SC439

Since the invention of lasers in 1960, various techniques such as mode-locking have been developed to reduce the duration of visible and infrared light pulses down to femtoseconds. The generation of attosecond pulses requires novel methods to produce broadband coherent electromagnetic waves in the UV to X-ray range. The discovery of high-order harmonic generation (HHG) in high intensity laser-atom interaction at the end of 1980s paved the way. In 2001, attosecond light pulses, a train of attosecond bursts or single isolated attosecond pulses, were measured for the first time. The temporal characterization was accomplished by first converting the attosecond photons to photoelectrons in a combination of weak extreme ultraviolet and strong infrared fields, and then retrieve the spectral phase of the attosecond pulse by reconstructing the photoelectron spectrum. Since then, various sub-optical-cycle gating schemes such as polarization gating and Double Optical Gating have been demonstrated to generation isolated attosecond pulses in the extreme ultraviolet region.  The new frontier in attosecond optics research is to extend the spectrum to water window (282 to 533 eV) X-rays, which requires high power femtosecond mid-infrared (MIR) lasers to drive high harmonic generation. This course covers: (1) Fundamentals of ultrafast optics.  (2) MIR Chirped Pulse Amplifier and Optical Parametric Chirped Pulse Amplifier. (3) Single-atom model of HHG.  (4) Phase-matching of HHG in partially ionized media. (5) Sub-cycle gating and attosecond pulse characterization. (6) Element-specific attosecond transient absorption spectroscopy in the water window.

This course should enable participants to understand how ultrafast laser pulses are described, generated and characterized. They will be able to specify parameters of femtosecond MIR driving lasers that are critical to the generation of single isolated attosecond X-ray pulses,  explain the principle and techniques of locking the carrier-envelope offset frequency of femtosecond oscillators and carrier-envelope phase of amplified pulses, define short and long trajectories in the attosecond generation process using the Strong Field Approximation in the Lewenstein model, estimate the cutoff photon energy and attosecond chirp using the semi-classical model, calculate ionization probability of atoms in an intense laser field with the Ammosov-Delone-Krainov (ADK) tunneling rate, describe the principle of attosecond streak camera for characterizing attosecond pulses, as well as identify the major factors that affects the phase matching of high harmonic generation in partially ionized media.

This short course targets senior undergraduate students, graduate students, postdoc fellows, scientists and engineers seeking to enter attosecond optics. The audience should have studied electromagnetism, optics, lasers, quantum mechanics and atomic physics at undergraduate or graduate levels. Prior knowledge of femtosecond lasers is required. Although basic theory is covered, it emphasizes on experimental aspects of attosecond optics, such as developing MIR driving lasers and designing grating spectrometers for attosecond transient absorption spectroscopy.

Zenghu Chang is a Trustee Chair and Distinguished Professor at the University of Central Florida, where he directs the Institute for the Frontier of Attosecond Science and Technology. His group generated 53-as X-ray pulses at the carbon K-edge. He is the author of the book “Fundamentals of Attosecond Optics.”