Fast piezoresistive sensor and UV photodetector based on Mn‐doped ZnO nanorods

A low cost hydrothermal synthesis method to synthesize Mn‐doped ZnO nanorods (NRs) with controllable morphology and structure has been developed. Ammonia is used to tailor the ammonium hydroxide concentration, which provides a source of OH^– for hydrolysis and precipitation during the growth instead of HMT. The morphological, chemical composition, structural, and electronic structure studies of the Mn‐doped ZnO NRs show that the Mn‐doped ZnO NRs have a hexagonal wurtzite ZnO structure along the c‐axis and the Mn ions replace the Zn sites in the ZnO NRs matrix without any secondary phase of metallic manganese element and manganese oxides observed. The fabricated PEDOT:PSS/Zn_0.85Mn_0.15O Schottky diode based piezoresistive sensor and UV photodetector shows that the piezoresistive sensor has pressure sensitivity of 0.00617 kPa^–1 for the pressure range from 1 kPa to 20 kP and 0.000180 kPa^–1for the pressure range from 20 kPa to 320 kPa with relatively fast response time of 0.03 s and the UV photodetector has both relatively high responsivity and fast response time of 0.065 A/W and 2.75 s, respectively. The fabricated Schottky diode can be utilized as a very useful human‐friendly interactive electronic device for mass/force sensor or UV photodetector in everyday living life. This developed device is very promising for small‐size, low‐cost and easy‐to‐customize application‐specific requirements. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

High‐stability organic red‐light photodetector for narrowband applications

Red‐light photodetectors without filters are in urgent need for narrowband applications such as full‐color imaging and multi‐output visible light communication (VLC). However, their development is hindered by the lack of small‐band‐gap and narrowband response materials. Without wavelength filters, a new type of photodetector with a simple single‐layer architecture is developed, based on a stable small‐band‐gap squarylium dye and characterized by a detectivity peak at 680 nm and full width at half maximum of 80 nm. The device, which exhibits high stability in air and humid conditions, shows a significantly low dark current of ∼2 nA·cm⁻² at −2 V and high specific detectivity of 3.2 × 10¹² Jones. The response current ratio of the device to red, green, and blue lights with a luminous flux amplitude ratio of 3:6:1 (standard ratio for white light) is 100:12:1.1. These properties indicate that the squarylium dye red‐light photodetectors are promising for VLC and other narrowband optoelectronic applications.

     

Drop‐cast and dye‐sensitized ZnO nanorod‐based visible‐light photodetectors

We report the facile fabrication of metal–semiconductor–metal (MSM) photodetectors with dye‐sensitized ZnO nanorods (NRs) operating at wavelengths of ～405–638 nm by a simple drop casting method. The ZnO NRs were synthesized by the hydrothermal synthesis method at 75 °C. The droplet of ethanol solution containing ZnO NRs was dropped between two metal electrodes and dried at 65 °C, which allows the ZnO NRs to be adhered and contacted to both metal electrodes. When a violet light of 405 nm was illuminated into the MSM ZnO NRs‐based photodetector, the photocurrent gain was obtained as ～3.9 × 10³ at the applied bias voltage of 5 V. By increasing the bias voltage from 10 V to 15 V, the device exhibited good recovery performance with a largely reduced reset time from 85.68 s to 2.47 s and an increased on–off ratio from 17.9 to 77.4. To extend the photodetection range towards the long visible spectral region, the ZnO NRs were sensitized by the N719 dye and then drop‐cast. The dye‐sensitized ZnO NRs‐based photodetector also exhibited good photocurrent switching under 638 nm of light illumination. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Binary response Se/ZnO p‐n heterojunction UV photodetector with high on/off ratio and fast speed

A high‐performance UV photodetector (PD) based on a p‐Se/n‐ZnO hybrid structure with large area (more than 1×1 cm) is presented in this study. The device is theoretically equivalent to a parallel‐connection circuit for its special structure and shows multifunction at different voltage bias, which means the output signal can be tailored by an applied voltage. The Se/ZnO PD shows binary response (positive and negative current output under on/off periodical light illumination) under small reverse bias (–0.05 V and –0.1 V) which efficiently reduces the negative effect of noise signal in weak‐signal detection applications. At zero bias, with the aid of a p‐n heterojunction, a high on/off ratio of nearly 10⁴ is achieved by this device at zero set bias under 370 nm (∼0.85 mW cm⁻²) illumination and this on/off ratio can be achieved in 0.5 s. The device also shows a fast speed with rise time of 0.69 ms and decay time of 13.5 ms measured by a pulse laser, much faster than that of a pure ZnO film. The Se/ZnO PD in this research provides a new pathway to fabricate multifunctional high‐speed, high signal‐to‐noise ratio, high detectivity and high selectivity UV photodetectors.

