Probing multiple molecular orbitals in an orthogonally polarized two-color laser field

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 moment is induced by the returning electron and the parent ion, emitting the harmonic radiation after the periodic repetition of the process. Characteristics of the bound electron, thus, can be imprinted in the high-harmonic radiation. The highest occupied molecular orbital (HOMO) is mostly ionized in a strong laser field and reveals its characteristics dominantly. Energetically low-lying molecular orbitals referred to HOMO-1 and HOMO-2 also contribute to the radiation. Multi-orbital contributions to the radiation distort proper information on a specific orbital. Resolving the contribution of each orbital is, thus, crucial for understanding molecular dynamics. By applying an orthogonally polarized two-color laser field that consists of the fundamental and its second-harmonic field, we show that high-harmonic radiation emitted from the two highest-occupied molecular orbitals, HOMO and HOMO-1, of aligned molecules can be resolved. The characteristics attributed to the two orbitals are found to be separately imprinted in odd and even harmonics. Two-dimensional high-harmonic spectroscopy using orthogonal odd and even harmonics may enable us to observe multi-orbital dynamics during chemical reactions. © Springer International Publishing AG 2017

Study of quantum-path interferences in the high harmonic generation process

High Harmonic generation can be used as a probe of the emitting medium with attosecond and Angström resolutions. We show that polarization-resolved pump-probe spectroscopy with high harmonics improves the detection sensitivity of rotationally excited molecules

Solid-state harmonics beyond the atomic limit

Strong-field laser excitation of solids can produce extremely nonlinear electronic and optical behaviour. As recently demonstrated, this includes the generation of high harmonics extending into the vacuum-ultraviolet and extreme-ultraviolet regions of the electromagnetic spectrum. High harmonic generation is shown to occur fundamentally differently in solids and in dilute atomic gases. How the microscopic mechanisms in the solid and the gas differ remains a topic of intense debate. Here we report a direct comparison of high harmonic generation in the solid and gas phases of argon and krypton. Owing to the weak van der Waals interaction, rare (noble)-gas solids are a near-ideal medium in which to study the role of high density and periodicity in the generation process. We find that the high harmonic generation spectra from the rare-gas solids exhibit multiple plateaus extending well beyond the atomic limit of the corresponding gas-phase harmonics measured under similar conditions. The appearance of multiple plateaus indicates strong interband couplings involving multiple single-particle bands. We also compare the dependence of the solid and gas harmonic yield on laser ellipticity and find that they are similar, suggesting the importance of electron-hole recollision in these solids. This implies that gas-phase methods such as polarization gating for attosecond pulse generation and orbital tomography could be realized in solids