Effect of length on the magnetic properties of Ni 300 nm wide nanowires

The Ni 300 nm wide nanowires were fabricated inside the as-synthesized nanochannels of anodic aluminum oxide (AAO) template by electrochemical deposition method. The angular dependence of coercivity and remanence of Ni nanowire arrays with various lengths have been comparatively studied. Investigation results demonstrate that the easy axis is along the wire axis for the longer nanowires of 4 μm or 9 μm in length due to their large shape anisotropy. However, the magnetostatic coupling is dominant for the shorter nanowires of 0.9 μm in length resulting in the change of magnetization behaviors, including coercivity and easy axis. A further explanation was also given by simple calculation in the paper.     

Quantum dot nanostructures for multi-band infrared detection

A dual-band (two-color) tunneling-quantum dot infrared photodetector (T-QDIP) structure, which provides wavelength selectivity using bias voltage polarity, is reported. In this T-QDIP, photoexcitation takes place in InGaAs QDs and the excited carriers tunnel through an AlGaAs/InGaAs/AlGaAs double-barrier by means of resonant tunneling when the bias voltage required to line up the QD excited state and the double-barrier state is applied. Two double-barriers incorporated on the top and bottom sides of the QDs provide tunneling conditions for the second and the first excited state in the QDs (one double-barrier for each QD excited state) under forward and reverse bias, respectively. This field dependent tunneling for excited carriers in the T-QDIP is the basis for the operating wavelength selection. Experimental results showed that the T-QDIP exhibits three response peaks at ～4.5 (or 4.9), 9.5, and 16.9 μm and selection of either the 9.5 or the 16.9 μm peak is obtained by the bias polarity. The peak detectivity (at 9.5 and 16.9 μm) of this detector is in the range of 1.0–6.0 × 10¹² Jones at 50 K. This detector does not provide a zero spectral crosstalk due to the peak at 4.5 μm not being bias-selectable. To overcome this, a quantum dot super-lattice infrared photodetector (SL-QDIP), which provides complete bias-selectability of the response peaks, is presented. The active region consists of two quantum dot super-lattices separated by a graded barrier, enabling photocurrent generation only in one super-lattice for a given bias polarity. According to theoretical predictions, a combined response due to three peaks at 2.9, 3.7, and 4.2 μm is expected for reverse bias, while a combined response of three peaks at 5.1, 7.8, and 10.5 μm is expected for forward bias.     

Bias-selectable mid-/long-wave dual band infrared focal plane array based on Type-II InAs/GaSb superlattice

In this paper, a mid-/long-wave dual-band detector which combined PπMN structure and unipolar barrier was developed based on type-II InAs/GaSb superlattice. A relevant 320 × 256 focal plane array (FPA) was fabricated. Unipolar barrier and PπMN structure in our dual band detector structure were used to suppress cross-talk and dark current, respectively. The two channels, with respective 50% cut-off wavelength at 4.5 μm and 10 μm were obtained. The peak quantum efficiency (QE) of mid wavelength infrared (MWIR) band and long wavelength infrared (LWIR) band are 53% at 3.2 μm under no bias voltage and 40% at 6.4 μm under bias voltage of −170 mV, respectively. And the dark current density under 0 and −170 mV of applied bias are 1.076 × 10⁻⁵ A/cm² and 2.16 × 10⁻⁴ A/cm². The specific detectivity of MWIR band and LWIR band are 2.15 × 10¹² cm·Hz^1/2/W at 3.2 μm and 2.31 × 10¹⁰ cm·Hz^1/2/W at 6.4 μm, respectively, at 77 K. The specific detectivity of LWIR band maintains above 10¹⁰ cm·Hz^1/2/W at the wavelength range from 4.3 μm to 10.2 μm under −170 mV. The cross-talk, selectivity parameter at 3.0 μm, about 0.14 was achieved under bias of −170 mV. Finally, the thermal images were taken by the fabricated FPA at 77 K.     

