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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.

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 β-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 β-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.

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.

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.

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.

Bi 2 O 3 is highly sensitive to low concentrations of NO 2 in ambient air but almost insensitive to most other common gasses. On the other hand, its sensing performance and detection limit need to be improved before it can be used more widely. This study examined the NO 2 gas sensing properties of β -Bi 2 O 3 nanobelts encapsulated with ZnO. Bi 2 O 3 -core/ZnO-shell nanobelts were fabricated using a two-step process involving the thermal evaporation of Bi powders and the atomic layer deposition of ZnO. The core–shell nanobelts were 100 to 300 nm in diameter with lengths ranging from a few tens to a few hundreds of micrometers with a mean shell layer thickness of ~ 20 nm. Multiple networked Bi 2 O 3 -core/ZnO-shell nanobelt sensors showed responses of 113–198% and 227–665% to 1–5 ppm NO 2 at room temperature in the dark and under UV illumination, respectively. These responses were 1.2–1.9 and 2.4–6.3 times larger, respectively, than those of pristine Bi 2 O 3 nanobelt sensors at 1–5 ppm NO 2 . The underlying mechanism of the enhanced response of the Bi 2 O 3 nanobelts to NO 2 gas by ZnO encapsulation and UV irradiation is discussed.

GaN is highly sensitive to low concentrations of H2 in ambient air and is almost insensitive to most other common gases. However, enhancing the sensing performance and the detection limit of GaN is a challenge. This study examined the H2-gas-sensing properties of GaN nanowires encapsulated with ZnO. GaN-core/ZnO-shell nanowires were fabricated by using a two-step process comprising the thermal evaporation of GaN powders and the atomic layer deposition of ZnO. The core-shell nanowires ranged from 80 to 120 nm in diameter and from a few tens to a few hundreds of micrometers in length, with a mean shell layer thickness of ~8 nm. Multiple-networked pristine GaN nanowire and ZnO-encapsulated GaN (or GaN-core/ZnO-shell) nanowire sensors showed responses of 120–147% and 179–389%, respectively, to 500–2,500 ppm of H2 at room temperature under UV (254 nm) illumination. The underlying mechanism of the enhanced response of the GaN nanowire to H2 gas when using ZnO encapsulation and UV irradiation is discussed.

In 2 Ge 2 O 7 nanofibers were synthesized using a thermal evaporation technique. Scanning electron microscopy, transmission electron microscopy, and X-ray diffraction revealed that the nanofibers were single crystals and 50–100 nm in diameter and up to a few hundreds of micrometers in length. The sensing properties of multiple networked In 2 Ge 2 O 7 nanofiber sensors toward NO 2 gas at room temperature under UV irradiation were examined. The In 2 Ge 2 O 7 nanofiber gas sensors showed a remarkably enhanced response to NO 2 gas at room temperature under ultraviolet (UV) light illumination. The response of the In 2 Ge 2 O 7 nanofibers to NO 2 gas at room temperature increased from ∼104 to ∼601% with increasing the UV light illumination intensity from 0 to 1.2 mW/cm 2 . The significant improvement in the response of the In 2 Ge 2 O 7 nanofibers to NO 2 gas by UV irradiation is attributed to the increased change in resistance due to the increase in the number of electrons participating in the reactions with NO 2 molecules by photo-generation of electron–hole pairs.

GaN nanowires and GaN-core/WO 3 -shell nanowires were synthesized by the thermal evaporation of GaN powders followed by the sputter-deposition of WO 3 and their gas sensing properties were examined. The multiple networked pristine GaN nanowire sensors showed responses of approximately 125%, 140%, 146%, 159%, and 183% to 1, 2, 3, 4, and 5 ppm NO 2 gases, respectively. These responses are comparable to those obtained previously using metal oxide semiconductor one-dimensional nanostructure sensors. The responses of the nanowires to 1, 2, 3, 4, and 5 ppm NO 2 gases were improved 1.3, 1.4, 1.6, 1.7 and 1.8 fold, respectively, further through the encapsulation of GaN nanowires with a WO 3 thin film. The improvement in the response of GaN nanowires to NO 2 gas by encapsulation is attributed to the modulation of electron transport at GaN–WO 3 heterojunction. The electron transport in the core-shell nanowires is modulated by the heterojunction with an adjustable energy barrier height, resulting in an enhanced sensing property of the core-shell nanostructures.

