High-order harmonic generation in graphene: Nonlinear coupling of intraband and interband transitions

We investigate high-order harmonic generation (HHG) in graphene with a quantum master equation approach. The simulations reproduce the observed enhancement in HHG in graphene under elliptically polarized light [N. Yoshikawa et al., Science 356, 736 (2017)]. On the basis of a microscopic decomposition of the emitted high-order harmonics, we find that the enhancement in HHG originates from an intricate nonlinear coupling between the intraband and interband transitions that are respectively induced by perpendicular electric field components of the elliptically polarized light. Furthermore, we reveal that contributions from different excitation channels destructively interfere with each other. This finding suggests a path to potentially enhance the HHG by blocking a part of the channels and canceling the destructive interference through band-gap or chemical potential manipulation

Belief and Intention towards Design Ethics among Design Undergraduates in Malaysian Higher Education Institutions

This paper reports on the findings of a study by design graduates on their belief and intention on ethical issues in the creative design industry. The study was conducted at two universities (University A and University B) in Malaysia, which included 120 undergraduates in design. A survey was conducted in which a questionnaire was distributed among respondents to determine their degree of agreement with respect to each argument in the questionnaire. The results of the data showed that the belief and intention of design graduates towards design ethics is poor, with the exception of the respondents of University A who have strong belief towards social, environmental and sustainability issues. These results indicate that a well-structured model of design ethics education and effective teaching mechanism for design ethics education should be in place. These would have an impact on students’ belief and intention towards design ethics. As such, the research results serve as a cornerstone from which the current practice of teaching and learning of design ethics education can be more critically examined, so that more changes can be made to the existing curriculum that can help to develop designers with ethical characteristics

Size-controlled quantum dots reveal the impact of intraband transitions on high-order harmonic generation in solids

Since the discovery of high-order harmonic generation (HHG) in solids1,2,3, much effort has been devoted to understand its generation mechanism and both inter- and intraband transitions are known to be essential1,2,3,4,5,6,7,8,9,10. However, intraband transitions are affected by the electronic structure of a solid, and how they contribute to nonlinear carrier generation and HHG remains an open question. Here we use mid-infrared laser pulses to study HHG in CdSe and CdS quantum dots, where quantum confinement can be used to control the intraband transitions. We find that both HHG intensity per excited volume and generated carrier density increase when the average quantum dot size is increased from about 2 to 3 nm. We show that the reduction in sub-bandgap energy in larger quantum dots enhances intraband transitions, and this—in turn—increases the rate of photocarrier injection by coupling with interband transitions, resulting in enhanced HHG

Optical properties of structurally-relaxed Si/SiO2_2 superlattices: the role of bonding at interfaces

We have constructed microscopic, structurally-relaxed atomistic models of Si/SiO$_2$ superlattices. The structural distortion and oxidation-state characteristics of the interface Si atoms are examined in detail. The role played by the interface Si suboxides in raising the band gap and producing dispersionless energy bands is established. The suboxide atoms are shown to generate an abrupt interface layer about 1.60 \AA thick. Bandstructure and optical-absorption calculations at the Fermi Golden rule level are used to demonstrate that increasing confinement leads to (a) direct bandgaps (b) a blue shift in the spectrum, and (c) an enhancement of the absorption intensity in the threshold-energy region. Some aspects of this behaviour appear not only in the symmetry direction associated with the superlattice axis, but also in the orthogonal plane directions. We conclude that, in contrast to Si/Ge, Si/SiO$_2$ superlattices show clear optical enhancement and a shift of the optical spectrum into the region useful for many opto-electronic applications.Comment: 11 pages, 10 figures (submitted to Phys. Rev. B

Direct observation of free excitons in luminescence spectra of xenon clusters

Luminescence of surface and free bulk excitons is detected in xenon for the first time for substrate-free rare-gas clusters. Xenon clusters were produced by the method of gas condensation in a supersonic jet emitted into vacuum. Optical study was accompanied by electron diffraction measurements to determine the structure of clusters.Comment: The more complete version of the paper is to be published in 'Low Temperature Physics' (2007

Purification of the NaI(Tl) crystal for dark matter search project PICOLON

Direct search for dark matter is one of the most important problems in astrophysics. Significant signal for dark matter will be a hint to clarify the origin of the universe. Only DAMA/LIBRA experiment with NaI(Tl) detector has ever suggested the presence of dark matter signal. Verifying the DAMA/LIBRA result by a NaI(Tl) detector is urgent and important task. We have tried to purify NaI(Tl) crystal to search for dark matter. In this presentation, the present status of purification will be discussed. The concentration of potassium is successfully reduced to desired sensitivity. The 210Pb, which is difficult to reduce, has been reduced effectively. Present status of low background measurement in Kamioka observatory will be shown

Physics of the Riemann Hypothesis

Physicists become acquainted with special functions early in their studies. Consider our perennial model, the harmonic oscillator, for which we need Hermite functions, or the Laguerre functions in quantum mechanics. Here we choose a particular number theoretical function, the Riemann zeta function and examine its influence in the realm of physics and also how physics may be suggestive for the resolution of one of mathematics' most famous unconfirmed conjectures, the Riemann Hypothesis. Does physics hold an essential key to the solution for this more than hundred-year-old problem? In this work we examine numerous models from different branches of physics, from classical mechanics to statistical physics, where this function plays an integral role. We also see how this function is related to quantum chaos and how its pole-structure encodes when particles can undergo Bose-Einstein condensation at low temperature. Throughout these examinations we highlight how physics can perhaps shed light on the Riemann Hypothesis. Naturally, our aim could not be to be comprehensive, rather we focus on the major models and aim to give an informed starting point for the interested Reader.Comment: 27 pages, 9 figure