Charged two-exciton emission from a single semiconductor nanocrystal

Effect of incident laser power and incorporating polariton effects in GaN microrods

When a UV photon is absorbed by a single semiconductor nanocrystal (NC), two or more excitons can be simultaneously generated through the carrier multiplication (CM) process. It is still highly debated whether the CM efficiency is truly enhanced in semiconductor NCs because all the routine CM measurements performed exclusively at the ensemble level are incapable of completely excluding the false CM signals contributed by the charged excitons. Here we place single CdSe NCs above an aluminum film and successfully resolve their UV-excited photoluminescence time trajectories where the true and false CM signals are contained in the blinking “on” and “off” levels, respectively. When the UV photon energy is ~2.46 times of the NC energy gap, an average CM efficiency of ~20.2% can be reliably estimated. The ability to detect UV-excited photoluminescence from a single NC will surely provide a great guidance for the CM applications in various light-to-electric conversion devices.

Approaching a metallic tip to a single quantum emitter quenches the photolumi- nescence by opening non-radiative decay channels for the excited-state. This is one of the main problems in nano-optics research which prohibits optical studies on single emitters in contact with the tip with high resolution and high sensitivity. In this thesis I have shown that a bowtie nanoantenna can be used to overcome the quenching problem at the single chromophore level. Bowtie antenna tip in- teracts with the dipole of the single emitter. This is most probably the ¯rst study on this type of measurements which opens new pathways for many disciplines. Semiconductor nanocrystals were selected as a single emitter system due to their relatively high photo-stability. Based on °uorescence confocal studies, satura- tion behavior of single CdSefZnSg nanocrystal (NC) is studied under one- and two-photon excitation. In one-photon excitation (1PE) laser wavelength of 532 nm and in two-photon excitation (2PE) laser wavelength of 830 nm were used to excite the °uorophore. Due to the broad distribution in photoluminescence (PL) intensity of nanocrystals, power dependence studies were done based on an average over » 90 nanocrystals. Using focused ion beam, bowtie antennas are sculptured at the apices of silicon nitride AFM tips, which were fully-coated with a homogeneous layer of 40 nm of aluminum ¯lm. Details of structuring procedures used to fabricate well-de¯ned bowtie antennas with smallest possible feedgaps are described. Interaction of bowtie nanoantennas with single semicon- ductor nanocrystals, is investigated using PL intensity and excited-state lifetime of the nanocrystal as two intrinsic signatures of the single emitter. Proximity of the feedgap of a bowtie nanoantenna to a single nanocrystal under one-photon excitation leads to enhanced emission in addition to enhanced excitation. This e®ect is shown, by increasing the PL intensity of the nanocrystal and shortened lifetime, in contact with the bowtie antenna feedgap. These results were com- pared with a fully-coated tip which lead to complete quenching of the nanocrystal PL. Thus, the observed e®ects in PL intensity and lifetime of the nanocrystal in contact with the antenna are originated from the metallic nanostructure. Under two-photon excitation, PL intensity is enhanced but there is no change in the lifetime of the nanocrystal in contact with the bowtie antenna. This is caused by enhanced excitation through the antenna, induced by enlarging the absorption cross section of the nanocrystal in contact with the antenna. Since, °uorescence of single nanocrystal in contact with the bowtie antenna is not quenched, more detailed studies on their interaction were performed. Under two-photon excita- tion absorption cross section of one nanocrystal was measured with and without the presence of antenna. Free nanocrystal showed a two-photon absorption cross section in the order of 6:3£10i37cm4s which in contact with the bowtie antenna increased to 20:2£10i37cm4s. This proves that enhanced excitation observed in 2PE is caused by a larger absorption cross section of the system induced by the antenna structure. Emission polarization of nanocrystals was studied under 1PE using polarization microscopy. From these studies, in- and out-of-plane angles as well as the absolute value of the projection of the transition dipole moment on the sample plane were determined. Results showed in contact with the bowtie antenna, the in-plane angle turns towards the orientation of the antenna. This is induced by the strong dipole of the antenna in contact with the nanocrystal. Moreover, modulation depth and the absolute value of the transition dipole were increased dramatically in contact with the bowtie nanostructure. The results show that antenna/NC system has a highly polarized emission, whose polar- ization direction is determined by the antenna dipole. Photon antibunching of nanocrystals under 1PE was done with and without the presence of the antenna tip. Shorter lifetime of the excited-state in contact with the bowtie antenna immediately appears in antibunching results. This shows that the dead time for single photon generation, caused by excitation-recombination cycles, is much shorter in contact with the antenna. Therefore, a nanocrystal in contact with the bowtie antenna is a more e±cient single photon source. Moreover, taking into account the emission polarization of the antenna/NC system, polarization of single photons generated from the nanocrystal in contact with the antenna can be tuned by antenna orientation. Thus, single photons provided by antenna/NC system can have strong potentials in quantum cryptography. As a result, coupling single quantum emitters (here nanocrystal) to bowtie nanoantennas will produce a new type of emitter with widely adjustable photophysical properties, which can be called a tunable superemitter. Emission characteristics of the antenna/NC system is highly determined by the coupling intra-superemitter.

