Self-seeding Technique Research Articles

Математическое моделирование процесса выращивания монокристалла CdTe методом Обреимова--Шубникова

Modeling the growth of CdTe crystal modified method Obreimov Shubnikov-using the of self-seeding techniques from the start cooling temperature (1100 °C) until the output to the mode stationary growth was carried out in this paper for the first time. Figures are given of changes in the crystallization front during crystal growth, which are confirmed by experiment using synchrotron radiation.

Improved self-seeding and carrier remodulation performance for WDM-PON by means of double RSOA erasure

In this work, we investigated a novel wavelength division multiplexing passive optical network (WDM-PON) configuration, based on cascaded reflective semiconductor optical amplifiers (RSOA) data erasure for combined self-seeding and wavelength reuse. Improved performance is achieved by enhanced erasure efficiency using a self-seeded double-RSOA cavity at the optical line terminal (OLT) and cascaded RSOAs at the optical network terminal (ONT). To carry out the study, our experimental results for our earlier combined topology layout operating at a symmetric transmission were used to validate the simulation framework for the proposed double-RSOA erasure configuration. Using our original single-cavity self-seeding WDM-PON architecture as a benchmark, downstream extinction ratio (ERD) margins of 6.5 dB and 9.0 dB are obtained, for generating self-seeded downstream signals and upstream remodulation, respectively. Also, bidirectional transmission of 1.25 Gb/s over links reach up to 45 km is achieved by the double-RSOA cavity, surpassing in 25 km our original combined topology. The results indicated that our proposition effectively addresses a fundamental trade-off for WDM-PONs based on the wavelength reuse and/or self-seeding techniques. Specifically, the double erasure procedure allows the use of relatively high levels of ERD, thereby providing improved bit error rate (BER) at the ONT, while assuring high-quality generation of the self-seeding carrier and loopback operation.

Preparing Semiconducting Nanoribbons with Tunable Length and Width via Crystallization-Driven Self-Assembly of a Simple Conjugated Homopolymer.

Precise control of width and length of one-dimensional (1D) semiconducting nanostructures has attracted much attention owing to its potential for optoelectronic applications. However, regulating both their length and width using conjugated polymers or even block copolymers is a huge challenge. To solve this problem, we synthesized a unique conjugated polyacetylene homopolymer by living cyclopolymerization, which spontaneously formed 1D nanoribbons via in situ nanoparticlization. Interestingly, their widths could be controlled from 8 to 41 nm, which were directly proportional to their degree of polymerization. Furthermore, a self-seeding technique via crystallization-driven self-assembly (CDSA) was used to control the length of the nanoribbons up to 5.2 μm with narrow distributions less than 1.1. Interestingly, adding a block copolymer unimer to these nanoribbons produced triblock comicelles by the living CDSA mechanism. Finally, these nanoribbons were visualized directly by super-resolution optical fluorescence microscopy. Now, one can modulate both length and width of 1D nanoribbons simultaneously.

Characterization of mixed halide scintillators: CsSrBrI2:Eu, CsCaBrI2:Eu and CsSrClBr2:Eu

Eu2+-doped mixed halide scintillators, CsSrBrI2, CsCaBrI2 and CsSrClBr2 were studied for gamma-ray spectroscopy applications. We grew Ø13 mm single crystals via the vertical Bridgman method using a self-seeded technique and investigated physical and scintillation properties. Single crystal X-ray diffraction and thermal analysis reveal that CsSrBrI2 and CsSrClBr2 have orthorhombic crystal structures and melt congruently at 611 °C and 752 °C, respectively, while CsCaBrI2 has a cubic structure and melts congruently at 671 °C. All three compounds have the typical Eu2+ 5d – 4 f single-peak emission centered between 445 and 462 nm with a scintillation decay time of a few microseconds. The optimal Eu2+ concentration to maximize the scintillation light yield was determined to be ~7–10%. All three mixed scintillators have an improved energy resolution compared to the non-mixed CsSrI3:Eu, CsCaI3:Eu and CsSrBr3:Eu. The best performing scintillator is CsSrBrI2:Eu 7% with light yield of 65,300 ph/MeV and energy resolution of 3.4% at 662 keV. Excellent proportional response for a wide range of gamma-ray energies was measured for all three scintillators.

