Kyung Taec Kim

Gwangju Institute of Science and Technology · Department of Physics and Photon Sciences

We demonstrate a novel pulse shaper in which an incident laser beam is angularly dispersed by a first prism, and then it is split into separate beams using multiple prisms. Since this new pulse shaper offers independent control of the amplitude and phase of the separate beams, it can produce pulses having desired temporal shapes. Furthermore, it im...

High-harmonic radiation can be generated when an ultra-intense laser beam is reflected from an over-dense plasma, known as a plasma mirror. It is considered a promising technique for generating intense attosecond pulses in the extreme ultraviolet and X-ray wavelength ranges. However, a solid target used for the formation of the over-dense plasma is...

An electron tunneled out in an intense laser field can be driven back and rescattered by the parent ion, leading to rich physical phenomena. Recently, it has been shown that the streaking measurement of the rescattered electron is capable of the in situ characterization of the laser field. Here, we demonstrate that the streaking of the rescattered...

Coherent extreme ultraviolet (EUV) line emissions can be generated from an atom excited through frustrated tunneling ionization (FTI). The phase variation of the EUV emission in a generation medium along the propagation direction is a critical parameter that determines the phase-matching condition of this new light source. Here we show that the EUV...

: The superposition of a fundamental laser pulse and its second harmonic can form an asymmetric laser field that is useful in many applications. The temporal characterization of the two-color laser field becomes necessary. However, the temporal characterization of the two-color laser pulse is a challenging task due to its broad bandwidth and a spec...

We present a method to estimate the backward rescattering time of a tunnel-ionized electron in an intense laser pulse. The idea of the classical backpropagation method is applied to determine the backward rescattering time and its dependence on various parameters such as the ionization potential of the parent ion, its spatial profile and the peak i...

We present a reconstruction algorithm developed for the temporal characterization method called tunneling ionization with a perturbation for the time-domain observation of an electric field (TIPTOE). The reconstruction algorithm considers the high-order contribution of an additional laser pulse to ionization, enabling the use of an intense addition...

When an atom or molecule is exposed to a strong laser field, an electron can tunnel out from the parent ion and moves along a specific trajectory. This ultrafast electron motion is sensitive to a variation of the laser field. Thus, it can be used as a fast temporal gate for the temporal characterization of the laser field. Here, we demonstrate a ne...

Output pulses of a 100 kHz mid-infrared OPCPA system are post-compressed from 4.7 cycles down to 2.3 cycles by using a combination of a dielectric and a semiconductor crystal in a hybrid thin plate setup. Efficient spectral broadening is demonstrated with 11 W average input power. After compression the output power reached 6.8 W with exceptional CE...

To generalize the applicability of the temporal characterization technique called “tunneling ionization with a perturbation for the time-domain observation of an electric field” (TIPTOE), the technique is examined in the multicycle regime over a broad wavelength range, from the UV to the IR range. The technique is rigorously analyzed first by solvi...

We describe an approximate solution to the Heisenberg operator equations of motion for an atom in a laser field. The solution is based on a quantum generalization of the physical picture given by the well-known Simple Man Model (SMM). We provide justification of the plausibility of this generalization and test its validity by applying it for the ca...

We apply information theoretic entropies of coordinate and velocity distributions in quantum mechanics for the description of the strong field ionization process. The approach is based on the properties of the entropies used in the information theory, viz., their ability to gauge the 'distance' between the probability distributions and thus to be s...

A single-cycle laser pulse was generated using a two-stage compressor and characterized using a pulse characterization technique based on tunnelling ionization. A 25-fs, 800-nm laser pulse was compressed to 5.5 fs using a gas-filled hollow-core fibre and a set of chirped mirrors. The laser pulse was further compressed, down to the single-cycle limi...

We describe an approach based on the view of the well-known strong-field approximation (SFA) as an evolution equation. From this point of view the SFA is a nonhomogeneous evolution equation with the inhomogeneous term which determines the departure of the approximate evolution driven by the SFA from the evolution driven by the exact ab initio time-...

An atom can be excited through tunneling when it is exposed to a strong laser field. This excitation process is known as frustrated tunneling ionization (FTI). Recently it has been reported that coherent extreme-ultraviolet emission can be achieved through FTI. In the present work we extend the strong-field-approximation (SFA) model developed by Le...