Effects of functionalization materials on the selectivity of SnO2 nanorod gas sensors were examined by comparing the responses of SnO2 one-dimensional nanostructures functionalized with CuO and Pd to ethanol and H2S gases. The response of pristine SnO2 nanorods to 500 ppm ethanol was similar to 100 ppm H2S. CuO-functionalized SnO2 nanorods showed a slightly stronger response to 100 ppm H2S than to 500 ppm ethanol. In contrast, Pd-functionalized SnO2 nanorods showed a considerably stronger response to 500 ppm ethanol than to 100 ppm H2S. In other words, the H2S selectivity of SnO2 nanorods over ethanol is enhanced by functionalization with CuO, whereas the ethanol selectivity of SnO2 nanorods over H2S is enhanced by functionalization with Pd. This result shows that the selectivity of SnO2 nanorods depends strongly on the functionalization material. The ethanol and H2S gas sensing mechanisms of CuO- and Pd-functionalized SnO2 nanorods are also discussed.

ZnS-core/ZnO-shell nanowires were synthesized by the thermal evaporation of ZnS powders followed by the atomic layer deposition of ZnO and then multiple networked ZnS-core/ZnO-shell nanowire gas sensors were fabricated. The morphology, crystal structure, and sensing properties of pristine ZnS nanowires and ZnS-core/ZnO-shell nanowires to NO2 gas at room temperature under ultraviolet (UV) illumination were examined. Pristine ZnO nanowires and ZnS-core/ZnO-shell nanowires showed responses ranging from ~236 to ~620 % and from ~439 to ~1,180 %, respectively, to 1–5 ppm NO2 under UV (254 nm, 1.2 mW/cm2) illumination, corresponding to increases by 2.3–5.8 and 3.8–10.2 times, respectively, compared to the responses in the dark. These results imply a synergistic effect of a combination of UV irradiation and core-shell nanostructures formation on the gas sensing properties of ZnS nanowires. The underlying mechanism for the enhanced gas sensing properties of ZnS-core/ZnO-shell nanowires toward NO2 gas under UV illumination is discussed.

Sensitivity is a very important parameter for biosensor applications. We have numerically and experimentally studied the sensitivity and linewidth responses of optical resonant reflective biosensors to the thickness of high refractive index materials coating the grating layer used to create a guided mode. Zero sensitivity, which may create serious problems in biosensors, is obtained when the coating material has a relatively low refractive index and a thin coating thickness (near the cutoff condition in which a high index layer is no longer capable of acting as a waveguide). The sensitivity and linewidth are greatly influenced by the thickness of the coating material. The thickness also has an optimal state, which is a very important factor for designing highly sensitive optical resonant reflective biosensors using a guided mode resonance filter.

In this letter, we present low-temperature grown GaAs (LTG-GaAs)-based photoconductive antennas for the generation and detection of terahertz (THz) waves. The growth of LTG-GaAs and the annealing temperatures are systematically discussed based on the material characteristics and the properties of THz emission and detection. The optimum annealing temperature depends on the growth temperature, which turns out to be to for the initial excess arsenic density of to .

Recently, the enhanced field emission, photocatalytic activity and blue shift in UV emission of branched GaN nanowire structures were reported. Nevertheless, enhanced luminescence in branched GaN nanowires have not been reported despite the importance of short-wavelength emission. This paper reports synthesis of self-catalytic brush-like GaN nanowires exhibiting enhanced near-ultraviolet (NUV) emission. The diameters of the branches in the brush-like nanowires ranged from 3 to 14 nm, whereas those of backbones ranged from 30 to 80 nm. In contrast, the diameters of pristine nanowires were in a similar range to those of the backbones in the brush-like nanowires. The brush-like GaN nanowires showed significantly enhanced NUV emission compared to pristine GaN nanowires. This enhanced NUV emission might be due to the quantum confinement of excitons caused by the branches in the brush-like nanowires with diameters smaller than the Bohr exciton radius.