Analog Radio-over-Fiber Fronthaul by a WDM-PON employing Double RSOA Self-Seeding and Carrier-Reuse Techniques

WDM-PON has been considered a promising solution for future fronthaul links concerning new mobile networks applications. However, in order to avoid inventory problems and reduce operation costs, there is a need for colorless sources, where the same basic optical source can be used regardless of the desired wavelength. Also, the digital RoF scheme, which has been employed so far, might require a bandwidth that is prohibitive even for optical fiber links. In this paper, we propose and numerically investigate a bidirectional colorless WDM-PON fronthaul transporting analog RoF signals as an alternative to meet these demands. All simulations were performed using a framework calibrated with experimental data. For the downstream, we employ a double-cavity self-seeding technique, while the upstream is performed by a cascaded-RSOAs carrier-reuse approach. BER and EVM simulation analysis are presented for various data rates, modulations formats and RF bands ranging from 1 to 5 GHz, demonstrating the feasibility of our proposed topology as a novel fronthaul approach.

Morphology and competitive growth during the development of the parallel lamellar structure by self-seeding in directionally solidified Ti-50Al-4Nb alloy

Bridgman-type directional solidification was conducted for the parallel lamellar structure of the TiAl-Nb alloys by self-seeding technique. The growth morphology and the competitive growth were investigated during the growth of the non-preferred crystals. The primary α phase and the final α2/γ lamellar structure are obtained at the selected growth rates. The non-preferred crystals with the parallel lamellar always have the asymmetry morphology with the tilting primary arms and the developed second arms at the front-lateral direction, which can affect the non-preferred growth advantage greatly. For the converging growth process the competitive growth takes on the struggle of the primary arms, while the second arms for the diverging growth, in which the parallel crystals always have the advantage to growth out and eliminate the misorientated grain formation at the crucible sidewall. Generally the parallel columnar crystals with the parallel lamellar structure can be achieved during the cellular-dendritic growth at the proper growth rates by self-seeding technique.

A comparison on self-seeding and isothermal crystallization of polyethylene in solution using small angle neutron scattering

We report a simultaneous study in self-seeding and isothermal crystallization of polyethylene (PE) in the solution using time resolved small angle neutron scattering (SANS). We observe that the SANS spectra obtained from self-seeding technique are significantly different from isothermal crystallization technique of PE crystallization in solution. The first correlation peak from structure factor for self-seeding crystallization is not clear compare to isothermal crystallization spectra, indicating that crystals are dominantly in single or few lamellae forms. Structural information, such as the lamellar long period, lamellar crystal thickness, amorphous layers between lamellae, invariant and crystallinity has been quantitatively determined based on model fitting. Avrami analysis depicts 2D lamellar structure formation.

Nascent lateral habits of solution crystallization of poly(ethylene glycol)-block-polystyrene diblock copolymers

Some special habits of dilute solution-grown single crystals were introduced by means of atomic force microscopy (AFM) and transmission electron microscopy (TEM) equipped with electron diffraction (ED). Well-defined block copolymers of poly(ethylene glycol)-block-polystyrene (PEG-b-PS) having predetermined molecular weight and narrow molecular weight distribution (1.06–1.08) were synthesized by atom transfer radical polymerization (ATRP). The single crystals of PEG-b-PS copolymers were grown in a mixed solvent comprised of chlorobenzene/octane and separately in amyl acetate, both using a self-seeding technique. We were capable to develop various growth fronts that resulted from different growth rates through infinitesimal temperature fluctuations. Elevated crystallization temperatures and a higher ratio of molecular weight of PS to that of PEG intensified the curvatures. The corresponding overall, crystalline, and amorphous thickness of a given single crystal, either in square and truncated shapes or with concave and convex lateral sides, had high consistency with each other.

Uniform, High Aspect Ratio Fiber-like Micelles and Block Co-micelles with a Crystalline π-Conjugated Polythiophene Core by Self-Seeding

Monodisperse fiber-like micelles with a crystalline π-conjugated polythiophene core with lengths up to ca. 700 nm were successfully prepared from the diblock copolymer poly(3-hexylthiophene)-block-polystyrene using a one-dimensional self-seeding technique. Addition of a polythiophene block copolymer with a different corona-forming block to the resulting nanofibers led to the formation of segmented B-A-B triblock co-micelles by crystallization-driven seeded growth. The key to these advances appears to be the formation of a relatively defect-free crystalline micelle core under the self-seeding conditions.

Resonance fluorescence in ultrafast and intense x-ray free-electron-laser pulses

The spectrum of resonance fluorescence is calculated for a two-level system excited by an intense, ultrashort x-ray pulse made available, for instance, by free-electron lasers such as the Linac Coherent Light Source. We allow for inner-shell hole decay widths and destruction of the system by further photoionization. This two-level description is employed to model neon cations strongly driven by x rays tuned to the $1s2{p}^{\ensuremath{-}1}\ensuremath{\rightarrow}1{s}^{\ensuremath{-}1}2p$ transition at $848\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$; the x rays induce Rabi oscillations which are so fast that they compete with Ne $1s$-hole decay. We predict resonance fluorescence spectra for two different scenarios: first, chaotic pulses based on the self-amplified spontaneous emission principle, like those presently generated at x-ray free-electron-laser facilities and, second, Gaussian pulses which will become available in the foreseeable future with self-seeding techniques. As an example of the exciting opportunities derived from the use of seeding methods, we predict, in spite of the above obstacles, the possibility to distinguish at x-ray frequencies a clear signature of Rabi flopping in the spectrum of resonance fluorescence.

