Attosecond relative delay measurement using transient-grating frequency-resolved optical grating
Authors: Huang Pei<sup>1\2\3</sup>  , Fang Shao-Bo  , Huang Hang-Dong  , Zhao Kun  , Teng Hao  , Hou Xun  , Wei Zhi-Yi<sup>2\3</sup>  ,   State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics of Chinese Academy of Sciences, Xi'an 710119, China;,   Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

The accurate and precise controlling of the attosecond time delay between the sub-pulses within a hundredth of an optical cycle is the key ingredient for the sophisticated custom-tailored coherent waveform synthesizer. The attosecond delay control technique commonly experiences the “complete” characterization of the ultrashort sub-cycle pulses, which includes the spatiotemporal pulse characterization of the synthesized waveform and the attosecond relative delay between the parent pulses. In this work, the relative time delay between spectrally separated ultrashort parent pulses is characterized in an interferometer scheme with a background-free transient-grating frequency-resolved optical grating (TG-FROG). The TG-FROG geometry accurately measures the full time-dependent intensity and phase of ultrashort laser pulses in a wide range of regime (from ultraviolet to infrared) and offers significant advantages over other nonlinear-optical processes geometries (i.e., the polarization-gate-FROG, the self-diffraction-FROG, the second-harmonic generation-FROG and the third-harmonic-generation-FROG). The attosecond measurement accuracy is achieved for the first time, to the best of our knowledge. In this experiment, the output of a carrier-envelope-phase-stable Ti:sapphire amplifier (sub-30-fs, over-1-mJ, 1 kHz) is spectrally broadened in a neon-filled hollow-core fiber with an inner diameter of 250μm. The transmission through the pressure-gradient hollow-core fiber results in an mJ-level octave-spanning whitelight supercontinuum, supporting a sub-3-fs Fourier transform-limited pulse. The supercontinuum is spectrally divided into two parent pulses by using a dichroic mirror. The sub-pulses are individually compressed by the custom-designed double-chirped mirrors and wedge pairs. The short and long wavelength pulses are separately compressed in few-cycle regime, yielding pulses with 6.7 fs and 9.8 fs, respectively. This technique overcomes the bottlenecks in the traditional delay measurement and should be applicable for many ultra-broadband pulse characterizations with extremely simple and alignment-free delay control device used. Furthermore, this new method will be easily adapted for the ultra-broadband two-dimensional electronic spectroscopy, the advanced temporal cloaking, and the field of sub-cycle arbitrary coherent waveform synthesizer for controlling strong-field interactions in atoms, molecules, solids, and nanostructures. We foresee that in the near future this novel technology will be very attractive for various applications in the next-generation light sources such as the Synergetic Extreme Condition User Facility in Beijing, China.