Reliability and optical properties of LED lens plates under high temperature stress

In this investigation the thermal degradation mechanisms of Bisphenol A Polycarbonate (BPA-PC) plates at the temperature range 100–140 °C are studied. The BPA-PC plates are currently used both in light conversion carriers in LED modules and optical lenses in LED-based products. In this study BPA-PC plates are aged at elevated temperature of 100–140 °C for a period up to 3000 h. Optical and chemical properties of the thermally-aged plates were studied using UV–Vis spectrophotometer, FTIR–ATR spectrometer, and integrated sphere. The results show that increasing the thermal ageing time leads to yellowing, loss of optical properties, and decrease of the light transmission and of the relative radiant power value of BPA-PC plates. The results also depict that there is not much discoloration within the first 1500 h of thermal ageing. The rate of yellowing significantly increases at the end of this induction period. Formation of oxidation products is identified as the main mechanism of yellowing. An exponential-based reliability model is also presented to calculate the rate of degradation reaction and to predict the life-time of BPA-PC plates.     

Characteristics of InAIAs/InP and InAIP/GaAs native oxides

Characteristics of InAlAs/InP and InAlP/GaAs wet oxidation layers were measured for the first time. These oxidation layers can be a current blocking or an optical confining layer in laser diodes. These layers were well made at 500–575 °C. The oxidation rates at 525 °C are approximately 340 nm/h and 120 nm/h for InAlAs and InAlP oxides, respectively. The refractive index are 1.82–1.90 for InAlAs oxide and 1.565–1.595 for InAlP oxide between 0.6 μm and 1.65 μm wavelength. The characteristics are not much varied with processing temperatures except the oxidation rate. And a 200 nm thick InAlAs oxidation layer has a current–voltage characteristic that currents rapidly flow at about 10 V, which is much lower than that of SiO₂.     

Contact and proximity lithography using 193 nm Excimer laser in Mask Aligner

In this paper we describe the use of an Excimer laser for full-field lithography in a Mask Aligner. The DUV light from the Excimer laser is homogenized by using micro lens based optical integrators instead of a macro lens array. A simulation of the intensity distribution for 5 μm squares was performed to visualize the diffraction effects and to show the potential of 193 nm illumination. It is demonstrated that compared to the conventional homogenization optics the MO Exposure Optics further improves the illumination uniformity, calculated as 1.8% for MO Exposure Optics and 2.9% for the A-Optics. Moreover the improved optical setup allows a modification of the angular spectrum by using exchangeable illumination filter plates (IFP). Compared to the A-Optics the main improvement effect of MO Exposure Optics is detectable in the patterning of layouts containing critical dimension from 8 μm down to 2 μm.     

High flowability monomer resists for thermal nanoimprint lithography

In this paper, we have been using polymer and thermally curable monomer resists in a full 8 in. wafer thermal nanoimprint lithography process. Using exactly the same imprinting conditions, we observed that a monomer solution provides a much larger resist redistribution than a polymer resist. Imprinting Fresnel zone plates, composed of micro- and nano-meter features, was possible only with the monomer resist. In order to reduce the shrinkage ratio of the monomer resists, acrylate–silsesquioxane materials were synthesised. With a simple diffusion-like model, we could extract a mean free path of 1.1 mm for the monomer resist, while a polymer flows only on distances below 10 μm in the same conditions.     

Tailoring chromatic dispersion in chalcogenide–tellurite microstructured optical fiber

We report fabrication of a highly nonlinear hybrid microstructured optical fiber composed of chalcogenide glass core and tellurite glass cladding. The flattened chromatic dispersion can be achieved in such an optical fiber with near zero dispersion wavelength at telecommunication wavelengths λ = 1.35–1.7 μm, which cannot be achieved in chalcogenide glass optical fibers due to their high refractive index, i.e. n > 2.1. We demonstrate a hybrid 4-air hole chalcogenide–tellurite optical fiber (Δn = 0.25) with flattened chromatic dispersion around λ = 1.55 μm. In optimized 12-air hole optical fiber composed of the same glasses, the chromatic dispersion values were achieved between −20 and 32 ps/nm/km in a broad wavelength range of 1.5–3.8 μm providing the fiber with extremely high nonlinear coefficient 86,000 km⁻¹W⁻¹. Hybrid chalcogenide/tellurite fibers pumped with the near infrared lasers give good promise for broadband optical amplification, wavelength conversion, and supercontinuum generation in the near- to mid-infrared region.     

