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Extremely efficient terahertz high-harmonic generation in graphene by hot Dirac fermions

Multiple optical harmonic generation-the multiplication of photon energy as a result of nonlinear interaction between light and matter-is a key technology in modern electronics and optoelectronics, because it allows the conversion of optical or electronic signals into signals with much higher frequency, and the generation of frequency combs. Owing to the unique electronic band structure of graphene, which features massless Dirac fermions(1-3), it has been repeatedly predicted that optical harmonic generation in graphene should be particularly efficient at the technologically important terahertz frequencies(4-6). However, these predictions have yet to be confirmed experimentally under technologically relevant operation conditions. Here we report the generation of terahertz harmonics up to the seventh order in single-layer graphene at room temperature and under ambient conditions, driven by terahertz fields of only tens of kilovolts per centimetre, and with field conversion efficiencies in excess of 10(-3), 10(-4) and 10(-5) for the third, fifth and seventh terahertz harmonics, respectively. These conversion efficiencies are remarkably high, given that the electromagnetic interaction occurs in a single atomic layer. The key to such extremely efficient generation of terahertz high harmonics in graphene is the collective thermal response of its background Dirac electrons to the driving terahertz fields. The terahertz harmonics, generated via hot Dirac fermion dynamics, were observed directly in the time domain as electromagnetic field oscillations at these newly synthesized higher frequencies. The effective nonlinear optical coefficients of graphene for the third, fifth and seventh harmonics exceed the respective nonlinear coefficients of typical solids by 7-18 orders of magnitude(7-9). Our results provide a direct pathway to highly efficient terahertz frequency synthesis using the present generation of graphene electronics, which operate at much lower fundamental frequencies of only a few hundreds of gigahertz

Nutrition in relation to organic aquaculture: Sources and strategies

Organic production is a system of farm management and food production that combines best environmental practices, a high level of biodiversity, the preservation of natural resources, the application of high animal welfare standards and a production method in line with the preference of certain consumers for products produced using natural substances and processes. Mie et al. (2017) reviewed existing evidence on the impact of organic food on human health and compared organic versus conventional food production with respect to parameters important to human health. The review emphasised several documented human health benefits associated with organic food production and production methods and concluded that it is likely to be beneficial within the conventional agriculture, for example, in integrated pest management and antibiotics. This chapter covers aspects of current use of formulated feeds, feed composition, aquafeed technology, sustainable alternatives to common feed ingredients, nutritional physiology and general nutritional principles and product quality in the context of the organic aquaculture. It reviews new knowledge and presents research results to update and may modify the criteria and standards for organic aquaculture in relation to nutrition and thus to provide high-quality products for the consumers. This chapter is based on the current European regulation on organic aquaculture, as well as on the proposed revision of the European regulation, which is currently being approved after a long process for getting the agreement of the European Parliament, European Council and the European Commission