Hard magnetic properties of rapidly annealed NdFeB thin films on Nb and V buffer layers

NdFeB thin films of the form A (20 nm)/NdFeB(d nm)/A(20 nm), where d ranges from 54 to 540 nm and the buffer layer A is Nb or V were prepared on a Si(1 0 0) substrate by magnetron sputtering. The hard Nd₂Fe₁₄B phase is formed by a 30 s rapid anneal or a 20 min anneal. Average crystallite size ranged from 20 to 35 nm with the rapidly annealed samples having the smaller crystallite size. These samples also exhibited a larger coercivity and energy product than those treated by a 20 min vacuum anneal. A maximum coercivity of 26.3 kOe at room temperature was obtained for a Nb/NdFeB (180 nm)/Nb film after a rapid anneal at 725°C. Initial magnetization curves indicate magnetization rotation rather than nucleation of reverse domains is important in the magnetization process. A Brown's equation analysis of the coercivity as a function of temperature allowed us to compare the rapidly annealed and 20 min annealed samples. This analysis suggests that rapid annealing gives higher quality crystalline grains than the 20 min annealed sample leading to the observed large coercivity in the rapidly annealed samples.     

Spectral characteristics of polymer micro-fiber MZI near 1550 nm

The authors have produced the polymer micro-fiber with a highly optical conductive efficiency of 83% and 89% for the pump light of 532 nm and 1550 nm, respectively. The authors constructed a Mach–Zehnder Interferometer (MZI) by a micro-manipulation method and measured the different interference spectra by micro-adjusting the path difference of the dual interference arms of MZI under a microscope. Due to the path difference, the coherent length of the corresponding spectrum continuously and slightly decreases from 20 μm, 13.5 μm, 10.6 μm to 8 μm. The relationships between this particular MZI structure and the surrounding temperature, as well as the refractive index changes can be determined via the evanescent field and the thermally induced expansion or contraction effect, which will be reflected in the interference spectrum.     

A thin film magnetic field sensor of sub-pT resolution and magnetocardiogram (MCG) measurement at room temperature

We developed a very sensitive high-frequency carrier-type thin film sensor with a sub-pT resolution using a transmission line. The sensor element consists of Cu conductor with a meander pattern (20 mm in length, 0.8 mm in width, and 18 μm in thickness), a ground plane, and amorphous CoNbZr film (4 μm in thickness). The amplitude modulation technique was employed to enhance the magnetic field resolution for measurement of the high-frequency field (499 kHz), a resolution of 7.10×10^?13 T/Hz^1/2 being achieved, when we applied an AC magnetic field at 499 kHz. The phase detection technique was applied for measurement of the low frequency field (around 1 Hz). A small phase change was detected using a dual mixer time difference method. A high phase change of 130°/Oe was observed. A magnetic field resolution of 1.35×10^?12 T/Hz^1/2 was obtained when a small AC field at 1 Hz was applied. We applied the sensor for magnetocardiogram (MCG) measurement using the phase detection technique. We succeeded in measuring the MCG signal including typical QRS and T waves, and compared the MCG with a simultaneously obtained conventional electrocardiogram (ECG) signal.     

320 × 256 Short-/Mid-Wavelength dual-color infrared focal plane arrays based on Type-II InAs/GaSb superlattice

Short-/Mid-Wavelength dual-color infrared focal plane arrays based on Type-II InAs/GaSb superlattice are demonstrated on GaSb substrate. The material is grown with 50% cut-off wavelength of 2.9 μm and 5.1 μm for the blue channel and red channel, separately at 77 K. 320 × 256 focal plane arrays fabricated in this wafer is characterized. The peak quantum efficiency without antireflective coating is 37% at 1.7 μm under no bias voltage and 28% at 3.2 μm under bias voltage of 130 mV. The peak specific detectivity are 1.51 × 10¹² cm·Hz^1/2/W at 2.5 μm and 6.11x10¹¹ cm·Hz^1/2/W at 3.2 μm. At 77 K, the noise equivalent difference temperature presents average values of 107 mK and 487 mK for the blue channel and red channel separately.     