Vertically aligned ZnO nanowires prepared by thermal oxidation of RF magnetron sputtered metallic zinc films

ZnO nanowires have been successfully grown by thermal oxidation of metallic zinc films at 430 °C. Polycrystalline zinc films were deposited on Si (100) substrates by RF magnetron sputtering utilizing discharge power from 70 to 180 W. Experimental results show that 70 W discharge power results in the formation of porous zinc nanoparticles that prevent zinc atom from diffusion and thus does not result in the formation of ZnO nanowires by subsequent thermal oxidation. By increasing discharge power to 120 W the zinc film transforms to Zone II with a columnar structure, while further increase in discharge power to 180 W results in re-crystallization and formation of micron-sized hexagonal structures on the surface. Vertically aligned ZnO nanowires can only be obtained by thermal oxidation of columnar zinc films that exhibit a field emission threshold of 5.3 V/μm (at a current density of 10 μA/cm²) with a field enhancement factor of 1834. A target current density of 0.75 mA/cm² is achieved with a bias field less than 10 V/μm.     

Crosslinkable organic glasses with quadratic nonlinear optical activity

In this paper, the use of a pure benzimidazole based dimethacrylic push–pull chromophore has been investigated for the preparation of thin films exhibiting quadratic nonlinear optical (NLO) properties. To stabilize these NLO properties, the chromophore orientation has been frozen in a noncentrosymmetric arrangement by cross-linking the material using thermal polymerization induced by suitable initiators. The films present a very stable quadratic optical activity up to T = 152 °C with SHG coefficient d₃₃ of 14 pm/V measured at 1.9 μm.     

Modulation instability initiated high power all-fiber supercontinuum lasers and their applications

High average power, all-fiber integrated, broadband supercontinuum (SC) sources are demonstrated. Architecture for SC generation using amplified picosecond/nanosecond laser diode (LD) pulses followed by modulation instability (MI) induced pulse breakup is presented and used to demonstrate SC sources from the mid-IR to the visible wavelengths. In addition to the simplicity in implementation, this architecture allows scaling up of the SC average power by increasing the pulse repetition rate and the corresponding pump power, while keeping the peak power, and, hence, the spectral extent approximately constant. Using this process, we demonstrate >10 W in a mid-IR SC extending from ～0.8 to 4 μm, >5 W in a near IR SC extending from ～0.8 to 2.8 μm, and >0.7 W in a visible SC extending from ～0.45 to 1.2 μm. SC modulation capability is also demonstrated in a mid-IR SC laser with ～3.9 W in an SC extending from ～0.8 to 4.3 μm. The entire system and SC output in this case is modulated by a 500 Hz square wave at 50% duty cycle without any external chopping or modulation. We also explore the use of thulium doped fiber amplifier (TDFA) stages for mid-IR SC generation. In addition to the higher pump to signal conversion efficiency demonstrated in TDFAs compared to erbium/ytterbium doped fiber amplifier (EYFA), the shifting of the SC pump from ～1.5 to ～2 μm is pursued with an attempt to generate a longer extending SC into the mid-IR. We demonstrate ～2.5 times higher optical conversion efficiency from pump to SC generation in wavelengths beyond 3.8 μm in the TDFA versus the EYFA based SC systems. The TDFA SC spectrum extends from ～1.9 to 4.5 μm with ～2.6 W at 50% modulation with a 250 Hz square wave. A variety of applications in defense, health care and metrology are also demonstrated using the SC laser systems presented in this paper.     