Interplay of laser power and pore characteristics in selective laser melting of ZK60 magnesium alloys: A study based on in-situ monitoring and image analysis

Influence of Ni coating on interfacial reactions and mechanical properties in laser welding-brazing of Mg/Ti butt joint

Summary This paper describes an investigation into the behaviour of helium bubbles in helium-doped austenitic stainless steel weldments by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and numerical calculations. The stainless steel was helium ion implanted and welded with a YAG laser apparatus. Helium ion implantation in the sample was performed with an 8 MeV implantation apparatus. The samples were doped with 1.0 × 1019 atoms/m2 at 5 MeV and then doped with 2.45 × 1019 atoms/m2 at 6 MeV. The results obtained may be summarised as follows. 1. At a low laser power, large helium bubbles of 3–4 μm size are formed at the weld bead/base metal interface, i.e. on the weld metal side of the weld bond region. The amount of weld metal increases, and the helium concentration decreases, with an increasing laser power. The helium bubbles therefore tend to have a diameter as small as under 1 μm. The results of the numerical calculations suggest that the helium concentration in the weld metal is more than 10 appm. Helium-free samples contain no helium bubbles. 2. Helium bubbles with a diameter of less than 0.2 μm are also formed at the grain boundaries in the heat-affected zone (HAZ). The helium bubble region increases with an increasing laser power. 3. The results of the TEM observations suggest that fine helium bubbles of under 0.2 μm size are formed at the dendrite cell boundaries in the weld metal. Their size tends to increase with an increasing laser power. 4. When helium ion implanted samples are laser-welded, helium bubbles are formed in the weld metal and HAZ. Any increases in the laser power and welding speed decrease the helium concentration. Even at helium concentrations of 2.9 and 3.8 appm, helium bubble-induced microcracking occurs at the dendrite cell boundaries in the weld metal.

• Increasing the laser power in LP-DED enlarged the melt pools in NASA HR-1 alloy. • Heat treatment reduced Ti segregation, occurring after NASA HR-1 deposition. • Material hardness increased with stress relief due to formed precipitate nuclei. • NASA HR-1 hardness and ductility improved with conducting heat treatment. This study investigated the microstructure and tensile properties of the NASA HR-1 (Fe-Ni-Cr) alloy using the laser powder directed energy deposition (LP-DED) process. Laser power and spot size were varied for deposition, and it was noted that low laser power could result in higher cooling rates and finer microstructure. The melt pool depth and width and the defect content increased with the increase in laser power. Out of several laser power (i.e., 350, 750, 1070, 2000, and 2620 W), the samples deposited using 750 W demonstrated the highest material density. In addition, the effects of heat treatment on the microstructure and tensile properties were investigated. It was found that a heat treatment schedule consisting of stress relief, homogenization, solution treatment, and aging could thoroughly homogenize the microstructure (i.e., remove the as-solidified dendritic microstructure), result in relatively uniform, equiaxed grains, and increase the material hardness. Importantly, this heat treatment schedule efficiently reduced titanium segregation, preventing the formation of deleterious needle-shaped η (Ni 3 Ti) precipitates in NASA HR-1, known to be detrimental to mechanical properties and resulted in almost comparable tensile properties with varying laser power.