WO 3 nanotubes were synthesized using TeO 2 nanowire templates. Transmission electron microscopy revealed the nanotubes to have tube diameters, lengths, and wall thicknesses ranging from 100–200 nm, 3–4 μm, and 20–30 nm, respectively. The multiple networked WO 3 nanotube sensors showed responses of 144–677% in the NO 2 concentration range of 1–5 ppm at 300 °C. These responses were approximately double those observed for the WO 3 nanorod sensors over the same NO 2 concentration range. A model describing the gas sensing mechanism of WO 3 NTs is also proposed.

Au-functionalized ZnSe nanorods were synthesized by the thermal evaporation of ZnSe powder followed by Au sputter-deposition and thermal annealing. Photoluminescence (PL) showed that the intensity of near-band edge (NBE) emission of ZnSe nanorods was enhanced remarkably by Au-coating and annealing in a H2 atmosphere. The intensity ratio of NBE emission to the deep level emission, INBE/IDL of Au-coated ZnSe nanorods after annealing in a H2 atmosphere was ∼68 times higher than that of the pristine (unannealed, uncoated) ZnSe nanorods. The increase in INBE/IDL might be due to a combination of carrier transfer from the defect level to the Fermi level of Au nanoparticles, surface plasmon resonance in Au nanoparticles and hydrogen passivated deep level defects.

This paper reports the synthesis of ZnTe nanostructures with a range of morphologies and sizes using a novel, simple thermal evaporation technique and their luminescence properties. Photoluminescence measurements showed that pure ZnTe nanostructures synthesized at 600 °C by the thermal evaporation of ZnTe nanopowders had an intense emission band centered at ∼810 nm, whereas those of the ZnTe nanostructures obtained at ∼500 and ∼700 °C showed weak emission bands centered at ∼820 and ∼880 nm, respectively. Energy-dispersive X-ray spectroscopy revealed that the intense near-infrared emission was attributed to localized states associated with zinc vacancies.

Au-functionalized ZnS nanowires were prepared for use as gas sensors. Scanning electron microscopy revealed the diameters and lengths of the nanowires ranging from 20 to 100 nm and from a few tens to a few hundreds of micrometers, respectively. X-ray diffraction and transmission electron microscopy revealed that ZnS nanowires and Au nanoparticles were monocrystalline and polycrystalline, respectively. The responses of multiple networked ZnS nanowire sensors to 1–5 ppm NO2 were increased 1.6- to 1.9-fold and 2.8- to 5.4-fold by Au functionalization and a combination of Au functionalization and UV irradiation, respectively. Au-functionalized ZnS nanowire sensors showed responses of 168–313% to 1–5 ppm NO2 at 300 °C in the dark. These values are comparable or superior to most semiconductor oxide one-dimensional nanostructures-based sensors reported in the literature. Furthermore, Au-functionalized ZnS nanowire sensors showed far higher responses (291–884%) to 1–5 ppm NO2 at 300 °C under UV illumination. The NO2 gas sensing mechanism of the Au-functionalized ZnS nanowire sensors under UV illumination is discussed.

Branched ZnO nanowires were synthesized using a three-step process: 1) synthesis of ZnO nanowires by the thermal evaporation of a mixture of ZnO and graphite powders on Au-coated C-plane sapphire substrates for 1 h, 2) deposition of Au on the as-synthesized ZnO nanowires, and 3) resumption of the thermal evaporation process for 10 min. The multiple-networked branched ZnO nanowire sensors exhibited far superior NO2 gas sensing properties than the unbranched nanowires. The origin of the enhanced NO2 gas sensing properties is also discussed.

