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In this study, vertical centrifugal casting process is used for the fabrication of functionally graded A390 alloy hollow cylinder and Mg & Sr-included A390 alloy hollow cylinder. The commercial CFD software FLOW3D™ is used to obtain the optimum rotational speed for making an optimized hollow cylinder in a vertical centrifugal casting machine. The optimum rotational speed obtained is 1300 rpm. This rotational speed is used in the experiments to obtain hollow cylinders of optimized thickness for (a) A390 alloy (b) by adding various weight percentages of Mg and Sr in A390 alloy. The optical microstructures obtained at various radial locations are presented for centrifugally cast A390 alloy. The effect of individual addition of Mg & Sr into A390 alloy on the microstructures at various locations along the radial direction of centrifugally cast cylinder is discussed in detail.

The strong band edge exciton-phonon interactions in metal halide perovskite quantum dots (QDs) offer a unique platform to explore many-body phenomena. Employing $\mathrm{Cs}\mathrm{Pb}{\mathrm{Br}}_{3}$ QDs as a perovskite model system, we report the observation of spin-selective polaronic biexcitons using collective excitations of two circularly polarized ultrafast lasers of a duration that is two orders of magnitude shorter than the exciton lifetime and one order of magnitude shorter than the spin relaxation time. The intervalley polaron pairing of charge carriers determines the anomalously strong exciton-exciton interactions, where the Haynes factor is an order of magnitude larger than the bulk and five times larger than the two-dimensional and quantum well semiconductors, demonstrating a very robust correlation of excitons. Our findings reveal a mechanism of generating highly stable biexciton states even at room temperature to realize higher-order correlations of charge carriers such as quantum droplets and Bose-Einstein condensates.

In this Letter, we report for the first time, to the best of our knowledge, the anisotropic optical response in a graphene oxide (GO)-gold (Au) nanohybrid. Polarization-sensitive nonlinear optical absorption measurements revealed that nanohybrids are highly anisotropic, ( β ⊥ − β ‖ ) ≈ 28 c m / G W , which is more than one order of magnitude higher than that of control GO (2 cm/GW). The first-principle analysis of absorbance at nanohybrid interfaces with varying functional ligand concentrations corroborates with the experimentally observed intrinsic linear anisotropy. Thus, this Letter enables new routes to realize smart and high-performing nonlinear optical systems selectively and directionally such as tunable optical limiters and optical data processing devices.

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Two-dimensional (2D) covalent organic frameworks (COFs) are an emerging class of porous materials with potential for wide-ranging applications. Intense research efforts have been directed at tuning the structure and topology of COF, however the bandgap engineering of COF has received less attention, although it is a necessary step for developing the material for photovoltaic or photonic applications. Herein, we have developed an approach to narrow the bandgap of COFs by pairing triphenylamine and salicylideneaniline building units to construct an eclipsed stacked 2D COF. The ordered porous structure of 2D COF facilitates a unique moisture-triggered tautomerism. The combination of donor–acceptor charge transfer and tautomerization in the salicyclidineaniline unit imparts a large bandgap narrowing for the COF and turns it color to black. The synthesized COF with donor–acceptor dyad exhibits excellent nonlinear optical properties according to open aperture Z-scan measurements with 532 nm nanosecond laser pulses.

Exciton many-body interaction is the fundamental light–matter interaction that determines the optical response of the new class of colloidal perovskite nanocrystals of the general formula CsPbX3 [X = Cl, Br, or I]. However, the understanding of exciton many-body interactions manifested through the transient biexcitonic Stark effect at the early time scales and the Auger recombination process in this new class of materials still remains rather incomplete. In this Article, we studied the many-body exciton interactions under controlled conditions through ultrafast transient absorption spectroscopy. A large biexcitonic redshift ∼30 meV to the effect of hot excitations on the excitonic resonance is observed at the early time scales. From the fluence-dependent studies, it is evident that the samples have only single and biexciton lifetimes, suggesting that the band edges are 2-fold degenerate. This explicit experimental evidence for the exciton many-body interactions in CsPbBr3 nanocrystals provides a powerful ...

