for tunable optical beat sources using a fiber coupled CW THz measurement system to confirm the feasibility of using our frequency swept optical beat source as a CW THz radiation source. The THz output signal falls to the thermal noise level of the low-temperature grown (LTG) InGaAs photomixer beyond 1.0 THz. The rapidly frequency-swept optical beat source will be useful for generating high-speed tunable CW THz radiation.

Optical fiber-coupled InGaAs-based terahertz time-domain spectroscopy system.

The successful demonstration of an optical fiber-coupled terahertz time-domain spectroscopy (THz-TDS) system is described in this study. The terahertz output power of the emitter with two optical band rejection filters was 132 nW, which is an improvement of 70% over the output power without any filters. This improvement is due to the suppression of an optical modulated signal that is reverse-generated when an alternating current bias exceeding a certain threshold is applied to the emitter. Under the optimal alignment conditions, the terahertz detector in a fiber-coupled THz-TDS system clearly measured water vapor dips in the free space.