ZnO/ZnS core-shell nanowires were synthesized by using the thermal evaporation of a mixture of ZnO and graphite powders followed by the thermal evaporation of ZnS powder. Then, multiple networked ZnO/ZnS core-shell nanowire gas sensors were fabricated. The morphology, crystal structure, and sensing properties of the ZnO/ZnS core-shell nanowires to NO2 gas at 300 °C were examined. The responses of the ZnO/ZnS core-shell nanowires to NO2 gas were 2.0–2.6 fold higher compared to those for ZnO nanowires at NO2 concentrations ranging from 1 to 5 ppm. In addition, the underlying mechanism for the enhanced gas-sensing properties of ZnO/ZnS core-shell nanowires for NO2 gas is discussed.

TiO2-core/ZnO-shell nanorods were synthesized using a two-step process: the synthesis of TiO2 nanorods using a hydrothermal method followed by atomic layer deposition of ZnO. The mean diameter and length of the nanorods were ∼300 nm and ∼2.3 μm, respectively. The cores and shells of the nanorods were monoclinic-structured single-crystal TiO2 and wurtzite-structured single-crystal ZnO, respectively. The multiple networked TiO2-core/ZnO-shell nanorod sensors showed responses of 132–1054 % at ethanol (C2H5OH) concentrations ranging from 5 to 25 ppm at 150 ∘C. These responses were 1–5 times higher than those of the pristine TiO2 nanorod sensors at the same C2H5OH concentration range. The substantial improvement in the response of the pristine TiO2 nanorods to C2H5OH gas by their encapsulation with ZnO may be attributed to the enhanced absorption and dehydrogenation of ethanol. In addition, the enhanced sensor response of the core–shell nanorods can be attributed partly to changes in resistance due to both the surface depletion layer of each core–shell nanorod and the potential barriers built in the junctions caused by a combination of homointerfaces and heterointerfaces.

Pt-capped ZnO nanorods were synthesized by the thermal evaporation of a mixture of ZnO and graphite powder at 900 °C followed by Pt sputter-deposition and thermal annealing. Photoluminescence (PL) showed that the intensity of near-band edge (NBE) emission of ZnO nanorods was enhanced significantly by Pt-capping and annealing in a H 2 atmosphere. The intensity ratio of NBE emission to the deep level emission, I NBE / I DL of the Pt-capped ZnO nanorods after annealing in a H 2 atmosphere was ∼11 times higher than that of the unannealed, uncapped ZnO nanorods. The increase of the I NBE / I DL might be due to a combination of carrier transfer from the defect level to the Fermi level of Pt nanoparticles, surface plasmon resonance in Pt nanoparticles and hydrogen passivating deep level defects.

In2O3 nanotubes were synthesized as gas sensors using TeO2 nanowires as a template. Scanning and transmission electron microscopy revealed the tubes to have diameters of a few hundred nanometers, wall thickness of ∼25 nm and lengths up to a few millimeters. Multiple networked Au-doped In2O3 nanotube sensors showed responses of 187–1219% to 50–250 ppm C2H5OH at 300 °C. These responses are far superior to those obtained by undoped In2O3 nanotubes and stronger than those obtained by pure In2O3 nanowires at 370 °C. In addition, the ethanol sensing mechanism of the Au-doped In2O3 nanotube sensors is discussed.

CuO/In 2 O 3 core–shell nanorods were fabricated using thermal evaporation and radio frequency magnetron sputtering. X-ray diffraction and transmission electron microscopy showed that both the cores and shells were crystalline. The multiple networked CuO/In 2 O 3 core–shell nanorod sensors showed responses of 382–804%, response times of 36–54 s and recovery times of 144–154 s at ethanol (C 2 H 5 OH) concentrations ranging from 50 to 250 ppm at 300 °C. These responses were 2.3–2.8 times higher than those of the pristine CuO nanorod sensor over the same C 2 H 5 OH concentration range. The origin of the enhanced ethanol sensing properties of the core–shell nanorod sensor is discussed.

Gas sensors based on Ag–TeO2 composite nanorods were fabricated using thermal evaporation and sputtering techniques. The morphology, structure and phase composition of the as-prepared nanofibers were characterized by scanning electron microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD), respectively. TEM and XRD showed that the nanorods and nanoparticles on them were tetragonal-structured single crystal TeO2 and a mainly amorphous phase, respectively. The multiple-networked bare TeO2 nanorod sensors exhibited a response of ∼219% at 25 ppm C2H5OH at 300 °C, whereas the Ag-functionalized TeO2 nanorod sensors showed a response of ∼808% under the same conditions. The mechanism by which the sensing properties of the TeO2 nanorods were enhanced by functionalization with Ag is also discussed.

