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.

Multi-mW,few-cycle mid-infrared continuum spanning from 500 to 2250cm^(−1)

The demand for and usage of broadband coherent mid-infrared sources,such as those provided by synchrotron facilities,are growing.Since most organic molecules exhibit characteristic vibrational modes in the wavelength range between 500 and 4000 cm^(−1),such broadband coherent sources enable micro-or even nano-spectroscopic applications at or below the diffraction limit with a high signal-to-noise ratio1–3.These techniques have been applied in diverse fields ranging from life sciences,material analysis,and time-resolved spectroscopy.Here we demonstrate a broadband,coherent and intrinsically carrier-envelope-phase-stable source with a spectrum spanning from 500 to 2250 cm^(−1)(−30 dB)at an average power of 24mW and a repetition rate of 77 MHz.This performance is enabled by the first mode-locked thin-disk oscillator operating at 2μm wavelength,providing a tenfold increase in average power over femtosecond oscillators previously demonstrated in this wavelength range4.Multi-octave spectral coverage from this compact and power-scalable system opens up a range of time-and frequency-domain spectroscopic applications.