1,721,213 research outputs found
Terahertz-assisted excitation of the 1.5-mu m photoluminescence of Er in crystalline Si
No full textWe show that the 1.5-mum emission from Si:Er generated by continuous-mode band-to-band optical excitation can be dramatically enhanced by terahertz radiation from a free-electron laser. The effect is observed at cryogenic temperatures in samples prepared from FZ-Si by high-temperature implantation with Er ions and requires a high density of infrared photons (hOmegaapproximate to100 meV). Based on experimental characteristics of this effect, we argue that the excitation mechanism responsible for the enhancement is, in this case, different from the previously discussed free-electron-laser-induced optical ionization of trapped carriers. A theoretical model of the energy transfer path is developed. It involves participation of a higher-lying conduction c2 band of the Si host and excitation into the I-4(11/2) second excited state of Er3+ ion. Since formation of the Er-related level is not necessary in this mechanism, it opens a possibility to excite a large fraction of Er3+ ions, including also these which are not linked to a recombination level in the band gap. Possible implications of the proposed model toward realization of a true three-level scheme in Si:Er are pointed out. Finally, further experiments necessary for testing and confirmation of the theoretically developed model are proposed.
Nanosecond dynamics of the near-infrared photoluminescence of Er-doped SiO2 sensitized with Si nanocrystals
No full textWe report on an observation of a fast 1.5 mu m photoluminescence band from Er3+ ions embedded in an SiO2 matrix doped with Si nanocrystals, which appears and decays within the first microsecond after the laser excitation pulse. We argue that the fast excitation and quenching are facilitated by Auger processes related to transitions of confined electrons or holes between the space-quantized levels of Si nanocrystals dispersed in SiO2. We show that a great part-about 50%-of all Er dopants is involved in these fast processes and contributes to the submicrosecond emission.
Comparison of the Optical Properties of Graphene and Alkyl-terminated Si and Ge Quantum Dots
Full text linkSemiconductor quantum dots are widely investigated due to their size dependent energy structure.
In particular, colloidal quantum dots represent a promising nanomaterial for optoelectronic devices,
such as photodetectors and solar cells, but also luminescent markers for biotechnology, among other
applications. Ideal materials for these applications should feature efficient radiative recombination and
absorption transitions, altogether with spectral tunability over a wide range. Group IV semiconductor
quantum dots can fulfill these requirements and serve as an alternative to the commonly used direct
bandgap materials containing toxic and/or rare elements. Here, we present optical properties of butylterminated
Si and Ge quantum dots and compare them to those of graphene quantum dots, finding
them remarkably similar. We investigate their time-resolved photoluminescence emission as well as
the photoluminescence excitation and linear absorption spectra. We contemplate that their emission
characteristics indicate a (semi-) resonant activation of the emitting channel; the photoluminescence
excitation shows characteristics similar to those of a molecule. The optical density is consistent with
band-to-band absorption processes originating from core-related states. Hence, these observations
strongly indicate a different microscopic origin for absorption and radiative recombination in the three
investigated quantum dot systems
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