Zn 2 SnO 4 -core/ZnO-shell nanorods were synthesized using a two-step process: synthesis of Zn 2 SnO 4 nanorods the thermal evaporation of a mixture of ZnO, SnO 2 , and graphite powders, followed by atomic layer deposition (ALD) of ZnO. The nanorods were 50–250 nm in diameter and a few to a few tens of micrometers in length. The cores and shells of the nanorods were face-centered cubic-structured single crystal Zn 2 SnO 4 and wurtzite-structured single crystal ZnO, respectively. The multiple networked Zn 2 SnO 4 -core/ZnO-shell nanorod sensors showed a response of 173–498% to NO 2 concentrations of 1–5 ppm at 300 °C. These response values are 2–5 times higher than those of the Zn 2 SnO 4 nanorod sensor over the same NO 2 concentration range. The NO 2 sensing mechanism of the Zn 2 SnO 4 core/ZnO-shell nanorods is discussed.

SnO 2 -core/Ga 2 O 3 -shell nanowires were synthesized by one-step thermal evaporation of a mixture of SnO 2 , GaN and graphite powders at 1000 °C. Transmission electron microscopy and energy-dispersive X-ray spectroscopy concentration profiles of Sn, Ga, and O across the diameter of the nanowire indicated clearly that the nanowire consisted of a single crystal SnO 2 core and a single crystal Ga 2 O 3 shell with a shell layer thickness of ∼20 nm. A possible growth mechanism of SnO 2 -core/Ga 2 O 3 -shell nanowires in the one-step synthesis process is proposed.

The nanowires are highly sensitive to low concentrations of 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 encapsulation of nanorods with on the gas sensing properties. nanorods were fabricated by a two-step process comprising the thermal evaporation of powders and sputter-deposition of . Multiple networked nanorod sensors showed the responses of 12-156% at 1-5 ppm at . These response values were 1.3-2.7 times larger than those of bare nanorod sensors at 1-5 ppm . The enhancement in the response of nanorods to gas by the encapsulation by can be accounted for based on the space-charge model.

We report the use of a widely tunable detuned dual-mode laser diode (DML) as a compact and portable continuous-wave THz emitter. The wavelength difference between the two lasing modes of this DML can be tuned from 2.4 nm to 9.3 nm by using integrated microheaters. The power difference between these modes is less than 1 dB, and the side-mode suppression ratio is greater than 30 dB over the entire tuning range.

This study reports a portable and precision photonic biosensor reader that can measure the concentration of a particular antigen using an optical resonant reflection biosensor (ORRB). To create a compact biosensor reader, a compact tunable vertical-cavity surface-emitting laser (VCSEL) and a compact built-in wavelength meter were manufactured. The wavelength stability and accuracy of the compact built-in wavelength meter were measured to be less than 0.02 nm and 0.06 nm, respectively. The tunable VCSEL emission wavelength was measured with the compact built-in wavelength meter, it has a fast sweep time (~ 10 seconds) and a wide tuning range (> 4 nm) that are sufficient for biosensor applications based on ORRB. The reflection spectrum of a plastic based ORRB chip was measured by the fabricated portable photonic biosensor reader using the VCSEL and wavelength meter. Although the reader is the size of a palmtop device, it could make a precise measurement of the peak wavelength on equal terms with a conventional bulky optical spectrometer.

We demonstrate a commercialized injection molding method for nano patterning of polymethyl methacrylate (PMMA) for the purpose of mass production of a label-free optical biosensor chip. Scanning electron microscope (SEM) images clearly show uniform nanopattern transfer on the plastic chip fabricated by an injection molding method. A uniformly deposited silicon nitride layer having a high refractive index by a plasma enhanced chemical vapor deposition (PECVD) technique is also observed. A unique characterization method is described wherein optical resonance reflection is used accurately and quickly to characterize the geometrical sensor structure. The authors anticipate that this robust method will provide an excellent means of producing an optical resonance reflection biosensor (ORRB).

