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[en] It is well known that air plasma induced by ultrashort laser pulses emits broadband terahertz waves. The authors report the study of terahertz wave generation from the laser induced plasma where there is a preexisting plasma background. When a laser beam from a Ti:sapphire amplifier is used to generate a terahertz wave, enhancement of the generation is observed if there is another laser beam creating a plasma background. The enhancement of the terahertz wave amplitude lasts hundreds of picoseconds after the preionized background is created, with a maximum enhancement up to 250% observed
[en] InGaN/GaN heterostructures have been deposited onto (0 0 0 1) sapphire by our home-made low pressure MOVPE with different growth parameters. It has been noted that the indium incorporation depends by a complex way on a number of factors. In this work, the effect of substrate temperature, trimethylindium input flow and V/III ratio on the indium incorporation has been investigated. Finally, by optimizing the growth parameters, we made a series of single-phase InGaN samples with indium content from 10% up to 45%.
[en] Sapphire is widely used as a substrate for the growth of GaN epitaxial layer (EPI), but has several drawbacks such as high cost, large lattice mismatch, non-flexibility, and so on. Here, we first employ graphene directly grown on Si or sapphire substrate as a platform for the growth and lift-off of GaN-light-emitting diode (LED) EPI, useful for not only recycling the substrate but also transferring the GaN-LED EPI to other flexible substrates. Sequential standard processes of nucleation/recrystallization of GaN seeds and deposition of undoped (u-) GaN/AlN buffer layer were done on graphene/substrate before the growth of GaN-LED EPI, accompanied by taping and lift-off of u-GaN/AlN or GaN-LED EPI. This approach can overcome the limitations by the catalytic growth and transfer of graphene, and make the oxygen-plasma treatment of graphene for the growth of GaN EPI unnecessary.
[en] Complete test of publication follows. In the quest for a way to generate ultrashort, high-power, few-cycle laser pulses the discovery of optical parametric amplification (OPA) has opened up to the path towards a completely new regime, well beyond that of conventional laser amplification technology. The main advantage of this parametric amplification process is that it allows for an extremely broad amplification bandwidth compared to any known laser amplifier medium. When combined with the chirped-pulse amplification (CPA) principle (i.e. OPCPA), on one hand pulses of just 10 fs duration and 8 mJ pulse energy have been demonstrated. On the other hand, pulse energies of up to 30 J were also achieved on a different OPCPA system; the pulse duration in this case, however, was 100 fs. In order to combine ultrashort pulse durations (i.e. pulses in the few-cycle regime) with high pulse energies (i.e. in the Joule range) we propose tu pump on OPCPA chain with TW-scale short pulses (100 fs - 1 ps instead of > 100 ps of previous OPCPA systems) delivered by a conventional CPA system. This approach inherently improves the conditions for generating high-power ultrashort pulses using OPCPA in the following ways. Firstly, the short pump pulse duration reduces the necessary stretching factor for the seed pulse, thereby increasing stretching and compression fidelity. Secondly, also due to the shortened pump pulse duration, a much higher contrast is achieved. Finally, the significantly increased pump power makes the use of thinner OPCPA crystals possible, which implies an even broader amplification bandwidth, thereby allowing for even shorter pulses. We carried out theoretical investigations to show the feasibility of such a set-up. Alongside these studies we will also present preliminary experimental results of an OPCPA system pumped by the output of our Ti:Sapphire ATLAS laser, currently delivering 350 mJ in 43 fs. An insight into the planned scaling of this technique to petawatt levels (i.e. the Petawatt-Field-Synthesizer project at the MPQ) will also be given.
[en] The diffusion of indium in α-axis sapphire amorphised with high dose In implantation was studied. During isothermal annealing at 600 deg C, diffusion behaviour was strongly dependent on dose. For dose of 6x1016In/cm2, implanted layer still remained amorphous, the indium profile broadened, and there was a long diffusion tail into underlying crystal. For lower dose of 1x1016 and 3x1016In/cm2 no such diffusion was observed
[en] The paper Wisniewski-Barker E et al (2013 New J. Phys. 15 083020) is intended to distinguish experimentally between two mechanisms of pulse delay in ruby and to provide evidence in favor of the slow-light model. The proposed test is based on the idea of monitoring time delay of a ‘dark pulse’ or ‘intensity null’, rather than that of some Gaussian-like pulse. We show that, because of certain experimental inconsistencies, the results of the measurements do not allow one to prefer one of the models and, thus, are interpreted inadequately. In this comment, we propose and realize a simple modification of the experiment Wisniewski-Barker E et al (2013 New J. Phys. 15 083020), which allows us to unambiguously resolve this dilemma. We show that the effect of pulse delay in ruby is perfectly described by the simple model of pulse reshaping and does not require invoking the coherent population oscillation-based slow-light effects. (comment)
[en] We have designed and developed a CW ring-type Ti:sapphire laser based on the ABCD- matrix analysis. The laser cavity was composed of two concave mirrors, two flat mirrors and a Brewster-cut Ti:sapphire crystal. The folding angle at the concave mirrors was calculated to compensate the astigmatism caused by the a Brewster-cut crystal. At a pump power of 10 W, the output power was 2.37 W and slope efficiency was 28%. The output wavelength could be turned from 743 nm to 770 nm by using a birefringent filter.
[en] Ion beam modification of thermal shock resistance of sapphire single crystals with various crystallographic faces is experimentally investigated. The temperature threshold of fracture is determined in both implanted and unimplanted crystals by measuring the fragment contraction on cooling from fracture temperature. Optical and SEM microscopy are used to analyse fracture morphology and thermal shock behaviour on the (0 0 0 1), (1 1-bar 0 2) and (1 1 2-bar 0) faces in sapphire crystals implanted with 70 keV Si- ions and then subjected to thermal stress testing using pulsed plasma. The most stable crystal faces in terms of stress resistance are established. Ion implantation is shown to reduce the temperature threshold of fracture for all faces tested. The (1 1-bar 0 2) face proved to be the most stable for both implanted and unimplanted crystals. The results are discussed on the basis of fracture mechanics principles and the implantation-induced crack nucleation process
[en] Recently a new generation of picosecond dissectors were created on the basis of the PIF-01/S1 picosecond streak-image tube designed and manufactured at the GPI Photoelectronics Department. The results of the measurements of instrument characteristics of the new dissector, which were carried out in the static mode, showed that temporal resolution of the dissector can be better than 3-4 ps (FWHM). The results of temporal resolution calibration of the new-generation picosecond dissector carried out at the specialized set-up based on a femtosecond Ti:sapphire laser are given in this work
[en] The phase locking of diode lasers to the frequency component (mode) of the output radiation of a highly stable femtosecond Ti:sapphire laser, resulting in the amplification of the mode power by several orders of magnitude, is reported. The possibilities of using femtosecond lasers for creation of a new generation of miniature optical clocks and for synthesis and measurements of absolute laser frequencies are discussed. (femtosecond technologies)