We apply information theoretic entropies of coordinate and velocity distributions in quantum mechanics for the description of the strong field ionization process. The approach is based on the properties of the entropies used in the information theory, viz., their ability to gauge the "distance" between the probability distributions and thus to be s...

The notion of the instantaneous ionization rate (IIR) is often employed in the literature for understanding the process of strong field ionization of atoms and molecules. This notion is based on the idea of the ionization event occurring at a given moment of time, which is difficult to reconcile with the conventional quantum mechanics. We describe...

Coherent extreme-ultraviolet emission can be obtained through high-harmonic generation and multiphoton excitation from atoms exposed to a strong laser field. We report the generation of a new kind of coherent extreme-ultraviolet emission from He atoms excited by intense few-cycle laser pulses. An atom can be excited after tunnelling in a strong las...

We present experimental results on the strong-field ionization of Ar atoms in the intensity range of 10^14 -10^15 W/cm^2 to investigate the relativistic effects. We compare these measurements to a theoretical model based on the time-dependent Dirac equation (3D-TDDE). This is the first theoretical study of the non-dipole effects in this regime, usi...

Temporal characterization of a laser pulse is an essential task in many applications. Temporal characterization methods that are currently available support only a limited spectral bandwidth without information on the carrier-envelope phase (CEP) of the laser pulse or require complicated equipment in a vacuum environment. Here we demonstrate that a...

We describe an approach defining instantaneous ionization rate (IIR) as a functional derivative of the total ionization probability. The definition is based on physical quantities which are directly measurable, such as the total ionization probability and the waveform of the pulse. The definition is, therefore, unambiguous and does not suffer from...

High harmonic light sources with attosecond duration in the EUV and soft X-ray spectral range can be applied to probe ultrafast dynamics of atoms and molecules. By tuning harmonic wavelength, He atoms can be excited to the 1s3p state that can be further ionized by a time-delayed infrared femtosecond laser pulse. The interference, between the direct...

We present a study of the photoelectron spectra for the ionization process driven by counterrotating and corotating circularly polarized fundamental and odd-order harmonic fields. The main features of the spectra, such as symmetric lobed structures, are understood using simple arguments based on the strong field approximation (SFA) picture of ioniz...

An arbitrary optical waveform can be measured using the technique called the 'petahertz optical oscilloscope' in which an electron trajectory in the process of high harmonic generation is used as a fast temporal gate. In the petahertz optical oscilloscope, a signal laser pulse that needs to be measured is superposed on a fundamental laser pulse tha...

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...

This paper proposes and verifies methods to improve the isolation of the attosecond pulses generated via the attosecond lighthouse technique in gases. We find that the converging fundamental field can compensate for the inherent diverging wavefront of the attosecond pulses, but is only valid over a limited portion of the spectrum and sacrifices the...

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. Th...

High-harmonic radiation emitted from gaseous molecules is a coherent extreme ultraviolet (EUV) radiation that carries information on electronic structure and dynamics of the molecule. High-harmonics are generated when an electron, ionized and accelerated in a strong laser field, recombines with the parent ion. During the recombination, a dipole mom...

We present a study of the relativistic nondipole effects in strong-field tunneling ionization process driven by a linearly polarized laser pulse. We consider the role of these effects in breaking the symmetry with respect to the inversion of the momentum component perpendicular to the laser-field polarization direction, which the dipole differentia...

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 sof...

We show that the Gabor transform provides a convenient tool allowing one to study the origin of the low-energy structures (LES) in the process of the strong-field ionization. The classical trajectories associated with the stationary points of the Gabor transform enable us to explicate the role of the forward scattering process in forming LES. Our a...

An arbitrary optical waveform can be measured using a perturbative approach known as the petahertz optical oscilloscope, in which an electron trajectory in the process of high-order-harmonic generation is used as a temporal gate. Here, we report that the petahertz optical oscilloscope exhibits a significant frequency dependence for the short wavele...

The response of electrons in atoms to ultrashort optical light pulses has been probed by measuring the ultraviolet light emitted by the atoms. This reveals that a finite time delay occurs before the response.

