Speaker
Description
High harmonic generation (HHG) – the signature of strong field physics – has been demonstrated in transparent solids. The underlying mechanisms in solids are commonly classified as interband emission, arising from laser-driven electron–hole recombination, and intraband emission, originating from nonlinear carrier motion within non-parabolic bands. In contrast, HHG from metals has remained unexplored.
In metals, due to the poor penetration of light, the electrons experience a much weaker field than the incident laser field. Hence, HHG from metals has remained largely unexplored. Recently, observation of HHG in plasmonic titanium nitride (TiN) suggested intraband-dominated emission occurs at laser intensities close to the damage threshold, opening the question of whether and how harmonics can be generated in conventional metals. Here, we report vacuum ultraviolet HHG from epitaxial thin films and bulk single-crystal silver, a noble metal.
In the experiment, few-cycle near-infrared (NIR) pulses centered at 780 nm are focused onto 65 nm thick epitaxial Ag films in reflection geometry, and the emitted harmonics are detected in the far field. Bright harmonics up to the 11th order, corresponding to photon energies near 19 eV, are observed. Under identical illumination conditions, films deposited on silicon withstand intensities of 30 TW/cm², surpassing the damage threshold of TiN before lattice modification occurs. Our theoretical analysis indicates that, at near-damage intensity, the dominant contribution to harmonic emission in silver arises from transitions involving d-electrons.
We further investigate polarization and crystal orientation effects. Using circularly polarized NIR field, efficient harmonics are generated in accordance with space–time symmetry selection rules, and the harmonic yield exhibits a six-fold rotational symmetry reflecting the Ag (111) crystal structure.
These results demonstrate that HHG is a universal response of metals driven near their damage threshold and extend ultrafast strong-field spectroscopy into the metallic regime, where lattice order and plasma formation intersect. HHG from metals is promising in the development of integrated VUV photonics. Metals pumped with conventional laser sources can now fill all roles from the source of the VUV to heat sinking the nonlinear material to a thermal bath.
| Keyword-1 | Attosecond science in solids |
|---|---|
| Keyword-2 | Strong field Physics |
| Keyword-3 | High harmonic generation |