We report an experimental demonstration of 40 Gbps all-optical 3R regeneration with all-optical clock recovery based on InP semiconductor devices. We also obtain alloptical non-return-to-zero to return-to-zero (NRZ-to-RZ) format conversion using the recovered clock signal at 10 Gbps and 40 Gbps. It leads to a good performance using a Mach-Zehnder interferometric wavelength converter and a self-pulsating laser diode (LD). The self-pulsating LD serves a recovered clock, which has an rms timing jitter as low as sub-picosecond. In the case of 3R regeneration of RZ data, we achieve a 1.0 dB power penalty at 10 -9 BER after demultiplexing 40 Gbps to 10 Gbps with an eletroabsorption modulator. The regenerated 3R data shows stable error-free operation with no BER floor for all channels. The combination of these functional devices provides all-optical 3R regeneration with NRZ-to-RZ conversion.

All-optical clock recovery is performed using a multisection semiconductor laser that can generate two kinds of self-pulsation due to compound cavity mode beating and passive mode locking. To investigate factors to influence quality of optical clock, optical clocks recovered from an optical data are compared by using various self-pulsations with different characteristics. It is experimentally demonstrated that characteristics of optical clock depend on free-running characteristics of a RF linewidth and amplitude noise. In particular, optical clock with a subpicosecond jitter is recovered by use of a self-pulsation with a narrow linewidth and low amplitude noise of the monolithic semiconductor laser in the 10 GHz regime. The free-running characteristics can be utilized as a criterion of a potential of self-pulsating semiconductor lasers for all-optical clock recovery.

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.

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.

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.

Our recent studies in regards of developing portable THz scanner for imaging and spectroscopy systems are presented. In the course, high power tunable continuous wave (CW) THz emitter and high sensitivity THz receiver platforms are presented. Those platforms can be realized with tunable optical beating source, broadband photomixer, arrayed photomixer and Schottky barrier diode, evanescently-coupled photodiodes with high saturation current, and semiconductor optical amplifier (SOA) integrated optical beating source. On the system level, our recent THz thickness measurement systems and the THz line scanner imaging system are presented.

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.

An optical triplex transceiver (TRx) module, which consists of thin-film filter (TFF)-attached wavelength-division multiplexer (WDM) and photodiode (PD) carriers, has been fabricated using a silica planar lightwave circuit (PLC) hybrid integration technology. Two types of TFFs were attached to a diced sidewall of a silica-terraced PLC platform to realize the TFF-attached WDM. The PD carriers with a 45deg mirror, on which receiving surface-illuminated PDs were bonded, were assembled with the PLC platform to form receiver (Rx) parts. As the main performances of the packaged TRx module, a very clear transmitter (Tx) eye pattern and minimum Rx sensitivity of -25.7 dBm were obtained under a 1.25-Gb/s Tx-Rx operation for digital applications. For an analog Rx application, a module responsivity of about 0.8 A/W was achieved, and a second-order intermodulation distortion value of less than -70 dBc at an optical modulation index of 40% was obtained under a two-tone test of 400 and 450 MHz

We report characterization and experimental realization of recent InP-related semiconductor optical devices for all-optical 3R regeneration. A Mach-Zehnder interferometric wavelength converter (MZ IWC) with preamplifiers gives an extinction ratio (ER) improvement and a negative power penalty for all-optical reamplification and reshaping (2R) of pseudorandom bit sequence (PRBS) nonreturn-to-zero (NRZ) data at 10 Gb/s. In addition, we briefly describe the mechanism for injection locking of the multisection laser diode with a distributed feedback (DFB) reflector. Using this device, we demonstrate clock recovery from PRBS return-to-zero (RZ) and NRZ data. The achieved rms timing jitter is less than 1 ps for the input of 11.727 Gb/s PRBS RZ and NRZ. All-optical 3R regeneration leads to a good performance using the MZ IWC and multisection DFB LD. The combination of these functions provides all-optical 3R regeneration with NRZ to RZ conversion. For the realization of a small and compact 3R regeneration module, we propose the hybrid module packaging process, which is composed of 2R and 1R semiconductor chip in single butterfly-type module