The attosecond lighthouse is a method of using ultrafast wavefront rotation with high-harmonic generation to create a series of coherent, spatially separated attosecond pulses. Previously, temporal measurements by photoelectron streaking characterized isolated attosecond pulses created by manipulating the single-atom response. The attosecond lighth...

We report a study of the effect of relativity on the tunneling time for the process of the tunneling ionization. Our results indicate that the relativistic effects make the tunneling time nonzero. A time interval of a few attoseconds is required for the electron to traverse the barrier. This finding agrees with the basic postulate of the special re...

We examine the instantaneous ionization amplitudes and instantaneous ionization rates for the process of tunneling ionization. We show that the endpoint contribution usually neglected in the asymptotic evaluation of the amplitudes, may be significant. For weak fields the instantaneous ionization rate is largely defined by this contribution. For hig...

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 d...

High-harmonic radiation emitted from molecules in a strong laser field contains information on molecular structure and dynamics. When multiple molecular orbitals participate in high-harmonic generation, resolving the contribution of each orbital is crucial for understanding molecular dynamics and for extending high-harmonic spectroscopy to more com...

A beam with an angular-dependant phase Φ=ℓϕ about the beam axis carries an orbital angular momentum of ℓℏ per photon. Such beams are exploited to provide superresolution in microscopy. Creating extreme ultraviolet or soft-x-ray beams with controllable orbital angular momentum is a critical step towards extending superresolution to much higher spati...

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...

Sources of attosecond-duration light pulses provide the fastest time resolutions available today for observing ultrafast phenomena in atoms, molecules and condensed matter. The measurement of such pulse durations is challenging because the spectrum lies in the vacuum ultraviolet or soft X-ray range. Two classes of pulse duration measurements now ex...

The time-dependent field of an electromagnetic pulse can be measured if there is a fast enough gate. For terahertz radiation, femtosecond photoinjection of free carriers into a semiconductor in the presence of the terahertz radiation can serve as the gate1. For visible or infrared radiation, attosecond photoionization of a gas target in the presenc...

High harmonic generation in gas targets leads to the production of attosecond pulses. The process of high harmonic generation requires that the gas be ionized by an intense femtosecond laser field. The highest photon energy produced is related to the laser intensity times the wavelength squared. This cutoff is reached only if good phase matching is...

We demonstrate an all-optical technique that measures instantaneous field of unknown optical pulses. We show that the field can be imprinted onto the deflection of the attosecond XUV pulse.

High harmonic radiation, produced when intense laser pulses interact with matter, is composed of a train of attosecond pulses. Individual pulses in this train carry information on ultrafast dynamics that vary from one half-optical-cycle to the next. Here, we demonstrate an all-optical photonic streaking measurement that provides direct experimental...

Attosecond extreme-ultraviolet pulses have a complex space-time structure. However, at present, there is no method to observe this intricate detail; all measurements of the duration of attosecond pulses are, to some extent, spatially averaged. A technique for determining the full space-time structure would enable a detailed study of the highly nonl...

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 me...

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.

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.

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.

We present a novel spatio-temporal characterization method for the attosecond pulses produced through high harmonic generation. A spatio-temporal profile is reconstructed from the spatial modulation of the harmonic spectra in the far-field

The duration of attosecond pulses, produced through high harmonic generation from gaseous atoms, was found to be much longer than the transform-limited duration. The inherent attosecond chirp existing in the temporal structure of the attosecond pulses was analysed to obtain a scaling factor in terms of laser intensity and wavelength. We explored th...

Ultrafast atomic processes, such as excitation and ionization occurring on the femtosecond or shorter time scale, were explored by employing attosecond high-harmonic pulses. With the absorption of a suitable high-harmonic photon a He atom was ionized, or resonantly excited with further ionization by absorbing a number of infrared photons. The elect...

We present a novel spatio-temporal characterization method for the attosecond pulses produced through high harmonic generation. A spatio-temporal profile is reconstructed from the spatial modulation of the harmonic spectra in the far-field.

Attosecond high-harmonic pulses obtained from Ar were characterized by the two methods — RABITT and FROG CRAB. The comparison of the two results revealed the capabilities of the two methods well.

