[Paper Review] Enhancement of molecular coherent anti-Stokes Raman scattering with silicon nano-antennas

Shamsul Abedin, Li Yong+3 more|arXiv (Cornell University)|May 25, 2022

Gold and Silver Nanoparticles Synthesis and Applications55 references3 citations

TL;DR

This study demonstrates the first experimental enhancement of molecular coherent anti-Stokes Raman scattering (CARS) using all-dielectric silicon nano-antennas, replacing plasmonic gold nanostructures to overcome thermal instability and morphological degradation. The Si-based antennas provide a 10-fold signal enhancement with dramatically improved stability and reduced two-photon excited luminescence (TPEL) background, enabling reproducible, high-speed biomolecular sensing.

ABSTRACT

Surface-enhanced coherent anti-Stokes Raman scattering (SE-CARS) takes advantage of surface plasmon resonances supported on metallic nanostructures to amplify the coherent Raman response of target molecules. While these metallic antennas have found significant success in SE-CARS studies, photo-induced morphological changes to the nanoantenna under ultrafast excitation introduce significant hurdles in terms of stability and reproducilibty. These hurdles need to be overcome in order to establish SE-CARS as a reliable tool for rapid biomolecular sensing. Here we address this challenge by performing molecular CARS measurements enhanced by nanoantennas made from high-index dielectric particles with more favorable thermal properties. We present the first experimental demonstration of enhanced molecular CARS signals observed at Si nano-antennas, which offer much improved thermal stability compared to their metallic counterparts.

Motivation & Objective

• To address the instability and irreproducibility of surface-enhanced CARS (SE-CARS) using plasmonic metallic nanoantennas under ultrafast excitation.
• To overcome photo-induced morphological changes and local heating in metallic nanostructures that degrade signal reproducibility.
• To explore high-index dielectric materials like silicon as a stable alternative for enhancing CARS signals.
• To demonstrate improved signal stability and reduced background luminescence in all-dielectric nanojunctions compared to plasmonic gold nano-dumbbells.
• To enable reliable, high-speed biomolecular screening through stable, reproducible SE-CARS platforms.

Proposed method

• Fabrication of silicon nano-antennas using 150 nm diameter, pyridine-functionalized Si nanoparticles spin-cast on BK7 coverslips.
• Formation of nanojunctions via spontaneous particle aggregation during spin-coating, creating nanocavities for field confinement.
• Use of all-dielectric Si nano-antennas to generate strong displacement currents and confine optical fields via high refractive index and low optical loss.
• Comparison with reference gold-based nano-dumbbells (90 nm Au nanospheres with BPE linker and SiO2 shell) for signal enhancement and background assessment.
• Measurement of CARS signals using ultrafast laser excitation at 800 nm and 1040 nm, with detection of anti-Stokes signal at 420 nm.
• Quantitative analysis of signal enhancement, stability over time, and two-photon excited luminescence (TPEL) background levels.

Experimental results

Research questions

• RQ1 Can all-dielectric silicon nano-antennas provide comparable or superior CARS signal enhancement to plasmonic gold nano-antennas?
• RQ2 Does the use of silicon nano-antennas significantly reduce thermal degradation and morphological changes under ultrafast excitation compared to metallic antennas?
• RQ3 To what extent does the TPEL background in CARS measurements decrease when replacing plasmonic gold with dielectric silicon?
• RQ4 Can stable, reproducible CARS signals be achieved with dielectric nanojunctions for high-speed biomolecular sensing applications?
• RQ5 How does the signal-to-background ratio of dielectric-enhanced CARS compare to plasmonic-enhanced CARS under identical experimental conditions?

Key findings

• The study reports the first experimental demonstration of enhanced molecular CARS using all-dielectric silicon nano-antennas, achieving a signal enhancement factor of approximately 10 compared to bare substrates.
• The Si nano-antennas exhibit dramatically improved signal stability over time, with minimal degradation after repeated laser exposure, unlike plasmonic gold nano-dumbbells that show rapid signal decay.
• The two-photon excited luminescence (TPEL) background in the Si-based system is substantially reduced compared to the gold-based reference system, enabling cleaner molecular signal detection.
• The use of high-index dielectric materials like silicon enables strong optical field confinement via displacement currents and Mie resonances, without the high local heating and electron tunneling effects seen in metals.
• The measured CARS signal from Si nanojunctions remains stable over multiple laser scans, indicating robustness against photo-induced damage and morphological changes.
• The results demonstrate that dielectric nano-antennas can outperform plasmonic nanostructures in terms of signal reproducibility and long-term stability for coherent Raman sensing applications.