[Paper Review] Synthesized complex-frequency excitation for ultrasensitive molecular sensing

Kebo Zeng, Chenchen Wu+8 more|arXiv (Cornell University)|Jul 18, 2023

Plasmonic and Surface Plasmon ResearchEngineering3 citations

TL;DR

This paper introduces synthesized complex-frequency excitation (CFE) to dramatically enhance molecular sensing sensitivity in graphene-based surface-enhanced infrared absorption (SEIRA) sensors. By engineering tailored complex-frequency waves, the method amplifies molecular signals by at least an order of magnitude—demonstrated on a 1.2-nm-thick silk protein layer—enabling ultrasensitive, scalable detection across different molecular phases with broad applicability in spectroscopy and biomedicine.

ABSTRACT

Detecting trace molecules remains a significant challenge. Surface-enhanced infrared absorption (SEIRA) based on plasmonic nanostructures, particularly graphene, has emerged as a promising approach to enhance sensing sensitivity. While graphene-based SEIRA offers advantages such as ultrahigh sensitivity and active tunability, intrinsic molecular damping weakens the interaction between vibrational modes and plasmons. Here, we demonstrate ultrahigh-sensitive molecular sensing based on synthesized complex-frequency waves (CFW). Our experiment shows that CFW can amplify the molecular signals (~1.2-nm-thick silk protein layer) detected by graphene-based sensor by at least an order of magnitude and can be universally applied to molecular sensing in different phases. Our approach is highly scalable and can facilitate the investigation of light-matter interactions, enabling diverse potential applications in fields such as optical spectroscopy, metasurfaces, optoelectronics, biomedicine and pharmaceutics.

Motivation & Objective

• To overcome the limitation of intrinsic molecular damping in plasmonic sensors that weakens light-matter interactions.
• To develop a scalable, tunable excitation method that enhances sensitivity in molecular sensing beyond conventional plasmonic approaches.
• To enable practical, high-sensitivity detection of trace molecules in various phases using graphene-based SEIRA platforms.
• To explore the potential of complex-frequency waves in manipulating light-matter interactions for advanced sensing applications.

Proposed method

• The authors design and implement synthesized complex-frequency waves (CFW) with tailored real and imaginary frequency components to excite plasmonic modes in graphene nanostructures.
• CFW is engineered to match the resonant response of target molecular vibrations, enhancing the coupling between plasmons and molecular dipole transitions.
• The method leverages active tunability of graphene to dynamically adjust plasmonic resonances for optimal excitation of specific vibrational modes.
• Experimental validation is performed using a graphene-based SEIRA sensor with a 1.2-nm-thick silk protein layer as a test sample.
• The system measures signal enhancement by comparing response amplitudes under CFW excitation versus conventional monochromatic excitation.
• The approach is validated across different molecular phases, confirming universal applicability.

Experimental results

Research questions

• RQ1 Can complex-frequency excitation significantly enhance molecular signal response in graphene-based SEIRA sensors?
• RQ2 How does CFW compare to conventional excitation in terms of signal amplification and sensitivity?
• RQ3 To what extent can CFW be universally applied across different molecular phases in sensing?
• RQ4 What is the role of plasmon-molecular vibration coupling in determining the enhancement factor under CFW?
• RQ5 Can the method be scaled and adapted for use in diverse applications such as biomedicine and optoelectronics?

Key findings

• The synthesized complex-frequency excitation (CFW) amplifies molecular signals by at least one order of magnitude compared to conventional excitation in a graphene-based SEIRA sensor.
• The method achieves significant signal enhancement even for a 1.2-nm-thick silk protein layer, demonstrating ultrasensitive detection of trace molecular layers.
• The enhancement is robust across different molecular phases, confirming the universal applicability of CFW in molecular sensing.
• The approach is highly scalable and compatible with existing plasmonic and metasurface platforms for integrated sensing.
• The technique enables active tuning of plasmonic responses to maximize coupling with specific molecular vibrational modes.
• The results open new pathways for high-sensitivity optical spectroscopy, biomedicine, and pharmaceutical analysis.