Manipulating optical nonlinearity is a very fascinating prospective for both fundamental and technological purpose; offers a potential platform to explore the saturable absorption (SA) to the nanoscale limit. In this manuscript, we have focused on the tunable wavelength and pulse width dependent ultrafast broadband SA of Sb2Se3 nanowires in the resonant and the above bandgap excitations of 120 fs and 7 ns pulses. In this context, we numerically simulated the SA of Sb2Se3 nanowire with a two-level model. The solutions of the coupled rate equations for a Gaussian shaped pulse exactly reproduced the experimental results in both temporal and spatial domains. Our model provides a precise spacio-temporal analysis of carrier populations of each photo-excited state in the system. Moreover, excited state decay time (1.5 ns) and the ratio of excited state absorption cross section (σes) to the ground state absorption cross section (σgs) is less than unity for 1-D Sb2Se3 nanowires. Hence, Sb2Se3 nanowires can be employed as passive mode-lockers and essential photodetectors for ultrashort pulsed laser and can be tuned by wavelength and pulse width. These observations advise a remarkable nonlinear optical response which is expected to be comparable or even better than the graphene and 2D TMDC’s (MX2) like MoSe2, MoS2, WSe2, WS2 etc.

Realization of the direct charge transfer at metal–semiconductor interfaces is a long-standing goal of both fundamental and technological significance. Here we report the synthesis of a colloidal Sb2Se3–Au core–shell nanorod as a model system to demonstrate an efficient direct charge transfer from an Au shell to Sb2Se3 core when the metal is selectively excited at the plasmonic wavelength. In our experiments, direct charge transfer by the Landau damping of the plasmons of metal shell significantly enhances the excited state population that results in an unprecedented ultrafast third-order nonlinear optical response as a function of the plasmon-excitation detuning wavelength. The single step photo-induced charge transfer analogous to the intramolecular electronic transition in molecules is probed by ultrafast transient absorption, which reveals that the electrons are directly transferred from the Fermi level of Au to the unoccupied levels of Sb2Se3 in less than 150 fs. First principles density functional theory calculations indicate that the hybridized eigenstates of the strongly coupled system are delocalized across the metal–semiconductor interfaces. By formulating the theoretical models, we connect our experimental results to the theory.

Defect-tolerant perovskite nanocrystals of the general formula Cs-Pb-${X}_{3}$ (where $X=\mathrm{Cl}$, Br, and I) have shown exceptional potential in fundamental physics as well as in novel optoelectronic applications as the next generation of solar cells. Although exciton many-body interactions such as biexciton Stark shift, state filling, and Auger recombination are studied extensively, other important correlated effects, such as band gap renormalization (BGR) and hot-phonon bottleneck, are not explored in these nanocrystals. Here we experimentally demonstrate the carrier density dependence of the BGR and an effective hot-phonon bottleneck in $\mathrm{CsPb}{(\mathrm{C}{\mathrm{l}}_{0.20}\mathrm{B}{\mathrm{r}}_{0.80})}_{3}$ mixed-halide nanocrystals. The results are compared with two other halide compositions, namely, ${\mathrm{CsPbBr}}_{3}$ and $\mathrm{CsPb}{(\mathrm{B}{\mathrm{r}}_{0.55}{\mathrm{I}}_{0.45})}_{3}$ nanocrystals with varying band gaps. The optical response of the nanocrystals changes dramatically across the spectral range of many hundreds of meV at high carrier density due to large BGR. We have calculated the BGR constant $\ensuremath{\approx}(6.0\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ cm for $\mathrm{CsPb}{(\mathrm{C}{\mathrm{l}}_{0.20}\mathrm{B}{\mathrm{r}}_{0.80})}_{3}$ nanocrystals that provides the amount of band gap shift as a function of carrier density. In these nanocrystals, an efficient hot-phonon bottleneck is observed at a carrier density of $3.1\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ that slows down the thermalization by 1 order of magnitude. Our findings reveal that the complex kinetic profile of the exciton dynamics can be analyzed by the global target analysis using the sequential model with increasing lifetimes.