A novel self-seeding scheme for hard X-ray FELs

Typical SASE XFEL pulses exhibit poor longitudinal coherence, a characteristic inherited from the start-up from shot noise. Self-seeding schemes are an answer to the call for improved longitudinal coherence. If applied to already working or designed XFELs, these schemes are subject to constraints, including minimal change to the baseline design, and possibility to recover the baseline mode of operation. In this work we propose a novel single-bunch self-seeding method, based on a particular kind of monochromator that satisfies these constraints. We will limit ourselves to the analysis of the simplest possible configuration, consisting of an input undulator and an output undulator, separated by our novel monochromatization stage. Such a stage consists of a weak few-meter long chicane, acting as a tunable delay stage, washing out the electron beam microbunching, and creating a transverse offset for the monochromator. In essence, the monochromator consists of a single crystal in Bragg-transmission geometry, which operates as a bandstop filter for the transmitted X-ray SASE radiation pulse. When the incident angle and the spectral contents of the incoming beam satisfy the Bragg diffraction condition, the temporal waveform of the transmitted radiation pulse shows a long monochromatic tail, whose duration is inversely proportional to the bandwidth of the absorption line in the transmittance spectrum. The magnetic chicane is tuned to shift the electron bunch on top of the monochromatic wake created by the bandstop filter thus selecting (temporal windowing) a part of the wake. By this, the electron bunch is seeded with a radiation pulse characterized by a bandwidth much narrower than the natural FEL bandwidth. The output power from our setup can be further increased by tapering the magnetic field of the undulator, yielding a tremendous increase in peak brightness with respect to the baseline mode of operation. In this work we present a feasibility study and exemplifications for the LCLS, and we discuss advantages of our method compared to other self-seeding techniques.

Large-Scale and Highly Oriented Liquid Crystal Phase in Suspensions of Polystyrene-block-poly(l-lactide) Single Crystals

A large number of lozenge-shaped and sandwiched polystyrene-block-poly(L-lactide) (PS-b-PLLA) single crystals were prepared by the self-seeding technique. The single crystals were nearly monodispersed in both thickness and diameter. They are well-dispersed because of the steric stabilization offered by tethered PS in p-xylene, which is a good solvent for PS. The suspensions were observed to separate into a transparent upper phase and a turbid lower phase. The lower phase showed uniform iridescent stripes extending over the whole tube between crossed polarizers. The birefringence demonstrates the liquid crystal order, and the uniform stripes reveal that the phase is a well-oriented single domain. The phase-transition concentration is rather low. Polarizing light microscopy (PLM) images show Schlieren texture and thread-like texture. Small-angle X-ray scattering (SAXS) results showed that the single crystals in the liquid crystal phase oriented horizontally with a vertical repeat distance of about 70 nm. Additionally, the possible structure of the liquid crystal phase is being discussed. The novel disclike colloidal particle might be useful for anisotropic photonic materials.

High-pressure vertical Bridgman growth of lead magnesium niobate–lead titanate single crystal

A modified high-pressure vertical Bridgman (HPVB) method has been used to grow single crystals of lead magnesium niobate–lead titanate (PMN–PT) using self-seeding. The starting charge was prepared close to stoichiometric composition of (1− x)Pb(Mg 1/3Nb 2/3)O 3– xPbTiO 3, ranging from x=0.29 to x=0.32. The ingots grown had a top diameter of ∼114 mm and weighing ∼1.9 kg with a volume of ∼230 cc. Standard form off-the-shelf platinum crucibles from vendors with a wall thickness ∼0.34±0.01 mm were used. The thinner platinum helped to attain a larger radial thermal gradient as well as enhancing the cost effectiveness of the process by reducing usage of precious metals. An oxygen partial pressure of 8000–10 000 ppm was used and controlled during the entire growth process. Self-seeding technique was used in order to attempt to grow large diameter single crystals of PMN–PT. The composition variation in the ingot is dependent on the growth rate, thermal gradients and oxygen partial pressure. The segregation coefficient of Ti was 0.86±0.008 for the starting composition of x=0.32.

