Scalable high-repetition-rate sub-half-cycle terahertz pulses from spatially indirect interband transitions

Intense phase-locked terahertz(THz)pulses are the bedrock of THz lightwave electronics,where the carrier field creates a transient bias to control electrons on sub-cycle time scales.Key applications such as THz scanning tunnelling microscopy or electronic devices operating at optical clock rates call for ultimately short,almost unipolar waveforms,at megahertz(MHz)repetition rates.Here,we present a flexible and scalable scheme for the generation of strong phase-locked THz pulses based on shift currents in type-ll-aligned epitaxial semiconductor heterostructures.The measured THz waveforms exhibit only 0.45 optical cycles at their centre frequency within the full width at half maximum of the intensity envelope,peak fields above 1.1 kVcm^(-1) and spectral components up to the mid-infrared,at a repetition rate of 4 MHz.The only positive half-cycle of this waveform exceeds all negative half-cycles by almost four times,which is unexpected from shift currents alone.Our detailed analysis reveals that local charging dynamics induces the pronounced positive THz-emission peak as electrons and holes approach charge neutrality after separation by the optical pump pulse,also enabling ultrabroadband operation.Our unipolar emitters mark a milestone for flexibly scalable,next-generation high-repetition-rate sources of intense and strongly asymmetric electric field transients.

全球果汁和饮料——饱和市场中的小幅度发展

2014年，全球果汁和饮料产品的消费量出现了小幅度的增长。整体0．3％的增长率主要归功于饮料品类销量的增长（其增长率为1.8％），而果汁品类的增长率事实上则降低了0．8％。饮料品类的消费增长主要源于原材料价格的变化，这使得制造零售饮料的成本大幅降低，进而吸引了大量对价格敏感的消费者。亚洲和非洲市场是驱动饮料品类增长的主要动力。