High-order femtosecond vortices up to the 30th order generated from a powerful mode-locked Hermite-Gaussian laser

Femtosecond vortex beams are of great scientific and practical interest because of their unique phase properties in both the longitudinal and transverse modes,enabling multi-dimensional quantum control of light fields.Until now,generating femtosecond vortex beams for applications that simultaneously require ultrashort pulse duration,high power,high vortex order,and a low cost and compact laser source has been very challenging due to the limitations of available generation methods.Here,we present a compact apparatus that generates powerful high-order femtosecond vortex pulses via astigmatic mode conversion from a mode-locked Hermite-Gaussian Yb:KGW laser oscillator in a hybrid scheme using both the translation-based off-axis pumping and the angle-based non-collinear pumping techniques.This hybrid scheme enables the generation of femtosecond vortices with a continuously tunable vortex order from the 1st up to the 30th order,which is the highest order obtained from any femtosecond vortex laser source based on a mode-locked oscillator.The average powers and pulse durations of all resulting vortex pulses are several hundred milliwatts and<650 fs,respectively.In particular,424-fs 11th-order vortex pulses have been achieved with an average power of 1.6 W,several times more powerful than state-of-the-art oscillator-based femtosecond vortex sources.

Complete characterization of ultrafast optical fields by phase-preserving nonlinear autocorrelation

Nonlinear autocorrelation was one of the earliest and simplest tools for obtaining partial temporal information about an ultrashort optical pulse by gating it with itself.However,since the spectral phase is lost in a conventional autocorrelation measurement,it is insufficient for a full characterization of an ultrafast electric field,requiring additional spectral information for phase retrieval.Here,we show that introducing an intensity asymmetry into a conventional nonlinear interferometric autocorrelation preserves some spectral phase information within the autocorrelation signal,which enables the full reconstruction of the original electric field,including the direction of time,using only a spectrally integrating detector.We call this technique Phase-Enabled Nonlinear Gating with Unbalanced Intensity(PENGUIN).It can be applied to almost any existing nonlinear interferometric autocorrelator,making it capable of complete optical field characterization and thus providing an inexpensive and less complex alternative to methods relying on spectral measurements,such as frequency-resolved optical gating(FROG)or spectral phase interferometry for direct electric-field reconstruction(SPIDER).More importantly,PENGUIN allows the precise characterization of ultrafast fields in non-radiative(e.g.,plasmonic)nonlinear optical interactions where spectral information is inaccessible.We demonstrate this novel technique through simulations and experimentally by measuring the electric field of~6-fs laser pulses from a Ti:sapphire oscillator.The results are validated by comparison with the well-established FROG method.