Poly(ethylene oxide) Crystal Orientation Change under 1D Nanoscale Confinement using Polystyrene-block-poly(ethylene oxide) Copolymers: Confined Dimension and Reduced Tethering Density Effects

In this study, we systematically investigated the effects of confined dimension (the thickness of PEO layer) and reduced tethering density (σPEO, defined by σπRg2, where σ is the tethered chain density and Rg is the radius of gyration of the tethered PEO chain in its end-free state in the melt) on the PEO crystal orientation change within a 1D nanoscale confinement. We realized this confinement by utilizing two symmetric polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer single crystals grown at different crystallization temperatures (Tx) in dilute solution via the self-seeding technique. After these single crystals were collected as mats, the thicknesses of both the PEO single crystal (dPEO) and PS glassy layer were measured using small-angle X-ray scattering (SAXS) coupled to the 1D correlation function. Because TgPS is higher than TmPEO in those PS-b-PEO single crystal mats, 1D confinements having different dPEO and σPEO values can be prepared by changing Tx values. The PEO block sin...

Cloning polymer single crystals through self-seeding

In general, when a crystal is molten, all molecules forget about their mutual correlations and long-range order is lost. Thus, a regrown crystal does not inherit any features from an initially present crystal. Such is true for materials exhibiting a well-defined melting point. However, polymer crystallites have a wide range of melting temperatures, enabling paradoxical phenomena such as the coexistence of melting and crystallization. Here, we report a self-seeding technique that enables the generation of arrays of orientation-correlated polymer crystals of uniform size and shape ('clones') with their orientation inherited from an initial single crystal. Moreover, the number density and locations of these cloned crystals can to some extent be predetermined through the thermal history of the starting crystal. We attribute this unique behaviour of polymers to the coexistence of variable fold lengths in metastable crystalline lamellae, typical for ordering of complex chain-like molecules.

Production of structurally perfect single crystals of CdTe and CdZnTe

Single-crystal ingots of CdTe and Cd 0,96 Zn 0.04 Te 60-100 mm in diameter were grown by directional solidification using the self-seeding technique. The microstructure of the crystals was cheeked by optical microscopy, electron microscopy (TEM, SEM and EBIC), cathodoluminescence and X-ray diffraction. It was found that crystal perfection depends to a large measure on the temperature schedule during Lpostgrowth crystal cooling: special caution should be exercised in the temperature interval including the polymorphous transformation. A direct relationship between optical and electronic properties on the one hand and crystal microstructure on the other hand was established.

One-dimensional nanostructures of chalcogens and chalcogenides

This paper presents an overview of solution-phase methods for generating one-dimensional (1D) nanostructures of chalcogens and chalcogenides. The first part describes self-seeding techniques for the formation of pure Se, pure Te, and alloy nanostructures. Se and Te are interesting for their inherent properties such as chirality, photoconductivity, piezoelectricity, and high reactivity. The specific morphology (i.e., nanowires, nanorods, and nanotubes) and dimensions of these nanostructures could be tightly controlled. The second part describes a versatile template-engaged technique for topotactic transformation of chalcogen nanostructures into chalcogenides. This transformation enables the formation of 1D nanostructures of materials that do not have intrinsically anisotropic crystal structure. These 1D nanostructures will play an important role in the fabrication of nanoscale devices and in the study of electronic size-confinement effects.

Simple method to generate nanosecond UV pulse trains

By use of a self-seeding technique, energetic nanosecond UV pulse trains of adjustable characteristics (number of pulses, pulse energy, interpulse delay) can be easily produced from an oscillator-amplifier XeCl laser system.

Growth of Self-Seeded Aluminum Nitride by Sublimation-Recondensation and Substrate Preparation

ABSTRACTBulk aluminum nitride boules have been grown at driving rates of 0.9mm/h by the self-seeded sublimation-recondensation technique. Up to 15mm diameter substrates cut from those boules present large single crystal grains that have been analyzed using different techniques. X-ray double crystal diffraction shows a full-width-at-half-maximum of around 100 arcsec and X-ray topography reveals extensive areas with a density of dislocations less than 104 cm−2. These substrates have been prepared by chemical mechanical polishing techniques to obtain a surface roughness of 1.4-1.6nm.

The growth of single crystals by the self-seeding technique

A self-seeding technique suitable for growing large single crystals of low-heat-conductivity materials from the melt is developed using industrially important CdTe and Cd 1− x Zn x Te crystals as an example. A method of controlling crystal composition by maintaining an “undersaturation” excessive partial pressure of a higher-volatility component over the melt, which was developed for preparing semiconductor materials, is applied to Cd-containing compounds for the first time. With these modifications, rather perfect large CdTe and Cd 1− x Zn x Te single crystals are grown by the Obreimov–Shubnikov (gradient freeze) method.