Novel ultracompact wavelength division demultiplexer based on photonic band gap

We propose ultra-compact wavelength division demultiplexer based on photonic band gap in two dimensional photonic crystals. The structure consists of two different types of lattice, which can separate two communication wavelengths, 1310 and 1490 nm. In addition, it also can be used to separate the wavelengths of 1310 and 1550 nm if we change the structural parameters. The total size of the present structure is only 12.5 μm × 12.5 μm. The transmission efficiency is above 97%, and the good performance is verified with plane wave expansion and finite-difference time-domain simulation.     

High temperature magnetoresistance in (Co,CoFe and NiFe)/Cu/CoFe/IrMn spin-valves

The MR characteristics of temperature variations ranging from room temperature to 570 K for IrMn spin-valves which consist of free layer Co, CoFe and NiFe were studied. Co-SV had the highest MR value in all temperature ranges among them and even at 510 K the MR ratio held more than half of room temperature value. Whereas CoFe-SV had a slightly higher MR than NiFe-SV below 450 K, above 450 K CoFe-SV showed a lower MR ratio than NiFe-SV. MR loops of large coercivity such as CoFe free magnetic layer collapsed in the lower elevated temperature compared to NiFe and Co. Small coercivity of the free magnetic layer would be preferable to spin-valves.     

A novel polarization splitter based on dual-core elliptical-holes hybrid photonic crystal fiber

A novel kind of polarization splitter in dual-core elliptical holes hybrid photonic crystal fiber is proposed. Numerical results show that the splitter can reach small coupling length ratio of 0.5, for wavelength from 1.15 μm to 1.9 μm. At wavelength 1.55 μm, the extinction ratio can achieve ?64 dB and the 1.92-mm-long splitter is suggested to achieve extinction ratio better than ?10 dB, a bandwidth of 150 nm. The fiber has small coupling length ratio, small coupling length and high extinction ratio and it is more suitable for fabricating polarization splitter.     

Magnetic properties of Ni nanoparticles on microporous silica spheres

Ni nanoparticles (~32 nm particle diameter) have been synthesized on the walls of microporous (~1 nm pore diameter) silica spheres (~2.6 μm sphere diameter) and characterised magnetically to potentially produce a new class of core (silica micro-spheres)-shell (nanometallic)-type nanocomposite material. These magnetic nanocomposite materials display a characteristic increase in coercivity with reducing temperature. The average particle size has been used to calculate the anisotropy constant for the system, K. The discussion postulates the potential mechanisms contributing to the difference between the calculated K value and the magnetocrystalline anisotropy constant of bulk Ni. Various factors such as surface anisotropy and interparticle interactions are discussed as possible contributing factors to the anisotropy values calculated in the paper.     

Zero bias PIN photodetector based on gradient band distribution and doping gradient profile

Zero bias photodetector which was suitable for top-illuminated and side-illuminated was fabricated. Maximal bandwidth-efficiency product (BEP) value could be achieved when the epitaxial layer structure was optimized. The 3-dB bandwidth of the zero bias was 12.27 GHz, which was numerically above 80% of that maximum value. The measured external quantum efficiency of the photodetector was 17% at the zero bias and 1550 nm. The dark current of the photodetector with 12-μm diameter was less than 9 × 10⁻⁸ A at a reverse bias of 0.1 V. The influence of doping gradient profile on photodetector performance was illustrated by simulation comparison. The important aspects of the design of the high-speed low-bias photodetector were explained. The phenomenon of the photodetector at the reverse bias which was not the higher the better was explained. The improvement in performance of the photodetector was discussed. The fabrication process and the testing process were described in detail.     