     

Single‐beam coherent anti‐Stokes Raman scattering (CARS) spectroscopy of gas‐phase CO2 via phase and polarization shaping of a broadband continuum

Coherent anti‐Stokes Raman scattering (CARS) spectroscopy of gas‐phase CO₂ is demonstrated using a single femtosecond (fs) laser beam. A shaped ultrashort laser pulse with a transform‐limited temporal width of ∼7 fs and spectral bandwidth of ∼225 nm (∼3500 cm⁻¹) is employed for simultaneous excitation of the CO₂ Fermi dyads at ∼1285 and ∼1388 cm⁻¹. CARS signal intensities for the two Raman transitions and their ratio as a function of pressure are presented. The signal‐to‐noise ratio of the single beam–generated CO₂ CARS signal is sufficient to perform concentration measurements at a rate of 1 kHz. The implications of these experiments for measuring CO₂ concentrations and rapid pressure fluctuations in hypersonic and detonation‐based chemically reacting flows are also discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Bandgap engineering of GaxZn1–xO nanowire arrays for wavelength‐tunable light‐emitting diodes

Wavelength‐tunable light‐emitting diodes (LEDs) of Ga_xZn_1–xO nanowire arrays are demonstrated by a simple modified chemical vapor deposition heteroepitaxial growth on p‐GaN substrate. As a gallium atom has similar electronegativity and ion radius to a zinc atom, high‐level Ga‐doped Ga_xZn_1–xO nanowire arrays have been fabricated. As the x value gradually increases from 0 to 0.66, the near‐band‐edge emission peak of Ga_xZn_1–xO nanowires shows a significant shift from 378 nm (3.28 eV) to 418 nm (2.96 eV) in room‐temperature photoluminescence (PL) measurement. Importantly, the electroluminescence (EL) emission of Ga_xZn_1–xO nanowire arrays LED continuously shifts with a wider range (∼100 nm), from the ultraviolet (382 nm) to the visible (480 nm) spectral region. The presented work demonstrates the possibility of bandgap engineering of low‐dimensional ZnO nanowires by gallium doping and the potential application for wavelength‐tunable LEDs.     

Intra‐laser‐cavity microparticle sensing with a dual‐wavelength distributed‐feedback laser

An integrated intra‐laser‐cavity microparticle sensor based on a dual‐wavelength distributed‐feedback channel waveguide laser in ytterbium‐doped amorphous aluminum oxide on a silicon substrate is demonstrated. Real‐time detection and accurate size measurement of single micro‐particles with diameters ranging between 1 µm and 20 µm are achieved, which represent the typical sizes of many fungal and bacterial pathogens as well as a large variety of human cells. A limit of detection of ∼500 nm is deduced. The sensing principle relies on measuring changes in the frequency difference between the two longitudinal laser modes as the evanescent field of the dual‐wavelength laser interacts with micro‐sized particles on the surface of the waveguide. Improvement in sensitivity far down to the nanometer range can be expected upon stabilizing the pump power, minimizing back reflections, and optimizing the grating geometry to increase the evanescent fraction of the guided modes.     

Self‐association and hydrogen bonding of propionaldehyde in binary mixtures with water and methanol investigated by concentration‐dependent polarized Raman study and DFT calculations

The Raman spectra of neat propionaldehyde [CH₃CH₂CHO or propanal (Pr)] and its binary mixtures with hydrogen‐donor solvents, water (W) and methanol (M), [CH₃CH₂CHO + H₂O] and CH₃CH₂CHO + CH₃OH] with different mole fractions of the reference system, Pr varying from 0.1 to 0.9 at a regular interval of 0.1, were recorded in the ν(CO) stretching region, 1600–1800 cm⁻¹. The isotropic parts of the Raman spectra were analyzed for both the cases. The wavenumber positions and line widths of the component bands were determined by a rigorous line‐shape analysis, and the peaks corresponding to self‐associated and hydrogen‐bonded species were identified. Raman peak at ∼1721 cm⁻¹ in neat Pr, which has been attributed to the self‐associated species, downshifts slightly (∼1 cm⁻¹) in going from mole fraction 0.9 to 0.6 in (Pr + W) binary mixture, but on further dilution it shows a sudden downshift of ∼7 cm⁻¹. This has been attributed to the low solubility of Pr in W (∼30%), which does not permit a hydrogen‐bonded network to form at higher concentrations of Pr. A significant decrease in the intensity of this peak in the Raman spectra of Pr in a nonpolar solvent, n‐heptane, at high dilution (C = 0.05) further confirms that this peak corresponds to the self‐associated species. In case of the (Pr + M) binary mixture, however, the spectral changes with concentration show a rather regular trend and no special features were observed. Copyright © 2010 John Wiley & Sons, Ltd.     

