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 scanni...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.展开更多
基金We thank M.Furthmeier,I.Laepple and I.Gronwald for assistance.Financial support from the Deutsche Forschungsgemeinschaft(DFG,German Research Foundation)through Project ID 422314695032-SFB 1277(Subprojects A01 and B02)Project ID 223848855-SFB 1083(Subproject B2)as well as Research Grant HU1598/8 and Research Grant CH660/8 is acknowledgedThe work in Ann Arbor was supported by the W.M.Keck Foundation,and through NSF DMREF award#2118809.
文摘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.
