# High-order harmonic generation in an organic molecular crystal

## TL;DR

Researchers showed that high-order harmonic generation can be used to study the electronic structure of organic molecular crystals like pentacene, revealing how neighboring molecules affect the process.

## Contribution

The study demonstrates HHG up to the 17th order in pentacene, a new class of organic molecular crystals.

## Key findings

- Pentacene crystals can sustain laser intensities for efficient HHG up to the 17th order.
- Higher harmonic orders are sensitive to both nearest- and next-nearest-neighbor molecular couplings.
- Weaker intermolecular interactions require higher harmonic orders to resolve crystal structure.

## 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.

High-harmonic generation (HHG) in organic molecular crystals was previously demonstrated only in the non-perturbative regime. Here, the authors demonstrate HHG up to the 17th order in pentacene crystals, with a strong orientational dependence caused by the neighbouring molecules.