Khuong Ba Dinh

Generation of extreme ultraviolet radiation with a Bessel–Gaussian beam

The generation of few high-order harmonics in the extreme ultraviolet is enhanced by focusing the fundamental femtosecond laser beam with a combination of lens and axicon in an infinite gas cell. We show that this combination leads to an improvement in the phase-matched generation of harmonics in the cutoff region, with a higher photon flux and a better spatial beam profile. A high atom density through the use of a high gas pressure can be used and the absorption limit of the harmonic generation is obtained for an interaction length of a few millimeters.

Wave-mixing with high-order harmonics in extreme ultraviolet region

We report studies of the wave-mixing process in the extreme ultraviolet region with two near-infrared driving and controlling pulses with incommensurate frequencies (at 1400 nm and 800 nm). A non-collinear scheme for the two beams is used in order to spatially separate and to characterise the properties of the high-order wave-mixing field. We show that the extreme ultraviolet frequency mixing can be treated by perturbative, very high-order nonlinear optics; the modification of the wave-packet of the free electron needs to be considered in this process.

High resolution coherent diffractive imaging with a table-top extreme ultraviolet source

We demonstrate a resolution of 45 nm with a sample size down to 3 μm × 3 μm is achieved in a short exposure time of 2 s, from the diffraction pattern generated by a table-top high harmonic source at around 30 nm. By using a narrow-bandwidth focusing mirror, the diffraction patterns quality is improved and the required exposure time is significantly reduced. In order to obtain a high quality of the reconstructed image, the ratio of the beam size to the sample size and the curvature of the focused beam need to be considered in the reconstruction process. This new experimental scheme is very promising for imaging sub-10 nm scale objects with a table-top source based on a small inexpensive femtosecond laser system.

Control of High Harmonic Generation with a Second Beam

We report the use of a second beam to vary the phase-matching condition of high harmonic generation in a semi- infinitive gas cell. The phase-matching can be improved or destroyed by the second beam. The enhancement of the phase-matching at high ionization rate with a second beam can be used to extend the cut-off photon energy in an atomic or molecular gas. The second beam with different carrier frequency can be used for the generation of continuous XUV spectra. In a molecular gas the second beam creates a vibrational and rotational wave packet which leads to a modulation of the HHG intensity and can be used to study the molecular structure

Coherent diffractive imaging microscope with a high-order harmonic source.

We report the generation of highly coherent extreme ultraviolet sources with wavelengths around 30 and 10 nm by phase-matched high-order harmonic generation (HHG) in a gas cell filled with argon and helium, respectively. We then perform coherent diffractive imaging (CDI) by using a focused narrow-bandwidth HHG source with wavelength around 30 nm as an illumination beam for two kinds of samples. The first is a transmission sample and the second is a absorption sample. In addition, we report the successful reconstruction of a complex absorption sample using a tabletop high-harmonic source. This will open the path to the realization of a compact soft x-ray microscope to investigate biological samples such as membrane proteins.

Phase-matched high-order harmonic generation in a semi-infinite gas cell with absorbing gaseous media

We experimentally study the pressure dependent high-order harmonic generation in a semi-infinite gas cell with different absorbing gaseous media in order to reveal the influence of absorption in the high harmonic generation (HHG) process. Theoretical fits to the experimental results allow us to measure the absorption cross-section of the harmonics and to investigate the pressure independent phase mismatches. Our findings provide a better understanding about HHG in absorbing media and show that a sharp and intense harmonic spectrum can be generated as phase-matched and absorption-limited in a long interaction length geometry.

Two color laser fields for studying the Cooper minimum with phase-matched high-order harmonic generation

We experimentally study the observation of the Cooper minimum in a semi-infinite argon-filled gas cell using two-color laser fields at wavelengths of 1400 nm and 800 nm. The experimental results show that the additional 800 nm field can change the macroscopic phase-matching condition through change of the atomic dipole phase associated with the electron in the continuum state and that this approach can be used to control the appearance of the Cooper minimum in the high-order harmonic spectrum in order to study the electronic structure of atoms and molecules.

Coherent diffractive imaging with table top high harmonic radiation at 13.5 nm

A highly coherent extreme ultraviolet source was obtained by phase-matched high-order harmonic generation in a semi-infinite gas cell filled with helium gas at ~550 Torr. The phase matching condition can be achieved for the generation of a few harmonic orders around 13.5 nm by variation of the gas pressure, focus position, and laser intensity. We demonstrate that the source is usable for coherent diffractive imaging and a spatial resolution of ~200 nm with short exposure times can be achieved.

Intensity dependent spectral features in high harmonic generation

We study the influence of the driving laser intensity on the spectral features of high harmonic generation in a semi-infinite gas cell. The effects of the harmonic dipole phase and the dispersion phase mismatches induced by the ionized medium during the harmonic generation process are revealed and the interplay between the macroscopic response and the single-atom response is discussed. We consider the conditions for generation of a narrow bandwidth, bright, and highly coherent high harmonic source.

High order harmonic generation for study of atomic and molecular dynamics

We present a pump-probe experiment based on the use of a second electrical field to modulate the intensity and the spatial profile of the phase-matched high-order harmonics radiation generated by the first strong driving field. The modification is caused by the variation of the phase-matching condition, e.g., change of electron trajectory and the effect of field gradient forces on the electron that reflect the dynamics of atomic and molecular gases. Atomic argon and molecular oxygen gases are used to demonstrate the feasibility of the technique.