Ultraviolet photodetector with bandpass characteristic based on a blend of PVK and PBD

We fabricate an ultraviolet photodetector based on a blend of poly (N-vinylcarbazole) (PVK) and 2- tert-butylphenyl-5-biphenyl-1, 3, 4-oxadiazole (PBD) using spin coating. The device exhibites a low dark current density of 2.2×10 3 μA/cm 2 at zero bias. The spectral response of the device shows a narrow bandpass characteristic from 300 to 355 nm, and the peak response is 18.6 mA/W located at 334 nm with a bias of –1 V. We also study the performances of photodetectors with different blend layer thicknesses. The largest photocurrent density is obtained with a blend of 90 nm at the same voltage.     

Electro‐optic polymer/titanium dioxide hybrid core ring resonator modulators

Compact electro‐optic (EO) modulators are desirable for a number of applications. In this study, a ring modulator has been fabricated in the titanium dioxide (TiO₂) core and EO polymer cladding waveguide structure. A 250‐nm thick TiO₂ core was utilized to minimize the ring radius down to 100 μm, to avoid using the top cladding between the EO polymer and the electrode, and to improve the poling efficiency. The resonance obtained by the ring modulator was observed to shift by 0.02 nm/V due to the enhanced in‐device EO coefficient of 105 pm/V. A modulation depth of 3 dB was observed at the frequency response function at 20 kHz using 2‐V V_p–p clock signal.     

Fast-speed self-powered PEDOT: PSS/α-Ga2O3 nanorod array/FTO photodetector with solar-blind UV/visible dual-band photodetection

The $\alpha $-Ga$_{2}$O$_{3}$ nanorod array is grown on FTO by hydrothermal and annealing processes. And a self-powered PEDOT:PSS/$\alpha $-Ga$_{2}$O$_{3}$ nanorod array/FTO (PGF) photodetector has been demonstrated by spin coating PEDOT:PSS on the $\alpha $-Ga$_{2}$O$_{3}$ nanorod array. Successfully, the PGF photodetector shows solar-blind UV/visible dual-band photodetection. Our device possesses comparable solar-blind UV responsivity (0.18 mA/W at 235 nm) and much faster response speed (0.102 s) than most of the reported self-powered $\alpha $-Ga$_{2}$O$_{3}$ nanorod array solar-blind UV photodetectors. And it presents the featured and distinguished visible band photoresponse with a response speed of 0.136 s at 540 nm. The response time is also much faster than the other non-self-powered $\beta $-Ga$_{2}$O$_{3 }$ DUV/visible dual-band photodetectors due to the fast-speed separation of photogenerated carries by the built-in electric field in the depletion regions of PEDOT:PSS/$\alpha $-Ga$_{2}$O$_{3}$ heterojunction. The results herein may prove a promising way to realize fast-speed self-powered $\alpha $-Ga$_{2}$O$_{3}$ photodetectors with solar-blind UV/visible dual-band photodetection by simple processes for the applications of multiple-target tracking, imaging, machine vision and communication.     

High‐gain Zn1–xMgx O‐based ultraviolet photodetectors on Al2O3 and LiGaO2 substrates