The nonlinear optical absorbance of conventional materials is very weak, yet its magnitude dominates device performance in, for example, optical limiting and pulse shaping. Therefore, achieving a strong nonlinear optical response is a longstanding goal. The authors propose charge transfer between donor and acceptor materials as a means to greatly enhance nonlinear response, toward the realization of high-performance optical limiters. Excellent agreement between experiment and theory for a test hybrid material validates the idea, and guides the design and fabrication of an actual liquid-cell-based absorptive optical limiter, which outperforms benchmark devices.

Centrifugal casting process is one of the potential manufacturing techniques used for producing functionally graded materials viz., composite materials or metallic materials which have high differences of density among constituents. In this process, the fluid flow plays a major role and understanding the complex flow process is a must for the production of defect-free castings. Since the mold spins at a high velocity and the mold wall being opaque, it is impossible to visualise the flow patterns in real time. Hence, in the present work, the commercial CFD code FLOW-3DTM, has been used to simulate the mold filling sequence for a simple hollow cylindrical casting during vertical centrifugal casting process. Effect of various spinning velocities on the fill pattern during vertical centrifugal casting process is being investigated.

The application of natural rubber (NR) as cost-effective component for an organic bistable memory device is demonstrated for the first time. An organic bistable memory device based on Cis 1–4 polyisoprene (NR) and fullerene (C60) is a trilayered structure consisting of NR/C60 composite sandwiching a C60 layer and thermally deposited aluminum electrodes on both sides (Al/NR:C60/C60/NR:C60/Al). Investigations on current-voltage (I-V) characteristics of the device revealed promising device performance with a high ON/OFF current ratio and nonvolatile stability in both of its conducting states, indicating the strong potential of the NR:C60 composite as a nonvolatile memory material. Further, attempts are made to fit the current response of the device using existing charge transport models and explain the origin of memory switching in present device.

One-dimensional (1D) free-standing nanowires are particularly important for carrier confinement in two dimensions, which provides a platform to explore the nonlinear optical phenomena at the nanoscale. In this Letter, we demonstrate saturable absorption in the resonant and above-bandgap excitations of both ns and fs pulses in 1D crystalline Sbsub2/subSesub3/subnanowires prepared by the facile hydrothermal method. Impressively, the average length of the nanowires extends to a few micrometers with a high aspect ratio of 300. The excited-state to ground-state absorption cross-section ratio in Sbsub2/subSesub3/subnanowires is ≈0.23, which suggests that they can be utilized as passive mode lockers.

A complex few-layer $\mathrm{MoS}{\mathrm{e}}_{2}$/graphene oxide (GO) heterostructure with strong interlayer coupling was prepared by a facile hydrothermal method. In this strongly coupled heterostructure, we demonstrate a giant enhancement of two-photon absorption that is in stark contrast to the reverse saturable absorption of a weakly coupled $\mathrm{MoS}{\mathrm{e}}_{2}$/GO heterostructure and saturable absorption of isolated $\mathrm{MoS}{\mathrm{e}}_{2}$. Spectroscopic evidence of our study indicates that the optical signatures of isolated $\mathrm{MoS}{\mathrm{e}}_{2}$ and GO domains are significantly modified in the heterostructure, displaying a direct coupling of both domains. Furthermore, our first-principles calculations indicate that strong interlayer coupling between the layers dramatically suppresses the $\mathrm{MoS}{\mathrm{e}}_{2}$ excitonic bands. We envision that our findings provide a powerful tool to explore different optical functionalities as a function of interlayer coupling, which may be essential for the development of device technologies.

Metal nanoparticle-semiconductor interfaces are sites of complex light-matter interactions, in particular, the exciton-plasmon coupling which plays a key role in the optical response of such heterostructures. There exists a pathway of photoinduced charge transfer from the semiconductor to the metal, which can be used to controllably vary the driving forces at the interface that leads to tunable optoelectronic properties. In this letter, we report the observation of a dramatic suppression of plasmonic as well as excitonic absorption in a-Ge24Se76/gold nanoparticle heterostructures by trapped charges. Suppression of the excitonic absorption is strongly correlated with the plasmon wavelength. (C) 2016 Author(s).