We have demonstrated a high-power widely tunable sampled grating distributed Bragg reflector (SGDBR) laser integrated monolithically with a semiconductor optical amplifier (SOA) having a lateral tapered waveguide, which is the first to emit a fiber-coupled output power of more than 10 dBm using a planar buried heterostructure (PBH). The output facet reflectivity of the integrated SOA using a lateral tapered waveguide and two-layer AR coating of TiO 2 and SiO 2 was lower than 3 × 10 - 4 over a wide bandwidth of 85 nm. The spectra of 40 channels spaced by 50 GHz within the tuning range of 33 nm were obtained by a precise control of SG and phase control currents. A side-mode suppression ratio of more than 35 dB was obtained in the whole tuning range. Fiber-coupled output power of more than 11 dBm and an output power variation of less than 1 dB were obtained for the whole tuning range.

Semiconductor devices based on photonics for the tunable continuous-wave THz generation and detection are explored. A tunable single-chip terahertz transceiver consists of beating source, broadband photomixer, and THz detector is developed. The realizable possibility of monolithically integrated single-chip THz transceiver which is main building block for the wide spread industrial applications will be addressed.

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.

A novel type of semiconductor beating source, a monolithically integrated dual-mode laser, and continuous-wave terahertz (THz) system adopting it will be investigated. The combined system of the beating source with broadbandantenna- integrated low-temperature-grown semiconductor photomixers shows the possibility of the realization of the cost-effective and compact continuous-wave THz systems. Such a system is highly-demanded to examine the THz finger prints of specimens without limitations. Since the optimized performance depends not only on the characteristics of functional devices but also module configurations, various approaches such as traveling-wave photomixers, Schottky barrier diodes, and nano-structure contained photomixers have been investigated to implement high-performance THz platforms as the main building blocks of a THz system. Semiconductor-based compact and cost-effective photonics technologies will envisage the bright future of THz systems.

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.

We have fabricated thermal-induced wavelength- tunable vertical-cavity surface-emitting lasers with a 780-nm emission wavelength that is tunable by using an internal device heater. A 5.36-nm tuning range was achieved by means of a 70-mA tuning current and a 3.5-mA driving current while maintaining continuous single-mode operation and the optical power variation is less than 2.5 dB in the tuning range. In addition, a multimode tuning range of 9.1 nm was achieved with a 90-mA tuning current and a 7-mA driving current.

Harmonic signal generation and frequency up-conversion are demonstrated using a hybrid mode-locked multisection distributed-feedback (MS-DFB) laser. Hybrid mode-locking is realized by direct injection of electrical local oscillator (LO) signals into the laser gain control section. The harmonic signals are generated when multiple optical modes produced by the hybrid mode-locked MS-DFB laser are detected in a photodiode (PD). In addition, if data signals at fIF are applied to the device along with LO signals at fLO, optical sidebands separated from the optical modes by fIF are generated and harmonic up-converted signals are obtained by mode-beating in PD. Using this method, we demonstrate generation of the third-harmonic millimeter-waves at 30.79 GHz with fLO at 10.263 GHz, and up-conversion of 12.5-Mb/s 32 quadrature amplitude modulation data at 300-MHz fIF into 30-GHz band

A travelling-wave photomixer was realized. The responsivity was 0.42A/W for 15μm length device. It showed a 3dB bandwidth of more than 200GHz, limited by transit-time of photo-generated carriers. Also, it showed a successful THz wave generation up to frequency range of more than 500GHz.

We characterize InGaAs Schottky barrier diodes (SBDs) with a variation in the anode size. High-efficiency terahertz (THz) pulse detection of the InGaAs SBDs is performed in the cases of anode diameters of 2 μm and 3 μm. The uniformity of the fabricated 1 × 20 InGaAs SBD array is measured to be fine. The THz imaging results using the 1 × 20 InGaAs SBD array are presented.