IPG QPM crystal delivers order-of-magnitude efficiency gain for 148 nm VUV comb at 40 μW

The team started with a 2.4 μm Cr:ZnS frequency comb and performed successive doublings to reach the 16th harmonic. The final stage used a newly developed, large-aperture, periodically poled nonlinear crystal grown at IPG that combines high χ², VUV transparency, and phase-matching uniformity over centimeters. This eliminated the need for enhancement cavities or atomic resonances while preserving the comb's narrow linewidth and 80 MHz spacing.

Power and efficiency figures were measured directly with calibrated photodiodes and a VUV spectrometer; the 40 μW output represents a concrete scaling milestone for applications such as Th-229 nuclear-clock spectroscopy and actinic inspection of EUV lithography masks. Unlike earlier demonstrations that relied on enhancement cavities or random quasi-phase matching, the new crystal supports straightforward power scaling by increasing pump intensity and aperture size without re-engineering the optical layout.

The approach also sidesteps the thermal and alignment fragility of cavity-enhanced schemes, offering a path to field-deployable or space-qualified VUV sources. Remaining engineering tasks include improving long-term crystal lifetime under high VUV fluence and reducing residual spectral sidebands below the 10^{-12} level required for clock operation.

Within two years, integration with existing Cr:ZnS oscillators already used in metrology labs could yield turnkey 148 nm combs, accelerating both fundamental tests of nuclear physics and industrial metrology tools that currently lack compact coherent sources at this wavelength.