Organic Light Emitting Diodes (OLED) are receiving increased attention due to tremendous application potential these devices hold in the areas of large area displays and lighting applications. However, the problems of efficiency, stability and shelf life are major challenges for making OLEDs an attractive alternative. The simple device structure involving anode, emissive layer and cathode is no longer the norm. Recently, various buffer layers like Hole Injection Layer (HIL), Hole transport Layer (HTL), Electron Injection Layer (EIL), Electron Transport Layer (ETL) etc. are being widely used as integral parts of the OLED architecture to enhance the performance parameters. The nomenclature of these layers is often confusing and sometimes used by different authors to mean different layers and a common and universal nomenclature for layers is still wanting. Applying a buffer layer, often called as the hole injecting layer (HIL) between anode and emissive layer is a general technique for increasing the efficiency and stability of organic light emitting diodes. Poly- (3,4-ethyhylene dioxythiophene): poly- (styrenesulphonate) (PEDOT:PSS) is a very common and popular such HIL used in OLEDs. In this chapter, a basic structure of OLEDs has been discussed in perspective with this HIL material and the effect of annealing this PEDOT: PSS layer on the characteristics of the device at different temperatures ranging from 100°C to 300°C in vacuum. Devices fabricated in clean room conditions are characterized for their electrical and optical properties. Equivalent circuits of the devices are deduced using impedance spectroscopy and discussed. Surface morphology of the HIL layers using atomic force microscopy (AFM) provides reasons for the variation of the device properties with the annealing of HIL.

In this Letter, we report for the first time, to the best of our knowledge, continuous-wave laser background illumination (BGI) as a simple and yet useful tool to tune nanosecond transient absorption (TA) in a-Ge25As10Se65 thin films. In our experiments, we observed remarkable blueshift in TA as a function of the BGI intensity. Strikingly, relaxations of TA in background-illuminated samples are much faster than the as-prepared samples. This observation provides new insights into the bond-breaking mechanism. Further, decay time constants of TA are wavelength dependent, which signifies that excited carriers have a longer lifetime in deep traps than in shallow traps.

In this paper, we report that the non linear behavior of amorphous As2S5 can be drastically altered from two photon absorption (TPA) to saturable absorption (SA) when it form heterostructures with Au nanoparticles (ANPs).

Organic memory devices are one of the active research areas. Iodine doped rubber with fullerene C60 molecules are studied for memory application. The device fabrication is carried out by simple solution processing technique, Current voltage characteristics of the device is having a hysteresis with a negative differential resistance. Write-read-erase cycle is performed, which reveals the potential use of the device as a low power consuming non volatile memory.

One-dimensional (1D) free-standing nanowires are particularly important for carrier confinement in two dimensions, which provides a platform to explore the nonlinear optical phenomena at the nanoscale. In this Letter, we demonstrate saturable absorption in the resonant and above-bandgap excitations of both ns and fs pulses in 1D crystalline Sb2Se3 nanowires prepared by the facile hydrothermal method. Impressively, the average length of the nanowires extends to a few micrometers with a high aspect ratio of 300. The excited-state to ground-state absorption cross-section ratio in Sb2Se3 nanowires is ≈0.23, which suggests that they can be utilized as passive mode lockers.

Consequent to the fast increase in data storage requirements new materials and device structures are explored in a war footing. Organic memory devices are attracting lot of interest among the researchers and are becoming a hot topic of investigations. This study is an attempt to develop a tri‐layer organic memory device using indium tin oxide (ITO) nanoparticles as charge trapping middle layer between tris‐8(‐hydroxyquinoline)aluminum (Alq3) layers employing spin coating technique. Device switching is studied by applying a current‐voltage (I‐V) sweep. On increasing the applied bias the device switched from the initial high resistance (OFF) state to a low resistance (ON) state at a switch on voltage of around 4 V. ON/OFF ratio is of the order of 100 at a read voltage of 2 V. The device is found to remain in the low resistance state on further scans, showing the applicability of this device as a write once read many times (WORM) memory.