Kyung Taec Kim

Applications of ultrafast wavefront rotation in highly nonlinear optics

This paper provides an overview of ultrafast wavefront rotation of femtosecond laser pulses and its various applications in highly nonlinear optics, focusing on processes that lead to the generation of high-order harmonics and attosecond pulses. In this context, wavefront rotation can be exploited in different ways, to obtain new light sources for time-resolved studies, called ‘attosecond lighthouses’, to perform time-resolved measurements of nonlinear optical processes, using ‘photonic streaking’, or to track changes in the carrier–envelope relative phase of femtosecond laser pulses. The basic principles are explained qualitatively from different points of view, the experimental evidence obtained so far is summarized, and the perspectives opened by these effects are discussed.

Controlling attosecond angular streaking with second harmonic radiation.

High harmonic generation, which produces a coherent burst of radiation every half cycle of the driving field, has been combined with ultrafast wavefront rotation to create a series of spatially separated attosecond pulses, called the attosecond lighthouse. By adding a coherent second harmonic beam with polarization parallel to the fundamental, we decrease the generating frequency from twice per optical cycle to once. The increased temporal separation increases the pulse contrast. By scanning the carrier envelope phase, we see that the signal is 2π periodic.

Exit point in the strong field ionization process

We analyze the process of strong field ionization using the Bohmian approach. This allows retention of the concept of electron trajectories. We consider the tunnelling regime of ionization. We show that, in this regime, the coordinate distribution for the ionized electron has peaks near the points in space that can be interpreted as exit points. The interval of time during which ionization occurs is marked by a quick broadening of the coordinate distribution. The concept of the exit point in the tunneling regime, which has long been assumed for the description of strong field ionization, is justified by our analysis.

Dynamic wavefront rotation in the attosecond lighthouse

Attosecond pulses propagating in different directions, generated in a rotating wavefront of a driving laser field, can provide a source of multiple isolated attosecond pulses. Clear spatial separation of the attosecond pulses is attained if the divergence of the individual attosecond pulse is smaller than their angular separation, which is limited by the bandwidth of the driving laser pulse. Here we demonstrate both experimentally and numerically that an additional wavefront rotation is imposed during the propagation of the driving laser pulse in a highly ionizing medium. This dynamic wavefront rotation enables the generation of the isolated attosecond pulse even in the case when the conditions derived from a linear diffraction theory do not permit the angular separation. The described nonlinear phenomenon has its roots in the half-cycle ionization events, and may open up new ways to study strong field processes in highly ionizing media.

Full characterization of an attosecond pulse generated using an infrared driver.

The physics of attosecond pulse generation requires using infrared driving wavelength to reach the soft X-rays. However, with longer driving wavelength, the harmonic conversion efficiency drops significantly. It makes the conventional attosecond pulse measurement using streaking very difficult due to the low photoionization cross section in the soft X-rays region. In-situ measurement was developed for precisely this purpose. We use in-situ measurement to characterize, in both space and time, an attosecond pulse produced by ultrafast wavefront rotation of a 1.8 μm fundamental beam. We confirm what models suggest – that each beamlet is an isolated attosecond pulse in the time domain. We get almost constant flat wavefront curvature through the whole photon energy range. The measurement method is scalable to the soft X-ray spectral region.

The Attosecond Lighthouse in Gas: Spatial Gating Technique for Isolated Attosecond Pulses generation

The spatially chirped laser pulses are used in high-order harmonics generation experiment to get a train of isolated attosecond pulses angularly separated—an attosecond lighthouse. Single attosecond pulse can be selected with a far-field aperture.

Transform-Limited Attosecond Pulse Generation Through Atto-Chirp Compensation by Material Dispersion

High harmonics are a unique light source with ultrashort duration and superb spatial coherence in the extreme ultraviolet and X-ray region. Though regularly spaced broad harmonic spectra are suitable for generating very short pulses, they suffer from the inherent chirp due to the harmonic generation process. Here we proposed and demonstrated the method to compensate for the attosecond chirp by propagating the harmonic pulses through a medium with negative group delay dispersion. In a single Ar gas cell, we showed that the chirp compensation could be achieved in the same harmonic generation cell, obtaining near transform-limited 206-as pulses. We could demonstrate also the generation of much shorter 63-as pulses by obtaining the harmonics from Ne and by compensating for the attosecond chirp in the second gas cell of Ar. The chirp compensation in gaseous media is, thus, very effective and powerful in obtaining near transform-limited attosecond pulses—very valuable for attosecond science.

An All-optical Characterization of the Attosecond Pulse in Space and Time

We demonstrate an all-optical spatio-temporal characterization method for the attosecond pulses produced through high harmonic generation. A spatio-temporal profile is retrieved from the spatial modulation of the harmonic spectra.

Generation and measurement of high harmonics with orbital angular momentum

We investigate high harmonics with orbital angular momentum by imparting orbital angular momentum to the driving beam. We calculate and measure the spatial profile of each harmonic and propose how to measure their phase structure.