Responsivity and lifetime of resonant-cavity-enhanced HgCdTe detectors

Resonant-cavity-enhanced HgCdTe structures have been grown by molecular beam epitaxy, and photoconductors have been modelled and fabricated based on these structures. Responsivity has been measured and shows a peak responsivity of 8 × 10⁴ V/W for a 50 × 50 μm² photoconductor at a temperature of 200 K. The measured responsivity shows some agreement with the modelled responsivity across the mid-wave infrared window (3–5 μm). The measured responsivity is limited by surface recombination, which limits the effective lifetime to ≈15 ns. The optical cut-off of the detector varies with temperature as modelled from 5.1 μm at 80 K to 4.4 μm at 250 K. There is strong agreement between modelled peak responsivity and measured peak responsivity with varying temperature from 80 to 300 K.     

Growth enhancement and field emission characteristics of one-dimensional 3,4,9,10-perylenetetracarboxylic dianhydride nanostructures on pillared titanium substrate

We have utilized the π–π interactions between 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) molecules and temperature-induced morphology changes to synthesize one-dimensional (1D) nanostructures of PTCDA on a heated (ca. 100 °C) titanium substrate through vacuum sublimation. Because of the pillared Ti structures and the presence of reactive Ti–Cl sites, the titanium substrate played a crucial role in assisting the PTCDA molecules to form 1D nanostructures. The average diameter of the nanofibers deposited on the Ti-CVD substrate, a Ti substrate formed by chemical vapor deposition (CVD), at 100 °C was ca. 84 nm, with lengths ranging from 100 nm to 3 μm. When the PTCDA nanofibers were biased under vacuum, the emission current remained stable. The turn-on electric field for producing a current density of 10 μA/cm² was 8 V/μm. The maximum emission current density was 1.3 mA/cm², measured at 1100 V (E = 11 V/μm). From the slope of the straight line obtained after plotting ln(J/E²) versus 1/E, we calculated the field enhancement factor β to be ca. 989. These results demonstrate the PTCDA nanofibers have great potential for applicability in organic electron-emitting devices.     

RF measurements to pinpoint defects in inkjet-printed,thermally and mechanically stressed coplanar waveguides

In this work 10-GHz-band RF measurement and microscopy characterizations were performed on thermally and mechanically long-term-stressed coplanar waveguides (CPW) to observe electrical and mechanical degradation in 1-mm-thick PPO/PPE polymer substrates with inkjet-printed Ag conductors. The structure contained two different CPW geometries in a total of 18 samples with 250/270 μm line widths/gaps and 670/180 μm line widths/gaps. A reliability test was carried out with three sets. In set #1 three 250 μm and three 670 μm lines were stored in room temperature conditions and used as a reference. In set #2 six samples were thermally cycled (TC) for 10,000 cycles, and in set #3 six samples were thermally cycled and bent with 6 mm and 8 mm bending diameters.Thermal stressing was done by cycling the samples in a thermal cycling test chamber operating at 0/100 °C with 15-minutes rise, fall, and dwell times, resulting in a one-hour cycle. The samples were analyzed during cycling breaks using a vector network analyzer (VNA). In addition to optical microscopy, field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) imaging were used to mechanically characterize the structures.The results showed that the line width of 670 μm had better signal performance and better long-term reliability than the line width of 250 μm. In this study, the average limit for proper RF operation was 2500 thermal cycles with both line geometries. The wide CPW lines provided more stable characteristics than the narrow CPW lines for the whole 10,000-cycle duration of the test, combined with repeated bending with a maximum bending radius of 6 mm. A phenomenon of nanoparticle silver protruding from cracks in the print of the bent samples was observed, as well as fracturing of the silver print in the CPW lines.     

All-fiber chalcogenide-based mid-infrared supercontinuum source

An all-fiber based supercontinuum source with emission covering the wavelength range of 1.9–4.8 μm is demonstrated. The laser source is based on a combination of silica commercial off-the shelf components and a chalcogenide-based nonlinear optical fiber. The system provides 10 dB spectral flatness from 2.0 to 4.6 μm, and ?20 dBm points from 1.9 to 4.8 μm. The output power is 565 mW but scalable by scaling the repetition rate. The limit on the long wavelength edge of the system is identified as an extrinsic absorption feature in the fiber used; confirming the system could be scaled to generated a broadband source even further in the infrared.     