We report the fabrication and characterization of highly responsive ZnMgO‐based ultraviolet (UV) photodetectors in the metal–semiconductor–metal (MSM) configuration for solar‐blind/visible‐blind optoelectronic application. MSM devices were fabricated from wurtzite Zn_1–xMg_x O/ZnO (x ～ 0.44) thin‐film heterostructures grown on sapphire (α‐Al₂O₃) substrates and w‐Zn_1–xMg_x O (x ～ 0.08), grown on nearly lattice‐matched lithium gallate (LiGaO₂) substrates, both by radio‐frequency plasma‐assisted molecular beam epitaxy (PAMBE). Thin film properties were studied by AFM, XRD, and optical transmission spectra, while MSM device performance was analyzed by spectral photoresponse and current–voltage techniques. Under biased conditions, α‐Al₂O₃ grown devices exhibit peak responsivity of ～7.6 A/W at 280 nm while LiGaO₂ grown samples demonstrate peak performance of ～119.3 A/W, albeit in the UV‐A regime (～324 nm). High photoconductive gains (76, 525) and spectral rejection ratios (～10³, ～10⁴) were obtained for devices grown on α‐Al₂O₃ and LiGaO₂, respectively. Exemplary device performance was ascribed to high material quality and in the case of lattice‐matched LiGaO₂ films, decreased photocarrier trapping probability, presumably due to low‐density of dislocation defects. To the best of our knowledge, these results represent the highest performing ZnO‐based photodetectors on LiGaO₂ yet fabricated, and demonstrate both the feasibility and substantial enhancement of photodetector device performance via growth on lattice‐matched substrates. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

An ultra‐compact multimode interference coupler with a subwavelength grating slot

Multimode interference couplers (MMIs) are fundamental building blocks in photonic integrated circuits. Here it is experimentally demonstrated, for the first time, a two‐fold length reduction in an MMI coupler without any penalty on device performance. The design is based on a slotted 2 × 2 MMI fabricated on a commercial silicon‐on‐insulator (SOI) substrate. The slot is implemented with a subwavelength grating (SWG) comprising holes fully etched down to the oxide cladding, thereby allowing single etch step fabrication. The device has been designed using an in‐house tool based on the Fourier Eigenmode Expansion Method. It has a footprint of only 3.5 μm x 23 μm and it exhibits a measured extinction ratio better than 15 dB within the full C‐band (1530 nm‒1570 nm). SWG engineered slots thus offer excellent perspectives for the practical realization of MMIs couplers with substantially reduced footprint yet with outstanding performance.     

Water‐Dispersible Silicon‐Quantum‐Dot‐Containing Micelles Self‐Assembled from an Amphiphilic Polymer

The dispersion of silicon quantum dots (Si QDs) in water has not been established as well as that in organic solvents. It is now demonstrated that the excellent dispersion of Si QDs in water with photoluminescence (PL) quantum yields (QYs) comparable to those for hydrophobic Si QDs can be realized by combining the processes of hydrosilylation and self‐assembly. Hydrogen‐passivated Si QDs are initially hydrosilylated with 1‐dodecence. The toluene solution of the resulting dodecyl‐passivated Si QDs is mixed with the water solution of the amphiphilic polymer of Pluronic F127 to form an emulsion. Dodecyl‐passivated Si QDs are encapsulated in the micelles self‐assembled from F127 in the emulsion. The size of the Si‐QD‐containing micelles may be tuned in the range from 10 to 100 nm. Although self‐assembly in the emulsion causes the PL QY of Si QDs to decrease, after a few days of storage in ambient conditions, Si QDs encapsulated in the water‐dispersible micelles exhibit recovered PL QYs of ≈24% at the PL wavelength of ≈680 nm. The intensity of the PL from Si QDs encapsulated in the water‐dispersible micelles is >90% of the original value after 60 min ultraviolet illumination, indicating excellent photostability.     

On‐chip silicon 8‐channel hybrid (de)multiplexer enabling simultaneous mode‐ and polarization‐division‐multiplexing

An 8‐channel hybrid (de)multiplexer to simultaneously achieve mode‐ and polarization‐division‐(de)multiplexing is proposed and demonstrated experimentally on a silicon‐on‐insulator platform to improve the link capacity of an on‐chip optical interconnect. The present hybrid (de)multiplexer has four channels for each polarization. A polarization beam splitter based on a three‐waveguide coupler is used to combine/separate the fundamental modes of TE‐ and TM‐polarizations (TE₀ and TM₀). Six asymmetric directional couplers are cascaded for (de)multiplexing the high‐order modes (TE₁, TE₂, TE₃, TM₁, TM₂, and TM₃). The experimental results show all eight channels have low loss and low crosstalk (<−10 dB) over a ∼ 30 nm wavelength range.     

