Chip-scale polydimethylsiloxane acousto-optic phase modulator boosts higher-resolution plasmonic comb spectroscopy

High-resolution optical spectroscopy is an essential tool in quantum optics, chemical reaction analysis, and precision metrology, as it enables detailed investigation of quantum states, energy levels, spin states, and vibrational modes in atoms and molecules. However, conventional diffraction grating-based spectrometers are limited by their large and complex optical configurations and face fundamental challenges in achieving sub-MHz spectral resolution. As an alternative, direct frequency comb spectroscopy (DFCS) based on frequency combs has gained attention due to its potential for compact, high-resolution spectral measurements. Nonetheless, the spectral resolution of DFCS is inherently constrained by the mode spacing of frequency combs, which typically ranges from tens of MHz to several GHz. Techniques such as extending the optical cavity length or employing pulse-picking methods to reduce the repetition frequency have been proposed, but these approaches often suffer from increased system complexity and limited operational stability. Thus, there is a growing need for a robust and scalable method to reduce the mode spacing of frequency combs for enhanced spectral resolution.

In this study, we introduce polydimethylsiloxane (PDMS) as a novel material platform to overcome the limitations of conventional solid-state acousto-optic phase modulators based on SiO₂or TeO₂. PDMS offers a unique combination of low acoustic velocity, high elasto-optic coefficient, and broadband optical transparency across the visible to mid-infrared range, making it well-suited for chip-scale photonic applications. Its low elastic modulus enables large refractive index variations under acoustic pressure, resulting in strong optical phase modulation. Moreover, PDMS allows for simple micro-structuring via mold casting and exhibits excellent compatibility with nanophotonic devices, including plasmonic nanostructures, enabling cost-effective and scalable fabrication of phase modulators.

Experimentally, we demonstrate that PDMS-based acousto-optic phase modulators achieve a fourfold increase in phase modulation index within the 0.2–2.0 MHz modulation frequency range while maintaining consistent performance regardless of modulation direction. Furthermore, by integrating the PDMS modulator with a Fabry–Pérot interferometer, we validate its capability to deliver sub-MHz spectral resolution, confirming the effectiveness and versatility of PDMS as a soft-matter platform for next-generation frequency comb modulation and high-resolution optical spectroscopy.

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