Ultraviolet photoresistors based on ZnO thin films grown by P-MBE

Ultraviolet photoresistors based on ZnO thin films were fabricated on sapphire substrates with MgO buffer layer by plasma-assisted molecular beam epitaxy. An extremely large dark resistance up to 4 × 10¹⁰ Ω was obtained and the dark/photo resistance ratio is up to 2.3 × 10⁵ with a light intensity of 1.3 mW/cm² at 370 nm. The spectral response shows a large responsivity of more than 1 Ω^?1 W^?1 in the UV region. The photo-resistance depends linearly on the reciprocal of the optical power density for more than two orders of magnitude. The transient response property shows a decay time of 167 μs and the relaxation mechanisms are also discussed.     

Printing and preparation of integrated optical waveguides for optronic sensor networks

Printing processes are employed to create three-dimensional transparent structures which serve as multimode optical waveguides in intelligent systems. The additive manufacturing techniques deliver integrated elements for optical signal transmission in optical communications and novel optronic sensor systems. These networks of sources, sensors and detectors consist of polymers and rely entirely on the conversion of light for space resolved measurements. Measured quantities are temperature, strain and chemical concentration in application scenarios such as structural health monitoring and the life sciences.Results show the suitability of flexographic and inkjet printing to generate parabolic shaped waveguides with a minimum lateral dimension of 50 μm and maximum height of 110 μm. We describe the geometrical properties of printed waveguides as basis for further integration of functional elements.End facets of waveguides serve as interfaces to adjacent functional elements. We compare the capabilities of several processes for end facet preparation of printed polymer waveguides. Stress-induced cleaving is presented as a highly effective procedure with a root mean square surface roughness smaller than 60 nm.     

Temperature-dependent ambipolar electrical characteristics of pentacene-based thin-film transistors: The impact of opposite-sign charge carriers

The temperature-dependent electrical and charge transport characteristics of pentacene-based ambipolar thin-film transistors (TFTs) were investigated at temperatures ranging from 77 K to 300 K. At room temperature (RT), the pentacene-based TFTs exhibit balanced and high charge mobility with electron (μ_e) and hole (μ_h) mobilities, both at about 1.6 cm²/V s. However, at lower temperatures, higher switch-on voltage of n-channel operations, almost absent n-channel characteristics, and strong temperature dependence of μ_e indicated that electrons were more difficult to release from opposite-signed carriers than that of holes. We observed that μ_e and μ_h both followed an Arrhenius-type temperature dependence and exhibited two regimes with a transition temperature at approximately 210–230 K. At high temperatures, data were explained by a model in which charge transport was limited by a dual-carrier release and recombination process, which is an electric field-assisted thermal-activated procedure. At T < 210 K, the observed activation energy is in agreement with unipolar pentacene-based TFTs, suggesting a common multiple trapping and release process-dominated mechanism. Different temperature-induced characteristics between n- and p-channel operations are outlined, thereby providing important insights into the complexity of observing efficient electron transport in comparison with the hole of ambipolar TFTs.     

D–A copolymer with high ambipolar mobilities based on dithienothiophene and diketopyrrolopyrrole for polymer solar cells and organic field-effect transistors

Donor–acceptor (D–A) type conjugated polymers have been developed to absorb longer wavelength light in polymer solar cells (PSCs) and to achieve a high charge carrier mobility in organic field-effect transistors (OFETs). PDTDP, containing dithienothiophene (DTT) as the electron donor and diketopyrrolopyrrole (DPP) as the electron acceptor, was synthesized by stille polycondensation in order to achieve the advantages of D–A type conjugated polymers. The polymer showed optical band gaps of 1.44 and 1.42 eV in solution and in film, respectively, and a HOMO level of 5.09 eV. PDTDP and PC₇₁BM blends with 1,8-diiodooctane (DIO) exhibited improved performance in PSCs with a power conversion efficiency (PCE) of 4.45% under AM 1.5G irradiation. By investigating transmission electron microscopy (TEM), atomic force microscopy (AFM), and the light intensity dependence of J_SC and V_OC, we conclude that DIO acts as a processing additive that helps to form a nanoscale phase separation between donor and acceptor, resulting in an enhancement of μ_h and μ_e, which affects the J_SC, EQE, and PCE of PSCs. The charge carrier mobilities of PDTDP in OFETs were also investigated at various annealing temperatures and the polymer exhibited the highest hole and electron mobilities of 2.53 cm² V⁻¹ s⁻¹ at 250 °C and 0.36 cm² V⁻¹ s⁻¹ at 310 °C, respectively. XRD and AFM results demonstrated that the thermal annealing temperature had a critical effect on the changes in the crystallinity and morphology of the polymer. The low-voltage device was fabricated using high-k dielectric, P(VDF-TrFE) and P(VDF-TrFE-CTFE), and the carrier mobility of PDTDP was reached 0.1 cm² V⁻¹ s⁻¹ at V_d = −5 V. PDTDP complementary inverters were fabricated, and the high ambipolar characteristics of the polymer resulted in an output voltage gain of more than 25.     