Self‐phase‐locked degenerate femtosecond optical parametric oscillator based on BiB3O6

A self‐phase‐locked degenerate femtosecond optical parametric oscillator (OPO) based on the birefringent nonlinear material, bismuth triborate, BiB₃O₆, synchronously‐pumped by a Kerr‐lens‐mode‐locked Ti:sapphire laser at 800 nm is described. By exploiting versatile phase‐matching properties of BiB₃O₆, including large spectral and angular acceptance for parametric generation and low group velocity dispersion in the optical xz plane, stable self‐phase‐locked degenerate OPO operation centered at 1600 nm is demonstrated using collinear type I (e → oo) interaction in a 1.5‐mm crystal at room temperature. The degenerate OPO output spectrum extends over 46 nm (∼5.4 THz) with 190 fs pulse duration for input pump pulses of 155 fs with a bandwidth of 7 nm. Phase coherence between the pump and degenerate output is verified using f‐2f interferometry, and discrete frequency beats caused by different carrier‐envelope‐offset frequencies are measured using radio frequency measurements. Photo shows a 1.5‐mm BiB₃O₆ crystal used as a nonlinear gain medium in a degenerate self‐phase‐locked femtosecond OPO operating at room temperature. The green beam is the result of non‐phase‐matched sum‐frequency mixing between the pump light and the sub‐harmonic OPO field at degeneracy.     

Fabrication of superconducting tunnel junctions for soft X‐ray spectroscopy

Superconducting tunnel junction (STJ) array detectors with a new design, which has a minimum junction edge coverage of an SiO₂ insulation, passivation layer and an asymmetric tunnel junction layer structure, have been fabricated for a soft X‐ray region between 100 eV and 1 keV. The sensitive area was patterned by removing the SiO₂ deposition layer by a lift‐off technique that ensured no contamination layer on the top Nb electrode surface. The width of the passivation rim was as narrow as 0.5 µm at the junction edge. The clean Nb surface and the narrow SiO₂ rim resulted in almost no artifact photon events in a low‐energy region. The asymmetric layer design is effective in solving a problem of double peak response to monochromatic X‐rays, which is commonly observed in STJ detectors. The performance of a 100 pixel array detector was investigated by the fluorescent X‐ray analysis of oxides and nitrides: an energy resolution of about 30 eV for the total absorption of the Kα lines of oxygen and nitrogen. We plan to realize an energy resolution of better than 20 eV and a counting rate of over 1 Mcps for fluorescence‐yield X‐ray absorption spectroscopy for light trace elements. Copyright © 2011 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering from gold‐coated germanium oxide nanowires

We utilized bulk‐synthesized nanowires (NWs) of germanium dioxide as nanoscale structures that can be coated with noble metals to allow the excitation of surface plasmons over a broad frequency range. The NWs were synthesized on substrates of silicon using gold‐catalyst‐assisted vapor–liquid–solid (VLS) growth mechanism in a simple quartz tube furnace setup. The resulting NWs have diameters of ∼100–200 nm, with lengths averaging ∼10–40 µm and randomly distributed on the substrate. The NWs are subsequently coated with thin films of gold, which provide a surface‐plasmon‐active surface. Surface‐enhanced Raman scattering (SERS) studies with near‐infrared (NIR) excitation at 785 nm show significant enhancement (average enhancement > 10⁶) with good uniformity to detect submonolayer concentrations of 4‐methylbenzenethiol (4‐MBT), trans‐1,2‐bis(4‐pyridyl)ethylene (BPE), and 1,2‐benzendithiol (1,2‐BDT) probe molecules. We also observed an intense, broad continuum in the Raman spectrum of NWs after metal coating, which tended to diminish with the analyte monolayer formation. Copyright © 2008 John Wiley & Sons, Ltd.     

High quantum efficiency UV detection using a‐Se based photodetector

High‐sensitivity ultraviolet (UV) photodetection has been attempted by utilizing the carrier multiplication effect in amorphous selenium. The prototype photodetector presented in this Letter showed an extremely high sensitivity to UV light, so that up to 1000 carriers are generated per incident photon. This result should lead to the development of an ultrahigh‐sensitivity photodetector that can be used for spectrometric applications covering the visible to UV region. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The simulation of temperature dependence of responsivity and response time for 6H-SiC UV photodetector

In this paper the temperature dependence of responsivity and response time for 6H-SiC ultraviolet (UV) photodetector is simulated based on numerical model in the range from 300K to 900K. The simulation results show that the responsivity and the response time of device are less sensitive to temperature and this kind of UV photodetector has excellent temperature stability. Also the effects of device structure and bias voltage on the responsivity and the response time are presented. The thicker the drift region is, the higher the responsivity and the longer the response time are. So the thickness of drift region has to be carefully designed to make trade-off between responsivity and response time.