Near transform-limited 63-as high-harmonic pulses were successfully generated by compensating positive attosecond chirp of harmonics using negative group delay dispersion of Ar in broad spectral region. Without proper chirp compensation, the harmonic pulses were split into multiple peaks due to the large phase variation, in addition to the broadeni...

The method of complete reconstruction of attosecond bursts has been demonstrated for attosecond high-harmonic pulse trains. The retrieved harmonic field provided detailed information about the envelope and the individual attosecond pulses contained in the attosecond pulse train. The time–frequency analysis revealed complicated spectral chirp struct...

By generating broadband high-harmonic pulses from neon and compensating for attosecond chirp by the material dispersion of argon, the generation of near transform-limited 63 as pulses was achieved. The spectral phase analysis showed that, without proper compensation, the attosecond chirp of the broadband harmonics caused splitting of attosecond hig...

A method to obtain near trans form-limited attosecond harmonic pulses is presented along with techniques to characterize attosecond pulses, especially complete temporal reconstruction of attosecond pulses based on the frequency-resolved optical gating algorithm.

High-harmonic-seeded x-ray laser became an important issue in x-ray laser development due to the possibility to obtain a highly coherent and polarized soft x-ray source. We performed theoretical investigations into amplification of high harmonic pulses in an x-ray lasing medium by using a model based on Maxwell-Bloch equations. From the theoretical...

The development in the field of X-ray laser and its applications at c-FAST is discussed in detail. First of all the recent progress in understanding of seeding X-ray laser with harmanics is presented. On the other hand, analysis of the kinetic processes by numerical simulations shows their dependence on the pump laser pulse structure. The latter cl...

The characteristics of attosecond-pulse trains (APT) obtained from high-order harmonics are investigated by using a nonadiabatic three-dimensional model. A time-dependent phase matching approach is used in order to analyze the macroscopic formation of the APT. Under high ionization conditions, the process of APT formation is found to be the result...

Self-compression of attosecond high-order harmonic pulses in the harmonic generation medium itself has been demonstrated. The attosecond pulses were generated in an argon-filled gas cell and compressed by exploiting the dispersion characteristics of argon. Since the harmonic generation medium itself was used as the compression medium, continuous ch...

The intrinsic chirp of attosecond harmonic pulse obtained from Ar was compensated by taking into account the material dispersion of the harmonic generation medium itself. The measured attosecond pulse duration was very close to the transform-limited value.

For complete temporal characterization of an attosecond harmonic pulse train the frequency-resolved optical gating technique is applied to the photoelectron spectra of He driven by high harmonic pulses and IR laser pulses with time delay, which shows the detailed temporal information of attosecond pulses in the attosecond pulse train.

The attosecond high harmonic pulses obtained from a long Ar-filled gas cell were characterized by two techniques - the reconstruction of attosecond beating by interference of two-photon transition (RABITT) and frequency-resolved optical gating (FROG) methods. The pulse durations obtained by RABITT and FROG methods agreed within 10 %.

Excitation to an autoionization level in O2 is achieved using a high harmonic light source. The 11th harmonic at 72.7 nm from Kr gas excites O2 in the ground state to the autoionizing level, and the dynamic behavior is investigated by laser-field-assisted photoionization with a pump-probe scheme.

Techniques to characterize attosecond harmonic pulses based on the cross-correlation between harmonic and IR laser pulses are presented. The selfcompression of an attosecond harmonic pulse in the harmonic generation itself has been demonstrated, obtaining near transform-limited attosecond harmonic pulses.

Autoionization dynamics in O2 is investigated using laser-field-assisted extreme ultraviolet (XUV) photoionization. The photoelectron spectrum is retrieved theoretically and the dynamic behaviors for three categories of photoelectron peaks are studied.

Photoionization of oxygen molecules by high-order harmonics was investigated. High-order harmonics of a Ti:sapphire laser produced in a Kr gas cell were used to excite autoionization states of O 2. Since the high-order harmonic source used contains several harmonic orders, the resulting photoelectron spectrum also showed multiple peaks coming from...

XUV harmonics are used to study the photoelectron spectrum of O₂. The autoionization states of O₂ are further investigated using XUV harmonic as pump and IR laser as probe.