Fluorosilicate and fluorophosphate superfluorescent multicore optical fibers co-doped with Nd3+/Yb3+

In the paper spectroscopic properties of two fluorosilicate and fluorophosphate glass systems co-doped with Nd³⁺/Yb³⁺ ions are investigated. As a result of optical excitation at the wavelength of 808 nm strong and wide emission in the 1 μm region corresponding to the superposition of optical transitions ⁴F_3/2 → ⁴I_11/2 (Nd³⁺) and ²F_5/2 → ²F_7/2 (Yb³⁺) can be observed. The optimization of Nd³⁺ → Yb³⁺ energy transfer in both glasses allows to manufacture multicore optical fibers with narrowing and red-shifting of amplified spontaneous emission (ASE) at 1.1 μm.     

Optical hydrogen sensor based on etched fiber Bragg grating sputtered with Pd/Ag composite film

A novel fiber optical fiber hydrogen sensor based on etched fiber Bragg grating coated with Pd/Ag composite film is proposed in this paper. Pd/Ag composite films were deposited on the side-face of etched fiber Bragg grating (FBG) as sensing elements by magnetron sputtering process. The atomic ratio of the two metals in Pd/Ag composite film is controlled at Pd:Ag = 76:24. Compared to standard FBG coated with same hydrogen sensitive film, etched FBG can significantly increase the sensor’s sensitivity. When hydrogen concentrations are 4% in volume percentage, the wavelength shifts of FBG-125 μm, FBG-38 μm and FBG-20.6 μm are 8, 23 and 40 pm respectively. The experimental results show the sensor’s hydrogen response is reversible, and the hydrogen sensor has great potential in hydrogen’s measurement.     

Surface and interface characterization of oxygen plasma activated anodic bonding of glass–ceramics to stainless steel

The anodic bonding of RAS glass–ceramics to stainless steel were carried out at 350–400 °C and 800–1000 V under atmosphere, the micro-topography and compositions of the bonding interfaces also were investigated. With the assistant of oxygen plasma activated, anodic bonding was achieved at 350 °C and 800 V for 23 min under atmosphere, and the anodic bonding strength was up to 1.23 MPa. Experiments results pointed out oxygen plasma could help forming the bonding with glass–ceramics to stainless steel, and the width of cation depletion layer about 50 μm, lithium iron oxide (LiFe₅O₈ and Li₅FeO₄) were observed on the surface of glass–ceramics after anodic bonding.     

Electronic and chemical properties of ZnO in inverted organic photovoltaic devices

Photo-conversion efficiency of inverted polymer solar cells incorporating pulsed laser deposited ZnO electron transport layer have been found to significantly increase from 0.8% to up to 3.3% as the film thickness increased from 4 nm to 100 nm. While the ZnO film thickness was found to have little influence on the morphology of the resultant ZnO films, the band structure of ZnO was found to evolve only for films of thickness 25 nm or more and this was accompanied by a significant reduction of 0.4 eV in the workfunction. The films became more oxygen deficient with increased thickness, as found from X-ray photoelectron spectroscopy (XPS) and valence band XPS (VBXPS). We attribute the strong dependence of device performance to the zinc to oxygen stoichiometry within the ZnO layers, leading to improvement in the band structure of ZnO with increased thickness.