Microstructural Evolution and Mechanical Properties of Laser Processed Ti6Al4V Sheets

Field-emission patterns from a clean tungsten tip apex induced by femtosecond laser pulses have been investigated. Strongly asymmetric field-emission intensity distributions are observed depending on three parameters: (1) the polarization of the light, (2) the azimuthal and (3) the polar orientation of the tip apex relative to the laser incidence direction. In effect, we have realized an ultrafast pulsed field-emission source with site selectivity of a few tens of nanometers. Simulations of local fields on the tip apex and of electron emission patterns based on photo-excited nonequilibrium electron distributions explain our observations quantitatively. Electron emission processes are found to depend on laser power and tip voltage. At relatively low laser power and high tip voltage, field-emission after two-photon photo-excitation is the dominant process. At relatively low laser power and low tip voltage, photoemission processes are dominant. As the laser power increases, photoemission from the tip shank becomes noticeable.

Effect of laser beam incident angle on welding of Ti6Al4V with fiber lasers

Influence of laser power on microstructure evolution and properties of laser cladded FeNiCoCrMo HEA coatings

This research investigates the influence of layer thickness, laser power, and sintering orientation on the mechanical properties of aged Polyamide-12 (PA-12) FS3300PA using the Selective Laser Sintering (SLS) 3D printing method. Specimens were sintered with three different layer thicknesses, laser powers, and sintering orientations using SLS. The study also aimed to examine the resulting powder morphology, mechanical properties, and tensile fracture behaviors of the aged (more than eight continuously sintering cycles) and virgin FS3300PA powders. The specimens divided into ten groups: nine groups of aged powder and one group of virgin powder as a benchmark. The nine groups of aged powder were sintered with three different layer thicknesses (0.07 mm, 0.12 mm, and 0.15 mm), laser powers (65 W, 70 W, and 75 W), and orientations (YZY 0°, YZY 90°, and XYY 0°). The selections of these laser power and layer thickness values for the sintering setting are due to machine and material parameter limitation. The results from these parameters then compared with those of the virgin powder, which sintered using the parameters provided by the manufacturer, in terms of powder morphology, mechanical properties, and tensile fracture behaviours. Observation made using scanning electron microscope (SEM) revealed that there were not many changes in shape, size, and distribution between the virgin and aged powder, but slightly larger sizes and the presence of cracks found in the aged powder. The tensile strength, elongation at break value, and Young’s modulus all shared a similar trend, increasing with higher laser power but decreasing with increased layer thickness. Regarding the fracture morphologies, the number of pores and dimples decreased with increased laser power but increased with thicker layer thickness. There was also the occurrence of un-molten powder, especially in specimens sintered at the YZY 90° orientation with lower laser power and thicker layer thickness.

Numerical and experimental study of RHEAs surface morphology and defect in selective laser melting

Microstructure evolution of 316L produced by HP-SLM (high power selective laser melting)

A Zr‐based amorphous alloy (thickness = 2 mm) is welded by laser welding. The phase composition and microstructure of the welding joints under different laser powers are analyzed by optical microscopy, X‐ray diffraction, and scanning electron microscopy. Mechanical properties of the welded joints under different laser powers are examined using microhardness and tensile testers. The glass forming ability and thermal stability of the welded joints are investigated using differential scanning calorimetry. With an increase in laser power, the degree of crystallization of welded joints increases, their microhardness is in the range 502.3–546.9 HV0.2, and tensile strength is up to 583 MPa (75% of that of the Zr‐based amorphous alloy). Through the research of the article, it can be found that when the laser power increases, the crystallization degree of the welded joint first increases rapidly and then tends to be stable. The lower laser power should be used as far as possible in the case of penetration.