Controlled fabrication of Si nanostructures by high vacuum electron beam annealing

Silicon nanostructures, called Si nanowhiskers, have been successfully synthesized on Si(1 0 0) substrate by high vacuum electron beam annealing (EBA). Detailed analysis of the Si nanowhisker morphology depending on annealing temperature, duration and the temperature gradients applied in the annealing cycle is presented. A correlation was found between the variation in annealing temperature and the nanowhisker height and density. Annealing at 935 °C for 0 s, the density of nanowhiskers is about 0.2 μm⁻² with average height of 2.4 nm grow on a surface area of 5×5 μm, whereas more than 500 nanowhiskers (density up to 28 μm⁻²) with an important average height of 4.6 nm for field emission applications grow on the same surface area for a sample annealed at 970 °C for 0 s. At a cooling rate of −50 °C s⁻¹ during the annealing cycle, 10–12 nanowhiskers grew on a surface area of 5×5 μm, whereas close to 500 nanowhiskers grew on the same surface area for samples annealed at the cooling rate of −5 °C s⁻¹. An exponential dependence between the density of Si nanowhiskers and the cooling rate has been found. At 950 °C, the average height of Si nanowhiskers increased from 4.0 to 6.3 nm with an increase of annealing duration from 10 to 180 s. A linear dependence exists between the average height of Si nanowhiskers and annealing duration. Selected results are presented showing the possibility of controlling the density and the height of Si nanowhiskers for improved field emission properties by applying different annealing temperatures, durations and cooling rates.     

Ultrasmall optical logic gates based on silicon periodic dielectric waveguides

An ultrasmall silicon periodic dielectric waveguides-based multimode interference all-optical logic gate has been proposed. The device consists of three 205 nm wide single-mode input waveguides, a 1.1 μm wide and 5.5 μm long multimode interference waveguide, and three 205 nm wide single-mode output waveguides. The total length and width of the device are 13.7 μm and 3.2 μm, respectively. By changing the states of the input optical signals and/or control signals launched into the device, multifunctional logic functions including OR, NAND, NOR, and NOT gates are performed, and each logic function can be realized at a specific output waveguide in accordance with the launched control signals. The ultrasmall multifunctional logic device has potential applications in high density photonic integrated circuits.     

Ferrofluidic masking of solid state nuclear track detectors during etching

This work presents a novel method for determining bulk etch rate of CR-39 during prolonged etching by masking the surface with a ferrofluidic film held in position by magnetostatic forces. The CR-39 etching conditions were 6.25 M NaOH solution for 24 h at temperatures ranging from 50 to 80 °C. After etching, the heights of the resulting un-etched plateaus were measured using a Talyscan 150 profilometer. The removed layer thicknesses ranged from 12 to 85 μm, giving corresponding bulk etch rates in the range 0.5–3.54 μm/h.     

Picosecond laser cutting and drilling of thin flex glass

We investigate the feasibility of cutting and drilling thin flex glass (TFG) substrates using a picosecond laser operating at wavelengths of 1030 nm, 515 nm and 343 nm. 50 μm and 100 μm thick AF32^®Eco Thin Glass (Schott AG) sheets are used. The laser processing parameters such as the wavelength, pulse energy, pulse repetition frequency, scan speed and the number of laser passes which are necessary to perform through a cut or to drill a borehole in the TFG substrate are studied in detail. Our results show that the highest effective cutting speeds (220 mm/s for a 50 μm thick TFG substrate and 74 mm/s for a 100 μm thick TFG substrate) are obtained with the 1030 nm wavelength, whereas the 343 nm wavelength provides the best quality cuts. The 515 nm wavelength, meanwhile, can be used to provide relatively good laser cut quality with heat affected zones (HAZ) of <25 μm for 50 μm TFG and <40 μm for 100 μm TFG with cutting speeds of 100 mm/s and 28.5 mm/s, respectively. The 343 nm and 515 nm wavelengths can also be used for drilling micro-holes (with inlet diameters of ⩽75 µm) in the 100 μm TFG substrate with speeds of up to 2 holes per second (using 343 nm) and 8 holes per second (using 515 nm). Optical microscope and SEM images of the cuts and micro-holes are presented.     

