High-Order Harmonic Generation in Organic Molecular Crystals

TL;DR

This paper demonstrates high-order harmonic generation in organic molecular crystals, specifically pentacene, revealing how harmonic yields depend on molecular interactions and offering a new optical method to probe their electronic structures.

Contribution

The study experimentally shows HHG in organic molecular crystals and analyzes how intermolecular interactions influence harmonic generation, providing a novel approach for electronic structure probing.

Findings

HHG observed up to the 17th order in pentacene crystals

Harmonic yield depends strongly on laser polarization and intermolecular interactions

Higher harmonic orders are needed to resolve weaker intermolecular couplings

Abstract

High-order harmonic generation (HHG) is a powerful tool for probing electronic structure and ultrafast dynamics in matter. Traditionally studied in atomic and molecular gases, HHG has recently been extended to condensed matter, enabling all-optical investigations of electronic and crystal structures. Here, we experimentally demonstrate HHG in a new class of materials: thin organic molecular crystals with perfectly aligned molecules, using pentacene as a model system. Organic molecular crystals, characterized by weak intermolecular coupling, flat electronic bands, and large unit cells, differ fundamentally from conventional covalent or ionic crystals and have attracted significant interest as promising candidates for organic electronics. We show that pentacene crystals endure laser intensities sufficient for efficient HHG up to the 17th order. The harmonic yield as a function of laser polarization reveals a strong dependence on intermolecular interactions, with higher harmonic orders particularly sensitive to both nearest- and next-nearest-neighbor couplings. Model calculations indicate that weaker intermolecular interactions necessitate probing with higher harmonic orders to resolve the crystal structure. These findings suggest that HHG may serve as a powerful tool for probing the electronic structure of organic molecular crystals, enhancing all-optical techniques for studying electronic properties and ultrafast dynamics in complex organic materials.