Rare earth free exchange spring magnet FeCo/FePt(001): Giant magnetic anisotropy and energy product

Using the full potential linearized augmented plane wave (FLAPW) method, we have investigated the thickness dependent magnetic properties of rare earth free exchange spring magnet FeCo/FePt(001). The FeCo adlayer thickness is increased from one monolayer (ML) to four ML coverage. It is observed that the FeCo adlayers and Fe atoms in FePt substrate show almost half metallic behavior, while an ordinary metallic feature is found in Pt atoms. The average magnetization increases with FeCo thickness and the estimated maximum energy product reaches 66 MGOe in FeCo(4 ML)/FePt(001). A giant perpendicular magnetocrystalline anisotropy (MCA) energy of 18.20 meV/cell is found in pure FePt(001) and it becomes 17.35 meV/cell even in FeCo(4 ML)/FePt(001). In addition, we find very large coercivity field in FeCo/FePt(001) systems. For instance, the calculated maximum coercivity field in FeCo(4 ML)/FePt(001) is about 188 kOe. Both energy product and coercivity field calculations may imply that the FeCo/FePt can be utilized for potential rare earth free exchange spring magnet material.     

Electro-optic metal–insulator–semiconductor–insulator–metal Mach-Zehnder plasmonic modulator

The performance of a CMOS-compatible electro-optic Mach-Zehnder plasmonic modulator is investigated using electromagnetic and carrier transport simulations. Each arm of the Mach-Zehnder device comprises a metal–insulator–semiconductor–insulator–metal (MISIM) structure on a buried oxide substrate. Quantum mechanical effects at the oxide/semiconductor interfaces were considered in the calculation of electron density profiles across the structure, in order to determine the refractive index distribution and its dependence on applied bias. This information was used in finite element simulations of the electromagnetic modes within the MISIM structure in order to determine the Mach-Zehnder arm lengths required to achieve destructive interference and the corresponding propagation loss incurred by the device. Both inversion and accumulation mode devices were investigated, and the layer thicknesses and height were adjusted to optimise the device performance. A device loss of <8 dB is predicted for a MISIM structure with a 25 nm thick silicon layer, for which the device length is <3 μm, and <5 dB loss is predicted for the limiting case of a 5 nm thick silicon layer in a 1.2 μm long device: in both cases, the maximum operating voltage is 7.5 V.     

Fabrication of single-mode Y-branch waveguides in photosensitive polymer with reduced Y-junction residue

Single-mode small-core (～2 μm × 2 μm) Y-branch waveguide structures in photosensitive polymer have been fabricated. Y-branch waveguides are designed by the beam propagation method and Y-branch waveguides are obtained on development after a cross-linkable negative tone epoxy SU-8 2002 polymer is exposed to UV through a photomask. Optical Adhesive NOA 61 is used as under- and over-clad. The fabrication process is optimized to avoid polymer residue at the Y-junction. The average insertion loss obtained for a 7.2 mm 1 × 2 device at chip-level is ～13 dB at 1550 nm.     

Step feature observed in the angular dependence of magnetization switching fields in GaMnAs micro-device

The magnetization switching phenomena of GaMnAs Hall devices have been investigated by using the planar Hall effect (PHE) measurement. Though two different sizes of Hall bar devices, width of 300 and of 10 μm, show very similar Curie temperature, their magnetization switching fields behave significantly different. While the angle dependence of magnetization switching field of the 300 μm device showed typical rectangular shape behavior with an applied magnetic field angle in the polar plot, that of the 10 μm device exhibited large step at 〈1 1 0〉 crystallographic directions, breaking the continuity of the switching field in angle dependence. Such unusual phenomenon observed in the 10 μm device was discussed in terms of the change in magnetic anisotropy by the fabrication of micro-device.